Alkyltrimethylammonium Surfactants Research Articles

Free-Standing and Oriented Periodic Mesoporous Organosilica Films with Variable Pore Size at the Air−Water Interface

Free-standing and oriented periodic mesoporous organosilica (PMO) films with variable pore size have been synthesized at the air−water interface using cationic alkyltrimethylammonium surfactants (alkyl chain length from 12 to 18 carbon atoms) as the structure-directing agents and 1,2-bis(triethoxysilyl)ethane as the organosilica precursor. After hydrothermal reaction at 95 °C for 8 h, C12TA−PMO, C16TA−PMO, and C18TA−PMO films grown at the air−water interface have a uniform thickness of ∼350, ∼670, and ∼400 nm, respectively. From the XRD patterns and TEM images, PMO films with hexagonal symmetry were identified. It was confirmed by 29Si and 13C CP MAS NMR experiments that the organic−inorganic moiety (−Si−CH2−CH2−Si−) is the basic structural unit in the films. The pore diameter and the surface area of the surfactant-extracted PMO films, e.g., C12TA−, C16TA−, and C18TA−PMO films, obtained from the N2 sorption isotherms were to be determined to be 24.3, 26.4, and 32.8 A and 890.3, 917.7, and 811.0 m2 g-1, re...

Synthesis of Large-Pore Mesostructured Micelle-Templated Silicas as Discrete Spheres

The synthesis of large-pore micelle-templated silica (MTS) materials as discrete spheres by pseudomorphic synthesis solves the double challenge: enlarging the pore size of MTS beyond the diameter of the micelles and controlling the morphology and the size of the particles. The pseudomorphic transformation of silica beads of the desired size into MTS is one of the more suitable routes to control both nanometric and micrometric scales of MTS synthesis. The most employed method to enlarge pore diameter of MTS synthesized from alkyltrimethylammonium surfactants is to use 1,3,5-trimethylbenzene (TMB) as a swelling agent in the micelles. Unfortunately this method leads to particle aggregation. By means of the pseudomorphic route combining an original set of chemical compositions (C18TAB/decane/TMB/NaI), under mild conditions (low temperature, without autoclaving), we successfully synthesized nonaggregated MTS beads of 10 μm with pore diameter ranging from 7 to 9 nm, 900 m2/g specific surface area, and 1.5 mL/g...

Synthesis of Mesostructured and Mesoporous Aluminum Organophosphonates Prepared by Using Diphosphonic Acids with Alkylene Groups

Mesostructured and mesoporous aluminum organophosphonates (AOPs) with 2-D hexagonal structures (AOP-2) can be synthesized by using alkylene diphosphonic acids in the presence of alkyltrimethylammonium (CnTMA) surfactants. In the aqueous system, mesostructured AOP-2 with short-range orderings was obtained by using methylene diphosphonic acid. Low-temperature calcination at 400 °C of the mesostructured AOP-2 led to the deformation of the mesostructures. In the ethanol−water system, highly ordered mesostructured AOP-2 can be prepared through the reaction of aluminum chloride and alkylene diphosphonic acids. The preparation of ordered mesoporous AOPs with methylene and ethylene groups was achieved by using the method to remove surfactant molecules by calcination at 400 °C. Mesoporous AOPs containing propylene groups cannot be obtained through a similar synthetic procedure containing the low-temperature calcination process. Thermal stabilities of the organic groups within the frameworks are significant for the...

Investigation of elutions from a surfactant immobilized solid phase extraction sorbent based upon the hydrophobicity of the trapped species

A unique solid phase extraction (SPE) sorbent having a removable “stationary phase” is presented. This removable phase consists of alkyltrimethylammonium surfactant, which is initially immobilized onto hydrophilic strong cation exchange resin. The surfactant chain through hydrophobic interactions extracts hydrophobic analytes in the same manner as conventional bonded alkyl moieties on silica-based non-polar sorbents. For the extraction of very hydrophobic species with conventional sorbents, solvents such methylene chloride and benzene are needed to break strong hydrophobic interactions for efficient elutions. These solvents however are toxic to the analyst and present a significant environmental concern. Using a removable “stationary phase”, hydrophobic interactions need not be broken between the analyte and the sorbent. In the presented approach, the surfactant (“stationary phase”) is removed via ion exchange with exchange ions in very mild aqueous-based and instrument compatible solutions. The analyte, being associated with the surfactant, is also removed in the process. Very efficient elutions of analytes, regardless of hydrophobicity, under mild and more favorable environmental conditions are a direct benefit of having a removable “stationary phase”. Rinse solution parameters explored include exchange cation type and concentration, and alcohol type and concentration. The extraction of three test molecules of varying hydrophobicity, naphthalene, pyrene and benzo(ghi)perylene, is investigated using this sorbent material.

Solubilization of Aromatic Molecules in Templating Micelles of Mesoporous Silicas Followed by 1H NMR

Liquid-phase 1H NMR measurements of M41S synthesis gels were performed to characterize the structure of the alkyltrimethylammonium salt micelles swelled with benzene or trisubstituted aromatic compounds. The chemical shift and the line width of the aromatic species allowed the determination of the locus of the aromatic compounds within the templating micelle. Benzene is incorporated and immobilized close to the surface of the micelle, whereas the more hydrophobic trisubstituted aromatic compounds are located in the hydrophobic core of the micelle, where they retain some mobility. The variation of the amount of the aromatic swelling agent further supports this idea. The decreasing mobility of the alkyltrimethylammonium surfactants in the course of the synthesis could be followed from the broadening of the signals in 1H NMR. It showed that the organic−inorganic mesophase is formed within the first 24 h of synthesis.

Synthesis of SBA-1 Mesoporous Silica Crystals with Tunable Pore Size Using Sodium Silicate and Alkyltrimethylammonium Surfactants

Abstract The spherical micelle-templated SBA-1 mesoporous silicas with different pore size were conveniently synthesized from commercially available alkyltrimethylammonium surfactants and sodium silicate components in dilute solution.

Influence of pore size of MCM-41 matrices on drug delivery rate

The influence of pore size of MCM-41 materials on drug delivery rate has been studied. In order to achieve this objective, small pore size MCM-41 materials have been synthesised from mixtures of two different alkyltrimethylammonium surfactants with chain length of 8 and 10 carbon atoms. The analgesic ibuprofen was introduced into the pore channels and its delivery to the media has been measured and compared with the delivery from bigger pore-sized MCM-41 obtained from surfactants of 12 and 16 atoms of carbon. This study has revealed that the delivery rate of ibuprofen in a simulating body fluid solution decreases as the pore size decreases in the range of 3.6–2.5 nm.

Thermal stability of the secondary structure of poly(alpha,L-glutamate) in self-assembled complexes as studied by molecular dynamics in chloroform solution.

The thermal stability of the self-assembled complexes formed by alkyltrimethylammonium surfactants and poly(alpha,l-glutamate) is investigated in dilute chloroform solution by molecular dynamics simulations. By applying temperatures ranging from 300 to 500 K, we observed an extraordinary stability of the alpha-helix adopted by the polypeptide during a total of 48 ns. The multiple electrostatic interactions formed between the surfactant chains and the polyanion play a crucial role in this stability since they are responsible for the inaccessibility of the solvent molecules to the helix even at high temperatures.

Synthesis of Periodic Mesoporous Phenylenesilica under Acidic Conditions with Novel Molecular Order in the Pore Walls

High-quality phenyl-bridged organosilicas (p6mm) were prepared under acidic conditions using nonionic alkyl poly(oxyethylene) oligomers as structure-directing agents. In addition to the long-range order of the pore system, these materials exhibited a novel molecular order of the phenylene rings within the pore walls different from that observed in samples prepared under basic conditions in the presence of alkyltrimethylammonium surfactants.

Determination of alkyltrimethylammonium surfactants in hair conditioners and fabric softeners by gas chromatography–mass spectrometry with electron-impact and chemical ionization

The commercial hair conditioners and fabric softeners were analyzed for the content of alkyltrimethylammonium compounds (ATMACs) by gas chromatography–mass spectrometry (GC–MS) with electron impact (EI) and low-pressure positive-ion chemical ionization (PICI) modes. The method involves mixed diluted samples (adjust pH to 10.0) with potassium iodide to enhance the extraction of iodide–ATMA + ion pairs by direct liquid–liquid extraction. The iodide–ATMA + pairs were then demethylated to their corresponding nonionic alkyldimethylamines (ADMAs) by thermal decomposition in a GC injection-port. A high abundance of ADMAs was detected at the temperature above 300 °C in the GC injection-port. The enhanced selectivity of quasi-molecular ion chromatograms of C 12–C 18-ADMA, obtained using methanol PICI-MS enables ADMAs to be identified. The accuracy and precision of the method was validated and was successfully applied to determine contents of ATMAC in commercial hair conditioners and fabric softeners. The contents of total measured ATMAC ranged from 0.4 to 6.9% for hair conditioners, and from 3.3 to 4.6% for fabric softeners.

Calcination behavior of different surfactant-templated mesostructured silica materials

The removal of the template by calcination from mesostructured M41S and SBA-type silica materials was studied by combining high temperature X-ray diffraction, thermogravimetry–differential thermal analysis and mass spectrometry, allowing detailed in situ investigations during the thermal treatment. A comparison was made between materials with different mesoscopic structures, resulting from different synthesis routes and chemical treatment. The in situ XRD studies showed a strong increase in scattering contrast observed for the low angle reflections occurring when the template is removed from the inside of the pores, irrespective of the type of mesostructure. In agreement with the XRD investigations, the TG–DTA/MS experiments proved that the removal of the surfactant is a stepwise mechanism. Marked differences in the scattering contrast variations and chemical reactions were observed depending on the synthesis conditions and the type of surfactant, which highlight the role of the silica–surfactant interfaces. MCM-41 and MCM-48 materials synthesized in the presence of alkyltrimethylammonium surfactant under alkaline conditions showed a template removal mechanism based on an Hofmann degradation at low temperatures, followed by oxidation and combustion reactions above 250 °C. On the other hand, acidic conditions employed for the synthesis of SBA-3 type materials seems to favor reactions of oxidations after the evaporation of water and hydrochloric acid at low temperature. In that case, large contraction of the hexagonal unit cell was usually observed. Most of the block-copolymer template is removed from SBA-15 at lower temperatures, in a single oxidation step. The SBA-15 framework possibly catalyzes the oxidation of the block copolymer template species. In addition, the presence of framework porosity or pore connectivities seems to be responsible for the strong scattering contrast variations observed below 250 °C. Residual carbonaceous species and water are removed from the structure upon heating from 300 °C up to 550 °C. During this subsequent process a large contraction of the hexagonal unit cell is observed, possibly due to further framework condensation. In addition, a brief survey of the previous investigations reported in the literature related to the decomposition of structure-directing agents is given.

Synthesis of Novel Mesoporous Aluminum Organophosphonate by Using Organically Bridged Diphosphonic Acid

A novel mesoporous aluminum organophosphonate with organic moieties homogeneously distributed within the framework was successfully synthesized by using methylene diphosphonic acid in the presence of alkyltrimethylammonium surfactants as an example of non-silica-based inorganic−organic hybrid mesostructured and mesoporous materials.

The influence of chain length and electrolyte on the adsorption kinetics of cationic surfactants at the silica–aqueous solution interface

The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica–aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.

Expanded Mesoporous Silicate Films Grown at the Air−Water Interface by Addition of Hydrocarbons

We have examined the effect of addition of saturated straight-chain alkanes, Cn (n even and n = 8−16), and the aromatics toluene and benzene on the formation of mesoporous silicate films, which form at the air−water interface. The silicate films self-assemble by slowly condensing silicate oligomers attaching to templates formed by the alkyltrimethylammonium surfactants CnTAX (n even and n = 12−18, and X = Cl- and Br-) in solution. The collection of X-ray and neutron reflectivity profiles from these solutions shows that there is a chain length dependency to the formation of alkane-swollen hexagonal silicate films, which is manifest as a linear increase of the pore sizes of the films with increasing alkane chain length for Cn, n > 8, up to a limit of a hexagonal cell dimension of a100 = 80.8 A. The expansion is greatest when using decane as expander for all surfactant chain lengths examined. For C12 chain length surfactants a cubic phase results, C14 and C16 chain lengths form expanded hexagonal films, whil...

Ordered benzene–silica hybrids with molecular-scale periodicity in the walls and different mesopore sizes

Mesoporous benzene–silica hybrid materials have been obtained via surfactant-mediated synthesis using alkyltrimethylammonium surfactants with alkyl chain lengths from 14 to 18. By combining N2 adsorption–desorption experiments and powder X-ray diffraction (PXRD) analysis we showed that these samples exhibited mesopore sizes in the range 3.2 to 3.9 nm and that the molecular-scale periodicity in the walls of these materials is maintained. Transmission electron microscopy (TEM), solid state nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA) have also been used to characterise the structure and stability of the samples.

Synthesis of mesoporous silica foams with hierarchical trimodal pore structures

A new type of mesoporous silica foam having extremely low bulk density (<0.01 g cm−3), high mesopore volume (>2 cm3 g−1) and specific surface area (>1000 m2 g−1) was prepared by the spraying of aqueous siliceous solutions containing alkyltrimethylammonium surfactants under high pH conditions. These remarkable properties were ascribed to macrovoids and two kinds of mesopores ranging around 3 and 40 nm. The macrocellular structure produced with air bubbles was stabilized through a rapid condensation reaction during drying. The framework of the macrovoids contained mesopores with a bimodal distribution derived from the surfactant micelles and the interparticle spaces of silica nanoparticles. The hierarchical self-assembly of surfactant micelles, silica nanoparticles and air bubbles provided the specific architecture with a trimodal pore size distribution.

Solid-phase extraction sorbent consisting of alkyltrimethylammonium surfactants immobilized onto strong cation-exchange polystyrene resin

Presented is a solid-phase extraction sorbent material composed of cationic alkyltrimethylammonium surfactants attached to a strong cation-exchange resin via ion-exchange. The original hydrophilic cation-exchange resin is made hydrophobic by covering the surface with alkyl chains from the hydrophobic portion of the surfactant. The sorbent material now has a better ability to extract hydrophobic molecules from aqueous samples. The entire stationary phase (alkyltrimethylammonium surfactant) is removed along with the analyte during the elution step. The elution step requires a mild elution solvent consisting of 0.25 M Mg 2+ in a 50% 2-propanol solution. The main advantage of using a removable stationary phase is that traditionally utilized toxic elution solvents such as methylene chloride, which are necessary to efficiently release strongly hydrophobic species from SPE stationary phases, may now be avoided. Also, the final extract is directly compatible with reversed-phase liquid chromatography. The performance of this procedure is presented using pyrene as a test molecule.

Surfactant-Assisted Synthesis of the SBA-8 Mesoporous Silica by Using Nonrigid Commercial Alkyltrimethyl Ammonium Surfactants

A new simple method for obtaining the intermediate ribbonlike SBA-8 mesoporous silica is presented and discussed. We show how the SBA-8 phase can be prepared with the help of commercial alkyltrimethylammonium nonrigid surfactants (such as cetyltrimethylammonium bromide) instead of rigid bolaform amphiphiles, which were used as templates in the only SBA-8 synthesis procedure previously reported. The method is based on the use of atrane complexes as silicon precursors, and we also show how bringing into play fundamental kinetic parameters (like reaction time and temperature at which the successive reaction steps are carried out) makes it possible to modulate the resulting material topology.

Sorption of Ion Associates on Polyurethane Foams and Its Application to Sorption–Spectroscopic and Test Methods of Analysis

The results of studying the sorption of various ion associates on polyurethane foams were generalized. The main sorption-affecting factors were found to be the nature, hydrophobicity, and charge of the associate ion; the nature and concentration of the counter ion; the composition of the polymer unit of the polyurethane foam; and the pH and salt composition of the aqueous phase. Correlation equations were proposed to relate the partition coefficients with the hydration energy of counter ions in the ion associates of cationic dyes and metal complexes of 1,10-phenanthroline and with the number of carbon atoms in the alkyl fragment of cationic alkyltrimethylammonium surfactants. A sorption scheme was proposed and substantiated. Examples were given of the practical use of sorption for determining anionic and cationic surfactants, phenols, 1-naphthol, Fe(III), and Ru(IV).

Synergistic Effect of Cationic Surfactant on Surface Properties of Anionic Copolymers of Maleic Acid and Styrene

Surface tension measurements have been made on aqueous solutions of anionic monoesters of an alternating copolymer of maleic acid and styrene, MAS-n ,with n = 0−10, in the presence of alkyltrimethylammonium surfactants. It was found that the surface activity of these mixtures is increased relative to that observed for the pure components. This synergistic effect was ascribed to the formation of a complex, and its dependence of the complex structure was studied. The standard free energy of adsorption per methylene group was found to be −1.4 kJ mol-1 for complexes formed with MAS. This value decreases for MAS-n, becoming equal to zero for longer linear alkyl chains. However, all the calculated values of the standard free energy were negative and lower than those obtained for the pure surfactant. These results indicate that the main contribution to this change of free energy is the neutralization of the negative charge by a hydrophobic cation.