Atomic view of how two liquids mix at the surface.

Whenever cream is poured into coffee, these two liquids form a homogeneous mixture, which is nearly impossible to separate again. Other liquids, such as water and oil, do not mix, instead forming emulsions, such as salad dressing.
A collaboration lead by physicists from Harvard University have used x-rays to look at how atoms of two miscible elements – bismuth and tin – mix at the surface of a liquid alloy. Despite forming a perfectly miscible solution in the bulk, near the surface the two elemental liquids separate into atomic layers with alternating compositions
While previous studies have observed segregation of the low-surface tension component in the surface monolayer, a phenomena known as Gibbs adsorption, extension of surface effects to subsequent sub-surface layers have never been observed in liquids before.

“The demixing we observe is somewhat of a paradox since the interactions between the two components are attractive, not repulsive as in the case of immiscible mixtures” explains Dr. Oleg Shpyrko, the leading author of the study. “Surface demixing was predicted in 1950 by Defay and Prigogine, but it eluded experimentalists for more than 50 years: liquids only demix within a nanometer-deep surface region, and there are very few experimental techniques capable of probing structure of liquid surfaces on such tiny length scales. But as we are learning about a variety of novel nanoscale materials where most atoms are near the surface, these and other interfacial effects are expected to play a dominant role.”

Shpyrko and colloborators have developed x-ray technique which allows independent measurements of atomic structure in the near surface region of the liquid, as well as the effects thermally induced capillary fluctuations of the surface, which can significantly obscure direct observation of surface structures on atomic scale. Over the past years, these developments by Harvard group have lead to discovery of surface-induced layering – a quasi-crystalline structure that appears at liquid-vapor interface of even simplest metallic fluids, but is apparently absent in dielectric liquids such as water.

In the most recent study, Shpyrko and co-workers added resonant x-ray scattering to obtain element-specific density profile, while retaining sub-nanometer spacial resolution – a method which can be applied to a wide range of multi-component liquids in the future studies.

The results are reported in this week’s issue of Physical Review Letters [Phys. Rev. Lett. 95, 106103 (2005)]

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