Measurement of the specific and the apparent molar volumes of some even chain length fatty acids in ethanol

Abstract

The specific and the apparent molar volumes of some fatty acids of even chain length from decanoic to octadecanoic acid were determined in ethanol. The objective was to understand the extent to which the acids can disturb the structure of the solvent as well as affect the micellar characteristics. The specific volumes were obtained by fitting the measured viscosity data to the Einstein equation while the apparent molar volumes were obtained using the pycnometric method. The specific volumes of the acids increased with the increase in the carbon chain length of the acids but were fairly constant within a narrow concentration and temperature ranges. Both the apparent molar volumes and apparent molar volumes at infinite dilution were temperature and carbon chain length dependent. The activation enthalpies for the viscous flows were independent of the carbon chain length suggesting little dissociation/association. The dissolution enthalpies were positive in conformity with the endothermicity of the process. The negative expansivity values were interpreted to originate from the dominant hydrophobic characters of the fatty acids. The combined analysis of (energy contribution of the solute to the free energy of Δµ 2 o the solution) and the Hepler constant dictated the classification of the acids as anomalous structure makers. Keywords : Viscosity, Specific Volume, Apparent Molar Volume, Activation Energy, Expansivity, Hepler Constant