Experiments are performed at 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15 K to measure the activities of water for S-(+)-Alanine + H2O system over a wide range of amino acid concentrations. The measured water activities are used to calculate the vapor pressure of solutions, activity coefficient of water, activity coefficient of alanin, and excess Gibbs energies of solution. Also, from the measured water activity data, the hydration number of alanin molecules is calculated at different temperatures. The effect of temperature on the obtained thermodynamic properties of aqueous solutions of S-(+)-Alanine is studied. It was found that, the temperature has very slightly effect on the water activity of aqueous Alanine solutions. This behavior of aqueous amino acid solutions is similar to the those of aqueous electrolyte solution. In fact, when the amino acids are dissolved in water, their carboxyl groups lose a proton and become negatively charged and their amino groups gain a proton and become positively charged, forming a zwitterion. The results are discussed on the basis of the iteractions exist in this system. Furthermore, a thermodynamic model is used for the correlation of the experimental water activity data.
