The theory of holes and the activation energy of viscosity in some nitrate melts at low temperatures

Abstract

The molar activation energy for viscous flow of some solutions of fused nitrates, in the temperature range 360-400 K, for solute mole fractions XM [math] 0.1, has been found to conform to the following equation :E* = X5 [math] + XM [math]where,[math] : activation energy of the solvent[math] : contribution of the solute to the activation energy of the solution.A good order of magnitude of the molar contribution of one component to the activation energy is obtained by assuming that this contribution is close to the energy of formation of the smallest hole, or vacant site, which gives the following equation :[math]with n = 1 for a monovalent nitrate and n = 2 for a divalent nitrateE* : contribution of the component to the activation energyTm : melting point of the componentΔSi : entropy contribution to the entropy of fusion which is involved in the process of incorporation of holes or empty sites at the melting pointθi : ratio of the number of empty sites to the total number of sites in the liquid phase at the melting point.θi and ∆Si are solutions of the following system of equations :[math]R : gas constantThe above two equations stem from a physical interpretation which considers the entropy variation associated with the incorporation of holes, or vacant sites, to be equivalent to the entropy of mixing of these vacant sites with the regular occupied sites. Simultaneously, the requirements is set that this entropy variation be a linear function of the volume variation originating from the vacant sites. Such a requirement being imposed on a function which has no linear part except in the vicinity of an inflection point, the coordinates of this point yield the values of θi and ∆Si needed to estimate E* :θi = 0.17 ∆Si = 23 JK-1 mol-1* whence the equation :E* = 56 n Tm n = 1 or 2E* in Jmol-1