Simulations of water nano-confined between corrugated planes.

Water confined to nanoscale widths in two dimensions between ideal planar walls has been the subject of ample study, aiming at understanding the intrinsic response of water to confinement, avoiding the consideration of the chemistry of actual confining materials. In this work, we study the response of such nanoconfined water to the imposition of a periodicity in the confinement by means of computer simulations, both using empirical potentials and from first-principles. For that we propose a periodic confining potential emulating the atomistic oscillation of the confining walls, which allows varying the lattice parameter and amplitude of the oscillation. We do it for a triangular lattice, with several values of the lattice parameter: one which is ideal for commensuration with layers of Ih ice and other values that would correspond to more realistic substrates. For the former, the phase diagram shows an overall rise of the melting temperature. The liquid maintains a bi-layer triangular structure, however, despite the fact that it is not favoured by the external periodicity. The first-principles liquid is significantly affected by the modulation in its layering and stacking even at relatively small amplitudes of the confinement modulation. Beyond some critical modulation amplitude, the hexatic phase present in flat confinement is replaced by a trilayer crystalline phase unlike any of the phases encountered for flat confinement. For more realistic lattice parameters, the liquid does not display higher tendency to freeze, but it clearly shows inhomogeneous behaviour as the strength of the rugosity increases. In spite of this expected inhomogeneity, the structural and dynamical response of the liquid is surprisingly insensitive to the external modulation. Although the first-principles calculations give a more triangular liquid than the one observed with empirical potentials (TIP4P/2005), both agree remarkably well for the main conclusions of the study.

Enhanced Configurational Entropy in High-Density Nanoconfined Bilayer Ice.

A novel kind of crystal order in high-density nanoconfined bilayer ice is proposed from molecular dynamics and density-functional theory simulations. A first-order transition is observed between a low-temperature proton-ordered solid and a high-temperature proton-disordered solid. The latter is shown to possess crystalline order for the oxygen positions, arranged on a close-packed triangular lattice with AA stacking. Uniquely among the ice phases, the triangular bilayer is characterized by two levels of disorder (for the bonding network and for the protons) which results in a configurational entropy twice that of bulk ice.

Computational approach to (ZnS)_{i} nanoclusters in ionic liquids.

Unique and attractive properties have been predicted for II-VI-type semiconductor nanoclusters within the field of nanotechnology. However, the low reaction kinetics within the usual solvents gives only thermodynamic control during their production process, making the obtention of different metastable polymorphs extremely difficult. The use of ionic liquids as solvents has been proposed to overcome this problem. Identifying how these nanoclusters are solvated within ionic liquids is fundamental if this strategy is to be pursued. While computational chemistry tools are best suited for this task, the complexity and size of the system requires a careful design of the simulation protocol, which is put forward in this work. Taking as reference the (ZnS)_{12} nanocluster and the [EMIM][EtSO_{4}] ionic liquid, we characterize the interactions between the nanoparticle and first solvation shell by density functional theory calculations, considering most of the solvent implicitly. The DFT results are consistent through different theory levels showing a strong interaction between the Zn atoms of the nanocluster and the [EtSO_{4}^{-}] anion of the ionic liquid. A more realistic representation of the system is obtained by classical MD calculations, for which various classical force fields were considered and several atomic interactions parameterized. This new set of parameters correctly describes the interaction of different (ZnS) nanoclusters, supporting its transferability. The resulting MD simulation shows the formation of a structured ionic liquid solvation shell around the nanocluster with no exchange of ions for at least 5 ns, in agreement with the strong interactions observed in the density functional theory calculations.

Continuous melting through a hexatic phase in confined bilayer water.

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquidlike throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems.