A triple system water-urea-L-α-alanine. Thermodynamic properties and intermolecular interactions

The heat of dissolution of L-α-alanine at 293, 298, 306, and 318 K in aqueous solutions of urea in the concentration range 0–7 mol urea/kg water has been determined by the microcalorimetric method. It was found that the heat of dissolving the amino acid linearly depends on the square root of the molality of the urea solution. It was shown that the enthalpy and entropy of the transfer of L-α-alanine from water into an aqueous solution of urea do not depend on temperature, and the heat capacity of transfer is equal to zero. Almost complete enthalpy-entropy compensation was noted. It was shown that the parameter characterizing the changes in the effective Gibbs energy of dissolving L-α-alanine in water and aqueous urea solutions has negative values due to the dominant entropy component. The system water-urea-L-α-alanine becomes less structured with increasing temperature and more structured as the urea concentration increases.