The Rates of Evaporation from Different Faces of Rhombic Sulphur

E. Rideal, P. M. Wiggins
1952 Proceedings of the Royal Society A  
The evaporation pressures of four faces of rhombic sulphur have been measured by a method which is a modification of the effusion technique for the measurement of vapour pressures of solids. The results in the temperature range 20 to 40° C $how that each face has a charac teristic, reproducible evaporation pressure, irrespective of the conditions under which the crystal was grown; that this evaporation pressure varies from face to face of the crystal; and that the activation energy for
more » ... on also varies, increasing with the reticular density of the face. From a consideration of the processes occurring at the crystal surface during evaporation, it was concluded that the condensation coefficients of the four faces are different, and that they have characteristic temperature coefficients. This was supported by some data on the growth form of crystals grown from solution at different temperatures, and it was concluded that it is possible that these four faces, with widely differing rates of evaporation, are all equilibrium faces of the crystal. Discontinuities were found to occur in the curves of log P against l/T for two of the four faces above 40° C. It was suggested that these are due to the onset of rotation on the crystal surfaces at the temperatures concerned, and that for each face a transition occurs at the temperature at which its condensation coefficient reaches unity. The mechanism of evaporation proposed assumes that an evaporating surface always has an inexhaustible supply of molecular terraces and that the rate of evaporation is controlled by the rate of formation of 'kinks' on the edge of such a step. Two faces of the crystal were investigated at temperatures above 90° C. Transitions were found to occur in each case at a temperature below that of the transformation of the rhombic into the monoclinic lattice. This was interpreted as a formation of monoclinic crystallites on the rhombic surface.
doi:10.1098/rspa.1952.0001 fatcat:pwwrqmbivzc2jgultvqrylgsju