Phenomena at aqueous interfaces

A. Pohorille, The Encyclopedia of Computational Chemistry, Schleyer, P. v. R.; Allinger, N. L., Clark, T.; Gasteiger, J.; Kollman, P. A.; Schaefer III, H. F.; Schreiner, P. R. (Eds.); John Wiley & Sons: Chichester, 1998, pp. 30-44.

Modern computer simulation of aqueous interfaces are only a decade old. In this short period, they have yielded new insights into the unique properties of interfacial systems, which distinguish them from bulk phases. Perhaps the most important of these properties is the existence of very different environments, polar and nonpolar, in direct proximity. As a result, aqueous interfaces tend to concentrate and organize organic material. In particular, they provide ideal surroundings for amphiphilic molecules, which can simultaneously have their polar parts immersed in water and nonpolar parts immersed in the organic liquid. Amphiphiles, however, are not the only solutes with the affinity towards interfaces. Many other polar molecules, which are not considered amphiphilic, also accumulate at the interface. This is primarily due to the balance between the electrostatic contribution to the excess chemical potential of the solute, which favors remaining in the proximity of the aqueous phase, and the chemical potential of cavity formation in the solvent, which is more favorable in the organic liquid.

Probably the best known example of organizing capabilities of aqueous interfaces is the formation of monolayers of organic molecules in this environment. Amphiphiles are typical examples of such monolayer-forming material, although molecules as complex as proteins can also arrange into monolayers. Other, biologically relevant examples of interfacial organization are sequence dependent conformational transitions of peptides or protein fragments from a random coil in aqueous solution to an ordered structure, such as an -helix or -strand, at the interface. Again, the formation of these structures is driven by a tendency to segregate polar and nonpolar moieties into the aqueous and organic liquid phase, respectively.

Molecular-level computer simulations clearly reveal that the interface should not be considered as a rigid plane but, instead, it is a rough, fluctuating surface. The significance of interfacial roughness is particularly evident in charge transfer across the interface. The inability to capture these deformations of the interface and the orientational anisotropy of solvent molecules in the interfacial region are the primary obstacles in successful applications of continuum models to describe interfacial phenomena.

Even though computer simulations of interfacial phenomena have been so far remarkably successful, many important problems still remain to be resolved. Studies on peptide folding in the interfacial environments have been initiated only recently and chemical reactions at interfaces have been investigated only sparingly. [, ] Interfacial electron transfer is still not well understood and proton transfer or, more generally, protonation of weak acids at interfaces have not been studied at all. There are also several methodological challenges to be faced. Among them are the inclusion of long-ranged and many-body effects in molecular simulations, the development of efficient parallel codes suitable for treating anisotropic, inhomogeneous systems, and improvements to continuum models that would extend their range of applicability in studies of interfacial phenomena. Due to the importance of these problems in chemistry and biology, there is no question that computer simulations of interfacial systems will remain an area of active and creative research for years to come.