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We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature Tmono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon 1 of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above Tmono the second liquid Bi-rich phase becomes stable and epsilon 1 changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.

Interfacial phase transitions like wetting and prewetting transitions are of considerable interest in physics and chemistry of condensed matter since they represent phase transitions in reduced dimensionality. Besides this interfacial properties are of profound practical and technological interest. Most systems studied experimentally in this respect are characterized by Van der Waals intermolecular interactions. However, in the last few years it was shown that Coulomb liquids like liquid alloys or metal molten salt solutions exhibit interfacial phase transitions similar to those known in Van der Waals systems. In order to get more insight into these phenomena the fluid‐vapor interface of two different alloy systems have been studied using ellipsometry. Gallium‐bismuth is a binary alloy with large positive deviations from Raoult's law, exhibiting a distinct miscibility gap. Approaching liquid‐liquid coexistence a Bi‐rich film completely wets the fluid‐vapor interface. As can be estimated from the ellipsometric results the film thickness jumps at the monotectic temperature to a value of about 50 Å. In contrast, gallium‐germanium shows continuous miscibility and deviates much less from ideal mixing. As the Ge concentration in liquid Ga increases along the solid‐liquid coexistence curve the optical properties at the surface also vary continuously, which can be modelled within a simple effective medium approach.

The authors report the first systematic study of the wetting behaviour in fluid alkali metal-alkali halide systems on the metal-rich side of the phase diagram. To this end the interface of fluid sample-inert substrate (sapphire) has been probed by ellipsometry. Of particular interest is the influence of differences of the bulk phase diagram on the wetting characteristics. If the bulk fluid phase exhibits homogeneous miscibility like Cs-CsCl the optical reflectivity changes continuously with composition consistent with metallic Drude type behaviour. However, in systems with a critical and a triple point like K-KCl and Na-NaCl a wetting transition is observed. This occurs in metal-rich solutions approaching the triple point along the phase boundary. In K-KCl a salt-rich wetting film of approximately=100 nm thickness and composition corresponding to K0.1KCl0.9 has been determined. This is the film thickness in thermal equilibrium as has been found by vigorous ultrasonic stirring.

First indications of wetting transitions in alkalimetal‐alkalihalide solutions have been obtained in previous optical studies. In order to study this transition continuously as a function of temperature and composition along the coexisting curve and in the homogeneous phase we have started SHG‐measurements on these systems at the fluid‐sapphire interface. First results of this experiment in comparison with the ellipsometric results will be presented. On the other hand, in order to exclude influences of the substrate on the interfacial behaviour we have investigated the liquid‐vapour interface in the liquid alloy Ga‐Bi (metallic system with miscibility gap) by spectroscopic ellipsometry. The main interest concentrates on possible discontinuous changes of the optical constants at the interface with varying composition and temperature. First results are presented.