Volumetric and ultrasonic studies of some amino acids in binary aqueous solutions of MgCl 2·6H 2O at 298.15 K

Abstract

Apparent molar volumes ( V ϕ ) and adiabatic compressibilities ( K ϕ,s ) of glycine, l-alanine and l-valine in binary aqueous solutions of MgCl 2·6H 2O (0.1–0.8 mol kg −1) have been determined at 298.15 K from precise density and sound speed measurements. Partial molar volumes ( V ϕ 0) and partial molar adiabatic compressibilities ( K ϕ,s 0) of these amino acids at infinite dilution were evaluated. These values are used for calculating the number of water molecules hydrated ( n H) to the amino acids. Group contributions for the partial molar volumes and adiabatic compressibilities have been determined from the amino acids. Transfer volumes (Δ V ϕ 0) and transfer adiabatic compressibilities (Δ K ϕ,s 0) at infinite dilution from water to aqueous magnesium chloride solutions have been calculated. Transfer parameters have been interpreted in terms of solute–cosolute interactions on the basis of a cosphere overlap model. Pair and triplet interaction coefficients have also been calculated from transfer parameters. All these results indicate that hydrophillic–ionic group interactions are predominant over hydrophillic–hydrophobic group interactions over the whole concentration range of magnesium chloride and decrease with the increase in size of the side chain of amino acids.