Influence of surface interactions on the dynamics of the glass former ortho-terphenyl confined in nanoporous silica

We present results on investigations of the dynamics of the glass forming ortho-terphenyl (oTP) confined in nanoporous silica. Calorimetry experiments showed that the glass transition temperature of the confined liquid, T_gconf, has a non-trivial pore size dependence and is strongly affected by surface interactions. Fluid-wall interactions introduce gradients of structural relaxation times in the pores. The molecules at the surface of the pores are slowed down compared to those at the center of the pores. We focus here on a pore diameter range (7 σ< d < 12 σ, where σ is the molecular diameter), where a large variety of dynamical behavior were observed. Depending on surface properties of the confined media, T _gconf may be smaller or larger than the bulk one. In a quite attractive matrix with a pore size of around 7 nm, the structural relaxation times gradient is important enough to allow the observation of two glass transitions for the same liquid. Effects of fluid wall interactions on the short time dynamics at high temperature were also investigated by quasielastic neutron scattering. The self and collective motions exhibit well above the bulk melting point the same dependence on fluid-wall interactions as at T_g.     

Probing interfacial dynamics of water in confined nanoporous systems by NMRD

The confined dynamics of water molecules inside a pore involves an intermittence between adsorption steps near the interface and surface diffusion and excursions in the pore network. Depending on the strength of the interaction in the layer(s) close to the surface and the dynamical confinement of the distal bulk liquid, exchange dynamics can vary significantly. The average time spent in the surface proximal region (also called the adsorption layer) between a first entry and a consecutive exit allows estimating the level of ‘nanowettablity’ of water. As shown in several seminal works, NMRD is an efficient experimental method to follow such intermittent dynamics close to an interface. In this paper, the intermittent dynamics of a confined fluid inside nanoporous materials is discussed. Special attention is devoted to the interplay between bulk diffusion, adsorption and surface diffusion on curved pore interfaces. Considering the nano or meso length scale confinement of the pore network, an analytical model for calculating the inter-dipolar spin–lattice relaxation dispersion curves is proposed. In the low-frequency regime (50?KHz–100?MHz), this model is successfully compared with numerical simulations performed using a 3D-off lattice reconstruction of Vycor glass. Comparison with experimental data available in the literature is finally discussed.     

NMR study of the vapor phase contribution to diffusion in partially filled silica glasses with nanometer and micrometer pores

The contribution of the vapor phase to molecular diffusion in porous silica glasses with nanometer (Vycor) and micrometer (VitraPor#5) pores partially filled with water (polar) or cyclohexane (nonpolar) was investigated with the aid of field-gradient NMR diffusometry. Due to the vapor phase, the effective diffusion coefficient of cyclohexane filling micrometer pores (VitraPor#5) increased up to 10 times relative to the value in bulk liquid upon reduction of the pore space filling factor. On the other hand, the effective diffusion coefficient of water first decreases and then increases when the liquid content is reduced. The dependence of the effective diffusion coefficient on the pore filling factor is strongly related to the pore dimension. A general two-phase exchange model is presented that is well accounting for all experimental diffusion features.     

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.     

Salt permeation and exclusion in hydroxylated and functionalized silica pores

We use combined ab initio molecular dynamics (AIMD), grand canonical Monte Carlo, and molecular dynamics techniques to study the effect of pore surface chemistry and confinement on the permeation of salt into silica nanopore arrays filled with water. AIMD shows that 11.6 A diameter hydroxylated silica pores are relatively stable in water, whereas amine groups on functionalized pore surfaces abstract silanol protons, turning into NH3+. Free energy calculations using an ab initio parametrized force field show that the hydroxylated pores strongly attract Na+ and repel Cl- ions. Pores lined with NH3+ have the reverse surface charge polarity. Finally, studies of ions in carbon nanotubes suggest that hydration of Cl- is more strongly frustrated by pure confinement effects than Na+.     

Phase transitions of interfacial water at 165 and 240 K. Connections to bulk water physics and protein dynamics

We are considering water adsorbed as a monolayer on Vycor, a porous silica glass. The interfacial water molecules interact with the substrate through hydrogen bonding with the numerous silanol (Si-OH) groups present all over the surface. This special form of water exhibits peculiar dynamical properties. A combined calorimetric, diffraction, high resolution quasi-elastic and inelastic neutron scattering study shows that interfacial water experiences a glass transition at 165 K and a liquid-liquid transition at 240 K from a low-density to a high density-liquid. We show that this unusual behaviour, compared to the bulk, is due to a strong weakening of the hydrogen-bond strength, possibly due to the reduced number of hydrogen-bonds engaged by water molecules when they are in an interfacial two dimensional situation. The connections of these findings to the physics of bulk water and protein dynamics are discussed.     

The Effect of Curing Temperature on Early Hydration of Gray Cement Via Fast Field Cycling-NMR Relaxometry

In the present work, we use fast field cycling (FFC) nuclear magnetic resonance relaxometry to evaluate the influence introduced by the curing temperature on the hydration process of gray cement. The main advantage of FFC relaxometry as compared with other relaxation studies performed at a specific frequency is that it is sensitive to a wider range of molecular motions and better separates the surface and bulk contributions from the global measured relaxation rate. In the case of cement hydration, the relaxation process is dominated by the interaction of water protons with the paramagnetic centers located on the surface of cement grains. This allows us in the frame of a two-phase exchange model to monitor the temperature dependence of the transverse diffusional correlation time at the surface of cement grains. An increase of the surface diffusion coefficient of water molecules with the temperature was revealed. Another outcome is that the surface-to-volume ratio of capillary pores continuously increases during the early hydration and this process is strongly enhanced by rising the temperature.     

Synthesis of silver nanocomposites by SCF impregnation of matrices of synthetic opal and Vycor glass by the Ag(hfac)COD precursor

Silver-containing nanocomposites were prepared by impregnating Vycor glass (a pore diameter of 4 nm) and synthesized opal matrices (an interstitial void size of 40 nm) with cyclooctadiene complex of silver hexafluoroacetylacetonate (Ag(hfac)COD), a silver precursor, dissolved in supercritical carbon dioxide and were examined by optical absorption spectroscopy, atomic force microscopy, and electron spin-resonance spectroscopy. It was demonstrated that the absorption spectra of Vycor glass and opal matrices impregnated with Ag(hfac)COD molecules and subjected to thermal treatment in air at temperatures above 50°C exhibit plasmon resonances characteristic of Ag nanoparticles at 420–430 nm. The peculiarities of the plasmon resonance band for both types of samples were attributed to the morphology of the pore space in which silver particles are formed. Paramagnetic Cu(hfac)₂ molecules (copper hexafluoroacetylacetonate) were used as a spectroscopic probe for estimating the distribution of the precursor in the pores of Vycor glass and opal matrices during supercritical fluid impregnation.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Freezing and glass transition of hard spheres in cavities

Dynamical and static properties of N=13-4000 hard spheres in spherical cavities with smooth and rough walls have been calculated by molecular-dynamics computer simulations. We use a dynamical criterion to distinguish between fluidlike and solidlike states. The associated crossover densities show a strong dependence both on the system size and on the surface roughness. For large N, these crossover densities tend to the bulk glass transition density for rough walls and to the bulk crystallization density for smooth walls. The crossover densities for finite N are found to be significantly smaller than the corresponding bulk densities. A detailed examination of the layer-resolved radial- and tangential mean-square displacements reveals qualitatively different dynamics for smooth and rough cavities.     

Structure and dynamics of water confined in a nanoporous sol-gel silica glass: a neutron scattering study

During recent years, the understanding of the modification of the structure and dynamics of water confined in different environments has been the focus of much interest in scientific research. This topic is in fact of great relevance in a lot of technological areas and, in living systems, essential water-related phenomena occur in restricted geometries in cells, and active sites of proteins and membranes, or at their surface. In this paper we report on the most recent up to date account of structural and dynamical properties of confined water in comparison with the bulk state. In particular, as far as structure is concerned, we present new neutron diffraction results on heavy water confined in a fully hydrated sol-gel silica glass (GelSil) as a function of the temperature. At low T, the nucleation of cubic ice superimposed to liquid water, already observed for water within Vycor glasses, is discussed. As far as the dynamics is concerned, we report results of a detailed spectroscopic analysis of diffusive relaxation and vibrational properties of water confined in nanopores of Gelsil glass, at different temperatures and hydration percentages, performed by our research group during recent years by means of incoherent quasi-elastic (IQENS) and inelastic (IINS) neutron scattering. IQENS spectra are analysed in the framework of the relaxing cage model (RCM). IINS spectra show the evolution of the one-phonon-amplitude weighted proton vibrational density of states (VDOS), Z(ω), when water loses its peculiar bulk properties and originates new structural environments due to its surface interactions.     

Slow dynamics in colloidal glasses and porous media as probed by NMR relaxometry: assessment of solvent levy statistics in the strong adsorption regime

Mesoscopic media such as porous materials or colloidal pastes develop large specific surface area which strongly influence the dynamics of the embedded fluid. This fluid confinement can be used either to probe the interfacial geometry (frozen porous media) or the particle dynamics (paste and colloidal glass). In the strong adsorption regime, it was recently proposed that the effective surface diffusion on flat surface is anomalous and exhibits long time pathology (Lévy walks). This phenomena is directly related to the time and space properties of loop trajectories appearing in the bulk between a desorption and a readsorption step. The Lévy statistics extends the time domain of the embedded fluid dynamics toward the low frequency regime. An interesting way to probe such a slow interfacial process is to use field cycling NMR relaxometry. In the first part of this paper, we propose a simple theoretical model of NMR dispersion which only involves elementary time steps of the solvent dynamics near an interface (loops, trains, tails in relation with the confining geometry). In the second part, field cycling NMR relaxometry is used to probe the slow solvent dynamics in two type of interfacial systems: (i) a colloidal glass made of thin and flat particles (ii) two fully saturated porous media, the Vycor glass and MCM48 respectively. Experimental results are critically compared to closed-form analytical expressions and numerical simulations.     

The dynamics of water in nanoporous silica studied by dielectric spectroscopy

Broad-band dielectric spectroscopy is used to investigate the dynamics of hydration water on the surface of the cylindrical pores of a nanostructured silica material (MCM-41, with pore diameter of 3.2 nm) at various hydrations, in the temperature range 250-150 K. We focus our attention on orientational relaxations that shift from 0.5 MHz at 250 K to less than 1 Hz at 150 K. The measurements distinguish the relaxation of the hydroxyl groups at the surface of silica from the orientational dynamics of hydration water which strongly depends on the degree of hydration. Although it is significantly faster than the dynamics of water in ice, the orientational relaxation of non-freezing water has an activation energy comparable to that in ice when the hydration layer is complete and approximately two-molecule thick.     

Water confined in mesoporous silica glasses: Influence of temperature on adsorption/desorption hysteresis loop and fluid structure

In natural as well as industrial processes, water is frequently confined in silica porous materials with pore sizes in the nanometer scale. Understanding the confinement effects on the fluid properties is a fundamental issue, helpful to optimize the industrial processes. The molecular simulation is a powerful tool to study complex polar fluid like water at the atomic scale. The water adsorption/desorption properties in a mesoporous silica glass are investigated by means of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN TrAZ potential are used to describe water-water and water-silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor (pore size distribution, pore interconnections, etc). The intermolecular potential is shown to reproduce the experimental data at 300K (adsorption isotherm and isosteric heat of adsorption). The water structure is analyzed and confinement effects are emphasized. The temperature influence is studied: the hysteresis loop is shown to shrink with an increase in temperature.     

Fluidity of hydration layers nanoconfined between mica surfaces

We perform molecular dynamics simulations to investigate the shear dynamics of hydration water nanoconfined between two mica surfaces at 1 bar pressure and 298 K. Newtonian plateaus of shear viscosity comparable to the bulk value for different hydration layers D=0.92-2.44 nm are obtained. The origin of this persistent fluidity of the confined aqueous system is found to be closely associated with the rotational dynamics of water molecules, accompanied by fast translational diffusion under this confinement.     

Phase coexistence and dynamic properties of water in nanopores

The dynamical properties of a confined fluid depend strongly on the (spatially varying) density. Its knowledge is therefore an important prerequisite for molecular-dynamics (MD) simulations and the analysis of experimental data. In a mixed Gibbs ensemble Monte Carlo (GEMC)/MD simulation approach we first apply the GEMC method to find possible phase states of water in hydrophilic and hydrophobic nanopores. The obtained phase diagrams evidence that a two-phase state is the most probable state of a fluid in incompletely filled pores in a wide range of temperature and level of pore filling. Pronounced variations of the average and local densities are observed. Subsequently, we apply constant-volume MD simulations to obtain water diffusion coefficients and to study their spatial variation along the pore radius. In general, water diffusivity slightly decreases in a hydrophilic pore and noticeably increases in a hydrophobic pore (up to about 40% with respect to the bulk value). In the range of gradual density variations the local diffusivity essentially follows the inverse density and the water binding energy. The diffusivity in the quasi-two-dimensional water layers near the hydrophilic wall decreases by 10 to 20% with respect to the bulk value. The average diffusivity of water in incompletely filled pore is discussed on the basis of the water diffusivities in the coexisting phases.Received: 1 January 2003, Published online: 14 October 2003PACS: 61.20.Ja Computer simulation of liquid structure - 64.70.Fx Liquid-vapor transitions     

水溶液中结合水的定义与量化

水溶液中溶质的结合水具有不同于远离溶质的自由水的结构和性质.结合水的存在对水和溶质结构和动力学性质均具有显著甚至决定性的影响.然而,对结合水动力学和热力学性质的定量理解在诸多方面一直存在争议甚至严重分歧,其中重点包括如何定义和量化结合水,如何表征结合水和自由水的动力学差别,结合水如何参与生物大分子各种生物功能过程,以及溶质或界面影响结合水结构与性质的途径等.给出结合水定义的物理学依据和量化方法,是深入理解上述问题的第一步.本文简述了各种不同谱学方法定义结合水的基本原理及量化的困难,强调具有不同时间和空间响应尺度的测试方法所得结合水数不必完全可比.此外,系列水溶液物性随浓度升高会明显改变其浓度依赖关系,相应拐点浓度常被用于量化稀溶液中的溶质结合水数.我们近期研究的水溶液玻璃化转变温度-浓度关系,为结合水的定义、量化和水溶液的三区划分提供了物理依据,同时揭示了上述利用性质-浓度关系拐点浓度量化结合水方法的不足.     

Role of oxygen functional groups for structure and dynamics of interfacial water on low rank coal surface: a molecular dynamics simulation

Molecular dynamics simulations were employed to study the effects of oxygen functional groups for structure and dynamics properties of interfacial water molecules on the subbituminous coal surface. Because of complex composition and structure, the graphite surface modified by hydroxyl, carboxyl and carbonyl groups was used to represent the surface model of subbituminous coal according to XPS results, and the composing proportion for hydroxyl, carbonyl and carboxyl is 25:3:5. The hydration energy with ?386.28 kJ/mol means that the adsorption process between water and coal surface is spontaneous. Density profiles for oxygen atoms and hydrogen atoms indicate that the coal surface properties affect the structural and dynamic characteristics of the interfacial water molecules. The interfacial water exhibits much more ordering than bulk water. The results of radial distribution functions, mean square displacement and local self-diffusion coefficient for water molecule related to three oxygen moieties confirmed that the water molecules prefer to absorb with carboxylic groups, and adsorption of water molecules at the hydroxyl and carbonyl is similar.     

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10⁻²–10⁷ Hz) over a large temperature interval (213–333K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.