Electrophoresis, surface charge and surface activity coefficient of ionic adsorbates at interfaces

Abstract

Historical development of measurements of electrophoretic mobilities of solid particles and proteins using moving boundary techniques have been discussed in brief. Results on micro-electrophoretic methods of mobility measurements of solid particles adsorbed with proteins and other biopolymers have also been elaborated and features of electrophoretic separation of biomolecules using various types of zone electrophoresis techniques have been mentioned. The corrections required in connecting electrophoretic mobilities with electrokinetic or zetapotential using Smoluchowski relation discussed by Henry, Booth, Ghosh and others have been mentioned. The pictures included in the electrical double layers proposed by Helmholtz, Gouy-Chapman and Stern have been discussed. From the adsorption experiments of surfactants at oil-water interfaces, the charge densities (σ) and diffuse double layer potentials (ψ δ ) at a given concentration have been evaluated. The data are observed to fit the Stern layer model with counter-ion binding at the charged surface. ψ δ is found to be significantly higher than zetapotential. From the boundary tension measurement, the surface tension lowering or surface pressure II for soluble monobasic and dibasic acids have been calculated at different values of area A per adsorbed molecule. The electrical pressure II e obtained for these acids in the monolayer does not fit the Gouy model of double layer but using Stern model, the extents of counter-ion binding in the monolayer can be calculated. Using the Langmuir surface balance experiments, II-A curves for insoluble monolayers of different behenate salts have been obtained. At pH 12.0, values of IT e depends upon extents of counter-ion binding, hydration and other factors. A new concept of surface activity coefficient based on Raoult's law, and Hennry's law have been introduced to explain deviation of II from their ideal values.