Ab Initio H2O in Realistic Hydrophilic Confinement

Perturbative vibration of the coupled hydrogen-bond (O:H-O) in water

Perturbation Raman spectroscopy has underscored the hydrogen bond (O:H-O or HB) cooperativity and polarizability (HBCP) for water, which offers a proper parameter space for the performance of the HB and electrons in the energy-space-time domains. The OO repulsive coupling drives the O:H-O segmental length and energy to relax cooperatively upon perturbation. Mechanical compression shortens and stiffens the O:H nonbond while lengthens and softens the HO bond associated with polarization. However, electrification by an electric field or charge injection, or molecular undercoordination at a surface, relaxes the O:H-O in a contrasting way to the compression with derivation of the supersolid phase that is viscoelastic, less dense, thermally diffusive, and mechanically and thermally more stable. The HO bond exhibits negative thermal expansivity in the liquid and the ice-I phase while its length responds in proportional to temperature in the quasisolid phase. The O:H-O relaxation modifies the mass densities, phase boundaries, critical temperatures and the polarization endows the slipperiness of ice and superfluidity of water at the nanometer scale. Protons injection by acid solvation creates the H↔H anti-HB and introduction of electron lone pairs derives the O:⇔:O super-HB into the solutions of base or H₂O₂ hydrogen-peroxide. The repulsive H↔H and O:⇔:O interactions lengthen the solvent HO bond while the solute HO bond contracts because its bond order loss. Differential phonon spectroscopy quantifies the abundance, structure order, and stiffness of the bonds transiting from the mode of pristine water to the perturbed states. The HBCP and the perturbative spectroscopy have enabled the dynamic potentials for the relaxing O:H-O bond. Findings not only amplified the power of the Raman spectroscopy but also substantiated the understanding of anomalies of water subjecting to perturbation.

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies

Molecular dynamics (MD) simulations are a powerful tool for detailed studies of altered properties of liquids in confinement, in particular, of changed structures and dynamics. They allow, on one hand, for perfect control and systematic variation of the geometries and interactions inherent in confinement situations and, on the other hand, for type-selective and position-resolved analyses of a huge variety of structural and dynamical parameters. Here, we review MD simulation studies on various types of liquids and confinements. The main focus is confined aqueous systems, but also ionic liquids and polymer and silica melts are discussed. Results for confinements featuring different interactions, sizes, shapes, and rigidity will be presented. Special attention will be given to situations in which the confined liquid and the confining matrix consist of the same type of particles and, hence, disparate liquid-matrix interactions are absent. Findings for the magnitude and the range of wall effects on molecular positions and orientations and on molecular dynamics, including vibrational motion and structural relaxation, are reviewed. Moreover, their dependence on the parameters of the confinement and their relevance to theoretical approaches to the glass transition are addressed.

Static field gradient NMR studies of water diffusion in mesoporous silica

NMR diffusometry is used to ascertain the pore-size dependent water diffusion in MCM-41 and SBA-15 silica over broad temperature ranges. Detailed analysis of 1H and 2H NMR stimulated-echo decays reveals that fast water motion through voids between different silica particles impairs such studies in the general case. However, water diffusion inside single pores is probed in the present approach, which applies high static field gradients to enhance the spatial resolution of the experiment and uses excess water in combination with subzero temperatures to embed the silica particles in an ice matrix and, thus, to suppress interparticle water motion. It is found that the diffusion of confined water slows down by almost two orders of magnitude when the pore diameter is reduced from 5.4 nm to 2.1 nm at weak cooling. In the narrower silica pores, the temperature dependence of the self-diffusion coefficient of water is well described by an Arrhenius law with an activation energy of Ea = 0.40 eV. The Arrhenius behavior extends over a broad temperature range of at least 207-270 K, providing evidence against a fragile-to-strong crossover in response to a proposed liquid-liquid phase transition near 225 K. In the wider silica pores, partial crystallization results in a discontinuous temperature dependence. Explicitly, the diffusion coefficients drop when cooling through the pore-size dependent melting temperatures Tm of confined water. This finding can be rationalized by the fact that water can explore the whole pore volumes above Tm, but is restricted to narrow interfacial layers sandwiched between silica walls and ice crystallites below this temperature. Comparing our findings for water diffusion with previous results for water reorientation, we find significantly different temperature dependencies, indicating that the Stokes-Einstein-Debye relation is not obeyed.

Confining a Protein-Containing Water Nanodroplet inside Silica Nanochannels

Incorporation of biological systems in water nanodroplets has recently emerged as a new frontier to investigate structural changes of biomolecules, with perspective applications in ultra-fast drug delivery. We report on the molecular dynamics of the digestive protein Pepsin subjected to a double confinement. The double confinement stemmed from embedding the protein inside a water nanodroplet, which in turn was caged in a nanochannel mimicking the mesoporous silica SBA-15. The nano-bio-droplet, whose size fits with the pore diameter, behaved differently depending on the protonation state of the pore surface silanols. Neutral channel sections allowed for the droplet to flow, while deprotonated sections acted as anchoring piers for the droplet. Inside the droplet, the protein, not directly bonded to the surface, showed a behavior similar to that reported for bulk water solutions, indicating that double confinement should not alter its catalytic activity. Our results suggest that nanobiodroplets, recently fabricated in volatile environments, can be encapsulated and stored in mesoporous silicas.

DMA study of water's glass transition in nanoscale confinement

Dynamic mechanical analysis (DMA) measurements of water confined in nanoporous silica have been performed as a function of temperature and frequency for different pore sizes (2.5-10 nm) at heating and cooling. Most of the data show three processes, P1, P2 and P3, where P1 and P2 depend on measurement frequency and P3 does not. The characteristic shift of P3 with pore size shows that this process corresponds to freezing/melting of "internal water", i.e. in the core of the pores. Thermal expansion data indicate - in agreement with e.g. [A. Taschin, P. Bartolini and R. Torre, Meas. Sci. Technol., 2017, 28, 014009] - that in all our nanoporous systems about 2 layers of water remain liquid much below the freezing point. Dynamic elastic measurements show clear signatures of glass freezing of this supercooled water in the vicinity of P1. Extrapolating the DMA data to the timescale (103 s) of adiabatic calorimetry unveils a systematic behaviour: P1(T) shows a clear size dependence for a broad range of pore diameters, i.e. 2.5 nm ≤ d ≤ 52 nm, implying (together with the corresponding activation energy 0.5 eV) that P1 corresponds to the glass-liquid transition of a few layers of supercooled water at Tg(d). An extrapolation of Tg(d) to d → ∞ yields Tg(∞) ≈ 136 K, the traditional value for bulk water. The small (liquid like) value of Young's modulus in a temperature region above P1 is most naturally explained assuming that the supercooled water in this range is still liquid, implying that Tg values of 160 K or even 210 K - as suggested by various authors - are unlikely.

Molecular Dynamics Simulations of Water, Silica, and Aqueous Mixtures in Bulk and Confinement

Aqueous systems are omnipresent in nature and technology. They show complex behaviors, which often originate in the existence of hydrogen-bond networks. Prominent examples are the anomalies of water and the non-ideal behaviors of aqueous solutions. The phenomenology becomes even richer when aqueous liquids are subject to confinement. To this day, many properties of water and its mixtures, in particular, under confinement, are not understood. In recent years, molecular dynamics simulations developed into a powerful tool to improve our knowledge in this field. Here, our simulation results for water and aqueous mixtures in the bulk and in various confinements are reviewed and some new simulation data are added to improve our knowledge about the role of interfaces. Moreover, findings for water are compared with results for silica, exploiting that both systems form tetrahedral networks.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Properties of Hydrogen-Bonded Liquids at Interfaces

Effects of interfaces on hydrogen-bonded liquids play major roles in nature and technology. Despite their importance, a fundamental understanding of these effects is still lacking. In large parts, this shortcoming is due to the high complexity of these systems, leading to an interference of various interactions and effects. Therefore, it is advisable to take gradual approaches, which start from well designed and defined model systems and systematically increase the level of intricacy towards more complex mimetics. Moreover, it is necessary to combine insights from a multitude of methods, in particular, to link novel preparation strategies and comprehensive experimental characterization with inventive computational and theoretical modeling. Such concerted approach was taken by a group of preparative, experimentally, and theoretically working scientists in the framework of Research Unit FOR 1583 funded by the Deutsche Forschungsgemeinschaft (German Research Foundation). This special issue summarizes the outcome of this collaborative research. In this introductory article, we give an overview of the covered topics and the main results of the whole consortium. The following contributions are review articles or original works of individual research projects.

Liquid Water Confined in Cellulose with Variable Interfacial Hydrophilicity

We investigate liquid water confined within nanoscale cellulose slabs by means of molecular dynamics simulations. Depending on the construction of the cellulose–water interface, two different surface structures with distinct levels of hydrophilicity are exposed to the water. The different philicities are reflected in the response of the water phase to this geometric confinement, both in terms of the density profile and in the strength of the aqueous hydrogen bonding network. At the smooth surface cut along the (010) axis of the cellulose crystal, water shows typical properties of a hydrophilic confinement: the density shows fluctuations that disappear further away from the wall, the water molecules orient themselves and the coordination numbers increases at the interface. As a consequence, the water becomes “harder” at the interface, with a considerably increased local ordering. At the zigzag-shaped surface along the (111) axis, the degree of hydrophilicity is reduced, and only small effects can be seen: the density shows weak fluctuations, and the orientation of the water molecules is closer to that of bulk water than to the smooth surface. The local coordination numbers remains constant over the whole confinement. Our work shows that the nature of the exposed cellulose interface has a strong influence on how the structure of adjacent water is modified. The different ways of surface construction yield distinct degrees of hydrophilicity and spatial accessibility regarding the hydrogen bond network, resulting in a notably different interfacial water structure.

Ab-Initio Molecular Dynamics Simulations and Calculations of Spectroscopic Parameters in Hydrogen-Bonding Liquids in Confinement (Project 8)

We investigate the effect of several nanoscale confinements on structural and dynamical properties of liquid water and binary aqueous mixtures. By means of molecular dynamics simulations based on density functional theory and atomistic force fields. Our main focus is on the dependence on the structure and the hydrogen-bonding-network of the liquids near the confinement interface at atomistic resolution. As a complementary aspect, spatially resolved profiles of the proton NMR chemical shift values are used to quantify the local strength of the hydrogen-bond-network.

Confined Water as Model of Supercooled Water

Water in confined geometries has obvious relevance in biology, geology, and other areas where the material properties are strongly dependent on the amount and behavior of water in these types of materials. Another reason to restrict the size of water domains by different types of geometrical confinements has been the possibility to study the structural and dynamical behavior of water in the deeply supercooled regime (e.g., 150-230 K at ambient pressure), where bulk water immediately crystallizes to ice. In this paper we give a short review of studies with this particular goal. However, from these studies it is also clear that the interpretations of the experimental data are far from evident. Therefore, we present three main interpretations to explain the experimental data, and we discuss their advantages and disadvantages. Unfortunately, none of the proposed scenarios is able to predict all the observations for supercooled and glassy bulk water, indicating that either the structural and dynamical alterations of confined water are too severe to make predictions for bulk water or the differences in how the studied water has been prepared (applied cooling rate, resulting density of the water, etc.) are too large for direct and quantitative comparisons.