Thin films of bicontinuous cubic mesostructured silica templated by a nonionic surfactant

Thin films ofbicontinuous cubic mesostructured silica were formed using the nonionic poly(oxyethylene)-alkyl ether surfactant Brij-56 as a structure-directing agent. The synthesis conditions were chosen such that the estimated volume fraction of surfactant in the silica/surfactant films corresponded approximately to the composition at which the bicontinuous cubic phase occurs in the water/surfactant phase diagram. Small-angle X-ray scattering and transmission electron microscopy measurements reveal that the cubic phase corresponds to the Ia3(-)d double-gyroid structure, with some distortion due to anisotropic film shrinkage. The cubic structure grows as faceted domains that are well-oriented with respect to the substrate and often occur in coexistence with a lamellar phase. By adjusting the temperature at which the films are aged, it is possible to create films with 2D hexagonal, cubic, or lamellar structures at a single composition.     

Evolution of packing parameters in the structural changes of silica mesoporous crystals: cage-type, 2D cylindrical, bicontinuous diamond and gyroid, and lamellar

Cage-type, two-dimensional (2D) cylindrical hexagonal (C), bicontinuous diamond (D), bicontinuous gyroid (G), and one-dimensional (1D) lamellar (L) structures of silica mesoporous crystals (SMCs) were obtained by using the anionic surfactant N-stearoyl-l-glutamic acid (C(18)GluA) as a template in the presence of the nonionic surfactant C(16)(EO)(10) (Brij-56). The mesostructures were controlled by the organic/inorganic interface curvature change induced by Brij-56. A synthesis-field diagram showed that the mesostructure changed in the sequence cage-type → C → intergrowth of C and D → intergrowth of C and G → D → G → L with increase of the amount of Brij-56. Mixed micelles were formed by the anionic and nonionic surfactants, the packing parameter g of which increased with increasing the addition amount of nonionic surfactant and the reaction temperature. The local g parameter was obtained from electron crystallography reconstruction results by calculating mean curvatures and Gaussian curvatures from the equi-electrostatic potential surface. The intergrowth of C and D and two kinds of intergrowth of C and G are also discussed.     

Oligomeric polymer surfactant driven self-assembly of phenylene-bridged mesoporous materials and their physicochemical properties

The detailed synthesis of highly hydrophobic average pore benzene-bridged hybrid mesoporous silicas under an acidic medium is described. With the use of a 1,4-bis(triethoxysilane)benzene silsesquioxane precursor and biodegradable alkyl polyoxyethylene (Brij-56 or Brij-76) nonionic surfactant oligomers as supramolecular templates, no molecular scale periodicity was observed. The well-defined mesoporous materials could be synthesized with two-dimensional hexagonal (p6mm) symmetries. The textural and surface properties (adsorption isotherms of water and benzene vapors) were estimated and were compared to the analogous benzene-bridged mesoporous silica with molecular scale periodicity which was prepared using cationic surfactant under basic conditions. Different textural properties resulted for the two kinds of materials and revealed some important insights regarding the structure and nature of the materials.     

Encapsulation of complementary model drugs in spray-dried nanostructured materials

Two model drugs of different physico-chemical and pharmaceutical properties (ibuprofen, acetaminophen) have been incorporated together or separately in silica-based microspheres using sol–gel and spray-drying processes. A variable amount of a neutral surfactant Brij-56© has also been added. The properties of the microspheres vary significantly depending on their composition. Three kinds of texture are identified: (1) silica containing spheroid nano-domains (formed by ibuprofen; diameters between 20 and 100 nm), (2) silica containing worm-like mesophases (formed by Brij-56© and both model drugs, typical correlation distances ~6 nm), (3) silica intimately mixed with the drug (acetaminophen) without visible phase-separation. The kinetics of drug release in simulated intestinal fluid strongly depend on these textures. The association of ibuprofen and acetaminophen in a single type of microsphere and without surfactant favours a concomitant release. Possible mechanisms of materials’ formation are discussed.     

Tuning the wall thickness and pore orientation in mesoporous titania films prepared with low-temperature aging

Porous titania thin films with well-ordered mesostructures are prepared by using Pluronic surfactant P123 as the pore template and aging the films in a high-humidity environment at −6 °C. These structures are stable enough to undergo calcination at 400 °C to generate nanocrystalline TiO₂ walls with retention of mesoporosity. Under the aging conditions used, the films have well-ordered mesostructures even with a molar ratio of P123 to titanium (R) as small as 0.006. Because the P123 micelle diameter remains constant across a range of compositions, the pore diameter also remains fixed but the wall thickness of the titania thin films increases as the P123 concentration decreases without decreasing the long-range order of the products. Furthermore, mesoporous titania thin films with hexagonal close-packed channels oriented perpendicular to the substrate can be obtained R values of 0.008–0.012 by sandwiching the as-prepared films between glass slides modified with crosslinked P123. Analysis of the mesophases obtained here indicates that a transition from films containing significant 2D hexagonal channels to 3D hexagonal structure occurs below P123/Ti = 0.008. This does not match the expected volume fraction for this transition based on the mesophases behavior of aqueous P123 at room temperature, suggesting that a more detailed model would be needed to predict mesostructure in titania films aged below the freezing point of water.     

Mesostructured silica containing conjugated polymers formed within the channels of anodic alumina membranes from tetrahydrofuran-based solution

The synthesis of mesostructured silica from a tetrahydrofuran (THF)-based sol gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation-induced self-assembly (EISA) method. Two different nonionic surfactants were used as structure-directing agents, the triblock copolymer Pluronic P123 and the oligomer surfactant Brij56. The effect of the relative humidity and surfactant concentration on the type of mesophase and orientation of the in-channel mesostructures was studied using transmission electron microscopy (TEM) and grazing incidence small angel X-ray scattering (GISAXS). The in-channel structures obtained in this study were primarily of the 2D hexagonal phase with a circular orientation in which the hexagonally packed cylinders form a spiral-like shape from the channel wall inward. In addition, a columnar orientation of the hexagonal phase, in which the axes of the hexagonally packed cylinders are oriented parallel to the channel axes, was also observed. Finally, the use of the THF-based synthesis allowed the in situ incorporation of the highly hydrophobic yellow-emitting conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] into the in-channel mesostructure upon its formation. The conjugated polymer was well distributed within the mesostructure and maintained its optical properties.     

In situ GISAXS study of the formation of mesostructured phases within the pores of anodic alumina membranes

The formation and subsequent transformations of mesostructured silica within the confined tubular environment of anodic alumina membrane (AAM) channels [porous alumina membrane (PAM) channels] were investigated for the first time in situ with grazing incidence small-angle X-ray scattering (GISAXS) techniques, in combination with ex situ transmission electron microscopy (TEM) of the same samples. A better understanding of the mesostructure formation mechanism within the confined space of the AAM pores is a direct result of this study. Three different surfactants were used as the structure-directing agents in acid-catalyzed silica synthesis solutions. With ionic cetyltrimethylammonium bromide acting as the structure-directing agent, a columnar hexagonal structure with mesopores oriented parallel to the AAM channels was observed to form directly from the beginning of the synthesis. In samples synthesized with the nonionic surfactants Brij 56 and Pluronic P123, a circular hexagonal structure was found to form first; here, the mesopores are aligned around the circumference of the AAM channels. The circular structure subsequently transforms directly into a columnar hexagonal (P123 surfactant), or a mixture of columnar hexagonal and a new curved lamellar phase with lamellae oriented parallel to the walls of the AAM channels (Brij 56 surfactant). These transformations occur after complete solvent evaporation and therefore differ from a simple evaporation-induced phase formation. The existence of a previously postulated lamellar phase could be proven by GISAXS and TEM investigations.     

Controlling the shape and alignment of mesopores by confinement in colloidal crystals: designer pathways to silica monoliths with hierarchical porosity

Monolithic pieces of hierarchically structured silica, containing both periodic macropores and mesopores with well-controlled architecture, are synthesized by dual templating methods. Colloidal crystal templating with close-packed arrays of poly(methyl methacrylate) spheres yields regular, highly interconnected macropores a few hundred nanometers in diameter, and templating with nonionic surfactants produces mesoporosity (2.5-5.1 nm pore diameters) in the macropore walls. Several distinct mesostructures can be achieved within the silica skeleton, depending on the choice of surfactant, co-surfactant, and processing conditions. In the three-dimensional (3D) confinement of the colloidal crystal template, wormlike channels, cubic (Pm3n), or two-dimensional (2D) hexagonal (P6mm) mesostructures are produced with the surfactant Brij 56 (C16H33(OCH2CH2)nOH (n approximately 10) and dodecane as cosurfactant. In the 2D hexagonal structure, channels are oriented perpendicular to the polymer spheres, thereby connecting adjacent macropores through the silica walls. This orientation contrasts with channel alignment parallel to latex spheres when the polymeric surfactant Pluronic P123 (EO20PO70EO20) is used. On the basis of high-resolution 3D transmission electron microscopy, scanning electron microscopy, small-angle X-ray scattering, and nitrogen sorption measurements, structural and textural properties of the monoliths are described in detail as a function of the synthesis parameters. The control over the mesoarchitecture of these silica-surfactant systems in 3D confinement is explained by considering the relative dimensions of the mesostructures with respect to the interstitial space in the latex template, interfacial interactions, entropic effects, and structural frustration.     

Surfactant-Assisted Ethane-Bridged Silica Coating for Capillary Electrochromatography

With surfactant P123 as structure directing reagent, 1, 2-bis (trimethoxysilyl) ethane was hydrolyzed under acid condition. The resulting ethane-bridged silica was coated onto the inner walls of fused silica capillaries and used as the stationary phase for capillary electrochromatography. The bridged ethyl silica provided hydrophobic groups for reversed-phase separation. A comparative coated capillary was fabricated without the use of surfactant in the preparation of the bonded silica. Separation of model neutral compounds was compared between these two kinds of capillaries. Surfactant-assisted organosilica-coated capillaries displayed much superior retention efficiency without obviously decreased electroosmotic flow. The existence of surfactant in the synthesis of the sol results in higher surface areas of the coating. Such ethane-bridged organosilica stationary phases can be used under basic conditions.     

Preparation of amino-functionalized mesoporous silica thin films with highly ordered large pore structures

Highly ordered amino-functionalized mesoporous silica thin films have been directly synthesized by co-condensation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) in the presence of triblock copolymer Pluronic P123 surfactant species under acidic conditions by sol-gel dip-coating. The effect of the sol aging on thin films organization is systematically studied, and the optimal sol aging time is obtained. The amino-functionalized mesoporous silica thin films exhibit a long-range ordering of 2D hexagonal (p6mm) mesostructure with a large pore size of 8.3 nm, a large Brunauer–Emmett–Teller (BET) specific surface area of 680 m² g⁻¹ and a large pore volume of 1.06 cm³ g⁻¹ following surfactant extraction as demonstrated by X-ray diffraction (XRD), Transmission electron microscope (TEM), and physical adsorption techniques. Based on BET surface area and weight loss, the surface coverage of amino-groups for the amino-functionalized mesoporous silica thin films is calculated to be 3.2 amino-groups per nm². Moreover, the functionalized thin films display improved properties for immobilization of cytochrome c in comparison with pure-silica mesoporous thin films.     

Mesostructured silica tubes and rods by templating porous membranes

Mesostructured silica tubes and rods were prepared by deposition of a prehydrolysed silica sol with surfactant onto the cylindrical pores of a polycarbonate (PC) membrane template, followed by calcination to remove the surfactant and membrane template. The tubular and rodlike morphologies of mesostructured silica were demonstrated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements. The tubes and rods show ordered hexagonal mesostructures, which were demonstrated by small angle X-ray diffraction (XRD) patterns and TEM measurements.     

Probing the impact of advanced melting and advanced adsorption phenomena on the accuracy of pore size distributions from cryoporometry and adsorption using NMR relaxometry and diffusometry

The penetration of compressed CO(2) in hydrocarbon and fluorocarbon regions of concentrated surfactant mesophases are interpreted from differences in the CO(2)-processed pore expansion of mesoporous silica thin films templated by three surfactants containing varying degrees of hydrocarbon and fluorocarbon functionality. Ordered silica thin films are synthesized for the first time using the 16-carbon (C(16)) partly fluorinated surfactant, 11,11,12,12,13,13,14,14,15,15,16,16,16-tridecafluorocetyl pyridinium bromide (HFCPB), as a templating agent. Silica films templated with surfactants containing a 8-carbon (C(8)) fluorocarbon tail (3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl pyridinium chloride (HFOPC)) and a 16-carbon (C(16)) hydrocarbon tail (cetyl pyridinium bromide (CPB)) and HFCPB (C(16)) are processed in compressed CO(2) (69-172 bar, 25 °C and 45 °C) during synthesis. CO(2) processing results in significant pore expansion for films templated with both fluorinated surfactants, while pore expansion is negligible for the hydrocarbon templated material suggesting that preferential CO(2) penetration occurs in the 'CO(2)-philic' fluorocarbon segments of the surfactant template. The effect of substrate surface energy on the final uniformity of the dip-coated films is studied by varying the substrate from unmodified glass to a fluorocarbon-capped substrate. The ability to create dip-coated thin films on low surface energy substrates through favorable interaction of surfactant template tail with the substrate surface functional groups is demonstrated.     

Fabrication of Two-Dimensional Gratings Using Photosensitive Gel Films and Their Characterization

Two-dimensional (2D) surface-relief gratings have been fabricated by the two-beam interference method on photosensitive gel films, which are derived from chemically modified metal-alkoxides, and characterized with respect to surface morphology and antireflection effect. Photosensitive ZrO₂ gel film was deposited on a silica glass substrate using the sol that was prepared from Zr-tetrabutoxide chemically modified with benzoylacetone. The gel film was exposed to two-beam interference fringe (1.0 m period) of He-Cd laser and then rotated by 90° in its own plane, followed by an additional exposure. Leaching of the gel film with ethanol gave lattice or island types of 2D-gratings depending on the dose of laser irradiation. The morphology of the gratings changed with irradiation time of laser, leaching conditions and so on. The formation of 2D-grating of island type on a silica glass substrate substantially reduced the reflection at its surface in a wavelength range of 1.3 to 2.6 m.     

Bent-core liquid crystal-functionalised flexible polymer substrates for liquid crystal alignment

ABSTRACT

A transparent flexible polymer film is chemically functionalised with a bent-core liquid crystal (BCLC) compound for effective alignment of the bulk BCLC sample at the substrate–LC interface. The surface attachment was achieved via a simple procedure which involved pre-treatment of the polymer film (commercial name: over head projector film) using piranha solution followed by chemically attaching the BCLC compound through silane condensation reaction. Surface characterisation of the unmodified and BC-modified flexible films was carried out through X-ray photoelectron spectroscopy (XPS), attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy, contact angle (CA) and atomic force microscopy (AFM) techniques. The BC-modified flexible substrates are analysed for their efficiency to orient the bulk LC sample. Remarkably, the chemically modified polymer substrates are highly efficient in vertically aligning both the BC and rod-like LC samples at the substrate–LC interface, in comparison to their unmodified counterparts. The described method is simple, reproducible, surface modified substrates are highly stable and more importantly reusable. The demonstrated method for the alignment of BCLCs advances a step forward towards the realisation of applications proposed for these fascinating compounds.     

Physicochemical investigations of anionic–nonionic mixed surfactant microemulsions in nonaqueous polar solvents: I. Phase behavior

Phase behaviors of AOT/heptane (Hp)/formamide (FA), ethylene glycol (EG), propylene glycol (PG), triethylene glycol (TEG) and glycerol (GLY) have been investigated in the absence and presence of a nonionic surfactant, polyoxyethylene(2) cetyl ether (Brij-52) at 303 K. The phase characteristics of (AOT+Brij-52)/Hp/(EG or PG or TEG) have been found to be different from that of AOT/Hp/FA systems in respect of both the area of monophasic domain and the appearance of other mesophases. The area of monophasic domain of (AOT+Brij-52)/Hp/EG depends on the content of Brij-52 (X _Brij-52) and shows a maximum at X _Brij-52=0.4. A negligible effect on the area of the monophasic domain has been shown by more hydrophobic surfactants, polyoxyethylene(2) stearyl ether (Brij-72) and polyoxyethylene(2) oleyl ether (Brij-92). The effect of oils (dodecane and hexadecane) on the mixed systems stabilized by (AOT+Brij-52) in EG has been investigated. The area of monophasic domain has been found to be dependent on the type of nonaqueous solvents and follows the order GLY>EG>PG>TG. A systematic investigation on the measurement of phase volumes of mixed surfactant systems [AOT+nonionic surfactant(s)] stabilized in oils of different chain lengths (heptane, dodecane and hexadecane) and polar solvent (EG) has been carried out at different compositions of the ingredients to identify the phase transitions of these systems as a function of X _Brij-52. The threshold point of phase transition (both W I→W IV and W IV→W II transitions) has been found to be a function of the configuration of added nonionic surfactant, nature of the polar solvent and oil. The conversion of the initial oil/EG droplets into EG/oil droplets with increasing X _nonionic has been facilitated for hydrophobic surfactants polyoxyethylene(4) lauryl ether (Brij-30), Brij-52, and Brij-72 in comparison to the hydrophilic surfactants polyoxyethylene(10) cetyl ether (Brij-56) and polyoxyethylene(20) cetyl ether (Brij-58).     

Flow-injection system for determination of critical micelle concentrations of ionic and nonionic surfactants

A practical technique is presented for the rapid, accurate determination of the critical micelle concentrations (CMCs) of ionic and nonionic surfactants. The precision, speed and instrumental simplicity of a flow-injection system are combined with a gradient chamber and flow-through conductance and absorbance detection to produce a system capable of determining the CMC of surfactant solutions in less than 30 min. The exponential response gradients from the resulting system are monitored by a chart recorder and simple manual calculations yield the CMC. The validity of the technique is verified by determination of the CMC values for both ionic (cetyltrimethylammonium bromide and chloride and sodium dodecyl sulfate) and nonionic (Brij-35, Brij-56, Brij-99, Triton X-100) surfactants. The proposed technique does not require the extensive solution preparation, repetitive measurements, complex instrumentation and data manipulation typical of other methods for determining CMCs.     

Evaluation of the Physical-Chemical Properties of Poly(ethylene oxide)-block-Poly(propylene oxide) by Different Characterization Techniques

Summary: A surfactant's efficiency for a given application is dependent on its chemical structure and physical-chemical properties in solution, including surfactant solubility as a function of concentration and temperature as well as adsorption and aggregation behavior. This review work describes the main physical-chemical properties ascertained by means of various characterization techniques, which can be used to study nonionic surfactants based on poly(ethylene oxide)-block-poly(propylene oxide) (PEO-PPO). Among these, some are widely used and others are relatively new for this type of application.     

Simulation-aided design and synthesis of hierarchically porous membranes

Free-standing silica membranes with hierarchical porosity (ca. 300 nm macropores surrounded by 6-8 nm mesopores) and controllable mesopore architecture were prepared by a dual-templating method, with the structural design aided by mesoscale simulation. To create a two-dimensional, hexagonal macropore array, polymeric colloidal hemisphere arrays were synthesized by a two-step annealing process starting with non-close-packed polystyrene sphere arrays on silicon coated with a sacrificial alumina layer. A silica precursor containing a poly(ethylene) oxide-poly(propylene oxide)-poly(ethylene) oxide (PEO-PPO-PEO) triblock-copolymer surfactant as template for mesopore creation was spin-coated onto the support and aged and then converted into the free-standing membranes by dissolving both templates and the alumina layer. To test the hypothesis that the mesopore architecture may be influenced by confinement of the surfactant-containing precursor solution in the colloidal array and by its interactions with the polymeric colloids, the system was studied theoretically by dissipative particle dynamics (DPD) simulations and experimentally by examining the pore structures of silica membranes via electron microscopy. The DPD simulations demonstrated that, while only tilted columnar structure can be formed through tuning the interaction with the substrate, perfect alignment of 2D hexagonal micelles perpendicular to the plane of the membrane is achievable by confinement between parallel walls that interact preferentially with the hydrophilic components (PEO blocks, silicate, and solvent). The simulations predicted that this alignment could be maintained across a span of up to 10 columns of micelles, the same length scale defined by the colloidal array. In the actual membranes, we manipulated the mesopore alignment by tuning the solvent polarity relative to the polar surface characteristics of the colloidal hemispheres. With methanol as a solvent, columnar mesopores parallel to the substrate were observed; with a methanol-water mixed solvent, individual spherical mesopores were present; and with water as the only solvent, twisted columnar structures were seen.     

PM-IRRAS spectroscopy for the characterization of polymer nanofilms: chains conformation,anisotropy and crystallinity

This paper is focused on the use of the Polarization-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) for studying thin polymer films at interfaces. When forming a polymer film on a metallic substrate, for instance by spin-coating, the characterization of the polymeric layer becomes very difficult given the small amount of matter deposited and also because of the contact with the metal. Among the techniques well adapted to surface and interface analyses, the PM-IRRAS spectroscopy represents an excellent tool to probe ultra-thin films. Different systems have been selected in this study such as polyamides (PA) and ethylene-co-vinyl acetate (EVA) nanofilms spin-coated onto chemically controlled surfaces (i.e. thiol self-assembled monloayers grafted onto gold coated glass slides). PM-IRRAS spectroscopy allowed us to characterize the polymer anisotropy (chains orientation and conformation), to suggest a model for chain organization at the polymer/substrate interface, and to calculate the orientation angles. Moreover, we were able to determine, by using PM-IRRAS, the degree of crystallinity of PA and EVA films of nanometric dimensions without any calibration procedure needed by other techniques.     

Fluorescent diazapyrenium films and their response to dopamine

Experimental protocols for the preparation of 2,7-diazapyrenium films on glass, quartz, and silica in one or two steps have been developed. The one-step procedures involve the adsorption of preformed 2,7-diazapyrenium dications with trimethoxysilane appendages to the hydroxylated substrates. The two-step procedures consist in the formation of interfacial polysiloxanes with pendent chloromethyl groups and their subsequent coupling to monoalkylated 2,7-diazapyrene derivatives. For the modification of the glass slides, the silane building blocks have been copolymerized with Si(OEt)4. The transmission absorption spectra of the coated glass and quartz slides all reveal the characteristic bands of the 2,7-diazapyrenium chromophores. Combustion analyses confirm the adsorption of the 2,7-diazapyrenium dications on the silica particles. A comparison of the surface coverages of all films indicates that the one-step procedures are significantly more efficient than their two-step counterparts. Furthermore, the copolymerization of the silane building blocks with Si(OEt)4 translates into an increase in 2,7-diazapyrenium surface coverage of approximately 1 order of magnitude. The emission and excitation spectra of all modified substrates reveal the characteristic bands of the 2,7-diazapyrenium fluorophores. The fluorescence quantum yield, however, decreases as the surface coverage increases. Presumably, interactions between adjacent fluorophores encourage nonradiative deactivation pathways. With the exception of the glass slides modified in two steps, all films respond to the presence of dopamine, in aqueous environments at neutral pH, with pronounced decreases in emission intensity. The association of the 2,7-diazapyrenium acceptors and dopamine donors at the solid/liquid interface is responsible for fluorescence quenching. The glass slides and silica particles modified in one step are the most sensitive substrates and respond to sub-millimolar concentrations of dopamine with large changes in emission intensity. Furthermore, their fluorescence is not affected by relatively large concentrations of ascorbic acid, which is the main interferent in conventional dopamine detection protocols. Thus, these results demonstrate that the supramolecular association of 2,7-diazapyrenium dications and pi-electron rich substrates can be reproduced successfully at solid/liquid interfaces and suggest that the unique properties of 2,7-diazapyrenium films might lead to dopamine-sensing schemes based on fluorescence measurements.