Water at functional interfaces
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oduction That water is a solvent medium, described simply as a continuum dielectric medium, is an old perspective. The molecular nature of water, its asymmetric charge distribution, its geometry, and the resultant tetrahedral hydrogen-bonding network together contribute to the fascinating behavior of water in its pure state, as well as in solution and near various complex interfaces.1,2 Numerous fundamental questions persist in our understanding of molecular-scale interactions of water with interfaces and their consequences for natural and synthetic systems.3 These include the following: How do the structure and dynamics of water respond to the chemical and topographical context provided by solutes, biomolecules, and organic/inorganic materials? For example, biomolecular surfaces illustrate a complex patterning of hydrophilic and hydrophobic regions whose importance is far from fully understood. What are the consequences of the altered structure and dynamics of water on water-mediated interactions and assembly? For instance, assembly of solutes in an aqueous environment often requires removal (i.e., desolvation) of hydrated water molecules prior to solute aggregation. Recent computer simulations demonstrate that barriers to desolvation are lowered near hydrophobic interfaces, providing for preferential solute aggregation at the interface (Figure 1). Finally, what are the implications of water structure and dynamics for interfacial reactions and catalysis? As an example, preferential interactions of ions with water
interfaces influence molecular conformations that ultimately determine molecular reactivity (Figure 2). Given that interfaces are the most important motif in bio- and nanotechnologies, answering the previous questions is central to many applications. To this end, a deeper understanding is emerging from a combination of state-of-the-art experiments and theory as well as molecular modeling and simulations.
Articles in this issue Stable and transient hydrogen-bonding interactions between water molecules and the molecular and particulate solutes that are usually present in aqueous media have numerous consequences for assembly and reactions within the aqueous environment. Although many experimental techniques can probe the macroscopic consequences of this underlying molecular structure and dynamics, only a few can reveal these effects on a molecular length scale.4 These techniques include high-resolution scattering of x-rays and neutrons, specialized spectroscopy techniques, and scanning probe microscopies. In their article in this issue, Fenter and Lee review x-ray scattering studies of water ordering at solid surfaces, focusing on the angstrom-scale organization of water molecules at mineral– water and graphene–water interfaces as well as the effects of adsorbed ions on this organization. They quantify the hydration of surfaces with different characteristics, including those that are charged, uncharged, reactive, and ionic.
Shekhar Garde, The Howard P. Isermann Department of Chemical and Biological Engineeri
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