(550d) Monte Carlo and Density Functional Study of the "Drying" Behavior of Adsorbed Yukawa Fluids On the Repulsive/Attractive Solid Surfaces

Lee, L. L., The California State University Pomona
Pellicane, G., University of Kwazulu-Natal
Gering, K. L., Idaho National Laboratory

We examine the adsorption of Yukawa fluids on two types of planar walls: (a) hard walls and (b) Yukawa walls. The first type (hard walls) is to establish the baseline; the second type is to mimic the ?electric double layers? of an ?electrode/electrolyte? interface.

Type (a) System:

?Bulk Fluid: (attractive Yukawa interaction) ?Wall: Hard wall.

Type (b) System:

?Bulk Fluid: equal molar mixtures of species A and B molecules. A-A and B-B pairs interact via the repulsive Yukawa potential; A-B pairs via the attractive Yukawa potential. ?Wall: interacting with A molecules via the attractive Yukawa potential; with B molecules via the repulsive Yukawa potential.

The Yukawa potential (plus a hard-sphere interaction uHS) is defined in terms of an attraction strength parameter K, and an exponential decay parameter Z. We note that as the decay range parameter Z of the Yukawa potential decreases to zero, the Coulomb potential is recovered.

Monte Carlo simulations are carried out at temperatures from T* = kT/å = 0.76 to 1.25, and densities ñ* = ñó3 = 0.4, 0.7, and 0.8. Type a wall exhibits ?drying? at the two low densities, but ?wetting? at the highest density. Type b walls show more complicated behavior: drying is attenuated by the species A molecules which are attracted to the wall. The quantitative relation is modeled by a newly developed star-function*-based density functional theory (S-DFT). We delineate the ?drying? behavior by exhibiting the underlying correlations in S-DFT, such as the cavity distribution functions and the chemical potential behavior that give substance to the observed phenomena. Implications for the electric double layers on the electrodes in battery electrochemistry are explored.

*L. L. Lee, ?Chemical potentials based on the molecular distribution functions: An exact diagrammatical representation and the star function?, J. Chem. Phys. 97, 8606 (1992).