Electric Birefringence of Electrolytes near Charged Surfaces, I

Abstract

We investigate electric birefringence of aqueous electrolyte solutions near a negatively charged silica glass surface using evanescent wave laser polarimetry (EWLP). By probing changes in the polarization state of visible light totally internally reflected at the solid-liquid interface, EWLP facilitates unprecedented measurements of electric-field-induced birefringence in a thin electrolyte film, O (150 nm), adsorbed on the glass surface. For aqueous NaCl and MgCl 2 solutions (1-100 mM), these measurements reveal Kerr law behavior over a range of pH and solution concentrations. The Kerr coefficients are, however, several orders of magnitude larger than values normally obtained from bulk electric birefringence measurements. We propose that formation of anisotropic, polarizable ionic clusters near the glass surface is the source of these observations. Orientation of the clusters by the electric field is confined in the plane defined by the glass/electrolyte interface and is attributed to rearrangement of the constituent ions. Birefringence relaxation measurements reveal relaxation times, r, many orders of magnitude larger than typically observed in molecular fluids. These measurements also show that r is independent of both the electrolyte concentration, c, and pH. We discuss these latter observations in terms of an ion redistribution process, wherein the intrinsic surface adsorption-desorption rates of ions in the clusters determine the time constant for relaxation of surface birefringence.