High pressure Raman spectroscopy investigation on acetonitrile and acetonitrile-water mixture

High-pressure Raman scattering studies on pure acetonitrile and an acetonitrile-water mixture at a molar ratio of (nCH(3)CN : nH(2)O) 1 : 7.25 were performed in a diamond anvil cell at room temperature. The structural transitions of pure acetonitrile from liquid to alpha phase, alpha to beta phase and beta to gamma phase were detected from Raman spectra variations at 0.2 GPa, 0.8 GPa and 4.95 GPa, respectively. The acetonitrile-water mixture presented a much higher solidification pressure of 1.25 GPa and the beta phase is found to be sustained up to 7 GPa. Through Raman analysis on the solid acetonitrile-water mixture, the acetonitrile clusters were found to exist as separated domains and surrounded by ice crystal domains due to large amounts of water in the mixture. Meanwhile the fluorescence was obviously depressed in the mixture and the Raman peaks can be detected up to 29.88 GPa, while the Raman peaks in pure acetonitrile are undetectable at 21 GPa due to its increased fluorescence.