Eötvös rule

The Hungarian physicist Loránd after (Roland ) Eötvös ( 1848-1919 ) named Eötvössche rule allows us to predict the surface tension of any liquid pure substance at all temperatures. Therefore, merely the density, the molecular weight and the critical temperature of the fluid must be known. At the critical point is the surface tension to zero.

The first statement of the rule is:

1, the surface tension is linearly dependent on the temperature.

But the Eötvös equation describes not only the function of the surface tension of a liquid by the temperature but applies In addition, a further important and comprehensive statement:

2 The temperature dependence of the surface tension can be applied for all liquids so that it approximately always the same straight line results. This requires either the molar mass and the density of the liquid or its molar volume must be known.

Using these two rules can predict the surface tension of any liquid at any temperature.

The Eötvös equation

Is the molar volume, and the critical temperature of the liquid, its surface tension is γ according to the simple equation Eötvös

The after Eötvös valid for all liquids Eötvös constant has a value of

The units

• J for Joule
• K for Kelvin
• Mol
• Erg ( sometimes still in use in the cgs system).

Slightly more accurate values ​​are obtained if one takes into account that already 6 K intersects the temperature axis straight usually before the critical point:

The molar volume is given by the molecular weight M and density ρ:

The term is also referred to as interfacial tension molar:

Thus, the Eötvös equation can be written as:

A meaningful representation that avoids the unfavorable appearance of the unit mol 2/3 is obtained with the help of the Avogadro constant NA:

As John Lennard -Jones and Corner 1940 have shown with statistical mechanics, the constant k 'is approximately equal to the Boltzmann constant:

Water

For water, the equation

Historical

Eötvös began as a student to deal with the surface tension. He developed a new way to determine the surface tension, the reflection method. The Eötvös equation was first found purely phenomenological and published in 1886. 1893 showed William Ramsay and Shields the improved version, which takes into account that intersects the temperature axis straight usually before the critical point. Albert Einstein also examined the temperature dependence of the surface tension. John Lennard -Jones Corner and published in 1940 a derivation of the equation of statistical mechanics. M. Katayama was empirically found in 1916 a variant Eötvös equation for the case that the density of the steam is not negligible as compared to the density of the liquid. Based on that gave EA Guggenheim in 1945 a further variant of the equation is known which is now called Katayama - Guggenheim equation: