EQUILIBRIUM PROPERTIES AND STRUCTURAL RELAXATION IN METHANOL TO 30.4 GPA

The velocity and attenuation of ultrasonic waves and the characteristic time for the Mountain mode relaxation in liquid and glassy methanol have been measured as a function of temperature, frequency, and pressure up to 30.4 GPa between 5 and 94 degrees C. The density, bulk modulus, structural relaxation times, and amplitudes are deduced from the acoustic data. The high-frequency limit of the shear modulus at 25 degrees C at the low end of the experimental pressure range can be derived from reported low-frequency viscosities and the acoustic relaxation times. It is found that, in addition to the Mountain mode, at least two relaxation times are required to describe the high-pressure ultrasonic data. The slowest of these times is 160 ps at 25 degrees C and 3.7 GPa; a volume of activation of 2 cm(3)/mol describes the pressure dependence of this characteristic time up to 10 GPa. The limitations set by the experimental relaxation times on dynamical simulation of the high-density fluid are briefly discussed.