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.     

Confinement Effects on the Benzene Orientational Structure

Liquids under confinement exhibit different properties compared with their corresponding bulk phases, for example, miscibility, phase transitions, and diffusion. The underlying cause is the local ordering of molecules, which is usually only studied using pure simulation methods. Herein, we derive experimentally the structure of benzene confined in MCM‐41 using total neutron scattering measurements. The study reveals a layering of molecules across a pore, and four concentric cylindrical shells can be distinguished for a pore with the radius of 18 Å. The nanoscale confinement of the liquid has a major effect on the spatial and orientational correlations observed between the molecules, when compared with the structure of the bulk liquid. These differences are most marked for molecules in parallel configurations, and this suggests differences in chemical reactivity between the confined and bulk liquids.     

Water in nanoconfinement between hydrophilic self-assembled monolayers

Molecular dynamics (MD) simulations of water confined to subnanometer thicknesses between carboxyl-terminated alkanethiol self-assembled monolayers (SAMs) on gold were performed to address conflicts in the literature on the structure and response of water in confinement. The amount of water was varied to yield submonolayer to bilayer structures. The orientation of the water is affected by the confinement, especially in the submonolayer case. We find that the diffusion coefficient decreases as the film becomes thinner and at higher pressures. However, in all cases studied, liquid diffusion is always found. At maximal suppression, the diffusion constant is 2 orders of magnitude smaller than the bulk value.     

Diffusion enhancement in core-softened fluid confined in nanotubes

We study the effect of confinement in the dynamical behavior of a core-softened fluid. The fluid is modeled as a two length scales potential. This potential in the bulk reproduces the anomalous behavior observed in the density and in the diffusion of liquid water. A series of NpT molecular dynamics simulations for this two length scales fluid confined in a nanotube were performed. We obtain that the diffusion coefficient increases with the increase of the nanotube radius for wide channels as expected for normal fluids. However, for narrow channels, the confinement shows an enhancement in the diffusion coefficient when the nanotube radius decreases. This behavior, observed for water, is explained in the framework of the two length scales potential.     

Phase behavior and local structure of a binary mixture in pores: mean-field lattice model calculations for analyzing neutron scattering data

We investigate the phase behavior of an asymmetric binary liquid A-W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.     

A diffusion model for the fluids confined in micropores

The self-diffusion coefficients were calculated by molecular dynamics simulations and the effects of pore width, temperature, and fluid density on diffusion behavior of simple fluid argon and polar fluid water confined in micropores were analyzed and studied. A mathematical model describing diffusion behavior of fluids confined in micropores was proposed from the theories of molecular dynamics and molecular kinematics, and validated on the basis of the simulation results at various conditions. The model indicates that the diffusion coefficient is proportional to the square root of the pore width and to the temperature divided by the density squared. It is applicable to either liquid or gas states and only two parameters are required.     

Structure and dynamics of a confined ionic liquid. Topics of relevance to dye-sensitized solar cells

The behavior of a model ionic liquid (IL) confined between two flat parallel walls was studied at various interwall distances using computer simulations. The results focus both on structural and dynamical properties. Mass and charge density along the confinement axis reveal a structure of layers parallel to the walls that leads to an oscillatory profile in the electrostatic potential. Orientational correlation functions indicate that cations at the interface orient tilted with respect to the surface and that any other orientational order is lost thereafter. The diffusion coefficients of the ions exhibit a maximum as a function of the confinement distance, a behavior that results from a combination of the structure of the liquid as a whole and a faster molecular motion in the vicinity of the walls. We discuss the relevance of the present results and elaborate on topics that need further attention regarding the effects of ILs in the functioning of IL-based dye-sensitized solar cells.     

Molecular dynamics of zigzag single walled carbon nanotube immersion in water

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes are reported. Using molecular dynamics simulation, we discuss the relation between the value of this enthalpy and tube diameters showing that the obtained plot can be divided into three regions. The structure of water inside tubes in all three regions is discussed and it is shown that the existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nm is due to freezing of water under confinement. The calculations of hydrogen bond statistics and water density profiles inside tubes are additionally reported to confirm the obtained results. Next, we show the results of calculation for the same tubes but containing surface carbonyl oxygen groups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparison to the initial tubes is observed. We also discuss the influence of charge distribution between oxygen and carbon atom forming surface carbonyls on the structure of confined water. It is concluded for the first time that the presence of surface oxygen atoms at the pore entrances remarkably influences the structure and stability of ice created inside nanotubes, and surface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effect is explained by the pore blocking leading to a decrease (compared to initial structure) in the number of confined water molecules after introduction of surface oxygen groups at pore entrances.     

Translational diffusion of cumene and 3-methylpentane on free surfaces and pore walls studied by time-of-flight secondary ion mass spectrometry

Mobility of molecules in confined geometry has been studied extensively, but the origins of finite size effects on reduction of the glass transition temperature, T(g), are controversial especially for supported thin films. We investigate uptake of probe molecules in vapor-deposited thin films of cumene, 3-methylpentane, and heavy water using secondary ion mass spectrometry and discuss roles of individual molecular motion during structural relaxation and glass-liquid transition. The surface mobility is found to be enhanced for low-density glasses in the sub-T(g) region because of the diffusion of molecules on pore walls, resulting in densification of a film via pore collapse. Even for high-density glasses without pores, self-diffusion commences prior to the film morphology change at T(g), which is thought to be related to decoupling between translational diffusivity and viscosity. The diffusivity of deeply supercooled liquid tends to be enhanced when it is confined in pores of amorphous solid water. The diffusivity of molecules is further enhanced at temperatures higher than 1.2-1.3 T(g) irrespective of the confinement.     

Confinement effect on the adsorption from a binary liquid system near liquid/liquid phase separation

The preferential adsorption of one component of a binary system at the inner surfaces of mesoporous silica glasses was studied in a wide composition range at temperatures close to liquid/liquid phase separation. Confinement effects on the adsorption were investigated by using three controlled-pore glass (CPG-10) materials of different mean pore size (10 to 50 nm). For the experimental system (2-butoxyethanol+water), which exhibits an upper miscibility gap, strong preferential adsorption of water occurs, as the coexistence curve is approached at bulk compositions, at which water is the minority component. In this strong adsorption regime the area-related surface excess amount of adsorbed water decreases with decreasing pore width, while the shift in the volume-related mean composition of the pore liquid shows an opposite trend, i.e., greatest deviation from bulk composition occurring in the most narrow pores. A simple mean-field lattice model of a liquid mixture confined by parallel walls is adopted to rationalize these experimental findings. This model reproduces the main findings of the confinement effect on the adsorption near liquid/liquid phase separation.     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Effect of confinement on the liquid-liquid phase transition of supercooled water

We report on an observation of the phase transition between two liquid phases of supercooled confined water in simulations. The temperature of the liquid-liquid transition of water at zero pressure slightly decreases due to confinement in the hydrophobic pore. The hydrophilic confinement affects this temperature in the opposite direction and shifts the critical point of the liquid-liquid transition to a higher pressure. As a result, in a strongly hydrophilic pore the liquid-liquid phase transition becomes continuous at zero pressure, indicating the shift of its critical point from negative to a positive pressure. These findings indicate that experimental studies of water confined in the pores of various hydrophobicity/hydrophilicity may clarify the location of the liquid-liquid critical point of bulk water.     

Confinement effect on phase transitions of a discotic liquid crystal: a calorimetric study

Differential scanning calorimetry was used to investigate the confinement effects on the phase transition behaviour of a discotic liquid crystal. The liquid crystal studied is the hexa-n-octanoate of rufigallol (RHO); Millipore membranes of various pore sizes were the confining materials. The polymorphism of RHO is affected by confinement. The transition from an enantiotropic columnar phase (D1) to a monotropic columnar phase (D2) is supressed in membranes with pore sizes 500 A. The transformation from D1 to the crystalline phase is also perturbed, particularly in the membrane having an average pore size of 250 A. In the first case the crystal formed displays a double-melting endotherm, with a distinct structure melting at lower temperatures; in the other, the induction period of isothermal crystallization becomes longer and the global rate of crystallization is slowed. However, confinement shows no effect on the overall crystallization mechanism; a similar Avrami constant of n ~ 3 was obtained for both confined and bulk RHO. An analysis of the results is presented.     

Water confined in nanotubes and between graphene sheets: a first principle study

Water confined at the nanoscale has been the focus of numerous experimental and theoretical investigations in recent years, yet there is no consensus on such basic properties as diffusion and the nature of hydrogen bonding (HB) under confinement. Unraveling these properties is important to understand fluid flow and transport at the nanoscale, and to shed light on the solvation of biomolecules. Here we report on a first principle, computational study focusing on water confined between prototypical nonpolar substrates, i.e., single-wall carbon nanotubes and graphene sheets, 1-2.5 nm apart. The results of our molecular dynamics simulations show the presence of a thin, interfacial liquid layer (approximately 5 A) whose microscopic structure and thickness are independent of the distance between confining layers. The properties of the HB network are very similar to those of the bulk outside the interfacial region, even in the case of strong confinement. Our findings indicate that the perturbation induced by the presence of confining media is extremely local in liquid water, and we propose that many of the effects attributed to novel phases under confinement are determined by subtle electronic structure rearrangements occurring at the interface with the confining medium.     

Pyrolysis of phenethyl phenyl ether tethered in mesoporous silica. Effects of confinement and surface spacer molecules on product selectivity

There has been expanding interest in exploring porous metal oxides as a confining environment for organic molecules resulting in altered chemical and physical properties including chemical transformations. In this paper, we examine the pyrolysis behavior of phenethyl phenyl ether (PPE) confined in mesoporous silica by covalent tethers to the pore walls as a function of tether density and the presence of cotethered surface spacer molecules of varying structure (biphenyl, naphthyl, octyl, and hexadecyl). The PPE pyrolysis product selectivity, which is determined by two competitive free-radical pathways cycling through the two aliphatic radical intermediates (PhCH·CH(2)OPh and PhCH(2)CH·OPh), is shown to be significantly different from that measured in the liquid phase as well as for PPE tethered to the exterior surface of nonporous silica nanoparticles. Tailoring the pore surface with spacer molecules further alters the selectivity such that the PPE reaction channel involving a molecular rearrangement (O-C phenyl shift in PhCH(2)CH·OPh), which accounts for 25% of the products in the liquid phase, can be virtually eliminated under pore confinement conditions. The origin of this change in selectivity is discussed in the context of steric constraints on the rearrangement path inside the pores, surface and pore confinement effects, pore surface curvature, and hydrogen bonding of PPE with residual surface silanols supplemented by nitrogen physisorption data and molecular dynamics simulations.     

Transport properties and distribution of water molecules confined in hydrophobic nanopores and nanoslits

The transport properties, including the diffusivity and viscosity, of water confined in hydrophobic nanopores and nanoslits were studied by molecular dynamics simulations. The results show that the diffusion coefficient in nanopores and nanoslits is markedly lower than that in the bulk. But the viscosity is much larger than that in bulk. The parallel diffusion coefficient is obviously larger than the perpendicular ones. The diffusion coefficient in the channel pore is ever less than that in the slit pore at the same pore width, but the viscosity is larger. The temperature and density affect significantly the diffusivity and viscosity in nanopores and nanoslits. Lower density water exhibits some special characteristics on density profiles in nanopores and nanoslits at lower temperatures, and the density profiles show a change from homogeneous to inhomogeneous as the pore width is reduced. Even clusters occurred in micropores.     

Density minimum of confined water at low temperatures: a combined study by small-angle scattering of X-rays and neutrons

A simple explanation is given for the low-temperature density minimum of water confined within cylindrical pores of ordered nanoporous materials of different pore size. The experimental evidence is based on combined data from in-situ small-angle scattering of X-rays (SAXS) and neutrons (SANS), corroborated by additional wide-angle X-ray scattering (WAXS). The combined scattering data cannot be described by a homogeneous density distribution of water within the pores, as was originally suggested from SANS data alone. A two-step density model reveals a wall layer covering approximately two layers of water molecules with higher density than the residual core water in the central part of the pores. The temperature-induced changes of the scattering signal from both X-rays and neutrons are consistent with a minimum of the average water density. We show that the temperature at which this minimum occurs depends monotonically on the pore size. Therefore we attribute this minimum to a liquid-solid transition of water influenced by confinement. For water confined in the smallest pores of only 2 nm in diameter, the density minimum is explained in terms of a structural transition of the surface water layer closest to the hydrophilic pore walls.     

Neutron scattering study on dynamics of water molecules in MCM-41. 2. Determination of translational diffusion coefficient

Quasielastic neutron scattering (QENS) spectra of water-filled MCM-41 samples (pore diameters: 21.4 and 28.4 Angstrom) were measured over the temperature range 238-298 K and the momentum transfer range 0.31-0.99 A(-1) to investigate the dynamics of confined water molecules. The spectra, which consist mainly of contributions from the translational diffusion of water molecules, were analyzed by using the Lorentzian and the stretched exponential functions. Comparison of the fits indicated that the latter analysis is more reliable than the former one. The fraction of immobile water molecules located in the vicinity of the pore walls, which give an elastic component, was found to be 0.044-0.061 in both pores. The stretch exponent beta was determined as 0.66-0.80. It was shown that the translational diffusion of water molecules in the pores is decelerated by confinement and that the deceleration becomes marked with a decrease in pore size. The ratios of the translational diffusion coefficient D(T) of confined water to that of bulk water at room temperature were within a range of 0.47-0.63.     

Thermal properties and vibrational density of states of a nanoconfined discotic liquid crystal

Neutron scattering is employed to investigate the vibrational density of states (VDOS) of the discotic liquid crystal 2,3,6,7,10,11-hexakis[hexyloxy] triphenylene (HAT6) confined to the pores of alumina oxide membranes with different pore sizes. Additionally, the phase transitions were studied by differential scanning calorimetry. The transitions were observed down to the smallest pore size. The decrease of the transition enthalpies versus inverse pore size for both transitions implies an increase of the amount of disordered amorphous material. By extrapolation of its pore size dependence, a critical pore diameter for structure formation of 17 nm is estimated. Similar to the bulk, excess contributions to the VDOS (Boson peak) are also observed for confined HAT6. The Boson peak gains in intensity and shifts to lower frequencies with decreasing pore diameter. This is discussed in the framework of a softening of HAT6 induced by the confinement due to a less-developed plastic crystalline state inside the pores compared to the bulk.     

Restricted diffusion in silica particles measured by pulsed field gradient NMR

The restricted diffusion coefficient of water through porous silica is measured by pulsed field gradient (PFG) NMR as a function of loading in order to develop a model for self-diffusion at full pore filling in sol-gel-made porous silica particles. This model describes the pore or intraparticle diffusion coefficient as a function of particle porosity, tortuosity, and the steric hindrance applied on the molecules by the pore space. The particle morphology is characterized by nitrogen adsorption and an appropriate tortuosity model is chosen in comparison with literature data. To characterize the material, NMR relaxation and diffusion studies at different degrees of pore filling were carried out in relation to the silica/water adsorption isotherm.