Thermodynamic studies on the interactions of diglycine with magnesium chloride in aqueous medium at different temperatures

Apparent molar heat capacities (C-P2,C-phi), apparent molar volumes (V-2,V-phi), and viscosities (eta) of diglycine in water and in aqueous magnesium chloride (MgCl2) solutions of molality m(S) approximate to (0.05 to 0.70) mol center dot kg(-1) over the temperature range T = (288.15 to 328.15) K have been determined using high sensitivity micro-differential scanning calorimeter, vibrating-tube digital density meter, and automatic viscosity measuring unit (AVS 350), respectively. The data have been used to calculate the partial molar heat capacities (C-P2(infinity)) and partial molar volumes (V-2(infinity)) at infinite dilution. The viscosity B-coefficients have also been obtained from viscosity data using Jones-Dole equation. The C-P2(infinity) and V-2(infinity) values of diglycine in aqueous MgCl2 solutions are higher than those in water and thus exhibit positive transfer functions (Delta C-tr(P2)infinity and Delta V-tr(2)infinity), which are indicative of strong interactions between diglycine and MgCl2. Corresponding viscosity B-coefficients of transfer are also generally positive. The transfer functions decrease with increase in temperature and increase with the concentration of MgCl2. The free energies, enthalpies and entropies of activation for viscous flow of diglycine in aqueous MgCl2 solutions have been obtained by using the Feakins transition-state theory. Partial molar expansibilities (partial derivative V-2(infinity)/partial derivative T-2), and (partial derivative V-2(2)infinity/partial derivative T-2)(p) at infinite dilution along with their temperature dependence, the interaction coefficients from the volume, heat capacity, and viscosity B-coefficients have been used to divulge the various kinds of plausible interactions between solute (diglycine) and cosolute (MgCl2) in solutions. (c) 2006 Elsevier Ltd. All rights reserved.