Structure and dynamics of liquid methanol confined within functionalized silica nanopores

Molecular dynamics simulations have been carried out to investigate the structure and dynamics of liquid methanol confined in 3.3 nm diameter cylindrical silica pores. Three cavities differing in the characteristics of the functional groups at their walls have been examined: (i) smooth hydrophobic pores in which dispersive forces prevail, (ii) hydrophilic cavities with surfaces covered by polar silanol groups, and (iii) a much more rugged pore in which 60% of the previous interfacial hydroxyl groups were replaced by the bulkier trimethylsilyl ones. Confinement promotes a considerable structure at the vicinity of the pore walls which is enhanced in the case of hydroxylated surfaces. Moreover, in the presence of the trimethylsilyl groups, the propagation of this interface-induced spatial ordering extends down to the central region of the pore. Concerning the dynamical modes, we observed an overall slowdown in both the translational and rotational motions. An analysis of these mobilities from a local perspective shows that the largest retardations operate at the vicinity of the interfaces. The gross features of the rotational dynamics were analyzed in terms of contributions arising from bulk and surface states. Compared to the bulk dynamical behavior, the characteristic timescales associated with the rotational motions show the most dramatic increments. A dynamical analysis of hydrogen bond formation and breaking processes is also included.     

Proton quantum coherence observed in water confined in silica nanopores

Deep inelastic neutron scattering measurements of water confined in nanoporous xerogel powders, with average pore diameters of 24 and 82 A, have been carried out for pore fillings ranging from 76% to nearly full coverage. DINS measurements provide direct information on the momentum distribution n(p) of protons, probing the local structure of the molecular system. The observed scattering is interpreted within the framework of the impulse approximation and the longitudinal momentum distribution determined using a model independent approach. The results show that the proton momentum distribution is highly non-Gaussian. A bimodal distribution appears in the 24 A pore, indicating coherent motion of the proton over distances d of approximately 0.3 A. The proton mean kinetic energy W of the confined water molecule is determined from the second moment of n(p). The W values, higher than in bulk water, are ascribed to changes of the proton dynamics induced by the interaction between interfacial water and the confining surface.     

Aqueous electrolytes confined within functionalized silica nanopores

Molecular dynamics simulations have been carried out to investigate structural and dynamical characteristics of NaCl aqueous solutions confined within silica nanopores in contact with a "bulk-like" reservoir. Two types of pores, with diameters intermediate between 20 A? and 37.5 A?, were investigated: The first one corresponded to hydrophobic cavities, in which the prevailing wall-solution interactions were of the Lennard-Jones type. In addition, we also examined the behavior of solutions trapped within hydrophilic cavities, in which a set of unsaturated O-sites at the wall were transformed in polar silanol Si-OH groups. In all cases, the overall concentrations of the trapped electrolytes exhibited important reductions that, in the case of the narrowest pores, attained 50% of the bulk value. Local concentrations within the pores also showed important fluctuations. In hydrophobic cavities, the close vicinity of the pore wall was coated exclusively by the solvent, whereas in hydrophilic pores, selective adsorption of Na(+) ions was also observed. Mass and charge transport were also investigated. Individual diffusion coefficients did not present large modifications from what is perceived in the bulk; contrasting, the electrical conductivity exhibited important reductions. The qualitative differences are rationalized in terms of simple geometrical considerations.     

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.     

Solidification and melting of cetane confined in the nanopores of silica gel

Synthetic silica gels with six different effective diameters of nano-pores (3–30 nm) were loaded with n-hexadecane (cetane) after the elimination of adsorbed water. Kinetics of the solidification and melting of cetane was studied by differential scanning calorimetry (DSC) above the room temperature. Two thermodynamically different states of cetane were found in the samples: the free (bulk)-cetane state and the confined-cetane state. As suspected, the third state of cetane can be amorphous. This has been indicated by the small total transformation heat. The complex crystallization effect of cetane has been found to obey the nucleation-and-growth kinetics and also to depend on the dimensions of confining pores of silica gel. The melting of cetane seems to vary only with the average diameter of silica gel pores, which satisfies the Gibbs–Thompson relation. The presented results validate the applicability of the DSC technique for the porometry. The cetane-medium calibration curve for the silica gel nano-thermoporometry has been determined.     

Structure and dynamics of water confined in dimethyl sulfoxide.

We study the structure and dynamics of hydrogen-bonded complexes of H2O/D2O and dimethyl sulfoxide (DMSO) by infrared spectroscopy, NMR spectroscopy and ab initio calculations. We find that single water molecules occur in two configurations. For one half of the water monomers both OH/OD groups form strong hydrogen bonds to DMSO molecules, whereas for the other half only one of the two OH/OD groups is hydrogen-bonded to a solvent molecule. The H-bond strength between water and DMSO is in the order of that in bulk water. NMR deuteron relaxation rates and calculated deuteron quadrupole coupling constants yield rotational correlation times of water. The molecular reorientation of water monomers in DMSO is two-and-a-half times slower than in bulk water. This result can be explained by local structure behavior.     

Melting and freezing of water in cylindrical silica nanopores

Freezing and melting of H(2)O and D(2)O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and (1)H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DeltaT(m) can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* approximately equal to 2.8 nm. It is concluded that D* represents a lower limit for first-order melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.     

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.     

Water dynamics in silica nanopores: the self-intermediate scattering functions

The dynamics of water molecules confined in approximately cylindrical silica nanopores is investigated using molecular simulation. The model systems are pores of diameter varying between 20 and 40 ? containing water at room temperature and at full hydration, prepared using grand canonical Monte Carlo simulation. Water dynamics in these systems is studied via molecular dynamics simulation. The results of the basic characterization of these systems have been reported in A. A. Milischuk and B. M. Ladanyi [J. Chem. Phys. 135, 174709 (2011)]. The main focus of the present study is the self-intermediate scattering function (ISF), F(S)(Q, t), of water hydrogens, the observable in quasi-elastic neutron scattering experiments. We investigate how F(S)(Q, t) depends on the pore diameter, the direction and magnitude of the momentum transfer Q, and the proximity of water molecules to the silica surface. We also study the contributions to F(S)(Q, t) from rotational and translational motions of water molecules and the extent of rotation-translation coupling present in F(S)(Q, t). We find that F(S)(Q, t) depends strongly on the pore diameter and that this dependence is due mainly to the contributions to the ISF from water translational motion and can be attributed to the decreased mobility of water molecules near the silica surface. The relaxation rate depends on the direction of Q and is faster for Q in the axial than in the radial direction. As the magnitude of Q increases, this difference diminishes but does not disappear. We find that its source is mainly the anisotropy in translational diffusion at low Q and in molecular reorientation at higher Q values.     

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.     

Molecular dynamics study of hydrated imogolite. 2. Structure and dynamics of confined water

The behaviour of water confined in an imogolite nanotube was studied by means of molecular dynamics simulations. The results of the study show an important difference between the interaction of water molecules with the internal and external surfaces of the nanotube. The analysis of the density profiles of confined molecules, of their spatial organisation, of the size of molecular clusters, of the lifetime of H-bonds in the system and of dynamical characteristics of molecules permits us to qualify the external imogolite surface as hydrophobic, whereas the internal surface reveals a hydrophilic character.     

Aqueous NaCl and CsCl solutions confined in crystalline slit-shaped silica nanopores of varying degree of protonation

All-atom molecular dynamics simulations were conducted to study the dynamics of aqueous electrolyte solutions confined in slit-shaped silica nanopores of various degrees of protonation. Five degrees of protonation were prepared by randomly removing surface hydrogen atoms from fully protonated crystalline silica surfaces. Aqueous electrolyte solutions containing NaCl or CsCl salt were simulated at ambient conditions. In all cases, the ionic concentration was 1 M. The results were quantified in terms of atomic density distributions within the pores, and the self-diffusion coefficient along the direction parallel to the pore surface. We found evidence for ion-specific properties that depend on ion-surface, water-ion, and only in some cases ion-ion correlations. The degree of protonation strongly affects the structure, distribution, and the dynamic behavior of confined water and electrolytes. Cl(-) ions adsorb on the surface at large degrees of protonation, and their behavior does not depend significantly on the cation type (either Na(+) or Cs(+) ions are present in the systems considered). The cations show significant ion-specific behavior. Na(+) ions occupy different positions within the pore as the degree of protonation changes, while Cs(+) ions mainly remain near the pore center at all conditions considered. For a given degree of protonation, the planar self-diffusion coefficient of Cs(+) is always greater than that of Na(+) ions. The results are useful for better understanding transport under confinement, including brine behavior in the subsurface, with important applications such as environmental remediation.     

Structure and adsorption of water in nonuniform cylindrical nanopores

Grand canonical Monte Carlo simulations are used to examine the adsorption and structure of water in the interior of cylindrical nanopores in which the axial symmetry is broken either by varying the radius as a function of position along the pore axis or by introducing regions where the characteristic strength of the water-nanopore interaction is reduced. Using the extended simple point charge (SPC∕E) model for water, nanopores with a uniform radius of 6.0 A? are found to fill with water at chemical potentials approximately 0.5 kJ∕mol higher than the chemical potential of the saturated vapor. The water in these filled pores exists in either a weakly structured fluidlike state or a highly structured uniformly polarized state composed of a series of stacked water clusters with pentagonal cross sections. This highly structured state can be disrupted by creating hydrophobic regions on the surface of the nanopore, and the degree of disruption can be systematically controlled by adjusting the size of the hydrophobic regions. In particular, hydrophobic banded regions with lengths larger than 9.2 A? result in a complete loss of structure and the formation of a liquid-vapor coexistence in the tube interior. Similarly, the introduction of spatial variation in the nanopore radius can produce two condensation transitions at distinct points along the filling isotherm.     

Solvation dynamics of coumarin 153 in alcohols confined in silica nanochannels

Solvation dynamics in alcohols confined in silica nanochannels was examined by time-resolved fluorescence spectroscopy using coumarin 153 (C153) as a fluorescent probe. Surfactant-templated mesoporous silica was fabricated inside the pores of an anodic alumina membrane. The surfactant was removed by calcination to give mesoporous silica (Cal-NAM) containing one-dimensional (1D) silica nanochannels (diameter, 3.1 nm) whose inner surface was covered with silanol groups. By treating Cal-NAM with trimethylchlorosilane, trimethylsilyl (TMS) groups were formed on the inner surface of the silica nanochannels (TMS-NAM). Fluorescence dynamic Stokes shifts of C153 were measured in alcohols (ethanol, butanol, hexanol, and decanol) confined in the silica nanochannels of Cal- and TMS-NAMs, and the time-dependent fluorescence decay profiles could be best fitted by a biexponential function. The estimated solvent relaxation times were much larger than those observed in bulk alcohols for both Cal- and TMS-NAMs when ethanol or butanol was used as a solvent, indicating that the mobility of these alcohol molecules was restricted within the silica nanochannels. However, hexanol or decanol in Cal- and TMS-NAMs did not cause a remarkable increase in the solvent relaxation time in contrast to ethanol or butanol. Therefore, it was concluded that a relatively rigid assembly of alcohols (an alcohol chain) was formed within the silica nanochannels by hydrogen bonding interaction and van der Waals force between the surface functional groups of the silica nanochannels and alcohol molecules and by the successive interaction between alcohol molecules when alcohol with a short alkyl chain (ethanol or butanol) was used as a solvent.     

Ultrafast optical Kerr effect spectroscopy of water confined in nanopores of the gelatin gel

We report on the investigation of a short-time collective dynamics of water confined in the pores of the gelatin gel, using the femtosecond optical Kerr effect spectroscopy. The ultrafast responses of water molecules obtained in bulk liquid and in three concentrations of gelatin gels are explained theoretically, both in a long time and in a short time regime, taking into account all molecular motions. We prove that the contribution of molecules involved in tetrahedral, strongly H-bonded structures stabilizing the gel network increases with the gel concentration. On the other hand the long-time relaxation of water molecules is significantly slowed down in the gel pores.     

Dynamics of small-molecule glass formers confined in nanopores

We report a comparative neutron scattering study of the molecular mobility and nonexponential relaxation of three structurally similar glass-forming liquids, isopropanol, propylene glycol, and glycerol, both in bulk and confined in porous Vycor glass. Confinement reduces molecular mobility in all three liquids, and suppresses crystallization in isopropanol. High-resolution quasielastic neutron scattering spectra were fit to Fourier transformed Kohlrausch functions exp[-(t∕τ)(β)], describing the α-relaxation processes in these liquids. The stretching parameter β is roughly constant with wavevector Q and over the temperature range explored in bulk glycerol and propylene glycol, but varies both with Q and temperature in confinement. Average relaxation times <τ(Q)> are longer at lower temperatures and in confinement. They obey a power law <τ(Q)> ∝ Q(-γ), where the exponent γ is modified by confinement. Comparison of the bulk and confined liquids lends support to the idea that structural and∕or dynamical heterogeneity underlies the nonexponential relaxation of glass formers, as widely hypothesized in the literature.     

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.     

NMR investigation of the porosity of hypercrosslinked polystyrene and the properties of water confined in its nanopores

The melting of water frozen preliminarily at 180 K in a free internal volume of water-swollen hypercrosslinked polystyrene networks with degrees of crosslinking ranging from 43 to 500% is studied by NMR. It is found that ice melts within a narrow range of low temperatures, 195?C225 K, demonstrating that the pores in the networks are small and uniform in size. It is, however, impossible to calculate the pore size via the Gibbs-Thomson equation, since the structure of water (and hence its properties) depend on a sample??s rate of freezing. We conclude that the activation of the orientational motions of water molecules starts to manifest itself already at 200 K; upon reaching 220?C230 K, the total mobility of water molecules is due largely to translational motions with an activation energy of 27?C28 kJ/mol.