Volumetric Investigations on Molecular Interactions of Glycine/l-alanine in Aqueous Citric Acid Solutions at Different Temperatures

Abstract

Apparent molar volumes (phi_{V}) of glycine/l-alanine in water and in aqueous citric acid (CA) solutions of varying concentrations, i.e. (0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) mol·kg−1 were determined from density measurements at temperatures T = (288.15, 298.15, 308.15, 310.15 and 318.15) K and at atmospheric pressure. Limiting partial molar volumes (phi_V^{text{o}}) and their corresponding partial molar volumes of transfer (Delta_{text{tr}} phi_{V} ) have been calculated from the phi_{V} data. The negative Delta_{text{tr}} phi_{V} values obtained for glycine/l-alanine from water to aqueous CA solutions indicate the dominance of hydrophilic–hydrophobic/hydrophobic–hydrophilic and hydrophobic–hydrophobic interactions over ion/hydrophilic–dipolar interactions. Further, pair and triplet interaction coefficients, i.e.(V_{text{AB}} );{text{and}}; (V_{text{ABB}} ) along with hydration number (n_{text{H}} ) have also been calculated. The effect of temperature on the volumetric properties of glycine/l-alanine in water and in aqueous CA solutions has been determined from the limiting partial molar expansibilities (partial phi_{V}^{text{o}} /partial T)_{p} and their second-order derivative (partial^{2} phi_{V}^{text{o}} /partial T^{2} )_{{P}}. The apparent specific volumes (nu_{phi} ) for glycine and l-alanine tend to approach sweet taste behavior both in the presence of water and in aqueous CA solutions. The nu_{phi} values for glycine/l-alanine increase with increase in concentration of CA at all temperatures studied. This reveals that CA helps in enhancing the sweet taste behavior of glycine/l-alanine which also supports the dominance of hydrophobic–hydrophobic interactions.