Improvement of the Derjaguin-Broekhoff-de Boer theory for capillary condensation/evaporation of nitrogen in mesoporous systems and its implications for pore size analysis of MCM-41 silicas and related materials

In this work, we propose an improvement of the classical Derjaguin-Broekhoff-de Boer (DBdB) theory for capillary condensation/evaporation in mesoporous systems. The primary idea of this improvement is to employ the Gibbs-Tolman-Koenig-Buff equation to predict the surface tension changes in mesopores. In addition, the statistical film thickness (so-called t-curve) evaluated accurately on the basis of the adsorption isotherms measured for the MCM-41 materials is used instead of the originally proposed t-curve (to take into account the excess of the chemical potential due to the surface forces). It is shown that the aforementioned modifications of the original DBdB theory have significant implications for the pore size analysis of mesoporous solids. To verify our improvement of the DBdB pore size analysis method (IDBdB), a series of the calcined MCM-41 samples, which are well-defined materials with hexagonally ordered cylindrical mesopores, were used for the evaluation of the pore size distributions. The correlation of the IDBdB method with the empirically calibrated Kruk-Jaroniec-Sayari (KJS) relationship is very good in the range of small mesopores. So, a major advantage of the IDBdB method is its applicability for small mesopores as well as for the mesopore range beyond that established by the KJS calibration, i.e., for mesopore radii greater than similar to4.5 nm. The comparison of the IDBdB results with experimental data reported by Kruk and Jaroniec for capillary condensation/evaporation as well as with the results from nonlocal density functional theory developed by Neimark et al. clearly justifies our approach. Note that the proposed improvement of the classical DBdB method preserves its original simplicity and simultaneously ensures a significant improvement of the pore size analysis, which is confirmed by the independent estimation of the mean pore size by the powder X-ray diffraction method.

Equivalence of the Peleg, Pilosof and Singh-Kulshrestha models for water absorption in food

The similarity between the Peleg, Pilosof –Boquet–Batholomai and Singh–Kulshrestha models was investigated using the hydration behaviours of whey protein concentrate, wheat starch and whey protein isolate at 30 °C in 100% relative humidity. The three models were shown to be mathematically the same within experimental variations, and they yielded parameters that are related. The models, in their linear and original forms, were suitable (r2 > 0.98) in describing the sorption behaviours of the samples, and are sensitive to the length of the sorption segment used in the computation. The whey proteins absorbed more moisture than the wheat starch, and the isolate exhibited a higher sorptive ability than the concentrate.

Estimation of water retention curve from mercury intrusion porosimetry and van Genuchten model

The water retention curve (WRC) is a hydraulic characteristic of concrete required for advanced modeling of water (and thus solute) transport in variably saturated, heterogeneous concrete. Unfortunately, determination by a direct experimental method (for example, measuring equilibrium moisture levels of large samples stored in constant humidity cells) is a lengthy process, taking over 2 years for large samples. A surrogate approach is presented in which the WRC is conveniently estimated from mercury intrusion porosimetry (MIP) and validated by water sorption isotherms: The well-known Barrett, Joyner and Halenda (BJH) method of estimating the pore size distribution (PSD) from the water sorption isotherm is shown to complement the PSD derived from conventional MIP. This provides a basis for predicting the complete WRC from MIP data alone. The van Genuchten equation is used to model the combined water sorption and MIP results. It is a convenient tool for describing water retention characteristics over the full moisture content range. The van Genuchten parameter estimation based solely on MIP is shown to give a satisfactory approximation to the WRC, with a simple restriction on one. of the parameters.

Pore size distribution analysis of activated carbons: Application of density functional theory using nongraphitized carbon black as a reference system

The application of nonlocal density functional theory (NLDFT) to determine pore size distribution (PSD) of activated carbons using a nongraphitized carbon black, instead of graphitized thermal carbon black, as a reference system is explored. We show that in this case nitrogen and argon adsorption isotherms in activated carbons are precisely correlated by the theory, and such an excellent correlation would never be possible if the pore wall surface was assumed to be identical to that of graphitized carbon black. It suggests that pore wall surfaces of activated carbon are closer to that of amorphous solids because of defects of crystalline lattice, finite pore length, and the presence of active centers.. etc. Application of the NLDFT adapted to amorphous solids resulted in quantitative description of N-2 and Ar adsorption isotherms on nongraphitized carbon black BP280 at their respective boiling points. In the present paper we determined solid-fluid potentials from experimental adsorption isotherms on nongraphitized carbon black and subsequently used those potentials to model adsorption in slit pores and generate a corresponding set of local isotherms, which we used to determine the PSD functions of different activated carbons. (c) 2005 Elsevier Ltd. All rights reserved.

Applications of the cyclone stickiness test for characterization of stickiness in food powders

The cyclone stickiness test (CST) technique was applied to measure the stickiness temperature and relative humidity of whey, honey, and apple juice powders. A moisture sorption isotherm study was conducted to analyze the surface moisture content of whey powder. The glass transition temperatures of the sample powder were analyzed using differential scanning calorimetry (DSC). The stickiness results of these products were found within 20 degrees C above their surface glass transition temperatures, which is well within the normal temperature range for glass transition in general. The results obtained by the CST technique were found consistent with DSC values.

Sorption and desorption behaviour of acetochlor in surface, subsurface and size-fractionated soil

Pesticides leaching through a soil profile will be exposed to changing environmental sorption and desorption conditions as different horizons with distinct physical and chemical properties are encountered. Soil cores were taken from a clay soil profile and samples taken from 0.0 to 0.3 m (surface), 1.0-1.3 m (mid) and 2.7-3.0 m (deep) and treated with the chloroacetanilide herbicide, acetochlor. Freundlich isotherms revealed that sorption and desorption behaviour varied with each depth sampled. As soil depth increased, the extent and strength of sorption decreased, indicating that the potential for leaching was increased in the subsoils compared with the surface soil. Hysteresis was evident at each of the three depths sampled, although no significant correlations between soil properties and the hysteresis coefficients were evident. Desorption studies using soil fractions with diameters of > 2000, 250-2000, 53-250, 20-53, 2-20, 0-2 and 0-1 mum separated from each of the three soil depths showed that differential desorption kinetics occurred and that the retention of acetochlor significantly correlated (R-2 = 0.998) with organic matter content. A greater understanding of the influence of soil components on the overall sorption and desorption potential of surface and subsurface soils is required to allow accurate prediction of acetochlor retention in the soil. In addition, it is likely that the proportion of each size fraction in a soil horizon would influence acetochlor bioavailability and movement to groundwater.

Non-additivity of attractive potentials in modeling of N-2 and Ar adsorption isotherms on graphitized carbon black and porous carbon by means of density functional theory

We present a new approach accounting for the nonadditivity of attractive parts of solid-fluid and fluidfluid potentials to improve the quality of the description of nitrogen and argon adsorption isotherms on graphitized carbon black in the framework of non-local density functional theory. We show that the strong solid-fluid interaction in the first monolayer decreases the fluid-fluid interaction, which prevents the twodimensional phase transition to occur. This results in smoother isotherm, which agrees much better with experimental data. In the region of multi-layer coverage the conventional non-local density functional theory and grand canonical Monte Carlo simulations are known to over-predict the amount adsorbed against experimental isotherms. Accounting for the non-additivity factor decreases the solid-fluid interaction with the increase of intermolecular interactions in the dense adsorbed fluid, preventing the over-prediction of loading in the region of multi-layer adsorption. Such an improvement of the non-local density functional theory allows us to describe experimental nitrogen and argon isotherms on carbon black quite accurately with mean error of 2.5 to 5.8% instead of 17 to 26% in the conventional technique. With this approach, the local isotherms of model pores can be derived, and consequently a more reliab * le pore size distribution can be obtained. We illustrate this by applying our theory against nitrogen and argon isotherms on a number of activated carbons. The fitting between our model and the data is much better than the conventional NLDFT, suggesting the more reliable PSD obtained with our approach.

Application of density functional theory to equilibrium adsorption of argon and nitrogen on amorphous silica surface

We present a new version of non-local density functional theory (NL-DFT) adapted to description of vapor adsorption isotherms on amorphous materials like non-porous silica. The novel feature of this approach is that it accounts for the roughness of adsorbent surface. The solid–fluid interaction is described in the same framework as in the case of fluid–fluid interactions, using the Weeks–Chandler–Andersen (WCA) scheme and the Carnahan–Starling (CS) equation for attractive and repulsive parts of the Helmholtz free energy, respectively. Application to nitrogen and argon adsorption isotherms on non-porous silica LiChrospher Si-1000 at their boiling points, recently published by Jaroniec and co-workers, has shown an excellent correlative ability of our approach over the complete range of pressures, which suggests that the surface roughness is mostly the reason for the observed behavior of adsorption isotherms. From the analysis of these data, we found that in the case of nitrogen adsorption short-range interactions between oxygen atoms on the silica surface and quadrupole of nitrogen molecules play an important role. The approach presented in this paper may be further used in quantitative analysis of adsorption and desorption isotherms in cylindrical pores such as MCM-41 and carbon nanotubes.

Do decomposing organic matter residues reduce phosphorus sorption in highly weathered soils?

Many studies have shown a reduction in P sorption in highly weathered soils when organic matter (OM) is applied, suggesting competition between OM decomposition products and P for sorption sites. However, such studies seldom consider the P released from the added OM. To delineate the effects of OM addition on P availability through sorption competition and P addition, water leachate from incubated soybean (SB) [Glycine mar (L.) Merr.] and Rhodes grass (RG) (Chloris gayana Knuth cv. Callide) was used in competitive P sorption studies both undiluted and after acidification (i.e., the fulvic acid [FA] component). Addition of two rates (0.2 and 2 mL) of SB leachate to an Oxisol significantly increased P sorption at the higher rate, while a similar trend was observed following RG leachate addition at the same rates. Extending the range of highly weathered soils examined (two Oxisols, an Ultisol, and an acidic Vertisol) resulted in no observed decrease in P sorption following addition of OM leachate. Surprisingly, SB leachate transiently increased P sorption in the two Oxisol soils. Addition of the FA component of the leachates resulted in a transient (< 6 d) decrease in P sorption in three of the four soils examined and constituted the only evidence in this study that decomposing OM residues reduced P sorption. This research provides further evidence contradicting the long held assumption that inhibition of P sorption by dissolved organic compounds, derived from decomposing OM, is responsible for increased P phytoavailability when P fertilizer and OM are applied together.

Competitive sorption reactions between phosphorus and organic matter in soil: a review

The incorporation of organic matter ( OM) in soils that are able to rapidly sorb applied phosphorus ( P) fertiliser reportedly increases P availability to plants. This effect has commonly been ascribed to competition between the decomposition products of OM and P for soil sorption sites resulting in increased soil solution P concentrations. The evidence for competitive inhibition of P sorption by dissolved organic carbon compounds, derived from the breakdown of OM, includes studies on the competition between P and (i) low molecular weight organic acids (LOAs), (ii) humic and fulvic acids, and (iii) OM leachates in soils with a high P sorption capacity. These studies, however, have often used LOAs at 1 - 100 mM, concentrations much higher than those in soils ( generally < 0.05 mM). The transience of LOAs in biologically active soils further suggests that neither their concentration nor their persistence would have a practical benefit in increasing P phytoavailability. Higher molecular weight compounds such as humic and fulvic acids also competitively inhibit P sorption; however, little consideration has been given to the potential of these compounds to increase the amount of P sorbed through metal - chelate linkages. We suggest that the magnitude of the inhibition of P sorption by the decomposition products of OM leachate is negligible at rates equivalent to those of OM applied in the field. Incubation of OM in soil has also commonly been reported as reducing P sorption in soil. However, we consider that the reported decreases in P sorption ( as measured by P in the soil solution) are not related to competition from the decomposition products of OM breakdown, but are the result of P release from the OM that was not accounted for when calculating the reduction in P sorption.

Equilibrium adsorption in cylindrical mesopores: A modified Broekhoff and de boer theory versus density functional theory

This paper presents a thermodynamic analysis of capillary condensation phenomena in cylindrical pores. Here, we modified the Broekhoff and de Boer (BdB) model for cylindrical pores accounting for the effect of the pore radius on the potential exerted by the pore walls. The new approach incorporates the recently published standard nitrogen and argon adsorption isotherm on nonporous silica LiChrospher Si-1000. The developed model is tested against the nonlocal density functional theory (NLDFT), and the criterion for this comparison is the condensation/evaporation pressure versus the pore diameter. The quantitative agreement between the NLDFT and the refined version of the BdB theory is ascertained for pores larger than 2 nm. The modified BdB theory was applied to the experimental adsorption branch of adsorption isotherms of a number of MCM-41 samples to determine their pore size distributions (PSDs). It was found that the PSDs determined with the new BdB approach coincide with those determined with the NLDFT (also using the experimental adsorption branch). As opposed to the NLDFT, the modified BdB theory is very simple in its utilization and therefore can be used as a convenient tool to obtain PSDs of all mesoporous solids from the analysis of the adsorption branch of adsorption isotherms of any subcritical fluids.