Measurements are reported for the self-diffusion under pressures up to 300 MPa of methanol from 278 to 328 K and methan(2H)ol from 214 to 343 K. Data are also given for self-diffusion of ethanol under pressure, and ethan(2H)ol at 0.1 MPa, at 298 K. In methanol there is a large effect due to the isotopic substitution [D(CH30H)/D(CH302H) z 1.11 and there appears to be a correlation with the square root of the moments of inertia of the two isotopic species. Ethanol shows a similar, but much smaller, effect. The data for methanol and methan(2H)ol are fitted well by a rough hard-spheres treatment with temperature-dependent roughness parameters and also by a smooth hard-spheres correlation with a temperature-dependent molecular diameter.

Self-diffusion measurements are reported for acetonitrile in the temperature range 238-343 K at pressures up to 300 MPa. Lack of reliable high-pressure density data has restricted tests of theory to the temperature range 268-328 K. Over that range the results show a breakdown of rough hard-sphere theory which is attributed to the strongly dipolar nature of the acetonitrile molecule. A correlation approach based on Enskog smooth hard-sphere theory is found to be less sensitive to the dipole-dipole interactions.

Data have been obtained for self-diffusion in methanol to approximately 250 MPa pressure and methan(D)ol to about 304 MPa at 283, 298 and 313 K and at atmospheric pressure at 328 K. The average value of the ratio of the diffusion coefficients at atmospheric pressure is given by ������������� DR=D(CH3OH)/D(CH3OD)=1.09�0.03 At each temperature there is an increase in DR with pressure to a maximum at 250 MPa which is c. 6 % greater than the value at atmospheric pressure. The value of DR at atmospheric pressure is much greater than would be expected from the classical (smooth hard spheres) behaviour. Analysis of these values of DR in conjunction with moments of inertia data suggests that the major influence on diffusion is rotation of the methanol molecule about the principal axis.

Tracer-diffusion measurements have been made with a diaphragm cell at pressures up to ca. 260 MPa for acetonitrile in methanol and methanol in acetonitrile at 283, 298 and 313 K and up to 300 MPa for carbon disulphide in acetonitrile at 298 K. A correlation treatment used previously for self-diffusion in pure fluids has been extended to cover this type of tracer diffusion. Analysis of the data in acetonitrile indicates that dipoledipole interactions between the tracer species and solvent may have a significant effect on the diffusion.As a result of work carried out over the last decade, there is now a reasonable quantity of accurate data available for self-diffusion under pressure in a number of liquid systems covering a range of molecular sizes and interactions. The rough hard-sphere theory developed by Chandler1 has proved a valuable method of summarising and interpreting the data; it has been useful also for liquids composed of non-spherical molecules and even in associated systems where the model was not expected to apply. A correlation treatment of self-diffusion data due to Dymond2 has also proved useful, particularly since it can be used outside the density range of the hard-sphere model. Both approaches provide an estimate of the equivalent hard-sphere diameter of the diffusing particle. Chandler's model has also been used for mutual diffusion in systems at atmospheric p r e s ~u r e . ~ However, Dymond and W001f4 have recently used the rough hard-sphere model for a qualitative interpretation of tracer-diffusion data for benzene, toluene, benzo-a-pyrene, carbon disulphide and acetonitrileinn-hexane at pressuresup to400 MPa. The results seemedinaccord with the theory but a quantitative comparison was hampered by the small amount of computer simulation results available for smooth hard-sphere model systems of two components in which the molecules have a significantly different size ratio.This paper reports tracer-diffusion measurements which have been made up to ca. 260 MPa for acetonitrile in methanol and methanol in acetonitrile at 283, 298 and 3 13 K and up to 300 MPa for carbon disulphide in acetonitrile at 298 K. Because the equivalent hard-sphere diameter ratios, G ~/ C T , , where 2 denotes the tracer and 1 the solvent species, are closer to unity than was generally the case for Dymond and Woolf, the data for these systems enable a more quantitative test than was possible in their systems. It has also been possible to extend Dymond's correlation approach for self-diffusion to interpret the tracer data.

An attempt has been made to predict gas-gas immiscibility using the Guggenheim and van der Walls equations of state. The stability conditions have been solved, using a digital computer, for * Ref. (l), section 8.5.Ref. (l), section 8.4.