Electrofreezing of confined water

We report results from molecular dynamics simulations of the freezing transition of TIP5P water molecules confined between two parallel plates under the influence of a homogeneous external electric field, with magnitude of 5 V/nm, along the lateral direction. For water confined to a thickness of a trilayer we find two different phases of ice at a temperature of T=280 K. The transformation between the two, proton-ordered, ice phases is found to be a strong first-order transition. The low-density ice phase is built from hexagonal rings parallel to the confining walls and corresponds to the structure of cubic ice. The high-density ice phase has an in-plane rhombic symmetry of the oxygen atoms and larger distortion of hydrogen bond angles. The short-range order of the two ice phases is the same as the local structure of the two bilayer phases of liquid water found recently in the absence of an electric field [J. Chem. Phys. 119, 1694 (2003)]. These high- and low-density phases of water differ in local ordering at the level of the second shell of nearest neighbors. The results reported in this paper, show a close similarity between the local structure of the liquid phase and the short-range order of the corresponding solid phase. This similarity might be enhanced in water due to the deep attractive well characterizing hydrogen bond interactions. We also investigate the low-density ice phase confined to a thickness of 4, 5, and 8 molecular layers under the influence of an electric field at T=300 K. In general, we find that the degree of ordering decreases as the distance between the two confining walls increases.     

Anomalous freezing and melting behaviour of capillary confined CO2

In this paper we present our recent positron annihilation study of the liquid»solid phase boundary for CO₂ confined in nanometer pores of VYCOR glass. We find that CO₂ remains liquid in the pores far below the bulk freezing temperature and there is pronounced hysteresis between freezing and melting compared to that seen at the gas-liquid boundary in the pores. On freezing we see evidence of open space created in the pores. This leads to complex melting behaviour possibly involving the formation of gas-liquid interfaces. We see that frezing in the pores is totally irreversible, so that any solid which forms (no matter how small) remains stable up to the higher melting temperature. In contrast melting is more reversible (possibly indicating nucleation centres which permit immediate re-freezing). Finally, the pre-frozen state in the pores is different to the post-melted state.     

Molecular modeling of freezing of simple fluids confined within carbon nanotubes

We report Monte Carlo simulation results for freezing of Lennard-Jones carbon tetrachloride confined within model multiwalled carbon nanotubes of different diameters. The structure and thermodynamic stability of the confined phases, as well as the transition temperatures, were determined from parallel tempering grand canonical Monte Carlo simulations and free-energy calculations. The simulations show that the adsorbate forms concentric molecular layers that solidify into defective quasi-two-dimensional hexagonal crystals. Freezing in such concentric layers occurs via intermediate phases that show remnants of hexatic behavior, similar to the freezing mechanism observed for slit pores in previous works. The adsorbate molecules in the inner regions of the pore also exhibit changes in their properties upon reduction of temperature. The structural changes in the different regions of adsorbate occur at temperatures above or below the bulk freezing point, depending on pore diameter and distance of the adsorbate molecules from the pore wall. The simulations show evidence of a rich phase behavior in confinement; a number of phases, some of them inhomogeneous, were observed for the pore sizes considered. The multiple transition temperatures obtained from the simulations were found to be in good agreement with recent dielectric relaxation spectroscopy experiments for CCl(4) confined within multiwalled carbon nanotubes.     

The freezing of supercooled water

The onset of crystallization of supercooled water upon lowering the temperature is highly unpredictable, depending strongly on the specific sample and its treatment. The mechanism causing this is to be investigated and may be found in terms of the dependence of the transition temperature on the shear acting in the convecting liquid. This effect of shear on the fluctuation spectrum is considered qualitatively.     

Magnetic nanorods confined in a lamellar lyotropic phase

The dilute lamellar phase of the nonionic surfactant C 12EO 5 was doped with goethite (iron oxide) nanorods up to a fraction of 5 vol %. The interaction between the inclusions and the host phase was studied by polarized optical microscopy (with or without an applied magnetic field) and by small-angle X-ray scattering. We find that, when the orientation of the nanorods is modified using the magnetic field, the texture of the lamellar phase changes accordingly; one can thus induce a homeotropic-planar reorientation transition. On the other hand, the lamellar phase induces an attractive interaction between the nanorods. In more concentrated lamellar phases (under stronger confinement) the particles form aggregates. This behavior is not encountered for a similar system doped with spherical particles, emphasizing the role of particle shape in the interaction between doping particles and the host phase.     

Thermodynamics of water confined in porous calcium-silicate-hydrates

Water within pores of cementitious materials plays a crucial role in the damage processes of cement pastes, particularly in the binding material comprising calcium-silicate-hydrates (C-S-H). Here, we employed Grand Canonical Monte Carlo simulations to investigate the properties of water confined at ambient temperature within and between C-S-H nanoparticles or "grains" as a function of the relative humidity (%RH). We address the effect of water on the cohesion of cement pastes by computing fluid internal pressures within and between grains as a function of %RH and intergranular separation distance, from 1 to 10 ?. We found that, within a C-S-H grain and between C-S-H grains, pores are completely filled with water for %RH larger than 20%. While the cohesion of the cement paste is mainly driven by the calcium ions in the C-S-H, water facilitates a disjoining behavior inside a C-S-H grain. Between C-S-H grains, confined water diminishes or enhances the cohesion of the material depending on the intergranular distance. At very low %RH, the loss of water increases the cohesion within a C-S-H grain and reduces the cohesion between C-S-H grains. These findings provide insights into the behavior of C-S-H in dry or high-temperature environments, with a loss of cohesion between C-S-H grains due to the loss of water content. Such quantification provides the necessary baseline to understand cement paste damaging upon extreme thermal, mechanical, and salt-rich environments.     

Dynamics of water confined within reverse micelles

We report structural and dynamical properties of water confined within reverse micelles (RMs) ranging in size from R = 10 A to R = 23 A as determined from molecular dynamics simulations. The low-frequency infrared spectra have been calculated using linear response theory and depend linearly on the fraction of bulklike water within the RMs. Furthermore, these spectra show nearly isosbestic behavior in the region near 660 cm(-1). Both of these characteristics are present in previously measured experimental spectra. The single dipole spectra for interfacial trapped, bound, and bulklike water within the RMs have also been calculated and show region-dependent frequency shifts. Specifically, the bound and trapped water spectra have a peak at lower frequencies than that for the inner core water. We therefore assign the low-frequency band in the IR spectra to bound and trapped interfacial water. Finally, region-dependent hydrogen bonding profiles and spatial distribution functions are also presented.     

Oriented confined water induced by cationic lipids

We report on attenuated total reflection Fourier-transform infrared (ATR FTIR) spectroscopic measurements on oriented lipid multilayers of N,N-dimethyl-N,N-dioctadecyl-ammonium halides (DODAX, X = F, Cl, Br, I). The main goal of this study is the investigation of the structure and spectroscopic properties of water absorbed to these model membranes. Intensities of the water stretch absorptions were used to determine the amount of bound water. At high water activity, DODAF membranes bind ~11 water molecules/lipid while DODAC and DODAB adsorb 1-2 water/lipid and DODAI was hydrophobic. By adjustment of DODAF hydration to ~2 water molecules, stretching absorptions from water of the first hydration shell were accessible for the fluoride, chloride, and bromide analogs. The polarized measurements demonstrate highly confined and oriented water with infrared (IR) order parameters ranging from 0.2 to -0.4. Resolved IR water band components are attributed to different hydrogen-bonded populations. Complementary molecular dynamics simulations of DODAB strongly support the existence of differently hydrogen-bonded and oriented water within DODAB multilayers. A combination of both techniques was used for an assignment of water stretch band components to structures. The described cationic lipid systems are a prototype for a bottom-up approach to understand the IR spectroscopy of structured water at biological interfaces since they permit a defined increase of hydrophilic water-anionic interactions leading to extended water networks at membranes.     

Temperature-dependent dynamics of water confined in nafion membranes

We performed a neutron scattering study to investigate the dynamical behavior of water absorbed in Nafion at low hydration level as a function of temperature in the range 200-300 K. To single out the spectral contribution of the confined water, the measurements were done on samples hydrated with both H(2)O and D(2)O. Due to the strong incoherent scattering cross section of hydrogen atoms with respect to deuterium, in the difference spectra, the contribution from the Nafion membrane is subtracted out and the signal originates essentially from protons in the liquid phase. The main quantities we extracted as a function of the momentum transfer are the elastic incoherent structure factor (EISF) and the line width of the quasielastic component. Their trend suggests that the motion of hydrogen atoms can be schematized as a random jumping inside a confining region, which can be related to the boundaries of the space where water molecules move in the cluster they form around the sulfonic acid site. Through the calculated EISF, we obtained information on the size of such a region, which increases up to 260 K and then attains a constant value. Above this temperature, the number of water protons that are dynamically activated in the accessible time window increases with a faster rate. The jump diffusion dynamics is characterized by a typical jumping time which is stable at 5.3 ps up to approximately 260 K and then gradually decreases. The ensemble of the findings indicates that, within the limits of the energy resolution of the present experiment, water absorbed in the Nafion membrane undergoes a dynamical transition at around 260 K. We discuss the possible relationship of this dynamical onset with the behavior of the electrical conductivity of the membrane as a function of the temperature.     

Fluidity of water confined down to subnanometer films

A surface force balance with extremely high sensitivity and resolution for measuring shear forces across thin films has been used to investigate directly the dynamic properties of salt-free water (so-called conductivity water) in a gap between two atomically smooth solid surfaces. Our results reveal that no shear stress can be sustained by water (within our resolution and shear rates) down to films of thickness D = D0 = 0.0 +/- 0.3 nm. At short range (D < 3.5 +/- 1 nm), an attractive van der Waals (vdW) force between the surfaces causes a jump into a flat adhesive contact at D0, at which the surfaces rigidly couple. Analysis of the jump behavior reveals that the viscosity of water remains within a factor of 3 or so of its bulk value down to D0. This contrasts sharply with the case of confined nonassociating liquids, whose effective viscosity increases by many orders of magnitude at film thicknesses lower than about five to eight monolayers. We attribute this to the fundamentally different mechanisms of solidification of organic liquids and of water. In the former case, the density increase induced in the films by the confinement promotes solidification, while, in the case of water, such densification (due to vdW attraction between the liquid molecules and the confining walls), in agreement with bulk behavior, suppresses the tendency of the water to solidify.     

Interfacial transport of evaporating water confined in nanopores

A semianalytical, continuum analysis of evaporation of water confined in a cylindrical nanopore is presented, wherein the combined effect of electrostatic interaction and van der Waals forces is taken into account. The equations governing fluid flow and heat transfer between liquid and vapor phases are partially integrated analytically, to yield a set of ordinary differential equations, which are solved numerically to determine the flow characteristics and effect on the resulting shape and rate of evaporation from the liquid-vapor interface. The analysis identifies three important parameters that significantly affect the overall performance of the system, namely, the capillary radius, pore-wall temperature, and the degree of saturation of vapor phase. The extension of meniscus is found to be prominent for smaller nanoscale capillaries, in turn yielding a greater net rate of evaporation per unit pore area. The effects of temperature and ambient vapor pressure on net rate of evaporation are shown to be analogous. An increase in pore-wall temperature, which enhances saturation pressure, or a decrease in the ambient vapor pressure result in enhancing the net potential for evaporation and increasing the curvature of the interface.     

Hydration structure of water confined between mica surfaces

We report further molecular dynamics simulations on the structure of bound hydration layers under extreme confinement between mica surfaces. We find that the liquid phase of water is maintained down to 2 monolayer (ML) thick, whereas the structure of the K(+) ion hydration shell is close to the bulk structure even under D = 0.92 nm confinement. Unexpectedly, the density of confined water remains approximately the bulk value or less, whereas the diffusion of water molecules decreases dramatically. Further increase in confinement leads to a transition to a bilayer ice, whose density is much less than that of ice Ih due to the formation of a specific hydrogen-bonding network.     

Unusual buffer action of free-standing nanoscopically confined water

The acid–base properties of nanoscopic water confined in the black soap films (BSFs), which were prepared from aqueous solutions of sodium dodecylsulphate (SDS) with the dye neutral red (NR) as a pH probe, were investigated using a combination of UV–vis and FTIR spectroscopy. For the SDS micellar solutions at pH 1.0–9.5 adjusted with HCl/NaOH solutions and at pH 9.4 with ammonium buffered solution, the aqueous core thicknesses in the corresponding BSFs ranged from 2.7 to 6.2 nm, and the nanoscopically confined water exhibits unusual buffer action resistant not only to acidic/alkaline solutions but also to standard buffer solution. In the heavily water-depleted confined zones, it is most likely that charge pairs in proton-transfer reactions could not be formed effectively and proton transfer was prohibited in the absence of sufficient solvating ability. Theoretical analyzes indicated that the buffer action of the nanoscopic water originated from the confinement effect of two charged surfaces of the BSFs. These results might inspire deeper understanding and further studies of biobuffering, enzyme superactivity, acid-catalyzed reactions, and Nafion fuel cell membranes.     

Homogeneous freezing of water starts in the subsurface

Molecular dynamics simulations of homogeneous ice nucleation in extended aqueous slabs show that freezing preferentially starts in the subsurface. The top surface layer remains disordered during the freezing process. The subsurface accommodates better than the bulk the increase of volume connected with freezing. It also experiences strong electric fields caused by oriented surface water molecules, which can enhance ice nucleation. Our computational results shed new light on the experimental controversy concerning the bulk vs surface origin of homogeneous ice nucleation in water droplets. This has important atmospheric implications for the microphysics of formation of high altitude clouds.     

Recovery of phenols from water by extraction freezing

A procedure was developed for the recovery of phenol from aqueous solutions into acetonitrile using extraction freezing. Experimental results and the revealed regularities were discussed in the context of the proposed theoretical model. The low temperature used in the sample preparation advantageously distinguishes this procedure from the conventional versions of the extraction recovery of organic compounds from water. Moreover, the procedure does not require special glassware or expendable materials.     

Enhancing the hydrophobic effect in confined water nanodrops

The distribution of hydrophobic solutes, such as methane, enclosed in a nanosized water droplet contained in a reverse micelle of diameter 2.82 nm is investigated using Monte Carlo simulations. The effect of the hydrophobic solute's atomic diameter on the solute-solute potential of mean force is also studied. The study reveals that confinement has a strong influence on the solute's tendency to associate. The potential of mean force exhibits only a single minimum, indicating that the contact pair is the only stable configuration between solutes. The solvent-separated pair that is universally observed for small solutes in bulk water is conspicuously absent. This enhanced hydrophobic effect is attributed to the lack of sufficient water to completely hydrate and stabilize the solvent-separated configurations. The study is expected to be important in understanding the role of hydrophobic forces during protein folding and nucleation under confinement.     

Fragile-to-strong liquid transition in deeply supercooled confined water

Confining water in lab synthesized nanoporous silica matrices MCM-41-S with pore diameters of 18 and 14 A, we have been able to study the molecular dynamics of water in deeply supercooled states, down to 200 K. Using quasielastic neutron scattering and analyzing the data with the relaxing cage model, we determined the temperature variation of the average translational relaxation time and its Q-dependence. We find a clear evidence of an abrupt change of the relaxation time behavior at T approximately equal to 225 K, which we interpreted as the predicted fragile-to-strong liquid-liquid transition.     

A role for confined water in chaperonin function

Chaperonins engulf other proteins and accelerate their folding by an unknown mechanism. Here, we combine all-atom molecular dynamics simulations with data from experimental assays of the activity of the bacterial chaperonin GroEL to demonstrate that a chaperonin's ability to facilitate folding is correlated with the affinity of its interior surface for water. Our results suggest a novel view of the behavior of confined water for models of in vivo protein folding scenarios.     

Hydrogen-bond dynamics of water confined in cyclodextrin nanosponges hydrogel

Cyclodextrin nanosponges (CDNS) are a very promising class of cross-linked polymers, made up of cyclodextrins. CDNS swollen in aqueous solution give rise to cyclodextrin-based hydrogel in different states—gel or liquid suspension—depending on the hydration level of the system. Here we present a thorough inspection of the vibrational dynamics of these hydrogel by Raman scattering experiments, with the aim of clarifying the role played by the hydrogen-bond dynamics of water molecules confined into the nano-sized pores of nanosponges in determining the rigidity of the hydrogel network and their maximum water-holding capacity. Changes occurring in the spectral shape of the OH stretching band of water were interpreted by accounting the connectivity pattern of water molecules concurring to the gelation process. Spectral deconvolution analysis gives evidence of the existence of a characteristic cross-over hydration level associated to the rearrangement of water molecules in more cooperative, bulk-like networks as a consequence of saturation sites of water confinement of nanosponges. This interpretation is further confirmed by the inspection of the estimated collective intensities. These findings also support the existence of a specific phase diagram of the cyclodextrin nanosponges hydrogel, where the molecular structure of the cross-linking agent used during the synthesis of nanosponge plays a fundamental role in defining the nano- and microscopic properties of the system.     

Phase diagram of supercooled water confined to hydrophilic nanopores

We present a phase diagram for water confined to cylindrical silica nanopores in terms of pressure, temperature, and pore radius. The confining cylindrical wall is hydrophilic and disordered, which has a destabilizing effect on ordered water structure. The phase diagram for this class of systems is derived from general arguments, with parameters taken from experimental observations and computer simulations and with assumptions tested by computer simulation. Phase space divides into three regions: a single liquid, a crystal-like solid, and glass. For large pores, radii exceeding 1 nm, water exhibits liquid and crystal-like behaviors, with abrupt crossovers between these regimes. For small pore radii, crystal-like behavior is unstable and water remains amorphous for all non-zero temperatures. At low enough temperatures, these states are glasses. Several experimental results for supercooled water can be understood in terms of the phase diagram we present.