The occurrence of the hexagonal phase in two long chain monodisperse carboxylic acids at high pressure

High-pressure differential thermal analysis (DTA) experiments have allowed the pressure-temperature phase diagrams to be constructed for the monoacid CH₃-(CH₂)₁₉₀-COOH and the diacid HOOC-(CH₂)₁₉₂-COOH. The current work follows on from previous work concerning the high-pressure phase of various monodisperse n-alkanes. The use of a diamond anvil cell calibrated from DTA data has allowed the morphology of each sample to be investigated as a function of pressure and temperature and for the crystallization, melting and hexagonal/orthorhombic transitions to be examined directly. It was shown that the monoacid displayed a similar behaviour to the n-alkane of twice its chain length due to end group pairing, whereas the diacid shows a wider hexagonal stable region, which extends to pressures as low as 0.35 GPa. This enhanced stability is thought to be due to increased configurational entropy due to unlimited end group association.