Volumetric and Viscosity Properties of α-Amino Acids and Their Groups in Aqueous Sodium Caproate Solutions

Abstract

Densities and viscosities have been measured for solutions of dl-α-valine or dl-α-leucine in aqueous sodium caproate solutions at 298.15 K and 308.15 K. On the basis of these data, the apparent molar volumes (Vm,2), standard partial molar volumes (V°m,2), standard volumes of transfer (ΔtV°), hydration number (nH), and viscosity B-coefficients of dl-α-valine and dl-α-leucine have been determined. Combined with the corresponding data of glycine, dl-α-alanine, and dl-α-amino-n-butyric acid reported recently, it has been shown that the standard partial molar volumes and the viscosity B-coefficients of the amino acids vary linearly with increasing number of carbon atoms in the alkyl chain of the amino acids. These properties have been split into contributions from the zwitterionic end group (NH3+, COO-) and CH2 groups of the amino acids by using linear relations. The volumetric data suggest that sodium caproate interacts strongly with the zwitterionic end group of the amino acids, and the electrolyte has a strong dehydration effect on the amino acids. Viscosity data have been discussed on the basis of structure effects of the amino acids and their groups in the solutions.