Eötvös rule
The Eötvös rule, named after the Hungarian physicist Loránd (Roland) Eötvös (1848–1919) enables the prediction of the surface tension of an arbitrary liquid pure substance at all temperatures. The density, molar mass and the critical temperature of the liquid have to be known. At the critical point the surface tension is zero.
The first assumption of the Eötvös rule is:
1. The surface tension is a linear function of the temperature.
The Eötvös rule also gives a relation of the surface tension behaviour of different liquids in respect to each other:
2. The temperature dependence of the surface tension can be plotted for all liquids in a way that the data collapses to a single master curve. To do so either the molar mass, the density, or the molar volume of the corresponding liquid has to be known.
If V is the molar volume and Tc the critical temperature of a liquid the surface tension γ is given by
where k is a constant valid for all liquids. The Eötvös constant has a value of 2.1×10−7J/(K·mol2/3).
More precise values can be gained when considering that the line normally passes the temperature axis 6 K before the critical point:
The molar volume V is given by the molar mass M and the density ρ
The term is also referred to as the "molar surface tension" γmol :
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