PRESSURE EFFECT ON TRANSFORMATION OF CIS-UNSATURATED FATTY-ACID POLYMORPHS .2. PALMITOLEIC ACID (CIS-9-HEXADECENOIC ACID)

The existence of two polymorphs of palmitoleic acid was observed by differential scanning calorimetry. Powder X-ray diffraction pattern showed that the two polymorphs correspond to the α and γ forms, which have bee observed in other cis-monounsaturated fatty acids, oleic acid and erucic acid. γ reversibly transformed to α at -18.7°C on heating and α melted at 2.0°C. The effect of hydrostatic pressure on the γ-α transformation temperature, Ttr and the α-melt transition temperature, Tm, were studied by means of high-pressure differential thermal analysis. Ttr and Tm were elevated by pressure, as expreessed by dT/dp at atmospheric pressures of 0.190 and 0.202 K/MPa, respectively. The volume increases were estimated to be 5.6 cm3/mol for the γ-α transformation and 23/.6 cm3/mol for the α-melt transition by applying the Clausius-Clapeyron equation. The results were discussed compared with those observed in oleic acid. © 1990.