Effect of KCl and KNO ₃ on partial molal volumes and partial molal compressibilities of some amino acids at different temperatures

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 KNO ₃ 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 ( φv) and partial molal isentropic compressibility ( φ) values have been computed. The increase in partial molal volume with temperature has been attributed to the volume expansion of hydrated zwitterions. The φ and φ values of l-alanine, l-proline, l-valine, and l-leucine in 2M aqueous KCl and KNO ₃ 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 KNO ₃ solutions have been ascribed to the formation of 'zwitterion-K ⁺/Cl ^-/NO ₃ ^- ' and 'K ⁺/Cl ^-/NO ₃ ^- -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/KNO ₃ solutions than the corresponding values in water have been attributed to the formation of 'zwitterion-ion' and 'ion-water dipole' incompressible entities in solutions.