Concentration-depleted layers due to colloidal force and its influence on the xanthan fractionation

Abstract

This paper considers the entropic and colloidal interactions between rodlike polymers and a nearby boundary in a narrow channel of confined space. A theoretical model has been developed to predict both the concentration depletion profile and the chain configuration of thin rod polymers relative to the boundary. This model is shown to accurately predict the concentration profile of rodlike polymers in a confined space without any adjustable parameters. As the ionic strength is decreased, the predicted concentration profile shifts toward the center region due to the increase in repulsive colloidal force, so that the mean depleted layer becomes enlarged. For illustration, capillary hydrodynamic fractionations of anionic xanthan polyelectrolyte with eluants of various ionic strengths were also carried out experimentally. The ionic strength affects the conformational properties of xanthan molecules, which modifies the electrostatic interaction potential. When the eluant velocity is sufficiently low, the velocity enhancement factor χ (i.e., the ratio of average polymer velocity to eluant velocity) decreases as the ionic strength of the eluant is increased, which is in good agreement with the predicted results. For a higher eluant velocity, on the other hand, the theoretical prediction loses its accuracy due to the hydrodynamic force effect on xanthan motion, which will be examined in the forthcoming study. ？ 1993 by Academic Press, Inc.