Fluctuations of the number of neighboring pores and appearance of multiple nonergodic states of a nonwetting liquid confined in a disordered nanoporous medium

Multiple nonergodic states have been observed for nonwetting liquid in the Fluka100 C18 and Fluka100 C8 porous media with broad pore size distributions having different widths. The dispersion transition where the volume of confined liquid depends critically on the degree of filling of the porous medium and temperature is observed in the temperature range 293–343 K under study for the Fluka100 C18 porous medium and is not observed for the Fluka100 C8 porous medium. A mechanism of the appearance of multiple nonergodic states has been proposed. It has been shown that fluctuations of the number of the nearest neighbors in the disordered system are mainly responsible for the features of confinement of nonwetting liquid and nonergodic states of nonwetting liquid in the nanoporous media under investigation with wide pore size distributions.     

Investigation of the percolation transition in a nonwetting liquid-nanoporous medium system

The flows of liquid into and out of a nanoporous medium are studied as processes leading to the fluctuation formation and the growth of fractal clusters of filled and empty pores, respectively. The conditions for stable growth of such fluctuations are analyzed as a function of the interfacial energy between the liquid and the porous medium and the surface energy of the liquid. Expressions are obtained for the pressure at which the barrier for fluctuation filling and emptying of the pores vanishes. In general, it is shown for porous media with a pore-size distribution that these processes can be interpreted as a percolation phase transition. The volume and susceptibility of a liquid-porous medium system near the transition points with inflow and outflow of the liquid are calculated. The phenomenon of nonoutflow of a nonwetting liquid from a porous medium and hysteresis of the flow of liquid into and out of a porous medium are explained on the basis of the mechanism considered. The results of an experimental investigation of these processes in the system liquid Wood’s alloy-silochrome 80 and silochrome 120 are presented. The experimental data obtained can be described on the basis of the proposed mechanism.     

Dispersion transition and the nonergodicity of the disordered nanoporous medium-nonwetting liquid system

The experiments in which a nonwetting liquid does not flow from a disordered nanoporous medium are described. The outflow is shown to depend on the degree of filling of the porous medium and its temperature in a critical manner. A physical mechanism is proposed where the transition of a system of liquid nanoclusters in a confinement into a metastable state in narrow filling and temperature ranges results from the appearance of a potential barrier due to the fluctuations of the collective “multiparticle” interaction of liquid nanoclusters in neighboring pores of different sizes at the shell of a percolation cluster of filled pores. The energy of a metastable state forms a potential relief with numerous maxima and minima in the space of a porous medium. The dispersed liquid volume in a metastable state is calculated with an analytical percolation theory for a ground state with an infinite percolation cluster. The outflow time distribution function of pores is calculated, and a power law is obtained for the decrease in nonwetting liquid volume retained in a porous medium with increasing time. The relaxation of the system under study is a multistage process accompanied by discontinuous equilibrium and overcoming of numerous local maxima of a potential relief. The formation of the metastable state of retained nonwetting liquid results from the nonergodicity properties of a disordered porous medium. The proposed model can describe the detected dependences of dispersed liquid volume on the degree of filling and temperature.     

Multiplicity of metastable nonergodic states of a dispersed nonwetting liquid in a disordered nanoporous medium

Three different metastable nonergodic states of a dispersed nonwetting liquid (water) in the Fluka 100 C8 and Fluka 100 C18 disordered porous media, as well as transitions between these states under variation of the temperature and the degree of filling, have been qualitatively described. It has been shown that the appearance of such states is due to spatial variations of the number of the nearest neighbors because of the broadening of the pore size distribution function f(R), fluctuations of various local configurations of neighbors in the system of pores, and fluctuations of a configuration of a pore and its environment consisting of filled and empty pores on a percolation cluster. These states and transitions are caused by the competition between the effective repulsion of the nonwetting liquid from the wall of the pore, which is responsible for the “extrusion” of the liquid from the pore, and the effective collective multiparticle attraction of the liquid cluster in the pore to clusters in the neighboring connected pores. The theoretical dependences obtained make it possible to qualitatively describe experimental data.     

Investigation of the dynamics of a percolation transition under rapid compression of a nanoporous body-nonwetting liquid system

The dynamics of infiltration of a nanoporous body with a nonwetting liquid under rapid compression is studied experimentally and theoretically. Experiments are carried out on systems formed by a hydrophobic nanoporous body Libersorb 23, water, and an aqueous solution of CaCl₂ at a compression rate of \(\dot p\) ≥ 10⁴ atm/s. It is found that the infiltration begins and occurs at a new constant pressure independent of the compression energy and viscosity of the liquid. The time of infiltration and the filled volume increase with the compression energy. A model of infiltration of a nanoporous body with a nonwetting liquid is constructed; using this model, infiltration is described as a spatially nonuniform process with the help of distribution functions for clusters formed by pores accessible to infiltration and filled ones. On the basis of the proposed system of kinetic equations for these distribution functions, it is shown that under rapid compression, the infiltration process must occur at a constant pressure p _c whose value is controlled by a new infiltration threshold θ _c = 0.28 for the fraction of accessible pores, which is higher than percolation threshold θ _c0 = 0.18. Quantity θ _c is a universal characteristic of porous bodies. In the range θ _c0 < θ < θ _c , infiltration of the porous body should not be observed. It is shown that the solution to the system of kinetic equations leads to a nonlinear response by the medium to an external action (rapid compression), which means the compensation of this action by percolation of the liquid from clusters of filled pores of finite size to an infinitely large cluster of accessible but unfilled pores. As a result of such compensation, infiltration is independent of the viscosity of the liquid. It is found that all experimental results can be described quantitatively in the proposed model.     

Correlation effects during liquid infiltration into hydrophobic nanoporous media

To explain the thermal effects observed during the infiltration of a nonwetting liquid into a disordered nanoporous medium, we have constructed a model that includes correlation effects in a disordered medium. It is based on analytical methods of the percolation theory. The infiltration of a porous medium is considered as the infiltration of pores in an infinite cluster of interconnected pores. Using the model of randomly situated spheres (RSS), we have been able to take into account the correlation effect of the spatial arrangement and connectivity of pores in the medium. The other correlation effect of the mutual arrangement of filled and empty pores on the shell of an infinite percolation cluster of filled pores determines the infiltration fluctuation probability. This probability has been calculated analytically. Allowance for these correlation effects during infiltration and defiltration makes it possible to suggest a physical mechanism of the contact angle hysteresis and to calculate the dependences of the contact angles on the degree of infiltration, porosity of the medium, and temperature. Based on the suggested model, we have managed to describe the temperature dependences of the infiltration and defiltration pressures and the thermal effects that accompany the absorption of energy by disordered porous medium-nonwetting liquid systems with various porosities in a unified way.     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

On the behaviour of supercooled liquids and polymers in nano-confinement

ABSTRACT

Size effects play an important role in structural phase transitions, melting transitions, in martensitic materials, glass transitions, etc. Very often the question arises, whether a measured size effect originates from the geometrical confinement itself, or if it appears due to the interaction with the limiting surface. Using dynamic mechanical analysis (DMA) technique we have studied various microphase segregated polymers, molecular glass forming liquids and supercooled water confined in nanoporous silica as well as in biological tissues. Here we show on some selected examples that DMA measurements can be used to study relaxation processes in detail and to disentangle in favourable cases pure pore size effects from effects that are induced by the confining surface.     

Observation of a dispersion transition and the stability of a liquid in a nanoporous medium

It has been found that the removal of overpressure is accompanied by a transition of some nonwetting-liquid nanoclusters to the stable state in narrow ranges of the filling factor and temperature. This means that the nonwetting liquid becomes ??wetting.??     

Percolation transition in an irreversible-surface reaction model

Monte Carlo simulation studies of percolation transition in a surface reaction model describing the oxidation of carbon mono-oxide on a catalytic surface are presented. The percolation transition for adsorbed oxygen atoms occurs below the poisoning transition where carbon mono-oxide completely covers the surface of the catalyst and takes place for an oxygen coverage of about 0.525 which is close to the percolation transition in an Ising lattice gas with nearest-neighbor attractive interactions. In several respects the oxygen clusters near the percolation threshold resemble those of the Ising lattice gas near its critical point.     

NMR magnetization transfer as a tool for characterization of nanoporous materials

The application of nuclear magnetic resonance magnetization transfer experiments to probe the surface-to-volume ratio and pore morphology of porous materials with characteristic pore sizes of 1-100 nm is described. The method is based on the phenomenon of incomplete freezing of liquids in small pores where a few monolayers adjacent to the pore walls remain liquid. Sufficient difference between the transverse relaxation times in the solid frozen core and liquid surface layer allows the initial preparation and subsequent re-equilibration of a solid-liquid magnetization grating. The method is demonstrated using model nanoporous materials with known characteristics. The ensuing problems of the mechanism of the magnetization transfer through the interface and within the frozen core are discussed and elucidated by pulsed-field-gradient NMR experiments.     

Inter- and intramolecular hydrogen bond in liquid polymers: a Fourier transform infrared response

Infrared (IR) absorption measurements are employed to investigate the vibrational spectra of propylene glycol (PG) and its oligomers the poly(propylene glycols) (PPGs) with different molecular weights (425 and 725 Da) in the OH stretching region. The goals are to identify the intramolecular, hydrogen bond imposed, sub-band and to connect the various sub-bands to different intermolecular aggregates originated by the existence of the hydrogen bond potential. The observed experimental data, compared with those of previous IR measurements on other similar polymers, ethylene glycol (EG) and poly(ethylene glycols) (PEGs), confirm the role played by the hydrogen bond in the structure of these liquids, discussed on the basis of current theories for associated systems.     

The instant-direct-templating synthesis of phenyl-functionalized HOM-type mesoporous silica materials

Two kinds of novel phenyl-functionalized mesoporous silica materials have fabricated for the first time by an instant-direct-templating method using poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) and tetraethyl orthosilicate as surfactant template and precursor, respectively. Samples were characterized by Fourier transform infrared spectroscopy, small-angle X-ray diffraction, thermogravimetric analysis, N₂ adsorption-desorption, scanning electron micrography and transmission electron micrography. The results show that phenyl groups are attached covalently to the pore wall of mesoporous materials after modification. The functionalized materials still preserve a desirable ordered hexagonal P6mm and cubic Ia3d mesophase structure, respectively, have high specific surface area, large pore volume and narrow pore size distribution.     

Single polymer molecules in a protein nanopore in the limit of a strong polymer-pore attraction

The capture and release of single poly(ethylene glycol) molecules by the alpha-Hemolysin pore are observed as time-resolved reversible steps in ion conductance. The capture on rate, inferred from the step frequency, decreases monotonically with polymer size. However, the polymer residence time shows a crossover behavior, first increasing and then decreasing with molecular weight. Our interpretation is that, in the case of polymers which are too large to be accommodated within the pore, the out-of-the-pore part of the molecule pulls on the trapped part, thus acting as an entropic spring.     

非水液体中由修正的刹管法所产生的瞬态单一空化气泡的发光研究

在除水之外的7种非水液体和甘油的水溶液中，用修正的刹管法产生了有效直径达厘米量级的较大的瞬态单一空化气泡，这些气泡在崩溃时几乎都产生不同强度的光辐射。在其中的6种液体中，其光辐射强度与该液体的饱和蒸汽压和粘度是相关的。但在乙二醇和丙二醇中的气泡发光并不遵循这个规律，两者中的气泡发光都很强，但乙二醇中的发光强度的变化显示出某种非通常的规律性。本文还对瞬态单一气泡与通常的稳态单一气泡的发光特性进行了比较。     

Ionization energies of liquids from energy distribution,quantum yield and second derivative curves

The energies of the lowest ionization band of eight liquids of low vapor pressure are determined from energy distribution curves (EDC), quantum yield spectra (collected electrons per incident photon as a function of photon energy), and second derivative curves (SDC) of retarding potential curves. Threshold energies from EDCs and quantum yield spectra agree if one takes into account a 0.15 e V shift caused by the spectrometer's rather low resolution and a small difference (0.15 eV or less) resulting from the use of approximate extrapolation methods. Threshold energies from EDCs and SDCs agree to within 0.1 eV after correction for the half-width of the high-energy branch of SDCs. Multiple ionization bands are exhibited by the SDCs of some of the liquids, and the observed splittings agree well with the results from gas-phase UPS spectra. A new spectrometer for the measurement of EDCs of liquids is described. The liquids studied are 6-chloro-1-hexanol, 2-ethyl-1-hexanol, ethylene cyanohydrin, ethylene glycol, 1,5-pentanediol, tetraglyme, triethylene glycol and tetraethylene glycol.     

Vibrational features of confined water in nanoporous TiO_2 by Raman spectra

Raman spectra of confined water adsorbed in nanoporous TiO_2 are obtained in experiment. TiO_2 samples with different pore diameters under different humidity conditions are investigated. The results indicate that the symmetric vibrational mode of water molecule is destroyed when relative humidity decreases. This indicates that the interaction between water molecules and surface of TiO_2 becomes stronger when the distance between water molecules and surface turns smaller, and the interaction plays a major role in depressing the symmetric vibrational peak. The spectra of confined water in TiO_2 and Vycor are compared. When filling fractions are the same, their spectra show distinctions no matter whether they are in partial filling condition or in full filling condition. The spectra of HDO confined in TiO_2 with different filling fractions are compared with each other. There is no clear distinction among their vibrational peaks, and the peaks mainly relate to asymmetric vibration. Therefore, the interaction between water molecules and the wall of pore decouples the symmetric vibrational mode only, and the influences on asymmetric vibrational mode show little differences among different filling fractions.     

Electrical and galvanomagnetic properties of nanoporous carbon samples impregnated with bromine

Nanoporous carbon samples with a large specific surface area can be filled with heavier elements or their compounds, which makes it possible to investigate the interaction of their electronic subsystems with carbon. One of the elements convenient for filling pores of carbon materials is bromine. Impregnation of nanoporous carbon samples with bromine causes the occurrence of the processes of micropore filling, monolayer adsorption, and intercalation. It has been found that samples impregnated with bromine substantially change their electrical and galvanomagnetic properties, and these changes depend on the structure of the samples. It has been shown that, if in the skeleton of a porous carbon sample there is a fraction of graphite clusters, the impregnation of the sample with bromine increases the concentration of charged carriers (holes). But when the sample has a quasi-amorphous structure, the injection of bromine into the sample leads to the appearance of a certain concentration of electrons in addition to charged mobile holes of the initial sample; i.e., the electrical conductivity becomes bipolar. In the former case, bromine molecules intercalate graphite clusters and, since bromine is an acceptor during intercalation of graphite, the hole concentration in the carbon skeleton network increases. In the latter case, bromine molecules can only be adsorbed on pore walls. As a result, the adsorption interaction between the electron shells of bromine molecules and the carbon surface leads to the formation of a donor layer near the surface and to the generation of electrons in the carbon skeleton network.     

Cluster self-organization of nanotubes in a nematic phase: The percolation behavior and appearance of optical singularities

The structural features, as well as the optical and electrophysical properties of a 5CB nematic liquid crystal with additions of multilayer carbon nanotubes, have been investigated in the concentration range C = 0.0025–0.1 wt %. The self-aggregation of nanotubes into clusters with a fractal structure occurs in the liquid crystal. At 0.025 wt %, the clusters are merged, initiating the percolation transition of the composite to a state with a high electric conductivity. The strong interaction of 5CB molecules with the surface of nanotube clusters is responsible for the formation of micron surface liquid crystal layers with an irregular field of elastic stresses and a complex structure of birefringence. They are easily observed in a polarization microscope and visualize directly invisible submicron nanotube aggregates. Their transverse size increases when an electric field is applied to the liquid crystal cell. Two mechanisms of the generation of optical singularities in the passing laser beam have been revealed. Optical vortices appear in the speckle fields of laser radiation scattered at the indented boundaries of the nanotube clusters, whereas the birefringence of the beam in surface liquid-crystal layers is accompanied by the appearance of polarization C points.