Effect of Nanoscale Confinement on Freezing of Modified Water at Room Temperature and Ambient Pressure

Understanding the phase behavior of confined water is central to fields as diverse as heterogeneous catalysis, corrosion, nanofluidics, and to emerging energy technologies. Altering the state points (temperature, pressure, etc.) or introduction of a foreign surface can result in the phase transformation of water. At room temperature, ice nucleation is a very rare event and extremely high pressures in the GPa–TPa range are required to freeze water. Here, we perform computer experiments to artificially alter the balance between electrostatic and dispersion interactions between water molecules, and demonstrate nucleation and growth of ice at room temperature in a nanoconfined environment. Local perturbations in dispersive and electrostatic interactions near the surface are shown to provide the seed for nucleation (nucleation sites), which lead to room temperature liquid–solid phase transition of confined water. Crystallization of water occurs over several tens of nanometers and is shown to be independent of the nature of the substrate (hydrophilic oxide vs. hydrophobic graphene and crystalline oxide vs. amorphous diamond‐like carbon). Our results lead us to hypothesize that the freezing transition of confined water can be controlled by tuning the relative dispersive and electrostatic interaction.     

Hydration of NaCl on glassy, supercooled-liquid, and crystalline water

Interactions of sodium chloride with amorphous and crystalline water films, leading to the possible formation of a dilute NaCl solution, were investigated using time-of-flight secondary ion mass spectrometry as a function of temperature. A monolayer of NaCl tends to remain on the surface or in subsurface sites of thick amorphous solid water films (200 monolayers); the Na+ ion is hydrated preferentially, whereas the Cl- ion is segregated at the surface. The hydration structure of NaCl is fundamentally unchanged for viscous liquid water that appears at temperatures higher than 136 K. The solubility of NaCl increases abruptly at 160 K because of the evolution of supercooled liquid water, which can hydrate the Cl- ion efficiently. However, the diffusion of the ions toward the bulk of supercooled liquid water is interrupted by crystallization; therefore, the dilute NaCl solution that is characterized by completely separated Na+-Cl- pairs may not be formed. When NaCl is deposited on the crystalline ice film, hydration of NaCl is enhanced above 160 K as well, indicating that a liquidlike phase coexists with crystals.     

Liquid to quasicrystal transition in bilayer water

The phase behavior of confined water is a topic of intense and current interest due to its relevance in biology, geology, and materials science. Nevertheless, little is known about the phases that water forms even when confined in the simplest geometries, such as water confined between parallel surfaces. Here we use molecular dynamics simulations to compute the phase diagram of two layers of water confined between parallel non hydrogen bonding walls. This study shows that the water bilayer forms a dodecagonal quasicrystal, as well as two previously unreported bilayer crystals, one tiled exclusively by pentagonal rings. Quasicrystals, structures with long-range order but without periodicity, have never before been reported for water. The dodecagonal quasicrystal is obtained from the bilayer liquid through a reversible first-order phase transition and has diffusivity intermediate between that of the bilayer liquid and ice phases. The water quasicrystal and the ice polymorphs based on pentagons are stabilized by compression of the bilayer and are not templated by the confining surfaces, which are smooth. This demonstrates that these novel phases are intrinsically favored in bilayer water and suggests that these structures could be relevant not only for confined water but also for the wetting and properties of water at interfaces.     

Magnetic freezing of confined water

We report results from molecular dynamic simulations of the freezing transition of liquid water in the nanoscale hydrophobic confinement under the influence of a homogeneous external magnetic field of 10 T along the direction perpendicular to the parallel plates. A new phase of bilayer crystalline ice is obtained at an anomalously high freezing temperature of 340 K. The water-to-ice translation is found to be first order. The bilayer ice is built from alternating rows of hexagonal rings and rhombic rings parallel to the confining plates, with a large distortion of the hydrogen bonds. We also investigate the temperature shifts of the freezing transition due to the magnetic field. The freezing temperature, below which the freezing of confined water occurs, shifts to a higher value as the magnetic field enhances. Furthermore, the temperature of the freezing transition of confined water is proportional to the denary logarithm of the external magnetic field.     

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.     

Molecular-dynamics study of photodissociation of water in crystalline and amorphous ices

We present the results of classical dynamics calculations performed to study the photodissociation of water in crystalline and amorphous ice surfaces at a surface temperature of 10 K. A modified form of a recently developed potential model for the photodissociation of a water molecule in ice [S. Andersson et al., Chem. Phys. Lett. 408, 415 (2005)] is used. Dissociation in the top six monolayers is considered. Desorption of H(2)O has a low probability (less than 0.5% yield per absorbed photon) for both types of ice. The final outcome strongly depends on the original position of the photodissociated molecule. For molecules in the first bilayer of crystalline ice and the corresponding layers in amorphous ice, desorption of H atoms dominates. In the second bilayer H atom desorption, trapping of the H and OH fragments in the ice, and recombination of H and OH are of roughly equal importance. Deeper into the ice H atom desorption becomes less important and trapping and recombination dominate. Motion of the photofragments is somewhat more restricted in amorphous ice. The distribution of distances traveled by H atoms in the ice peaks at 6-7 Angstroms with a tail going to about 60 Angstroms for both types of ice. The mobility of OH radicals is low within the ice with most probable distances traveled of 2 and 1 Angstrom for crystalline and amorphous ices, respectively. OH is, however, quite mobile on top of the surface, where it has been found to travel more than 80 Angstroms. Simulated absorption spectra of crystalline ice, amorphous ice, and liquid water are found to be in very good agreement with the experiments. The outcomes of photodissociation in crystalline and amorphous ices are overall similar, but with some intriguing differences in detail. The probability of H atoms desorbing is 40% higher from amorphous than from crystalline ice and the kinetic-energy distribution of the H atoms is on average 30% hotter for amorphous ice. In contrast, the probability of desorption of OH radicals from crystalline ice is much higher than that from amorphous ice.     

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.     

HCl adsorption and ionization on amorphous and crystalline H2O films below 50 K

Molecular beams were used to grow amorphous and crystalline H(2)O films and to dose HCl upon their surface. The adsorption state of HCl on the ice films was probed with infrared spectroscopy. A Zundel continuum is clearly observed for exposures up to the saturation HCl coverage on ice upon which features centered near 2530, 2120, 1760, and 1220 cm(-1) are superimposed. The band centered near 2530 cm(-1) is observed only when the HCl adlayer is in direct contact with amorphous solid water or crystalline ice films at temperatures as low as 20 K. The spectral signature of solid HCl (amorphous or crystalline) was identified only after saturation of the adsorption sites in the first layer or when HCl was deposited onto a rare gas spacer layer between the HCl and ice film. These observations strongly support conclusions from recent electron spectroscopy work that reported ionic dissociation of the first layer HCl adsorbed onto the ice surface is spontaneous.     

The structure and crystallization of thin water films on Pt(111)

When water is adsorbed on Pt(111) above 135 K several different ice structures crystallize, depending on the thickness of the ice layer. At low coverage water forms extended islands of ice with a (square root(37) x square root(37))R25(o) unit cell, which compresses as the monolayer saturates to form a (square root(39) x square root(39))R16(o) structure. The square root(39) low-energy electron diffraction (LEED) pattern becomes more intense as the second layer grows, remaining bright for films up of 10-15 layers and then fading and disappearing for films more than ca. 40 layers thick. The ice multilayer consists of an ordered square root(39) wetting layer, on which ice grows as a crystalline film which progressively loses its registry to the wetting layer. Ice films more than ca. 50 layers thick develop a hexagonal LEED pattern, the entire film and wetting layer reorienting to form an incommensurate bulk ice. These changes are reflected in the vibrational spectra which show changes in line shape and intensity associated with the different ice structures. Thin amorphous solid water films crystallize to form the same phases observed during growth, implying that these structures are thermodynamically stable and not kinetic phases formed during growth. The change from a square root(39) registry to incommensurate bulk ice at ca. 50 layers is associated with a change in crystallization kinetics from nucleation at the Pt(111) interface in thin films to nucleation of incommensurate bulk ice in amorphous solid water films more than 50 layers thick.     

Molecular dynamics study of tryptophylglycine: a dipeptide nanotube with confined water

To investigate the mechanism of structural changes of a peptide nanotube and water confined inside the channel, the helical peptide tryptophylglycine monohydrate (WG.H2O) was studied by molecular dynamics (MD) simulations using the three-dimension parallel MD program ddgmq (software package) and a consistent force field. Simulations were performed on both the water-containing system and a model system without water molecules. The details of the structural behavior with temperature are investigated for the entire simulated temperature range. Phase transitions were obtained at 115, 245, 270, 310, and 385 K, due to the contributions of both the peptide and the confined water subsystems. The crystalline, amorphous, liquidlike, liquid, and superheated phases of water were observed in the temperature ranges 40-115, 115-245, 245-310, 310-385, and >385 K, respectively. At 300 K, the diffusion constant of the confined water is 0.46 x 10-5 cm2 s-1, a value comparable to that of other peptide nanotubes. The empty peptide system melts at 440 K. Mechanisms of the negative thermal expansion (NTE) along the tube axis were investigated for different temperature ranges. The contraction of the crystalline water (or amorphous water) draws also the tube walls in and leads to NTE below 245 K. The other NTEs appear to be connected to the collapse of the ice network or the solid peptide network between 245 K and room temperature or from 310 to 440 K, respectively.     

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.     

Transformation of ice in aqueous KCl solution to a high-pressure, low-temperature phase

The changes in in situ Raman spectra of ice in aqueous KCl solution have been measured as a function of pressure at liquid nitrogen temperature (77 K). The ice that is formed abruptly transforms to a crystalline phase at 800 MPa. It has a spectrum close to that of ice VII′ to which high density amorphous (hda) ice transforms at about 4 GPa. This behavior contrasts with that of the ice in aqueous LiCl solution, which transforms to an amorphous phase at 500 MPa, as in the case of pressure-induced amorphization of ice I_h to hda.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).     

Radiation effects in water ice: a near-edge x-ray absorption fine structure study

The changes in the structure and composition of vapor-deposited ice films irradiated at 20 K with soft x-ray photons (3-900 eV) and their subsequent evolution with temperatures between 20 and 150 K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO(2) radicals, as well as the oxygen O(2) and hydrogen peroxide H(2)O(2) molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (I(cryst)) ice films. The evolution of their concentrations with the temperature indicates that HO(2), O(2), and H(2)O(2) result from a simple step reaction fuelled by OH, where O(2) is a product of HO(2) and HO(2) a product of H(2)O(2). The local order of ice is also modified, whatever the initial structure is. The crystalline ice I(cryst) becomes amorphous. The high-density amorphous phase (I(a)h) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (I(a)l). The phase I(a)h is thus peculiar to irradiated ice and does not exist in the as-deposited ice films. A new "very high density" amorphous phase-we call I(a)vh-is obtained after warming at 50 K the irradiated p-ASW ice. This phase is stable up to 90 K and partially transforms into crystalline ice at 150 K.     

Lupane-type pentacyclic triterpenes in Langmuir monolayers: a synchrotron radiation scattering study

Lupane-type pentacyclic triterpenes (lupeol, betulin, and betulinic acid) are natural products isolated from various plant sources. The terpenes exhibit a vast spectrum of biological activity and are applied in therapies for different diseases, among which the anticancer, anti-HIV, antihypercholesteremic, and antiinflammatory are the most promising. These chemicals possess amphiphilic structure and were proved to interact strongly with biomembranes, which can be the key stage in their mechanism of action. In our studies, we applied Langmuir monolayers as versatile models of biomembranes. It turned out that the three investigated terpenes are capable of stable monolayer formation; however, these monolayers differ profoundly regarding their physicochemical characteristics. In our research, we applied the Langmuir technique (surface pressure-mean molecular area (π-A) isotherm registration) coupled with Brewster angle microscopy (BAM), but the main focus was on the synchrotron radiation scattering method, grazing incidence X-ray diffraction (GIXD), which provides information on the amphiphilic molecule ordering in the angstr?m scale. It was proved that all the investigated terpenes form crystalline phases in their monolayers. In the case of lupeol, only the closely packed upright phase was observed, whereas for betulin and betulinic acid, the phase situation was more complex. Betulinic acid molecules can be organized in an upright phase, which is crystalline, and in a tilted phase, which is amorphous. The betulin film is a conglomerate of an upright crystalline monolayer phase, tilted amorphous monolayer phase, and a crystalline tilted bilayer. In our paper, we discuss the factors leading to the formation of the observed phases and the implications of our results to the therapeutic applications of the native lupane-type triterpenes.     

Studies of cavitation and ice nucleation in 'doubly-metastable' water: time-lapse photography and neutron diffraction

High-speed photographic studies and neutron diffraction measurements have been made of water under tension in a Berthelot tube. Liquid water was cooled below the normal ice-nucleation temperature and was in a doubly-metastable state prior to a collapse of the liquid state. This transition was accompanied by an exothermic heat release corresponding with the rapid production of a solid phase nucleated by cavitation. Photographic techniques have been used to observe the phase transition over short time scales in which a solidification front is observed to propagate through the sample. Significantly, other images at a shorter time interval reveal the prior formation of cavitation bubbles at the beginning of the process. The ice-nucleation process is explained in terms of a mechanism involving hydrodynamically-induced changes in tension in supercooled water in the near vicinity of an expanding cavitation bubble. Previous explanations have attributed the nucleation of the solid phase to the production of high positive pressures. Corresponding results are presented which show the initial neutron diffraction pattern after ice-nucleation. The observed pattern does not exhibit the usual crystalline pattern of hexagonal ice [I(h)] that is formed under ambient conditions, but indicates the presence of other ice forms. The composite features can be attributed to a mixture of amorphous ice, ice-I(h)/I(c) and the high-pressure form, ice-III, and the diffraction pattern continues to evolve over a time period of about an hour.     

Shear dynamics of hydration layers

Molecular dynamics (MD) simulations have been performed to investigate the shear dynamics of hydration layers of the thickness of D=0.61-2.44 nm confined between two mica surfaces. Emphases are placed on the external shear response and internal relaxation properties of aqueous films. For D=0.92-2.44 nm liquid phase, the shear responses are fluidic and similar to those observed in surface force balance experiments [U. Raviv and J. Klein, Science 297, 1540 (2002)]. However, for the bilayer ice (D=0.61 nm) [Y. S. Leng and P. T. Cummings, J. Chem. Phys. 124, 74711 (2006)] significant shear enhancement and shear thinning over a wide range of shear rates in MD regime are observed. The rotational relaxation time of water molecules in this bilayer ice is found to be as high as 0.017 ms (10(-5) s). Extrapolating the shear rate to the inverse of this longest relaxation time, we obtain a very high shear viscosity for the bilayer ice, which is also observed quite recently for D< or =0.6+/-0.3 nm hydration layers [H. Sakuma et al., Phys. Rev. Lett. 96, 46104 (2006)]. We further investigate the boundary slip of water molecules and hydrated K(+) ions and concluded that no-slip boundary condition should hold for aqueous salt solution under extreme confinement between hydrophilic mica surfaces, provided that the confined film is of Newtonian fluid.     

Investigation of vapor-deposited amorphous ice and irradiated ice by molecular dynamics simulation

With the purpose of clarifying a number of points raised in the experimental literature, we investigate by molecular dynamics simulation the thermodynamics, the structure and the vibrational properties of vapor-deposited amorphous ice (ASW) as well as the phase transformations experienced by crystalline and vitreous ice under ion bombardment. Concerning ASW, we have shown that by changing the conditions of the deposition process, it is possible to form either a nonmicroporous amorphous deposit whose density (approximately 1.0 g/cm3) is essentially invariant with the temperature of deposition, or a microporous sample whose density varies drastically upon temperature annealing. We find that ASW is energetically different from glassy water except at the glass transition temperature and above. Moreover, the molecular dynamics simulation shows no evidence for the formation of a high-density phase when depositing water molecules at very low temperature. In order to model the processing of interstellar ices by cosmic ray protons and heavy ions coming from the magnetospheric radiation environment around the giant planets, we bombarded samples of vitreous ice and cubic ice with 35 eV water molecules. After irradiation the recovered samples were found to be densified, the lower the temperature, the higher the density of the recovered sample. The analysis of the structure and vibrational properties of this new high-density phase of amorphous ice shows a close relationship with those of high-density amorphous ice obtained by pressure-induced amorphization.     

The effect of the incident collision energy on the phase and crystallization kinetics of vapor deposited water films

Molecular beam techniques are used to grow water films on Pt(111) with incident collision energies from 5 to 205 kJ/mole. The effect of the incident collision energy on the phase of vapor deposited water films and their subsequent crystallization kinetics are studied using temperature programmed desorption and infrared spectroscopy. We find that for films deposited at substrate temperatures below 110 K, the incident kinetic energy (up to 205 kJ/mole) has no effect on the initial phase of the deposited film or its crystallization kinetics. Above 110 K, the substrate temperature does affect the phase and crystallization kinetics of the deposited films but this result is also independent of the incident collision energy. The presence of a crystalline ice template (underlayer) does affect the crystallization of amorphous solid water, but this effect is also independent of the incident beam energy. These results suggest that the crystallization of amorphous solid water requires cooperative motion of several water molecules.     

Self-organization of substituted azacrowns based on their discoid and amphiphilic nature

Cyclame and hexacyclene derivatives, bearing four and six long-chain substituents respectively, were synthesized. They are discussed as monolayer-forming amphiphiles as well as liquid-crystalline-phase-forming thermotropic mesogens. The compounds investigated form ordered monolayers at the gas/water interface. In the monolayer the hydrophilic cyclic head group lies flat on the water surface, whereas the hydrophobic substituents are oriented perpendicularly with respect to the interface. Most derivatives fitted with aromatic substituents exhibit a solid condensed state exclusively. In contrast with this, solid condensed as well as expanded phases can be found when spreading the aliphatic-substituted compounds. In the latter case, the onset of the phase transition takes a bump-like shape, owing to kinetic reasons. A liquid-crystalline columnar order is only achieved with hexacyclenes bearing aromatic substituents, etherified with one alkyl chain. Besides this, the remaining derivatives melt from the crystalline state, transforming directly into the isotropic liquid, or show amorphous behaviour. Preliminary irradiating experiments in the columnar state of cinnamoyl-substituted hexacyclene Hex-7 were carried out in order to obtain polymeric tubes.