MOLECULAR ACTIVATION-ENERGIES FROM ULTRASONIC DISPERSION

Experimental values of ultrasonic dispersion are determined in eight unassociated organic liquids of low shear viscosity, using a kin mpt 10 echoskop, at 500 kHz and 24 MHz, at various temperatures. Both velocity (V) and it's dispersion (V-omega- V-0) are found to decrease with temperature. Expressions are worked out for the compressional relaxation time (tau), using the concepts of complex (*) and frequency (omega) dependent bulk viscosity coefficient and compressibility of the liquid, in terms of the dispersion (V-omega - V-0) and velocity (V-omega) of the wave in the medium. The value of tau is found to vary with [(V-omega - V-0)/V-omega](1/2). The velocity (V-omega) at the compressional relaxation frequency (i.e.. omega tau=1) is found to be twice the low frequency velocity (i.e. V-0). Temperature (T) variations of tau in various liquids, are used to calculate their molecular activation energies (W) by plotting the linear variations between log(10) tau and 1/TK. The ultrasonic (or compressional) relaxation times in these liquids are found to be between 10(-11) to 10(-10) s, comparing fairly well with available data Values of W. calculated from slopes of these linear plots, varies between 0.24 eV to 0.46 eV. They compare fairly well with available values.