The heat of vaporization of liquid metals

The standard decomposition of the heat of vaporization Q per unit mass of a liquid is Qu + Qw, where Qu denotes the internal energy added to the system and Qw represents the external work of expansion. It is shown that Q must receive a substantial contribution, Qs, from the overcoming of the forces owing to the surface tension of the liquid, so that the heat equation is Q = Qu + Qw + Qs. Considering the removal of molecular layers one by one during evaporation, we derive the relation Qs = σ/aρ, where σ, a, and ρ are respectively the surface tension, the average distance between two molecules, and the density of the liquid. By applying this theory to the case of the liquid alkali metals lithium, sodium, potassium, rubidium, and cesium at saturation over a wide temperature range, we drew the following important conclusions: (i) Qs is comparable to Qw in all cases; (ii) the sum Q' = Qs + Qw shows remarkable constancy with respect to temperature; and (iii) the functional dependence Q'/cal g-1 = 9304 (M/g)-1.208 holds, where M is the molecular weight of the metal. (author)