Effect of KCl and KNO3 on Partial Molal Volumes and Partial Molal Compressibilities of Some Amino Acids at Different Temperatures

Abstract

Density (ρ) and ultrasonic velocity (u) values of amino acids l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and 2M aqueous KNO3 solutions have been measured as a function of amino acid concentration at different temperatures (298.15 K, 303.15 K, 308.15 K, 313.15 K, 318.15 K, and 323.15 K). Using the ρ and u data, partial molal volume (\({\phi_{\rm v}^{\rm o}}\)) and partial molal isentropic compressibility (\({\phi_{\rm k}^{\rm o} }\)) values have been computed. The increase in partial molal volume with temperature has been attributed to the volume expansion of hydrated zwitterions. The \({\phi_{\rm v}^{\rm o}}\) and \({\phi_{\rm k}^{\rm o}}\) values of l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and KNO3 solutions have been found to be larger than the corresponding values in water. The larger partial molal volumes of l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and KNO3 solutions have been ascribed to the formation of ‘zwitterion-K+/Cl−/NO3−’ and ‘K+/Cl−/NO3−–water dipole’ aggregates in solutions. The formation of these entities in solutions causes the release of water associated with zwitterions to the bulk water. The larger partial molal compressibilities of l-alanine-/l-proline-/l-valine-/l-leucine–2M aqueous KCl/KNO3 solutions than the corresponding values in water have been attributed to the formation of ‘zwitterion–ion’ and ‘ion–water dipole’ incompressible entities in solutions.