The Effect of Protein Concentration on Electrophoretic Mobility.

The electrophoretic mobility of haemoglobin was measured in a novel membrane-electrophoresis cell and by electrophoretic light scattering. The effect of protein concentration was investigated at different ionic strengths in two different buffer systems. The results indicated that although the effect of the concentration is weak, the mobility did decrease linearly with an increase in volume fraction throughout the range of volume fractions investigated (φ<0.06). This dependence is more pronounced at lower kappaa values where double-layer interactions between the particles are more significant. Protein contribution to the solution ionic strength alone cannot explain the observed reduction in electrophoretic mobility. The theory of Shugai et al. (Shugai, A. A., Carnie, S. L., Chan, D. Y. C., and Anderson, J. L., J. Colloid Interface Sci. 191, 357 (1997)) was found to be adequate in describing particle interactions. The agreement with Shugai's theory is somewhat surprising considering the polydispersed nature of the samples, uncertainties in protein size, changes in ionic strength at high protein concentrations, and possible membrane-protein interactions not accounted for in the theory. Copyright 2000 Academic Press.

Direct Spectroscopic Measurement and Theoretical Modeling of the Diffusion of a Single Species in a Two-Phase Unstirred System

The diffusion of 8-hydroxyquinoline from an aqueous phase into an organic phase and vice versa has been measured using a static transfer cell. The total change in concentration in either phase can be accurately predicted by considering one-dimensional diffusion in the plane normal to the oil/water interface. It was found that any turbulent mixing that occurs during the initiation of the experiment dies out in the first few minutes. The relevance of this technique to the study of metal ion extraction from an aqueous phase into an organic phase is discussed.

A Kinetic Study of Copper Ion Extraction by Kelex 100 at a Heptane-Water Interface

A study of the kinetics of Cu2+ extraction by 7-(4-ethyl-1-methyloctyl)8-hydroxyquinoline (Kelex 100) has been undertaken in a static transfer cell, where the total concentration of the Cu2+-Kelex 100 complex in the organic phase is monitored in situ as the reaction proceeds using attenuated total internal reflectance spectroscopy. The temporal characteristics of the extraction of Cu2+ from water into heptane could be accounted for using a simple one-dimensional diffusion model coupled with a reaction-limited interaction between Cu2+ and Kelex 100. Rate constants for the reaction of Kelex 100 and Cu2+ at a model interface were obtained from stopped flow experiments using a neutral micelle (C12E8) system. These rate constants were used to model the reaction between Cu2+ and Kelex 100 at the heptane-water interface.

The Direct Measurement of the Forces of Interaction between a Colloid Particle and an Oil Droplet

An atomic force microscope was used to measure the forces between a silica sphere (diameter 10 μm) and a n-decane droplet (diameter 0.3 mm) in water. Force-distance profiles showed a weak attraction and adhesion due to van der Waals forces. When the anionic surfactant sodium dodecyl sulfate was added, there was electrostatic repulsion at all separations due to the adsorption of the anionic surfactant at the oil-water interface, and no adhesion of the sphere to the oil droplet was observed upon retraction. Fitting the repulsive curves to the nonlinear Poisson-Boltzmann equation for heterogeneous surfaces yielded surface potentials on the oil surface which were consistently lower than those values found from electrokinetic experiments on oil drops under similar conditions. The measured Debye lengths were also found to be significantly different from those calculated from the surfactant and electrolyte conditions employed. Possible reasons for the discrepancies are outlined.