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The activation volume of viscous flow in mixtures of paraffinic and naphthenic base oils of high and low viscosities, and in those of liquid fatty acids, their esters, and fatty oils have been determined experimentally. The results are used to test the applicability of the semiempirical formula which can estimate the activation volume from the molecular structure of the substance involved. Limitations in its applicability are also discussed.

The flow activation quantities were experimentally determined at 25.0°C with mineral lubricating oils blended with various concentrations of VI-improver. While polylauryl-methacrylate increases activation volume substantially and decreases activation entropy considerably, it does not affect activation enthalpy. Polyisobutylene changes the quantities only slightly. Molecular interpretation of these observations has been attempted.

Activation enthalpy, entropy, and volume of viscous flow were determined experimentally with 16 mineral lubricating oils and 4 blends. The relations between these quantities were examined and it was found that the relation between activation enthalpy and volume and that between the enthalpy and entropy were approximately linear. The later relation seems similar to the so-called compensation effect in chemical reactions. The activation enthalpy of the blends was shown to be additive when expressed in volume fraction.