Intermolecular/interionic interactions in l-isoleucine-, l-proline-, and l-glutamine-aqueous electrolyte systems

Speed of sound and density values for ternary systems (amino acid + salt + water): l-isoleucine/l-proline/l-glutamine in aqueous solutions of 1.5 M KCl, 1 M KNO3, and 0.5 M K2SO4 have been measured for several concentrations of amino acids at different temperatures (303.15, 308.15, 313.15, 318.15, and 323.15 K). Using speed of sound and density data, the thermodynamic parameters such as isentropic compressibility (κs), change in isentropic compressibility (Δκs) and relative change (Δκs/κ0) in isentropic compressibility have been computed. The isentropic compressibility values decrease with increase in the amino acid concentration as well as with temperature. The decrease in κs values with increase in concentration of l-isoleucine/l-proline/l-glutamine in 1.5 M KCl, 1 M KNO3, and 0.5 M K2SO4 has been ascribed to an increase in the number of incompressible zwitterions in solutions, and the formation of ‘zwitterions–ions’ and ‘zwitterions–water dipole’ entities in solutions. The decrease in κs values with increase in temperature has been attributed to the corresponding decrease of κrelax (a relaxational part of compressibility), which is dominant over the corresponding increase of κ∞ (an instantaneous part of compressibility). The trends of variation of Δκs and Δκs/κ0 with variations in solute concentration and temperature have also been discussed in terms of solute–solute and solute–solvent intermolecular/interionic interactions operative in the systems.