Volumetric, viscometric and molecular simulation studies of glycine in aqueous sodium sulphate solutions at different temperatures

The densities and viscosities of Glycine in five aqueous solutions of Sodium Sulphate, were measured in the temperature range from 283.15 to 308.15 K at intervals of 5 K, using a vibrating U-tube densimeter Anton Paar DMA 5000 and the Automatic Microviscosimeter AMVn, respectively. The measured densities were used to calculate the limit-apparent molar volume (Vϕ0), limit-transfer molar volume (ΔtrVφ0)and hydration number (nh). The partial volumes at infinite dilution of the glycine in aqueous solution of sodium sulfate were evaluated through extrapolation at each temperature. The behavior of ΔtrVϕ0 was interpreted in terms of solute-solvent interactions on the basis of the cosphere-overlap model. The hydration numbers were positive and interpreted in terms of dehydration and electrostriction effects. The relative viscosity data were adjusted by least-squares for obtain the Jones-Dole equation coefficients A, B and D. To accompany the experimental results, a series of small molecular complexes simulating the interactions present in the glycine + water/sodium sulfate systems were modeled by the Density Functional Theory with the basis function 6-31G(d). From viscosity dates, the activation parameters of the viscous flow (free molar energy of Gibbs ΔG≠, enthalpy ΔH≠, and entropy ΔS≠) were evaluated. The temperature dependence of the partial molar volume at infinite dilution and the viscosity B coefficient were discussed in terms of the dominant interactions in solution; it was found that the glycine has a making effect in the structure of the solvent, and at infinite dilution ion-solvent interactions are dominant between NH3+ and COO− groups of the amino acid with Na+ and SO42− ions. From the theoretical point of view, the complexes studied turned out to be stable and represent a good approximation to rationalize the intermolecular interactions that occur in glycine + water/sodium sulphate mixtures.