Micellization, acoustic and thermodynamic study of procaine in aqueous solution at different temperatures

Abstract

Critical micelle concentrations (CMC) of procaine hydrochloride (PHCl) in aqueous solution has been determined from conductivity (κ), density (ρ) and speed of sound (u) measurements using Williams and Philips methods at the temperature range 298.15–318.15 K intervals of 5 K.The CMC values found by these two methods using the speed of sound, density and conductivity data were in good agreement at all temperature. As the applied temperature increases, the CMC increases, owing to the lower probability of hydrogen bond formation at higher temperatures.The obtained values of the standard Gibbs energy of micellization (ΔGmo) and the standard enthalpy of micellization (ΔHmo) are negative at all investigated temperatures. Thus, one can say that the micellization process and the relationship between drug-water interactions and solute-solute interactions occur at different temperatures. Negative ΔGmo values were recorded when the systems become more hydrophobic. The negative increase in ΔHmo by the increased temperatures can be explained by the exothermic nature of the micellization process. The obtained values of the entropy for micellization (ΔSmo) are positive at all temperatures. The ΔSmo values decrease with temperature increase, this result interpreted as micellazation tend to be energy driven at higher temperatures.Apparent molar volume (V∅), apparent molal volumes at ınfinite dilution (V∅0), micellar apparent molal volumes (V∅mic), changes in apparent molal volumes upon aggregation (ΔVm), apparent molar isentropic compressibilities (K∅(S)), isentropic apparent molal adiabatic compressibilities at infinite dilution (K∅(S)0), micellar ısentropic apparent molal adiabatic compressibilities (K∅(S)m) and changes in isentropic apparent molal adiabatic compressibilities (ΔK∅(S)) of procaine in aqueous solution have been calculated by combining the speed of sound and density measurements. ΔVm are estimated and found to be negative except for 0.43 m3mol−1 (maximum value) at 318.15 K. The variations in these parameters were discussed for micellization process and drug-water interactions and solute-solute interactions at different temperatures.