Volumetric studies of aqueous solutions of local anesthetical drug compounds [hydrochlorides of procaine (PC HCl), lidocaine (LC HCl) and tetracaine (TC HCl)] at 298.15 K

Abstract

In order to probe micellization phenomenon and structural interactions in aqueous solutions of local anesthetical drug compounds procaine hydrochloride (PC HCl), lidocaine hydrochloride (LC HCl) and tetracaine hydrochloride (TC HCl), we carried out experimental measurement of densities as a function of drug concentration at 298.15 K. The derived properties like apparent molar volumes ( ϕ V ) and partial molar volume ( V ¯ 2 ) of the drug molecules were appropriately calculated, treating drug molecules as electrolytes. The limiting partial molar volumes ( V ¯ 2 0 ), critical micelle concentrations (cmc) and aggregation number ( n) are estimated and found to be 225.00⋅10 3 mm 3mol − 1 , 0.45 mol kg − 1 and 2 for PC HCl, 237.10⋅10 3 mm 3mol − 1 , 0.13 mol kg − 1 and 8 for LC HCl and 259.80⋅10 3 mm 3mol − 1 , 0.17 mol kg − 1 and 6 for TC HCl respectively. Applying the pseudo-phase separation model the volume changes due to micellization ( ΔV x m ) are estimated and are found to be 0.37⋅10 3 mm 3mol − 1 , −1.17⋅10 3 mm 3mol − 1 and 1.50⋅10 3 mm 3mol − 1 for PC HCl, LC HCl and TC HCl respectively. These results are explained on the basis of drug–water interactions and hydrophobic interactions (i.e. solute–solute interaction) which seem to depend upon the structural characteristic of the linkages involved and difference in the chain which binds the hydrophobic group and hydrophilic group in the studied drug molecules.