THE EFFECT OF THE NATURE OF THE ORGANIC COSOLVENT ON SURFACE-CHARGE DENSITY OF SILICA IN MIXED-SOLVENTS

Potentiometric titrations of silica in mixtures of water with alcohols , diols, polyalcohols, ketones, ethers, DMSO, DMF, and acetonitrile ha ve been carried out. Usually, the negative surface charge sigma(0) of silica at a given pH and ionic strength decreases when the concentrati on of the organic component in the mixed solvent increases. Polyalcoho ls are an exception: their presence does not affect sigma(0), and eryt hritol makes it even more negative. Generally, the absolute value of a o of silica at a given concentration of an organic cosolvent is lower for the cosolvents with a longer hydro-carbon chain, e.g., water > met hanol > ethanol, and higher for cosolvents with a higher number of pol ar groups, e.g., propanol < propanediol < propanetriol. The magnitude of oo at a given pH, ionic strength, and organic cosolvent concentrati on is correlated with Reichardt's E(T)(30) and Kosower's Z and Z' pola rity parameters; namely, the absolute value of sigma(0) increases when these parameters increase. Even better correlation can be achieved wh en the solvent is characterized by a linear combination of one of thre e above-mentioned solvent parameters or alpha hydrogen bond donation a bility with beta hydrogen bond acceptance ability or with DN Gutman's donor number. In solvents with high values of the later two parameters , the negative surface charge of silica is low. (C) 1996 Academic Pres s, Inc.