Surface Energetic Heterogeneity Research Articles

GCMC kernel for analyzing the pore size distribution of porous carbons based on a simplified slit-pore model considering surface energetic heterogeneity

GCMC kernel for analyzing the pore size distribution of porous carbons based on a simplified slit-pore model considering surface energetic heterogeneity

Adsorption of CO2, CO, H2, and N2 on Zeolites, Activated Carbons, and Metal-Organic Frameworks with Different Surface Nonuniformities

The single-component adsorption of CO2, CO, N2, and H2 at 25 and 35 °C was studied using microporous faujasite-framework zeolites (NaY and NaX), activated carbons (GCN and MSP), and metal–organic frameworks (A100 and Z1200) as starting points for the separation of CO2 from syngases produced by gasifying biomass-based solid wastes. The indicated adsorption isotherms and uptake of the adsorbates strongly depended on the adsorbates themselves as well as on the adsorbents because of significant differences in the surface features, such as surface nonuniformity, and in the molecular properties. The selectivity of CO2 to the other gases also varied with the adsorbents due to the distinctive energetic characteristics. The surfaces of the zeolites were the most energetically heterogeneous ones, yielding higher CO2 uptake at low pressures, while the two activated carbons and A100 had moderate surface heterogeneities, and MSP showed the highest CO2 uptake at high pressures, such as 6 bar, at which the micropore volume and surface area are important. Z1200, which has highly homogeneous surfaces and no high-affinity-binding sites, exhibited the lowest CO2 adsorption capacity regardless of equilibrated pressure. The surface nonuniformities of the six sorbents were consistent with the calculated isosteric heats of CO2 adsorption. CO2 could be reversibly adsorbed on NaY and MSP but not on GCN, with some metal impurities, although all these adsorbents showed a fully reversible process for CO adsorption. The estimated working capacity for CO2 adsorption at 25 °C was 0.78–6.50 mmol/g, depending on the sorbents used. The highest value was disclosed for MSP, the surface energetic heterogeneity of which was between that of zeolites and Z1200. Such a high working capacity bodes well for use in our later applications.

The polylogarithmic adsorption function: a linear energetic surface heterogeneity

Given a surface energetic heterogeneity as a linear distribution function on adsorption heat, the exact solution for a new adsorption function is obtained. It is demonstrated that the interpretation of numerous experimental results associated earlier with energetic even heterogeneity of surfaces (the Temkin case) on account of linear segments of adsorption functions (isotherms) in semilogarithmic coordinates may need crucial revision. Properties of the new adsorption function as well as its limiting cases with application to the experimental data for an inhibiting composition in the benchmark-medium of National Association of Corrosion Engineers are analysed.

An Asymptotically Exact Analytical Solution for the Isotherm Based on the Gaussian Distribution on Adsorption Heat

AbstractGiven a surface energetic heterogeneity as the normal distribution function, the approximate analytical solution has been obtained that resembles the true isotherm at both low‐ and high‐pressure limits as well as reproduces its intermediate, so‐called centrosymmetric, point. An asymptotic complement property that introduces a continuous mapping of the isotherm onto itself has been formulated, proved and shown to be applicable to a whole class of surfaces. The limiting or degenerative cases on energetically homogeneous (the Langmuir case), evenly heterogeneous (the Temkin case) and quasi‐passive surface have been considered. The role of the heterogeneity parameters of the overall Gauss isotherm has been studied. An algorithm to determine the adsorption heat characteristics of the surface has been built. Hints for future use of the asymptotic complement property have been given.

Adsorption characteristics of light gases on basalt rock-based zeolite 4A

The adsorption characteristics of light gases on basalt rock-based zeolite 4A (BR zeolite-4A) were systematically investigated to evaluate its potential application as an alternative adsorbent for adsorption-based separation processes. We used alkali fusion and hydrothermal procedure to prepare the nanostructured adsorbent, BR zeolite-4A, which was characterized with field emission scanning electron microscopy, X-ray diffraction, and carbon dioxide adsorption apparatus. The single component adsorption equilibrium for CO2, CH4, N2 and H2 on the BR zeolite-4A was volumetrically determined using a nanoPOROSITY adsorption analyzer at the temperature range from (288.15 to 308.15) K and pressure range from (0.1 to 110) kPa. The experimental results indicate that BR zeolite-4A showed higher adsorption capacities for CO2 compared to other light gases, indicating the suitable porous material for selective separation by adsorption. Three different isotherm equations, Langmuir, Toth, and Sips, were used to correlate the adsorption isotherm data and the most reasonable results obtained from the Sips model irrespective of the adsorption isotherm types. Isosteric heat of adsorption and adsorption energy distribution function values were calculated and used to further examine the surface energetic heterogeneity of BR zeolite-4A. The pure component adsorption isotherm results were also used to predict the adsorption selectivity for CO2/N2, CO2/CH4, CO2/H2, and CH4/H2 binary mixtures (50:50) at different pressure ranges using ideal adsorbed solution theory.

2D-NLDFT adsorption models for porous oxides with corrugated cylindrical pores

In this work, we develop two-dimensional models based on the non-local density functional theory (2D-NLDFT) for the analysis of pore size distribution (PSD) of oxide materials with cylindrical pores with rough and heterogeneous walls. The existing standard NLDFT models for porous oxides assume the smooth energetically uniform surface of the pore walls. Due to this assumption, the calculated theoretical isotherms show typical layering transitions, which are not consistent with the experimental adsorption isotherms measured on real oxide materials. As a result, the fits of standard NLDFT models to N2 or Ar adsorption isotherms show deviations from the experimental points in association with artifacts observed on the calculated PSD plots. The 2D-NLDFT framework allows us to improve the standard model by introducing the corrugation and energetic heterogeneity to the surface of cylindrical pores. The surface roughness and energetic heterogeneity are known characteristics of the oxide surfaces. With these assumptions we develop a comprehensive approach in which both branches of the adsorption isotherm may be used for the PSD analysis of mesoporous oxide materials. We validate this approach by using Ar data measured at 87 K on the reference set of MCM-41 silica samples (Kruk and Jaroniec, 2000). The generated kernels are smooth, do not show layering transitions and fit accurately the reference data.

Surface Energy Heterogeneity Profiles of Carbon Nanotubes with a Copolymer-Modified Surface Using Surface Energy Mapping by Inverse Gas Chromatography

Abstract The effectiveness and quantitative control of the surface transition of multi-walled carbon nanotubes (MWCNTs) was characterized by inverse gas chromatography (iGC). The surface energy profile of carbon nanotubes compatibilized with an olefin-maleic-anhydride-ester-amide (OMAEA)-type coupling agent was determined by a surface energy analyzer (SEA). The surface energetic heterogeneity with energy distributions of dispersive and specific (acid-base) components of the surface energy of the MWCNTs were determined at various surface coverages. The results of the surface energy mapping showed that surface treatment significantly reduced the dispersive surface energy of MWCNTs and increased the specific surface energy. Furthermore, the surface modification enhanced its Lewis basic character and simultaneously decreased the acidic character of MWCNTs. It has been demonstrated that the surface treatment modified the heterogeneity profiles of the energetic surface of the carbonaceous nanomaterials.

Adsorptive removal of anionic azo dye from aqueous solution using activated carbon derived from extracted coffee residues

This study reports the synthesis of activated carbon derived from extracted coffee residues (AC-ECRs) via wet impregnation using different ratios of potassium hydroxide as an activating agent. The influence of the impregnation ratio on physicochemical properties such as surface characteristics, structural properties, and energetic heterogeneity were evaluated using nitrogen adsorption isotherms and adsorption energy distributions (AEDs). The overall porosity and energetic heterogeneity were improved with increasing impregnation ratio, resulting in a positive impact towards the adsorption capacity of substrate. Particularly, the correlation between AEDs and adsorption capacity clearly indicated that mesopore distribution plays an essential role on the adsorption capacity of acid orange 7 as a wider pore size offers more favorable conditions for adsorbing the substrate. This is the first report documenting the effect of impregnation ratio on surface heterogeneity and the correlation between AEDs and adsorption capacity of an azo dye. Adsorption kinetics and equilibrium isotherm studies insinuate that the overall process follows the pseudo-second-order and Sips isotherm models, respectively. Thermodynamic and isosteric heat of adsorption analyses revealed that the adsorption process was exothermic and is primarily governed via physisorption.

Towards a better description of organosilane grafting onto silica particles using volumetric techniques based on molecular probing

The surface energetic heterogeneity of Aerosil OX50 silica is investigated before and after modification by the grafting of various amounts of hydrophobic hexadecyltrichlorosilane (HTS). A detailed study is conducted to obtain new insight into the grafting process using volumetric techniques based on molecular probing. The surface of the silica particles has been thoroughly characterized using classical N2 Brunauer–Emmett–Teller measurements and by low-pressure quasi-equilibrium volumetry using nitrogen and argon as probe molecules. The experimental data are analyzed using the derivative isotherm summation method in order to gain quantitative information on the specific surface area, the surface energy and the polarity of the bare and HTS-covered silica, in terms of adsorption energy sites and heterogeneity. The modification process is also studied by Fourier transform infrared spectroscopy (FTIR) and total carbon mass measurements performed on solid samples (TOC). The combination of these three techniques leads to a better description of organosilane grafting onto the particles. The bare silica surface appears energetically heterogeneous due to the use of 5 different adsorption sites to describe the low pressure adsorption results. The HTS molecules are mainly located on the polar high and medium energy domains. At the largest HTS coverage (2.1 µmol/m2), the surface becomes energetically homogeneous (one site of low energy) and fully apolar. The data obtained from the low-pressure gas adsorption have been employed to estimate the grafted organic content in contact with the gas. The surface coverages derived from volumetry analysis and TOC measurements are in acceptable agreement but some difference can be recognized. The discrepancy is discussed in terms of conformation of the HTS molecules at the solid surface. The proposed conformational changes are supported by the FTIR results.

Surface-Energetic Heterogeneity of Nanoporous Solids for CO2 and CO Adsorption: The Key to an Adsorption Capacity and Selectivity at Low Pressures.

This study has been focused on surface energetic heterogeneity of zeolite (H-mordenite, "HM"), activated carbon ("RB2") and metal-organic framework family ("Z1200") materials and their isotherm features in adsorption of CO2 and CO at 25 degrees C and low pressures ≤ 850 Torr. The nanoporous solids showed not only distinctive shape of adsorption isotherms for CO2 with relatively high polarizability and quadrupole moment but also different capacities in the CO2 adsorption. These differences between the adsorbents could be well correlated with their surface nonuniformity. The most heterogeneous surfaces were found with the HM that gave the highest CO2 uptake at all pressures allowed, while the Z1200 consisted of completely homogeneous surfaces and even CO2 adsorption linearly increased with pressure. An intermediate character was indicated on the surface of RB2 and thus this sorbent possessed isotherm features between the HM and Z1200 in CO2 adsorption. Such different surface energetics was fairly consistent with changes in CO2/CO selectivity on the nanoporous adsorbents up to equilibrated pressures near 850 Torr.

A novel colored talc filler: Preparation and surface property determination using two distinct methods

A better understanding of filler's surface properties is critical for determining the most effective polymer reinforcement fillers. In this work, a colored talc filler (CTF) was prepared and its surface properties were comparatively studied to those of talc filler (TF), using inverse gas chromatography (IGC) and contact angle methods—Owens–Wendt–Kaelble (OWK), van Oss–Chaudhury–Good (vOCG) and Wu. The results indicated that the dispersive component (γSD) for both samples contributed a major part (68–81%) of the total surface energy (γST), and displayed a decreasing trend with increasing surface coverage, meaning the fairly energetic surface heterogeneity for them. The γSD values determined by the contact angle methods were consistent, although lower than those using IGC analysis. The specific component (γSSP) calculated by the contact angle methods were also consistent and lower than those calculated using IGC. The specific Gibbs free energies of adsorption (ΔGsp) changed with surface coverages, further confirming the heterogeneous nature of both samples. The significantly lower γST value for CTF could reduce filler particle-particle interactions, allowing a better dispersion in matrix, and thus lead to an improvement in mechanical performance of CTF/polymer composite.

The influence of surface energetic heterogeneity on electrofluidic gating of a metal oxide surfaces

We developed a theoretical model which describes the influence of the applied potential to the wall of an oxide nanochannel on the diffuse layer potential in the solution filling the nanopore. It was assumed that the surface of oxide is energetically heterogeneous and electrolyte ions from the solution can adsorb on channel walls. We performed calculations for two oxides: SiO2 and Al2O3. It was found that energetically heterogeneous oxide surfaces strongly suppress gating potential. Moreover, taking into account ion adsorption considerably increased the electrokinetic potential and reduced the influence of applied potential on the electric double layer in the solution. The density of surface sites is the key parameter in the modeling of oxide nanochannels. Low density of surface hydroxyl groups significantly increases susceptibility on electrofluidic gating.

Surface structural and energetic heterogeneity of carbon materials prepared from corn grains using steam and H3PO4‐steam activations

The porous activated carbons (ACs) were prepared from corn grains through physical (steam) and chemical–physical (H3PO4‐steam) activations. The effects of steam activation temperature (700–900 °C) on pore development, surface roughness, and energetic heterogeneity were investigated in both activations. Also, the effect of prior carbonization on H3PO4‐steam activation was studied. The physical properties, surface fractal dimensions, and adsorption energy distributions of ACs were determined from nitrogen adsorption–desorption isotherm data. Both physical and chemical–physical activations show that the AC with higher surface area, relatively smoother surface, and energetically heterogeneous surface could be produced at a maximum steam activation temperature (900 °C). Copyright © 2015 John Wiley & Sons, Ltd.

Surface Energy Determined by Inverse Gas Chromatography as a Tool to Investigate Particulate Interactions in Dry Powder Inhalers.

Dry powder inhalers (DPIs) usually contain drug particles <6 µm which agglomerate and/ or adhere on the surfaces of large carriers particles. The detachment of drug particles from carriers and de-agglomeration of drug particles into primary particles is essential for drug deposition in the deep lung. These processes are influenced by the surface energy of particles. Inverse gas chromatography (IGC) has been used to determine the surface energy of powder particles used in DPI to characterize materials and to understand aerosolization behaviour. Early studies used an infinite dilution technique to determine nonpolar surface energy and free energy of adsorption for polar interactions separately. Although some correlations were observed with the change in nonpolar surface energy before and after micronization, milling and storage, a lack of consistency in the change of free energy of adsorption was common. Moreover, a consistent relationship between complex de-agglomeration behaviour and surface energy has not been established and there are even some examples of negative correlation. In fact, nonpolar surface energy at infinite dilution is an incomplete representation of powder surface characteristics. The techniques for measuring polar surface energy, total surface energy and surface energy distribution have provided more revealing information about surface energetics of powders. Surface energy distributions determined by IGC or surface energy analyser have been successfully used to understand energetic heterogeneity of surfaces, characterize different polymorphs and understand changes due to micronization, structural relaxation, dry coating and storage. Efforts have been made to utilize surface energy distribution data to calculate powder strength distribution and to explain complex de-agglomeration behaviour of DPI formulations.

Application of NRTL activity coefficient model to describe the kinetics of gas adsorption

Abstract The Non-Random Two-Liquid activity coefficient model is applied to describe the kinetics of pure gas adsorption on energetically heterogeneous solid surfaces. The surface energetic heterogeneity has been represented by the Gaussian-like function of the adsorption energy distribution. Two different kinetic isotherms have been presented. One of them, determined by using the statistical rate theory, has been critically discussed. The applicability of the presented approach has been demonstrated by a quantitative analysis of two sets of experimental data previously reported in the literature.

The Balance between Diffusional and Surface Reaction Kinetic Models: A Theoretical Study

In this work we considered the factors determining the rate of adsorption. By assuming the balance between the surface reaction and diffusional mechanisms we discussed the possibility to conclude which model is the rate-controlling step. By using the pseudo first-order Lagergren equation we showed the theoretical interpretation of the Lagergren coefficient. If this parameter is time-dependent, the diffusional model should be assumed to represent the kinetics of adsorption. Such property can be utilized practically to resolve the rate-determining step. We also discussed the influence of surface energetic heterogeneity on the mechanism of the rate of adsorption.

Development of a First-Order Kinetics Based Model, Equilibrium Studies, and Thermodynamics for the Adsorption of Methyl Orange onto a Lignocellulosic Anion Exchanger

The present article describes the adsorption behavior of methyl orange (MO) on polyacrylamide-grafted aminomethylated tamarind fruit shell anion exchanger. The optimum pH for MO removal was 3.0 and the equilibrium was attained within 2 h. The kinetic and isotherm data obtained at different pH and temperatures, could be fitted with the pseudo-first-order rate equation and the Sips isotherm model, respectively. The adsorption process was exothermic and spontaneous in nature. The decrease in isosteric heat of adsorption with increasing MO uploading on the adsorbent reflects the surface energetic heterogeneity of the adsorbent. The reusability of the adsorbent was demonstrated over 4 cycles using 0.2 M NaOH as the desorbing agent.

Phenol adsorption on α,α′-dichloro-p-xylene (DCX) and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMBP) modified XAD-4 resins from aqueous solutions

The commercial Amberlite XAD-4 resin was post-crosslinked by two crosslinking reagents, α,α′-dichloro-p-xylene (DCX) and 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMBP) through the Friedel–Crafts reaction and two post-crosslinked resins named DCX and BCMBP modified resins were prepared in this study. Characterization of the two modified resins indicated that DCX and BCMBP were connected on the surface of XAD-4 successfully and the pore diameter distribution of the modified resins was transferred from 2 to 14nm (XAD-4) to a narrower micro/mesopore region (1–9nm) and a large amounts of micropores were emerged for the two modified resins. Phenol adsorption experiments showed that the equilibrium adsorption capacity on the two modified resins was greatly enhanced due to emergence of the large number of micropores and the equilibrium adsorption isotherms could be well fitted by the Freundlich equation. The surface energy heterogeneity of the resins could be described as a function of the isosteric adsorption enthalpy and the tested three resins exhibited different surface energetic heterogeneity patterns. Both of the pseudo-first-order and pseudo-second-order rate equations could characterize the kinetic data of phenol adsorption on XAD-4 while only the pseudo-second-order rate equation was appropriate for the modified resins and the micropore diffusion models could describe the kinetic data on the modified resins rather than XAD-4. The dynamic adsorption capacity of phenol on XAD-4, DCX and BCMBP modified XAD-4 resins were predicted to be 80.6, 97.5 and 127.0mg/g dry resin, respectively, which were very close to the equilibrium adsorption capacity in the batch experiments and the resin column could be completely recovered by less than 7 BV of 75% of ethanol aqueous solution (w/v).

The influence of energetic surface heterogeneity on proton desorption during capillary filling of silica nanochannels

We applied the surface complexation model to describe the deprotonation of silica nanochannel walls as a function of pH and ionic strength of filling solution. We took into account energetic heterogeneity of silanol groups. We found that more heterogeneous walls of silica nanochannel will be more charged and can interact more strongly with ionic solutes. The modeling of nanochannels coated with 3-cyanopropydimethylchlorosilane is uncertain because charging mechanism of coated silica should take into account increased autolysis of interfacial water.

Theoretical prediction of gas adsorption kinetics based on equilibrium data

Based on the equilibrium adsorption isotherms, new expressions describing the rate of adsorption were developed. The theoretical model takes into account the influence of diffusional effects on the rate of adsorption. Other nonideal effects such as surface energetic heterogeneity or intermolecular interactions can be taken into account by using the adequate equilibrium isotherm. The accuracy of the presented rate equations was verified using real adsorption systems.