Effect of NaCl and NaNO3 on the Partial Molar Volumes and Partial Molar Isentropic Compressibilities of Some Amino Acids at Several Different Temperatures (298.15–328.15) K

The partial molar volumes, \(\varphi_{v}^{\mathrm{o}}\), and partial molar isentropic compressibilities, \(\varphi_{k}^{\mathrm{o}}\), values have been computed using density and speed of sound values of L-phenylalanine/ L-leucine/L-glutamic acid/L-proline in 2 mol ⋅ L−1 aqueous NaCl or NaNO3 solutions, measured as a function of the amino acid concentration at temperatures of 298.15, 303.15, 308.15, 313.15, 318.15, 323.15 and 328.15 K. The variation of \(\varphi_{v}^{\mathrm{o}}\) values among the amino acids studied was found to follow the order proline < glutamic acid < leucine < phenylalanine. The increase in the partial molar volume of an amino acid with increasing in temperature has been attributed to the volume expansion of the hydrated zwitterions. The \(\varphi_{v}^{\mathrm{o}}\) and \(\varphi_{k}^{\mathrm{o}}\) values of L-phenylalanine, L-leucine, L-glutamic acid, and L-proline in 2 mol ⋅ L−1 aqueous NaCl and NaNO3 solutions have been found to be larger than the corresponding values in water. The larger partial molar volumes of L-phenylalanine, L-leucine, L-glutamic acid, and L-proline in 2 mol ⋅ L−1 aqueous NaCl and NaNO3 solutions have been ascribed to the possible formation of ‘zwitterion–\(\mbox{Na}^{+}/\mbox{Cl}^{-}/\mathrm{NO}_{3}^{ -}\)’ and ‘\(\mbox{Na}^{+}/\mbox{Cl}^{-}/\mathrm{NO}_{3}^{ -}\)–water dipole’ entities in solutions. The formation of these entities in solutions causes the release of water associated with zwitterions to the bulk water. The larger partial molar compressibilities of L-phenylalanine, L-leucine, L-glutamic acid and L-proline in 2 mol ⋅ L−1 aqueous NaCl NaNO3 solutions compared to the corresponding values in water has been attributed to the formation of these ‘zwitterion–ion’ and ‘ion–water dipole’ entities.