Silica Surfactant Research Articles

Preparation and characterization of porous silica templated by a nonionic fluorinated systems

Taking advantage of our experience in the field of fluorinated surfactants, we report the preparation and characterization of both large pore mesoporous and macroporous silica by using fluorinated systems. Here, in particular we have investigated in detail the effect of fluorocarbon solubilization on the properties of materials. Our results show that the various investigated systems can be used for the synthesis of large pore mesoporous materials. Nevertheless, depending on different surfactants, we have to consider two different swelling mechanisms. Indeed, concerning R F 8(EO) 9, the pore size expansion occurs via the swelling of surfactant–silica hybrid mesophase. On the contrary, for the compounds prepared from R F 7(EO) 8, the mesopore enlargement takes place through the swollen micelles. In addition, macroporous silica could be design by using fluorinated emulsions as template. Our study also reveals that the formation of macroporous materials is correlated to the phase inversion temperature. We presume that the system with high PIT value conduce the formation of macroporous materials.

Synthesis of ordered mesoporous silica membrane on inorganic hollow fiber

This paper reports on a new method for the preparation of mesoporous silica membranes on alumina hollow fibers. A surfactant–silica sol is filled in the lumen of an α-alumina hollow fiber. The filtration technique combined with an evaporation-induced self-assembly (EISA) process results in the formation of a continuous ordered mesoporous silica layer on the outer side of α-alumina hollow fibers. X-ray diffraction (XRD), transmission electron microscopy (TEM), and nitrogen isothermal adsorption measurements reveal that these membranes possess hexagonal (P6mm) mesostructures with pore diameters of 4.48 nm and BET surfaces of 492.3 m 2 g −1. Scanning electron microscopy (SEM) studies show that the layers are defect free and energy-dispersive spectroscopy (EDS) mapping images further confirm the formation of continuous mesoporous silica layer on the outer side of α-alumina hollow fibers. Nitrogen and hydrogen permeance tests show that the membranes are defect free.

Self-assembled conjugated polymer–surfactant–silica mesostructures and their integration into light-emitting diodes

A self-assembly process for the preparation of functional mesoscopically ordered semiconducting polymer–silica nanocomposite thin films is reported. The nanocomposites are prepared by introducing pre-synthesized semiconducting polymers into a tetrahydrofuran (THF)–water homogeneous sol solution containing silica precursor species and a surface-active agent. Depending on the concentration of the surface-active agent, it was possible to prepare materials with three different types of mesostructural order: i) a 2D hexagonal mesophase silica with conjugated polymer guest species incorporated within the hydrophobic cylinders organized in domains aligned parallel to the substrate surface plane; ii) a lamellar mesophase silica with the layers oriented parallel to the substrate surface and the conjugated polymer guest species incorporated in the hydrophobic layers; or iii) an apparent intermediate phase consisting of a mixture of the hexagonal and lamellar phases in addition to worm-like aggregates with no appreciable orientational order. The continuous through-film conductive pathway provided by the intermediate phase has allowed the integration of ordered semiconducting polymer–silica nanocomposites into opto-electronic devices. By comparison, the lamellar mesostructure prevents through-film conduction, with the result that no light emission occurs. Blue-, green- and red-emitting diodes comprising blue-emitting poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), green-emitting poly(9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-(2,1′,3)-thiadiazole) (F8BT), and red-emitting poly[2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEHPPV) confined within the 2D hexagonal silica nanostructure were fabricated with luminances of ca. 3 cd m–2 at 15 V. Device performances provide criteria for optimizing the selection of synthesis chemistries, processing conditions, compositions, and structures, for light-emission properties sought.

Concentration of chlorophenols in water to dialkyated catinonic surfactant–silica gel admicelles

Chlorophenols including monochlorophenol, dichlorophenol, trichlorophenol, tetrachlorophenol, and pentachlorophenol in water were extracted into dialkylated cationic surfactant–silica gel admicelles. The dialkylated cationic surfactants such as didecyldimethylammonium bromide (DC10) and didodedyldimethylammonium bromide (DC12) sorbed on silica gel surfaces to form admicelles at pH 9. Approximately 200 mg of DC10 was quantitatively sorbed on 1 g of silica gel. The sorption further increased by further addition of DC10. This is in contrast to the fact that the maximum sorption of mono-alkylated cetyltrimethyammonium chloride (CTAC) was only ca. 100 mg. Based on the fluorescent spectra of a molecular probe, N-phenyl-1-naphthylamine, DC10- and DC12-silica gel admicelles were more hydrophobic than CTAC-silica gel admicelles. The extents of the extraction of chlorophenols into DC10-silica gel admicelles were greater than those into CTAC-silica gel admicelles. However, the extractions to DC12-silica gel admicelles were insufficient due to leakage of DC12 vesicles. Consequently, DC10-silica gel admicelles were the most adequate for concentrating chlorophenols in water. An admicelle column was prepared by passing aqueous buffer solution of DC10 through a Bond Elut Jr. ® silica gel solid-phase extraction cartridge. It was successfully applied to the 500-fold concentration of chlorophenols including hydrophilic mono-substituted chlorophenol in water samples prior to their HPLC analysis.

Deposition of thin mesoporous silica films on glass substrates from basic solution

Transparent thin (ca. 100 nm) films of silica–surfactant mesostructured materials were deposited on borosilicate glass plates and soda-lime glass tubes from aqueous solutions containing tetraethoxysilane, alkyltrimethylammonium chloride, ammonia, and methanol. By calcination in air, the films became mesoporous (BET surface area of 700–900 m2 g−1) with pore diameter 2.0–2.8 nm.

Incorporation of Dyes into Silica–Surfactant Mesostructured Nanoparticles as a Nanoscale Host Material for Organic Molecules

Abstract Various kinds of organic dye molecules were successfully incorporated into silica–surfactant nanoparticles with a hexagonally ordered mesostructure. The dye-incorporated composite particles, which had a diameter of ca. 30 nm, exhibited specific fluorescent properties and pH sensitivity derived from the organic molecules.

Formation of mesoporous silica thin films on oxide substrates by casting

The preparation of mesoporous silica thin films with bi-dimensional hexagonal pores whose channel axis oriented parallel to the plane of the film on oxide substrates has been studied using casting, X-ray diffraction (XRD), scanning electron spectroscopy (SEM), N 2 adsorption–desorption and thermogravimetry (TG). XRD and SEM observations show that the surfactant–silica mesostructured thin films are synthesized on the substrates using a cetyltrimethylammonium bromide (CTMABr)–tetraethyl orthosilicate (TEOS) gel under acidic conditions at ambient temperature. XRD and N 2 adsorption–desorption studies reveal that the powdered mesoporous silica composite remains structurally stable and ordered after template removal and that the supported thin films are plausibly stable after template removal but less stable and ordered than the powdered composite. TG analysis indicates that the template supercritical fluid extraction (SFE) efficiency reaches 87%. The influences of the surfactant-substrate and film-substrate interactions on the formation and structural stability of the thin films are discussed.

Ordered nanowire arrays in the mesoporous silica thin films

In the present study we report synthesis and investigation of the magnetic iron nanowire arrays in the mesoporous silica thin films. The films were deposited on SiC/Si(100) substrate by spin-coating technique, enabling one to obtain all mesopores parallel to the substrate surface and aligned radially by centrifugal force. Very high level of local alignment (90% on 2 cm 2) of the pores was proved by small angle X-ray scattering and neutron reflectivity. Magnetic iron nanowires were synthesized in the mesoporous silica channels by intercalation of a hydrophobic iron compound into a hydrophobic part of the silica–surfactant composite followed by decomposition of the complex and additional crystallization of iron in hydrogen flow. Nanocomposites were studied by TEM, ED, SAXS, BET, BJH and neutron reflectometry. It was found that an ordered structure preserves after the chemical modification, while iron nanowires uniformly fill porous structure, making this system highly promising for high-density data recording media.

Preparation and properties of ZnO nanoparticles in the mesoporous silica matrix

In the present work a novel variant of synthesis of ZnO nanoparticles in the mesoporous silica matrix was suggested. The method is based on the introduction of a hydrophobic Zn compound, Zn acetylacetonate, into the hydrophobic part of silica–surfactant composite, followed by thermal modification. The ZnO content in the as-synthesized composites attains 14.6 wt.%, which exceeds most previously reported values for ZnO/mesoporous silica nanocomposites. The samples were characterized by SAXS, chemical analysis, TEM, XRD, EDX, UV–Vis and photoluminescent (PL) spectroscopy. The UV–Vis spectra of the composites show a significant blue shift of an absorption band demonstrating an increase of ZnO band gap energy due to the quantum confinement effect. The PL spectra under the excitation with 337 nm light show a narrow UV emission band of ZnO at 370 nm, while emission in the visible range of spectra almost disappeared. The intensity ratio IUV/IVis for the as-synthesized nanocomposites is up to 65 (at 77 K).

On the specific filtration mechanism of a mesoporous silica membrane, prepared with non-connecting parallel pores

We report the singular filtration properties of an ultrafiltration membrane made with mesoporous silica that exhibits cylindrical pores aligned mostly normal to the support. This membrane supported on tubular commercial macroporous alumina supports was prepared by the interfacial growth mechanism between stable silica–surfactant hybrid micelles made of the association of silica oligomers with polyethyleneoxide-based (PEO) surfactants and sodium fluoride, a well-known silica condensation catalyst [Boissière et al., An ultrafiltration membrane made with mesoporous MSU- X silica, Chem. Mater. 15 (2003) 460–463]. It appears that the combined effect of the silica nature of the membrane, whose surface charge can be easily adjusted by changing the pH and the non-connected cylindrical shape of the pores provides a new behavior in the retention properties, as proved by the filtration of polyoxyethylene polymers (PEO) with different molecular weights. Depending on the filtration conditions, a rejection rate of 80% and a steep cut-off at 2000 Da can be obtained or, on the reverse, polymers three times bigger than the pore diameter can diffuse through the membrane. This new filtration mechanism, which opens up new modes of separation modes, is explained in the light of both topology of the porous network and pH-dependent interactions between PEO polymers and silica porous media.

Solubilization of pentachlorophenol in micelles and confined surfactant phases

The solubilization of pentachlorophenol (PCP) in cationic surfactant micelles and in surfactant–silica mesostructures is studied. The micelle–water distribution coefficient is derived from both theoretical models and experiments. The sorption isotherm in the mesostructure is determined by the solution depletion method and compared to that of phenol, mono-, di- and tri-chlorophenol. The solubilization or sorption energies of PCP in the various surfactant phases are obtained by microcalorimetry. The interaction between the anionic form of the PCP molecule and the cationic surfactant is so strong that the precipitation of a phenolate form of the surfactant is observed. This alkyltrimethylammonium phenolate form can be considered as a new chemical species that replaces the bromide form of the cationic surfactant. In the case of the non-ionic form of polychlorophenols, the water–micelle distribution coefficients, the solubilization enthalpies in micelles and the sorption enthalpies in the mesostructure increase with increasing hydrophobicity. This latter which is defined by the octanol–water partition coefficient increases itself with the number of chloride molecules in the phenol derivative. On the other hand the affinity for the mesostructure follows a non-monotonous evolution with the same parameter. This could be due to steric effects that would appear with an increase of the phenol derivative molecular size.

Preparation of strontium hexaferrite nanowires in the mesoporous silica matrix (MCM-41)

We report a novel method of synthesis of ordered magnetic strontium hexaferrite nanowires in the mesoporous silica matrix by incorporation of metal complexes into the freshly prepared mesoporous silica–surfactant composite. The shape and size of obtained nanowires are consistent with the dimensions of the porous framework. The obtained nanocomposites are characterized by high blocking temperatures up to 160K. Mesoporous silica serves as nanoreactor for the formation of nanowires.

Fragmented structure of adsorbed layer of non-ionic surfactant on colloidal silica

The structure of the adsorbed layer of a non-ionic surfactant on small colloidal silica beads in water at high pH is investigated by small-angle neutron scattering (SANS). Experiments of fluorescence decay on porous silica, neutron reflectivity on wafers and SANS on colloids suggest a fragmented structure in some case, like e.g. at high pH. Here, measurement which unambiguously rule out all continuous layer models will be presented. First, the adsorption isotherm was constructed by centrifugation and subsequent spectroscopic determination of the concentration of non-adsorbed surfactant in the supernatant. Second, the shape of the surfactant–silica complexes was measured by SANS in dilute solutions. External contrast variation was used to enhance scattering from the silica and the surfactant, respectively. Finally, a model of decorated silica beads has been developed. It will be shown to agree quite nicely with the experimentally observed intensities.

Self-assembly of a silica–surfactant nanocomposite in a porous alumina membrane

A mesoporous membrane composed of nanochannels with a uniform diameter has a potential use for precise size-exclusive separation of molecules. Here, we report a novel method to form a hybrid membrane composed of silica-surfactant nanocomposite and a porous alumina membrane, by which size-selective transport of molecules across the membrane becomes possible. The nanocomposite formed inside each columnar alumina pore was an assembly of surfactant-templated silica-nanochannels with a channel diameter of 3.4 nm; the channel direction being predominantly oriented along the wall of the columnar alumina pore. Molecules could be transported across the membrane including the silica-surfactant nanocomposite with a capability of nanometre-order size-exclusive separation. Our proposed membrane system has a potential use not only for separation science, but also catalysis and chip technologies.

Heat capacities and volumes of suspensions in the presence of surfactants

Density and heat capacity measurements of water–surfactant–silica ternary systems were carried out as functions of the silica and the surfactant concentrations at 298 K. From these properties, the apparent molar volume and heat capacity of the surfactant in a given water+silica mixture were evaluated. As well, the apparent specific volume and heat capacity of silica, at a given concentration, in the water+surfactant mixtures as functions of the surfactant concentration were calculated. Some surfactants (decyltrimethylammonium bromide, sodium octanoate, sodium decanoate, sodium dodecanoate, sodium decylsulfate, sodium perfluorooctanoate and N, N-dimethyldodecylamine- N-oxide) were chosen to study the effect of the head-group and the hydrophobicity. As a general feature, the hydrophobicity of the surfactant does not essentially play a role on the silica–surfactant interactions whereas the surfactant head-group does. Finally, the apparent specific properties of silica are very sensitive in detecting the destabilization of suspensions whereas the apparent molar properties of the surfactant are not.

Ordered iron nanowires in the mesoporous silica matrix

We report a novel synthetic method for preparation of ordered magnetic iron nanowires in mesoporous silica matrix. The method is based on the incorporation of a hydrophobic metal compound, Fe(CO) 5 , into the hydrophobic part of a freshly prepared mesoporous silica–surfactant composite. Shape and size of obtained iron nanowires is consistent with the dimensions of porous framework. Particles are uniform and well ordered in the silica matrix. Thus, mesoporous silica serves as nanoreactor for the formation of Fe-nanoparticles.

Morphological and textural control of spray-dried mesoporous silica-based spheres

Micrometric ordered mesoporous silica spheres are synthesised by spray-drying alcoholic sols. For this synthetic route, the important effect of siloxane chemistry on both the morphological and the textural properties of spray-dried silica particles is demonstrated. This is achieved by correlating data obtained on the sols prior to atomisation (29Si liquid state NMR, SAXS) with data coming from a multi-scale characterisation of the resulting powders (SEM, XRD, 1H high resolution solid state NMR). The morphology (agglomerated or separated spheres, 1 to 3 µm of diameter) and texture (disordered or ordered p6mm mesophases), as well as the silica–surfactant interface, are sensitive to variations in the proportion of solvent or in the ageing time of the sols. These effects are related to the relative extents of siloxane hydrolysis and condensation reactions as evidenced by the variations in the radii of gyration of the siloxane oligomers, and in the remaining non-hydrolysed Si-OR alkoxy groups. We propose a scheme for the formation of the spray-dried mesoporous spheres that takes into account the main role of these reactions, and their effects on the determining physico-chemical processes: solid formation, fragmentation and aggregation of droplets, micellisation, mesophase formation and extension.

Aerosol‐Assisted Formation of Mesostructured Thin Films

A new aerosol‐assisted surfactant self‐assembly approach has been developed for large‐scale mesostructured silica thin film fabrication. The formation mechanism involves coalescence of semisolid surfactant–silica mesostructured particles on substrates (see Figure) and subsequent reassembly of the mesostructures to adapt the morphology transformation from spherical to planar shape.

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.

Evaporation-Controlled Self-Assembly of Silica Surfactant Mesophases

Surfactant templated silica mesophases belong to the class of self-assembled materials that exhibit long range ordered two-dimensional (2D) hexagonal, three-dimensional (3D) hexagonal, or 3D cubic mesostructures when the composition of the initial deposited solution and its aging time have been optimized. Thin films of such mesophases with a thickness typically less than 300 nm are now routinely obtained by dip coating. In this study a reference solution was used with a chemical composition leading to the formation of thin films with a 3D hexagonal (P63/mmc) mesostructure by dip coating. Thick films (about 10 μm thick) were alternatively prepared by a evaporation-controlled self-assembly (ECSA) process and studied with grazing incidence small-angle X-ray scattering (GISAXS). In the ECSA process, the mesostructure was developed under slower evaporation conditions than for dip-coated films by finely tuning the ethanol/H2O evaporation rate of the reference solution horizontally deposited on a silicon wafer. It is shown that the evaporation rate of the solvent is one of the key parameters that control the final mesostructure and can, under certain conditions, promote the formation of the cubic mesophase. The ability to precisely control such a structural arrangement on the mesoscale is of major interest for making sensor arrays, nanoreactors, photonic and fluidic devices, and low dielectric constant films.