Pore Surface Roughness Research Articles

Understanding capillary condensation and hysteresis in porous silicon: Network effects within independent pores

The ability to exert a significant degree of pore structure control in porous silicon materials has made them attractive materials for the experimental investigation of the relationship between pore structure, capillary condensation, and hysteresis phenomena. Using both experimental measurements and a lattice gas model in mean field theory, we have investigated the role of pore size inhomogeneities and surface roughness on capillary condensation of N2 at 77K in porous silicon with linear pores. Our results resolve some puzzling features of earlier experimental work. We find that this material has more in common with disordered materials such as Vycor glass than the idealized smooth-walled cylindrical pores discussed in the classical adsorption literature. We provide strong evidence that this behavior comes from the complexity of the processes within independent linear pores, arising from the pore size inhomogeneities along the pore axis, rather than from cooperative effects between different pores.

Protein fouling of ultrafiltration membranes — investigation of several factors relevant for tertiary wastewater treatment

The fouling of two different ultrafiltration membranes by solutions of three globular proteins with different molecular weights and isoelectric points was investigated at low concentrations relevant to secondary effluents (10 mg·L–1). The results reveal the increased fouling potential of macromolecules being small enough to enter the membrane pores due to the resulting blockage and (or) constriction of the pores, compared to a moderate increase of the filtration resistance due to gel layer formation by larger compounds. Differences between the applied membranes in terms of fouling behaviour, protein retention, as well as attachment of macromolecules to membrane surface and pore walls demonstrate the impact of pore size distribution, zeta potential, surface roughness, and hydrophobicity of the membranes on the fouling mechanism. Comparative ultrafiltration experiments using two different pumps indicate the influence of type and dimensioning of the pumping system on membrane fouling due to structural changes of solutes by pump-induced shear forces.

The influence of dispersing and stabilizing of indium tin oxide nanoparticles upon the characteristic properties of thin films

Stable suspensions of indium tin oxide nanoparticles were prepared by dispersing for 1 h in ethanol and stabilizing with 2-[2-(2-Methoxyethoxy)ethoxy]acetic acid. Particle size, morphology, and structure were characterized by a combination of dynamic light scattering, high resolution transmission electron microscopy, and X-ray diffraction. Spin coating of the stable suspensions resulted in densely packed films with a high transparency. Dispersing times exceeding 1 h led to a significant “overdispersing” effect, i.e. an increasing aggregate size of the suspensions was observed. Zeta-potential, pH, and X-ray photoelectron spectroscopy measurements were conducted to achieve a detailed understanding of the effect of “overdispersing”. With an increasing time of dispersion, i.e. with a decreasing stability of the suspensions, the thin films fabricated by spin coating displayed an increasing pore size distribution and surface roughness. In addition a strong decrease in transmittance and in conductivity was observed.

Morphology and photoelectrochemical properties of TiO 2 electrodes prepared using functionalized plant oil binders

Chemically functionalized plant oils, viz. acrylated epoxidized soybean oil (AESO) and maleinized acrylated epoxidized soybean oil (MAESO), were used as bio-based binders for the TiO 2 electrodes of dye-sensitized solar cells (DSSC). The surface roughness and number of appropriate pores were increased in the TiO 2 films prepared using the plant oil binders in comparison with the film prepared using polyethylene glycol (PEG), due to the larger number of functionalities. The short circuit photocurrent ( I SC) and open circuit photovoltage ( V OC) were increased, and the conversion efficiency was significantly improved, in the cell using the plant oil binders.

Aggregate slaking during rapid wetting: Hydrophobicity and pore occlusion

The slaking process after rapid wetting is a key factor controlling soil structural stability in dry soil, and an understanding of the relative importance of the different mechanisms involved in slaking may help in the design of management strategies aimed at maintaining a stable surface soil structure. Slaking has been linked to, among other factors, rapid pressure build-up in aggregate, and previous work has emphasized the role of organic matter to hamper that pressure build-up, possibly due to hydrophobicity, reducing rapid water entry within aggregates and hence the build-up. This study emphasizes this latter aspect linked to slaking. The evolution of the intra-aggregate pressure, the matter lost by slaking and the expelled air after rapid wetting of two soils of different textures (clay loam soil and silty-clay loam soil) amended with different types of paper sludge were studied. Hydrophobicity effects were also studied using a tensio-active solution. The results of these experiments showed that when aggregates were submitted to sudden wetting, those treated with paper sludge had an improved resistance to the destructive action of rapid wetting. The lower pressures measured in the aggregates from the amended soils and having less slaking resulted most likely from slow water entry and reduced swelling. Detailed investigation on the link between hydrophobicity and water entry revealed that the true hydrophobic effect (modification of contact angle) was non-existent for the silty-clay loam and minor for the clay loam. This study, rather, suggests that changes in the water potential at the wetting front following organic matter addition and aggregate immersion most likely depend on pore occlusion and on changes in pore surface roughness. Key words: Aggregate stability, organic matter, slaking, pressure, swelling, wettability

The influence of hydrophobicity, roughness and charge upon ultrafiltration membranes for black tea liquor clarification

This paper investigates the clarification of black tea using UF membranes. The efficiency of separation and the final product quality were investigated using regenerated cellulose (RC) and fluoropolymer (FP) membranes of different nominal molar mass cut-off values. Changes during filtration in the hydrophobicity, surface charge on the pore walls, surface roughness and chemical properties were investigated. Results indicate that both of the membrane materials studied produced clarified tea liquors with polyphenol transmission rates of ca. 90%. Increased fouling was present on rougher, more hydrophobic FP surfaces. When roughness, charge and hydrophobicity were similar (i.e. for the RC membrane), variation in pore size was not found to affect the filtration properties significantly over the range investigated. Porous structures with isoelectric points at the pH of the liquor being filtered were linked to an increased fouling tendency. All RC and FP100 kg mol −1 membranes were cleaned relatively easily. FP10 kg mol −1 and FP30 kg mol −1 membranes were found to have undergone surface modification, including increased negative charge and changes in hydrophobicity. The dominant fouling mechanism during all ultrafiltration experiments was found to be cake filtration. The cake formed on RC membranes is thought to dominate filtration properties due to similar transmissions detected through varied pore sizes.

Pore Occlusion by Sugars and Lipids as a Possible Mechanism of Aggregate Stability in Amended Soils

Understanding the underlying mechanisms of structural stability and the contribution of specific organic fractions to such mechanisms is critical in designing new soil and water conservation strategies relying on organic amendments. The objective of this work was to study the role of neutral and uronic sugars and lipids in affecting key mechanisms (swelling rate, pressure evolution) involved in the stabilization of individual aggregates. A 48‐wk incubation study was performed on a clay loam and a silty clay loam amended with either deinking–secondary sludges, primary–secondary sludges, or composted deinking sludges at rates ranging from 8 to 24 Mg dry matter ha−1 Different structural stability indices were measured during the incubation, along with CO2 evolved, neutral and uronic sugar, and lipid contents. Significant increases in all stability indices were measured for both soil types. These improvements were linked to a very intense phase of C mineralization and highly correlated with neutral and uronic sugars as well as lipid contents. Paper sludge amendments also resulted in significant decreases in maximum internal pressure of aggregates and aggregate swelling following immersion in water, two mechanisms affecting structural stability. Overall, the results suggest that reduction in maximum internal pressure induced by organic amendments probably resulted from increases in pore surface roughness and pore occlusion rather than an increase in surface wetting angles. This study also supports the view of a nonspecific action of the lipids and neutral and uronic sugars on aggregate stability to rapid wetting.

Argon and krypton adsorption on templated mesoporous silicas: molecular simulation and experiment

In this work we report molecular simulation results for argon and krypton adsorption on atomistic models of templated mesoporous silica materials. These models add atomistic levels of detail to mesoscale representations of these porous materials, which were originally generated from lattice Monte Carlo simulations mimicking the synthesis process of templated mesoporous silicas. We generate our atomistic pore models by carving out of a silica block a ‘mathematically-smooth’ representation of the pores from lattice MC simulations. Following that procedure, we obtain a model material with mean mesopore and micropore diameters of 5.4 nm and 1.1 nm, respectively (model A). Two additional model materials were considered: one with no microporosity, and with mesopores similar to those of model A (model B), and a regular cylindrical pore (model C). Simulation results for Ar and Kr adsorption on these model materials at 77 K and 87 K shows that model A provides the best agreement with experimental data; however, our results suggest that fine-tuning the microporosity and/or the surface chemistry (i.e., by decreasing the density of OH groups at the pore surface) of model A can lead to better agreement with experiments. The filling of the mesopores in model materials A and B proceeded via a classical capillary condensation mechanism, where the pores fill at slightly different pressures. This observation contrasts with what was observed in our previous study (Coasne, et al. in Langmuir 22:194–202, 2006), where we considered atomistic silica mesopores with an important degree of surface roughness at length scales below 10 A, for which we observed a quasi-continuous mesopore filling involving intermediate phases with liquid-like “bridges” and gas-like regions. These results suggest that pore surface roughness, and other morphological features such as constrictions, play an important role in the mechanism of adsorption and filling of the mesopores.

Surface analysis of cryofixation-vacuum-freeze-dried polyaluminum chloride–humic acid (PACl–HA) flocs

The powder of polyaluminum chloride–humic acid (PACl–HA) flocs was prepared by cryofixation-vacuum-freeze-drying method. The FTIR spectra show that some characteristic functional groups in polyaluminum chloride (PACl), humic acid (HA), and kaolin still existed in the dried flocs. X-ray diffractometry (XRD) patterns indicate that these flocs are amorphous. Nitrogen adsorption–desorption isotherms were obtained for different samples of the dried PACl–HA flocs. The BET specific surface area, BJH cumulative absorbed volume and BJH desorption average pore diameter of them were determined. The peak values of 8.4–11.2 nm (pore diameter) for pore size distribution (PSD) curves indicate that the pores of the dried flocs are mostly mesopores. The surface fractal dimensions D s and the corresponding fractal scales determined from both SEM images and nitrogen adsorption–desorption data sets reveal the multi-scale surface fractal properties of the dried PACl–HA flocs, which exhibited two distinct fractal regimes: a regime of low fractal dimensions (2.07–2.26) at higher scales (23–387 nm), mainly belonging to exterior surface scales, and a higher fractal dimensions (2.24–2.37) at lower scales (0.80–7.81 nm), falling in pore surface scales. Both HA addition and kaolin reduction in dried floc can decrease the irregularity and roughness of external surface. However, for the irregularity and roughness of pore surface, the addition of HA or kaolin in dried floc can increase them. Furthermore, some difference was found between the pore surface fractal dimensions D s calculated from nitrogen adsorption and desorption data. The pore surface D s values calculated through thermodynamic model were much greater than three.

Effects of marine biofouling on gas sensor membrane materials

The use of underwater gaseous sensors has increased rapidly in the last 10 years. The majority of such sensors employ a thin membrane through which the gas diffuses. These sensors are potentiometric gas-sensing probes and essentially they are ion-selective electrodes. The deployment time of these membranes is curtailed by the formation of biofouling on the membrane leading to erroneous results. The physical properties of a variety of commonly used membranes were investigated using SEM and AFM. This showed that there were differences in topography between the PTFE membranes, such as pore sizes and surface roughness, which may be attributed to the manner in which they are manufactured. The pore size of the PTFE membranes varied greatly, ranging from circular pores with a diameter of 500 nm to elongated pores measuring 1 x 22 microm. The contact angle of each membrane showed that they were all hydrophobic. The amount of fouling on each was also observed and its affect on oxygen diffusion was monitored. Fouling slowed down the response of the instrument and caused reduced diffusion through the membranes. The amount of fouling varied between the membranes with the YSI membrane fouling least. Some of the membranes tested did foul less than others and there could be lifetime advantages of choosing a membrane with a smoother surface and a small pore size.

Density Functional Theory Model of Adsorption on Amorphous and Microporous Silica Materials

We present a novel quenched solid density functional theory (QSDFT) model of adsorption on heterogeneous surfaces and porous solids, which accounts for the effects of surface roughness and microporosity. Within QSDFT, solid atoms are considered as quenched component(s) of the solid-fluid system with given density distribution(s). Solid-fluid intermolecular interactions are split into hard-sphere repulsive and mean-field attractive parts. The former are treated with the multicomponent fundamental measure density functional. Capabilities of QSDFT are demonstrated by drawing on the example of adsorption on amorphous silica materials. We show that, using established intermolecular potentials and a realistic model for silica surfaces, QSDFT quantitatively describes adsorption/desorption isotherms of Ar and Kr on reference MCM-41, SBA-15, and LiChrosphere materials in a wide range of relative pressures. QSDFT offers a systematic approach to the practical problems of characterization of microporous, mesoporous, and amorphous silica materials, including an assessment of microporosity, surface roughness, and adsorption deformation. Predictions for the pore diameter and the extent of pore surface roughness in MCM-41 and SBA-15 materials are in very good agreement with recent X-ray diffraction studies.

Significance of straining in colloid deposition: Evidence and implications

Considerable research suggests that colloid deposition in porous media is frequently not consistent with filtration theory predictions under unfavorable attachment conditions. Filtration theory was developed from an analysis of colloid attachment to the solid‐water interface of a single spherical grain collector and therefore does not include the potential influence of pore structure, grain‐grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory. This conclusion is based upon pore size distribution information, size exclusion, time‐ and concentration‐dependent deposition behavior, colloid size distribution information, hyperexponential deposition profiles, the dependence of deposition on colloid and porous medium size, batch release rates, micromodel observations, and deposition at textural interfaces. The implications of straining in unsaturated and heterogeneous systems are also discussed, as well as the potential influence of system solution chemistry and hydrodynamics. The inability of attachment theory predictions to describe experimental colloid transport data under unfavorable conditions is demonstrated. Specific tests to identify the occurrence and/or absence of straining and attachment are proposed.

Pore surface fractal analysis of palladium-alumina ceramic membrane using Frenkel–Halsey–Hill (FHH) model

The alumina ceramic membrane has been modified by the addition of palladium in order to improve the H 2 permeability and selectivity. Palladium-alumina ceramic membrane was prepared via a sol–gel method and subjected to thermal treatment in the temperature range 500–1100 °C. Fractal analysis from nitrogen adsorption isotherm is used to study the pore surface roughness of palladium-alumina ceramic membrane with different chemical composition (nitric acid, PVA and palladium) and calcinations process in terms of surface fractal dimension, D. Frenkel–Halsey–Hill (FHH) model was used to determine the D value of palladium-alumina membrane. Following FHH model, the D value of palladium-alumina membrane increased as the calcinations temperature increased from 500 to 700 °C but decreased after calcined at 900 and 1100 °C. With increasing palladium concentration from 0.5 g Pd/100 ml H 2O to 2 g Pd/100 ml H 2O, D value of membrane decreased, indicating to the smoother surface. Addition of higher amount of PVA and palladium reduced the surface fractal of the membrane due to the heterogeneous distribution of pores. However, the D value increased when nitric acid concentration was increased from 1 to 15 M. The effect of calcinations temperature, PVA ratio, palladium and acid concentration on membrane surface area, pore size and pore distribution also studied.

Processing of Composite Thin Film Solid Oxide Fuel Cell Structures

A solid oxide fuel cell (SOFC) structure is proposed in which a composite thin film cathode substrate supports a dense thin film electrolyte with a thickness of less than 1 μm. The cathode substrate has a graded porosity achieved through the partial sintering of a spin‐coated CeO2 colloidal suspension. The resulting surface has a pore size and surface roughness which allowed a fully dense ZrO2:16%Y (YSZ) electrolyte to be spin‐coated directly from a polymeric precursor without capillary forces removing the precursor from the surface of the porous substrate. Using this process, fuel cell structures were constructed with temperatures not exceeding 800°C. The porous CeO2 interlayer should allow for decreased ohmic losses, as well as decreased reactions between the YSZ and the cathode substrate. In addition, the nanocrystalline grain sizes should allow for increased catalytic activity on the cathode. Calculated ohmic losses indicated the resistance of the CeO2 interlayer limited the power of the structure, which was minimized by impregnating the porous layer with a mixed‐conducting perovskite. The final structure shows significantly reduced ohmic losses as calculated at 400°C.

Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy

Oxide coatings were produced on AM60B magnesium alloy substrate making use of microarc oxidation (MAO) technique. The effect of KF addition in the Na 2SiO 3–KOH electrolytic solution on the microarc oxidation process and the structure, composition, and properties of the oxide coatings was investigated. It was found that the addition of KF into the Na 2SiO 3–KOH electrolytic solution caused increase in the electrolyte conductivity and decrease in the work voltage and final voltage in the MAO process. Subsequently, the pore diameter and surface roughness of the microarc oxidation coating were decreased by the addition of KF, while the coating compactness was increased. At the same time, the phase compositions of the coatings also varied after the addition of KF in the electrolytic solution, owing to the participation of KF in the reaction and its incorporation into the oxide coating. Moreover, the coating formed in the electrolytic solution with KF had a higher surface hardness and better wear-resistance than that formed in the solution without KF, which was attributed to the changes in the spark discharge characteristics and the compositions and structures of the oxide coatings after the addition of KF.

Seasonal Changes in the Structure of RBC Heterotrophic Biofilms

Heterotrophic Biofilms, growing in the successive three treatment stages of the RBC reactor in a full-scale wastewater treatment plant in the summer of 2002, were investigated and compared to those growing in the fall of 2001. The seasonal changes in biofilm structure were studied in terms of biofilm thickness, dry solid density, internal pore distribution and patterns, and surface roughness. The comparison results indicated that due to the higher wastewater flow rate in the summer of 2002, these biofilms had thinner, denser and less rugged surface structure than the biofilms sampled in the fall of 2001; furthermore, they showed less heterogeneous internal pore distribution. However, biofilms from both seasons showed some similarities: biofilms in the treatment stages 2 and 3 had similar structure; and their internal pore distribution and pore surface roughness didn't show any correlation with the biofilm depth.

Small-angle neutron scattering from a nano-layered synthetic silicate

The combined analysis of the power-law behavior of small-angle scattering and of the relative scattered intensity for different scattering vectors makes it possible to obtain information on both surface roughness and shape of pores in layered silicates. It was found that low and moderate pressures of compression (up to 2.6MPa) during the sedimentation stage of preparation of packed dehydrated fluorohectorite has little effect on the final small-angle scattering behavior and thus on the pore characteristics of the samples used in this study. The anisotropically shaped pores showed an average correlation length ratio of approx. 1:2 between the direction of axial compression and the direction perpendicular to this, and the pore structure seems to be determined mainly by the characteristics of the clay platelets themselves (aspect ratio, polydispersity), and less by the pressure applied during sedimentation. However, some indication of increased irregularities, possibly linked to deformation or fracture of parts of the clay grains, is observed at the highest pressure used (26MPa).

Impact of DEA solutions with and without CO 2 loading on porous polypropylene membranes intended for use as contactors

Two types of polypropylene microporous hollow fiber membranes, Celgard X40-200 and Celgard X50-215, were used to expose to diethanolamine (DEA) aqueous solutions with and without CO 2 loading, in order to investigate the impact of amine absorbents on membrane surfaces during the process of CO 2 capture in a membrane contactor. The effects of liquids on the membrane surface morphology and surface tension were studied by means of SEM, AFM as well as a Contact Angle Goniometer. Possible interactions between the liquids and the membranes were monitored by XPS. It was found that both Celgard X40-200 and Celgard X50-215 hollow fibers suffered changes in terms of pore structure and surface roughness after being exposed to DEA aqueous solutions. In particular, Celgard X40-200 hollow fiber membranes underwent significant deformation as observed in surface morphology during the initial stage of contact. However, dissolved CO 2 was found to cause less deformation of the micro-pores and to retard surface roughening, likely due to the formation of carbamate ions from interactions between CO 2 and DEA. XPS analysis suggests that possible chemical reactions between the membranes and DEA may be responsible for the alteration of the surface tension of membranes thereby reducing their hydrophobicity. Therefore, it is possible that polypropylene membranes were partially wetted during CO 2 capture, leading to performance deterioration in the membrane contactor.

Microstructure evolution during solid-state foaming of titanium

Solid-state foaming of commercially pure titanium was achieved by high-temperature expansion of high-pressure argon bubbles trapped in titanium by a powder-metallurgy technique. The foaming step was performed at constant temperature, where creep of the titanium matrix controls pore expansion, or during thermal cycling around the α⧸β allotropic temperature, which induces transformation superplasticity of the matrix. Superplastic foaming led to significantly faster pore growth and higher terminal porosity than isothermal creep foaming. During thermal cycling, the porosity remains nearly fully closed to the surface of the specimen up to the point where the maximum porosity (44%) is obtained, despite the presence of some internal pore coalescence. With continued thermal cycling, pores coalesce further by fracture of thin interpore walls and pores finally open to the surface, but without a significant increase in the amount of total porosity. The remnants of these walls result in a jagged pore morphology. Under isothermal conditions, pores remain small, equiaxed and unconnected with no pore surface roughness. However, after long annealing times, they exhibit faceting due to surface diffusion.

Photochemical modification of membrane surfaces for (bio)fouling reduction: a nano-scale study using AFM

Biofouling, due to microbial growth on membranes, is a common problem during the operation of water treatment membrane plants. It leads to an increase in operation and maintenance costs due to the deterioration of membrane performance and ultimately shortening membrane life. In an attempt to develop membranes with lower fouling properties in this paper we used the photoinduced grafting technique for the modification of membrane surfaces. Two different hydrophilic monomers: 2-acrylamido-2-methyl-l-propanesulfonic acid (AMPS) and quaternary 2-dimethylaminoethylmethacrylate (gDMAEMA) were photografted to the surface of commercial polyethersulfone (PES) microfiltration membranes (Millipore). The modified membranes were characterised using atomic force microscopy (AFM) by visualisation and the measurement of pore size, pore size distribution and surface roughness. A direct quantification of the force of adhesion using silica colloid probe technique and comparisons with unmodified samples were also made. The membrane affinity to biofouling was tested in the presence of Escherichia coli bacteria. It was found that the number of bacterial cells able to proliferate from countable colonies was much less for qDMAEMA-grafted samples compared with unmodified PES membranes. Thus these modified membranes could be potentially more resistant to biofouling.