Interactions of phenylalanine, tyrosine and histidine in aqueous caffeine solutions at different temperatures

Densities, p, viscosities, eta, and refractive indices, n(D) of aqueous caffeine (0.5 M) and of solutions of amino acids, I-phenylalanine (Phe), 1-tyrosine (Tyr) and l-histidine (His), (0.01-0.05 M) in aqueous-caffeine have been measured at 298.15, 303.15, 308.15 and 313.15 K. From these experimental data, apparent molar volume,,, limiting partial molar volume, phi(v)degrees and the slope, S,, transfer volume, Falkenhagen coefficient, A, Jones-Dole coefficients, B, free energies of activation per mole of solvent, Delta mu(1)degrees and per mole of solute, Delta mu degrees (#), enthalpy, Delta H-* and entropy, Delta S-* of activation of viscous flow, and molar refraction, R. were calculated. The results are interpreted from the point of view of solute-solvent and solute-solute interactions in these systems. It has been observed that there exist strong solute-solvent and weak solute-solute interactions in these systems. Further, the solute-solvent interactions decrease, whereas solute-solute interactions increase with rise in temperature. It is observed that these amino acids act as structure-makers in aqueous-caffeine solvent. The thermodynamics of viscous flow have also been discussed.