Molecular interactions of local anesthetics in aqueous MgCl2 solutions: A volumetric, acoustic and spectroscopic analysis

Abstract

Investigations of the physicochemical properties of drugs in the presence of co-solutes in aqueous solutions have always been a keen interest of the researchers. Such properties are very important to derive the information about solute–solute and solute–solvent type interactions. Therefore, in this communication, various physicochemical properties of two local anesthetics viz. Procaine hydrochloride (PC) and Tetracaine hydrochloride (TC) in aqueous and in aqueous solutions of MgCl2 (mB = 0.024, 0.047, 0.070 mol kg−1) have been studied using density and sound velocity measurements at different temperatures (288.15, 298.15, 308.15, and 318.15 K) along with 1H NMR spectroscopy. Various parameters like apparent molar volume (V) and isentropic compressibility (Ks) of solutes, apparent molar volume (V°) and isentropic compressibility (Ks°) of solutes at infinite dilution, apparent molar expansibility (E°) and their double derivatives (∂2V°/∂T2)P have been evaluated using density and sound velocity data. The transfer volumes (ΔtrV°) of PC and TC drugs from water to aqueous MgCl2 solutions and the various acoustical parameters like acoustic impedance (Z), intermolecular free length (Lf), and relative association (RA) have also been computed. All these parameters have been used to throw light on the effect of temperature as well as the effect of co-solute (MgCl2) concentration on the various solute–solvent/co-solute interactions with the help of co-sphere overlap model. The structure breaker/or maker ability of both local anesthetics in the presence of MgCl2 has also been analyzed. The spectroscopic results indicate the presence of hydrophobic–ionic interactions at higher concentrations of co-solute.