Effect of temperature on the partial molar volumes and the partial molar compressibilities of α-amino acids in water and in aqueous solutions of strong electrolytes

Abstract

Density and sound velocity of glycine, alanine, α-aminobutyric acid, norvaline and norleucine in water and in aqueous solutions of strong electrolytes have been determined in the temperature range from 293.15 to 313.15K at intervals of 5K. The electrolytes used are lithium chloride, sodium chloride, potassium chloride, ammonium sulfate and sodium sulfate. The apparent molar volumes and the apparent molar compressibilities were determined as a function of composition at each temperature from the experimental density and sound velocity data. The limiting values of the partial molar volumes and the partial molar adiabatic compressibilities at infinite dilution of the amino acids in water and in aqueous electrolyte solutions were obtained at each temperature. The results confirm that glycine and alanine behave as hydrophilic solutes while aminobutyric acid, norvaline and norleucine exhibit a hydrophobic behavior and this behavior is also observed in the presence of salts. The addition of electrolytes led to an increase of the partial molar volumes and the partial molar compressibilities at infinite dilution suggesting that the cations and anions of the salts interact directly with the zwitterionic group of the α-amino acids, removing water molecules from hydration sphere of the amino acids, therefore reducing their hydration number. The decrease of hydration number reflects a decrease of electrostriction which leads to an increase of the partial molar properties. The results suggest that the dehydrating ability of the salts used shows a very good agreement with the increase of the partial molar volume as well as the partial molar compressibility of the α-amino acids.