The High-Pressure Hexagonal Phase of Long Chain Dicarboxylic Acids in Water

Abstract

The high pressure phase of several kinds of dicarboxylic acid (DCA) has been studied by X-ray diffraction up to 700 MPa and compared with the hexagonal phase of polyethylene (PE) under high pressure. Eicosanedioic acid (DCA-20) is the shortest DCA investigated that has the high pressure phase. The phase diagram of DCA-20 is determined. Water plays an essential role for the transition from the monoclinic to the hexagonal form; in the absence of water, the monoclinic form directly melts without passing through the hexagonal form. Low molecular weight PE treated by fuming nitric acid is a DCA with a long methylene chain and also has the hexagonal form only in the water as a pressure transmitting fluid. The mechanism of the transition is discussed in the light of hydration of the carboxyl groups on the crystal surface and flexibility of the molecule depending on the length of methylene chain.