Physicochemical analysis on the concentration distribution of water-soluble polymer in microcapillary flows

Abstract

In the vicinity of an interface, the properties of polymer are strongly modified relative to the properties of bulk solution. Polymers are entropically excluded from the vicinity of a neighboring wall in confined spaces, which results in a concentration depleted layer. A theoretical model of the concentration profile and the depleted layer thickness for thin rods under the action of the colloidal forces has been formulated by extending the previous work (Hoagland,1990). To develop this model, both the simple pairwise summation for van der Waals attraction and the linear superposition approximation for electrostatic repulsion were applied. As the ionic strength (i.e., ionic concentration of eluant medium) is decreased, the predicted concentration profile shifts toward the center region due to the increase in a repulsive colloidal force, so that the mean depleted layer becomes enlarged. The alteration of penetration depth is closely related to the rod orientation, which can be characterized by estimation of the order parameter. When the rod exists near the wall, repulsive forces create the preferential alignment of rod segments perpendicular to the wall surface, while the parallel alignment is favored without these interactions. From experiments for the model polymer itself, the ionic strength dependence of intrinsic viscosity as a function of molecular weight was observed substantially in the anionic xanthan polyelectrolyte. From a stiffness parameter (i.e., ratio of Kuhn segment length to contour length), as the molecular weight decreases, the stiff chain of short degraded xanthan becomes rodlike. Surface potential of xanthan is determined from the hydrodynamic equivalent length and the surface charge density measured with the titration technique. Our experimental work on the capillary flows of native and sonicated xanthans using a capillary hydrodynamic fractionation system shows that the velocity enhancement factor x is obviously affected by the ionic stren...