Strong Critical Adsorption

At the liquid/vapor or liquid/solid interface of a binary liquid mixture the liquid component with the lowest surface free energy will adsorb at the surface, in preference to the other component. This preferential adsorption is found even in the two-phase region of the liquid mixture provided a wetting layer does not form. It is important to obtain a quantitative understanding of adsorption because it influences the interactions between colloidal particles and is important in fluid flow through porous media. The variation in the local volume fraction v(z), with depth z into the liquid mixture, is described by a universal surface scaling function P_±(z/x_±) which takes differing forms in the one- (+) and two-phase (-) regions where x_± represents the bulk correlation length.
 

Further readings:  J. H. Carpenter, B. M. Law and D. S. P. Smith, Phys. Rev. E 59, 5655 (1999).
                           J. H. Carpenter, J. –H. J. Cho and B. M. Law, Phys. Rev. E 61, 532 (2000).
                           J.-H. J. Cho, B. M. Law and K. Gray, J. Chem. Phys. 116, 3058 (2002).
 

Plot of the universal strong surface scaling functions P+(x) and P-(x)-1 versus x for different models: P1 (dashed line), P3 (solid line), and EP model (dotted line).

Comparison of our critical surface adsorption scaling function from the P1 model (shown in the left figure) with experimental data for the critical mixtures aniline + cyclohexane (AC) and 1,1,2,2-tetrabromoethane + n-dodecane (DDTB) in the one- (squares) and two- phase (circles) regions.