Although certain liquids show Arrhenius viscosity behavior over the whole liquid range, most liquids have a substantial nonhyphen;Arrhenius range in which their viscosity is described by the freehyphen;volume equation. In a few supercooled liquids, there are two distinct nonhyphen;Arrhenius regions. From a calculation of the volumes required for free rotation of molecules in a liquid, it is shown that the transition from one type of dependence of viscosity on temperature to another is due to the restriction of the rotation of molecules in the liquid. In the Arrhenius region, molecules are free to rotate many times about at least two axes during the time between translational jumps. In the higher temperature nonhyphen;Arrhenius region, molecules are able to rotate about only one axis, while in the lower temperature, nonhyphen;Arrhenius region rotation occurs primarily as a result of molecular translational motion.
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