Tetra point wetting of liquid K–KCl mixtures: Spectroscopic characterization of mesoscopic wetting and prewetting films

Abstract

The wetting and prewetting transitions at the metal-rich K–KCl melt–sapphire interface have been investigated by spectroscopic ellipsometry in combination with normal incidence reflectivity in the spectral range 0.8⩽ℏω⩽2.2 eV at temperatures up to 730 °C. Along the coexistence curve a salt-rich liquid wetting film is observed which is identified by the spectral features of the liquid state F-center. Unusually thick wetting films are found ranging from 30 nm near 540 °C to 300 nm approaching the monotectic temperature of 751 °C. Their composition has been determined from the absorption coefficients of the F-center band and it corresponds to about 90 mole % salt. At conditions off coexistence and near the prewetting line, similar mesoscopically thick wetting films exist. Crossing the prewetting line towards metal-rich solutions, the optical properties at the interface agree with those of the nearly free electron metal. The high thickness of the prewetting films is qualitatively explainable by charging and double layer formation at the interface. The occurrence of liquid F-center-like states up to 200 K below the monotectic temperature gives evidence of a strong undercooling of the wetting films with respect to the bulk phase. These characteristics of the wetting transition in a metal–molten salt solution can be described by the tetra point wetting scenario for binary fluid mixtures.