Stabilization of hydroxyl-group-terminated SERS-marker molecules on microAg particles by silanization

Surfactant modified montmorillonitic clays synthesized by ion exchange using the hydrothermal reaction method have been compared using XRD and thermal analysis. X-ray diffraction (XRD) shows the changes in the surface properties of organoclays through expansion with surfactant loading. A polynomial relationship exists between the basal spacing and the CEC loading described by the equation y=0.3232x(2) + 0.2052x+1.2834 with R(2)=0.9955. Different arrangements of the surfactant molecules in the organoclays are inferred from the changes in basal spacings. para-Nitrophenol also causes the expansion of the montmorillonite clay and affects the arrangements of the surfactant molecules within the clay layers. Changes in the surfactant molecular arrangements were analyzed by thermogravimetry. Additional thermal decomposition steps were observed when para-nitrophenol is adsorbed on the organoclay.     

Thermal stability of organoclays with mono- and di-alkyl cationic surfactants

In this study, mono- and di-alkyl cationic surfactants were used to prepare organoclays through ion exchange and the prepared organoclays were characterised by X-ray diffraction (XRD) and thermogravimetric analysis (TG). Larger basal spacings were observed in the interlayer of the organoclays intercalated with DDDMA than organoclays intercalated with DDTMA. The DTG curves identify the thermal stability of organoclays intercalated with two different types of surfactants (DDTMA and DDDMA) and the different arrangements of the surfactant molecules intercalated in the montmorillonite. Both organoclays intercalated with organic surfactant molecules proved to be thermally stable at high temperature. This study provides an understanding of the structure and properties of organoclays, which will enhance the potential applications of organoclays in environmental remediation.     

Characterisation of organoclays and adsorption of p-nitrophenol: environmental application

Organoclays were synthesised through ion exchange of a single surfactant for sodium ions, and characterised by a range of method including X-ray diffraction (XRD), BET, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). The change in surface properties of montmorillonite and organoclays intercalated with the surfactant, tetradecyltrimethylammonium bromide (TDTMA) were determined using XRD through the change in basal spacing and the expansion occurred by the adsorbed p-nitrophenol. The changes of interlayer spacing were observed in TEM. In addition, the surface measurement such as specific surface area and pore volume was measured and calculated using BET method, this suggested the loaded surfactant is highly important to determine the sorption mechanism onto organoclays. The collected results of XPS provided the chemical composition of montmorillonite and organoclays, and the high-resolution XPS spectra offered the chemical states of prepared organoclays with binding energy. Using TGA and FT-IR, the confirmation of intercalated surfactant was investigated. The collected data from various techniques enable an understanding of the changes in structure and surface properties. This study is of importance to provide mechanisms for the adsorption of organic molecules, especially in contaminated environmental sites and polluted waters.     

A thermoanalytical assessment of an organoclay

High resolution thermogravimetric analysis (TG) has attracted much attention in the synthesis of organoclays and its applications. In this study, organoclays were synthesised through ion exchange of a single cationic surfactant for sodium ions, and characterised by methods including X-ray diffraction (XRD) and TG. The changes of surface properties in montmorillonite (MMT) and organoclays intercalated with surfactant were determined using XRD through the changes in the basal spacing. The TG was applied in this study to investigate more information of the configuration and structural changes in the organoclays with thermal decomposition. There are four different decompositions steps in differential thermogravimetric curves. The obtained TG steps are relevant to the arrangement of the surfactant molecules intercalated in MMT and the thermal analysis indicates the thermal stability of surfactant modified clays. This investigation provides new insights into the properties of organoclays and is important in the synthesis and processing of organoclays for environmental applications.     

Sorption of Bisphenol A as Model for Sorption Ability of Organoclays

The arrangement of bisphenol A molecules into organoclays and their interactions with the intercalated surfactant were studied. The organoclays were prepared via solid-state intercalation of four cationic surfactants, such as dodecyltrimethyl-, tetradecyltrimethyl-, hexadecyltrimethyl-, and didodecyldimethyl-ammonium, as bromide salts, at different loading levels into the interlayers of two natural clays. The natural clays, the prepared organoclays, and the spent organoclays were characterized by X-ray powder diffraction, infrared spectroscopy, and scanning electron microscopy. X-ray powder diffraction measurements showed successive interlayer expansions of the d₀₀₁ basal spacing due to the intercalation of the cationic surfactants and the bisphenol A sorption. The increased d₀₀₁ basal spacing of the organoclays after bisphenol A sorption indicates that the molecules are integrated between the alkyl chains of the surfactant in the organoclays interlayers. Infrared spectroscopy was employed to probe the intercalation of the cationic surfactants and the sorbed bisphenol A. New characteristic bands attributed to the bisphenol A phenol rings appear in the range 1518–1613 cm⁻¹ on the infrared spectra of the spent organoclays, proving the presence of bisphenol A in the hydrophobic interlayers. Scanning electron microscopy of the organoclays before and after BPA sorption shows that their morphology becomes fluffy and that the presence of the organic molecules expands the clay structure.     

Influence of cationic surfactant removal on the thermal stability of organoclays

The microstructure, thermal stability, surface energy, and swelling characteristics of two kinds of commercial organoclays, before and after washing treatment with a mixture of H2O/ethanol, were investigated using X-ray diffraction (XRD), thermogravimetric analysis (TG/DTG), wettability measurement, and swelling measurement. This study demonstrates that the external-surface physically adsorbed surfactant can be removed after washing treatment, resulting in an increase in thermal stability and a decrease in surface energy of the resultant organoclays. Organoclays are difficult to be introduced into a solvent when their surface energies are lower than that of the solvent. On the other hand, the organoclay with gamma(organoclay) < gamma(solvent) is easier to be swollen and expandable by the solvent. The swelling and basal spacing measurements of the organoclays introduced into organic media indicate that the swelling factor and the interlayer swelling are two independent parameters. Both the polar character of the solvent and the swelling capacity of clay have a prominent effect on the interlayer swelling of the organoclays.     

Secondary adsorption of nitrobenzene and m-nitrophenol by hexadecyltrimethylammonium-montmorillonite thermo-XRD-analysis

In the present research we studied the effect of the solvent used, whether it was polar water or a non-polar organic solvent (n-hexane or n-hexadecane), on the basal-spacing and bulk structure of the sorbate-sorbent complexes obtained by the secondary adsorption of nitrobenzene and m-nitrophenol by two types of organo-montmorillonites. X-ray measured basal spacings before and after thermal treatments up to 360°C. The organo-clays were synthesized, with 41 and 90% replacement of the exchangeable Na⁺ by hexadecyltrimethylammonium (HDTMA), with mono-and bilayers of HDTMA cations in the interlayer space, labelled OC-41 and OC-90, respectively. After heating at 360°C both organo-clays showed spacing at 1.25–1.28 nm, due to the presence of interlayer-charcoal, indicating that in the preheated organo-clays the HDTMA was located in the interlayer. The thermo-XRD-analysis of Na-clay complexes showed that from organic solvents both sorbates were adsorbed on the external surface but from water they were intercalated. m-Nitrophenol complexes of both organo-clays obtained in aqueous suspensions contain water molecules. Spacings of nitrobenzene complexes of OC-41 and OC-90 and those of nitrophenol complexes of OC-41 showed that the adsorbed molecules were imbedded in cavities in the HDTMA layers. Adsorption of m-nitrophenol by OC-90 from water and n-hexane resulted in an increase of basal spacing (0.21 and 0.29 nm, respectively) suggesting the existence of a layer of nitrophenol molecules sandwiched between two parallel HDTMA layers.     

Structure of organoclays--an X-ray diffraction and thermogravimetric analysis study

X-ray diffraction has been used to study the changes in the surface properties of a montmorillonitic clay through the changes in the basal spacings of montmorillonite (SWy-2) and surfactant-intercalated organoclays. Variation in the d-spacing was found to be a step function of the surfactant concentration. High-resolution thermogravimetric analysis (HRTG) shows that the thermal decomposition of SWy-2-MMTs modified with the surfactant octadecyltrimethylammonium bromide takes place in four steps. A mass-loss step is observed at room temperature and is attributed to dehydration of adsorption water. A second mass-loss step is observed over the temperature range 87.9 to 135.5 degrees C and is also attributed to dehydration of water hydrating metal cations such as Na+. The third mass loss occurs from 178.9 to 384.5 degrees C and is assigned to a loss of surfactant. The fourth mass-loss step is ascribed to the loss of OH units through dehydroxylation over the temperature range 556.0 to 636.4 degrees C. A model is proposed in which, up to 0.4 CEC, a surfactant monolayer is formed between the montmorillonitic clay layers; up to 0.8 CEC, a lateral-bilayer arrangement is formed; and above 1.5 CEC, a pseudotrimolecular layer is formed, with excess surfactant adsorbed on the clay surface.     

Modification of Wyoming montmorillonite surfaces using a cationic surfactant

Surfaces of Wyoming SWy-2-Na-montmorillonite were modified using ultrasonic and hydrothermal methods through the intercalation and adsorption of the cationic surfactant octadecyltrimethylammonium bromide (ODTMA). Changes in the surfaces and structure were characterized using X-ray diffraction (XRD), thermal analysis (TG), and electron microscopy. The ultrasonic preparation method results in a higher surfactant concentration within the montmorillonite interlayer when compared with that from the hydrothermal method. Three different molecular environments for surfactants within the surface-modified montmorillonite are proposed upon the basis of their different decomposition temperatures. Both XRD patterns and TEM images demonstrate that SWy-2-Na-montmorillonite contains superlayers. TEM images of organoclays prepared at high surfactant concentrations show alternate basal spacings between neighboring layers. SEM images show that modification with surfactant reduces the clay particle size and aggregation. Organoclays prepared at low surfactant concentration display curved flakes, whereas they become flat with increasing intercalated surfactant. Novel surfactant-modified montmorillonite results in the formation of new nanophases with the potential for the removal of organic impurities from aqueous media.     

Thermal stability of octadecyltrimethylammonium bromide modified montmorillonite organoclay

Organoclays are significant for providing a mechanism for the adsorption of organic molecules from potable water. As such their thermal stability is important. A combination of thermogravimetric analysis and infrared emission spectroscopy was used to determine this stability. Infrared emission spectroscopy (IES) was used to investigate the changes in the structure and surface characteristics of water and surfactant molecules in montmorillonite, octadecyltrimethylammonium bromide and organoclays prepared with the surfactant octadecyltrimethylammonium bromide with different surfactant loadings. These spectra collected at different temperatures give support to the results obtained from the thermal analysis and also provide additional evidence for the dehydration which is difficult to obtain by normal thermoanalytical techniques. The spectra provide information on the conformation of the surfactant molecules in the clay layers and the thermal decomposition of the organoclays. Infrared emission spectroscopy proved to be a useful tool for the study of the thermal stability of the organoclays.     

Organoclays from alkaline-treated acid-activated clays

The cetyltrimethylammonium hydroxide (C16TMAOH) solution was proposed for the preparation of organoclays. Montmorillonite clay was acid activated at different acid/clay (a/c) (in mass) ratios, then treated with alkaline (sodium hydroxide) solution before being reacted with C16TMAOH solution. The acid activation caused a reduction in the number of cation exchange sites, and hence improved the exfoliation of the silicate sheets at higher pH values. The basal spacing increased significantly from 2.20 to 4.01 nm, and depended on the a/c ratios. The acid-activated clays with a/c ratios greater than 0.3 adsorbed significant amounts of C16TMA cations with a basal spacing of 4.01 nm compared with the non-acid-activated montmorillonite (2.51 nm). Meanwhile, the treatment of NaOH solution yielded clays with similar properties to that of the raw used clay. The XRF data, FT-IR, and ²⁹Si MAS-NMR techniques confirmed that the resulting amorphous silica during the acid activation was dissolved, and accompanied by a dramatical reduction in the surface areas. Similar amounts of C16TMA cations were adsorbed, i.e., close to 1 mmol g^?1, with a single basal spacing of 2.52 nm, independently of the treated acid-activated clays. The in-situ powder XRD studies revealed that an increase of the basal spacing to 4.20 nm was observed at intermediate temperatures ranging from 50 to 150 °C for organo-acid-activated clays with basal spacing of 4.01 nm, while a continuous decrease of the basal spacing was observed for organoclays with a basal spacing of 2.52 nm. At higher temperatures greater than 250 °C, the decomposition of the surfactant occurs, and the basal spacing decreases to a value of about 1.4 nm.     

Thermal properties of different transition metal forms of montmorillonite intercalated with mono-, di-, and triethanolammonium compounds

In the present study, different transition metal forms of montmorillonite have been intercalated with mono-, di-, and triethanolammonium cations via d coordination mechanism to investigate their thermal behavior, structural characteristics, surface properties, and elemental composition using TG, XRD, BET, and CHNS techniques. Thermogravimetric analysis showed two thermal transition steps for transition metal-exchanged montmorillonites, which attributed to desorption of the physically adsorbed water and hydrated water, and dehydroxylation of the structural water; whereas for ammonium-montmorillonite complexes, the TG curves showed three thermal transition steps which attributed to desorption of the adsorbed water and dehydration, decomposition of the ammonium cations in the interlayer space of montmorillonite, and the dehydroxylation of the structural water. The thermal analysis of ammonium-montmorillonites affirmed that the molar mass of amine compounds used affects both desorption temperature (position) and the amount of the adsorbed water (intensity). XRD results revealed that the molar mass of amine used has linear relation with the basal spacings of the corresponding ammonium-montmorillonites, indicating structural changes. BET results showed that the molar mass of amines has an inverse effect on the surface area of the studied samples. CHNS analysis for the studied samples quantitatively confirmed the intercalation of ammonium cations into the interlayer space of montmorillonite.     

Investigation of structure and thermal stability of surfactant-modified Al-pillared montmorillonite

For combining the properties of organoclays and pillared clays, inorganic–organic clays have attracted much attention in recent years. In this study, Al Keggin cation pillared montmorillonites (Al-Mts) were first prepared and parts of Al-Mts were calcined at different temperatures (C-Al-Mts). The inorganic–organic montmorillonites were synthesized by intercalating Al-Mts and C-Al-Mts with the cationic surfactant, hexadecyltrimethyl ammonium bromide (HDTMAB). The products were characterized by X-ray diffraction, X-ray fluorescence, and simultaneous thermogravimetric analysis. For HDTMAB-modified uncalcined Al Keggin cation pillared montmorillonites (H-Al-Mts), the basal spacing increased with the increment of surfactant loading level, but the Al content of H-Al-Mts decreased simultaneously, indicating that the intercalated surfactant replaced some Al Keggin cations in the interlayer space. However, in the case of C-Al-Mts, the interlayer spaces could not be further expanded after surfactant modification, implying that the neighboring montmorillonite layers were “locked” by the aluminum pillars which were formed by dehydroxylation of Al Keggin cation pillars during thermal treatment. The thermal stability of HDTMAB-modified C-Al-Mts (H-C-Al-Mts) was much better than that of H-Al-Mts. The major mass loss of H-C-Al-Mts occurred at ca. 410 °C, corresponding to decomposition of intercalated surfactant cations. In contrast, H-Al-Mts displayed two mass loss temperatures at ca. 270 and 410 °C, corresponding to the evaporation of surfactant molecules and the decomposition of surfactant cations in the interlayer space, respectively.     

Removal of phenols from water and petroleum industry refinery effluents by activated carbon obtained from coconut coir pith

Coir pith obtained from the coir industry as waste biomass was used to prepare activated carbon by chemical activation using phosphoric acid (H₃PO₄). The influences of activation temperature and lasting time of activation on specific surface areas (SSA) of the activated carbons were observed. Physical characteristics of the activated carbon were investigated using X-ray diffraction (XRD), infra-red spectroscopy (IR), surface area analyzer, scanning electron microscopy (SEM), thermal analysis and potentiometric titration. The feasibility of using activated carbon for the removal of phenol (P), p-chlorophenol (PCP) and p-nitrophenol (PNP) from water and petroleum refinery industry effluents was investigated. The effects of contact time, adsorbent dose, ionic strength and initial concentration on the adsorption of phenols onto the activated carbon were investigated. The optimum pH for the maximum removal of phenols was 6.0. The equilibrium adsorption data of phenols were correlated to Langmuir and Freundlich isotherm models, the latter being the best fit of the experimental data. Dynamics of the sorption process and mass transfer were investigated using McKay and Urano-Tachikawa models. Adsorption kinetic data fits the Urano-Tachikawa kinetic model. The utility of the adsorbent was tested by using petroleum refinery industry effluent. The adsorbed phenols can be recovered by treatment with 0.1 M NaOH solution.     

The adsorption of salicylic acid onto γ-alumina and kaolinite from solution in hexane studied using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT)

It was possible to determine the maximum loading of salicylic acid adsorbed onto γ-alumina and kaolinite clay after exposure to salicylic acid dissolved in hexane by examination using diffuse reflectance infrared Fourier transform infrared spectroscopy (DRIFTS). The maximum surface loading of salicylic acid (which resisted washing with fresh hexane) on γ-alumina was four times that observed using water as a solvent (approximately 3.0 compared with 0.7 molecules/nm²). Washing the sample with water removed the organic which was in excess to the maximum level observed for samples prepared with aqueous solution. The spectra of samples prepared with a loading up to the maximum observed with aqueous solution showed no significant differences to those of samples where the organic had been adsorbed from hexane (with the same surface loading). New peaks were observed for loadings greater than 1 molecules/nm², but the salicylic acid was still present as carboxylate (with no clear evidence for the carbonyl group). Salicylic acid adsorbed more readily to the surface of kaolinite from solution in hexane than from aqueous solution (up to maximum average loading of 2 molecules/nm²). Washing the samples with water removed the organic to a loading in the region of 0.2 molecules/nm², independent of the initial loading. Salicylic acid was adsorbed onto kaolinite as the carboxylate. The findings indicate that uptake is mediated by a surface water layer even in the absence of bulk water.     

Preparation and characterization of organoclays based on an amphoteric surfactant

The uptake of the amphoteric surfactant, cocamidopropyl betaine (CAB) by a sodium montmorillonite clay was studied with respect to concentration and pH. A series of organoclays was prepared in which the basal spacings were found to depend on both parameters. Adjusting the solution pH during preparation influenced the adoption of either 1.8 or 2.0 nm spacing, whereas the 3.9 nm spacing in the same sample was unaffected. The presence of the carboxyl group in the CAB molecule enabled dispersion of the clay at higher pH by which means pure organoclays of high spacing could be obtained, whereas, without dispersing the clay, mixed populations were obtained. The results indicate the optimum parameters for preparing organoclays of desired spacings for use in clay-reinforced nanocomposites.     

FTIR investigation of CTAB-Al-montmorillonite complexes

In this study, CTAB-Al-montmorillonite complexes were synthesized by pre-modifying montmorillonite using different concentrations of surfactant (resulting in different surfactant loadings and basal spacings), then pillaring the organoclays with hydroxy-Al cations. The resultant inorganic-organic montmorillonite complexes were characterized using FTIR, with a combination of XRD, TG and chemical analysis. This study indicates that the basal spacings of the CTAB-Al-montmorillonite complexes and the amounts of Al-contained pillars strongly depend on the surfactant loadings in the clay interlayer space, resulted from the mobility variation of the intercalated surfactants. During pillaring hydroxy-Al cations into clay interlayer space, part of the intercalated surfactants were removed, resulting in a decrease of the ordering of alkyl chains and the frequency shifts of Si(Al)-O, Si-O-Al and (M-O)(Td) stretching vibrations. The hydrophobicity of the CTAB-Al-montmorillonite complex also strongly depends on the surfactant loading whereas that of the CTAB-Al-montmorillonite complex is relative lower than that of the corresponding organoclay, indicated by the frequency shift of the vibrations corresponding to the sorbed water and their contents estimated by TG curves. With the decrease of the sorbed water content, the frequency of the band of H-O-H bending (nu(2)) shifts to higher frequency while the O-H stretching vibration (nu(1) and nu(3)) shifts to lower frequency, indicating that H(2)O is less hydrogen bonded. Meanwhile, the ordered conformations of the alkyl chains in CTAB-Al-montmorillonite complex decrease when compared with that of the corresponding organoclay.     

Surface structure of CTMA modified bentonite and their sorptive characteristics towards organic compounds

This work was to examine the correlation between surface structure and sorptive characteristics of cetyltrimethylammonium cations (CTMA⁺) modified bentonite, which will provide novel information for exploring the sorptive mechanisms of organoclays. Various amounts of CTMA⁺ (0.21–1.98 mmol/g) were intercalated into bentonite to prepare a series of organobentonites with different structures. N₂ adsorption–desorption isotherms were plotted for the organobentonites to obtain the surface structure information, and sorption capacities of these organobentonites toward phenol, aniline, nitrobenzene and naphthalene were examined. It was shown that surface areas, pore volumes and surface fractal dimension of the organobentonites decreased with increasing CTMA⁺ loading amount. Sorption capacities of the organobentonites towards the four organic compounds have no evident correlation with their surface structures, and K_oc values of the organic compounds were shown to first increase until the maximum and then decrease as CTMA⁺ loading amount further increased. Combining with the surface structure and sorption capacities of the organobentonites, we proposed that the solute molecules were penetrated into the CTMA⁺ aggregates, and partition rather than adsorption mechanism dominated the sorption processes. The CTMA⁺ aggregates formed optimal partition phases in the intermediate surfactant loading range.     

Influence concentration of modifying agent on properties of natural rubber/organoclay nanocomposites

Sodium-montmorillonite (Na-MMT) nanoclay was modified with different concentrations of octadecylamine organic modifying agent at 0.5, 1.0 and 1.5 times the CEC of Na-MMT. Influence of concentration of modifying agent on properties of the organoclays and natural rubber/organoclay nanocomposites was investigated. It was found that the optimum concentration of modifying agent was 1.5 times the CEC of Na-MMT. That is, at this concentration, larger d-spacing of organoclay particles and higher degree of clay dispersion in natural rubber matrix were observed. Larger interlayer d-spacing also caused enhancement of the mechanical properties of the NR/organoclay nanocomposites. Additionally, the NR/organoclay nanocomposites with higher concentration of modifying agent exhibited faster curing reaction with higher crosslink density. Furthermore, the organoclays with larger d-spacing and higher degree of dispersion in the natural rubber matrix exhibited enhancement of the mechanical and dynamic properties and thermal stability of natural rubber/organoclay nanocomposites.