Contact interactions free energy isotherms

Figures 2.13 and 2.14 show the isotherms of the free energy of contact interaction (V2 F) between methylated glass spheres as a function of surfactant concentration in aqueous solution for two model ionic surfactants SDS and CTAB, respectively.

In practice, the experimental procedure is quite long and fastidious referring to the successive determinations of the Gibbs energy of adhesion Aadh G ibetween a given solid and an apolar or polar liquid, which requires, in accordance with Eq. 6.17, measurement of the surface tension ylg, contact angle , and vapour adsorption isotherm (to calculate for each solid-liquid couple. To make matters worse, the very precise measurement of is possible only for atomically smooth surfaces. Finally, the additive approximation expressed by Eqs.6.9a, 6.9b and 6.12 is better suited to calculation of the enthalpy term than to that of the free energy, since the interactions may have both mechanical and entropic contributions [41]. 

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