Thermodynamics of dilution of L-cystine in aqueous solution of sodium hydroxide, potassium hydroxide or hydrochloric acid

The enthalpies of dilution of L-cystine in solutions of two strong alkalis and one strong acid have been determined by isothermal flow-mixed microcalorimetry at the temperatures of 298.15, 303.15, 308.15, 313.15 and 318.15 K. Empirical equations of apparent enthalpies of dilution varying with the concentration of the amino acid have been obtained from the experimental data in terms of the improved McMillan-Mayer theory. Enthalpic interaction coefficients, h(2), h(3), and h(4), and pairwise entropic interaction coefficient s(2) have been gotten. The values of pairwise enthalpic interaction coefficient, h(2), have been discussed in the light of solute-solute and solute-solvent interactions. The results indicate that medium effect on the bivalent cation of L-cystine is stronger than on the bivalent anion. The values of h(2) show quite well straight-line dependence on temperature and pairwise entropic interaction coefficient, s(2), is a constant in the experimental range of temperature.