Physical properties of fatty acids. I. Some dilatometric and thermal properties of stearic acid in two polymorphic forms

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Abstract

Summary Purified stearic acid (freezing point 69.20°C., melting point 69.5°C.) when slowly crystallized from benzene at room temperature, yielded transparent monoclinic crystals exhibiting an X‐ray diffraction pattern (long spacing) corresponding to the B‐form of the acid. Crystallization of stearic acid from hot glacial acetic acid resulted in a product exhibiting an X‐ray diffraction pattern (shorter spacing) corresponding to the C‐form of the acid. Rapid chilling of a melted sample of stearic acid likewise yielded a product exhibiting the diffraction pattern characteristic of the C‐forms of the acid. The specific volume, expansion in both solid and liquid states, and melting dilation of stearic acid in both B‐and C‐forms were calculated from dilatometric data. The irreversible transition of stearic acid from the B‐ to the C‐form was rapid and visible at 52.9°C. but also occurred at 35.2°C. when sufficient time was allowed for the transition to occur. The form of stearic acid which was obtained was found to be dependent upon the temperature at which crystallization occurred. When crystallization was carried out at temperatures below the transition point of the acid, the B‐form was obtained, and above the transition point only C‐form of the acid was obtained, irrespective of the solvent used for crystallization. The specific heat of stearic acid was determined, and equations developed for expressing the specific heat of the acid in both solid and liquid states. From the calorimetric data, the heat of fusion, and the entropy of stearic acid were calculated.

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