A Volumetric Study of Tetracaine Hydrochloride in Aqueous and in Aqueous Electrolyte Solutions at Different Temperatures

The experimental densities of a local anesthetical drug, tetracaine hydrochloride (TC·HCl) in aqueous binary (H2O + TC·HCl) solutions and aqueous ternary electrolytic solutions (H2O + NaCl/KCl + TC·HCl) are reported at 288.15, 293.15, 298.15, 303.15 and 308.15 K in the dilute concentration region. The densities, were used to calculate apparent molar volumes ( $$V_{\phi }$$ ) at the finite concentrations of TC·HCl at the studied temperatures. The coefficient of thermal expansion (α) of solutions and the apparent molar expansivity ( $$E_{\phi }$$ ) of the solute, at 293.15, 298.15 and 303.15 K are also obtained. The limiting apparent molar volume ( $$V_{\phi }^{0}$$ ) and limiting apparent molar expansivity ( $$E_{\phi }^{0}$$ ) of TC·HCl were estimated using extrapolation methods. It is noted that the solute expansivity goes through a minimum in water as a function of temperature and at lower concentrations of electrolytes. By using the $$V_{\phi }^{0}$$ data of the studied binary and ternary systems, the values of transfer volumes, that is, volume changes due to transfer ( $$\Delta_{{{\text{tr}}}} V_{\phi }^{0}$$ ) from aqueous to mixed electrolyte solutions, are estimated. The observed $$\Delta_{{{\text{tr}}}} V_{\phi }^{0}$$ values are found to be both temperature and concentration dependent. The excess molar volume of mixing ( $$\Delta_{\text{m}} V^{{\text{E}}}$$ ) are also calculated at 298.15 K. The volume changes due to micellization are small and the sign and magnitude depend upon the structure making and breaking abilities of the salt. It is suggested that the micelles involved are of the stacking type and not in the form of linear aggregates. The hydration shell water structure around the cations seems to be controlled by temperature and the nature of the perturbing ions. The results are explained in terms of unique stacking type interaction, akin to micelles but differing in details on the mode of interactions.