The Three-Dimensional Structure of Water Confined in Nanoporous Vycor Glass

Probing the Behaviour of Fluids Confined in Porous Materials by Neutron Scattering: Applications to CO2 Sequestration and Enhanced Oil and Gas Recovery

The current review presents a discussion on the utility of neutron scattering, with emphasis on neutron total scattering and small-angle neutron scattering (SANS), to explore the structural properties and the phase behaviour of fluids confined in nanopores. The effectiveness of contrast matching SANS on the evaluation of accessibility of porous materials to invading fluids is highlighted too. This review provides also an overview regarding the neutron scattering studies on the structure and the accessibility of greenhouse gases in the complex pore network of geomaterials, with applications to CO2 geological sequestration and enhanced oil and gas recovery.

The Effects of External Interfaces on Hydrophobic Interactions I: Smooth Surface

External interfaces, such as the air-water and solid-liquid interfaces, are ubiquitous in nature. Hydrophobic interactions are considered the fundamental driving force in many physical and chemical processes occurring in aqueous solutions. It is important to understand the effects of external interfaces on hydrophobic interactions. According to the structural studies on liquid water and the air-water interface, the external interface primarily affects the structure of the topmost water layer (interfacial water). Therefore, an external interface may affect hydrophobic interactions. The effects of interfaces on hydrophobicity are related not only to surface molecular polarity but also to the geometric characteristics of the external interface, such as shape and surface roughness. This study is devoted to understanding the effects of a smooth interface on hydrophobicity. Due to hydrophobic interactions, the solutes tend to accumulate at external interfaces to maximize the hydrogen bonding of water. Additionally, these can be demonstrated by the calculated potential mean forces (PMFs) using molecular dynamic (MD) simulations.

Ultrasonic study of water adsorbed in nanoporous glasses

Thermodynamic properties of fluids confined in nanopores differ from those observed in the bulk. To investigate the effect of nanoconfinement on water compressibility, we perform water sorption experiments on two nanoporous glass samples while concomitantly measuring the speed of longitudinal and shear ultrasonic waves in these samples. These measurements yield the longitudinal and shear moduli of the water-laden nanoporous glass as a function of relative humidity that we utilize in the Gassmann theory to infer the bulk modulus of the confined water. This analysis shows that the bulk modulus (inverse of compressibility) of confined water is noticeably higher than that of the bulk water at the same temperature. Moreover, the modulus exhibits a linear dependence on the Laplace pressure. The results for water, which is a polar fluid, agree with previous experimental and numerical data reported for nonpolar fluids. This similarity suggests that irrespective of intermolecular forces, confined fluids are stiffer than bulk fluids. Accounting for fluid stiffening in nanopores may be important for accurate interpretation of wave propagation measurements in fluid-filled nanoporous media, including in petrophysics, catalysis, and other applications, such as in porous materials characterization.

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

The role of metal ions in the behavior of bovine serum albumin molecules under physiological environment

Metal ions released from metallic implants can affect the conformation and structural stability of proteins in biological fluids, which eventually affects the biocompatibility of implants. The present study aimed at understanding the interactions between the metal ions (Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Cu²⁺, and Zn²⁺) and bovine serum albumin (BSA) molecules in physiological context. The structural information of BSA molecules and the microenvironment of functional groups were investigated using UV, Raman, and circular dichroism spectroscopy. The results revealed that addition of Fe³⁺, Fe²⁺, and Cu²⁺ ions alters the tertiary structure of BSA molecules and exposes the aromatic heterocyclic hydrophobic group of BSA amino acid residues. The addition of Fe³⁺ and Cu²⁺ ions results in increased viscosity and decreased intensity of the water peak in the BSA solution. Furthermore, Fe³⁺ and Cu²⁺ ions evidently promote the α-helix to β-sheet transformation of BSA molecules due to decreased disulfide bond stability. Tryptophan residues of BSA and metal ions containing BSA (Me⁺/BSA) solutions were found to be in a hydrophilic environment. Moreover, the addition of metal ions to BSA results in several of tyrosine residues acting as hydrogen-bond donors. Coomassie brilliant blue staining revealed that the addition of Cu²⁺ ions promotes the aggregation of BSA molecules. The findings of this study will be helpful for evaluating the biocompatibility of metallic implants.

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

X-ray scattering study of water confined in bioactive glasses: experimental and simulated pair distribution function

Temperature-dependent total X-ray scattering measurements for water confined in bioactive glass samples with 5.9 nm pore diameter have been performed. Based on these experimental data, simulations were carried out using the Empirical Potential Structure Refinement (EPSR) code, in order to study the structural organization of the confined water in detail. The results indicate a non-homogeneous structure for water inside the pore, with three different structural organizations of water, depending on the distance from the pore surface: (i) a first layer (4 Å) of interfacial pore water that forms a strong chemical bond with the substrate, (ii) intermediate pore water forming a second layer (4-11 Å) on top of the interfacial pore water, (iii) bulk-like pore water in the centre of the pores. Analysis of the simulated site-site partial pair distribution function shows that the water-silica (O_w-Si) pair correlations occur at ∼3.75 Å. The tetrahedral network of bulk water with oxygen-oxygen (O_w-O_w) hydrogen-bonded pair correlations at ∼2.8, ∼4.1 and ∼4.5 Å is strongly distorted for the interfacial pore water while the second neighbour pair correlations are observed at ∼4.0 and ∼4.9 Å. For the interfacial pore water, an additional O_w-O_w pair correlation appears at ∼3.3 Å, which is likely caused by distortions due to the interactions of the water molecules with the silica at the pore surface.

Water as a tuneable solvent: a perspective

Water is the most sustainable solvent, but its polarity limits the solubility of non-polar solutes. Confining water in hydrophobic nanopores could be a way to modulate water solvent properties and enable using water as tuneable solvent (WaTuSo).

Water properties under nano-scale confinement

Water is the universal solvent and plays a critical role in all known geological and biological processes. Confining water in nano-scale domains, as encountered in sedimentary rocks, in biological, and in engineered systems, leads to the deviations in water’s physicochemical properties relative to those measured for the non-confined phase. In our comprehensive analysis, we demonstrate that nano-scale confinement leads to the decrease in the melting/freezing point temperature, density, and surface tension of confined water. With increasing degree of spatial confinement the population of networked water, as evidenced by alterations in the O-H stretching modes, increases. These analyses were performed on two groups of mesoporous silica materials, which allows to separate pore size effects from surface chemistry effects. The observed systematic effects of nano-scale confinement on the physical properties of water are driven by alterations to water’s hydrogen-bonding network—influenced by water interactions with the silica surface — and has implications for how we understand the chemical and physical properties of liquids confined in porous materials.

Counterion-Regulated Dynamics of Water Confined in Lyotropic Liquid Crystalline Morphologies

The dynamics of confined water is of fundamental and long-standing interest. In technologically important forms of confinement, such as proton-exchange membranes, electrostatic interactions with the confining matrix and counterions play significant roles on the properties of water. There has been recent interest on the dynamics of water confined to the lyotropic liquid crystalline (LLC) morphologies of Gemini dicarboxylate surfactants. These systems are exciting because the nature of confinement, for example, size and curvature of channels and surface functionality is dictated by the chemistry of the self-assembling surfactant molecules. Quasielastic neutron scattering experiments have shown an interesting dependence of the water self-diffusion constant, D_α, on the identity (denoted α) of the counterion: at high hydration, the magnitude of the water self-diffusion constant is in the order D_TMA < D_Na < D_K, where TMA, Na, and K refer to tetramethyl ammonium, sodium, and potassium counterions, respectively. This sequence is similar to what is seen in bulk electrolyte solutions. At low hydrations, however, the order of water self-diffusion is different, that is, D_Na < D_TMA < D_K. In this work, we present molecular dynamics simulations for the dynamics of water in the LLC phases of dicarboxylate Gemini surfactants. The simulations reproduce the trends seen in experiments. From an analysis of the trajectories, we hypothesize that two competing factors play a role: the volume accessible to the water molecules and the correlations between the water and the counterion. The excluded volume effect is the largest with TMA⁺, and the electrostatic correlation is the strongest with Na⁺. The observed trend is a result of which of these two effects is dominant at a given water to surfactant ratio.

A neutron spin echo study of low-temperature water confined in the spherical silica pores of SBA-16

The dynamic properties of heavy water (D₂O) and light water (H₂O) confined in porous silica SBA-16 were studied over a temperature range of 210–290 K by neutron spin echo measurements.

Behavior of Bovine Serum Albumin Molecules in Molecular Crowding Environments Investigated by Raman Spectroscopy

The behavior of proteins in crowded environments is dominated by protein-crowder interactions (the entropic/excluded volume effect) and protein-protein interactions (the soft chemical effect). The details of these interactions, however, are not fully understood. In this study, the behavior of bovine serum albumin (BSA) in crowded environments, including high protein concentrations and in the presence of another protein, was investigated by Raman spectroscopy. A detailed analysis of the water, Tyr, and Phe Raman bands revealed that the excluded volume effect with an increase in the protein concentration changed the local environment of hydrophobic residues. In contrast, no specific changes to the secondary structure were observed from the analysis of the concentration dependence of the amide I band. BSA was experimentally shown to adopt a more compact state in the presence of the crowding agent. Moreover, H-D exchange experiments of the amide I band revealed that the intramolecular hydrogen bonds of BSA were strengthened in the presence of the protein crowder. Thus, the Raman spectroscopy results have revealed the molecular behavior of proteins in crowded environments by extracting information about the excluded volume effect, soft chemical interactions, and the hydration effect.

Positron annihilation and nuclear magnetic resonance study of the phase behavior of water confined in mesopores at different levels of hydration

We investigated the molecular origin of the phase behavior of water confined in MCM 41 mesopores at different levels of hydration using positron annihilation spectroscopic and nuclear magnetic resonance techniques. The level of hydration influenced the phase behavior of the nanoconfined water. Two transitions above and below the bulk freezing temperature were observed depending on the level of hydration. At the highest level of hydration, nucleation seemed to predominate over the effect of confinement, leading to the complete freezing of water, whereas disrupted H-bonding dominated at the lowest level of hydration, leading to the disappearance of the transitions. A transition at c. T = 188 K (close to the reported glass transition temperature of interface-affected water) was observed at intermediate hydration level. This study suggests that the H-bonding network within nanoconfined water, which can be tampered by the degree of hydration, is the key factor responsible for the phase behavior of supercooled water. This study on the phase behavior and associated transitions of nanoconfined water has implications for nanofluidics and drug-delivery systems, in addition to understanding the fundamentals of water in confinement.

Radical re-appraisal of water structure in hydrophilic confinement

The structure of water confined in MCM41 silica cylindrical pores is studied to determine whether confined water is simply a version of the bulk liquid which can be substantially supercooled without crystallisation. A combination of total neutron scattering from the porous silica, both wet and dry, and computer simulation using a realistic model of the scattering substrate is used. The water in the pore is divided into three regions: core, interfacial and overlap. The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density. There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K. The radical proposal is made here that water in hydrophilic confinement is under significant tension, around -100 MPa, inside the pore.

Protein crowding affects hydration structure and dynamics

The effect of protein crowding on the structure and dynamics of water was examined from explicit solvent molecular dynamics simulations of a series of protein G and protein G/villin systems at different protein concentrations. Hydration structure was analyzed in terms of radial distribution functions, three-dimensional hydration sites, and preservation of tetrahedral coordination. Analysis of hydration dynamics focused on self-diffusion rates and dielectric constants as a function of crowding. The results show significant changes in both structure and dynamics of water under highly crowded conditions. The structure of water is altered mostly beyond the first solvation shell. Diffusion rates and dielectric constants are significantly reduced following linear trends as a function of crowding reflecting highly constrained water in crowded environments. The reduced dynamics of diffusion is expected to be strongly related to hydrodynamic properties of crowded cellular environments while the reduced dielectric constant under crowded conditions has implications for the stability of biomolecules in crowded environments. The results from this study suggest a prescription for modeling solvation in simulations of cellular environments.

The effect of confinement on water structure

Neutron diffraction experiments with hydrogen isotope substitution on water confined in MCM41-S15 have been performed at temperatures of 300 and 210 K. Data are analyzed at a microscopic level using a revised version of the empirical potential structure refinement technique. It is found that the influence of the substrate on the water structure is not negligible and depends on the temperature: owing to the geometrical constraints and the symmetry breaking induced by the wall, comparison with the corresponding bulk phases is not straightforward and standard analysis should be replaced by a more suitable one.

Role of hydration and water coordination in micellization of Pluronic block copolymers

Raman, attenuated total reflectance FTIR, near-infrared spectroscopy, and DFT calculations have been used in a study of aqueous solutions of three tri-block copolymers poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) or PEO-PPO-PEO with commercial names Pluronic PE6200, PE6400 and F68. It is shown that the process of micellization as a response to increased temperature is reflected in the hydroxyl stretching region of infrared and Raman spectra, which contains information both about restructuring of water and changes of polymer chains in polymer/water aggregates. Raman spectra exhibit differences between individual Pluronics even at temperatures below the critical micellization temperature (CMT). According to the attenuated total reflection (ATR) FTIR spectra, the same five water coordination types defined by the number of donated/accepted hydrogen bonds are present in interacting water as in bulk water. It indicates that models considering mixed states of water with different hydrogen bonding environments provide appropriate descriptions of bound water both below and above the CMT. Above the CMT, aggregate hydration increases in the order PE6400 < PE6200 < F68, although that does not fully correspond to the EO/PO ratio, and points to the differences in microstructure of aggregates formed by each copolymer. This study relates nanoscale phenomena (hydrophobic and hydrophilic hydration) with the mesoscale phenomenon of micellization.

The impact of hydration water on the dynamics of side chains of hydrophobic peptides: from dry powder to highly concentrated solutions

Elastic and quasielastic neutron scattering experiments are used to investigate the dynamics of side chains in proteins, using hydrophobic peptides, from dry and hydrated powders up to solutions, as models. The changes of the internal dynamics of a prototypical hydrophobic amino acid, N-acetyl-leucine-methylamide, and alanine amino acids are investigated as a function of water/peptide molecular ratio. While previous results have shown that, in concentrated solution, when the hydrophobic side chains are hydrated by a single hydration water layer, the only allowed motions are confined and can be attributed to librational/rotational movements associated with the methyl groups. In the present work we observe a dynamical evolution from dry to highly hydrated powder. We also observe rotational and diffusive motions and a dynamical transition at approximately 250 K for long side chain peptides while for peptides with short side chains, there is no dynamical transition but only rotational motions. With a local measurement of the influence of hydration water dynamics on the amino acid side chains dynamics, we provide unique experimental evidence that the structural and dynamical properties of interfacial water strongly influence the side chain dynamics and the activation of diffusive motions. We also emphasize that the side chain length has a role on the onset of dynamical transition.

Singlet oxygen chemistry in water. 2. Photoexcited sensitizer quenching by O2 at the water-porous glass interface

Insight into the O2 quenching mechanism of a photosensitizer (static or dynamic) would be useful for the design of heterogeneous systems to control the mode of generation of 1O2 in water. Here, we describe the use of a photosensitizer, meso-tetra(N-methyl-4-pyridyl)porphine (1), which was adsorbed onto porous Vycor glass (PVG). A maximum loading of 1.1 x 10(-6) mol 1 per g PVG was achieved. Less than 1% of the PVG surface was covered with photosensitizer 1, and the penetration of 1 reaches a depth of 0.32 mm along all faces of the glass. Time-resolved measurements showed that the lifetime of triplet 1*-ads was 57 microseconds in water. Triplet O2 quenched the transient absorption of triplet 1*-ads; for samples containing 0.9 x 10(-6)-0.9 x 10(-8) mol 1 adsorbed per g PVG, the Stern-Volmer constant, K(D), ranged from 23,700 to 32,100 M(-1). The adduct formation constant, Ks, ranged from 1310 to 510 M(-1). The amplitude of the absorption at 470 nm decreased slightly (by about 0.1) with increased O2 concentrations. Thus, the quenching behavior of triplet 1*-ads by O2 was proposed to be strongly dependent on dynamic quenching. Only approximately 10% of the quenching was attributed to the static quenching mechanism. The quenching of triplet 1*-ads was similar to that observed for photosensitizers in homogeneous solution which are often quenched dynamically by O2.

Water/hydrocarbon interfaces: effect of hydrocarbon branching on single-molecule relaxation

Water/hydrocarbon interfaces are studied using molecular dynamics simulations in order to understand the effect of hydrocarbon branching on the dynamics of the system at and away from the interface. A recently proposed procedure for studying the intrinsic structure of the interface in such systems is utilized, and dynamics are probed in the usual laboratory frame as well as the intrinsic frame. The use of these two frames of reference leads to insight into the effect of capillary waves at the interface on dynamics. The systems were partitioned into zones with a width of 5 A, and a number of quantities of dynamical relevance, namely, the residence times, mean squared displacements, the velocity auto correlation functions, and orientational time correlations for molecules of both phases, were calculated in the laboratory and intrinsic frames at and away from the interface. For the aqueous phase, translational motion is found to be (a) diffusive at long times and not anomalous as in proteins or micelles, (b) faster at the interface than in the bulk, and (c) faster upon reduction of the effect of capillary waves. The rotational motion of water is (a) more anisotropic at the interface than in the bulk and (b) dependent on the orientation of the covalent O-H bond with respect to the plane of the interface. The effect of hydrocarbon branching on aqueous dynamics was found to be small, a result similar to the effect on the interfacial water structure. The hydrocarbon phase shows a larger variation for all dynamical probes, a trend consistent with their interfacial structure.