EXPERIMENTAL-DATA AND MODELING OF APPARENT MOLAR VOLUMES, ISENTROPIC COMPRESSIBILITIES AND REFRACTIVE-INDEXES IN AQUEOUS-SOLUTIONS OF GLYCINE PLUS NACL

Experiments have been performed at 298.15 K to measure the density, so und velocity and refractive index of glycine in aqueous solutions of N aCl over a wide range of both glycine and NaCl concentrations. The val ues of apparent molar volume and isentropic compressibility of glycine were calculated from the measured data. The results show a positive t ransfer volume of glycine from an NaCl solution to a more concentrated NaCl solution. This indicates that the size of a glycine molecule is larger in a solution with higher NaCl concentration. The negative valu es of apparent isentropic compressibility imply that the water molecul es around the glycine molecules are less compressible than the water m olecules in the bulk solution. These effects are attributed to the dou bly charged behaviour of glycine and to the formation of physically bo nded ion-pairs between the charged groups of glycine and sodium and ch loride ions. The formation of ion-pairs, whose extents of binding reac tions depend on the concentrations of both NaCl and glycine, alter the hydration number of glycine. This also explains the reason for the in crease in the size of glycine with an increase in the NaCl concentrati on. A model based on the Fitter formalism has been developed to correl ate the activity coefficient, apparent molar volume and isentropic com pressibility of glycine in aqueous solutions of NaCl. The results show that the model can accurately correlate the interactions in aqueous s olutions of glycine and NaCl. (C) 1998 Elsevier Science B.V. All right s reserved.