ZETA POTENTIAL CHARACTERIZATION OF HOLLOW FIBER … fileZeta potential characterization of hollow...

Master thesis

ZETA POTENTIAL CHARACTERIZATION OF HOLLOW FIBER MEMBRANES

May, 2018 Vita Petek Regoršek

Vita Petek Regoršek

Zeta potential characterization of hollow fiber membranes

Master thesis

Maribor, 2018

Zeta potential characterization of hollow fiber

membranes

Master thesis study program of II. level

Student: Vita Petek Regoršek

Study program: Master study program II. level Chemistry

Estimated professional title: Master of Chemistry

Advisor: Assist. Prof. Irena Petrinić, PhD

Coadvisor: Thomas Luxbacher, PhD

Maribor, 2018


I

Contents

Contents ..................................................................................................................................... I Statement ................................................................................................................................. II

Acknowledgement .................................................................................................................. III Abstract ................................................................................................................................... IV Povzetek ................................................................................................................................... V List of tables ........................................................................................................................... VI List of figures ........................................................................................................................ VII

List of abbreviations and symbols ....................................................................................... VIII 1 Introduction ....................................................................................................................... 1

1.1 Polymer membranes ................................................................................................... 1 1.2 Surface characterization ............................................................................................. 3 1.3 Literature review ........................................................................................................ 6 1.4 Goal of the thesis ....................................................................................................... 7

2 Theoretical section ............................................................................................................ 8

3 Experimental section ....................................................................................................... 12

3.1 Materials and methods ............................................................................................. 12 3.1.1 Microfiltration and ultrafiltration membranes .................................................. 12 3.1.2 Chemicals ......................................................................................................... 14

3.1.3 Surface zeta potential analysis .......................................................................... 15 3.2 3.2 Characterization steps on SurPASS ................................................................... 18

4 Results ............................................................................................................................. 21 4.1 Measurements of dI/dp, conductance, zeta potential of FS PES, HF PES and HF

PVDF 21 4.2 Influence of pH on the zeta potential ....................................................................... 28

5 Discussion ....................................................................................................................... 30

5.1 Effect of membrane porosity.................................................................................... 30 5.2 Effect of ionic strength ............................................................................................. 36

5.3 Effect of pH .............................................................................................................. 38 6 Conclusion ....................................................................................................................... 40

7 Literature ......................................................................................................................... 41


II

Statement

I declare, that I have created a master's thesis by myself, and the contributions of others are

specially marked. I reviewed literature in the field of master thesis in the following

Source: Web of Knowledge (apps.webofknowledge.com)

Keywords: Number of

references

hollow fiber AND flat sheet AND zeta potential 34

streaming current AND streaming potential 48

ultrafiltration AND hollow fiber membrane 101

microfiltration AND hollow fiber membrane 57

Source: COBISS/OPAC (http://www.cobiss.si/scripts/cobiss?ukaz=getid, COBIB.SI)

Keywords: Number of

references

membrane characterization AND surface zeta potential 36

membrane porosity AND hollow fiber 16

membrane porosity AND flat sheet 29

Total number of reviewed articles: 134

Total number of reviewed books: 12

Maribor, may 2018 Vita Petek Regoršek

signature


III

Acknowledgement

I would first like to thank my thesis advisor Assist. Prof. Irena

Petrinić, PhD of the Faculty of Chemistry and Chemical

Engineering at University Maribor. The door to Prof. Petrinić

office was always open whenever I ran into a trouble spot or had

a question about my research or writing. She consistently allowed

this paper to be my own work but steered me in the right direction

whenever she thought I needed it.

I would also like to acknowledge Thomas Luxbacher, PhD of the

company Anton Paar GmbH in Graz as the second reader of this

thesis, and I am very gratefully indebted to him for his very

valuable comments on this thesis. Without his participation and

input, the validation survey could not have been successfully

conducted.

Finally, I must express my very profound gratitude to my parents

for providing me with unfailing support and continuous

encouragement throughout my years of study and through the

process of researching and writing this thesis. This

accomplishment would not have been possible without them.

Thank you.


IV


Abstract

The zeta potential receives increasing significance in the characterization of the surface

properties and performance of filters and membranes used in various water purification

processes. Besides flat sheet membranes, polymer membranes in the shape of capillaries

(hollow fiber membranes) are used more frequently. The complex geometry of hollow fiber

membranes as well as the importance to characterize both the inner and outer surfaces

challenge the analytical method of the surface zeta potential. While the analysis of the surface

charge of flat sheet membranes by means of streaming potential and streaming current

measurements is well established today, the characterization of the outer surface of hollow

fiber membranes still remains qualitative. The aim of this thesis is to obtain quantitative zeta

potential results for the outer surface of polymer hollow fiber (HF) membranes. A correlation

of results obtained for a series of HF membranes for ultrafiltration with those obtained for flat

sheet membranes made of the same polymer shall elucidate the effects of sample mounting

and membrane porosity on the determination of the zeta potential. In order to investigate this

problem, the zeta potential was determined for the three polymeric materials: polyethersulfone

(flat sheet), polyethersulfone (HF) and polyvinylidene fluoride (HF) for microfiltration and

ultrafiltration. Results of zeta potential for HF membranes confirmed that characterization of

outer surface of HF membrane is possible.

Key words: hollow fiber membranes, porous membranes, membrane characterization,

electrokinetic characterization, zeta potential, streaming current.

UDK: [544.638:678.74-486.32]:66.081.6(043.2)


V

Karakterizacija zeta potenciala votlo-vlaknastih membran

Povzetek

Zeta potencial dobiva vse večji pomen pri karakterizaciji površinskih lastnosti in učinkovitosti

filtrov in membran, ki se uporabljajo v različnih procesih čiščenja vode. Poleg ploskovnih

membran so pogosteje uporabljene polimerne membrane v obliki kapilar (membrane votlih

vlaken). Kompleksna geometrija votlo-vlaknastih membran ter pomembnost karakterizacije

notranje in zunanje površine izzove analitsko metodo površinskega zeta potenciala. Do danes

je analiza površinskega naboja ploščatih membran s pomočjo pretočnega potenciala in

pretočnih meritev že zelo dobro raziskana, medtem ko je karakterizacija zunanje površine

votlo-vlaknastih membran še ostaja nepojasnejena. Namen naloge je pridobiti kvantitativne

rezultate zeta poteniciala za zunanjo površino votlo-vlaknastih (ang. hollow fibre (HF))

polimernih membran. Primerjava dobljenih rezultatov za serijo HF membran za ultrafiltracijo

s tistimi, dobljenimi za ploščate membrane iz istega polimera, bodo razjasnile učinke vgradnje

vzorcev v merilno celico za določanje zeta potenciala. Da bi raziskali ta problem, smo določili

zeta potencial za tri polimerne materiale: polietersulfon (ploska membrana), polietersulfon

(HF) in poliviniliden fluorid (HF) za mikrofiltracijo ter ultrafiltracijo. Rezultati zeta potenciala

za HF membrane so potrdili, da je možna karakterizacija zunanje površine HF

membrane. Rezultati so pokazali tudi večje razlike med zeta potencialom zunanje strani FS in

HF PES membrane. Pri merjenju pH v odvisnosti od zeta potenciala smo določili izoelektrično

točko tako za FS kot tudi za HF membrane.

Ključne besede: votlo-vlaknaste membrane, porozne membrane, karakterizacija membran,

elektrokinetična karakterizacija, zeta potencial, pretočni tok.

UDK: [544.638:678.74-486.32]:66.081.6(043.2)


VI

List of tables

Table 1-1. Characterization methods for membranes. .............................................................. 3

Table 3-1. Characteristics of membranes given by manufacturer. [20] .................................. 14

Table 4-1. Measured values of MicroPES in 1, 5 and 10 mM KCl solution. (a) at gap height

100 μm, (b) Fig.1, (c) Fig.2, (d) for calculation see discussion. ............................................. 23

Table 4-2. Measured and calculated values of soaked and dry PES membrane in 1, 5 and 10

mM KCl solution. (a) at gap height 100 μm, (b) Fig.1, (c) Fig.2, (d) for calculation see

discussion. ............................................................................................................................... 25

Table 4-3. Measured and calculated values of soaked and dry hollow fiber PVDF membrane

in 1, 5 and 10 mM KCl solution. (a) at gap height 100 μm, (b) Fig.1, (c) Fig.2, (d) for

calculation see discussion. ...................................................................................................... 27

Table 5-1. More effective gap heights for HF PES membrane. .............................................. 32

Table 5-2. Corrected streming current offsets for HF PES membrane. .................................. 33


VII

List of figures

Figure 1-1. Cross section SEM image of polymer of an Express PLUS Membrane PES Filter

(Millipore GPWP02500) and Durapore Membrane PVDF Filter (Millipore GVWP02500).

Skin layer thickness of membrane [3]. ..................................................................................... 2

Figure 1-2. Water drop behavior and contact angle values at solid of different hydrophobicity.

.................................................................................................................................................. 4

Figure 1-3. Selection of electrokinetic effects at a solid-liquid interface [1]. .......................... 5

Figure 2-1. Schematic drawing of a rectangular slit channel formed between solid samples

with a flat surface such as flat sheet membranes. ..................................................................... 9

Figure 3-1. SEM image of the cross-section of the ultrafiltration PES membrane [2]. ......... 12

Figure 3-2. Chemical structure of polyethersulfone [18]. ...................................................... 13

Figure 3-3. Chemical structure of polyvinylidene fluoride [19]............................................. 13

Figure 3-4. SurPASS electrokinetic analyzer used for measuring streaming potential and

streaming current. Components: (a) 3-way valve, (b) syringes for electrolyte transport, (c)

pressure transducers, (d) measuring cell, (e) pH electrode, (f) conductivity probe. ............... 15

Figure 2-2. Schematic representation of mounting a flat sheet membrane inside the adjustable

gap cell [22]. ........................................................................................................................... 16

Figure 3-7. Flow rate vs. pressure for MicroPES membrane at 80 μm. ................................. 19

Figure 4-1. Streaming current coupling coefficient dIstr/dp for FS PES membrane at different

gap height................................................................................................................................ 21

Figure 4-2. Conductance vs. gap height. ................................................................................ 22

Figure 4-3. Average zeta potential measuring dU/dp and dI/dp at 100 μm, and dI/dp correct.

................................................................................................................................................ 22

Figure 4-4. The streaming current of hollow fiber PES membranes as a function of gap height

in 0,001 M, 0,005 M and 0,01 M KCl solution. ..................................................................... 23

Figure 4-5. Conductance of hollow fiber PES membranes at different gap heights and different

solution concentrations. .......................................................................................................... 24

Figure 4-6. Zeta potential at pH 6 of polymeric membranes obtained with AGC cell. .......... 24

Figure 4-7. Measured streaming current of hollow fiber PVDF membrane as a function of gap

height. ..................................................................................................................................... 25

Figure 4-8. Conductance of hollow fibre PVDF membranes as a function of gap height at 0,001

M, 0,005 M and 0,01 M KCl solution. ................................................................................... 26

Figure 4-9. Zeta potential measuring streaming potential, streaming current and streaming

current correct at 100 μm. ....................................................................................................... 26

Figure 4-10. The zeta potential of flat sheet PES membranes as a function of pH in 0,001 M,

0,005 M and 0,01 M KCl solution. ......................................................................................... 28

Figure 4-11. The zeta potential of the hollow fiber PES membrane at various pH values. ... 29

Figure 4-12. pH dependence of zeta potential for hollow fiber PVDF membrane................. 29

Figure 5-1. More effective gap height. ................................................................................... 33

Figure 5-2. After measurements hollow fibers are squeezed together. .................................. 34

Figure 5-3. Streaming potential coefficient as a funtion of pH for flat sheet PES. ................ 39


VIII

List of abbreviations and symbols

Latin symbols

C Conductance (S)

H Gap height (m)

I Ionic strength (mol/l for 1:1 electrolyte)

Istr Streaming current (A)

L Length (m)

p Pressure (Pa)

R Ohm resistance (Ω)

Ustr Streaming potential (V)

W Width (m)

dm Membrane thickness (m)

Greek symbols

Conductivity of electrolyte (S/m)

Membrane porosity (/)

Zeta potential (V)

e Zeta potential of the external membrane surface (V)

p Conductivity of electrolyte solution in the pores (S/m)

peff Effective zeta potential inside pores (V)

𝜀0 Vacuum permittivity (8.85410-12 F/m)

ε Dielectric constant (/)

η Viscosity (Pa s)

ϕstr Streaming potential coupling coefficient (V/Pa)

Abbreviations

AGC Adjustable Gap Cel

CLC Clamping Cell

FM Fairbrother-Mastin

FS Flat sheet

FSP Filtration streaming potential

HF Hollow fibers

H-S Helmholtz-Smoluchowski

ID Inner diameter

IEP Isoelectric point

MBR Membrane bioreactor

MF Microfiltration


IX

NF Nanofiltration

OD Outer diameter

PES Polyethersulfone

PSf Polysulfone

PVDF Polyvinylidene fluoride

UF Ultrafiltration


1

1 Introduction

1.1 Polymer membranes

Clean water accessibility is a serious concern in all aspects of society, generating social,

economic and environmental problems. Obtaining purified water for drinking or industrial

purposes is becoming an enormous challenge.

A variety of different filtration and membrane processes for separation and purification have

been developed. The challenge for all membrane and filter applications is the optimization of

rejection, flux, energy consumption, lifetime, etc. Membrane and filter parameters such as

pore size, porosity, surface roughness, wettability, and surface charge determine the

applicability of the filter or membrane to a different extent. [1]

Membranes and filters are made of different materials and exist in different shapes. The most

common forms are polymer membranes and ceramic filters. We find flat sheet (FS) and hollow

fibre (HF) membranes and filters. Membranes can be made of a single material or prepared as

a composite of different materials. Flat sheet microfiltration (MF) membranes are symmetric

in terms of their pore size distribution and applicable for filtration using both sides. UF

membranes usually have an asymmetric pore size distribution and occur as FS and HF

membranes. Nanofiltration (NF) and reverse osmosis (RO) membranes are composites of a

dense polyamide-based thin film deposited on a polyurethane UF membrane. NF and RO

membranes are mechanically supported by a polyester nonwoven.[1]

Ultrafiltration (UF) as a versatile membrane separation technology has been widely applied in

water purification processes for the removal of particles, turbidity, microorganisms, and

natural organic matter from surface water and groundwater. This method offers several

advantages such as consistently high quality of water, capable of removing a wide range of

substances, and fewer additional chemicals for feed water pretreatment. The UF technology

was established with large plants installed worldwide since 1980 and rapidly expanding due

to the need for purfying drinking water. Researchers today still develop high performance UF

membrane modules for drinking water treatment. Membranes are fabricated and modified with

various methods with the purpose to obtain UF membranes with high flux, high rejection, low

fouling propensity, good chemical resistance, and mechanical stability. [2]

Modern separation technologies such as sterilization filtration, haemodialysis, and

applications of separation technologies such as production of fine chemicals, processes of the

dairy industry and wastewater treatment, etc., are predominantly based on using porous

polymer membranes. The required process conditions demand high chemical and physical

stability of the membrane material. Therefore, polymer membranes are fabricated from robust

synthetic materials such as polyethersulfone (PES), polysulfone (PSf), or polyvinylidene

fluoride (PVDF), which offer high stability within a broad range of process conditions.

However, membranes made from these polymers are prone to fouling, which is caused by


2

hydrophobic interactions of the membrane surface with biomolecules or colloids in the feed

solution and results in an irreversible adsorption, aggregation, ripening, and finally in a

reduced filtration performance.

The separation performance depends on membrane properties such as hydrophilicity, pore size

and pore distribution, surface charge, and membrane thickness. The hydrophilicity, porosity

and skin layer thickness of a membrane can be modified by the addition of additives to the

casting solution such as pyrrolidine, polyethylene glycol, etc. [1] Figure 1-1. shows membrane

composition of membrane for microfiltration. It shows that on the microporous support lies a

selective thin layer - surface that requires analysis. Figure 1-1. shows an SEM image of the

PES filter provided by the Millipore website.

Membranes by Millipore contain a dull side and a shiny side, which should not affect

performance for most applications (Millipore). The shiny side of the membrane is the tighter

side, reflecting the asymmetric structure of the filters. [3]

Figure 1-1. Cross section SEM image of polymer of an Express PLUS Membrane PES Filter

(Millipore GPWP02500) and Durapore Membrane PVDF Filter (Millipore GVWP02500). Skin layer

thickness of membrane [3].

HF membranes have been generally used in membrane bioreactor (MBR) technology as they

are mechanically self-supporting and easily assembled in modules for different membrane

applications. Although HF membranes have many advantages, their applications are

sometimes limited by problems of low tensile strength. [4]


3

1.2 Surface characterization

In recent years, the surface engineering of polymer membranes through surface modification

and surface functionalization has received significant attention. Since the properties of a

membrane surface are very important for practical applications, it is important to have the

means to characterize and measure these surface properties. Surface characterization is

important for understanding the membrane structure and its properties. It is well known that

various aspects of a membrane surface, which include chemical composition, morphology and

topography, wettability, and surface charge, can affect the properties and applications

remarkably.

Many kinds of surface characterization techniques may be applied to study the surface

properties of polymer membranes. Techniques for the characterization of a polymer membrane

surface are attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy,

X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SSIMS),

energy dispersive X-ray spectroscopy (EDS), optical microscopy, laser confocal scanning

microscopy (LCSM), scanning electron microscopy (SEM), environmental scanning electron

microscopy (ESEM), atomic force microscopy (AFM), contact angle measurement.[1]

Characterization techniques (Table 1-1.) can be classified into static and dynamic techniques.

The static techniques mainly give information on membrane morphology and structure,

chemical and physical properties. The dynamic techniques are of fundamental importance

when investigating membrane performance. Some characterization techniques are destructive

for the membrane, while the non-destructive ones are applied also to monitor the membrane

performance during its use. Table 1 shows the main characterization tests. [6]

Table 1-1. Characterization methods for membranes.

Method Characteristics

Bubble pressure Maximum pore size

Scanning Electron Microscopy (SEM) Top layer thickness, surface porosity

Atomic force microscopy (AFM) Surface roughness, surface porosity

Transmission Electron Microscopy (TEM) Pore size distribution, qualitative structure

analysis

SEM + X-Ray microanalysis (EDS) Surface/chemical studies

Infrared Spectroscopy (FT-IR) Functional group analysis, surface studies

Contact angle measurement Surface studies

Gas and liquid displacement methods

(GLDP, LLDP)

Pore size distribution

X-ray photoelectron spectroscopy (XPS) Detection of elements

Bubble pressure was initiated by Bechhold in 1908 and is based on the fact that the pressure

(p) necessary to blow a gas through a liquid-filled capillary is inversely proportional to the


4

Figure 1-2. Water drop behavior and contact angle values at solid of different hydrophobicity.

capillary radius (r). Unfortunately, the test does not give information on the membrane pore

size distribution and because of the very high air-water surface tension high pressure (which

can cause membrane compression) is needed for gas permeation through small pores (in fact

the lower limit of the measurable pore diameter is about 13 nm). The liquid-liquid porosimetry

(LLDP) is a method that can be used to provide information on the pore size distribution of

membranes with small pores. The procedure is based on the same principles of the air- liquid

displacement or extended bubble point technique, both methods using the correlation between

the applied pressure and the pore radius open to flux. Permeability of a membrane for a certain

liquid can be considered as a characteristic parameter; often the so-called hydraulic radius is

calculated from the measured fluxes.

The permporometry (pore size distribution) is a method based on the fact that the vapour

pressure at the surface of a liquid depends on its curvature. The vapour is capillary-condensed

in membrane pores thus blocking the permeation of the non-condensable gas in the feed. [8]

The aim of electron microscopy method is to acquire visual information about the membrane

structure and porosity through magnification by scanning electron microscope (SEM) or

transmission electron microscopy (TEM).

Contact angle is a widely used analysis for the characterization of the membrane’s

hydrophilic/hydrophobic behavior (Figure 1-2.), the effect of chemical modification like

cross-linking on the latter, or when introducing a hydrophilic or hydrophobic solvent-stable

polymeric material.

Atomic force microscopy can be used to image single molecules, proteins, monitor surface

topography, and measure the forces that hold biological structures together. This technique

gives invaluable insight into specific biological processes, none of them provides chemically

specific information. X-ray photoelectron spectroscopy (XPS), also called electron


5

spectroscopy for chemical analysis (ESCA), is the most widely used ultrahigh-vacuum surface

analysis technique. The adsorption of the X-rays by atoms in the sample leads to the ejection

of core and valence electrons (photoelectrons). XPS detects all elements except H and He. [7]

The zeta potential, a. k. a. the electrokinetic potential describes the charging behavior at

interfaces. Most of scientific papers and practical applications use and report the zeta potential

for the characterization of the solid-liquid interface. The zeta potential is an interfacial

property that is of great importance for understanding the behaviour of solid materials in many

technical processes. It gives insight into the charge and adsorption characteristics of solid

surfaces. The determination of the zeta potential involves the measurement if an electrokinetic

effect, which depends on the size and type of the solid material. The most prominent

electrokinetc effects are schematically shown in the Figure 1-3.

Figure 1-3. Selection of electrokinetic effects at a solid-liquid interface [1].

The electrokinetic effect is a coupling of a mechanical and an electrical force where the driving

force for the movement may either be of mechanical or electrical nature. Electrophoresis,

electro-osmotic flow and electrokinetic sonic amplitude are electrokinetic phenomena where

the driving force in an applied electric field. E.g., in electrophoresis a direct current is used to

motivate charged colloidal solid or liquid particles immersed in a liquid to move towards the

electrode of the opposite sign. [1] A mechanical force is applied to create the electrical

response of the streaming potential (by a pressure gradient) and the sedimentation potential

(by gravitational force).


6

1.3 Literature review

The majority of the scientific literature that includes solid surface zeta potential data reports

on polymer membranes for various water treatment applications, and on the analyses of

membranes used in hemodialysis and for biotechnological processes.

Childress et al. were the first to intensively characterize the membrane surface chemistry. They

correlated the performance of membranes with the zeta potential. [22]

Most studies of functionalization have been done on FS membranes for water desalination,

including: sustainable green synthesis of metallic nanoparticles (NPs) with good effectiveness

and low toxicity [9]; enzyme incorporation by layer-by-layer deposition [10]; pH and

temperature responsive applications [11]. Many of these functionalization processes are

carried out by the cross-linking of polymers (hydrogels) on top of the porous structure of the

membrane, without affecting the physicochemical properties of the material.

However, most sponge-like membrane functionalization was done on other membranes than

those made of PVDF, and there are relatively few studies describing functionalization on this

material [12]. PVDF membranes are commonly produced in the shape of hollow fibers (HF).

Most HF membranes are hydrophobic due to their fabrication by thermally induced phase

separation (TIPS). Hollow fibers show good mechanical strength and porous performance due

to their thickness and high surface area.

To increase the hydrophilicity of PVDF, numerous scientific studies have been made, from

blending to surface modification [13]. In the casting modification, hydrophilic materials such

as polyethylene glycol (PEG), poly(methyl methacrylate) (PMMA) or polyvinylpyrrolidone

(PVP) have been incorporated. Surface modification can be made by cross-linking poly(vinyl

alcohol) (PVA) or PVP; by covalent bonding of hydrophilic moieties following alkaline

pretreatment, or by grafting them onto the polymer chain.

Salgin et al. conclude in their study that at high pH values, anion adsorption on the membrane

surface had a more potent effect on zeta potential, while at low pH values, cation adsorption

on the membrane surface had also possessed more potent effect on zeta potential. [23] Coday

at al. conclude that in general, zeta potential decreased with increasing ionic strength due to

the compression of electrical double layer. [15]

Some studies have been done to promote the mechanical properties of hollow fiber

membranes, such as reinforced membrane, which displays good tensile strength and a self-

supporting compatibility. However, a problem remains that the separation layer could be easily

peeled off from the reinforced matrix of heterogeneous reinforced hollow fiber membrane, as

the separation layer and reinforced matrix show too little compatibility. It is desirable that the

homogenous reinforced HF membrane uses the same material between the reinforced matrix

and the separation layer to enhance the interfacial bonding. [4]


7

There have been a few studies comparing the performance of PES and PVDF membranes,

mainly focusing in the filtration efficiency. A study comparing the performance of PVDF and

PES in filtering viral suspensions have been reported [16]. This study by Moce-Llivina tested

the filtering capacity of both types of membranes by comparing the filtration rate and volume

that could be filtered before clogging the system. Results concluded that PES membranes were

as affective as the often used PVDF membranes for sewage samples. However, PES allowed

higher filtration rate and clogged more slowly. In addition, the group recommended the use of

PES membrane filters due to their lower cost and a more efficient method for high recovery

of viruses after decontamination by filtration of viral suspensions. Both membranes in this

study were purchased from Millipore (Bedford, MA). [16]

Arahnam et al. show that PES membranes have an excellent chemical resistance, a desirable

mechanical strength, and are applicable in a wide temperature range. This polymer is widely

used in membrane preparation for a various applications. [1]

The knowledge of zeta potential also provides valuable information to explain the mechanisms

of interfacial interactions that occur at flat macroscopic surfaces such as polyethersulphone

membrane. [23]

1.4 Goal of the thesis

The surface properties of filters and membranes (flat sheet, asymmetric, thin-film composite,

and hollow fiber polymer membranes) determine their separation performance. Surface charge

is among those properties, which are extremely important especially for microfiltration and

ultrafiltration.

Surface characterization of HF is difficult because of their tubular geometry. This master thesis

focuses on the characterization of the outer (shell) side of HF membranes. The streaming

potential method enables an almost non-destructive assessment of the surface charge for both

inner and outer hollow fiber membrane surfaces. Goal of this thesis is to characterize the outer

surface, which requires more effort for sample preparation and knowledge about the effect of

the wavy surface of a grid of aligned HF membranes on the zeta potential analysis. Goal of

this thesis is to prove a method, which was proposed by prof Tzahi Cath of Colorado School

of Mines, US, for a reliable and reproducible analysis of the zeta potential for the

characterization of the shell side of HF membranes.


8

2 Theoretical section

Every solid object in solution has a surface charge, and so a distribution of ions near the surface

occurs. Passing a liquid over the surface disrupts this distribution and creates a potential

difference, the streaming potential. In this technique, fluid is passed over the solid sample at

different pressures, and the streaming potential is measured. This is then converted to zeta

potential. The surface charge at the solid/water interface determines the electrostatic

interactions between the solid surface and dissolved components in the aqueous phase. The

zeta potential is an important parameter to describe solid surfaces and the interaction with the

surrounding liquid.

The surface charge of solid samples such as flat sheet and hollow fiber membranes is

commonly assessed by the surface zeta potential . The zeta potential is defined as the electric

potential difference between the solid-water interface and the bulk aqueous solution. It is

located at the shear plane of the electric double layer. The surface zeta potential is calculated

from the measurement of either the streaming potential Ustr or the streaming current Istr.

At the interface between a solid material surface and a liquid, most commonly an aqueous

solution, a charge distribution is generated, which differs from the charge (ion) distribution in

the bulk solution. Water interacts with the solid surface and introduces surface or interfacial

charge (either by dissociation or protonation of surface functional groups or by the adsorption

of water ions). This charge is compensated by ions of opposite charge that accumulate at the

solid-water interface.

When the aqueous solution is forced to move along the charged solid surface by applying a

pressure gradient, a streaming current Istr is generated. The liquid flow moves the charge-

compensating ions in the flow direction and leads to a charge separation. The charge separation

introduces an electric field, which opposes a backflow current of ions to the streaming current.

When streaming and backflow currents are in equilibrium, a potential difference exists, which

is called the streaming potential Ustr.

For the measurement of streaming potential or streaming current, the solid sample needs to be

arranged in such a way that a capillary channel (flow channel) is formed between sample

surfaces. For flat sheet porous membranes, a set of two identical membrane samples may be

mounted opposite of each other thereby creating a flow channel with a rectangular cross-

section (Figure 2-1). The streaming potential/current may then be measured along the

membrane surface (tangential streaming potential/current).


9

Figure 2-1. Schematic drawing of a rectangular slit channel formed between solid samples with a flat

surface such as flat sheet membranes.

Alternatively, the measuring solution may pass the pores of the membrane, which introduce a

network of capillaries (pores). The measurement then takes place by streaming the liquid

through the pores (filtration streaming potential/current).

For hollow fiber membranes the liquid may pass through the inner volume (lumen) of these

capillary membranes and the zeta potential of the inner surface becomes accessible. For the

characterization of the outer surface of hollow fiber membranes, two grids of membrane pieces

aligned in parallel are mounted again opposite of each other and the aqueous solution

permeates the flow channel, whose complex geometry may be approximated by a rectangular

slit channel.

For flat porous membranes, streaming potential is easily measured either along the membrane

surface (tangential streaming potential or TSP) or through the membrane (filtration streaming

potential or FSP). [1]

For the calculation of the surface zeta potential, the streaming current is related to the geometry

of the flow channel. The simplest case is either a cylindrical capillary with known length and

cross-section (diameter) or a rectangular slit channel, whose geometry is determined by its

length L, width W, and height H (Figure 2.1). The zeta potential is then calculated according

to the equation by Helmholtz and von Smoluchowski,

𝜁 =𝑑𝐼𝑠𝑡𝑟

𝑑𝛥𝑝×

𝜂

𝜀𝑟×𝜀0×

𝐿

𝑊×𝐻 Eq. 3

and r are the viscosity and dielectric coefficient of water, and 0 is the vacuum permittivity.

The derivation of Eq. 3 exceeds the scope of this thesis and is available, e.g., in Werner et al.

[ref]. The length L and width W of the slit channel are determined by the size of the sample

(e.g., L = 20 mm and W = 10 mm for flat sheet membranes), and the gap height H is the


10

distance between sample surfaces. The zeta potential is a solid-liquid interfacial property,

which is confirmed by its dependence on the liquid viscosity and dielectric property. The

viscosity determines the shear force, which the moving liquid acts on the interfacial charge,

and the dielectric coefficient of the solvent determines its ability to create and stabilize charges

(surface charge and dissolved ions).

For a given solid material and an aqueous solution, i.e., at a fixed zeta potential, the streaming

current coefficient dIstr/dp depends linearly on the gap height H,

𝑑𝐼𝑠𝑡𝑟

𝑑𝛥𝑝= 𝜁 ×

𝜀𝑟×𝜀0

𝜂×

𝑊

𝐿× 𝐻 Eq. 4

The streaming current is a d.c. current generated by electrokinetic means and thus related to

the streaming potential (a d.c. voltage) by Ohm’s law,


𝑑𝛥𝑝=

𝑑𝑈𝑠𝑡𝑟𝑑𝛥𝑝

𝑅 Eq. 5

with R being the Ohm resistance inside the capillary channel. By inserting Eq. 5 in Eq. 3 we

obtain a relation between the surface zeta potential and the streaming potential,

𝜁 =𝑑𝑈𝑠𝑡𝑟

𝑑𝛥𝑝×

𝜂

𝜀𝑟×𝜀0×

𝐿

𝑊×𝐻×

1

𝑅 Eq. 6

which requires the additional knowledge of the electric resistance. Using the relation between

resistance and conductivity ,

𝜅 =1

𝑅×

𝐿

𝑊×𝐻 Eq. 7

which is also used for the classical measurement of the electric conductivity of aqueous

solutions, Eq. 6 transfers into

𝜁 =𝑑𝑈𝑠𝑡𝑟

𝑑𝛥𝑝×

𝜂

𝜀𝑟×𝜀0× 𝜅𝐵 Eq. 8

The index B refers to the conductivity of the bulk aqueous solution since the conductivity

inside the capillary channel cannot be measured directly. The validity of Eq. 8 implies the

assumption that the conductivity inside the flow channel is carried by the aqueous solution

only, which is true for non-conductive and non-porous material surfaces that do not undergo


11

swelling in water. Even for material surfaces with such an ideal behaviour, the conductivity

inside the flow channel may differ from the conductivity of the bulk aqueous solution. The

accumulation of surface-charge-compensating ions at the solid-liquid interface gives rise to an

interfacial conductance (also referred to as the surface conductance), whose effect on the

calculation of the surface zeta potential by Eq. 8 is suppressed at a sufficiently high ionic

strength of the aqueous solution and a sufficiently large distance between sample surfaces. For

low ionic strength (I < 0.001 mol/l) or small distances between sample surfaces (H < 30 µm),

Eq. 8 gives an apparent zeta potential with a magnitude estimated too low. Fairbrother and

Mastin [ref] suggested an experimental approach to correct for the influence of surface

conductance for sample arrangements with capillaries of unknown geometry such as bundles

of fibers. The cell constant, i.e., the ratio of channel length and cross-section, is then

determined using Eq. 5 and an ionic strength, which equals the surface conductance, such as I

= 0.1 mol/l.

The approach by Fairbrother and Mastin, however, is not applicable to compensate ionic

conductance introduced by a porous material in contact with water. For flat sheet membranes,

the calculation of the zeta potential from streaming current measurement according to Eq. 3 is

applicable. However, depending on the membrane pore size, an additional contribution to the

streaming current occurs, which requires an extension of the Helmholtz-Smoluchowski

equation. Besides the streaming current inside the flow channel, the pressure gradient along

the porous membrane provokes a streaming current inside pores. The extended Helmholtz-

Smoluchowski equation includes the zeta potential inside pores and may be written as [23].


𝑑𝛥𝑝=

𝜀𝑟×𝜀0

𝜂×

𝑊

𝐿× (𝐻 × 𝜁𝑒 + 2 × 𝑑𝑚 × 𝛾 × 𝜁𝑝

𝑒𝑓𝑓) Eq. 9

e and peff denote the zeta potential of the external membrane surface and the effective zeta

potential inside pores, respectively, dm is the membrane thickness, and represents the

membrane porosity including the pore tortuosity. An analogue approach is used to assess the

conductivity m inside membrane pores. An extension of Eq. 7 gives

𝐶 =1

𝑅=

𝑊

𝐿× (𝐻 × 𝜅𝐵 + 2 × 𝑑𝑚 × 𝛾 × 𝜅𝑚), Eq. 10

For the zeta potential analysis of flat sheet and hollow fiber membranes (outer surface), we

apply Eqs. 9 and 10 to distinguish between contributions of the outer membrane surface and

the membrane pores to the zeta potential and the electric conductivity.

For the determination of the isoelectric point (IEP, i.e., pH where = 0 mV), we use Eq. 4 at

a fixed gap height between sample surfaces.


12

3 Experimental section

3.1 Materials and methods

3.1.1 Microfiltration and ultrafiltration membranes

All the experiments in this work were carried out with a commercial flat sheet polymeric

microfiltration membrane (MicroPES 1F EL, polyethersulfone (Membrana GmbH, Germany),

and two prototype hollow fiber membranes (PES batch 61B (T20), polyethersulfone; PVDF

batch 35B (T20), polyvinylidene fluoride). The HF membranes were provided by Fraunhofer

Institute for Interfacial Engineering and Biotechnology (FhG IGB, Stuttgart, Germany).

Polysulfone (PSf) and polyethersulfone (PES) are widely used for the preparation of

microfiltration (MF), ultrafiltration (UF), and gas separation membranes. They show the

favorable characteristics of a wide temperature range, wide pH tolerances, and a fairly good

chlorine resistance. PSf and PES make it easy to fabricate membranes in a wide variety of

configurations and modules with a wide range of pore sizes available for UF and MF

applications.

Figure 3-1. SEM image of the cross-section of the ultrafiltration PES membrane [29].


13

Figure 3-2. Chemical structure of polyethersulfone [18].

Polyvinylidene fluoride (PVDF) is valued for its toughness, stability, and distinct engineering

advantages. PVDF is the homopolymer of 1,1-di-fluoro-ethene. Its highly desirable

insolubility and electrical properties result from the polarity of alternating CH2 and CF2 groups

on the polymer chain. It is unaffected by long-term exposure to sunlight and other sources of

ultraviolet radiation. It retains its properties in high vacuum and gamma radiation and is

resistant to most acids and alkalis.

Figure 3-3. Chemical structure of polyvinylidene fluoride [19].

Some characteristics of the membrane as presented by the manufacturer are summarized in

Table 3-1 and the chemical structure is shown in Figures 3-2 and 3-3.


14

Table 3-1. Characteristics of membranes given by manufacturer.

Characteristic Polyethersufone

(FS)

Polyethersufone

(HF)

Polyvinylidene

fluoride (HF)

Pore size 0,1 µm / /

Outer diameter / 1,495 ± 0,039 mm 1,996 ± 0,067 mm

Inner diameter / 1,096 ± 0,033 mm 1,511 ± 0,058 mm

Thickness 110 ± 10 µm 202,01 ± 10,99 µm 240,5 ± 16,1 µm

3.1.2 Chemicals

The analysis of the membrane zeta potential was performed in aqueous solutions of potassium

chloride (KCl, Sigma Aldrich, Germany) at various KCl concentration (various ionic strength)

and at various pH. The aqueous solutions were prepared by dissolving the appropriate amount

of KCl in ultrapure water (ASTM I grade, Milli-Q water purification unit, Millipore, USA).

The solution pH was adjusted using hydrochloric acid (HCl, Sigma Aldrich, Germany) and

sodium hydroxide (NaOH, Sigma Aldrich, Germany). A concentration of 0,05 mol/l was used

for the acid and base stock solutions.


15

3.1.3 Surface zeta potential analysis

Streaming potential and streaming current measurements were performed with the SurPASS

electrokinetic analyzer (Anton Paar GmbH, Austria).

Figure 3-4. SurPASS electrokinetic analyzer used for measuring streaming potential and streaming

current. Components: (a) 3-way valve, (b) syringes for electrolyte transport, (c) pressure transducers,

(d) measuring cell, (e) pH electrode, (f) conductivity probe.

Components of the SurPASS electrokinetic analyzer are presented on Figure 3-4. With this

instrument, the streaming potential or streaming current is recorded with increasing pressure

difference. The “SurPASS” instrument is equipped with an integrated dosing unit for an


16

automatic pH adjustment, an analyzer, and a measuring cell appropriate for the solid sample,

and linked to a PC for software control.

For membrane zeta potential analysis, this instrument measures the streaming current and

streaming potential resulting from the pressure-driven flow of an electrolyte solution that

passes through a thin slit channel formed by two identical sample surfaces. The zeta potential

of flat surfaces can be determined using two different rectangular measuring cells: the

“Clamping Cell” (CLC) and the “Adjustable Gap Cell” (AGC). For CLC, two samples with

55 mm × 25 mm are mounted opposite of each other and separated by a spacer. In the CLC,

an area of only 25 mm × 5 mm of each sample contributes to the measurement (9% of the total

sample area). Bukšek et al.[17] have demonstrated that the CLC is not suited for the zeta

potential analysis of porous membranes.

For the AGC two samples with 20 mm × 10 mm are fixed on sample holders using double-

sided adhesive tape. The distance between the sample surfaces is then adjusted continuously.

For AGC, the complete sample surface is used for the measurement (i.e., 100% of the surface

of a flat sheet membrane). A cross-section of the AGC is schematically shown in Figure 3-5

and the sequence of sample mounting for FS and HF membranes is illustrated in Figure 3-6.

Figure 2-2 shows the shematics of an adjustable gap cell.

Figure 3-5. Schematic representation of mounting a flat sheet membrane inside the adjustable gap

cell [22].


17

(a) (b)

(c) (d)

(e) Figure 3-6. Schematic drawing of sample preparation in the adjustable gap cell: (a) sample holder,

(b) double-sided adhesive tape, (c) placing a flat sheet membrane or (d) a hollow fibre membrane

on the sample holder, (e) assembled AGC

Slit channel between

membrane surfaces.


18

3.2 3.2 Characterization steps on SurPASS

Membrane sample

Dry Soaked

Mounting

Fill

Rinse

𝑑𝐼

𝑑𝑝

𝑑𝑈

𝑑𝑝

Gap adjustment

- Flat sheet membrane MicroPES

- Hollow fibre membrane PES

- Hollow fibre membrane PVDF

- Soaking time (24 h)

- Fill time (200 s)

- Rinse time (200 s)

- Pressure (300 mbar)

- Equilibrate pH

- Nitrogen purging

- Zeta potential

- Flow directions

Figure 3-7. Sample preparation and measurement steps for membrane

characterization with SurPASS.


19

Sample preparation and measurement steps are presented in Figure 3-7. Prior to a new sample

measurement, the electrolyte circuit of SurPASS was first cleaned with warm tap water, then

with Milli-Q water. A clean instrument was confirmed when observing the target pH 5,5 - 6

for a fresh aqueous KCl solution.

Soaked membranes were hydrated and stored in Milli-Q water at room temperature for 24 h.

The membrane samples were mounted in an adjustable gap cell. Figure 3-5 shows the

schematics of this measuring cell. Flat sheet membranes and hollow fibre membranes were

fixed on sample holders using double-sided adhesive tape.

Before the measurements we checked flow rate to confirm linear dependence on pressure.

Figure 3-8. Flow rate vs. pressure for MicroPES membrane at 80 μm.

The measuring points were obtained via averaging over two four repeated measurements with

alternating flow directions. All measured results were collected by instrument software

versions VisioLab for SurPASS v. 2.30 and Attract v. 2.1. The software automatically

calculates the zeta potential and displays the results both as graphs and tables. All the data was

exported in Microsoft Excel for further analysis and data processing.

Streaming potential and streaming current were measured continuously with the pressure

difference increasing to 300 mbars in 20 seconds. Several measured quantities are permanently

accessible.

Measurements of streaming potential and streaming current were performed

• at different distances (gap height) between adjacent membrane samples in the

adjustable gap cell (120, 110, 100, 90, 80 µm),


20

• using different ionic strength of the aqueous KCl solution (0,001 mol/l, 0,005 mol/l,

0,01 mol/l),

• and in the pH range of pH 2-9 (at a fixed gap height) to determine the isoelectric

point.

For the analysis of the zeta potential at different pH, intermediate rinse cycles (180 min rinse

at a pressure difference of 300 mbar corresponding to a flow rate of 100 ml/min) were used

for equilibrating the pH at the membrane-water interface.

,


21

4 Results

4.1 Measurements of dI/dp, conductance, zeta potential of FS PES, HF PES and HF PVDF

In this section the results of membrane zeta potential for dry and soaked membranes are given.

Figure 4-1 shows the dependences of the streaming current coupling coefficient for the FS

PES membrane on the channel height. The dependences are linear, as predicted by eqs 11 and

13. The degree of linearity is high (R2 ranging from 0,882 to 0,988) for the streaming current.

The linearity of the data makes possible reliable determination of the slopes and intercepts.

Figure 4-2 shows cell electric conductance against channel height. Mathematically,

conductance is the reciprocal of resistance. The greater is the resistance, the less is the

conductance. From Figure 4-2 it is seen that the highest resistance has FS PES membrane

soaked in 1 mM KCl. The low resistance shows the results for a soaked and also for dry PES

membrane measured in a 1 mM KCl solution. The results of both samples are quite similar

(the lines in the graph are overlapping).

By comparing values of zeta potential in Figure 4-3 give us contribution of the internal

structure. Mentioned figures show zeta potential against channel height at different measurable

conditions. With an increasing KCl molarity, the zeta potential of the membrane is decreasing.

We see how the values of zeta potential are reducing by decreasing the columns in the graph.

Figure 4-1. Streaming current coupling coefficient dIstr/dp for FS PES membrane at different gap

height.


22

Figure 4-2. Conductance vs. gap height.

Figure 4-3. Average zeta potential measuring dU/dp and dI/dp at 100 μm, and dI/dp correct.


23

Table 4-1. Measured values of MicroPES in 1, 5 and 10 mM KCl solution: (a) at gap height 100 μm,

(b) Fig.1, (c) Fig.2, (d) for calculation see discussion.

Membrane

Condition

Ionic

strength

KCl

[mol/l]

ζ based

on dU/dp

[mV]

(a)

dI/dpOFFSET

[nA/mbar]

(b)

ConductanceOFFSET

[μS]

(c)

ζcorr.

[mV]

(d)

Fla

t sh

eet

Mic

roP

ES

Dry 0,001 -32,20 -0,24 8,55 -14,22

0,005 -25,24 -0,18 4,42 -10,77

0,01 -21,94 -0,17 1,39 -1,83

Soaked 0,001 -33,31 -0,27 1,46 -15,41

0,005 -29,91 -0,19 7,68 -17,25

0,01 -26,49 -0,18 6,82 -11,26

Results for HF membrane PES are given in Figures 4-4, 4-5 and 4-6. From Figure 4-4 it is

seen that membrane soaking has an influence on surface properties, because of high values

dI/dp at soaked membrane comparing to dry ones. Figure 4-5 shows linear dependence of cell

electric conductance on channel height. In Figure 4-6 the zeta potential for soaked and dry

membrane in 1 mM, 5 mM and 10 mM KCl ionic solution are given.

Figure 4-4. The streaming current of hollow fiber PES membranes as a function of gap height in

0,001 M, 0,005 M and 0,01 M KCl solution.


24

Figure 4-5. Conductance of hollow fiber PES membranes at different gap heights and different

solution concentrations.

Figure 4-6. Zeta potential at pH 6 of polymeric membranes obtained with AGC cell.


25

Table 4-2. Measured and calculated values of soaked and dry PES membrane in 1, 5 and 10 mM KCl

solution. (a) at gap height 100 μm, (b) Fig.1, (c) Fig.2, (d) for calculation see discussion.

Membrane

Condition

Ionic

strength

KCl

[mol/l]

ζ based

on dU/dp

[mV]

(a)

dI/dpOFFSET

[nA/mbar]

(b)

ConductanceOFFSET

[μS]

(c)

ζcorr.

[mV]

(d)

Holl

ow

fib

er

PE

S

Dry 0,001 -17,79 0,05 -0,07 -49,55

0,005 -18,66 0,08 -8,42 -51,32

0,01 -14,77 0,06 -11,42 -46,82

Soaked 0,001 -5,81 0,01 -0,10 -15,16

0,005 -11,33 0,06 -1,04 -35,82

0,01 -9,45 0,05 -8,75 -26,20

Figure 4-7 shows the dependences of streaming current on the channel height for HF PVDF

membrane. The slopes are linear, but with different positive offsets. Figure 4-8 shows cell

electric conductance against channel height. Values of zeta potential are given in Figure 4-9.

Table 4-3 shows important measured and calculated values of zeta potential based on dU/dp,

dI/dpOFFSET, conductanceOFFSET and corrected zeta potential for HF PVDF.

Figure 4-7. Measured streaming current of hollow fiber PVDF membrane as a function of gap height.


26

Figure 4-8. Conductance of hollow fibre PVDF membranes as a function of gap height at 0,001 M,

0,005 M and 0,01 M KCl solution.

Figure 4-9. Zeta potential measuring streaming potential, streaming current and streaming current

correct at 100 μm.


27

Table 4-3. Measured and calculated values of soaked and dry hollow fiber PVDF membrane in 1, 5

and 10 mM KCl solution: (a) at gap height 100 μm, (b) Fig.1, (c) Fig.2, (d) for calculation see

discussion.

Membrane

Condition

Ionic

strength

KCl

[mol/l]

ζ based

on dU/dp

[mV]

(a)

dI/dpOFFSET

[nA/mbar]

(b)

ConductanceOFFSET

[μS]

(c)

ζcorr.

[mV]

(d)

Holl

ow

fib

er

PV

DF

Dry 0,001 -12,77 0,05 1,53 -43,96

0,005 -11,24 0,06 -6,92 -33,42

0,01 -8,13 0,01 -8,35 -11,89

Soaked 0,001 -11,99 -0,05 -1,03 -4,95

0,005 -11,28 0,03 -2,68 -20,82

0,01 -8,36 0,01 -2,31 -15,30

The discussion of the results will be given in chapter 5.


28

4.2 Influence of pH on the zeta potential

Figure 4-10 shows the pH dependence of the zeta potential for the flat sheet PES membrane

in KCl solution at three ionic strength of 0,001 M, 0,005 M and 0,01 M KCl. The values of

ionic strength are equal to its salt concentration, because of the 1:1 salt solution. Figure 4-11

shows zeta potential vs. pH for hollow fiber PES. Figure 4-12 shows the zeta potential of

hollow fiber PVDF membranes as a function of pH in 0,001 M, 0,005 M and 0,01 M KCl

solution. Results will be discussed in chapter 5.

Figure 4-10. The zeta potential of flat sheet PES membranes as a function of pH in 0,001 M, 0,005 M

and 0,01 M KCl solution.


29

Figure 4-11. The zeta potential of the hollow fiber PES membrane at various pH values.

Figure 4-12. pH dependence of zeta potential for hollow fiber PVDF membrane.


30

5 Discussion

5.1 Effect of membrane porosity

In all kinds of membranes such as microfiltration or ultrafiltration, membrane porosity, pore

size, pore density are the important key factors for evaluation membrane performance and its

separation. The results for the evaluation of the different membrane samples, were obtained

on three different samples for each of the selected concentration of electrolyte solution. Two

measurements were performed for soaked sample in order to determine measurement

repeatability. Flat sheet membrane MicroPES, hollow fiber membrane PES and hollow fibre

membrane PVDF were used for investigating the membrane zeta potential in 0,001 M, 0,005

M and 0,01 M KCl. The results were obtained mainly on two samples for each of the selected

polymeric materials. Individual measurements were repeated to exclude the effect of

measuring time. This procedure added up to 5 measuring points for every specimen. As a

representative example, Figure 4-1 shows a series of 6 streaming current coupling coefficient

for flat sheet PES membrane (coming from two different batches) at various channel heights

(gap heights) soaked and unsoaked membranes. As expected, the streaming current measured

through the cell varies linearly with the channel height. For soaked and unsoaked the lines do

not pass through the origin. This gives evidence that an additional streaming current flows

through the porous structures of membranes, the value of which is obtained from extrapolation

to zero gap height (y-intercept in Fig. 4-1). In Figures 4-3, 4-6 and 4-9 first column at each

series of measurement displays value of zeta potential by measuring streaming potential at 100

μm, second column displays zeta potential when streaming current was measured at 100 μm,

and third column shows value of zeta potential, when we take into account corrected streaming

current. Corrected streaming current is explained further in this chapter.

These findings confirm the recent results obtained by Yaroshchuk and Luxbacher who

demonstrated the occurence of a streaming current through the porous body of homogenous

membranes during tangential electrokinetic experiments. [23] Also experimental

measurements in study by Szymczyk et al. complete useful information on the advanced

electrokinetic characterization of polyethertsulfone membrane. Figure 4-1 shows that all lines

dIstr/dp is a linear function of gap height. Value of current coupling coefficient is decreasing

with increasing gap height. For comparison, Figure 4-2 shows the dependence of cell electric

conductance on the channel height. The dependences of both streaming current and

conductance are linear, as predicted by Eqs 8 and 9. The decrease of streaming current

coupling is characteristic of filters with larger pores and can be caused by the fact zeta-

potential of internal pore surface is larger (in absolute value) than that of the external surface

of the membrane filter. In the theory section we have discussed that with sufficiently large

pores the classical Helmholtz-Smoluchowski formula can be applied (and the streaming

potential coefficient is independent of gap height) provided that the zeta potential of external

and internal surfaces are the same. Our experiments have revealed that the suitable range of

channel heights in the adjustable gap cell of SurPASS instrument is from ca. 60 to 120 μm.

By using the extended Helmholtz-Smoluchowski equation for the correct calculation of the

zeta potential at the external membrane surface, we get:


31

(𝑑𝐼𝑠𝑡𝑟

𝑑𝑝)𝐶𝑂𝑅𝑅 = (


𝑑𝑝)𝑀 − (


𝑑𝑝)𝑂𝐹𝐹𝑆𝐸𝑇 (11)

𝜁𝐶𝑂𝑅𝑅 =𝜁𝑀


𝑑𝑝)𝑀

× (𝑑𝐼𝑠𝑡𝑟

𝑑𝑝)𝐶𝑂𝑅𝑅 (12)

Where is corrected streaming current difference between the measured (M) value of streaming

current vs. pressure and y-intercept (OFFSET) of dIstr/dp slope. In Table 3 are presented

calculated values of correct zeta potential. By using the slopes of linear dependences of

streaming current coefficient on the channel height, one can calculate the zeta potential of

external film surface. Here is example of calculation for correct zeta potential of dry MicroPES

membrane in 0,001 M KCl.


𝑑𝑝)𝐶𝑂𝑅𝑅 = −0,30 nA/mbar − (−0,24

nA

mbar) = −0,06nA/mbar (13)

ζCORR =−76,80 mV

−0.30 nA.mbar× (−0,06 nA/mbar) = −14,22 mV (14)

Conductance is the inverse of electrical resistance, C = 1/R. If the conductance of

the membrane to a particular ion is low, then the resistance to movement of that ion across

the membrane is high. The cell electric conductance can be estimated in two different ways.

First, it could be directly measured by alternating current or could be calculated. Equation for

the gap height dependence of conductance is:

1

𝑅𝑠𝑙𝑜𝑝𝑒= 𝐶𝑠𝑙𝑜𝑝𝑒 = κ𝐵

𝑤 𝐻

𝐿 (15)

1

𝑅𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙= κ𝐵

𝐻

2 (16)

We know that 𝜅𝐵 is the specific conductivity of the bulk electrolyte, when analyzing flat sheet

membrane PES conductivity for 0,001 M KCl is 15,81mSm⁄ , width (w) is 1 cm and length

(L) is 2 cm of gap channel, H is gap height. Figure 4-2 shows interesting results, that show the

same trend of conductance and gap height. Slope for 0,001 M KCl shows smaller specific

conductivity of KCl as slope in higher concentration of KCl. Results show that soaked samples

have lower electric resistance than unsoaked ones.

Applying the approach of the extended H-S equation to HF membrane gives us quite similar

results as for flat sheet. Figure 4-4 shows much higher slope coefficient, that means much

steeper slope. For mostly hollow fibers offset of streaming current is positive. In Figure 4-5

and 4-8 it is seen that the dependences of conductance on gap height have negative offset,


32

which would formally correspond to a negative membrane conductance, which is physically

unreasonable.

To get more effective gap height, calculation of theoretical conductance has to be done.

Figures 5-1 and 5-2 show experimental results of conductance and streaming current for

hollow fiber PES soaked in 5 mM KCl solution and for dry membrane in 10 mM KCl solution.

When analyzing soaked hollow fiber PES membrane conductivity for 0,005 M KCl is 72 mS/m

and for dry HF PES membrane conductivity in 0,01 M KCl is 140 mS⁄m. From equation 15

we can calculate the conductance at 1 μm. By fixing the point at gap height 80 μm and new

calculated value for conductance, we can get trend line, which shows us more effective gap

height.

For HF PES 5 mM KCl soaked:

C = 0,075μS

𝜇𝑚.×

0,01 𝑚

0,02 𝑚× 1 𝜇𝑚 = 0,0360 μS (17)

For HF PES 10 mM KCl dry:

C = 0,140μS

μm×

0,01 𝑚

0,02 𝑚× 1 𝜇𝑚 = 0,070 μS (18)

From equations for trend lines we can calculate the more effective gap heights. For example,

if we compare how gap height 120 μm changes for soaked HF PES in 5 mM KCl with dry HF

PES membrane in 10 mM KCl, we get resultes that are seen in table 5-1.

Table 5-1. More effective gap heights for HF PES membrane.

HF PES

Gap height

(experimental)

[μm]

Gap height

(corrected)

[μm]

5 mM KCl dry 120 126

10 mM KCl soaked 120 157

Figure 5-1 and table 5-2 shows differences between gap height before and after correction.

Higher effective gap height reaches trend line for soaked HF PES membrane.


33

Figure 5-1. More effective gap height.

Because of correction for gap height, we can also correct offsets for streming current. Figure

5-2 shows corrected offsets, which were considered by corrected gap heigts. It is seen that

more effective offsets are more negative than previous.

Table 5-2. Corrected streaming current offsets for HF PES membrane.


34

The following assumptions are made to correct the gap height for the calculation of the zeta potential

of hollow fiber membranes:

• The lowest effective gap height of 80 µm is assumed correct since the hollow fiber

membrane samples get squeezed and simulate a continuous flat surface (see Fig. 5-2).

• The conductance at a fully closed gap is assumed C = 0 µS.

• The dependence of the conductance on the effective gap height is assumed linear.

Calculated values of gap height corrected are comparing to experimental one are different.

That is obvious, because hollow fibers have curvy surface and there is not constant distance

between samples. First reason of deviation for theoretical gap height is that hollow fiber

membrane has some own thickness, which reduces distance between samples. Second reason

is the wavy surface of the grid of HF membranes.

The channel height was varied between 80 and 120 μm by means of micrometric screws and

its value was determined from volume flow rate (dV/dt) measurements performed at various

ΔP by means of the Hagen–Poiseuille relation which reads as follows for parallelepipedic

channels (considering that the contribution of porous structures to QV is negligible and

neglecting edge phenomena):

H = √12𝜂𝐿

𝑤×

𝑑𝑉𝑑𝑡⁄

∆𝑃

3

(20)

Adjustable gap cell is designed for suitable flow rate and pressure to ensure laminar flow. The

height of the rectangular channel H is determined from the measurement of flow rate dV/dt

and the differential pressure according to eq. 20. Where w is the width of the streaming

channel. The deviation between the measured and effective gap height was found up to 37 μm.

Because of the hollow fiber geometry and curvy surface, the length of the channel is extended.

Beside that the flow is constant, it goes also into the pores and into small blank spaces among

fibers. In experimental section we confirm the linear dependence of volume flow rate on

pressure difference. That is seen in Figure 3-7.

Figure 5-2. After measurements hollow fibers are squeezed together.


35

Reasonable length based on correct gap height is larger at hollow fiber membranes as flat sheet

membrane. Flat sheet membrane has surface without some obstacles, where flow can easily

go through channel. Hollow fibers create some wavy surface where flow is not just linear, but

it could be also turbulent near pores and blank spaces between fibres.


36

5.2 Effect of ionic strength

The membranes were soaked prior to measurement to achieve wetting of the membrane pores.

Before each measurement, the two pieces of membrane were soaked for at least 24 hours in

the high-purity water used during the measurement.

Ionic strength has considerable impacts on membrane zeta potential. Electrostatic charge

shielding and compression of the diffuse layer due to increasing ionic strength reduces the

negative zeta potential of semipermeable polymeric membranes because of interactions at the

membrane-liquid interface. Studies have shown that the membrane zeta potential is larger in

magnitude when measured in dilute electrolyte solutions, thus overprediction the effect of

electrostatic forces and providing unrealistic information when comparing the electrostatic

forces of different polymeric membranes at environmentally relevant ionic strengths.

Figure 4-4 shows red and blue slopes. Red ones represent nonsoaked membrane and blue ones

are for soaked membrane. By comparing nonsoaked and soaked values we can conclude that

surface soaking gives different results. Also, by increasing ionic strength doesn’t have a strong

effect on the slope. In Figures 4-3, 4-6 and 4-9 it is seen the common dependence of zeta

potential on the ionic strength of the aqueous solution. By increasing ionic strenght of KCl the

value of zeta potential decreases. For the flat sheet membrane zeta potential of soaked

membrane reaches higher zeta potential than for unsoaked. The opposite effect is seen in

analyzing zeta potential for hollow fibers, where zeta potential of unsoaked membranes

reaches higher values.

Bulk electrolyte conductivity was also chosen for the linear regression because it was used to

calculate zeta potential from measured and extrapolated streaming potential using the

Helmholtz–Smoluchowski Equation 6. This equation is appropriate for investigating

electrolyte solutions with ionic strengths greater than 0,001 M. Below 0,001 M, the

conductivity of the bulk electrolyte solution must be corrected for the contribution of the

membrane–liquid interfacial conductance.

The development of a net charge at surface affect the distribution of ions in the surrounding

interfacial region, resulting in an increased concentration of counter ions close to surface. The

liquid layer surrounding the solid surface consists of an inner region called the Stern layer and

an outer region called the diffuse layer. Electrical double layer consists of Stern layer and

diffuse layer. The electrical double layer thickness or Debye screening length, 𝜅−1, is

calculated using:

𝜅−1 = (𝜀0𝜀𝑟𝑘𝐵𝑇

2000𝑒2𝐼𝑁)0,5 (21)

The dependence of the Debye length, which represents the extension of the electric double

layer, on the inverse square root of the ionic strength explains the corresponding dependence

of the zeta potential. Where 𝜀0 is the dielectric constant of free space, 𝜀𝑟 is the dielectric

constant of water, 𝑘𝐵 is Boltzmann′s constant, T is the absolute temperature, e is the magnitude

of the electron charge, N is Avogadro′s number and I is the ionic strength of the salt solution.


37

Comparing ZP corrected with ZP measure by streaming potential gives contribution of dU/dp

from inner surface. A high ionic strength makes the salt solution more conductive. Moreover,

at higher ionic strength, few counter-ions in the diffuse layer result in a smaller zeta potential.

At low ionic strengths, the membrane zeta potential is less screened, which leads to the

presence of more counter ions in the diffuse layer and then to more negative zeta potential

value.

The ionic strength has also an effect on the porosity effect, which is indicated by the offsets in

graphs dIstr/dp. Flat sheet membranes have more negative offsets than hollow fibres. In all

cases with increasing ionic strength, the offset dIstr/dp in graphs is higher. That shows some

special trend that is seen in Fig. 4-1, 4-4 and 4-7.


38

5.3 Effect of pH

The zeta potential changes of PES and PDVF membranes in KCl salt solution as a function of

pH are shown in Figures 4-10, 4-11 and 4-12. Both flat sheet PES and hollow fiber PES and

PVDF membranes mostly have negative zeta potential values under the studied conditions due

to anion adsorption to hydrophobic PES and PVDF surfaces [27]. With decreasing pH, the

zeta potential of all membranes slightly increased at some ranges more sharply and at others

more slightly.

In the range of pH 2-9 values, flat sheet PES membrane did not give an isoelectric point, that

is the pH value at which the zeta potential of membrane is zero. Figures 4-4 and 4-7 show that

offsets dIstr/dp for hollow fibre membranes are near 0 nA/mbar such that a porosity effect is

not observed, and the zeta potential determined by the conventional H-S approach is correct.

For FS PES, comparing ZP corrected for with ZP by streaming potential gives contribution of

dU/dp from inner surface. The ratio between the second and third column given in Figure 4-3

is 18,14% in 1 mM KCl solution, 25,88% in 5 mM KCl solution and 19,73% in 10 mM KCl

solution. This percentage gives us information about different surface inside pores. For HF

PES ratio between the inner and outer surface (comparing ZP corrected and ZP) is 129,3 % in

1 mM KCl solution, 181,84 % in 5 mM KCl solution and 206,29 % in 10 mM KCl solution.

Results confirmed different surfaces of inner and outer side of HF membrane.

The pH dependence of the zeta potential for the FS membrane in Figure 4-3 does not show the

isoelectric point. The reason for this unexpected result are the simultaneous contributions of

streaming current inside pores and of the external membrane surface to the zeta potential. For

a reliable estimation of the IEP, Figure 5-3 therefore displays the dependence of the streaming

potential coefficient on the electrolyte pH. The streaming potential is less affected by a

contribution inside membrane pores and by the parallel effect of varying membrane

conductance. The IEP at pH 2.5 is reasonable for PES.


39

Figure 5-3. Streaming potential coefficient as a funtion of pH for flat sheet PES.

For HF membranes there is not so obvious a porosity effect, mainly because of the smaller

pore size of HF UF membranes and because there is no interconnectivity along the series of

aligned membrane pieces.

For hollow fiber PES the IEP is between pH 3 - 4 and for hollow fibre PVDF membrane is at

around pH 3.

The lower IEP for FS PES as compared to the IEP for HF PES is explained by different

polymer composition e.g. by blending PES with PVP.


40

6 Conclusion

In this study, we presented the zeta potential characterization of PES membranes (hollow fiber,

flat sheet) and PVDF membranes (hollow fiber). We focused on the effect of membrane

porosity, effect on ionic strength and effect on pH. Results presented in this work showed that

pH and the ionic strength are influent factors in zeta potential values.

Through our experiments, it was found that the zeta potentials could be determined of both -

inner and outer surface of hollow fiber membranes film. Zeta potential values calculated from

experimental data and extrapolations have confirmed the reasonable explanation of external

and internal surface. Flat sheet PES membrane has more similar zeta potential on both sides

of the analysed material. According to the curvy surface of hollow fiber membrane, we

corrected gap height. Because of the nonlinear surface, the corrected values are more

reasonable.

The quality of linear fits of experimental data has been found to be good, and extrapolation

procedures were quite reliable. Ionic strength has some influences on membrane zeta potential.

The membrane zeta potential is more negative in dilute KCl solution. These results show the

assumption that the membrane charge is neutralized at high ionic strengths.

This study directly compares outer surface of the hollow fiber and flat sheet. These results can

be used in a number of applications in bio-engineering, biochemical engineering, and water

treatment. In water treatment, these results can be very useful for outside/in filtration.


41

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