Hydrophobic Nanopores Research Articles

低次元カーボン細孔中の水・水和構造の解明

Water has unique properties in the nanopores of activated carbons and carbon nanotubes, which are apparently different from those of typical adsorbed molecules such as N2 and Ar. Here the mechanism of water vapor adsorption in carbon nanopores and the adsorbed structures are reviewed using adsorption isotherms, X-ray scattering, and molecular simulations. Water cluster formation in hydrophobic carbon nanopores promoted self-adsorption of water by gaining stability. Water stabilization is a result of anomalously strong hydrogen bonding between water molecules in such carbon nanopores. On the other hand, in the case of an aqueous electrolyte solution, the formation of a hydration shell around ions is dominant in carbon nanopores. These findings promote our understanding of water nanoscience and nanotechnology as well as electrochemical nanoscience.

Confinement of Water in Hydrophobic Nanopores: Effect of the Geometry on the Energy of Intrusion

Water confinement in the hydrophobic nanopores of highly siliceous zeolite having MFI and CHA topology is investigated by high pressure manometry coupled to differential calorimetry. Surprisingly, the intrusion of water is endothermic for MFI but exothermic for CHA. This phase transition depends on the geometry of the environment in which water is confined: channels (MFI) or cavities (CHA). The energy of intrusion is mainly governed by the change in the coordination of water molecules when they are forced to enter the nanopores and to adopt a weaker, hydrogen-bonded structure. At such a nanoscale, the properties of the molecules are governed strongly by geometrical restraints. This implies that the use of classical macroscopic equations such as Laplace-Washburn will have limitations at the molecular level.

Enhanced Ionic Transport Mechanism by Gramicidin A Confined Inside Nanopores Tuned by Atomic Layer Deposition

The confinement and the understanding of ion transport through ionic channels when they are confined inside solid-state nanopores smaller than 10 nm remains a challenge. Here we report on the fabrication of biomimetic nanopores with high length (5 μm)/diameter (smaller than 10 nm) ratio obtained using both a track-etched technique and atomic layer deposition on flexible membranes. These membranes present uniform hydrophobic nanopores with a low roughness inside the pores. Gramicidin A is then confined inside nanopores (diameter 10.6, 5.7, and ∼2 nm) leading to the NaCl ionic transport mechanism through a hybrid nanopore similar to the biological ones especially for small diameter (5.7 and ∼2 nm).

Confinement of tert-Butanol Nanoclusters in Hydrophilic and Hydrophobic Silica Nanopores

From hydrogen bonds, tert-butanol molecules self-organize in micelle-like supermolecular clusters in the liquid state. While these nanoclusters have been largely investigated in the bulk phase, the confined situation remains seldom. However, from a relevant combination of neutron scattering and molecular simulation, it has been shown that these clusters persisted under hydrophilic cylindrical confinement [Ghoufi, A.; Hureau, I.; Lefort, R.; Morineau, D. J. Phys. Chem. C, 2011, 115, 17761]. In this work, we provide a structural, dynamic, and dielectric characterization of nanoconfined tert-butanol in both hydrophilic and hydrophobic silica nanopores. Differences between hydrophilic and hydrophobic pores with respect to the bulk phase are correlated to the modulation in the hydrogen-bonding network induced by the nanometric confinement and the chemical nature of the solid surface.

Molecular Simulation of CO2 Adsorption in the Presence of Water in Single-Walled Carbon Nanotubes

The adsorption of carbon dioxide in the presence of water in single-walled carbon nanotubes is studied using Monte Carlo simulation, at 300, 325, and 350 K. We also investigate the influence of the diameter and chirality of the nanotubes on the adsorption isotherms of CO2. It is observed that increasing the nanotube diameter from 1.36 nm (10, 10) to 2.03 nm (15, 15) leads to enhanced CO2 capacity, while change in chirality has little effect on the adsorption capacity of carbon nanotubes. Our results show that the influence of preadsorbed water on CO2 adsorption is dependent on both the effects of excluded volume and H2O–CO2 interactions. The maximum adsorbed amount of CO2 decreases linearly with the loading of water, and drops more rapidly in narrower nanotubes. The structure of water in hydrophobic nanopores is in the form of hydrogen-bonded clusters, and its adsorption does not affect the arrangement and orientation of CO2 molecules (i.e., it does not affect the mechanism of CO2 adsorption). The average size of water clusters coexisting with CO2 depends strongly on the adsorbed amount of CO2; however, it is shown that splitting large water clusters into smaller ones can lead to significant enhancement of CO2 adsorption, due to the resulting stronger water–CO2 interaction. The maximum percentage increase in the excess adsorption of CO2 is as high as 53.4% when a single cluster is split into multiple smaller clusters. This finding demonstrates that the efficiency of CO2 capture from flue gas can be significantly improved by controlling the structure of coexisting water in carbon nanotubes.

Voltage-Gating of Ions and Water in Hydrophobic Nanopores

The behavior of water in nanopores is very different from bulk water and strongly depends on the chemical characteristics of the pore walls. If the pore walls are lined with hydrophobic groups the density of water was predicted to be significantly lower than in the bulk and if a pore is sufficiently narrow, the water can even evaporate stopping any transport through the membrane. We provided an experimental evidence for the nano-induced water evaporation and condensation by chemically modifying polymer nanopores with decylamines. Hydrophobic properties of the pore walls were confirmed by contact angle measurements of a polymer film subjected to the same modification as the nanopores. The hydrophobic pores are closed for water and ionic transport unless a sufficiently high transmembrane potential is applied. The off state of the pore corresponds to the pore filled with water vapor. The pores conduct ion current only if the water undergoes condensation. The switching between open and closed was found fully reversible over many up to 20 cycles. For the intermediate voltages the pore would fluctuate between two conducting states. We also found that hydrophobic interactions tune rectifying properties of the pores. Understanding hydrophobic properties in nano-confinement is important in the light of recent findings of the role of hydrophobic interactions in the function of voltage-gated channels. Application of hydrophobic-gating in building drug-delivery systems was investigated with multi-pore membranes.

Preparation and photocatalytic activity of porous spherical TiO2 particles comprised of H3PW12O40 in hydrophobic nanopores

Porous TiO2 sphere particles were prepared by impregnating titanium (IV) oxysulfate solutions into organic monolith particles, with subsequent calcination in air. The powder possessed pores of meso-order. Its crystalline phase was anatase. Then, 12 Tungsto(VI) phosphoric acid (PW12) was present in the mesopores. The entire surface was modified to be hydrophobic by octadecylphosphonic acid (ODP). Subsequent UV illumination in water removed the PW12 and ODP adsorbed onto the outer sphere surface. The obtained spherical TiO2 particles with hydrophobic acidic nanopores exhibited high photocatalytic performance against the removal of 1,4-dioxane in water. Detailed analysis revealed that this material removed both 1,4-dioxane and ethylene glycol diformate (EGDF), the main intermediate of the photocatalysis of 1,4-dioxane. Results suggest that hydrophobic acidic nanopores enhance adsorption of EGDF in water and play an important role in the overall photocatalytic performance of this system.

Activated drying in hydrophobic nanopores and the line tension of water

We study the slow dynamics of water evaporation out of hydrophobic cavities by using model porous silica materials grafted with octylsilanes. The cylindrical pores are monodisperse, with a radius in the range of 1-2 nm. Liquid water penetrates in the nanopores at high pressure and empties the pores when the pressure is lowered. The drying pressure exhibits a logarithmic growth as a function of the driving rate over more than three decades, showing the thermally activated nucleation of vapor bubbles. We find that the slow dynamics and the critical volume of the vapor nucleus are quantitatively described by the classical theory of capillarity without adjustable parameter. However, classical capillarity utterly overestimates the critical bubble energy. We discuss the possible influence of surface heterogeneities, long-range interactions, and high-curvature effects, and we show that a classical theory can describe vapor nucleation provided that a negative line tension is taken into account. The drying pressure then provides a determination of this line tension with much higher precision than currently available methods. We find consistent values of the order of -30 pN in a variety of hydrophobic materials.

Modeling methyl methacrylate free radical polymerization: Reaction in hydrophilic nanopores

In previous work, we developed a simplified model for the diffusion controlled bulk polymerization of methyl methacrylate and extended the model to capture the reaction under nanoconfinement. The calorimetric conversion versus time data in bulk and in silanized hydrophobic nanopores was well captured by the model. Here we further extend the model to capture the reaction in native hydrophilic controlled pore glass (CPG) nanopores accounting for catalysis by surface silanol groups. The ability of the model to describe experimental data is tested. In order to fit the data, the parameters describing monomer and active chain diffusion differ from that in hydrophobic pores.

Modeling methyl methacrylate free radical polymerization: Reaction in hydrophobic nanopores

Free radical polymerization of methyl methacrylate in nanopores has been shown to result in a decrease in the time for the onset of autoacceleration. In this work, we simplify our previous kinetic model of nanoconfined methyl methacrylate polymerization, which was based on the work of Verros and coworkers, and incorporate diffusion effects into the model using the Doolittle free volume theory. The simplified model well describes the experimental calorimetric conversion versus time data for isothermal bulk methyl methacrylate polymerization, capturing autoacceleration and the dependence of the limiting conversion on temperature. In order to model the reaction in nanopores, we assume that the diffusion coefficient scales with molecular size to the −3 power and with nanopore diameter to the 1.3 power. Experimental calorimetric conversion versus time data for polymerization in hydrophobic nanopores are well captured by the model, including the decrease in the time to reach autoacceleration with decreasing pore size. The scaling assumed is consistent with that predicted using molecular simulations for good solvent conditions by Avramova and Milchev and by Cui, Ding, and Chen. According to the fit of the experimental data, chain diffusivity is 20–50% of the bulk value in 13 nm-diameter pores.

Liquid-Ice Coexistence below the Melting Temperature for Water Confined in Hydrophilic and Hydrophobic Nanopores

We use molecular dynamics simulations to investigate the coexistence between confined ice and liquid water as a function of temperature for a series of cylindrical nanopores with water–wall interactions ranging from strongly hydrophilic to very hydrophobic. In agreement with previous results from experiments, we find that the ice formed in the nanopores is a hybrid ice I with stacks of cubic and hexagonal layers and that the melting temperature of the nanoconfined ice is strongly dependent on the radius of the pore but rather insensitive to the hydrophilicity of the pore surface. We find a premelted liquid layer in coexistence with the confined ice down to the lowest temperatures of this study, 50 K below the melting temperatures of the confined ices. The fraction of water in the premelted liquid layer decreases with increasing hydrophobicity of the pore wall, but it does not vanish even for the most hydrophobic nanopores. The simulations suggest that the decrease in the fraction of water in the liquid layer with increasing hydrophobicity corresponds partly to a decrease in its width but also to a decrease in its effective density: the premelted liquid layer becomes sparser on decreasing the water–wall attraction. Our results indicate that agreement in the melting temperatures of water nanopores functionalized with different moieties does not imply identical fraction of nonfreezable water in these materials.

Hydrophobic Gating in Single Synthetic Nanopores

In nature, nanopores play a critical role in a number of vital biological functions. These pores can be ion selective based on their size and/or surface charge, but further functionality is achieved by modulating, or gating, their conductance state. The conductivity of a particular nanochannel can be controlled in a number of ways, including mechanically, chemically and electrically. By studying these phenomena in model systems, we may be able to create abiotic analogues of these biostructures with similar transport properties. Here, we present what we believe is first study to show the spontaneous opening and closing of a synthetic hydrophobic nanopore with no leakage current in the closed state. Hydrophobic gating has been observed before in biological channels.Single conically shaped nanopores in polyethylene terephthalate prepared by the track-etching technique were functionalized with hydrophobic groups either throughout the pore or on the outside of the tip region. These hydrophobic pores would be closed at low voltages below ∼1V but would open up for ionic transport when a transmembrane potential was increased above a threshold value. At the threshold voltages the pores would fluctuate between conducting and non-conducting states, which we attributed to reversible wetting and dewetting of the pores. Prior to the hydrophobic modification, aqueous electrolyte solutions were able to conduct readily through the structures for all voltages. Another feature of nanopores with a local hydrophobic layer is a strong dependence of their gating behavior on pH. At pH 8 the pore would typically conduct the current, but at pH 3 the pore was either closed or rectifying in the opposite direction. Transport properties of hydrophobic nanopores also provide information on hydrophobic interactions at the nanoscale.

Self-assembling subnanometer pores with unusual mass-transport properties

A long-standing aim in molecular self-assembly is the development of synthetic nanopores capable of mimicking the mass-transport characteristics of biological channels and pores. Here we report a strategy for enforcing the nanotubular assembly of rigid macrocycles in both the solid state and solution based on the interplay of multiple hydrogen-bonding and aromatic π-π stacking interactions. The resultant nanotubes have modifiable surfaces and inner pores of a uniform diameter defined by the constituent macrocycles. The self-assembling hydrophobic nanopores can mediate not only highly selective transmembrane ion transport, unprecedented for a synthetic nanopore, but also highly efficient transmembrane water permeability. These results establish a solid foundation for developing synthetically accessible, robust nanostructured systems with broad applications such as reconstituted mimicry of defined functions solely achieved by biological nanostructures, molecular sensing, and the fabrication of porous materials required for water purification and molecular separations.

Electric-field-induced wetting and dewetting in single hydrophobic nanopores

The behaviour of water in nanopores is very different from that of bulk water. Close to hydrophobic surfaces, the water density has been found to be lower than in the bulk, and if confined in a sufficiently narrow hydrophobic nanopore, water can spontaneously evaporate. Molecular dynamics simulations have suggested that a nanopore can be switched between dry and wet states by applying an electric potential across the nanopore membrane. Nanopores with hydrophobic walls could therefore create a gate system for water, and also for ionic and neutral species. Here, we show that single hydrophobic nanopores can undergo reversible wetting and dewetting due to condensation and evaporation of water inside the pores. The reversible process is observed as fluctuations between conducting and non-conducting ionic states and can be regulated by a transmembrane electric potential.

Melting and Crystallization of Ice in Partially Filled Nanopores

We investigate the melting and formation of ice in partially filled hydrophilic and hydrophobic nanopores of 3 nm diameter using molecular dynamics simulations with the mW water model. Above the melting temperature, the partially filled nanopores contain two water phases in coexistence: a condensed liquid plug and a surface-adsorbed phase. It has been long debated in the literature whether the surface-adsorbed phase is involved in the crystallization. We find that only the liquid plug crystallizes on cooling, producing ice I with stacks of hexagonal and cubic layers. The confined ice is wetted by a premelted liquid layer that persists in equilibrium with ice down to temperatures well below its melting point. The liquid-ice transition is first-order-like but rounded. We determine the temperature and enthalpy of melting as a function of the filling fraction of the pore. In agreement with experiments, we find that the melting temperature of the nanoconfined ice is strongly depressed with respect to the bulk T(m), it depends weakly on the filling fraction and is insensitive to the hydrophobicity of the pore wall. The state of water in the crystallized hydrophilic and hydrophobic pores, however, is not the same: the hydrophobic pore has a negligible density of the surface-adsorbed phase and higher fraction of water in the ice phase than the hydrophilic pore. The widths of the ice cores are nevertheless comparable for the hydrophobic and hydrophilic pores, and this may explain their almost identical melting temperatures. The enthalpy of melting ΔH(m), when normalized by the actual amount of ice in the pore, is indistinguishable for the hydrophobic and hydrophilic pores, insensitive to the filling fraction, and within the error bars, the same as the difference in enthalpy between bulk liquid and bulk ice evaluated at the temperature of melting of ice in the nanopores.

Voltage-Gated Hydrophobic Nanopores

Hydrophobicity is a fundamental property that is responsible for numerous physical and biophysical aspects of molecular interactions in water. Peculiar behavior is expected for water in the vicinity of hydrophobic structures, such as nanopores. Indeed, hydrophobic nanopores can be found in two distinct states, dry and wet, even though the latter is thermodynamically unstable. Transitions between these two states are kinetically hindered in long pores but can be much faster in shorter pores. As it is demonstrated for the first time in this paper, these transitions can be induced by applying a voltage across a membrane with a single hydrophobic nanopore. Such voltage-induced gating in single nanopores can be realized in a reversible manner through electrowetting of inner walls of the nanopores. The resulting I-V curves of such artificial hydrophobic nanopores mimic biological voltage-gated channels.

Photolithographic fabrication of solid–liquid core waveguides by thiol-ene chemistry

In this work we demonstrate an efficient and cleanroom compatible method for the fabrication of solid–liquid core waveguides based on nanoporous polymers. We have used thiol-ene photo-grafting to tune and pattern the hydrophilicity of an originally hydrophobic nanoporous 1, 2-polybutadiene. The generated refractive index contrast between the patterned water-filled volume and the surrounding empty hydrophobic porous polymer allows for light confinement within the water-filled volume—the solid–liquid core. The presented fabrication process is simple and fast. It allows a high degree of flexibility on the type and grade of surface chemistry imparted to the large nanoporous area depending upon the application. The fabrication does not need demanding chemical reaction conditions. Thus, it can be readily used on a standard silicon lithography bench. The propagation loss values reported in this work are comparable with literature values for state-of-the-art liquid-core waveguide devices. The demonstrated waveguide function added to the nanoporous polymer with a very high internal surface area makes the system interesting for many applications in different areas, such as diagnostics and bio-chemical sensing.

Effect of Electric Field on Liquid Infiltration into Hydrophobic Nanopores

Understanding the variation of nanofluidic behavior in the presence of an external electric field is critical for controlling and designing nanofluidic devices. By studying the critical infiltration pressure of liquids into hydrophobic nanopores using molecular dynamics (MD) simulations and experiments, important insights can be gained on the variation of the effective liquid-solid interfacial tension with the magnitude and sign of electric field, as well as its coupling with the pore size and the solid and liquid species. It is found that the effective hydrophobicity reduces with the increase of electric intensity and/or pore size, and the behavior is asymmetric with respect to the direction of the electric field. The underlying molecular mechanisms are revealed via the study of the density profile, contact angle, and surface tension of confined liquid molecules.

Urea-Induced Drying of Hydrophobic Nanotubes: Comparison of Different Urea Models

In a previous study, we performed the molecular dynamics (MD) simulations of various carbon nanotubes solvated in 8 M urea and observed a striking phenomenon of urea-induced drying of hydrophobic nanotubes, which resulted from the stronger dispersion interaction of urea than water with nanotube (Das, P.; Zhou, R. H. J. Phys. Chem. B2010, 114, 5427-5430). In this paper, we have compared five different urea models to investigate if the above phenomenon is sensitive to the urea models used. We demonstrate through MD simulations that the drying phenomenon and its physical mechanism are qualitatively independent of the urea models. Consistent with our previous study, our current analyses with both interaction potential energy and association free energy indicate that there is a "dry state" inside the carbon nanotubes, which is caused by the urea's preferential binding to nanotubes through stronger dispersion interactions. These results also have implications for understanding the urea-induced protein denaturation by providing further evidence of the potential existence of a "dry globule"-like transient state during protein unfolding and the "direct interaction mechanism" (whereby urea attacks protein directly, rather than disrupts water structure as a "water breaker"). In addition, our study highlights the crucial role of dispersion interaction in the selective absorption of molecules in hydrophobic nanopores and may have significance for nanoscience and nanotechnology.

State-resolved THz spectroscopy and dynamics of crystalline peptide–water systems

Vibrationally state-resolved THz spectra are obtained at cryogenic temperatures for three crystalline peptide-water systems that represent different structural motifs. The systems include two types of secondary structures and a hydrophobic peptide nanopore structure. Almost all of these systems are shown to undergo exchange with water at room temperature that alters the hydrogen bonding network in ways easily detectable in the THz region at cryogenic temperatures. Stark differences are observed in the spectra of model alpha-helical and beta-sheet structures upon water removal at hydrophilic binding sites. However, within the confined pore of a hydrophobic nanotube, water in the form of helical wires has a subtle but significant impact on the phonon modes of the tube. The THz spectra are shown to easily distinguish between the different hydration states of the system that have been independently characterized by mass change measurements. Spectral comparisons with quantum chemical predictions of fully relaxed crystal structures confirm the hydration states and give detailed information about the free energies associated with dehydration. The vibrational free energies are shown to make significant contributions to the overall energy balance of the dehydration processes.