Dimitrios Argyris

Ion-Specific Effects under Confinement: The Role of Interfacial Water

All-atom molecular dynamics simulations were employed for the study of the structure and dynamics of aqueous electrolyte solutions within slit-shaped silica nanopores with a width of 10.67 A at ambient temperature. All simulations were conducted for 250 ns to capture the dynamics of ion adsorption and to obtain the equilibrium distribution of multiple ionic species (Na+, Cs+, and Cl(-)) within the pores. The results clearly support the existence of ion-specific effects under confinement, which can be explained by the properties of interfacial water. Cl(-) strongly adsorbs onto the silica surface. Although neither Na+ nor Cs+ is in contact with the solid surface, they show ion-specific behavior. The differences between the density distributions of cations within the pore are primarily due to size effects through their interaction with confined water molecules. The majority of Na+ ions appear within one water layer in close proximity to the silica surface, whereas Cs+ is excluded from well-defined water layers. As a consequence of this preferential distribution, we observe enhanced in-plane mobility for Cs+ ions, found near the center of the pore, compared to that for Na+ ions, closer to the solid substrate. These observations illustrate the key role of interfacial water in determining ion-specific effects under confinement and have practical importance in several fields, from geology to biology.

Hydration structure on crystalline silica substrates

The structure of interfacial water at the silica solid surfaces was investigated using molecular dynamics simulations. Different degrees of surface hydroxylation were employed to assess the effect of the surface chemistry on the structure of interfacial water. Density profiles, in-plane radial distribution functions, in-plane density distribution, and hydrogen-bond profiles were calculated. Our results show that the surface hydroxylation affects the structure, orientation, and hydrogen-bond network of interfacial water molecules. Data analysis suggests that the degree of hydroxylation controls the amount of water molecules in the first interfacial layer as well as the distance between the first adsorbed layer and the substrate. Well-organized and uniform structures of interfacial water appear on the homogeneously hydroxylated surface, while a heterogeneous interfacial structure, characterized by extensive water-water hydrogen bonds, forms on the partially hydroxylated surface. We demonstrate that both the local surface chemistry and water-water hydrogen bonds are the dominant factors that determine the structural properties of interfacial water.

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.

Aqueous NaCl and CsCl Solutions Confined in Crystalline Slit-Shaped Silica Nanopores of Varying Degree of Protonation

All-atom molecular dynamics simulations were conducted to study the dynamics of aqueous electrolyte solutions confined in slit-shaped silica nanopores of various degrees of protonation. Five degrees of protonation were prepared by randomly removing surface hydrogen atoms from fully protonated crystalline silica surfaces. Aqueous electrolyte solutions containing NaCl or CsCl salt were simulated at ambient conditions. In all cases, the ionic concentration was 1 M. The results were quantified in terms of atomic density distributions within the pores, and the self-diffusion coefficient along the direction parallel to the pore surface. We found evidence for ion-specific properties that depend on ion-surface, water-ion, and only in some cases ion-ion correlations. The degree of protonation strongly affects the structure, distribution, and the dynamic behavior of confined water and electrolytes. Cl(-) ions adsorb on the surface at large degrees of protonation, and their behavior does not depend significantly on the cation type (either Na(+) or Cs(+) ions are present in the systems considered). The cations show significant ion-specific behavior. Na(+) ions occupy different positions within the pore as the degree of protonation changes, while Cs(+) ions mainly remain near the pore center at all conditions considered. For a given degree of protonation, the planar self-diffusion coefficient of Cs(+) is always greater than that of Na(+) ions. The results are useful for better understanding transport under confinement, including brine behavior in the subsurface, with important applications such as environmental remediation.

Structure and Orientation of Interfacial Water Determine Atomic Force Microscopy Results: Insights from Molecular Dynamics Simulations

Massive all-atom molecular dynamics simulations were employed to study hydration forces near α-Al2O3 (0001) surfaces as sampled during a hypothetical AFM force spectroscopy experiment conducted using a (28,0) single-walled carbon nanotube as the tip at ambient conditions. The results provide the force acting on the carbon nanotube tip, as well as detailed properties of interfacial water, as a function of the nanotube-surface distance. As the tip approaches the solid substrate, interfacial water undergoes conformational and structural changes. These changes are responsible for the features observed in the force profiles, including the range at which forces can be measured (up to two hydration shells), the intensity of the forces experienced by the AFM tip, and their oscillatory character. Our detailed analysis shows that heterogeneous surface chemical composition results in appreciably different force profiles. This observation may explain the variability of AFM data sampling hydration forces even on atomically smooth substrates. In addition, our results suggest that sufficiently accurate AFM force spectroscopy could be used to study how hydration forces depend on surface heterogeneous properties and on the orientation and local density of interfacial water, which could aid our understanding of interfacial phenomena and lead to significant scientific breakthroughs.

Interfacial Water Properties in the Presence of Surfactants.

Water, because of its fundamental role in biology, geology, and many industrial applications and its anomalous behavior compared to that of simple fluids, continues to fascinate and attract extensive scientific interest. Building on previous studies of water in contact with different surfaces, in this study, we report results obtained from molecular dynamics simulations of water near hydrophilic and hydrophobic interfaces in the presence of nonionic and ionic amphiphilic molecules, hexaethylene glycol monododecyl ether (C12E6) and sodium dodecyl sulfate (SDS). We elucidate how these surfactants affect the packing (i.e., density profiles) and orientation of interfacial water. The results highlight the interplay of both surfactant charges and the substrate charge distribution predominantly with respect to the orientation of water molecules, up to distances longer than those expected based on simulation results on flat solid surfaces. We also quantify the dynamics of interfacial water molecules by computing the residence probability for water in contact with various substrates. We compare our results to those previously obtained for interfacial water on silica and graphite and also with experimental sum-frequency vibrational spectroscopy results at the air-water interface in the presence of surfactants. Our analysis could be useful for a better understanding of interfacial water not only near solid substrates but also near self-assembled/aggregated molecules at a variety of interfaces.