Understanding the structure and dynamics of molecularly thin films or the state of water confined to nanoscale dimensions is an active field of research and has wide applications in areas ranging from biology to geology. The issues concern fundamental aspects related to the manner in which a substrate influences the organization of water, origin of forces present when water is confined to nanoscale dimensions, and the influence on the structure and dynamics of water adjacent to a surface. The focus of this thesis lies in examining the thermodynamics and transport properties of interfacial and confined water. As a prelude to studying the structure of water confined between two mica surfaces, we first investigated the structuring of water adjacent to a single mica surface using grand canonical Monte Carlo (GCMC) simulations. The adsorption isotherm reveals three distinct stages as the relative vapor pressure in increased. The derived film thickness, isotherm shape, and heats of adsorption are in excellent agreement with recent experimental data. Our study does not support the 2D ice hypothesis and indicates that beyond the first adsorbed layer water is liquid-like. The characteristics of water confined to nanometer dimensions between two hydrophilic surfaces are investigated to assess the influence of chemical functionality of the hydrophilic surface on the structure of confined water. 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 is able to retain its hydrogen bond network displaying bulk-like structural features down to surface separations as small as 0.45 nm. An oscillatory solvation force response is observed only for water confined between silica surfaces. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions and the solvation forces as a function of the surface separation. Recent experimental studies have shown that even for subnanometer confinement, the shear viscosity of water between mica surfaces is only three times larger than the free water viscosity. The dynamics of confined water between mica surfaces is evaluated using molecular dynamics simulations. Our analysis shows that the residence time for water in the contact layer is about two orders of magnitude larger than water in the central bulk-like regions between the surfaces. The K+ ions have a strong influence on the dynamics of confined water, leading to a decoupling in the translation and orientational motions. Our analysis also shows the presence of orientational jump dynamics in the contact layer near the mica surface. We also investigate the influence of confinement on the hydration characteristics of NaCl solutions both as a function of the salt concentration and the surface separation, H between graphite surfaces. A hydration limit is defined as the concentration at which a rapid drop in the hydration number is observed with increasing salt concentration. Despite a high degree of confinement, ions are able to form a quasi two-dimensional hydration shell between the two surfaces. The hydration number, reduces to about 4.15 at a pore width of H =8 A, when compared with the bulk hydration number of 6.25. In many practical situations, surfaces that are separated by an intervening fluid can be dissimilar giving rise to the so called Janus interface. In order to probe the fluid structure in such systems, we studied non-polar fluids confined between two asymmetric surfaces. By varying the degree of asymmetry between the two surfaces a wide variety of adsorption situations are examined using GCMC simulations and a mean field lattice model. The degree of asymmetry is found to influence the presence of frozen phases and can also support co-existing liquid and solid phases.

This thesis deals with several aspects of translational and reorientational dynamics of water molecules confined inside narrow carbon nanotubes. Water molecules confined in a non-polar, nanoscopic pore exhibit extremely unusual structural and dynamical properties. Adding to the list of anomalies which are already present in bulk liquid water, the confined water “chains” and “shells” springs many more surprises. The relatively weak interaction with the surrounding walls in conjuction with the strong inter-water hydrogen bonds lead o several novel structural and dynamical features, very special to this “strange” phase of water. In this thesis, we present our findings on the detailed molecular level description of translational and reorientational dynamics of this novel phase of anomalously “soft” water. Chapter 1 introduces the varied theoretical, numerical and experimental attempts to demystify the properties of bulk, interfacial and confined water. It also motivates the aspects of diffusion in low dimensional systems, which are often termed “anomalous”. In Chapter 2, we study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solid-like ordering at room temperature, which we quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not sub-diffusive. We also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. We find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal-mode diffusion inside the nanotube. We attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length. In Chapter 3, we study the diffusion of water molecules confined inside narrow (6,6) carbon nanorings. The water molecules form two oppositely polarised clusters. It is shown that the effective interaction between these two clusters is repulsive in nature. The computed mean-squared displacement (MSD) clearly shows a scaling with time, which is consistent with single file diffusion (SFD). The time up to which the water molecules undergo SFD is shown to be the lifetime of the water molecules inside these clusters. The inter-cluster repulsive interactions are electrostatic and hence long-ranged, which is in complete contrast with shorter ranged steric repulsion in other systems which exhibit SFD. In Chapter 4, we study the anisotropic orientational dynamics of water molecules confined in narrow carbon nanotubes and nanorings. We find that confinement leads to strong anisotropy in the orientational relaxation. The relaxation of the aligned dipole moments, occurring on a timescale of nanoseconds, is three order of magnitude slower than that of bulk water. In contrast, the relaxation of the vector joining the two hydrogens is ten times faster compared to bulk, with a timescale of about 150 femtoseconds. The slow dipolar relaxation is mediated by the hopping of orientational defects, which are nucleated by the water molecules outside the tube, across the linear water chain. In Chapter 5, we study the reorientational dynamics of water molecules confined inside narrow carbon nanotubes immersed in a bath of water. Our simulations show that the confined water molecules exhibit bistability in their reorientational relaxation, which proceeds by angular jumps between the two stable states. The energy barrier between these two states is about 2kBT. The effect of non-Markovian jumps shows up in the ratio of the timescales o the first and second order reorientational correlation functions, which exceeds the value of the ratio in the diffusive limit. The analytical solution of a proposed model is also presented, which qualitatively explains this “unusual” relaxation. These results will have important implications in understanding proton conduction in water-filled ion channels. In Chapter 6, we report the thermodynamic aspects of the translational and re-orientational dynamics of the strongly confined water molecules. Considering the energetics it is surprising that the water molecules spontaneously fill up the nanotube. Thus the thermodynamics of entry of water molecules in the hydrophobic cavity of nanotube. This is generally attributed to the rotational entropy gain by the water molecules on entering the tube, a fact which has not been demonstrated quantitatively so far. We show that the gain in rotational component of the entropy compensates the loss of energy of the water molecules upon entering the nanotube. In Chapter 7, we conclude by summarising the work done in the previous chapters and discuss the future course of actions. We would like to extend the studies on the diffusion of water inside finite nanotubes in the presence of bathwater outside, to nanotube lengths, where it is possible to observe the cross-over from an initial “single file” to and eventual, centre of mass dominated, “normal” diffusion. The mean field estimate of the length of the nanotube required so that one observes a crossover from the initial “single file” to “normal” diffusion at 100 ps is about 700 ˚A. Simulation of such a system would possibly provide an unambiguous answer to the question, whether it is possible to observe SFD in finite carbon nanotubes, filled with water. Regarding the reorientational dynamics, we would like to extend our understanding of the reorientational relaxation of water chains to more more complicated structures. Depending on the diameter of the confining nanotube water molecules form polygons of ice. In the present situation each water molecule can be in only two possible states of orientation. Hence, it would be interesting to predict the reorientational dynamics for other ice structures, where each water molecule can be more “orientational states”. In Chapter 8, we report a work which is unrelated to the rest of this thesis. The work has been done in collaboration with Prof. T. V. Ramakrishnan and Prof. Vijay B. Shenoy. We report a novel method for the calculation of elastic constants of a solid in the frame work of Ramakrishnan-Youssouf density functional theory. The structural aspect of the liquid to solid transition and how it affects the elastic constants of the solids is brought out very clearly. The calculation is analytical and we obtain explicit expressions for the elastic constants. The description of the solid is in terms of the structure factor, S(G), of the coexisting liquid. The elastic constants are expressed as a function of equilibrium parameters, such as c(0), relatedto the compressibility of the liquid. Another important quantity on which the elastic constants depend is the curvature, c"(|G|), of c(|q|)curve at its peak (q= G). These quantities are known experimentally for many systems, and can also be calculated accurately. The shear modulus depends only on c"(|G|), while the bulk modulus has contributions from both c"(|G|) and c(0). The obtained elastic constants do not satisfy the Cauchy relations, in that C12 is not equal to C44. Calculations have been performed for two-dimensional square and triangular lattices as well as bcc and fcc lattices in three dimensions. It is seen that in order to get good agreement between the theoretical and the experimental results of the elastic constants, three body correlations have to be introduced in the calculations for the bcc and the fcc lattices. For the last, in which two shells of reciprocal lattice vectors are appropriate, we point out the modifications needed for choosing the lattice parameter in the unstrained freezing problem. We obtain a new, first principles, quasiuniversal relation for elastic constants, scaled by the melting temperature, that is experimentally satisfied. It is similar to the famous Verlet criterion that S(|G|) = 2.9 at freezing and is free of some of the unphysical aspects of previous work.

<p> Inclement challenges of a changing climate and humanity's desire to explore extraterrestrial environments both necessitate efficient methods to obtain freshwater. To accommodate next generation water technology, there is a need for understanding and defining the energy efficiency for unconventional water sources over a broad range of environments. Exergy analysis provides a common description for efficiency that may be used to evaluate technologies and water sources for energy feasibility. This work uses robust thermodynamic theory coupled with atmospheric and planetary data to define water capture efficiency, explore its variation across climate conditions, and identify technological niches and development needs.  </p> <p><br></p> <p> We find that desalinating saline liquid brines, even when highly saline, could be the most energetically favorable option for obtaining water outside of Earth. The energy required to access water vapor may be four to ten times higher than accessing ice deposits, however it offers the capacity for decentralized systems. Considering atmospheric water vapor harvesting on Earth, we find that the thermodynamic minimum is anywhere from 0x (RH≥ 100%) to upwards of 250x (RH<10\%) the minimum energy requirement of seawater desalination. Sorbents, modelled as metal organic frameworks (MOFs), have a particular niche in arid and semi-arid regions (20-30%). Membrane-systems are best at low relative humidity and the region of applicability is strongly affected by the vacuum pumping efficiency. Dew harvesting is best at higher humidity and fog harvesting is optimal when super-saturated conditions exist. Component (e.g., pump, chiller, etc.) inefficiencies are the largest barrier in increasing process-level efficiency and strongly impact the regions optimal technology deployment. The analysis elucidates a fundamental basis for comparing water systems energy efficiency for outer space applications and provides the first thermodynamics-based comparison of classes of atmospheric water harvesting technologies on Earth.</p>