Interlamellar Surfaces Research Articles

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology.

Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.

Fatty acids promote fulvic acid intercalation by montmorillonite

Organic matter (OM) adsorbed to the interlamellar surfaces of swelling clay minerals is particularly stable, with residence times as long as 2000 to 10,000years. Intercalation of OM by smectitic clay minerals may therefore increase the stable C reservoir in both soils and sediments. However, the precise mechanisms of adsorption and intercalation of the myriad organic constituents by swelling clay minerals in soils and sediments remain unresolved. Model adsorption experiments with montmorillonite (Mt), a ubiquitous expanding layer silicate, showed that environmentally relevant fatty acids possessing long-chain alkyl C facilitate the adsorption and subsequent intercalation of fulvic acid (FA). Palmityl palmitate, comprising two C16:0 chains, promotes greater FA adsorption than does stearic acid, the latter possessing a single C18:0 chain. Furthermore, maximum FA interlayer adsorption was observed where the fatty acid was adsorbed to Mt prior to introduction of the humic material; the alkyl C chains of the fatty acid evidently serving as attachment points to enhance uptake and optimize placement of FA at the margins of the interlayer space. These data reveal a previously unknown mechanism through which humic materials become intercalated by expanding layer silicates, thus contributing to the stable OM pool in soils and sediments.

Structures of hexamethonium exchanged bentonite and the sorption characteristics for phenol

The role and contribution of siloxane surface and exchanged organic cations on sorption process of organic contaminants is critical for the designing of high efficient organoclay adsorbents. In this study, organobentonites were synthesized using hexamethonium bromide (HM) and tetramethyl ammonium bromide (TMA). And their structures and sorption characteristics for phenol were examined. It was suggested that HM molecules lay parallel to the silicate planes, and were isolated from each other in the interlamellar surfaces. Sorption of phenol by HM-Bent and TMA-Bent were dominanted by adsorption process. At low phenol concentrations, adsorption capacity of phenol on 30HM-Bent is higher than that on 60TMA-Bent, while it is lower at high phenol concentrations. The adsorption capacity of phenol on HM-Bent increased with increasing HM loading under 0.40CEC (cationic exchanged capacity), but which decreased when HM loading over 0.40CEC. The observed results suggested that in HM-Bent, the exposed siloxane surface was the effective adsorption sites for phenol, while the organic cations contributed to enhancing hydrophobic environment and the affinity for organic contaminants.

SORPTION OF BENZENE AND NAPHTHOL TO ORGANOBENTONITES INTERCALATED WITH SHORT CHAIN CATIONIC SURFACTANTS

This work studies the sorption of benzene and naphthol by bentonites exchanged with quaternary ammonium surfactants tetramethylammonium (TMA) ion, benzyltrimethylammonium (BTMA) ion, tetraethylammonium (TEA) ion, and benzyltriethylammonium (BTEA) ion to elucidate how exchanged short chain organic cations affect the mechanistic function of the modified bentonite. Local high charge density areas are found at interlamellar surfaces of bentonite and intercalated short chain organic cations aggregate preferentially at these sites to form organic carbon phase effective in nonionic organic compounds (NOC) uptake. Experimental results indicate that the amount of benzene uptake decreases as the size of intercalated organic cation increases from TMA to BTMA to TEA to BTEA, presumably due to the different structures of organic carbon phase formed in organobentonite. In addition, benzene sorption capacity of organobentonite modified with short chain organic cation is highly sensitive to the cation exchange capacities (CEC) of bentonite used.

Transformation of 2-(trifluoromethyl)aniline over ion-exchanged montmorillonites: formation of a dimer and cyclic trimer

The adsorption of 2-(trifluoromethyl)aniline (2TFMA) on the interlamellar surfaces of KSF, K-10, Cu 2+-, Ni 2+-, Fe 3+- and Al 3+-montmorillonites has been investigated. Structures of the dimer and cyclic trimer species, formed via condensation reactions, were determined using NMR and mass spectral data. The catalytic activity of montmorillonite was influenced by the specific cation present with the Ni 2+-montmorillonite producing the largest number of transformation products. The highest yield of 2-{difluoro-[2′-(trifluoromethyl)anilino]methyl}aniline (dimer) was obtained for Al 3+-montmorillonite. The Cu 2+-montmorillonite demonstrated the greatest activity in producing a 12-membered ring compound, (2-CF 2C 6H 4NH) 3, and gave the highest ratio of trimer to dimer yields. The presence of residual water in the Al 3+-montmorillonite allows conversion of some 2TFMA to a carboxylic acid.

Spectroscopic study of the adsorption of rhodamine 6G on clay minerals in aqueous suspensions

AbstractAbsorption and fluorescence spectroscopies were applied to study the adsorption of rhodamine 6G on several smectite-type clays in aqueous suspension. The dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. The presence of these species and their evolution with the stirring time of the sample and the relative dye/clay concentration depend on the nature of the smectite, the clay concentration and the particle size, factors that also affect the dispersion degree of the clay particles in water.

Physicochemical properties of protein-smectite and protein-Al(OH)x-smectite complexes

AbstractProteins (catalase, albumin, pepsin and lysozyme with different molecular weights and isoelectric points) were differently adsorbed at pH 7.0 on the clay fraction of three raw Na-saturated smectites (Crook and Uri montmorillonites and one hectorite). The adsorption isotherms of proteins on clay minerals showed typical Langmuir characteristics. Lysozyme was adsorbed under the effect of electrostatic interactions between the opposite charges of clay surfaces and protein molecules, whereas catalase and albumin were adsorbed under the effect of non-electrostatic forces. Pepsin was held in relatively high amounts only on the surfaces of hectorite. Proteins were intercalated in the interlayers spaces of smectites, usually undergoing extensive unfolding. Protein-smectite complexes showed different behaviour to heating treatment. Some complexes remained practically unchanged after heating at 200°C. Presence of ‘wrecks’ of interlayered materials was found after heating at 500°C for two hours. The amounts of proteins adsorbed on the external and interlamellar surfaces of clay minerals, partially coated with OH-Al species, were much lower than those fixed on the clean clays. Only lysozyme was intercalated in chlorite-like complexes.

Adsorption of Rhodamine 6G on saponite. A comparative study with other Rhodamine 6G-Smectite aqueous suspensions

Absorption and fluorescence spectroscopies are applied to study the adsorption of rhodamine 6G on saponite in aqueous suspensions. The interpretation of the experimental results suggests that the dye can be adsorbed as the monomeric and the dimeric forms on both the external and the interlamellar surfaces of the clay. A mechanism for the adsorption is proposed on the basis of the evolution of these species with the stirring time and the relative dye/clay concentration. The present results are compared to those previously reported for the adsorption of rhodamine 6G on laponite, montmorillonite ofWyoming, and hectorite in aqueous suspensions, and the effect of the nature of the clay and the particle size is discussed.

Ionic migration and charge reduction in Ni2+-, Co2+- and Zn2+-exchanged Texas montmorillonite

Controlled calcination of ion-exchanged Texas montmorillonite leads to layer charge reduction. Detailed chemical analyses of both exchangeable and unexchangeable metal species lead to the conclusion that, in complete contrast to the situation with Wyoming bentonite, Ni2+, Co2+ and Zn2+ are all capable of migrating from the interlamellar space into the octahedral region of the sheets where they are (i) isomorphously exchanged for Mg2+, and to a proportionately smaller extent, for Fe2+, (ii) trapped in the octahedral vacancies. In addition, they are converted to unexchangeable species on the interlamellar sheet surfaces by high temperature hydrolysis leading to hydroxide or oxide formation. It is suggested that protons within the octahedral region are bound as -OH+2rather than simply physically trapped.

Anion-exchange reactions of layered double hydroxides

Layered double hydroxides are antitypes of 2/1 clay minerals. In spite of a high surface charge density, as in micas, they react easily with various types of organic anions. Short- and long-chain anions are rapidly exchanged for the inorganic interlayer anions, and high degrees of reaction are attained. The pronounced preference of primary long-chain compounds as typical of the more highly charged clay minerals is not observed. The small equivalent area forces the alkyl chain compounds (fatty acid anions, dicarboxylates, alkyl sulfates) to point away from the interlamellar surfaces and to form monomolecular films of high regularity

Kinetics of Spinodal Decomposition in the TiO2‐SnO2 System: The Effect of Aliovalent Dopants

Dense samples in the TiO2‐SnO2 system were fabricated by pressureless sintering in air using commercial powders as well as powders obtained by coprecipitation. Phase equilibria in the system were examined using X‐ray diffraction on samples that were annealed for long periods of time. Samples of near equi‐molar composition made with controlled amounts of Al and Ta (separately) as dopants were annealed inside the coherent spinodal. X‐ray diffraction and electron microscopy were the principal characterization tools used to follow the kinetics of phase transformation. The composition modulations occurred along the [001] direction consistent with prior work. During the early stages of the decomposition process, interlamellar surfaces were coherent. At later stages, strain mismatch was accommodated by interface dislocations. The kinetics of decomposition were strongly influenced by the type and the amount of dopant. Specifically, trivalent aluminum enhanced the decomposition while pentavalent tantalum suppressed it. The kinetics in a sample doped with 0.5 mol% Al2O3 were more than 3 orders of magnitude more rapid than a sample doped with 1.0 mol% Ta2O5. These observations are rationalized on the premise that cation interstitial mobility is greater than cation vacancy mobility.

Interaction of Aliphatic Amines with Montmorillonite to Enhance Adsorption of Organic Pollutants

AbstractX-ray powder diffraction studies were performed with propyl-, dodecyl-, and dodecyldiam-monium bentonites saturated with aqueous mixtures of eleven organic pollutants at concentrations of 0.01 to 100%. The alkylammonium salt treatment separated the montmorillonite unit layers, which increased the interlamellar volume and exposed the interlamellar surfaces for adsorption of the organic pollutants. Washing the salt-treated clays with distilled water removed excess, physically adsorbed alkylammonium ions that could interfere with adsorption of the organic pollutants. Adsorption isotherms conducted at organic pollutant concentrations of 100 and 1000 mg/liter indicated the three alkylammonium montmorillonites, especially propylammonium montmorillonite, were effective adsorbents from aqueous solution, but to a lesser extent than adsorbents currently used in the water and wastewater fields.

Smectite clay minerals: properties and uses

The physicochemical properties of smectite clay minerals that determine their industrial utilization are reviewed. Smectite is the name used for a group of phyllosilicate mineral species, the most important of which are montmorillonite, beidellite, nontronite, saponite and hectorite. These and several other less common species are differentiated by variations in chemical composition involving substitutions of Al for Si in tetrahedral cation sites and Al, Fe, Mg and Li in octahedral cation sites. Smectite clays have a variable net negative charge, which is balanced by Na, Ca, Mg and, or, H adsorbed externally on interlamellar surfaces. The structure, chemical composition, exchangeable ion type and small crystal size of smectite clays are responsible for several unique properties, including a large chemically active surface area, a high cation exchange capacity, interlamellar surfaces having unusual hydration characteristics, and sometimes the ability to modify strongly the flow behaviour of liquids. In terms of major industrial and chemical uses, natural smectite clays can be divided into three categories, Na smectites, Ca-Mg smectites and Fuller’s or acid earths. Large volumes of Na smectites and Na-exchanged Ca-M g smectites and Fuller’s earth are directly used in the foundry, oil well drilling, wine, and iron ore and feed pelletizing industries, and are also used in civil engineering to impede water movement. Significant volumes of Na smectites are used for various purposes in the manufacturing of many industrial, chemical and consumer products. Large quantities of Ca-M g smectites are used directly in iron foundries, in agricultural industries and for filtering and decolorizing various types of oils. A significant fraction of the Ca-M g smectites used for decolorizing has been acid treated. Large volumes of Fuller’s or acid earths are commercially used for preparing animal litter trays and oil and grease absorbents, as carriers for insecticides, and for decolorizing of oils and fats. Natural Na smectites occur in commercial quantities in only a few places, but Ca-M g smectite and Fuller’s earth deposits of considerable size occur on almost every continent.

Intracrystalline grafting on layer silicic acids

Silanol groups (Si–OH) present at the surface of amorphous silica and microcrystalline silicates can react with organosilicon compounds to form stable organic derivatives1–3. The grafting reaction always occurs at the external surfaces of these materials. The present work considers the intracrystalline grafting on internal Si–OH groups in certain layer silicic acids. For the reaction to occur, the silicic acid must have been expanded by the intercalation of appropriate organic polar molecules such as dimethylsulphoxide; in this way, the individual layers of the silicic acid can be separated and the internal Si–OH groups become accessible to the grafting reagents. The resulting grafted products are organosilicic compounds which retain the lamellar structure of the starting silicic acids, but with the interlamellar surfaces covered by organic groups homogeneously distributed. These materials can be thought of as planar silicones.

Quantitative determination of the interlamellar volume in an interstratified mica-smectite soil clay

AbstractTwo well-characterized soils containing substantial amounts of interstratified mica-smectite have been examined by a combination of techniques in an attempt to correlate the characteristics of the interlamellar surfaces with other properties. The techniques used included XRD and chemical analyses, determination of surface area by nitrogen sorption and by desorption of water and ethylene glycol monoethyl ether vapour, and determination of cation exchange capacity. Reasonable agreement was found between the results obtained for the proportion of expanding or smectite-like surfaces, although estimates by XRD, especially when coarser material (> 2/µm) was included, were generally lower than those obtained by the other methods. The mean density of charge on the surfaces of the soil clays appeared to be similar to that of smectite. The decrease in porosity observed for both soils dried by the critical point method was largely accounted for by the decrease in volume associated with the loss of interlamellar water from the smectite-like interlamellar surfaces included within the clay fraction.

Intercalation of Silica in Smectite

AbstractSilica has been intercalated in swelling clays by hydrolysis and/or oxidation of tris(acetylacetonato)siIicon(IV) cations, Si(acac)3+, and polychlorosiloxanes, (-SiOCl2-)n, in the interla-mellar regions of the minerals. The Si(acac)3+ ions have been placed on the interlamellar surfaces by ion exchange and by in situ reaction of the acetylacetone-solvated clays with SiCl4. The (-SiOCl2-)n polymers were formed in the interlayers by reaction of adsorbed benzaldehyde with SiCl4. The optimum (001) spacing observed after firing the clays in air at 500°C (12.6 Å) corresponds to the presence of a monolayer of siloxane chains. Nitrogen BET surface areas range from 40 to 240 m2/g, depending on the amount of internal surface covered by the intercalated silica. In some cases, highly ordered products were formed which exhibit four orders of (00ℓ) reflection. Interstratified products with d(001) values between 9.6 and 12.6 Å exhibit surface areas consistent with the presence of a random mixture of totally collapsed interlayers and interlayers containing siloxane monolayers. Attempts to achieve silica intercalation by hydrolysis of SiCl4 in the clay interlayers were not productive.

Synthesis and characterization of the nickel(II)-rubeanic acid complex on interlamellar surfaces of molybdenum trioxide

Synthesis and characterization of the nickel(II)-rubeanic acid complex on interlamellar surfaces of molybdenum trioxide

Sorption-desorption characteristics of methyl parathion by clays.

Methyl parathion (O,O-dimethyl O-(4-nitrophenyl) phosphorothioate) adsorption was studied on montmorillonite, kaolinite, halloysite, natural zeolite, ion exchange resins and calcium carbonate. Methyl parathion was highly adsorbed by montmorillonite, followed by zeolite, and very little adsorption was obtained on kaolinite and halloysite. Calcium carbonate did not exhibit any adsorption. The values of the partial molar free energy "delta G" were calculated for all systems. For the montmorillonite-methyl parathion system, the calculated partial molar heat of adsorption "delta H", and the conformity of the data to Freundlich equation indicated a possible physical mechanism of adsorption. Increasing acetone concentration decreases methyl parathion adsorption and dehydration increased adsorption in the nonaqueous system. Moreover, the increased adsorption on the swollen clay indicated that methyl parathion was adsorbed on the interlamellar surfaces of the clay. This was also in agreement with the results of the desorption studies, since the insecticide was not desorbed using the same aqueous acetone solution. This indicated that methyl parathion was adsorbed as a water-insoluble organic compound.

Adsorption of Aluminum by a Smectite: I. Surface Hydrolysis During Ca2+‐Al3+ Exchange

AbstractThe exchange of Ca by Al in montmorillonite suspensions adjusted to low pH (∼4) was investigated by measurement of Ca, Al, and pH in the solution phase before and after the addition of clay. Adsorption of Al was accompanied by a release of protons to solution, indicating that the clay surfaces promoted hydrolysis of Al3+ when only partial coverage of exchange sites by Al had occurred. Calculations showed a two‐ to nine fold increase in the degree of hydrolysis at the surface relative to the value for solution. In suspensions with pKsp values near 33.3, Al(OH)3 was precipitated on interlamellar surfaces. However, at lower pH, the predominant form of adsorbed Al was Al3+, with hydroxy ‐Al monomers and/or polymers also present. The preference of the ions for the clay surface followed the order: hydroxy‐Al>Al3+>Ca2+>H3O+. The exchange systems were complicated by proton uptake by the montmorillonite, resulting from H3O+‐Ca2+ exchange as well as proton adsorption by edges concurrent with slow mineral dissolution reactions.

NEW INTERCALATED COMPLEXES OF MPS3 (M=Mg, Zn, Mn) WITH n-ALKYLAMINES

Abstract New types of intercalated complexes are synthesized by the reaction of MPS3 layer crystals (M=Mg, Zn, Mn) with n-alkylamines (CnH2n+1NH2, n=1–8). The amines are adsorbed on the interlamellar surfaces of the MPS3 crystals to form two different types of complexes of increased values in basal spacing. The spacing value of the first type of complexes is independent of the number of carbon atoms in alkyl chain of the amines, and the value of the second type of complexes increases with the number of the carbon atoms. The possible arrangements of the amines in the interlayer region are proposed for these types of complexes.