Surface Chemical Heterogeneity Modulates Silica Surface Hydration
A.M. Schrader, J.I. Monroe, R. Sheil, H.A. Dobbs, T.J. Keller, Y. Li, S. Jain, M.S. Shell, J.N. Israelachvili, and S. Han, PNAS 115, 2890 (2018). DOI:10.1073/pnas.1722263115
An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and many other applications. However, details of its surface chemistry and interactions with water are notoriously complicated and unclear––partially due to its tunable surface chemistry comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica–silica interaction forces across water using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica–silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0–2.9 nm−2). We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.
Spatially Heterogeneous Surface Water Diffusivity around Structured Protein Surfaces at Equilibrium
R. Barnes, S. Sun, Y. Fichou, F.W. Dahlquist, M. Heyden, and S. Han, Journal of the American Chemical Society 139, 17890 (2017). DOI:10.1021/jacs.7b08606
Hydration water on the surface of a protein is thought to mediate the thermodynamics of protein–ligand interactions. For hydration water to play a role beyond modulating global protein solubility or stability, the thermodynamic properties of hydration water must reflect on the properties of the heterogeneous protein surface and thus spatially vary over the protein surface. In this study, we employ Overhauser dynamic nuclear polarization (ODNP) to probe the equilibrium hydration water dynamics at select sites on the surface of Chemotaxis Y (CheY) in dilute solution. ODNP reports on site-specific hydration water dynamics within 5–10 Å of a label tethered to the biomolecular surface on two separate time scales of motion, corresponding to diffusive water (DW) and protein–water coupled motions, referred to as bound water (BW). We find DW dynamics to be highly heterogeneous across the surface of CheY. The heterogeneity in the hydration water dynamics suggests that structured proteins have the capacity to encode information into the surrounding hydration shell.
Bicontinuous Fluid Structure with Low Cohesive Energy: Molecular Basis for Exceptionally Low Interfacial Tension of Complex Coacervate Fluids
K.-Y. Huang, H.Y. Yoo, Y. Jho, S. Han, and D.S. Hwang, ACS Nano 10, 5051 (2016). DOI:10.1021/acsnano.5b07787
An exceptionally low interfacial tension of a dense fluid of concentrated polyelectrolyte complexes, phase-separated from a biphasic fluid known as complex coacervates, represents a unique and highly sought-after materials property that inspires novel applications from superior coating to wet adhesion. Despite extensive studies and broad interest, the molecular and structural bases for the unique properties of complex coacervates are unclear. Here, a microphase-separated complex coacervate fluid generated by mixing a recombinant mussel foot protein-1 (mfp-1) as the polycation and hyaluronic acid (HA) as the polyanion at stoichiometric ratios was macroscopically phase-separated into a dense complex coacervate and a dilute supernatant phase to enable separate characterization of the two fluid phases. Surprisingly, despite up to 4 orders of magnitude differing density of the polyelectrolytes, the diffusivity of water in these two phases was found to be indistinguishable. The presence of unbound, bulk-like, water in the dense fluid can be reconciled with a water population that is only weakly perturbed by the polyelectrolyte interface and network. This hypothesis was experimentally validated by cryo-TEM of the macroscopically phase-separated dense complex coacervate phase that was found to be a bicontinuous and biphasic nanostructured network, in which one of the phases was confirmed by staining techniques to be water and the other polyelectrolyte complexes. We conclude that a weak cohesive energy between water–water and water–polyelectrolytes manifests itself in a bicontinuous network, and is responsible for the exceptionally low interfacial energy of this complex fluid phase with respect to virtually any surface within an aqueous medium.