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Colloid chemistry
Lecture 8: electrokinetic phenomena
Diffuse Layer:Also called Electrical Double Layer:Ionic concentration not the same as in the bulk solution; there is a gradient in concentration of ions outward from the particle until it matches the bulk
Stern Layer: Rigid layer of ions tightly bound to particle; ions travel with the particle
Plane of hydrodynamic shear: Also called slipping plane: Boundary of the Stern layer: ions beyond the shear plane do not travel with the particle.
Particle surface
Characteristics of Surface Charge: Definitions
Zeta () potential: the electrical potentialthat exists at the slipping plane
Characteristics of Surface Charge: Definitions
diffuse double layer
St
o
Characteristics of Surface Charge: Definitions
Characteristics of Surface Charge
Electrokinetic phenomena result from the differential movement of two phases where the interface is an electrical double layer. The region containing the double layere issheared at some distance from the solid surface creating a thin film associated with the solid. The electrical potential at the shearing plane is the zeta potential.
Positive particle with negative ion atmosphere
St
shear plane
x
Electrokinetic phenomena
surface charge density (amountof surface charge determining
ions adsorbed ) plotted against ion concentration c
Plateau: maximum amount adsorbed (max)at c* implies maximum surface charge
density (max) which, in turn impliesmaximum surface potential (omax)
surface potential oplotted against the
electrolyte concentrationc in the bulk solution
distance x from thesolid surface
towards the bulk solution
zeta potential plotted against the
electrolyte concentrationc in the bulk solution
surface potential oStern potential St
zeta-potential
*
% o
Elektrokinetic phenomena; the electrokinetic (zeta, ) potential
Electrokinetic phenomena
A solid surface in contact with a solution of an electrolyte usually carries an electric charge, o. This gives rise to an electric potential, o, at the surface, and a decreasing potential, , as we move through the bulk solution away from the surface, and in turn this effect the distribution of ions in the liquid.
Two regions are of primary importance: the Stern layerimmediately adjacent to the surface where ion size is important; and outside this region there is a diffuse layer.
Because of difference in charge between the diffuse layer and the solid suface, movement of one relative to the other will cause charge separation and hence generate a potential difference, or alternatively, application of an electrical potential will cause movement of one relative to the other.The relative movement of the solid surface and the liquid occurs at a surface of shear. The potential at the shear planeis known as the zeta () potential and its value can be determined by measurement of electrokinetic phenomena. Zeta potential is almost identical with the Stern potential thus gives a measure of the potential at the beginning of the diffuse layer.
Shear planeShear plane
Electrokinetic Phenomena
Isoelectric point
Zeta Potential Effect of pH
OH-H+
+-+-+-+-+-+-+
+-+-+-+-+-+-+
ShearPlane
ShearPlane
ShearPlane
Low pH: not enough OH- so
there is an excess positive charge.
+-+-+-+-+-+-+
High pH: not enough H+ so there is an excess negative
charge.
There is a point at which there are
enough OH- and H+ions to completely
balance out the surface charge
Zeta Potential Effect of pH
+-+-+-+-+-+-+
+-+-+-+-+-+-+
+-+-+-+-+-+-+
pH
-
(
m
V
)
+ Isoelectric Point (IEP):pH at which = 0
-
(
m
V
)
+
pH
Zeta Potential Effect of pH and ion concentration
Isoelectric point of several common proteins
Electrophoresis a suspended, charged particle moves as a result of an applied electrical field
Sedimentation potential an electrical potential created by the movement of charged particles through a liquid by gravity
Electrosmosis a liquid flows along a charged surface when an electric field is applied parallel to the surface
Streaming potential an electric potential created when a liquid is forced to move along a charged surface
Electrokinetic phenomena
Elektrokinetic phenomena
Electrophoresis - Movement of particle in a stationary fluid by an applied electric field.
Electroosmosis - Movement of liquid past a surface by an applied electric field
Streaming potential - Creation of an electric field as a liquid moves past a stationary charged surface
Sedimentation potential - Creation of an electric field when a charged particle moves relative to stationary fluid
Elektrokinetic phenomena Electrophoresis - Movement of particle in a stationary
fluid by an applied electric field. Electroosmosis - Movement of liquid past a surface by
an applied electric field Streaming potential - Creation of an electric field as a
liquid moves along a stationary charged surface Sedimentation potential - Creation of an electric field
when a charged particle moves relative to stationary fluid
the movement of a charged particle through a liquid under theinfluence of an applied potential difference (electric current)
Electrophoresis
Voltage source
Moving boundary electrophoresis
charged colloidal particles
Paper electrophoresis, continuous separation
sample buffer solution
(Horizontal electrophoresis)
support media: filter paper; cellulose acetate;
gel media: starch gels; polyacrylamide gels; agarose gels; polyacrylamide-agarose gels
Zone electrophoresis (from1950)Zone electrophoresis (from1950)
Schematic of capillary electrophoresis
Detector
Measurement of potential by microelectrophoresis
electrophoreticmobility(V/cm)
m/s)( [ xE
elfvelfu
]==
relfu
=
Smoluchowskiequation
relfu
23=
Hckelequation
(big particlethin double layer)
(small particlethick double layer)
m/s [ tdxd
elfv ]==
/cmV [ UxE ]== lelectric field
strength
electric driftvelocity
The movement of a liquid relative to an immobile charged surfaceof a capillary tube under the influence of an electric field
Electroosmosis
ElectroosmosisMovement of liquid past a surface by an applied electric field
steady state conditions:E = streaming potentiali = flow current
i
glass capillary
Streaming potentialcreation of an electric field as a liquid moves along
a stationary charged surface
glass capillary glass capillary pressure pressure
E = sedimentation potential
EE
Sedimentation potential (Dorn-effect) creation of an electric field when a charged particle
moves relative to stationary fluid
Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential
Electrophoresis - Zeta Mark 21;particles ~ 0.10 - 300 m
Electroacoustic - Colloidal Dynamics Acoustosizer; powders/slurries < 10nm - up to 50 weight%
Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential
Photocorrelation Spectroscopy - Brookhaven Zeta Plus;dilute suspensions; < 5 m; light scattering technique
Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential
Streaming Potential Paar Physica EKA; granular particles, surfaces
Equipment for Measuring Zeta Potential Equipment for Measuring Zeta Potential
Normal protein patterns
Abnormal protein patterns Abnormal protein patterns
Diagnostic applicationsof electrophoresis
Proteins are essential for metabolismidentified by electrophoresis based onsize, shape, and charge of particles.
Applications: clinical diagnosis.