Wettability and contact angle

For liquid and vapour in equilibrium, these values of AG are equal and  

The Kelvin equation can be applied to the solubility of spherical particles by replacing the ratio p/p0 by a/a0 where a0 is the activity of dissolved solute in equilibrium with a large flat surface and a is the activity in equilibrium with a small spherical surface. If we consider an ionic solute of formula MmXn,the activity of a dilute solution is related to the molar solubility S by  

When a drop of liquid is placed on a solid surface the liquid may form a bead on the surface, or it may spread to form a film. A liquid having a strong affinity for the solid, i.e., if its surface tension is less than the critical surface tension of the surface, yc, will seek to maximize its contact (interfacial area) and spread to form a film. A liquid with much weaker affinity, i.e., if its surface tension is above yc, will form into a bead. The critical surface tensions of solids range from 18 mN/m for Teflon to about 46 mN/m for nylon. 

The solid is completely wetted if 6= 0 and only partially wetted otherwise. Several practical points should be noted. Although in theory complete non-wetting would be 6= 180°, but values larger than about 140° are not actually seen in practice. Values of 0 90° are often considered to represent non-wetting whereas values of 0 9O° are 

Two common methods for measuring contact angles are the sessile drop and tilting plate methods. 

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A reversible photocontrol of wettability of polymeric materials is possible by a technique developed by Irie and Iga When a copolymer of butyl methacrylate and (2-hydroxyphenyl)-a-(4-vinylphenyl)benzyl alcohol is irradiated with ultraviolet light, fliere is a large increase in the contact angle and wettability of the material. This reverses back to the original structure in the dark... 

Hansen, G Hamouda, A.A., and Denoyel, R. (2000) The effect of pressure on contact angles and wettability in the mica/water/n-decane system and the calcite + stearic acid/water/n-decane system. Colloids and Surfaces A Physicochemical and Engineering Aspects, 172, 7-16. 

The surface structures are schematically summarized in Figure 7.12 [8]. There are various methods to characterize these surface structures, for example, ultravacuum techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy, Raman spectroscopy and infrar spectroscopy, contact angle and wettability, scanning tunneling microscopy, and thermal desorption mass spectrometry. 

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