Ultraacoustic and conductometric studies on the interactions of l-glutamic acid with potassium sorbate and sodium benzoate in aqueous media

Abstract

The objective of this investigation is to study and understand the nature of interactions between different ionic, polar and alkyl segments of l-glutamic acid (Glu) and food preservatives with carboxylate ions, e.g. Potassium sorbate (PS) and sodium benzoate (SB), with each other and with water. Acoustic properties were determined by measuring the velocity of ultrasonic sound (U) passing through the aqueous solutions of different concentration of Glu and PS/SB at 298.15 K. Compressibility parameters like isentropic compressibility (Ks), apparent molar isentropic compressibility (Ks,ϕ) and isothermal compressibility (KT) were analysed to elucidate the type of ion–solvent and ion-ion interactions and structural changes of water molecules in the solvation shells with the influence of solute (Glu) and cosolute (PS/SB) ions. Variation of relative association (RA), internal pressure (πi) and acoustic impedance (Z) with concentration of solute and cosolute in the solutions provide knowledge of ion-ion and ion–solvent interactions, structure building/breaking behavior of solute and cosolutes and formation of ion pairs or triplets. Additionally, electrical conductance (К) of the solutions was measured at different temperatures (T = 293.15–313.15 K with 5 K interval). Molar conductance values (Λm), Walden factor (Λm0η0), thermodynamic variables for the process of ion association (ΔGA,ΔHAandΔSA) and energy of activation (EA) for transportation of electrical charges were studied to explore the spontaneity of ion association and mobility of the charged particles through the solution. Critical evaluation of all these physicochemical and thermodynamic data leads to the conclusion that stronger ion–solvent interactions prevail in the solutions, Glu acts as a structure builder in attendance of PS/SB and the process of ion pair formation is spontaneous.