Additional Useful Thermodynamic Relationships for Wetting

Although the mathematical relationships encountered in wetting phenomena are usually quite simple, they are found to be very useful in many practical applications. Their combinations and variations have given rise to still more relationships, which further expand their utihty without expanding the amount of information necessary for their application. Two thermodynamic relationships that can be useful in the analysis of wetting and spreading phenomena are the works of cohesion and adhesion. 

The work of cohesion, Wc, is defined as the reversible work required to separate two surfaces of unit area of a material with surface tension s, given simply as 

The related quantity, the work of adhesion, W, is similarly defined as the work required to separate unit area of interface between two different materials or phases to leave a bare solid surface  

The subscript S12 is employed here to emphasize the fact that, as the two surfaces (S and liquid 2) are separated, two new interfaces are formed—S2 and 1-2. If the idea of the spreading pressure 77s2,i is included, Equation (17.18) becomes 

Working from Equation (17.19), one can define a new quantity, the reversible work of adhesion, which includes ffs2,i  

---