Computer Simulation of Macroion Layering in a Wedge Film

Abstract

The layering of macroions confined to a wedge slit formed by two uncharged hard walls is studied using a canonical Monte Carlo method combined with a simulation cell that contains both wedge-shaped slit and bath regions. The macroion solution is modeled within a one-component fluid approach that in an effective way incorporates the double layer repulsion due to simple electrolyte ions as well as the discrete nature of an aqueous solvent. The layer formation under a wedge confinement is analyzed by carrying out separate simulation runs for a set of consecutive wedge segments designed to represent a single wedge slit. As the wedge thickness progressively increases, the sequence of regions along the wedge film with distinct features of macroion layering has been established. This sequence comprises (i) a wedge region of the thickness smaller than the macroion diameter that is free of macroions; (ii) a region with a one-dimensional macroion chain along the wedge corner at a wedge thickness of a one macroion diameter; (iii) a region comprising a low-ordered macroion monolayer that extends until the wedge thickness slightly above two macroion diameters; (iv) a region comprising a pair of well-defined two-dimensional configurations of macroions segregated on each of the wedge walls; and (v) a free-of-macroions wedge region between two surface monolayers that now originates from an electrostatic repulsion imposed by the surface macroions, which is followed by (vi) a well-defined macroion monolayer film between two surface monolayers, a less defined bilayer film, a three-layer film, and so on up to the bulk solution. Once formed, the macroion surface monolayers persist for all remaining wedge thicknesses up to the bulk, forming in such a way effective charged wedge boundaries. Such a formation of the macroion surface monolayers on the uncharged confining walls is related to the haloing mechanism for regulating the stability in colloidal suspensions [Tohver et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 8951] and is discussed as well. Finally, the estimated boundary of the free of macroion region between surface monolayers correlates well with the location of the boundary of the so-called "vacuum" phase that has been observed experimentally in an aqueous suspension of charged polystyrene spheres bounded by electrostatically repulsive glass walls [Pieransky et al. Phys. Rev. Lett. 1983, 50, 900].

The colloid structural forces as a tool for particle characterization and control of dispersion stability

“…Possible future directions could include the extension and application of the above approach to other systems such as mixed micelles, ABA block copolymers, and polyelectrolyte/ surfactant systems. In this respect, the greatest challenge is the extension of the approach to ionic micelles and other charged particles, for which computer simulations by means of the canonical Monte Carlo method have been successfully applied, 42,78 but accurate analytical expressions, like those in Section 4, are still missing. Furthermore, the effect of divalent and multivalent counterions on the structuring of charged particles in liquid films could be investigated.…”

Charged colloidal particles in a charged wedge: do they go in or out?

“…This would allow for a systematic understanding of many-body effects and for a more quantitative comparison between experiment and theory. We would expect that layering transitions would then play an important role, similar to those found for neutral particles in neutral wedges [40,41]. Then one could, in principle, think about using charged colloids in charged wedges as a model system for wetting transitions such as the wedge filling transition [51][52][53].…”

Wetting and Spreading of Nanofluids on Solid Surfaces Driven by the Structural Disjoining Pressure: Statics Analysis and Experiments

“…Nanoparticles inside the wedge film tend to form more ordered structures (due to the collective interactions) in the confined region than those in the bulk meniscus. [3][4][5][6][7][8][9][10][11][12][13][14] This structuring is a consequence of the fact that the ordering increases the entropy of the overall dispersion by permitting greater freedom for the nanoparticles in the bulk liquid. The result is that these ordered microstructures exert excess pressure (i.e., the disjoining pressure) in a film relative to that in the bulk solution, separating the two surfaces confining the nanofluid.…”