Temperature and Volume: The Independent Variables of State for Viscosity

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Abstract
The viscosity coefficients of molten tetrapropylammonium tetrafluoroborate and its phosphonium and arsonium analogs at 1 atm pressure show an anomalous relation to each other. When temperature and volume are used to define the state of the system, the relations between the viscosity coefficients become consonant with theory. Differences in molecular size and potential energy, so small that the viscosity is insensitive to them, can cause differences of several hundred atmospheres in the pressure of different fluids confined at the same volume and temperature. It is this sensitivity of the pressure to molecular parameters which exhibits itself in slight differences in molar volume at 1 atm pressure. These volume differences cause the “anomalous” behavior of the viscosity. A modified hard‐sphere equation of a state with a Coulomb term based on a pseudolattice adequately describes the PVT isotherms at high pressure and predicts the thermal expansion coefficient and Coulomb energy.

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