Partial molar volumes of some α-amino acids in aqueous sodium acetate solutions at 308.15 K

Abstract

The apparent molar volumes V 2,φ have been determined for glycine, dl-α-alanine, dl-α-amino-n-butyric acid, dl-valine and dl-leucine in aqueous solutions of 0.5, 1.0, 1.5 and 2.0 mol kg −1 sodium acetate by density measurements at 308.15 K. These data have been used to derive the infinite dilution apparent molar volumes V 0 2,φ for the amino acids in aqueous sodium acetate solutions and the standard volumes of transfer, Δ t V 0, of the amino acids from water to aqueous sodium acetate solutions. It has been observed that both V 0 2,φ and Δ t V 0 vary linearly with increasing number of carbon atoms in the alkyl chain of the amino acids. These linear correlations have been utilized to estimate the contributions of the charged end groups (NH 3 +,COO −), CH 2 group and other alkyl chains of the amino acids to V 0 2,φ and Δ t V 0. The results show that V 0 2,φ values for (NH 3 +,COO −) groups increase with sodium acetate concentration, and those for CH 2 are almost constant over the studied sodium acetate concentration range. The transfer volume increases and the hydration number of the amino acids decreases with increasing electrolyte concentrations. These facts indicate that strong interactions occur between the ions of sodium acetate and the charged centers of the amino acids. The volumetric interaction parameters of the amino acids with sodium acetate were calculated in water. The pair interaction parameters are found to be positive and decreased with increasing alkyl chain length of the amino acids, suggesting that sodium acetate has a stronger dehydration effect on amino acids which have longer hydrophobic alkyl chains. These phenomena are discussed by means of the cosphere overlap model.