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Electrokinetic phenomena Istvan Banyai University of Debrecen Dept of Colloid and Environmental Chemistry http://dragon.unideb.hu/~kolloid/ The electrical double layer at a charged surface A solid surface in contact with a solution of an electrolyte usually carries an electric charge, σ0. This gives rise an electric potential, ψ0, at the surface, and a decreasing potential, ψ, as we move through the liquid away from the surface, and in turn this effect the distribution of ions in the liquid. Two regions: The Stern Layer immediately adjacent to the surface where ion size is important; and outside this is a diffuse layer. Because of difference in charge between the diffuse layer and the solid surface, 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 plane is 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 ψ = ψ St exp ( −κ ( x − xSt ) ) xSt Plane of shear xSt Electrokinetic potential ψ = ψ St exp ( −κ ( x − xst ) ) ψSt ζ Shear plane xst or xd~ distance of Stern plane from the surface Electrokinetic potential or zeta potential is the electrostatic potential in the plane of shear Positive particle with negative ion atmosphere ζ ≈ ψ St The shear plane is located close the outer edge of the Stern layer so Stern potential is close to the zeta potential at low electrolyte concentration Electrokinetic potential of particles Thickness of diffuse layer δ= 1/κ within the slipping plane the particle acts as a single entity bulk ζ2<ζ1 ζ1 ζ2 Stern plane interface An electrical double layer exists around each particle. The liquid layer surrounding the particle exists as two parts; an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated Within this diffuse layer is a notional boundary known as the slipping plane, within which the particle acts as a single entity 1 ψ0 ψ St Electrokinetic potential Shear plane Iron oxide pH PZC ~6.5 1. Iron oxide 0,01 M KCl pH 4 2. Iron oxide 0.0001 M KCl pH 5 2 distance 3. Iron oxide 0.001 MKCl pH 8.5 + cationic tenzid The value of zeta potential may differ significantly from ψ0 but it has the same sign as ψSt 3 ζ1 = ζ2 = ζ3 Stern plane 1. A high positive surface potential with a low to moderate adsorption of an ionic solute at the Stern plane but with supporting electrolyte concentration to yield a thin diffuse layer. 2. Lower surface potential but still positive, little Stern layer adsorption and low concentration of electrolyte so that there is considerable extension of diffuse layer. 3. Negative but small surface potential, strong super-equivalent adsorption in the Stern plane and moderate extension of diffuse layer, i.e. moderate concentration of supporting electrolyte Electrokinetic phenomena Technique What Is measured What Moves What Causes Movement Electrophoresis Velocity particles move applied electric field Electroosmosis Velocity liquid moves in capillary applied electric field Streaming Potential Potential liquid moves pressure gradient Sedimentation Potential Potential particles move gravity = gΔρ 1. electrophoresis Particles move 2. electroosmosis Liquid moves in capillary 3. Streaming potential The moving liquid generates potential (reverse of electroosmosis) 4. Sedimentation potential Moving particles generate potential Electrophoretic mobility Fel = QE κ Ffric = fv since Fel = Ffric QE mobility v= f ze ze = u= 6πη r kT / D a v Q u= = E f εε0ζ μe = C(κa) η where Fel the direct electric force, E is the magnitude of the electric field, and Q is the particle charge, μe electrophoretic mobility V/m, ε is the dielectric constant of the dispersion medium, ε0 is the permittivity of free space (C² N m-2), η is dynamic viscosity of the dispersion medium (Pas), and ζ is zeta potential (i.e., the electrokinetic potential of the slipping plane in the double layer) in V. Electrophoretic mobility Biochemical proof of protein-DNA interactions using EMSA (electrophoretic mobility shift assay) The method bases on the property that unbound DNA in a non-denaturated gel exhibits a higher electrophoretical mobility than protein-bound DNA. Gel Electrophoresis Polyacrylamide Gel Electrophoresis (PAGE) http://www.steve.gb.com/science/chromatography_electrophoresis.html Isoelectric focusing (IEF) Isoelectric focusing employs a pH gradient extending the length of an electrophoresis gel. A protein stops migrating when it enters the zone in which the surrounding pH equals its isoelectric point, pI. At any other point in the gradient, the protein acquires a charge which causes it to migrate toward its pI (green and blue arrows). http://www.biochem.arizona.edu/classes/bioc462/462a/N OTES/Protein_Properties/protein_purification.htm The stable pH gradient between the electrodes is formed by including a mixture of low molecular weight 'carrier ampholytes' in the inert support. These are synthetic, aliphatic polyaminopolycarboxylic acids available commercially whose individual pI values cover a preselected pH range Isoelectric focusing (IEF) μe is electrophoretic mobility (EPM) It is important to avoid molecular sieving effects so that the protein separation occurs solely on the basis of charge The isoelectric point is the pH at which the zeta potential is zero. It is usually determined by pH titration: measuring zeta potential as a function of pH. The point of zero charge is the pH at which the positive and negative charges of a zwitteric surface are balanced. Capillary electrophoresis 1 Capillary electrophoresis 2. http://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm Move in capillary Neutral electrophoretic mobility: surface potential (zeta potential), size Electroosmotic Flow Schematic illustrating electroosmosis in a capillary. The circles indicate molecules and ions of the indicated charges, as well as their migration speed vector Flow profiles in microchannels. (a) A pressure gradient, -∇P, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule. (b) In electroosmotic (EO) flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness λD. . On the right is an experimental measurement of the distortion of a volume of fluid in an EO flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule. Electroosmosis http://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm The capillary wall can be pretreated with a cationic surfactant and the EOF will be reversed, that is, toward the anode (LB layers) Another way to control the EOF (electro osmotic flow) is to modify the wall with coatings So far –The charged surface stands liquid moves Summary Electroosmosis electrophoresis Streaming potential http://membranes.nist.gov/ACSchapter/toddPAGE.html http://zeta-potential.sourceforge.net/zeta-potential.shtml Sedimentation potential and Electrodeposition Non-stoichiometric or ionic exchange The exchange takes place in a "resin bed" made up of tiny bead-like material. The beads, having a negative charge, attract and hold positively charged ions such as sodium, but will exchange them whenever the beads encounter another positively charged ion, such as calcium or magnesium minerals. XR + KA ↔ KR + XA RY + KA ↔ RA + KY Cation, anion exchange, acid exchange, amphoteric surfaces Water softener A water softener reduces the dissolved calcium, magnesium, in hard water. Zeolit, clays, resins It can be regenerated