Pressure study on thermal transitions of oleic acid polymorphs by high-pressure differential thermal analysis

Transition temperatures for oleic acid polymorphs (alpha, beta and gamma) and the melt were measured under various pressures up to 200 MPa by means of high-pressure differential thermal analysis. The pressure dependences of the transition temperatures were analyzed by applying the Clapeyron equation, and the volume changes associated with the phase transitions were estimated as follows; delta V gamma-alpha = 5.9 cm3/mol, delta V alpha-melt = 29.2 cm3/mol and delta V beta-melt = 41.1 cm3/mol. The volume changes for the gamma-alpha and alpha-melt transitions were discussed in relation to the crystal structures of the alpha and gamma modifications. The experiments showed that the beta modification, whose structural data are lacking, is the most dense. This correlates with the fact that the beta form is thermodynamically most stable among the three modifications of oleic acid.