Synthesis of highly-ordered mesoporous silica particles using mixed cationic and anionic surfactants as templates

We applied a molecular assembly formed in an aqueous surfactant mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium octylsulfate (SOS) as templates of mesoporous silica materials. The hexagonal pore size can be controlled between 3.22 and 3.66 nm with the mixed surfactant system. In addition, we could observe the lamellar structure of the mixed surfactants with precursor molecules, which strongly shows the possibility of precise control of both the pore size and the structure of pores by changing the mixing ratio of surfactants. Moreover, use of the cationic surfactant having longer hydrophobic chain like stearyltrimethylammonium bromide (STAB) caused the increase in d(100) space and shifted the point of phase transition from hexagonal phase to lamellar phase to lower concentration of SOS.     

Poly(ethylene oxide)-based surfactants as templates for the synthesis of mesoporous silica materials

Various mesoporous silica solids were prepared by using poly(ethylene oxide)-based surfactants as templates in a neutral, fluoride, or moderately acidic medium, and their properties examined by different physical techniques. Precipitation in an acid or neutral medium provided materials of pore size in between those of micropores and mesopores irrespective of the molecular size of the surfactant. On the other hand, syntheses in a fluoride-containing medium yielded mesoporous materials with pore diameters over the range 36-84 A that increased with increasing surfactant size. All materials possessed specific surface areas above 650 m(2)g(-1) and high pore volumes-particularly those obtained in a fluorinated medium. The conditions used in the syntheses and the fact that all produced highly disordered porous materials suggest that their mechanism of formation is essentially of the N(0)I(0) neutral type. The materials obtained in the presence of fluoride ion, which promote the condensation of siliceous species, retain greater amounts of surfactant and exhibit increased cross-linking and decreased particle sizes, which results in textural mesoporosity.     

Controlled synthesis of bionic microstructure PbS crystals by mixed cationic/anionic surfactants

Hexagonal starfish-like PbS crystals have been synthesized by the reaction of lead acetate and thioacetamide (TAA) controlled by mixture of cetyltrimethylammonium bromide/sodium dodecyl sulfate (STAB/SDS) at the molar ratio of 5: 1 through a hydrothermal process at 80°C. It has been found that the hexagonal starfish-like PbS single crystal is in cubic phase, and the six arms of the star extend along the six 〈100〉 directions. By changing reaction conditions, such as the molar ratio of CTAB/SDS, temperature, and reaction time, the amounts of TAA and lead sources, the morphology and structure of the PbS crystals can be controlled. Furthermore, possible formation mechanism was preliminarily illustrated. The room-temperature photoluminescence spectra in solid state of the PbS single crystals obtained after different reaction times were investigated, which demonstrated that the PbS single crystals presented excellent optical properties. This work may open a novel route to the shaped-controllable synthesis of semiconductor crystals with various morphologies.     

Foams stabilized by mixed cationic gemini/anionic conventional surfactants

The mixed adsorption of a cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide) (abbreviated as 12-2-12), and an anionic conventional surfactant, sodium dodecyl sulfate (SDS), was examined using surface tension measurements. The viscoelastic properties of the mixed films were investigated by dilational interfacial rheology technique. The results showed that the addition of SDS promoted the close packing of adsorbed molecules at the interface, which increased the dilational elasticity of the mixed films. The stability of the foams was determined by the half-life of foam height collapse. The foams generated by 12-2-12/SDS mixtures were more stable than that formed by pure 12-2-12. In the presence of sodium bromide, the foam stability was further enhanced and the surfactant concentration required to attain the maximum effect in stabilizing foams was greatly reduced. The high foam stability could well relate to the high elasticity of the film.     

Ion chromatography on reversed-phase materials coated with mixed cationic and nonionic surfactants

Columns suitable for use in anion chromatography can be prepared by coating a packed reversed-phase HPLC column (C18 silica or polystyrene particles) with a cationic surfactant. The efficiency is improved dramatically by first coating the column with a nonionic surfactant and then subsequently with the cationic surfactant. The thickness of the first coated layer as well as the chemical structure of the surfactant have a major effect on the column performance. Actual separations are included to demonstrate the convenience and practical use of the coated columns. Using this approach, columns with 12,900 theoretical plates for the 15-cm column (or 86,000 plates/m) were produced, giving well shaped peaks with an average asymmetry factor of 1.09. The coated layers were found to be stable, giving retention times with an average relative standard deviation of 1.6% for 12 consecutive runs.     

Molecular interactions of cationic and anionic surfactants in mixed monolayers and aggregates

The properties of anionic-rich and cationic-rich mixtures of CTAB (cetyltrimethylammonium bromide) and SDS (sodium dodecyl sulfate) were investigated with conductometry and surface tension measurements and by determining the surfactant NMR self-diffusion coefficients. The critical aggregate concentration (CAC), surface tension reduction effectiveness(gamma(CAC)), surface excess(Gamma(max)), and mean molecular surface area (A(min)) were determined from plots of the surface tension (gamma) as a function of the total surfactant concentration. The compositions of the adsorbed films (Z) and aggregates (chi) were estimated by using regular solution theory, and then the interaction parameters in the aggregates (beta) and the adsorbed film phases (beta(sigma)) were calculated. The results showed that the synergism between the surfactants enhances the formation of mixed aggregates and reduces the surface tension. Further, the nature and strength of the interaction between the surfactants in the mixtures were obtained by calculating the values of the following parameters: the interaction parameter, beta, the size parameter, rho, and the nonrandom mixing parameter, P*. These results indicate that in ionic surfactant mixtures the optimized packing parameter has the highest value and that the size parameter can be used to account for deviations from the predictions of regular solution theory. It was concluded that, for planar air/aqueous interfaces and aggregation systems, this nonideality increases as the temperature increases. This trend is attributed to the increased dehydration of the surfactant head groups that results from increases in temperature. Further, our conductometry measurements show that the counterion binding number of mixed micelles formed in mixtures with a high CTAB content is different to those with a high SDS content. This difference is due to either their different aggregation sizes or the different interactions between the head groups and the counterions.     

Temperature-dependent vesicle formation of aqueous solutions of mixed cationic and anionic surfactants

The phase behavior of aqueous solutions of mixed cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) was examined at different temperatures (20, 30, 40, and 50 degrees C). While stable vesicles were formed in a narrow composition range on the SOS-rich side at 20 degrees C, the range widened remarkably when the temperature was raised to 30 degrees C. Thus, the vesicle region extended to cover almost the entire composition range, CTAB:SOS = 0.5:9.5-5.0:5.0, at the total surfactant concentrations of 50-70 mM on the SOS-rich side. To analyze the temperature dependence of this phase behavior of the mixed surfactant system, DSC and fluorescence polarization measurements were performed on the system. The experimental findings obtained revealed that pseudo-double-tailed CTAB/SOS complex, the major component of the bimolecular membrane formed by the surfactant mixture, undergoes a gel (Lbeta)-liquid crystal (Lalpha) phase transition at about 26 degrees C. This phenomenon was interpreted as showing that the bimolecular membrane has no curvature and is rigid and easy to precipitate at temperatures below the phase transition point, whereas it has a curvature and is loose enough to disperse in the solution as vesicles at temperatures above the phase transition point. Vesicles formed by the anionic/cationic surfactant complex were then stable at temperatures above the phase transition temperature of the complex.     

Critical aggregation concentration in mixed solutions of anionic polyelectrolytes and cationic surfactants

We have examined the polymer/surfactant interaction in mixed aqueous solutions of cationic surfactants and anionic polyelectrolytes combining various techniques: tensiometry, potentiometry with surfactant-selective electrodes, and viscosimetry. We have investigated the role of varying polymer charge density, polymer concentration, surfactant chain length, polymer backbone rigidity, and molecular weight on the critical aggregation concentration (Cac) of mixed polymer/surfactant systems. The Cac of these systems, estimated from tensiometry and potentiometry, is found to be in close agreement. Different Cac variations with polymer charge density and surfactant chain length were observed with polymers having persistence lengths either smaller or larger than surfactant micelle size, which might reflect a different type of molecular organization in the polymer/surfactant complexes. The surfactant concentration at which the viscosity starts to decrease sharply is different from the Cac and probably reflects the polymer chain shrinkage due to surfactant binding.     

Effect of water hardness on the interaction of cationic dye with anionic surfactants

ABSTRACT

The effect of water hardness causing ions on the critical micelle concentration (cmc) of surfactants was studied using conductivity measurement. Spectrophotometric and conductometric studies of dye and surfactant interaction under the influence of water hardness causing ions were also investigated. It was found that with increasing hardness in water, cmc of the surfactants tends to decrease. The change in the colour of the solution was observed as the degree of hardness increases. Hardness of water can consider as a major contributor of fading or change in colour during washing and cleaning of dyed material. Dye in a surfactant solution containing varying amount of hardness causing ions undergoes a blue shift in the visible region and red shift in ultraviolet region. The comparison between sodium dodecylsulphate (SDS) and saponin natural surfactant showed that turbidity was observed only in the aqueous solution of SDS in the presence of very hard water.     

Wormlike micelles formed by mixed cationic and anionic gemini surfactants in aqueous solution

The formation and the properties of wormlike micelles in aqueous solutions of mixed cationic and anionic gemini surfactants, 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3(OH)-12) and O,O'-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C(12)?C(12)), have been studied by steady-state and dynamic rheological measurements at 25°C. With the addition of a small amount of C(12)?C(12) into the solution of 12-3(OH)-12, the total surfactant concentration of which was always kept at 80 mmol L(-1), the solution viscosity was strongly enhanced and its maximum was much larger than that of the mixed system of propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3-12) and C(12)?C(12). The results of dynamic rheology measurements showed that 12-3(OH)-12/C(12)?C(12) formed longer wormlike micelles in comparison with 12-3-12/C(12)?C(12). This was attributed to the effect of hydrogen bonding occurring between 12-3(OH)-12 molecules, which was an effective driving force promoting micellar growth. As few C(12)?C(12) participated in the micelles, the electrostatic attraction between the oppositely charged head groups of 12-3(OH)-12 and C(12)?C(12) made the molecules in the aggregates pack more tightly. This reinforced the hydrogen-bonding interactions and greatly promoted the micellar growth.     

Interactions between cationic starch and anionic surfactants

The surface tensions and the phase equilibria of dilute aqueous cationic starch (CS)/surfactant systems were investigated. The degree of substitution of the CS varied from 0.014 to 0.772. The surfactants investigated were sodium dodecyl sulphate (SDS), potassium octanoate (KOct), potassium dodecanoate (KDod) and sodium oleate (NaOl). The concentrations of CS were 0.001, 0.01 and 0.1 w%.Critical association concentrations (cac) occur at surfactant concentrations well below the critical micelle concentrations of the surfactants, except for KOct, KDod and NaOl at the lowest CS concentrations investigated (0.001 w%). The surface tensions of CS/surfactant solutions decrease strongly already below the cac. This is attributed to the formation of surface active associates by ion condensation. Associative phase separation of gels formed by CS and surfactant takes place at extremely low concentrations when the surfactant/polymer charge ratio is somewhat larger than 1. The gel is higly viscous and contains 40–60% water, depending on the concentration of electrolyte, the surfactant hydrocarbon chain length and the nature of the polar head of the surfactant.The concentration at which the phase separation occurs decreases with increasing surfactant chain length and the concentration of simple electrolyte, factors that promote micelle formation. This indicates that the gels are formed by association of CS to surfactant micelles. When surfactant well in excess of charge equivalence is added, the gels dissolve because the CS/surfactant complexes acquire a high charge.     

Transitions of organized assemblies in mixed systems of cationic bolaamphiphile and anionic conventional surfactants

The transition from vesicles to tubelike structures has been studied in mixed systems of cationic bolaamphiphile BPHTAB [biphenyl-4,4'-bis(oxyhexamethylenetrimethylammonium bromide)] and its oppositely charged conventional surfactants with transmission electron microscopy (TEM) and dynamic light scattering (DLS). This transition can be attributed to the fact that tube-like structures are more stable aggregates than vesicles because of the special molecular packing in the aggregates of the mixed systems. The effects of temperature and salt addition on this transition have also been investigated, and the rate of the transition was found to be strongly dependent on temperature. Addition of the appropriate amount of NaBr will accelerate the transition from vesicles to tube-like structures, but the vesicles will transform into micelles at higher salt concentration. Moreover, the micelle-vesicle transition can be realized by addition of n-octanol in the mixed system of BPHTAB/sodium caprate (SC) at higher salt concentrations.     

Mixed micellar properties of cationic trimeric-type quaternary ammonium salts and anionic sodium n-octyl sulfate surfactants

The properties of quaternary ammonium salt-type cationic trimeric surfactants (m-2-m-2-m, m represents the carbon atom number in alkyl chain lengths of 8, 10, and 12) and oppositely charged anionic monomeric surfactant, sodium n-octyl sulfate (SOS), were characterized by employing several techniques such as static surface tension, fluorescence spectroscopy, and dynamic light-scattering measurements. The critical micelle concentrations (cmc) of m-2-m-2-m were much lower than those of the corresponding dimeric and monomeric surfactants, and decreased with increasing chain length. The addition of SOS to m-2-m-2-m solutions resulted in a further decrease of the cmc. The mixed surfactants showed higher efficiencies in lowering the surface tension than the individual surfactants. The fluorescence measurements suggested the formation of mixed micelles with a hydrophobic environment in the solutions even at lower concentrations. The dynamic light-scattering study indicated the presence of two different kinds of aggregates with different hydrodynamic diameters. The larger one was attributed to the mixed micelle of m-2-m-2-m and SOS. These results indicated a decline of the electrostatic repulsion between cationic head groups through the incorporation of anionic surfactant into the mixed surfactants.     

阴/阳离子表面活性剂复配体系的中相微乳液研究

阴离子表面活性剂双-2-乙基己基磺化琥珀酸钠(简称AOT), 和阳离子表面活性剂十六烷基三甲基溴代铵(简称CTAB), 在有醇、正辛烷、盐水存在的情况下,能形成多相微乳液。本文系统地研究了阴/阳离子表面活性剂配比、醇的种类、醇的浓度对该体系的中相微乳液的形成及特性的影响, 得到了中相微乳液的特性参数(最佳含盐量S^*, 最佳中相微乳液体积V^*, 界面张力r~E、盐宽△S等)。这些性质对与阴/阳离子表面活性剂复配体系, 三次采油及日用化工上的应用开发具有重要意义。最后还开展单独阴离子表面活性剂体系和阴/阳离子表面活性剂复配体系进行了比较, 得到一些有价值规律, 并从理论上进行了探讨。     

Ion decontamination with a mixture of cationic and anionic surfactants

The composition of mixed micelles and mixed micelle — solution interfaces changes with the concentration and molar ratio of the cationic and anionic surfactants present. The micelle — solution interface includes besides the headgroups of both surfactants, the counterions of the surfactant in excess. The finding of an enhanced binding of counterions to mixed micelles may be of some practical importance in decontamination.     

Adsorption of anionic and cationic surfactants on anionic colloids: supercharging and destabilization

We present herein a study on the adsorption of anionic (SDS), cationic (CTAB), and nonionic (C(12)E(5)) surfactants onto anionic silica nanoparticles. The effects of this adsorption are studied by means of the static structure factor, S(q), and the collective diffusion coefficient, D(c), obtained from small-angle X-ray scattering and dynamic light scattering measurements, respectively. The effective charge on the particles was determined also from classical electrophoresis and electroacoustic sonic-amplitude measurements. The surface tension of the sample was also investigated. Of particular note is the adsorption of SDS onto the silica nanoparticles, which leads to supercharging of the interface. This has interesting repercussions for structures obtained by the layer-by-layer (LbL) technique, because emulsions stabilized with supercharged and hydrophobized silica are perfect candidates for use in a multilayer system.     

The miscibility of dodecyltrihydroxyethylammonium bromide with cationic, nonionic and anionic surfactants in mixed monolayers and micelles

In this paper were analyzed the surface properties of surfactants and the miscibility and interactions between components of adsorbed monolayers and micelles formed from mixed systems. The investigated compounds differ in the structure of the polar head and represented cationic (dodecyltrihydroxyethylammonium bromide—DTEAB, dodecyltrimethylammonium bromide DTMAB), anionic (sodium dodecyl sulfate—SDS), and nonionic (dodecyl-β-d-glucoside—DG) surfactant. The experiments were based on the measurements of the surface tension of the aqueous solutions of the investigated compounds and their mixtures (cationic/nonionic—DTEAB/DG, cationic/cationic—DTEAB/DTMAB and cationic/anionic—DTEAB/SDS). The composition of the mixed films and micelles as well as the free energies of mixing values, which are a measure of the molecular interactions, was calculated basing on the equations resulting from the Motomura theory. The obtained results indicate that all the investigated systems mix nonideally both in the monolayers and micelles. The magnitude of the deviations from ideal behavior is strongly dependent on the type of the investigated mixture and increases in the following order: DTEAB/DTMAB < DTEAB/DG  DTEAB/SDS.     

Separation and sweeping of flavonoids by microemulsion electrokinetic chromatography using mixed anionic and cationic surfactants

In this report, a novel means for the separation and sweeping of flavonoids (quercetin, rutin, calycosin, ononin and calycosin-7-O-β-d-glucoside) by microemulsion electrokinetic chromatography using mixed anionic and cationic surfactants as modified pseudostationary phase was presented. The optimized background electrolyte consisted of 0.5% (w/v) ethyl acetate, 2.0% (w/v) SDS, 9 mM DTAC, 4.0% (w/v) 1-butanol and 10 mM sodium borate or 25 mM phosphoric acid. We systematically investigated the separation and preconcentration conditions, including the concentrations of surfactant, types of sweeping, sample matrix, the effect of high salt or acetonitrile, and sample injection volume. It was found that the use of mixed surfactants significantly enhanced the separation efficiency through the change of the efficient electrophoretic mobility of analytes. Compared with normal sample injection, 185-508-fold sensitivity enhancement in terms of limit of detection was achieved through effective sweeping of large sample volume at 50 mbar pressure (up to 45% capillary length). At last, the proposed method was suitable for the determination of Radix Astragali sample.     

Synthesis of lamellar mesoporous silica materials using LCs as templates

The study focuses on the synthesis of mesoporous silica materials using liquid crystals (LCs) formed in an aqueous mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) as templates and tetrathoxysilane (TEOS) as precursor. For this purpose, the phase behavior and range of LC areas were determined at different temperatures, concentrations, and ratios of CTAB/SDS. It was found that LCs became denser with the increased of concentration of surfactants. The mesoporous materials were synthesized using LCs as templates at various temperatures, surfactant concentrations, and pH values. The mesoporous samples were characterized using SEM and nitrogen sorption analysis. The research results showed that the structure of synthesized samples were lamellar and their surface areas increased significantly with the increase of temperature in the temperature range of LCs, reaching about 900?m²/g at 60°C. The surfactant concentrations affect the thickness of pore wall and thereby the specific surface area of products. The specific surface area and the order of mesoporous sample increased gradually with the decrease of pH.     

Interaction of anionic dyes and cationic surfactants with ionic liquid character

The interactions of an imidazolium based ionic liquid (IL), namely 1-dodecyl-3-methylimidazolium chloride [C12 mim][Cl] with two sulfonated anionic dyes, azocarmine G and methyl orange, are studied spectrophotometrically in both acidic and basic media. ILs (with some surface active character) can interact with the above dyes and cause considerable shifts in their spectra. These interactions are then compared with some surfactant-dye interactions. Evolving factor analysis (EFA) and multivariate curve resolution-alternating least squares (MCR-ALS) are used for complete resolution of the measured spectrophotometric data. The concentration and spectral profiles of all species were calculated without any assumption of the chemical models. The spectral variation of dye solutions as a function of IL concentrations below and above the critical aggregation concentration (CAC) is analyzed using MCR-ALS as a soft-modeling technique. The ion pair formation constants between ILs and dyes were calculated using the obtained concentration profiles.