Atomic and electronic transport properties and the molar volume of monatomic liquids

Abstract

Experimental data on the coefficients of self-diffusion and shear viscosity of liquid argon and methane at saturated vapour pressure and on the thermal conductivity of argon expressed as D|T 1 2 , T 1 2 |η and T 1 2 |λ , respectively, are shown to obey a linear increase with the molar volume in a substantial fraction of the liquid range. Though the linear relationships are consistent with the interpretation of transport properties in terms of an appropriate hard-sphere fluid, the calculated diffusion and thermal conductivity in argon are too large by about 20% compared with the experimental values. This is most likely due to the fact that non-uniformities in the attractive- potential energy surface are not taken into account in the hard-sphere model. The atomic-mass transport coefficients of the liquid alkali metals and mercury behave in a similar way, though in this case the calculated diffusion is too small by about 15% compared with experiment. For the liquids, the relationship between D and η in combination with the temperature-dependent hard-core size that can be derived from either transport coefficient is found to be in nearly quantitative agreement with the Stokes-Einstein equation. It turns out that also the electrical resistivity and the thermal conductivity of the alkalis expressed as ϱ|T 1 2 and T 1 2 |λ increase linearly with the molar volume over a substantial range of temperatures, which in the case of cesium extends from room temperature (solid Cs) up to at least 0.9 T c (liquid Cs at saturated vapour pressure). The ratio of electrical and thermal conductivity closely resembles the Lorenz number for free electrons. A remarkable feature is that electron transport properties in the liquid alkalis meet relationships with the molar volume similar to those derived for atomic transport coefficients in hard-sphere kinetic theory.