Adhesion, entropy and surface forces

The adhesion energy at contact between two surfaces can be seen as due to a sum of a positive direct interaction term and a term of entropic origin appearing for a free surface exposed to a solvent. When the former dominates we have a lyophobic system while if the entropic term wins the system is lyophilic. These statements are illustrated by three examples (i) a solid ionizing surface; (ii) a liquid-like surface with protrusion modes; (iii) a surface with solvent polarization. It is pointed out that a theory of surface forces also have implications for adhesion energies. In particular it is found that the Marçelja theory of hydration forces leaves as unexplained the absence of a strong adhesive minimum at contact.