Intercalated Surfactant Molecules Research Articles

Intercalation of montmorillonite with dialkylammonium cationic surfactants

Three montmorillonite (Mt) clays with different cation exchange capacity (CEC) were intercalated by a series of dialkyl ammonium cations with carbon numbers (Nc=) 8, 10, 12 and 18 in each alkyl chain. The intercalated clays were characterized by X-ray Diffraction, Thermogravimetric Analysis, Fourier Transform Infrared Spectroscopy and Transmission Electron Microscopy. The basal spacing of intercalated Mts increased gradually with increasing chain length. Interlayer arrangement of dialkyl surfactant cations in the intercalated clay gallery changed from flat bilayer to paraffinic and reached as high as 3.57 nm for the highest CEC clay intercalated with cations having Nc= 18 in each alkyl chain. For dialkylammonium intercalants, the increase in basal spacing was predominantly guided by the intercalant's structure and organic loading values lacked such direct correlation. Mt with low CEC showed large expansion in the clay interlayer with increasing alkyl chain length even with marginal increment in the organic loading. The highest basal spacing was observed always with the highest CEC clay for every intercalant irrespective of alkyl chain length. The wide expansion of clay gallery for the longest dialkyl intercalant in this series for all clays might be attributed to presence of intercalated surfactant molecules along with physically adsorbed surfactant cations in the clay interlayer. The change in conformations of intercalated dialkyl ammonium intercalants with increasing alkyl chain length was analyzed from FTIR study. With increasing alkyl chain length, methylene symmetric and asymmetric stretching frequency shifted to lower wavenumber for all Mt clays- which indicated greater ordered conformations obtained with increasing alkyl chain length.

Structure and Dynamics of Nonionic Surfactant Aggregates in Layered Materials.

The aggregation of surfactants on solid surfaces as they are adsorbed from solution is the basis of numerous technological applications such as colloidal stabilization, ore flotation, and floor cleaning. The understanding of both the structure and the dynamics of surfactant aggregates applies to the development of alternative ways of preparing hybrid layered materials. For this purpose, we study the adsorption of the triethylene glycol mono n-decyl ether (C10E3) nonionic surfactant onto a synthetic montmorillonite (Mt), an aluminosilicate clay mineral for organoclay preparation with important applications in materials sciences, catalysis, wastewater treatment, or as drug delivery. The aggregation mechanisms follow those observed in an analogous natural Mt, with the condensation of C10E3 in a bilayer arrangement once the surfactant self-assembles in a lamellar phase beyond the critical micelle concentration, underlining the importance of the surfactant state in solution. Solid-state 1H nuclear magnetic resonance (NMR) at fast magic-angle spinning (MAS) and high magnetic field combined with1H-13C correlation experiments and different types of 13C NMR experiments selectively probes mobile or rigid moieties of C10E3 in three different aggregate organizations: (i) a lateral monolayer, (ii) a lateral bilayer, and (iii) a normal bilayer. High-resolution 1H{27Al} CP-1H-1H spin diffusion experiments shed light on the proximities and dynamics of the different fragments and fractions of the intercalated surfactant molecules with respect to the Mt surface. 23Na and 1H NMR measurements combined with complementary NMR data, at both molecular and nanometer scales, precisely pointed out the location of the C10E3 ethylene oxide hydrophilic group in close contact with the Mt surface interacting through ion-dipole or van der Waals interactions.

Effect of the synthesis method on the properties and catalytic activities of Au/Pd nanoparticles supported on organophilic clay

Au and Pd mono and bimetallic nanoparticles supported on organoclay were prepared using two different approaches. In the first method (POST), the intercalated surfactant molecules were used as capturing and stabilizing agents of metal cations to prepare metal nanoparticles. In the second method (PRE), pre-made surfactant-stabilized metal nanoparticles were directly incorporated in the cationic clay. The structure and composition of the materials were probed by XRD, ICP-AES, XPS, CHN, TGA, and UV–Vis spectroscopy, while morphology was characterized by SEM. It was revealed that by the POST method, the particles remained majoritarily on the outer surface of the clay, while by the PRE method, the particles were intercalated. The materials were tested on the oxidation of benzyl alcohol, and the results showed that monometallic catalysts synthesized by the PRE method were more active. For the bimetallic materials, the activity was higher for the POST material, which was composed of AuPd alloy nanoparticles, while the PRE material was composed of isolated Au and Pd nanoparticles.

Structural characterisation and environmental application of organoclays for the removal of phenolic compounds

Modified montmorillonite was prepared at different surfactant (HDTMA) loadings through ion exchange. The conformational arrangement of the loaded surfactants within the interlayer space of MMT was obtained by computational modelling. The conformational change of surfactant molecules enhance the visual understanding of the results obtained from characterization methods such as XRD and surface analysis of the organoclays. Batch experiments were carried out for the adsorption of p-chlorophenol (PCP) and different conditions (pH and temperature) were used in order to determine the optimum sorption. For comparison purpose, the experiments were repeated under the same conditions for p-nitrophenol (PNP). Langmuir and Freundlich equations were applied to the adsorption isotherm of PCP and PNP. The Freundlich isotherm model was found to be the best fit for both of the phenolic compounds. This involved multilayer adsorptions in the adsorption process. In particular, the binding affinity value of PNP was higher than that of PCP and this is attributable to their hydrophobicities. The adsorption of the phenolic compounds by organoclays intercalated with highly loaded surfactants was markedly improved possibly due to the fact that the intercalated surfactant molecules within the interlayer space contribute to the partition phases, which result in greater adsorption of the organic pollutants.

The influence of alkyl chain length on surfactant distribution within organo-montmorillonites and their thermal stability

Organically modified clay minerals with high thermal stability are critical for synthesis and processing of clay-based nanocomposites. Two series of organo-montmorillonites have been synthesized using surfactants with different alkyl chain length. The organo-montmorillonites were characterized by X-ray diffraction and differential thermogravimetry, combining with molecule modelling. For surfactant with relatively short alkyl chain, the resultant organo-montmorillonite displays a small maximum basal spacing (ca. 1.5 nm) and most surfactants intercalate into montmorillonite interlayer spaces as cations with a small amount of surfactant molecules loaded in the interparticle pores with “house-of-cards” structure. However, for surfactant with relatively long alkyl chain, the resultant organo-montmorillonite displays a large maximum basal spacing (ca. 4.1 nm) and the loaded surfactants exist in three formats: intercalated surfactant cations, intercalated surfactant molecules (ionic pairs), and surfactant molecules in interparticle pores. The surfactant molecules (ionic pairs) in interparticle pores and interlayer spaces will be evaporated around the evaporation temperature of the neat surfactant while the intercalated surfactant cations will be evaporated/decomposed at higher temperature.

Adsorption of Cationic Monomeric and Gemini Surfactants on Montmorillonite and Adsolubilization of Vitamin E

Adsorption of a cationic gemini surfactant (1,2-bis(dodecyldimethylammonio) ethane dibromide, 12-2-12) and the corresponding monomeric surfactant (dodecyltrimethylammonium bromide, DTAB) on montmorillonite has been characterized with a combination of adsorption isotherm, interlayer spacing and FT-IR spectroscopic data. Adsolubilization of vitamin E into the adsorbed surfactant layers has also been studied. The adsorption isotherm data reveal that the adsorption of the two surfactants is driven by the two factors: one is the cation exchange that occurs on the interlayer basal planes and the other is the hydrophobic interaction between hydrocarbon chains of the surfactants. Although the adsorbed amount measured in the saturation region (in mol g(-1)) is almost identical for the two surfactants, the conformation of the intercalated surfactant molecules differs significantly from each other. The adsorption of DTAB results in a lateral bilayer arrangement in the limited interlayer space, whereas 12-2-12 gives a normal bilayer arrangement in the expanded interlayer space. Adsolubilization of vitamin E takes place into the adsorbed surfactant layers, and interestingly, all the vitamin E molecules added in the montmorillonite suspensions are hybridized at lower surfactant concentrations due to the great specific surface area of the clay material. Since the maximum adsolubilization amount is usually obtained just below the critical micelle concentration, the gemini surfactant is deemed to be more efficient than the corresponding monomeric one to achieve the great adsolubilization amount.

Interactions between nonionic Triton X surfactants and cholesterol-containing phosphatidylcholine liposomes

Solubilization kinetics experiments were developed to study the effects of the polyethylene glycol chain length of Triton X surfactants on their interactions with the cholesterol-containing phosphatidylcholine vesicles. An empirical liposome stability ratio was used to describe the vesicle solubilization process. The effectiveness of Triton X surfactants in solubilizing vesicles decreases with increasing polyethylene glycol chain length of surfactants. It was also shown that vesicles containing the intercalated surfactant molecules with the largest number of ethylene glycol units per molecule exhibited the exceedingly retarded solubilization behavior. Independent experiments based on a thermodynamic approach provide supporting evidence for the conclusions obtained from solubilization kinetics experiments.

Molecular order and disorder of surfactants in clay nanocomposites

Disorder of intercalated surfactant molecules in clay minerals causes gradual swelling, rather than commonly assumed swelling in discrete steps.

Characterization of organic phases in the interlayer of montmorillonite using FTIR and 13C NMR

The molecular conformation and mobility of the intercalated surfactant molecules cetyltrimethylammonium bromide (CTMAB) have been studied using Fourier transform infrared spectroscopy (FTIR) and high-resolution single-pulse 13C magic angle spinning nuclear magnetic resonance ( 13C SP MAS NMR) spectroscopy. The conformation and mobility of alkyl chains were found to be a function of the surfactant concentration. The splitting of the methylene scissoring mode at 1473–1463 cm −1 and the rocking mode at 730–720 cm −1 in FTIR are considered to be diagnostic of the packing density increase of the intercalated surfactants within the clay gallery. Compared with the 13C SP MAS NMR spectrum of CTMAB in the bulk state, 1–3 ppm upfield chemical shifts for end-methyl ( δC 16) and methylene ( δC 15, δC 2–14) of the intercalated surfactant molecules in the hybrids indicate a freer conformational situation. For these hybrids, the conformational freedom decreases with increased of surfactant concentration. In addition, on ∼2 ppm downfield shift for the C 1 carbon atom in the hybrids with higher surfactant content suggests a special local environment. This study demonstrates the different mobility of carbon atoms in the intercalated alkyl chain.

Conformation of Surfactant Molecules in the Interlayer of Montmorillonite Studied by 13C MAS NMR

AbstractThe ordering conformation of surfactant molecules in intercalated montmorillonite prepared at various concentrations was investigated by 13C MAS NMR. The 13C MAS NMR study demonstrates the coexistence of ordered and disordered chain conformations. Two main resonance peaks are associated with the backbone alkyl chains: the resonance at 33 ppm corresponds to the ordered conformation (all-trans), and the resonance at 30 ppm corresponds to the disordered conformation (mixture of trans and gauche). Deconvolution of 13C MAS NMR spectra indicates that the ordering conformation of surfactant molecules within the gallery of montmorillonite depends very much on their orientation and packing density. When amine chains are oriented parallel to the silicate layers, the amount of all-trans conformer decreases with the increase of amine concentration. However, the amount of all-trans conformer increases with the increase of amine concentration when amine chains radiate from the silicate layers. Furthermore, 13C MAS NMR spectra show that the intercalated surfactant molecules in the clay minerals never attained the complete liquidlike or solidlike behavior.

Silicalite-1 Zeogrid: A New Silica Molecular Sieve with Super- and Ultra-Micropores

Spherical, micrometer-sized particles with a layered structure were obtained by precipitation of a Silicalite-1 zeolite nanoslab suspension upon addition of cetyltrimethylammonium bromide (CTMABr) and subsequent calcination. The material had a specific micropore volume of 0.69 cm3 g–1, distributed over super- and ultra-micropores. The formation process of this peculiar microporous solid was studied using X-ray diffraction (XRD), 29Si MAS NMR spectroscopy, thermogravimetry (TG), and nitrogen adsorption. In the precipitate, the Silicalite-1 nanoslabs were laterally fused into nanoplates and stapled into layers with intercalated surfactant molecules. Removal of the surfactant through calcination caused facial fusion, besides additional lateral fusion, of the nanoplates. Empty spaces left lying laterally between individual nanoplates were responsible for the super-microporosity. The ultra-micropores were zeolitic channels inside the fused nanoplates. The potential of these Silicalite-1 zeogrids as molecular sieves was demonstrated with pulse gas-chromatographic separation of alkane mixtures. The mass-transfer resistance of a packed bed of zeogrid particles was considerably lower than of compacted zeolite powder.

Molecular dynamics simulations of organically modified layered silicates

Molecular dynamics (MD) simulations are used to study the static and dynamic properties of 2:1 layered silicates ion exchanged with alkyl-ammonium surfactants. These systems are in the form of oligomeric alkanes grafted by cationic groups on atomically smooth crystalline layers 10 Å thick and several microns wide. The organically modified layers self-assemble parallel to each other to form alternating, well-ordered organic/inorganic multilayers. By studying the systems at the experimentally measured layer separations, computer modeling directly provides the structure and dynamics of the intercalated surfactant molecules. The grafted-chain conformations are also expressed through the trans-gauche conformer ratios and transition frequencies which compare well with Fourier transform infrared spectroscopy (FTIR) experiments.