Adsorption-Desorption Flow of Condensable Vapors through Mesoporous Media: Network Modeling and Percolation Theory

Flow of condensable vapors in mesoporous media is investigated theoretically and experimentally during adsorption and desorption processes. A typical permeability curve of a condensable vapor is strongly enhanced in the capillary condensation region. This is because additional capillary pressure gradients are imposed on the capillary-condensed pores, which act as "good" conductors compared to the noncondensed pores, which are considered "poor" conductors. The percolation scaling properties that hold for a system of "good" and "poor" conductors are confirmed for the cases examined. As the ratio of gas flow/capillary-enhanced flow decreases, the rise of permeability with pressure becomes sharper. The network connectivity has a strong impact on the maximum permeability value and on the width of the scaling law regions. The contribution of surface flow does not affect the permeability in the peak region, but results in a shrinkage of the scaling law regions. During desorption, a marked hysteresis in the permeability curves is found and it is attributed only to thermodynamic hysteresis. The maximum permeability values in this case are higher and shifted to lower relative pressures. Copyright 2000 Academic Press.     

Separation of HT and HTO in the atmosphere with porous Vycor glass tube

The continuous selective monitoring of tritium gas (HT) in the air containing HT and tritiated water vapor (HTO) was investigated by using the separation cell made of porous Vycor glass tube. On admitting the air containing HT or HTO into the separation cell, HT permeated immediately through the Vycor tube depending on the partial pressure, however, HTO permeated very slowly through the Vycor tube after initial induction period. The initial induction period was elongated with a rise of the temperature of separation cell and then the permeability of HTO decreased remarkably. In the air containing HT and water vapor, the permeation of HT through the Vycor tube was considerably restricted by the water vapor adsorbed on the Vycor tube at lower temperature (at 25 degrees C) but it was hardly affected by water vapor at higher temperature (greater than 50 degrees C) since water vapor was difficult to adsorb on the Vycor tube. These results indicated that HT in the air containing HT and water vapor can be continuously monitored by using the separation cell made of the porous Vycor glass tube.     

Fluorescence spectra and chemical species of fluorescein molecules adsorbed on a calcinated porous Vycor glass

Properties of fluorescence and the excitation spectra of fluorescein molecules adsorbed onto a calcinated (773 K) porous Vycor glass have been investigated as a function of the amount of adsorbed dye (the surface coverage, θ = 0.00051 and 0.0098). The fluorescence and fluorescence-excitation spectra of fluorescein adsorbed onto Vycor glass showed the spectrum only due to cation species at θ = 0.00051. On the other hand, the spectra observed at θ = 0.0098 suggested the presence of cation, anion, and dianion species on the surface of Vycor glass. These results indicated the existence of at least two different types of adsorption sites involving the Brønsted acid site on the surfaces of Vycor glass.     

Adsorption of supercritical CO2 in aerogels as studied by small-angle neutron scattering and neutron transmission techniques

Small-angle neutron scattering (SANS) has been used to study the adsorption behavior of supercritical carbon dioxide (CO2) in porous Vycor glass and silica aerogels. Measurements were performed along two isotherms (T=35 and 80 degrees C) as a function of pressure (P) ranging from atmospheric up to 25 MPa, which corresponds to the bulk fluid densities ranging from rho(CO2) approximately 0 to 0.9 gcm3. The intensity of scattering from CO2-saturated Vycor porous glass can be described by a two-phase model which suggests that CO2 does not adsorb on the pore walls and fills the pore space uniformly. In CO2-saturated aerogels an adsorbed phase is formed with a density substantially higher that of the bulk fluid, and neutron transmission data were used to monitor the excess adsorption at different pressures. The results indicate that adsorption of CO2 is significantly stronger in aerogels than in activated carbons, zeolites, and xerogels due to the extremely high porosity and optimum pore size of these materials. SANS data revealed the existence of a compressed adsorbed phase with the average density approximately 1.07 gcm3, close to the density corresponding to closely packed van der Waals volume of CO2. A three-phase model [W. L. Wu, Polymer 23, 1907 (1982)] was used to estimate the volume fraction phi3 of the adsorbed phase as a function of the fluid density, and gave phi3 approximately 0.78 in the maximum adsorption regime around rho(CO2) approximately 0.374 gcm3. The results presented in this work demonstrate the utility of SANS combined with the transmission measurements to study the adsorption of supercritical fluids in porous materials.     

A polytetrafluoroethylene capillary viscometer

A polytetrafluoroethylene(PTFE) capillary Ubbelohde viscometer was designed and constructed. The relative viscosities of aqueous solutions of a polyethylene oxide and a polyvinylpyrrolidone sample were carefully determined down to an extremely dilute concentration region. In comparison with the data obtained from the common glass capillary viscometer, slippage is believed to occur in the PTFE capillary due to its hydrophobic nature. While for the glass capillary viscometer, conventional viscous flow is operative and adsorption phenomena occur since both the solvent water and aqueous solution are wet and/or adsorbed onto the glass capillary surface due to the existence of hydroxyl groups on glass surface. The data were analyzed with a recently developed wall-effect theory and satisfactory results were obtained.     

Experimental studies in relation to a new theoretical description of the permeation of dilute adsorbable gases through porous membranes

Precise data on the permeability of porous silica and alumina membranes to dilute gases are reported as a function of the nature of the gas and of temperature. It is shown that the unusual permeability behaviour previously observed only in “Vycor” porous glass at high temperatures [8-10] is a more general phenomenon. These results cannot be accounted for by conventional “surface diffusion” theory [1, 2] even qualitatively, but can be understood on the basis of recent, more advanced, theoretical treatments [3, 4, 7]. The present data provide an experimental test (not possible on the basis of previous data) of the general correlation between permeability and extent of sorption (including both the nature of the gas and temperature) predicted by the new theoretical approach, which is shown to be remarkably successful. Differences in the detailed permeability behaviour noted here, and in the previous porous glass study [8-10], are also satisfactorily accounted for in terms of differences in the mean effective pore size of the respective membranes.     

ELECTRON AND ENERGY TRANSFER IN ILLUMINATED POROUS VYCOR GLASS

Abstract— Porous Vycor glass samples containing adsorbed molecules were illuminated at 77 K by a mercury lamp jacketed by a filter cutting off wavelengths below 250 nm. Oxygen or carbon dioxide on Vycor produces an asymmetric electron paramagnetic resonance (EPR) signal best described as holes trapped in the glass. Methyl bromide produces an identical EPR signal plus four other lines due to methyl radicals. Evidence is presented that the products result from excitonic energy transfer from the Vycor to the adsorbed materials. Triphenylamine (TPA) adsorbed on Vycor can also be photoionized by similar illumination, and the cation radical TPA⁺ can be stabilized at 77 K if an electron acceptor is also adsorbed. Attachment of the photoejected electron by carbon dioxide forms CO₂^-, and that by methyl bromide leads to methyl radicals. The CH₃ radical yield is dependent on the surface separation between the electron donor (TPA) and the acceptor (CH₃Br). By monitoring the relative quantum yield of the methyl radicals as a function of distance separating the TPA and CH₃Br, it is shown that the photoelectron is capable of migrating on the Vycor glass surface.     

A Photochemical Approach to Integrated Optics

Highly resolved refractive index patterns or patterns of iron oxide are obtained by photolysis of (CH₃)₃SnI or Fe(CO)₅ absorbed onto Coming's code 7930 porous Vycor glass followed by consolidation to a nonporous glass. The photochemistries of the molecules on the glass surface, as well as the distribution and relative sizes of the photodeposited metal oxides, are described. Rutherford backscattering, small-angle x-ray scattering, and scanning electron microscopy show that the glass consolidates about the iron oxide particle but not about the tin oxide particle. Tin oxide chemically modifies the glass surface so that it does not consolidate at temperatures as high as 1200°C.     

Transport of organic vapors through poly(1-trimethylsilyl-1-propyne)

Poly(1-trimethylsilyl-1-propyne) [PTMSP], a high-free-volume glassy polymer, has the highest gas permeability of any known synthetic polymer. In contrast to conventional, low-free-volume, glassy polymers, PTMSP is more permeable to large, condensable organic vapors than to permanent gases. The organic-vapor/permanent-gas selectivity of PTMSP based on pure gas measurements is low. In organic-vapor/permanent-gas mixtures, however, the selectivity of PTMSP is much higher because the permeability of the permanent gas is reduced dramatically by the presence of the organic vapor. For example, in n-butane/methane mixtures, as little as 2 mol% n-butane (relative n-butane pressure 0.16) lowers the methane permeability 10-fold from the pure methane permeability. The result is that PTMSP shows a mixed-gas n-butane/methane selectivity of 30. This selectivity is the highest ever observed for this mixture and is completely unexpected for a glassy polymer. In addition, the gas mixture n-butane permeability of PTMSP is considerably higher than that of any known polymer, including polydimethylsiloxane, the most vapor-permeable rubber known. PTMSP also shows high mixed-gas selectivities and vapor permeabilities for the separation of chlorofluorocarbons from nitrogen. The unusual vapor permeation properties of PTMSP result from its very high free volume - more than 20% of the total volume of the material. The free volume elements appear to be connected, forming the equivalent of a finely microporous material. The large amount of condensable organic vapor sorbed into this finely porous structure causes partial blocking of the small free-volume elements, reducing the permeabilities of the noncondensable permanent gases from their pure gas values.     

Comparison of permeability coefficients of organic vapors through non-porous polymer membranes by two different experimental techniques

The permeability coefficients of saturated and non-saturated vapors of benzene, hexane and cyclohexane through flat polymer membranes (low density polyethylene BRALEN FB2-30 and polyether-block-amide PEBA 4033-PE) by two different experimental techniques at 298.15 K are reported. The permeation data have been obtained using the differential flow permeameter and sorption ones by glass sorption apparatus with McBain’s spiral balance. The so-called stationary (steady) diffusion theory has been applied for evaluating the permeability coefficients from sorption (equilibrium) data and obtained values have been compared with the permeability coefficients from permeation (steady-state) measurements. In the case of relative lower vapors sorption in polymers (hexane and cyclohexane) good agreement between permeability coefficients from sorption and permeation is obtained. Hence, this paper proves the possibility to estimate the permeability coefficients of organic vapors from sorption data without need of performing the permeation experiments.     

Adsorption hysteresis of nitrogen and argon in pore networks and characterization of novel micro- and mesoporous silicas

We report results of nitrogen and argon adsorption experiments performed at 77.4 and 87.3 K on novel micro/mesoporous silica materials with morphologically different networks of mesopores embedded into microporous matrixes: SE3030 silica with worm-like cylindrical channels of mode diameter of approximately 95 angstroms, KLE silica with cage-like spheroidal pores of ca. 140 angstroms, KLE/IL silica with spheroidal pores of approximately 140 angstroms connected by cylindrical channels of approximately 26 angstroms, and, also for a comparison, on Vycor glass with a disordered network of pores of mode diameter of approximately 70 angstroms. We show that the type of hysteresis loop formed by adsorption/desorption isotherms is determined by different mechanisms of condensation and evaporation and depends upon the shape and size of pores. We demonstrate that adsorption experiments performed with different adsorptives allow for detecting and separating the effects of pore blocking/percolation and cavitation in the course of evaporation. The results confirm that cavitation-controlled evaporation occurs in ink-bottle pores with the neck size smaller than a certain critical value. In this case, the pressure of evaporation does not depend upon the neck size. In pores with larger necks, percolation-controlled evaporation occurs, as observed for nitrogen (at 77.4 K) and argon (at 87.3 K) on porous Vycor glass. We elaborate a novel hybrid nonlocal density functional theory (NLDFT) method for calculations of pore size distributions from adsorption isotherms in the entire range of micro- and mesopores. The NLDFT method, applied to the adsorption branch of the isotherm, takes into account the effect of delayed capillary condensation in pores of different geometries. The pore size data obtained by the NLDFT method for SE3030, KLE, and KLE/IL silicas agree with the data of SANS/SAXS techniques.     

Temperature variations of average o-Ps lifetime in porous media

Modification of the Tao–Eldrup model is proposed in order to extend its usefulness to the case of porous media. The modification consists in the transition from spherical to capillary geometry and in inclusion of pick-off annihilation from the excited states of a particle in the well. Approximated equations for pick-off constant in these states are given. The model was tested by observing the temperature dependences of o-Ps lifetime in various media. In the case of silica gels and Vycor glass with narrow pores, the model seems to work well, while for larger pores in Vycor unexpectedly long lifetimes appear in the range of lowest temperatures.     

A network model for the permeability of condensable vapours through mesoporous media

The paper aims at introducing a discrete (network) approach to modelling transport of condensable vapours in mesoporous structures. Such models possess the potential for improving the understanding of the mechanisms responsible for the observed transport behaviour. The basic elements of a typical pore network representing a mesoporous medium are summarized and the current state concerning the simulation of the related phenomena is given. The main processes involved (adsorption, mass diffusion, surface flow, capillary condensation) are simulated over the entire range of relative pressure. Finally, the effects of material structural parameters (average pore radius, pore size distribution and standard deviation, pore connectivity) and other relevant factors (relative pressure, temperature, resistance to surface flow, total pressure drop) on vapour permeability are presented and the future research directions are pointed out.     

Spectral luminescent study of orientation states of dye molecules adsorbed on a nonuniform surface

Brownian reorientation motion of molecules results in an equilibrium of orientation states of adsorbed molecules. The character of the orientation equilibrium depends on the total concentration of adsorbed molecules. Since optical properties of molecules are sensitive to the intermolecular interaction with an adsorbent, each orientation state has an individual luminescence spectrum. A change in the equilibrium results in complicated concentration and temperature dependences of the spectra of adsorbed molecules. These dependences have been experimentally observed for several dyes adsorbed on microporous silicate glass. Equilibrium constants and fractal dimensions of the spatial distribution of dye molecules in porous matrices have been determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1284–1288, July, 1995.     

Isotherms of Capillary Condensation Influenced by Formation of Adsorption Films

Isotherms of capillary condensation are often used to determine the vapor sorption capacity of porous adsorbents as well as the pore size distribution by radii. In this paper, for calculating the volume of capillary condensate and of adsorption films in a porous body, an approach based on the theory of surface forces is used. Adsorption isotherms and disjoining pressure isotherms of wetting films are presented here in an exponential form discussed earlier. The calculations were made for straight cylindrical capillaries of different radii and slit pores of different width. The mechanisms of capillary condensation differ in cylindrical and slit pores. In cylindrical pores capillary condensation occurs due to capillary instability of curved wetting films on a capillary surface, when film thickness grows. In the case of slit pores, coalescence of wetting films formed on opposite slit surfaces proceeds under the action of attractive dispersion forces. Partial volumes of liquid in the state of both capillary condensate and adsorbed films are calculated dependent on the relative vapor pressure in a surrounding media. Copyright 2000 Academic Press.     

A grand canonical Monte Carlo study of capillary condensation in mesoporous media: effect of the pore morphology and topology

We study by means of Grand Canonical Monte Carlo simulations the condensation and evaporation of argon at 77 K in nanoporous silica media of different morphology or topology. For each porous material, our results are compared with data obtained for regular cylindrical pores. We show that both the filling and emptying mechanisms are significantly affected by the presence of a constriction. The simulation data for a constricted pore closed at one end reproduces the asymmetrical shape of the hysteresis loop that is observed for many real disordered porous materials. The adsorption process is a quasicontinuous mechanism that corresponds to the filling of the different parts of the porous material, cavity, and constriction. In contrast, the desorption branch for this pore closed at one end is brutal because the evaporation of Ar atoms confined in the largest cavity is triggered by the evaporation of the fluid confined in the constriction (which isolates the cavity from the gas reservoir). This evaporation process conforms to the classical picture of "pore blocking effect" proposed by Everett many years ago. We also simulate Ar adsorption in a disordered porous medium, which mimics a Vycor mesoporous silica glass. The adsorption isotherm for this disordered porous material having both topological and morphological defects presents the same features as that for the constricted pore (quasicontinuous adsorption and steep desorption process). However, the larger degree of disorder of the Vycor surface enhances these main characteristics. Finally, we show that the effect of the disorder, topological and/or morphological, leads to a significant lowering of the capillary condensation pressure compared to that for regular cylindrical nanopores. Also, our results suggest that confined fluids isolated from the bulk reservoir evaporate at a pressure driven by the smallest size of the pore.     

Correlation of isothermal and non-isothermal flow of dilute adsorbable gases in porous media

The general correlation has been established between isothermal and non-isothermal transport of a Knudsen gas through a porous medium where the resultant flux comprises several components. On this basis it is possible to decide between existing conflicting treatments [4, 6, 10] of non-isothermal flow of dilute adsorbable gases in porous media. With the aid of our recent treatment [1] for isothermal dilute gas flow, the basic predictions of the calibration gas approach for non-isothermal flow of moderately to strongly adsorbed gases have been confirmed, and a realistic interpretation of the behaviour of moderately to weakly adsorbed gases has been found.     

Ellipsometric determination of the structure of nanoscale porous layers

A new ellipsometric method is proposed to measure adsorption in thin layers directly in a stream of vapors of volatile liquid and inert gas at atmospheric pressure. The method enables the determination of the main structural parameters of nano- and microporous materials: the average pore size, pore surface, size pore distribution, and the total porosity of sorbents. A procedure to find the Young modulus in nanoscale porous layers with the use of a spectroellipsometer is described.