104516804-Lpiquid-Liquid-Interfaces.pdf

7/28/2019 104516804-Lpiquid-Liquid-Interfaces.pdf

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Liquid-Liquid Interfaces

By:

Ashoordin Ashoormaram

Capstone Project

2011-2012 Academic Year


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Abstract

A scientific issue of significance to society is the separation of different chemical species. Real

life examples occur in mining processes where by-products need to be filtered out; treatment of

nuclear waste, and environmental cleanup for the removal of toxic metals. The separation

processes take place at the oil-water interface. Since our current understanding of the physics of

the interface is limited, my research is focusing on this area.

I am studying the effect of 3Er+ ions on the ordering of extractant molecules at the interface

between an organic solution of /Dodecane DHDP and an aqueous solution of 3 /ErBr HBrwith

2.5pH . I am testing the hypothesis whether formation of a monolayer at the interface between

two phases would allow us to understand better the ion-extractant interaction.

A typical graph of interfacial tension versus temperature for such an interface consists of two

parts: (1) A linear relationship between interfacial tension and temperature; as the temperature

increases the surface tension correspondingly increases linearly; (2) A plateau region

characterized by slight changes in the surface tension. The distinction between these two regions

is a sudden change in the slope of the plot at some particular temperature named transition

temperature. It is basically a temperature at which the interaction forces between surfactant

molecules are broken as the interface undergoes an increase in the temperature passing from an

ordered phase to a disordered phase.

Thermodynamically, the high positive slope below the transition shows that the entropy of

molecules at the interface is highly less than the entropy of the same molecules in the bulk phase.

In other words, transition is between a highly ordered and a slightly disordered interface. Also,

there is an offset between the plots for different concentrations of 3ErBr accompanied by a shift

in the transition temperature which is an indication of 3Er+ ions being absorbed into interface.


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Introduction

For many practical reasons, an understanding of the properties of aqueous solutions at interfaces

is very important. Some reactions take place only at an interface. Membranes and the electrodes

of an electrochemical cell are some obvious examples. A scientific issue of significance to

society is the separation of different chemical species. Real life examples occur in mining

processes where by-products need to be filtered out; treatment of nuclear waste, which is

associated with separation of radionuclides; and environmental cleanup for the removal of toxic

metals. The separation processes take place at the interface of two liquids, typically oil and

water. The target species is in the water phase, along with many other chemical species. An

organic molecule, called an extractant, is placed into the oil phase. The extractant will journey to

the interface, where it grabs hold of the target species, and drags it into the oil phase. In this way,

the target species is separated from other chemicals in the water phase and can then be further

processed. Improving this process involves an understanding of the chemical interactions of the

target species, the extractant and the oil and water phases. Much of this understanding relies

upon chemistry. However, an important part of this process is the interaction that takes place at

the interface. Our current understanding of the physics governing the interface is inadequate1 and

my research focuses on this area.

My efforts go toward developing a model system to probe the interaction of metal ions with

extractants. Currently, I am working with Professor Schlossman of UIC, whose research group

has shown that the interaction of ions with extractants does not result in a buildup of the ion-

extractant complex at the interface.2


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Questions, Problems and Objectives

I am investigating the role of multivalent (multiple charge) ions on molecular ordering at liquid-

liquid interfaces. The electric field near multivalent ions is much larger than near monovalent

ions. Although this higher field is believed to have a significant effect on the nearby ordering of

other ions and molecules, it is not well understood and is a topic of current research. The specific

question that I am asking is: What is the effect of Er3+ ions on the ordering of extractant

molecules at the interface between a bulk oil phase (Dodecane) and bulk water? I am testing the

hypothesis whether formation of a monolayer at the interface between two phases would allow

us to understand better the ion-extractant interaction.

My faculty mentors research group is studying the role of electrostatic interactions at soft

interfaces. My studies contribute to this field of research by their investigation of the role of

multivalent ions in molecular ordering at interfaces. Finding an appropriate surfactant that has a

similar chemical interaction with the ion as the extractant in a stable layer at the interface is

important because it will allow us to study the structure of this ion-surfactant complex with x-ray

scattering. This information will then allow us to understand the ion-extractant interaction and,

as a result, we will have more practical methods in separation processes in industry.

For this purpose, we are looking for the formation of a stable monolayer or bilayer formed at the

interface between two phases namely, the lower aqueous solution of 3 /ErBr HBrand the upper

organic solution of /DHDP Dodecane . It is assumed that the formation of this layer is implied

through a considerable decrease in the slope of interfacial tension versus temperature graph

below the transition temperature as the sample cools down in its first cycle. In other words, any

decrease in the slope, in cooling stage of each cycle of measurements, is an indication of the

Er3+ ions being absorbed into the interface. It is believed that this absorption happens via DHDP


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surfactants at the interface at the time during which the sample is beyond the transition

temperature in the first cycle because the interface has a less ordered structure in the plateau

region. According to this hypothesis, then, the time that the sample is stayed beyond the

transition temperature is of crucial importance as it characterizes the chance being given to the

Er3+ ions to be absorbed into the interface. So, to show this is the case, we need to have a

control over this time period by changing it on our will. However, before modifying the Python

program associated with the tension measurements, we had no control over the amount of time

that the sample is stayed beyond the transition temperature. For this purpose, we chose to do the

extreme case in which the sample was heated up beyond the transition temperature for about a

complete day, giving more chance to the Er3+ ions leaving the lower phase and, hence, changing

its composition causing a shallower surface tension versus temperature plot below the

transition temperature.

Physics and Chemistry of Interfaces

An interface between two immiscible liquids is an inhomogeneous6 environment. The molecular

environment at the interface is different than in the bulk.12A liquid-liquid interface is not an

infinitesimal sharp boundary in the direction of its normal, but rather, it has a certain

thickness.5In other words, the density profile ( )z normal to the surface will exhibit a behavior

similar to Fig. 1 adapted from Ref. [6]. The orientation of the molecules at the interfaces also

affects the thickness of an interface.8

Figure 1: A schematic illustration of the interface between two immiscible liquids, showing the total density variation.

The dotted line is the intrinsic step function profile, and the solid line is the result of the capillary broadening.


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Surface tension and surface potential are the only macroscopic properties exhibiting some

information about separation of different chemical species mentioned earlier. However, standing

alone, these techniques lack the ability to provide a detailed understanding at the microscopic

level.(Benjamin, 2). This is why X-ray reflectivity has been employed as a complementary

technique. However, in this paper I try to address the macroscopic understanding of the

interfaces through utilizing the most relevant thermodynamic property of the interface, namely

surface tension, and draw a conclusion (if possible at all) about the structural ordering at

microscopic level.

Physical Interpretation of Surface Tension

It is energetically favorable for molecules to be surrounded by other molecules. They attract each

other through different electrostatic interactions such as van Der Waals andhydrogen bonding.

However, at the boundary surface of a liquid, molecules are partially enclosed by other

molecules and, hence, the number of neighboring molecules is less than that of in the bulk. This

is energetically unfavorable.5In order to bring molecules from the bulk to the surface, work has

to be done. The energy required for bringing molecules from the bulk to the surface for creating

new surface area is called surface tension, labeled with the typical range of 20-80 mN/m for

most liquids.5Since is a direct indication of the magnitude of intermolecular forces, it is an

important property of a liquid.12Thus, the Statistical mechanical expression for the surface

tension adapted from Ref. [6], is

( ) ( ) ( )1 1 , , , , , ,2

zz xx yy

i j

dW V V V whereV f i j R i j with x y zdA A

, the phase behavior of a system of particles is schematically shown in Fig. 5 adapted from Ref.

[14]. This phase diagram is a result of competing between an interparticle attraction whose

strength is controlled by a parameter 0 > and a repulsion whose strength is controlled by a

parameter 0A > based on the assumption 1 2z z> .

Thus, surfactants at water-oil interfaces display a phase

structure consisting of homogenous and inhomogeneous

phases encompassing solid, liquid and, gas monolayer

regions.10The transition temperature of a water-oil interface is

one in which a drastic change in the molecular ordering and

phase behavior of surfactants occur. It is basically a

temperature at which the interaction forces between surfactant

molecules are broken as the interface undergoes an increase in

the temperature passing from an ordered phase to a disordered

phase. It is shown10 that the fluorocarbon alkanols with rigid rod tails form ordered solid phases

and the hydrocarbon alkanols with flexible tails form disordered liquid phases even though both

form solid phases at the water-vapor interface. Whether the interface is a monolayer or a

multilayer depends,10 on molecular length of both the alkane solvent and the alkanol surfactants.

Also it is shown15 that at the water-vapor interface, n alkanols andn alkanoic acids of

adequate tail length form monolayer phases containing ordered tails but, at the water-hexane

interfaces, due to the hydrogen bonding interactions in surfactant monolayers, the molecular

ordering is different; they form monolayer phases with disordered tails. In the same reference, it

Figure 5: A schematic representation of a fullphase diagram based on SALR potential model


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is proved that a small variation in the headgroup structure results in a considerable change from

disordered to ordered tailgroups.

Sometimes, the adsorption of amphiphilic molecules at the surface of a liquid is so strong that a

monolayer is formed. If the amphiphiles do not dissolve in water, they form insoluble

monolayers like phospholipids.5On the other hand, introducing a solute to a pure liquid, will

lower the surface tension and, hence by the Gibbs equation, the solute must be adsorbed at the

interface. This adsorption amounts to formation of a monolayer of solute on the

surface.3Monolayers show ordered phases similar to three-dimensional systems and their

structural molecular ordering is investigated by X-ray reflectivity; X-ray reflectivity determines

the film thickness and electron density distribution normal to the water surface. The surface

tension reduces with surfactant concentration and with the increase in the surfactant tail.

A large effort had been put2 to establish the fact that for a water/Dodecane interface, the surface

tension decreases with an increase in the concentration of the surfactant. (See Fig. 6 adapted

from Ref. [2]) This could be explained by the fact that any increase in the surfactant will result in

a denser interface formed by the surfactant in which the polar heads of the DHDP establish new

and stronger hydrogen bondings with the polar water molecules compensating the loss of

neighboring contacts at the interface minimizing the Gibbs free energy of the surface. In other

words, surface molecules have more available neighbors to bind with.

Figure 6: Interfacial tension measurements for water/Dodecane systems using Wilhelmy Plate method atdifferent concentrations of DHDP surfactant. As seen above, the excess interfacial entropy changes slightlyin different directions of heating/cooling procedures and the number of the cycle.


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Methodology

I am using interfacial tension measurements to determine if the surfactant/extractant forms a

stable single layer of molecules at the interface between two phases. Interfacial tension is

proportional to the integrated density profile across the interface and is sensitive to the net effect

of all molecules that adsorb to the interface. X-ray reflectivity can measure the density profile on

the Angstrom length scale. These techniques characterize the molecular ordering at the interface.

They are used to characterize this ordering both with and without Er3+ ions. As part of my

project, I am developing a model system to probe the interaction of metal ions with extractants

between two phases. I am in search of a surfactant that has a similar chemical interaction with

the ion as the extractant, but will exist in a stable layer at the interface. For this purpose, I am

studying the variation of interfacial tension with temperature at different concentrations of the

3ErBrbeing used in the lower aqueous solution. Comparing these profiles with the images

obtained through X-ray reflectivity methods will justify our hypothesis mentioned earlier.

Experimental methods for measuring surface tension involve the contact of three phases, usually

glass, air and the liquid being studied. This implies that we must understand the properties of

three interfaces namely, the liquid/gas, the liquid/solid and, the solid/gas. However, as mentioned

by Fawcett,12 the interface is usually curved because in such simple system the cohesion forces

in the liquid are not necessarily the same as adhesion forces between the glass and the liquid.

Moreover, since curved interfaces have complicated properties compared with the flat surfaces,

we have to establish a flat interface in order to measure its surface tension. For this purpose, we

use a Mylar strip in contact with the glass containing the sample at a level where the interface

will be formed. In other words, the macroscopic boundary line between two phases must be

leveled with the bottom line of the Mylar strip supported by a stainless steel strip from inside.


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The presence of the Mylar strip will prevent the formation of curved boundary along the glass by

reducing the adhesion forces between the liquid and the walls, providing a flat interface.

The Wilhelmy-plate method is applied in order to measure the surface tension. (See Fig. 7

adapted from Ref. [9]). This technique uses a thin filter plate that is weighed before and after

being placed in contact with the interface. The difference in weight, as measured by a sensitive

electronic balance, is proportional to the interfacial tension namely, 2F L= whereL is the

length of the plate. It is important that the plate is wetted by the liquid and, also close to the

three-phase contact line the liquid surface is oriented almost vertically (provided that contact

angle is 0o

). Otherwise, this method is simple and no correction factors are needed provided that

plates are clean by preventing contamination in air.5

The organic solution of our sample is made up of anionic surfactant solute named dihexadecyl

phosphate, (also known as DHDP), with the chemical formula

dissolved in a liquid alkane hydrocarbon solvent of the paraffin series named Dodecane, (also

known as dihexyl, bihexyl or, adakane 12), with the chemical formula CH3(CH2)10CH3. And, the

aqueous solution of our sample is made up of Erbium (III) Bromide dissolved in the acidic

solution of HBr with a pH of about 2.5.

Figure 7: A schematic Demonstration of Wilhelmy Method
http://en.wikipedia.org/wiki/Chemical_formulahttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Chemical_formula


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First of all, the organic and aqueous solutions are made separately. The organic solution is heated

up to 0 055 60 C using a hotplate. The hot solution must remain at that temperature for at least one

hour until it is needed later on. Then two stainless steel and Mylar strips which are already

cleaned are put together: the latter on top of the former. They are bent in a circle configuration

and then placed inside a small cleaned dish letting them slowly expand getting in direct contact

with the walls of the dish. Using a reference level, the bottom line of the outer Mylar strip must

be leveled; it is where the interface between two phases will form. However, in order to have a

better look at the interface at any time, it is important that the stainless steel strip be raised for

about half a centimeter above the bottom line of the Mylar strip. Then, the dish is put inside the

system mounted on top of a leverage surrounded by the thermostat. A clean small stir bar is

placed inside the dish along with the cleaned filter plate hanging on a sensitive microbalance.

Using a pipette, we transfer the aqueous solution first into the dish until it reaches about two

millimeters above the bottom line of the Mylar. We turn on the stir bar at an intermediate rate

and then turn the heater on using the Lake Shore connected to the thermostat covering the

system. The setpoint temperature should be about 028 Cin order to pre heat the aqueous solution

before adding the organic solution on top of it. It is worthwhile to mention that during the whole

process of heating the aqueous solution, the filter plate must be immersed wholly into the water

phase with the voltmeter connected to the microbalance turned on. This makes the filter plate

completely wet before being ready for any later measurements. The system will reach

equilibrium in about an hour and then we turn off the stir bar motor so that to give some time

(about half an hour) giving the impurities inside the aqueous solution some chance to be

collected on the surface.


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From now on, the procedures must be followed as fast and careful as possible for reasons

mentioned later on. We aspirate the surface of the aqueous solution using an aspirator with a

cleaned pipette so that the surface of the solution is leveled exactly with the bottom line of the

Mylar strip. As soon as the temperature of the beaker containing the flask of organic solution

reaches to some temperature about 030 C, we turn on the stir bar motor again and, we add the

organic solution on top of the aqueous solution using another cleaned pipette in the amount of

( )65 75 mL . Then, we cover the dish with the provided parts and the thermostat together in

order to cover the whole system minimizing any possible loss of heat transferred to the dish

containing two phases. We put two temperature probes in place: one in the upper part of the

thermostat which controls the temperature of the system namely, channel A and the other inside

the solution through a hole provided on the upper part of the thermostat namely, channel B to

read the actual temperature of the system inside the dish. Then, quickly and carefully, we set the

temperature to 028 Cand wait for the system to reach thermal equilibrium in about an hour.

Now, it is the time to explain the reason of being fast and careful in the last paragraph in

preparing the sample for interfacial tension measurements. One of the drawbacks of this

procedure is that for some, yet unknown, reasons precipitation of the crystalline DHDP

constituent of the organic solution occurs at the interface especially at low temperatures. The

existences of these precipitations affect the interfacial tension and will cause a lot of ambiguities

about the actual meaning of the data. Since we usually heat up the sample starting with 028 C,

this temperature is of great importance because it is the minimum temperature that the sample

will be exposed to and hence very prone to precipitation. The time during which the heater is

turned off, the temperature of the aqueous solution is going down and the final equilibrium

temperature of the system after adding the organic solution will be lowered, probably helping the


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precipitation takes place on the interface. This is why it is crucial to minimize the time the heater

is turned off by doing the last procedures as fast as possible and, more importantly, to check the

interface at about equilibrium temperature just before running the experiment; if there is any

precipitation present on it, the measurement will have no meaning and the sample must be

disregarded. This actually happened in some cases and I had to prepare new samples for

interfacial tension measurements.

For interfacial tension measurements, for both time and temperature scaling, a program is written

in Python in which all measurements are taken automatically as the temperature of the sample is

changed over time. Before any measurements, the software must be connected to both

instruments namely, the Lake Shore Temperature Controller and the Voltmeter Weight

Controller. Once the program is ready, we can enter the size of the filter plate and the channel

corresponding to the actual temperature of the sample inside the dish namely, channel B. After

reaching equilibrium, we take the filter plate out of the aqueous solution and we adjust it such

that it is just above the interface before being drawn into it. Then, we measure the reference

voltage which is needed for measuring the reference weight of the plate by pressing the first

measure button of the opened window on the left. Then, we bring the plate slowly down such

that it just grabs the interface. At this point we press the second measure button of the opened

window on the right to read the instantaneous interfacial tension. Using the surface tension vs.

time feature of the program, we can monitor the tolerance of the surface tension values at

equilibrium temperature in order to set the sample settings of the software. After being satisfied

with the tolerance of the surface tension values at thermal equilibrium, we basically are ready to

run the Surface Tension vs. Temperature experiment by pressing the Start button of the last

part of the opened window.


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Notes:

1. Contamination is the main source of error for the measurement procedures. So, it is of greatimportance to follow the procedures in a very systematic and clean manner. Before using any

laboratory accessories or parts including, but not limited to, flasks, dishes, pipettes, filter

plates, stir bars, strips, and temperature probes, we must be sure that they are already cleaned

according to the Cleaning Protocol of Laboratory Glassware.

2. The stir bar motor must be working constantly during the whole process which may take upto 4 days in each run. And, whenever the temperature is changed by an increment of 01 C, it is

recommended to check the interface making sure of the cleanness of the interface regularly.

Measurements and Results

The appendix figures (8 through 25) provide the whole set of our interfacial-tension

measurements. They display the variation of interfacial tension with temperature at different

concentrations of the 3ErBr as well as initial conditions namely, the initial temperatures of the

two phases just before mixing.

The aqueous phase of the samples that I used contained 3/HBr ErBr with three different

concentrations of 3ErBrbut, a fixed value for the DHDP concentration of the organic solution

which was chosen to be ( ) 41.07 0.03 10 M . As seen from Figs. 8 through 25,

Instead of having a falling region after the transition temperature, we encountered a slightlyrising region, considered basically a plateau;

For the cases where we used stainless steel and Mylar strips, (1) the excess interfacialentropies of the systems before transition temperatures were slightly different; there were

consistent shifts in the location of the transition temperatures; And, the plots were not smooth


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enough to be used for any possible explanation for the behavior of the system in different

conditions. These all led us to our first hypothesis that during the first heating of the sample,

new impurities are introduced somehow to the system which manipulates the interactions

between the constituents of the interface by changing the composition of the system

thereafter. By removing the stainless steel strip from the procedures, and conducting the

measurements using only Mylar strip, we eliminated the inconsistency. Plots became smooth

with only slight shifts in the transition temperature and the linear slope of the plots;

For the cases where we used only Mylar strip, we observed the following trends:1.

The slope of the plot above the transition temperature, with or without 3ErBr , no matter in

what stage of the cycle, was constant; (See Figs. 17-19, 22-25).

2. In the absence of 3ErBr in the lower phase, the system is reproducible; all three stages:first heating, first cooling, and second heating have almost identical plots. (See Fig. 22).

3. When there is 3ErBr in the lower phase, aside from the first heating stage which has thelowest slope, all other stages have small difference in appearance. (See Figs. 17-19, 22-

25).

4. Introducing 3ErBr to the lower phase causes a decrease in the interfacial tension and anincrease in the transition temperature of the interface; (See Figs. 22-25).

5. For systems containing 3ErBr in the lower phase, the first heating stage of the first cyclehas smaller slope than the same systems without 3ErBr . However, in the first cooling

stage, the difference is almost vanished. Interestingly enough, in the second heating stage,

the trend is reversed; the slope is slightly larger than the same systems without 3ErBr .

(See Figs. 22-25).


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Note:

Two important features of each plot are the slope of the linear region below transition

temperature and the location of transition temperature itself. These two parameters are found

using best linear fit feature of M.S. Excel. Each plot is modeled as two straight lines with

different slopes andy-intersections. The line with the higher slope and smaller y-intersection

corresponds to the linear region of the plot below transition, and the line with lower slope and

larger y-intersection corresponds to the plateau region. The x-component of the intersection

point of the two modeled lines will represent the transition temperature. The uncertainties of the

two features which are included in each plot for different (heating/cooling) stages are found

using the following tricks:

I.Linear slope: the points below transition region that are best described as being followed bya single line are chosen. Then, the best linear fit is found using all of them; the slope of this

line will be the mean value of the actual slope. However, by changing the number of the points

being considered, which is at least equal to three, we found the minimum and the maximum

value of the slope. Of course, one of these extreme has maximum distance from the mean

value. The difference between the mean value and that extremum is the uncertainty of the

measurement.

II.Transition temperature: always is chosen somewhere in the range (0.2-0.5) C based on thesharpness of the transition region. The smoothest transition will have the largest uncertainty,

e.g. 0.5C and the sharpest transition will have the smallest uncertainty, e.g. 0.2C. Thus, this

uncertainty is qualitatively extracted by the amount of deviation between the modeledstraight

lines and the actualplot; the closer the modeled lines to the actual behavior of the system are,

the less uncertainty of the transition temperature is.


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Conclusion

Since, interfacial tension is the most relevant thermodynamic property of the interface,

Interfacial tension vs. temperature plots are valuable means for studying the thermodynamics of

the oil/water interfaces. Even though, it is a direct indication of the magnitude of intermolecular

forces, but surfaces-tension measurements, alone, cannot determine the structural ordering at

microscopic level. This is why x-ray reflectivity has been employed as a complementary

technique to study the structure of the ion-surfactant complex at oil/water interfaces. However,

here, the most relevant concept used to study the macroscopic characteristics of oil/water

interfaces is the excess interfacial entropy, which is the negative derivative of the tension with

temperature at constant pressure and surface area.

For majority of pure liquids, the surface tension decreases with increasing temperature. The

entropy on the surface is thus increased, which implies that the molecules at the surface are less

ordered than in the bulk liquid. However, this is not the case in the interfaces under study. Here,

the highly positive slope below the transition temperature indicates that the excess interfacial

entropy is highly negative; the entropy of molecules at the interface is much smaller than the

entropy of the same molecules in the bulk phase. Similarly, the slowly increasing plateau

region above the transition temperature, indicates that the excess interfacial entropy is still

negative, indicating that the interfacial molecules have slightly smaller entropy than in the bulk.

In other words, the interface is always more ordered than the two bulks. But, the drastic change

in the slope at the transition temperature indicates that two regions behave differently. The

qualitative analysis shows that the transition is between an ordered low-temperature phase and a

disorderedhigh-temperature phase at the interface.


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Secondly, the plots ofany individual system are almost identical above the transition temperature

for different (heating/cooling) stages independent of the presence of Er3+

ions in the aqueous

solution. In other words, once the system is above the transition temperature, the variations in the

structural order of the interface are very small.

Thirdly, in the absence of Er3+

, the plots of different (heating/cooling) stages ofany individual

system are almost identical even for the lower-temperature phase. In other words, when there is

no Er3+

in the lower phase, the excess interfacial entropy, more or less, remains unchanged as the

system undergoes a transition in any (heating/cooling) stage of measurements.

However, as Er3+

is added to lower phase, the linear region of the plot for any system is

considerably varied for different (heating/cooling) stages of measurements. More precisely, once

the system undergoes a transition in the first (heating) stage, the composition of the interface

changes in such a way as to decrease the excess interfacial entropy. A promising explanation for

this decrease in the entropy is that, depending on the initial concentration of Er3+

ions in the

aqueous solution, at least some have been transferred to the oil phase once the system is above

the transition temperature for many hours in the first heating stage. And, this mechanism requires

a positive adsorption of Er3+

ions at the interface while the sample is below the transition

temperature. Based on Gibbs adsorption equation ( )/ lnT

c RT = , one can measure the

excess interfacial concentration of Er3+

by measuring interfacial tension versus natural logarithm

of the concentration of Er3+

at constant temperature. Since we had used only two different

concentrations, it is implausible to extract a complete analysis.

Introducing Er3+

to aqueous solution causes a decrease in the interfacial tension and an increase

in the transition temperature of the system. This could be explained only by the fact that more


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Er3+

ions are present at the interface surrounding the interfacial DHDP molecules. And, the

increase in the transition temperature is understood through the fact that ionic interactions

between Er3+

andOOH head of the DHDP surfactants need more thermal energy to be broken

than the previous interactions in the absence of Er3+

.

Finally, in the presence of Er3+

, the slope of the linear region of the plots has the smallestvalue

for the first heating stage of an individual sample. In other words, the ionic interactions

mentioned earlier are so strong that once some portion of Er3+

ions are transferred to oil phase,

successive (heating/cooling) stages are unable to bring these ions back to the lower phase; once

the first transition occurs, it is impossible to bring the Er3+

ions back to the lower phase under

normal conditions. All this phenomena together could explain why for systems containing Er3+

,

(1) The first heating stage has smallerslope than the same systems without Er3+

; (2) The first

cooling stage has more or less the same slope; and, (3) The second heating stage has slightly

largerslope. Put it all together, these three observations indicate that once the system undergoes

the transition corresponding to the first heating stage, the molecular structure of the interface

along with both bulk phases changes irreversibly; the heating stage of the first cycle is not

reproducible.

______________________________________________________________________________

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11.Popov, Alexander N. The Interface Structure and Electrochemical Processes at theBoundary Between Two Immiscible Liquids. Berlin, Heidelberg, New York, London,

Paris, Tokyo: Springer-Verlag, 1987. 179.

12.Fawcett, W. Ronald.Liquids, Solutions, and Interfaces: From Classical MacroscopicDescriptions to Modern Microscopic Details. Oxford, New York: Oxford University

Press, Inc., 2004. 383-443. Print.

13.Tikhonov, Aleksey M., Ming Li, and Mark L. Schlossman. Phase Transition Behavior ofFluorinated Monolayers at the Water-Hexane Interface.Journal of Physical Chemistry

B. 105.34 (2001): n. page. Print.

14.Archer, Andrew J., and Nigel B. Wilding. "Phase behavior of a fluid with competing
http://staff.science.nus.edu.sg/~parwani/c1/node52.htmlhttp://staff.science.nus.edu.sg/~parwani/c1/node52.html


28/37

attractive and repulsive interactions." Physical Review E: statistical, nonlinear, and

soft matter Physics. 76.031501 (2007): n. page. Print.

.

15.Tikhonov, Aleksey M., Harshit Patel, Shekhar Garde, and Mark L. Schlossman. TailOrdering Due to Headgroup Hydrogen Bonding Interactions in Surfactant Monolayers

at the WaterOil Interface.Journal of Physical Chemistry B. 110. (2006): 19093-

19096. Web. 12 Mar. 2012. .

16.Snyder, Kendra. United States, New York. Office of Science of the U.S. Department ofEnergy (DOE).Expanding the Degrees of Surface Freezing: Molecular ordering

phenomenon found at interface between complex liquids and solids. Upton:

Brookhaven National Laboratory, 2011. Web.

.


29/37

APPENDIX A Interfacial Tension vs. Temperature Plots

Concentration and Temperature of the phases

( ) 41.07 0.03 10DHDP M= At ( )41 1o

C

( ) 73 3.0 0.1 10ErBr M= , 2.5pH = At ( )21 1

oC

Control Settings:Temperature Tolerance 0.005( )o C=

Tension Tolerance 0.03( / )mN m=

Wait Time 10min.= Figure 8




37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

27 28 29 30 31 32 33 34 35 36 37 38

SurfaceTensi

on(mN/m)

Temperature (C)

Increasing Temperature (1)Decreasing Temperature (1)Increasing Temperature (2)Decreasing Temperature (2)

Increasing Slope=0.730.08 (mN/mC) Transition Temperature=(32.30.5) (C)Decreasing Slope=0.840.05 (mN/mC) Transition Temperature=(31.00.4) (C)

Decreasing Slope=0.720.05 (mN/mC) Transition Temperature=(31.80.4) (C)

37.437.637.8

3838.238.438.638.8

3939.239.439.639.8

4040.240.440.640.8

4141.2

27 28 29 30 31 32 33 34 35 36 37 38

SurfaceTe

nsion(mN/m)

Temperature (C)

Increasing Temperature (1)

Decreasing Temperature (1)


Increasing Slope=0.820.03 (mN/mC) Transition Temperature=(31.10.3) (C)

Decreasing Slope=0.820.14 (mN/mC) Transition Temperature=(30.00.4) (C)Increasing Slope=0.720.10 (mN/mC) Transition Temperature=(30.30.5) (C)


30/37

37.437.637.8

3838.238.438.638.8

3939.239.4

39.639.8

4040.240.440.640.8

4141.241.441.641.8

4242.242.4

27 28 29 30 31 32 33 34 35 36 37 38

SurfaceTe

nsion(mN/m)

Temperature (C)




Increasing Slope=0.90 (mN/mC) Transition Temperature=(32.30.5) (C)

Decreasing Slope=0.80 (mN/mC) Transition Temperature=(30.70.5) (C)


( ) 41.07 0.03 10DHDP M= At ( )36 1 o C

( ) 73 3.0 0.1 10ErBr M= , 2.5pH = At ( )21 1

oC


Tension Tolerance 0.03( / )mN m= Wait Time 10min.=

Figure 10

36.837

37.2

37.437.637.8

38

38.238.4

38.638.8

3939.239.439.639.8

4040.240.4

27 28 29 30 31 32 33 34 35 36 37 38

SurfaceTen

sion(mN/m)

Temperature (C)






Figure 11


31/37

35.836.236.6

3737.437.838.238.6

3939.439.840.240.6

4141.441.842.2

27 28 29 30 31 32 33 34 35 36 37 38

SurfaceTension(mN/m)

Temperature (C)







( ) 41.07 0.03 10DHDP M= At ( )36 1o

C

( ) 73 1.0 0.1 10ErBr M= , 2.5pH= At ( )21 1

oC


Tension Tolerance 0.03( / )mN m= Wait Time 10min.=


( ) 41.07 0.03 10DHDP M= At ( )36 1o

C

73 5.0 10ErBr M

= , 2.5pH = At ( )21 1o

C


Tension Tolerance0.02( / )mN m=


35.4

35.8

36.2

36.6

3737.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

41.4

27 28 29 30 31 32 33 34 35 36 37 38


Temperature (C)



Increasing Slope=0.75 (mN/mC) Transition Temperature=(34.50.5) (C)Decreasing Slope=0.81 (mN/mC) Transition Temperature=(33.30.5) (C)

Figure 12


32/37

Control Settings:

Temperature Tolerance 0.005( )o C=


Wait Time 10min.= Concentration and Temperature of the phases

( )4

1.07 0.03 10DHDP M

= At ( )30 1o

C

( ) 73 5.0 0.1 10ErBr M= , 2.5pH = At ( )28 1

oC

Figure 14

Concentration and Temperature of the phases( ) 41.07 0.03 10DHDP M= At ( )33 1 o C

( ) 73 5.0 0.1 10ErBr M= , 2.5pH = At ( )28 1

oC

Figure 15

36.2

36.6

37

37.437.8

38.2

38.6

39

39.4

39.8

40.2

40.6

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Surface

Tension(mN/m)

Temperature (C)

Increasing Temperature

Decreasing Temperature



34

34.4

34.8

35.2

35.6

3636.4

36.8

37.2

37.6

38

38.4

38.8

39.2

39.6

40

40.4

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Surface

Tension(mN/m)

Temperature (C)







33/37

Concentration and Temperature of the phases( ) 41.07 0.03 10DHDP M= At ( )29 1

oC

( ) 73 5.0 0.1 10ErBr M= , 2.5pH = At ( )28 1

oC

Figure 16

Figure 17: Only Mylar

34.2

34.6

35

35.4

35.8

36.2

36.6

37

37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Surfa

ceTension(mN/m)

Temperature (C)





Decreasing Slope=0.850.03 (mN/mC) Transition Temperature=(32.00.5) (C)Increasing Slope=0.770.05 (mN/mC) Transition Temperature=(32.70.5) (C)

37

37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

41.4

41.8

42.2

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

InterfacialTension(mN/m)

Temperature (C)




Increasing Slope = 0.630.02 (mN/mC) Transition Temperature = (31.60.4) (C)

Decreasing Slope = 0.870.03 (mN/mC) Transition Temperature = (31.70.4) (C)Increasing Slope = 0.820.03 (mN/mC) Transition Temperature = (32.00.4) (C)


34/37

37

37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

41.4

41.8

42.2

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

InterfacialTension(mN/m)

Temperature (C)





Decreasing Slope = 0.850.01 (mN/mC) Transition Temperature = (31.30.3) (C)


37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

40.6

41

41.4

41.8

42.2

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

SurfaceTemsion(mN

/m)

Temperature (C)





Decreasing Slope = 0.930.05 (mN/mC) Transition Temperature = (31.00.3) (C)


Figure 18 (Only Mylar)



35/37


oC

3 0 ( 2.67)ErBr M HBr pH= + = , At ( )28 1o

C

Figure 20


oC

3 0 ( 2.5)ErBr M HBr pH= + = , At ( )28 1o

C

Figure 21

35.2

35.6

36

36.4

36.8

37.2

37.6

38

38.4

38.8

39.2

39.6

40

40.4

40.8

41.2

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45


Temperature (C)






34.2

34.6

35

35.4

35.8

36.2

36.6

37

37.4

37.8

38.2

38.6

39

39.4

39.8

40.2

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

Surface

Tension(mN/m)

Temperature (C)








36/37

38

38.4

38.8

39.2

39.6

40

40.4

40.8

41.2

41.6

42

42.4

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45


Temperature (C)




Increasing Slope = 0.720.01 (mN/mC) Transition Temperature = (30.90.2) (C)Decreasing Slope = 0.800.04 (mN/mC) Transition Temperature = (30.90.2) (C)




37/37

38

38.4

38.8

39.2

39.6

40

40.4

40.8

41.241.6

42

42.4

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

InterfacialTension(mN/m

)

Temperature (C)

First Heating Stage of First Cycle

No ErBr35.0*10^(-7) M ErBr3

Linear Slope =0.72 0.01 (mN/mC) Transition Temperature = (30.90.2) (C)

Linear Slope =0.630.02 (mN/mC) Transition Temperature = (31.60.4) (C)

36.837.237.6

3838.4

38.839.2

39.640

40.4

40.841.2

41.642

42.4

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

In

terfacialTension(mN/m)

Temperature (C)

First Cooling Stage of First Cycle

5.0*10^(-7) M ErBr3

No ErBr3


Linear Slope=0.80 0.04 (mN/mC) Transition Temperature = (30.90.2) (C)



36.8

37.2

37.6

38

38.4

38.8

39.2

39.6

40

40.4

40.8

41.2

41.6

42

42.4

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

InterfacialTe

nsion(mN/m)

T (C)

Second Heating Stage of Second Cycle

No ErBr3

5*10^(-7) M ErBr3




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