Thermodynamic studies on the interactions of diglycine with magnesium chloride in aqueous medium at different temperatures Bhajan S. Lark a , Poonam Patyar a , Tarlok S. Banipal b, * a Department of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India b Department of Applied Chemistry, Guru Nanak Dev University, Amritsar 143 005, India Received 19 January 2006; received in revised form 13 February 2006; accepted 31 March 2006 Available online 18 April 2006 Abstract Apparent molar heat capacities (C P2,/ ), apparent molar volumes (V 2,/ ), and viscosities (g) of diglycine in water and in aqueous mag- nesium chloride (MgCl 2 ) solutions of molality m S  (0.05 to 0.70) mol Æ 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 vis- cosity measuring unit (AVS 350), respectively. The data have been used to calculate the partial molar heat capacities ðC 1 P2 Þ and partial molar volumes ðV 1 2 Þ at infinite dilution. The viscosity B-coefficients have also been obtained from viscosity data using Jones–Dole equa- tion. The C 1 P2 and V 1 2 values of diglycine in aqueous MgCl 2 solutions are higher than those in water and thus exhibit positive transfer functions (D tr C 1 P2 and D tr V 1 2 ), which are indicative of strong interactions between diglycine and MgCl 2 . Corresponding viscosity B-coef- ficients of transfer are also generally positive. The transfer functions decrease with increase in temperature and increase with the concen- tration of MgCl 2 . The free energies, enthalpies and entropies of activation for viscous flow of diglycine in aqueous MgCl 2 solutions have been obtained by using the Feakins transition-state theory. Partial molar expansibilities ðoV 1 2 =oT Þ P and ðo 2 V 1 2 =oT 2 Þ 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 (MgCl 2 ) in solutions. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Diglycine; Magnesium chloride; Apparent molar volumes; Apparent molar heat capacities; Viscosity; Viscosity B-coefficients; Gibbs free energy of activation; Hydration numbers 1. Introduction The structural and functional behaviour of biomolecules and ions is crucially dependent on their interactions with the solvent. Biochemical processes take place in water and it is important to consider solvent effects on their thermody- namic behaviour. Peptides are among the building units of complex biomolecules such as proteins. They are also important due to their wide range of applications in drug production hinged on to their ability to act as hormones and for their role as signal transmittance. Thus there has been considerable interest in determination of various ther- modynamic properties for aqueous solutions of amino acids, small peptides, and their derivatives. Salt solutions have large effects on the structure and properties of proteins including their solubility, denaturation, dissociation into subunits, and the activity of enzymes [1,2]. One can find in the literature many reports on thermodynamic [3–5], elec- trochemical [6–8], and numerical studies [9–11] of amino acids and peptides in aqueous solutions of electrolytes, espe- cially those containing sodium and potassium ions. Thus it seems appropriate to extend these studies to the systems containing electrolytes with bivalent ions such as Mg 2+ that is considered the element of life along with Ca 2+ , Na + , and K + ions. Mg 2+ ions as components of all body fluids are indispensable, among others, for a correct functioning of 0021-9614/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2006.03.022 * Corresponding author. Tel.: +91 183 2258802–9x3319; fax: +91 183 2258819/8820. E-mail address: tsbanipal@yahoo.com (T.S. Banipal). www.elsevier.com/locate/jct J. Chem. Thermodynamics 38 (2006) 1592–1605