Calculations of the intermolecular potential and surface energies of stearic acid

Abstract

Intermolecular potential calculations have been performed for the B and C polymorph of stearic acid, CH 3(CH 2) 16-COOH. The potential used was a Lennard-Jones 6–12 potential with corrections for the Coulomb forces of the charged carboxylic group. The parameters for the potential were directly taken from the literature without further adjustments. The heat of sublimation deduced from the calculations is 178 kJ/mol, which can be compared with an experimental value of 166 kJ/mol. The heat of the polymorphic transition of B into C - experimentally determined to lay between 4 and 6 kJ/mol - was calculated to be approximately 5 kJ/mol (±2 kJ/mol). The surface energies were calculated for the (100), (010), (110) and (001) faces. From a Wulff plot, the equilibrium forms were obtained. The B crystals are limited by (010) and (110) faces and the C crystals by (100) and (110) faces. The edge energies were obtained on the (100), (010), (110) and (001) faces for a variety of low indexed steps.