Interactions of Phenylalanine, Tyrosine and Histidine in Aqueous Caffeine Solutions at Different Temperatures

摘要

Densities, ρ, viscosities, η, and refractive indices, n_D of aqueous caffeine (0.5 M) and of solutions of amino acids, l-phenylalanine (Phe), l-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, φ_v, limiting partial molar volume, φ_v~o and the slope, S_v, transfer volume, φ_(v,tr)~o, Falkenhagen coefficient, A, Jones-Dole coefficients, B, free energies of activation per mole of solvent, Δμ_1~(o#) and per mole of solute, Δμ_2~(o#), enthalpy, ΔH~* and entropy, ΔS~* of activation of viscous flow, and molar refraction, R_m 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.