Apparent molal volumes of pectic acid and some tetra-N-alkylammonium salts of pectic acid

From commercially available pectin (OBIPEKTIN AG), pectic acid was isolated by alkaline catalysed hydrolysis and physico-chemically characterised. The densities of aqueous solutions of tetramethyl-, tetraethyl-, tetra-n-propyl- and tetra-n-butylammonium pectate were determined at concentrations up to 0.016 g . cm(-3) with a vibrating tube densimeter at 298 K. The concentration dependence of the derived apparent molal volumes and the influence of the nature of the counterion on the apparent molal volume was investigated. Thereby, a linear increase in the apparent molal volume, determined at the reference concentration, with increasing number of carbon atoms in the counterion was found. The experimental values of the apparent molal volumes approached a logarithmic dependence on concentration. The theoretical expression for the concentration dependence of the apparent molal volumes was interpreted on the basis of the cell-model of a polyelectrolyte solution and on the basis of infinite-line-charge theory. The experimental values of the apparent molal volumes were found to be higher than theoretical ones in the concentration range investigated. (C) 1997 Elsevier Science Ltd. All rights reserved.