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Effect of liquid composition on the double layer of polystyrene model colloids

Abstract:

The interpretation of electrophoretic mobility of polymer colloids (in liquids of variable composition) in terms of double layer relaxation is the subject of this work. Therefore, we have studied the microelectrophoretic behaviour of cationic polystyrene beads in solvents of different composition (methanol-, ethanol-, 1-propanol-water mixtures). A typical maximum for the electrophoretic mobility μe is observed at moderate ionic strengths. As a general result, this maximum is less pronounced for lower dielectric permittivity and fluidity of the liquid phase. This result might support the explanation of the electrokinetic behaviour of polymer colloids on the basis of an anomalous ionic conductance in the diffuse layer inside the shear-plane. In order to establish the real influence of this surface conductance effect on the electric double layer (e.d.l.) polarization, the conversion of μe data into ζ-potential values has been made by different theoretical approaches. In this study we have chosen those developed by O'Brien and White (ζow) and Dukhin and Semenikhin (ζDS). The greater values of ζDS in comparison with ζow at low electrokinetic radius are readily explained by that in the second theory, the contribution to polarization from all ions of the diffuse layer is taken into account, whereas O' Brien and White account for only the ions of the diffuse layer part of the e.d.l.