Notes on the Rate Process Theory of Flow

This paper deals with the separation of the contributing terms of the energy and entropy of activation for viscous flow and the identification of some of the molecular constants entering into these terms, as a step toward the prediction of the viscosity of liquids from first principles. The energy of activation for hole formation ΔH‡h is found to be determined mainly by the magnitude of the dispersion energy and by the extent of displacement of the hole forming molecules from their equilibrium position. The latter, in the form of the viscosity-pressure coefficient ΔV‡, is, at present, not predictable from any molecular constant. The energy of activation for motion into the hole ΔH‡i was found for many substances to be numerically equal to the excess energy of vaporization ΔEvapi and is thus an additional measure of restricted external rotation. The cases in which ΔH‡i > ΔEvapi offer strong evidence for the requirement of deformation of the flowing molecule against internal potential energy barriers. The rotational, translational, and cooperative terms which contribute to the entropy of activation have been separated, but can so far be determined only from viscosity data. Numerical examples show that the stereometric arrangement of molecular structure determines the magnitude of ΔV‡ and ΔH‡i and thereby the viscosity of liquids to a far greater extent than chemical composition (except for OH groups). The existence of aliphatic fatty acids as double molecules in the liquid state over a wide range of temperatures is confirmed viscosimetrically, while the viscosity data of aliphatic alcohols suggest the presence of distinguishable multiple molecules only at very low temperatures but the existence of a continuous OH-bond network structure at ordinary temperatures. One of the important consequences of the rate process theory of flow is the recognition that the viscosity of a liquid is determined by the (very small) concentration of molecules in relatively shallow potential energy walls from which the activated molecules are preferentially recruited. Viscosity is therefore not a bulk property in the commonly accepted sense and depends only to a minor extent on the structure of a liquid, i.e., on its state of order.