Effect Of Capillary Condensation Research Articles

Recovery of Homogeneous Polyoxometallate Catalysts from Aqueous and Organic Media by a Mesoporous Ceramic Membrane without Loss of Catalytic Activity

The recovery of homogeneous polyoxometallate (POM) oxidation catalysts from aqueous and non-aqueous media by a nanofiltration process using mesoporous gamma-alumina membranes is reported. The recovery of Q(12)[WZn(3)(ZnW(9)O(34))(2)] (Q=[MeN(n-C(8)H(17))(3)](+)) from toluene-based media was quantitative within experimental error, while up to 97 % of Na(12)[WZn(3)(ZnW(9)O(34))(2)] could be recovered from water. The toluene-soluble POM catalyst was used repeatedly in the conversion of cyclooctene to cyclooctene oxide and separated from the product mixture after each reaction. The catalytic activity increased steadily with the number of times that the catalyst had been recycled, which was attributed to partial removal of the excess QCl that is known to have a negative influence on the catalytic activity. Differences in the permeability of the membrane for different liquid media can be attributed to viscosity differences and/or capillary condensation effects. The influence of membrane pore radius on permeability and recovery is discussed.

Synthesis and characterization of MCM-48 tubular membranes

MCM-48 membranes have been prepared on alumina supports of different pore sizes. A battery of characterization techniques has been used to study the physical properties and the quality of the membranes prepared. The highest quality membranes were prepared on supports with pore size of up to 60 nm. The MCM-48 membranes were tested in the separation of gas phase mixtures and a cyclohexane/O 2 selectivity higher than 270 was obtained. The selective separation of organic compounds from inert components is a result of the cooperative effects of capillary condensation in MCM-48 pores and of the specific interactions of the permeating compounds and the membrane material.

Chemical reaction equilibrium in nanoporous materials: NO dimerization reaction in carbon slit nanopores

We present a molecular-level simulation study of the effects of confinement on chemical reaction equilibrium in nanoporous materials. We use the reaction ensemble Monte Carlo (RxMC) method to investigate the effects of temperature, nanopore size, bulk pressure, and capillary condensation on the nitric oxide dimerization reaction in a model carbon slit nanopore in equilibrium with a bulk reservoir. In addition to the RxMC simulations, we also utilize the molecular-dynamics method to determine self-diffusion coefficients for confined nonreactive mixtures of nitric oxide monomers and dimers at compositions obtained from the RxMC simulations. We analyze the effects of the temperature, nanopore width, bulk pressure, and capillary condensation on the reaction equilibrium with respect to the reaction conversion, fluid structure, and self-diffusion coefficients. We show that the influence of the temperature, nanopore size, and capillary condensation on the confined reaction equilibrium is quite dramatic while the effect of the bulk pressure on the reaction equilibrium in the carbon slit nanopore is only moderate. This work is an extension of previous work by Turner et al. [J. Chem. Phys. 114, 1851 (2001)] on the confined reactive nitric oxide system.

Flux decline during ultrafiltration of protein-fatty acid mixtures

Proteins frequently co-exist with fatty acids and have specific binding interactions, forming complexes which can affect membrane performance. While specific and non-specific binding to proteins occurs at lower concentrations of fatty acid, at higher concentrations, excess fatty acids can produce additional interactions by adsorption in the protein cake or in the membrane pores. The effect of the fatty acid on ultrafiltration is multifold. The fatty acids can behave as a hydrophobic solute at low pH and a charged highly soluble solute at high pH. Using caprylic acid and bovine serum albumin (BSA) as a model fatty acid and protein, this study examines the flux decline caused by protein-fatty acid mixtures in the pH range of 3–10 and the potential effect of capillary condensation of fatty acids on membranes with proteins present as co-foulants. The flux and retention effects of increasing excess of fatty acids were particularly examined with respect to potential removal of residual fatty acids in the presence of proteins for food or environmental applications. Experiments on flux were carried out by diafiltration of solutions containing BSA and caprylic acid, using a 30 kDa polyethersulphone membrane at 50 kPa. BSA-fatty acid binding parameters were evaluated in bulk and compared with fatty acid flux and retention through BSA pre-fouled membranes. The fatty acid retention by the protein deposit exceeded specific and non-specific binding sites by several orders of magnitude, thus supporting a capillary condensation mechanism. Despite the high retention of fatty acid, the flux decline did not become significant until at high fatty acid concentrations relative to the saturation, suggesting that condensation occurred initially in small and dead end pores which did not carry the bulk of the flow. The possible effect of the protein conformation on this process is also discussed.

External Capillary Condensation and Adsorption of VOCs onto Activated Carbon Fiber Cloth and Felt

Adsorption isotherms of some volatile organic compounds (VOCs), present in air at concentrations ranging from 10 to 2500 mg m−3, were performed onto activated carbon fiber cloth and felt. The VOCs loading air were isopropanol, toluene, acetone, methyl ethyl ketone, ethyl acetate and dichloromethane. Isotherm adsorption data were generated. These experimental values were modeled by the equation of Dubinin and co-workers and by the Langmuir relation. Model parameters are discussed as a function of adsorbent materials and VOC structure. Adsorption capacity was generally found higher for the cloth than for the felt. An effect of external capillary condensation onto the felt of compounds with high surface tension was demonstrated for VOC concentrations above 1500 mg m−3.

Effects of capillary condensation on the caking of bulk sucrose

Caking in sucrose is a major problem for the sugar industry, especially during transportation. Liquid bridging in sucrose is the first step towards caking. This paper demonstrates, by measuring the strength of the liquid bridges and then the strength of the subsequently formed solid bridges under different water activity conditions that the critical water activity for initiation of caking is about 0.8. The identification of this point allows the conditions under which bulk sucrose is stored and transported to be specified to prevent the occurrence of water activities going above this critical level to eliminate caking.

Determination of Contact Angles on Microporous Particles Using the Thin-Layer Wicking Technique

The properties of particle-stabilized emulsions, especially with regard to phase inversion, are very dependent on the contact angle that the particles experience at the oil-water interface. For the very small particles used for such emulsions (often a few tens of nm), it is impossible to measure this contact angle directly. Its value could be calculated if it were possible to determine the components of the solid surface free energy. To establish a method suitable for such particles, we have investigated the imbibition of five probe liquids into a porous bed of silica (commercial TLC plates) using the thin-layer wicking technique. For all liquids, the difference between wicking rate for bare plates and for those pre-contacted with the vapors is large but it is not due to an advancing angle effect on bare plates. Our analysis shows that it is due to the diversion of flowing liquid into blind pores which are already filled in the pre-contacted case. Thus a new model is proposed describing wicking in a porous medium with very small blind pores by introducing a parameter into the Washburn equation that corrects for this capillary condensation effect. The parameter needed is determined independently using gravimetric adsorption measurements. When this modified Washburn equation is used, the difference between advancing and receding contact angle is actually quite small. When the averages are used as the Young's contact angles, values for the surface energy components of silica are obtained that are completely consistent between the five liquids and have magnitudes expected for this type of silica surface.

Photoluminescence quenching of porous silicon in gas and liquid phases – the role of dielectric quenching and capillary condensation effects

We present a systematic study of porous silicon photoluminescence quenching in the presence of precisely controlled amounts of linear aliphatic alcohols (from methanol to hexanol) in gas and liquid phases. From the concentration dependence of photoluminescence quenching response we determined sensitivity of porous silicon sensor for studied analytes. The sensor sensitivity revealed nearly monotonous change with the length of alcohol molecule within the homological set of alcohols in both gas and liquid phases. However, while in gas phase the sensor sensitivity rose with the length of alcohol chain, in liquid phase we observed the opposite behaviour. Photoluminescence quenching behaviour in liquid phase is very well explained by exciton dielectric quenching mechanism. In gas phase photoluminescence quenching depends both on analyte dielectric constant and analyte equilibrium concentration inside porous matrix which is controlled by capillary condensation effect.

Applying ACF to Desulfurization Process from Flue Gas

Inasmuch as the status of environmental pollution caused by SO2 is more and more serious and the policy of environmental protection is executed more and more strictly, desulfurization from flue gas (FGD) is introduced to a wide-spread field of national economy. By a comparison with lime-limestone method, the application of adsorption method in FGD is more effective in desulfurization and more adapted to the situation of our country in respect of its more valuable byproduct. However, the technique of adsorption method is limited by the large amount of adsorbent used. In this paper, activated carbon fiber (ACF) is proposed as a new type of adsorbent to apply in FGD. A series of experiments have been made in order to compare the performances between ACF and granular activated carbon (GAC) which has been mostly used. Experiments show that under the same working conditions ACF's adsorption capacity is 16.6 times as high as that of GAC, mass loss rate is 1/12 of GAC's, desorption efficiency of ACF can reach 99.9%. The theory of micropore adsorption dynamics is adopted to analyze the characteristics of both adsorbents. It is indicated that adsorbability and perfectibility of desorption are tightly related to the distribution of pores and the surface micromechanism of adsorbent surface. The accessibility of pores for specified adsorptive and the effects of capillary condensation are crucial factors to influence the process of FGD. According to the research of different adsorbents, conclusion can be drawn that ACF is a kind of good material with a strong selectivity for SO2. Compared with the traditional methods of FGD, the use of ACF can greatly economize the consumption of adsorbent and obviously reduce the introduction of new adsorbent, and at the same time keep down the equipment investment and operating cost.

Effects of capillary condensation on adsorption and thermal desorption dynamics of water in zeolite 13X and layered beds

The effects of capillary condensation on the adsorption and thermal desorption dynamics of water in zeolite 13X beds and layered beds with zeolite 13X/silica gel or zeolite 13X/alumina were experimentally and theoretically studied. As the equilibrium isotherm of water on zeolite 13X pellet was found to be most favorable at a low relative humidity and indicated capillary condensation at a high relative humidity, it was possible to construct a non-isothermal model that included capillary condensation and that could successfully predict plateaus of temperature and concentration profiles in thermal regeneration. In adsorption breakthrough, by using a feed in the capillary condensation range of the isotherm on zeolite 13X, the breakthrough curve showed a shock wave in the low concentration and a proportionate pattern in the high concentration. In thermal desorption breakthrough, the desorbed water at the upper part of the bed was re-adsorbed at the lower part of the bed, and that re-adsorption mainly occurred in the capillary condensation range of the isotherm. Therefore, even though an adsorption was performed at a feed in the favorable range of the isotherm, and could be well predicted with type I isotherm, its desorption dynamics should be predicted by using the isotherm model with its consideration of capillary condensation. The layered bed with silica gel or alumina did not have any advantage over the zeolite 13X bed with respect to adsorption breakthrough performance. However, compared to the zeolite 13X bed, the complete regeneration time in the layered bed was drastically shortened due to a greater variation of the amount of equilibrium adsorption of water under temperature on both silica gel and alumina. In addition, since an increase in temperature led to a greater decrease of the amount of equilibrium adsorption of water on silica gel than on alumina, a layered bed with silica gel obviously could be regenerated more efficiently than a layered bed with alumina.

Mechanisms of photoluminescence sensor response of porous silicon for organic species in gas and liquid phases

Photoluminescence response of porous silicon in presence of single organic analytes scales within the linear dynamic range with concentration of detected species. We present systematic study of changes in porous silicon photoluminescence intensity in the presence of precisely controlled amounts of linear aliphatic alcohols (from methanol to n-hexanol) in gas and liquid phases. From the concentration dependence of photoluminescence quenching we determined sensitivity of porous silicon sensor response. The sensor response sensitivity revealed nearly monotonous change within the homological set of n-alcohols in both gas and liquid phases. However, while in gas phase the sensitivity of sensor response rose with the length of alcohol chain, in liquid phase we observed the opposite behavior. The mechanism of sensor response in gas and liquid phases is explained by photoluminescence dielectric quenching. The strength of photoluminescence quenching is directly determined by dielectric constant and concentration of analyte in liquid phase whereas in gas phase it primarily depends on effective concentration of analyte inside porous silicon matrix. The thermodynamic equilibrium concentration of analyte inside porous silicon matrix is controlled by capillary condensation effect. A very good correlation between gas phase concentration and room temperature saturated vapor pressure of studied analytes and porous silicon photoluminescence quenching response was obtained.

Interaction of aerosol particles composed of protein and saltswith water vapor: hygroscopic growth and microstructural rearrangement

Abstract. The interaction of aerosol particles composed of the protein bovine serum albumin (BSA) and the inorganic salts sodium chloride and ammonium nitrate with water vapor has been investigated by hygroscopicity tandem differential mobility analyzer (H-TDMA) experiments complemented by transmission electron microscopy (TEM) and Köhler theory calculations (100-300nm particle size range, 298K, 960hPa). BSA was chosen as a well-defined model substance for proteins and other macromolecular compounds, which constitute a large fraction of the water-soluble organic component of air particulate matter. Pure BSA particles exhibited deliquescence and efflorescence transitions at 35% relative humidity () and a hygroscopic diameter increase by up to 10% at 95% in good agreement with model calculations based on a simple parameterisation of the osmotic coefficient. Pure NaCl particles were converted from near-cubic to near-spherical shape upon interaction with water vapor at relative humidities below the deliquescence threshold (partial surface dissolution and recrystallisation), and the diameters of pure NH4NO3 particles decreased by up to 10% due to chemical decomposition and evaporation. Mixed NaCl-BSA and NH4NO3-BSA particles interacting with water vapor exhibited mobility equivalent diameter reductions of up to 20%, depending on particle generation, conditioning, size, and chemical composition (BSA dry mass fraction 10-90%). These observations can be explained by formation of porous agglomerates (envelope void fractions up to 50%) due to ion-protein interactions and electric charge effects on the one hand, and by compaction of the agglomerate structure due to capillary condensation effects on the other. The size of NH4NO3-BSA particles was apparently also influenced by volatilisation of NH4NO3, but not as much as for pure salt particles, i.e. the protein inhibited the decomposition of NH4NO3 or the evaporation of the decomposition products NH3 and HNO3. The efflorescence threshold of NaCl-BSA particles decreased with increasing BSA dry mass fraction, i.e. the protein inhibited the formation of salt crystals and enhanced the stability of supersaturated solution droplets. The H-TDMA and TEM results indicate that the protein was enriched at the surface of the mixed particles and formed an envelope, which inhibits the access of water vapor to the particle core and leads to kinetic limitations of hygroscopic growth, phase transitions, and microstructural rearrangement processes. The Köhler theory calculations performed with different types of models demonstrate that the hygroscopic growth of particles composed of inorganic salts and proteins can be efficiently described with a simple volume additivity approach, provided that the correct dry solute mass equivalent diameter and composition are known. A parameterisation for the osmotic coefficient of macromolecular substances has been derived from an osmotic pressure virial equation. For its application only the density and molar mass of the substance have to be known or estimated, and it is fully compatible with traditional volume additivity models for salt mixtures.

Influence of capillary condensation effects on mass transport through porous membranes

The importance of capillary condensation effects in connection with properly quantifying the rate of mass transport through porous media has been already described (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259; J. Membr. Sci. 182 (2001) 91). However, there is still not enough work done in order to understand, quantify and possibly exploit this phenomenon. In this work permeation and binary diffusion experiments have been performed at ambient temperature in combination with measurements of the adsorption isotherms using porous Vycor glass possessing a mean pore radius of approximately 4 nm. Main focus was set on a comparison of the transport of (a) permanent gases (helium, nitrogen, methane and hydrogen) and (b) condensable gases (butane, propane and freon 112) as a function of the pressure conditions in the membrane. The adsorption isotherms of the condensable gases were measured using a volumetric method. The results indicate the occurrence of capillary condensation at a reduced pressure P/P0 of approximately 0.8 for all three gases. There is a good agreement between the volume of condensate at saturation and the pore volume determined in independent porosimetric studies. Results of a large set of experiments with butane revealed that the permeation is only a function of the mean gas pressure in the membrane. This contradicts the available models suggested in (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259) predicting that the transport mechanism in pores changes with different pressure conditions. Finally, in binary isobaric diffusion experiments with pure helium on one side of the membrane and pure butane on the other, a remarkable increase in selectivity of the transport process with increasing pressure in the membrane was observed.

Numerical study of multilayer adsorption on fractal surfaces.

We report a numerical study of van der Waals adsorption and capillary condensation effects on self-similar fractal surfaces. An assembly of uncoupled spherical pores with a power-law distribution of radii is used to model fractal surfaces with adjustable dimensions. We find that the commonly used fractal Frankel-Halsey-Hill equation systematically fails to give the correct dimension due to crossover effects, consistent with the findings of recent experiments. The effects of pore coupling and curvature dependent surface tension were also studied.

Influence of ceramic support on pervaporation characteristics of IPA/water mixtures using PDMS/ceramic composite membrane

The pervaporation of IPA(isopropanol)–water mixture was carried out by using PDMS (poly(dimethylsiloxane))/ceramic composite membrane. In the pervaporation using homogeneous PDMS membrane without ceramic support, selectivities of IPA were inversely proportional to operating temperature. However, selectivities of IPA in the pervaporation using PDMS/ceramic composite membrane increased with operating temperature in the range from 0 to 20 wt% of IPA in feed mixture. In this composition range, the effect of capillary condensation of water in ceramic pores on selectivities is more dominant than that of plasticization of PDMS by IPA. In more than 20 wt% IPA in feed mixtures the plasticization of PDMS by IPA in feed is dominant, thereby the selectivities of IPA is inversely proportional to the operating temperature. These coupling effects containing capillary condensation and plasticizing effect were investigated by measuring permeation ratio of each component. In the case of PDMS/ceramic composite membrane, fluctuation of permeation ratio with operating temperature was observed around 0–10% of IPA in feed mixture. It is explained by the blockage in ceramic pore due to capillary condensation of water. No severe fluctuation was observed at any feed concentration range by using homogeneous PDMS membrane without ceramic support.

Synchrotron X-ray Diffraction Studies of Multilayer Nitrogen Physisorbed on Porous Graphite Foam

We use synchrotron X-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. We provide structural information and concentrations of two- and three-dimensional solid phases at a coverage of ϑ = 8 ML (ϑ ≡ 1 ML for a √3 × √3 monolayer structure) for temperatures from below the bulk α−β transition temperature [Tα-β = 34 ± 0.5 K] to above the bulk triple point [Ttp = 63 K]. Our data indicate that layering begins near Tα-β, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film layers by 48 K at ϑ = 8 mL. Our results are consistent with ellipsometry studies of nitrogen on highly oriented pyrolytic graphite which found multilayer nitrogen on single-crystal graphite to undergo a series of layering transitions above the bulk nitrogen α−β structural transition. [Volkmann, U. G.; Knorr, K. Phys. Rev. Lett. 1991, 66, 473.] The effect of adsorption on a graphite foam substrate, which results in capillary condensation and finite size effects, is limited to a broadening and overlap of the discrete transition temperatures observed on a single-crystal substrate. A phase diagram for coverages above 2 ML is proposed, summarizing this and previous work.

HYSTERESIS AND PHASE TRANSITIONS IN A SINGLE PARTIALLY INTERNALLY WETTED CATALYST PELLET: THERMOGRAVIMETRIC STUDIES

The hydrogenation of cyclohexene over palladium supported in a microporous γ-alumina pellet is studied thermogravimetrically with a view to measuring the extent of partial internal wetting associated with the different steady state branches. As many as three steady state branches having significantly different degrees of internal wetting and reaction rates, with transitions between them, are confirmed from observations of catalyst weight change. It is seen that with reduction in catalyst activity the middle branch, obtained by condensation from a vapor filled pellet, is much more prominent without showing an evaporative transition for the range of hydrogen partial pressures used here. The catalyst activity is therefore an important parameter affecting the structure of the steady state branches. Hysteresis effects are found to occur, and the thermogravimetric results also confirm the importance of history in determining the catalyst state. The measured degree of wetting is in accordance with that estimated from a mathematical model incorporating capillary condensation effects in addition to reaction-diffusion phenomena. The same model also satisfactorily interprets the reaction rate variations and transitions seen in the present work.

Auto-adhesion of carbon fibres

This study deals primarily with the experimental measurement and analysis of the auto-adhesion which occurs between two contacting fibres in various gas and vapour phase environments. An experimental technique has been developed for measuring this adhesion which involves studying the auto-adhesive contact formed between two orthogonal monofilaments or ‘fibres’. One fibre is constrained at both ends, while the other (the so-called cantilever fibre) is held at one end only and used as a forcesensitive transducer. A static gravimetric method was employed to obtain adsorption isotherms of decane vapour on the carbon fibres at ambient temperature in vacuum. The specific surface area of the fibres was calculated from the method of Brunauer, Emmet and Teller. The measured forces of adhesion and surface energies are reported for the contact between two carbon fibres (about 7 μm in diameter) under the influence of various conditions which include air, vacuum and in the presence of condensing vapours such as decane and methanol. The measured forces required to separate the fibres are generally affected when the surfaces are exposed in such environments. The trends in the data are rationalized in terms of two effects: surface free energy modification through vapour adsorption on the fibre surfaces and capillary condensation effects which occur at the contact junction.

The effect of capillary condensation and liquid bridging on the bonding of hydrophobic particles in shear-flocculation

A mechanism of bonding two spherical particles suspended in a solution containing surfactant is presented. In the mechanism, the adhesion between particles which develop hydrophobic surfaces after adsorbing the surfactant is achieved by the action of capillary condensation of the surfactant in solution at the point of contact, and by the subsequent bridging of the condensed surfactant. The mechanism is consistent with experimental results obtained in shear-flocculation for the scheelite-oleate system. From the Kelvin equation, an order-of-magnitude estimate of the radius of curvature of the bond is made. The area of contact, the bond strength, and the bond energy are also estimated.

Carbon black porosity. I. Interparticle capillary condensation in pelletized carbon blacks

Comparison of nitrogen adsorption isotherms for fluffy and pelletized blacks revealed the presence of interparticle capillary condensation in commercially pelletized samples. The extent of capillary condensation in such samples is a function of particle size, structure level, and the amount of pelletizing work to which the sample has been subjected. Grinding of pellets in a high-speed coffee mill did not eliminate capillary condensation effects. Porosity values for fluffy blacks, calculated using the Lippens t-values, lie in the 28–32% range; values for the pelletized blacks range from 0–26%, depending upon the aforementioned factors. The results indicate that the majority of prior work on carbon black porosity measurement is of doubtful validity.