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1. Investigations of water/oxide interfaces by molecular dynamics simulations

   https://doi.org/10.1002/wcms.1537  

   Wang, Ruiyu ; Klein, Michael L. ; Carnevale, Vincenzo ; Borguet, Eric ( June 2021 , WIREs Computational Molecular Science)

   Water/oxide interfaces are ubiquitous on earth and show significant influence on many chemical processes. For example, understanding water and solute adsorption as well as catalytic water splitting can help build better fuel cells and solar cells to overcome our looming energy crisis; the interaction between biomolecules and water/oxide interfaces is one hypothesis to explain the origin of life. However, knowledge in this area is still limited due to the difficulty of studying water/solid interfaces. As a result, research using increasingly sophisticated experimental techniques and computational simulations has been carried out in recent years. Although it is difficult for experimental techniques to provide detailed microscopic structural information, molecular dynamics (MD) simulations have satisfactory performance. In this review, we discuss classical and ab initio MD simulations of water/oxide interfaces. Generally, we are interested in the following questions: How do solid surfaces perturb interfacial water structure? How do interfacial water molecules and adsorbed solutes affect solid surfaces and how do interfacial environments affect solvent and solute behavior? Finally, we discuss progress in the application of neural network potential based MD simulations, which offer a promising future because this approach has already enabled ab initio level accuracy for very large systems and long trajectories.

   This article is categorized under:

   Theoretical and Physical Chemistry > Spectroscopy

   Molecular and Statistical Mechanics > Molecular Interactions

   Structure and Mechanism > Molecular Structures

    

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2. “Solvent hydrogen‐bond occlusion”: A new model of polar desolvation for biomolecular energetics

   https://doi.org/10.1002/jcc.24740  

   Bazzoli, Andrea ; Karanicolas, John ( March 2017 , Journal of Computational Chemistry)

   Water engages in two important types of interactions near biomolecules: it forms ordered “cages” around exposed hydrophobic regions, and it participates in hydrogen bonds with surface polar groups. Both types of interaction are critical to biomolecular structure and function, but explicitly including an appropriate number of solvent molecules makes many applications computationally intractable. A number of implicit solvent models have been developed to address this problem, many of which treat these two solvation effects separately. Here, we describe a new model to capture polar solvation effects, called SHO (“solvent hydrogen‐bond occlusion”); our model aims to directly evaluate the energetic penalty associated with displacing discrete first‐shell water molecules near each solute polar group. We have incorporated SHO into the Rosetta energy function, and find that scoring protein structures with SHO provides superior performance in loop modeling, virtual screening, and protein structure prediction benchmarks. These improvements stem from the fact that SHO accurately identifies and penalizes polar groups that do not participate in hydrogen bonds, either with solvent or with other solute atoms (“unsatisfied” polar groups). We expect that in future, SHO will enable higher‐resolution predictions for a variety of molecular modeling applications. © 2017 Wiley Periodicals, Inc.

    

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3. Surface solvation and hindered isomerization at the water/silica interface explored with second harmonic generation

   Purnell, Grace E. Walker ( April 2019 , Journal of chemical physics)

   Resonantly enhanced second harmonic generation (SHG) spectra of Coumarin 152 (C152) adsorbed at the water-silica interface show that C152 experiences a local dielectric environment slightly more polar than that of bulk water. This result stands in contrast to recently reported time-resolved fluorescence experiments and simulations that suggest a alkane-like permittivity for interfacial water at strongly associating, hydrophilic solid surfaces. Taken together, these results imply that while the static electric field across the aqueous-silica interface may be large, restricted water dynamics lead to apparent nonpolar solvation behavior similar to that experienced by solutes in confinement. Resonance-enhanced SHG spectra and time-resolved fluorescence of C152 adsorbed to aqueous-hydrophobic silica surfaces show that when water’s ability to hydrogen bond with the silica surface is eliminated, a solute’s interfacial solvation and corresponding ability to photoisomerize converge to an intermediate limit similar to that experienced in bulk acetone or methanol. While water structure and dynamics at solid-liquid interfaces have received considerable attention, results presented below show how strong solvent-substrate interactions can create conflicting pictures of solute reactivity across buried interfaces. 

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4. In Situ Electron Microscopy of Poly(ethylene glycol) Crystals Grown in Thin Ionic Liquids Films

   https://doi.org/10.1002/pol.20190120  

   Srivastava, Satyam ; Ribbe, Alexander E. ; Russell, Thomas P. ; Hoagland, David A. ( January 2020 , Journal of Polymer Science)

   Crystals of poly(ethylene glycol) grown in thin films of the room temperature ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium ethyl sulfate were examined by electron microscopy as a first step toward exploiting nonvolatile liquids for nanoscale imaging of solvated/dissolved polymeric materials. The crystals were generated by cooling supported (over surfaces of varied polarity) and freestanding solution films to room temperature. This “open,” that is, without liquid cell, microscopy was performed on unstained, as‐grown crystals in the presence of the IL. A variety of nearly two‐dimensional crystal morphologies were observed, including rods, fibers, spherulites, compact faceted single crystals, and interconnected networks, with characteristic sizes ranging from tens of nanometers to tens of microns. Electron diffraction patterns for the rods and fibers revealed single crystal‐like long‐range order. The nature of the IL support little affected the morphology, but film thickness and cooling rate proved important. To assess the role of solvent polarity, crystals were also grown from 1‐ethyl‐3‐methylimidazolium ethyl sulfate mixed with the second IL, the less polar ethyl‐tributyl‐phosphonium diethyl phosphate; here, although the morphologies were similar to those made with pure IL, fibrillar morphologies were more prevalent. © 2020 Wiley Periodicals, Inc. J. Polym. Sci.2020,58, 478–486

    

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5. Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

   https://doi.org/10.1073/pnas.2020205118  

   Monroe, Jacob I. ; Jiao, Sally ; Davis, R. Justin ; Robinson Brown, Dennis ; Katz, Lynn E. ; Shell, M. Scott ( December 2020 , Proceedings of the National Academy of Sciences)

   Performance of membranes for water purification is highly influenced by the interactions of solvated species with membrane surfaces, including surface adsorption of solutes upon fouling. Current efforts toward fouling-resistant membranes often pursue surface hydrophilization, frequently motivated by macroscopic measures of hydrophilicity, because hydrophobicity is thought to increase solute–surface affinity. While this heuristic has driven diverse membrane functionalization strategies, here we build on advances in the theory of hydrophobicity to critically examine the relevance of macroscopic characterizations of solute–surface affinity. Specifically, we use molecular simulations to quantify the affinities to model hydroxyl- and methyl-functionalized surfaces of small, chemically diverse, charge-neutral solutes represented in produced water. We show that surface affinities correlate poorly with two conventional measures of solute hydrophobicity, gas-phase water solubility and oil–water partitioning. Moreover, we find that all solutes show attraction to the hydrophobic surface and most to the hydrophilic one, in contrast to macroscopically based hydrophobicity heuristics. We explain these results by decomposing affinities into direct solute interaction energies (which dominate on hydroxyl surfaces) and water restructuring penalties (which dominate on methyl surfaces). Finally, we use an inverse design algorithm to show how heterogeneous surfaces, with multiple functional groups, can be patterned to manipulate solute affinity and selectivity. These findings, importantly based on a range of solute and surface chemistries, illustrate that conventional macroscopic hydrophobicity metrics can fail to predict solute–surface affinity, and that molecular-scale surface chemical patterning significantly influences affinity—suggesting design opportunities for water purification membranes and other engineered interfaces involving aqueous solute–surface interactions.

    

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