Ateeque Malani

Influence of hydrophilic surface specificity on the structural properties of confined water.

The influence of chemical specificity of hydrophilic surfaces on the structure of confined water in the subnanometer regime is investigated using grand canonical Monte Carlo simulations. The structural variations for water confined between hydroxylated silica surfaces are contrasted with water confined between mica surfaces. Although both surfaces are hydrophilic, our study shows that hydration of potassium ions on the mica surface has a strong influence on the water structure and solvation force response of confined water. In contrast to the disrupted hydrogen bond network observed for water confined between mica surfaces, water between silica surfaces retains its hydrogen bond network displaying bulklike structural features down to surface separations as small as 0.45 nm. Hydrogen bonding of an invariant contact water layer with the surface silanol groups aids in maintaining a constant number of hydrogen bonds per water molecule for the silica surfaces. As a consequence, water depletion and rearrangement upon decreasing confinement is a strong function of the hydrophilic surface specificity, particularly at smaller separations. An oscillatory solvation force response is only observed for water confined between silica surfaces, and bulklike features are observed for both surfaces above a surface separation of about 1.2 nm. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions, and solvation forces as a function of the surface separation.

Adsorption Isotherms of Water on Mica: Redistribution and Film Growth

Adsorption isotherms of water on muscovite mica are obtained using grand canonical Monte Carlo simulations over a wide range of relative vapor pressures, p/p(0) at 298 K. Three distinct stages are observed in the adsorption isotherm. A sharp rise in the water coverage occurs for 0 < p/p(0) < 0.1. This is followed by a relatively slow increase in the coverage for 0.1 < or = p/p(0) < or = 0.7. Above p/p(0) = 0.7, a second increase in the coverage occurs due to the adsorption of water with bulklike features. The derived film thickness and isotherm shape for the simple point charge (SPC) water model is in excellent agreement with recent experiments of Balmer et al. [ Langmuir 2008 , 24 , 1566 ]. A novel observation is the significant redistribution of water between adsorbed layers as the water film develops. This redistribution is most pronounced for 0.1 < or = p/p(0) < or = 0.7, where water is depleted from the inner layers and film growth is initiated on the outer layer. During this stage, potassium hydration is found to play a dominant role in the rearrangement of water near the mica surface. The analysis of structural features reveals a strongly bound first layer of water molecules occupying the ditrigonal cavities between the potassium ions. In-plane structure of oxygen in the second layer, which forms part of the first hydration shell of potassium, reveals a liquidlike structure with the oxygen-oxygen pair correlation function displaying features similar to bulk water. Isosteric heats of adsorption were found to be in good agreement with the differential microcalorimetric data of Rakhmatkariev ( Clays Clay Miner. 2006 , 54 , 402 ), over the entire range of pressures investigated. Both SPC and extended simple point charge (SPC/E) water models were found to yield qualitatively similar adsorption and structural characteristics, with the SPC/E model predicting lower coverages than the SPC model for p/p(0) > 0.7.

Relaxation and jump dynamics of water at the mica interface.

The orientational relaxation dynamics of water confined between mica surfaces is investigated using molecular dynamics simulations. The study illustrates the wide heterogeneity that exists in the dynamics of water adjacent to a strongly hydrophilic surface such as mica. Analysis of the survival probabilities in different layers is carried out by normalizing the corresponding relaxation times with bulk water layers of similar thickness. A 10-fold increase in the survival times is observed for water directly in contact with the mica surface and a non-monotonic variation in the survival times is observed moving away from the mica surface to the bulk-like interior. The orientational relaxation time is highest for water in the contact layer, decreasing monotonically away from the surface. In all cases the ratio of the relaxation times of the 1st and 2nd rank Legendre polynomials of the HH bond vector is found to lie between 1.5 and 1.9 indicating that the reorientational relaxation in the different water layers is governed by jump dynamics. The orientational dynamics of water in the contact layer is particularly novel and is found to undergo distinct two-dimensional hydrogen bond jump reorientational dynamics with an average waiting time of 4.97 ps. The waiting time distribution is found to possess a long tail extending beyond 15 ps. Unlike previously observed jump dynamics in bulk water and other surfaces, jump events in the mica contact layer occur between hydrogen bonds formed by the water molecule and acceptor oxygens on the mica surface. Despite slowing down of the water orientational relaxation near the surface, life-times of water in the hydration shell of the K(+) ion are comparable to that observed in bulk salt solutions.

Hydration of ions under confinement

Molecular dynamics simulations are used to examine the changes in water density and hydration characteristics of NaCl solutions confined in slit-shaped graphitic pores. Using a structural signature, we define the hydration limit as the salt concentration at which a sharp drop in the hydration number is observed. At small pores (H = 8.0–10 Å), confined water does not possess bulk-like features and remains in a layered arrangement between two surfaces. Despite this high degree of confinement, ions are able to form a quasi-2D hydration shell between two surfaces. Our results indicate the strong propensity of ions to form the first hydration shell, even under extremely confined aqueous environments. The hydration of ions is seen to weakly perturb the oxygen density distributions between two surfaces. The hydration number of Na+ reduces to about 4.15 at a pore width of H = 0.8 nm, when compared with the bulk hydration number of 6.25. At larger pore widths, above H = 16 Å, where bulk-like water densities are observed in the central regions of the pore, the hydration number is above 6.

Confined fluids in a Janus pore: influence of surface asymmetry on structure and solvation forces

Density distribution, fluid structure and solvation forces for fluids confined in Janus slit-shaped pores are investigated using grand canonical Monte Carlo simulations. By varying the degree of asymmetry between the two smooth surfaces that make up the slit pores, a wide variety of adsorption situations are observed. The presence of one moderately attractive surface in the asymmetric pore is sufficient to disrupt the formation of frozen phases observed in the symmetric case. In the extreme case of asymmetry in which one wall is repulsive, the pore fluid can consist of a frozen contact layer at the attractive surface for smaller surface separations (H) or a frozen contact layer with liquid-like and gas-like regions as the pore width is increased. The superposition approximation, wherein the solvation pressure and number density in the asymmetric pores can be obtained from the results on symmetric pores, is found to be accurate for , where is the Lennard-Jones fluid diameter and within 10% accuracy for smaller surface separations. Our study has implications in controlling stick slip and overcoming static friction ‘stiction’ in micro and nanofluidic devices.

CO2 adsorption and separation in covalent organic frameworks with interlayer slipping

The energetic stability of layered covalent organic frameworks (COFs) in slipped structures and the experimental control of interlayer slipping (Q. Fang, Z. Zhuang, S. Gu, R. B. Kaspar, J. Zheng, J. Wang, S. Qiu and Y. Yan, Nat. Commun., 2014, 5, 4503) suggest that the interlayer slipping could be used as a design parameter to enhance the gas adsorption and separation properties of COFs. In this work, we have systematically studied the effect of interlayer slipping on CO2 adsorption and CO2/N2 separation in microporous TpPa1 and mesoporous TpBD and PI-COFs using quantum mechanical and grand canonical Monte Carlo simulations. We found that the slipping affects the number of preferred CO2 adsorption regions and corresponding adsorption energies, resulting in a drastic variation in the adsorption uptake. Our detailed analysis of the heat of adsorption, density distribution and energy landscape reveals that the effect of slipping on CO2 uptake is non-monotonous. We explain this behavior using a simplified model that also provides an optimal range of slipping distances to increase the gas storage performance of COFs. Our results show that the optimized slipped COF structures have approximately three times higher CO2 working capacity and CO2/N2 selectivity as compared to eclipsed structures. The highest CO2 working capacity of 5.8 mol kg−1 and CO2 : N2 separation selectivity of 197 (at 1 bar and 298 K) were observed for slipped PI-COF-2 and TpBD COFs, respectively, which are higher than those for any other COFs reported to date. The molecular insight presented here is qualitatively applicable to other similar slipped COFs and is useful for the development of COFs for enhanced gas storage and separation applications.