Relationships applicable for the prediction of self-diffusivities of gases and liquids have been developed from available experimental values and an application of dimensional analysis. Separate relationships were found to apply for gases a t normal pressures and for gases a t elevated pressures. For the liquid state a different dependence on the conditions of temperature and pressure was observed and was taken into account to develop the relationship for this state of aggregation.Theoretical relationships for the transport properties of fluids have received considerable attention in the last 50 yr. For gases at moderate pressures, expressions derived from kinetic theory can be used to calculate the transport properties with sufficient accuracy. Generalized relationships for the calculation of these properties have recently been developed (12, 27, 36, 37), and they offer a sound alternative to the somewhat involved theoretical relationships of Chapman and Enskog (7,16). Such relationships have been developed from existing data for the viscosity (36, 37) and the thermal conductivity (27) of gases at normal pressures.The generalized dimensional approach can be extended to consider the transport properties of dense gases and liquids. So far, the application of kinetic theory principles to a gas at high pressures has not succeeded in producing expressions that adequately describe its behavior (7, 16). Along these lines, Enskog (11) in 1922 published what can be considered the best theoretical approach to date. The Enskog treatment, however, is limited by the assumption that the gas is composed of rigid spheres. Attempts to overcome this limitation have been advanced from statistical mechanical concepts by Rice, Kirkwood, Ross, and Zwanzig (31), and Snider and Curtiss (35). Unfortunately, these more recent treatments do not yield expressions that exhibit a definite improvement over that advanced by Enskog (11 ).The application of a dimensional analysis approach to available high-pressure data offers a more direct means for establishing the dependence of a transport property on the conditions of the systems. Such an approach has been successfully applied for the prediction of viscosity (19, 39) and thermaI conductivity (38) of gases at high pressures and also for the liquid state. Following this line of reasoning, it should be possible to develop a generalized method capable of defining the self-diffusivity of gases at normal and elevated pressures and of liquids as well. The difficulty of obtaining reliable self-difisivity measurements has contributed largely to the present lack of a dependable method capable of predicting this transport property in the dense gaseous and liquid states. However, the recent experimental contributions of Durbin ( l o ) , Naghizahdeh and Rice (28), Becker, Vogell, and Zigan ( 2 ) , and OHern and Martin (30) have produced sufficient reliable self-diffusivities to make possible the development of a generalized method for predicting this transport property.
