Volumetric and Viscometric Behaviour of Glycine, L-Alanine and L-Valine in Aqueous MgCl₂ and CaCl₂ Solutions at T = (293.15–313.15) K

Kalpana Shahi; Monika Singh

Volumetric and Viscometric Behaviour of Glycine, L-Alanine and L-Valine in Aqueous MgCl₂ and CaCl₂ Solutions at T = (293.15–313.15) K

Author(s)	Dr. Kalpana Shahi, Monika Singh
Country	India
Abstract	Densities and dynamic viscosities of ternary systems containing glycine (Gly), L-alanine (L-Ala) and L-valine (L-Val) in aqueous magnesium chloride and calcium chloride have been measured at salt molalities of 0.25, 0.50, 0.75 and 1.00 mol·kg⁻¹ over the temperature range T = (293.15 to 313.15) K at intervals of 5 K. Apparent molar volumes (Vφ) were derived from the density data and extrapolated to infinite dilution using the Masson equation to obtain limiting apparent molar volumes (Vφ°) and experimental slopes (Sᵥ). Volumes of transfer from water to the aqueous electrolyte, ΔₜVφ°, become progressively more positive with increasing salt molality for all three amino acids and are larger in MgCl₂ than in CaCl₂ at matched ionic strength. This pattern is interpreted through the Friedman–Krishnan co-sphere overlap model as a dominance of ion–zwitterion interactions over ion–hydrophobic interactions. Regression of Vφ° against the number of side-chain carbon atoms (nc = 0, 1 and 3 for Gly, L-Ala and L-Val respectively) resolves the zwitterionic end-group contribution from the methylene increment, giving V°(CH₂) in the range 14.4 to 16.8 cm³·mol⁻¹. Limiting apparent molar expansibilities are positive and the second temperature derivative (∂²Vφ°/∂T²)ₚ is negative throughout, so by Hepler's criterion all three solutes behave as structure-makers in both media. Viscosity B-coefficients obtained from the Jones–Dole equation are positive and decrease with rising temperature, corroborating the volumetric assignment. Activation parameters for viscous flow computed within the Feakins transition-state formalism indicate an enthalpy-controlled process whose magnitude increases with side-chain length and with cation charge density. The results quantify how the two principal physiological divalent cations differ in their action on amino-acid hydration and provide baseline thermodynamic data for comparison with transition-metal chlorides.
Keywords	apparent molar volume; viscosity B-coefficient; transfer volume; amino acids; magnesium chloride; calcium chloride;
Published In	Volume 7, Issue 2, July-December 2016
Published On	2016-12-03

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Volumetric and Viscometric Behaviour of Glycine, L-Alanine and L-Valine in Aqueous MgCl₂ and CaCl₂ Solutions at T = (293.15–313.15) K

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