Partial molar volumes of some drugs in water and ethanol at 35 °C

Abstract

Partial molar volumes, V0, of six drugs, mostly anaesthetics, viz., cinchocaine HCl, lidocaine HCl, mepivacaine HCl, procaine HCl, propranolol HCl, tetracaine HCl, in water and ethanol, calculated from precision densities obtained at 35 °C from a vibrating tube densitometer, are reported in this work. The data represent the smaller volumes of drug molecules in ethanol than in water. Volume contribution of hydrochloride part were calculated and excluded from V0 to assess the volume of free base component of the solutes. Volumes of free bases were also found smaller in ethanol, although much closer to those in water. The differences in volumes are interpreted as due to hydrophobicity of solutes. Relative hydrophobicities were estimated from volumes of transfer from aqueous to organic media. Correlation of V0 with van der Waal volumes are also reported. The hydrophobicity of these compounds is proposed to play a key role in the resulting drug action. Possible mechanism of drug binding with the membrane structure is also discussed.