CTAB Systems Research Articles

Automation of liquid crystal phase analysis for SAXS, including the rapid production of novel phase diagrams for SDS-water-PIL systems.

Lyotropic liquid crystal phases (LCPs) are widely studied for diverse applications, including protein crystallization and drug delivery. The structure and properties of LCPs vary widely depending on the composition, concentration, temperature, pH, and pressure. High-throughput structural characterization approaches, such as small-angle x-ray scattering (SAXS), are important to cover meaningfully large compositional spaces. However, high-throughput LCP phase analysis for SAXS data is currently lacking, particularly for patterns of multiphase mixtures. In this paper, we develop semi-automated software for high throughput LCP phase identification from SAXS data. We validate the accuracy and time-savings of this software on a total of 668 SAXS patterns for the LCPs of the amphiphile hexadecyltrimethylammonium bromide (CTAB) in 53 acidic or basic ionic liquid derived solvents, within a temperature range of 25-75 °C. The solvents were derived from stoichiometric ethylammonium nitrate (EAN) or ethanolammonium nitrate (EtAN) by adding water to vary the ionicity, and adding precursor ions of ethylamine, ethanolamine, and nitric acid to vary the pH. The thermal stability ranges and lattice parameters for CTAB-based LCPs obtained from the semi-automated analysis showed equivalent accuracy to manual analysis, the results of which were previously published. A time comparison of 40 CTAB systems demonstrated that the automated phase identification procedure was more than 20 times faster than manual analysis. Moreover, the high throughput identification procedure was also applied to 300 unpublished scattering patterns of sodium dodecyl-sulfate in the same EAN and EtAN based solvents in this study, to construct phase diagrams that exhibit phase transitions from micellar, to hexagonal, cubic, and lamellar LCPs. The accuracy and significantly low analysis time of the high throughput identification procedure validates a new, rapid, unrestricted analytical method for the determination of LCPs.

Interaction of cationic surfactant with acid yellow dye in absence/presence of organic and inorganic additives: conductivity and dye solubilization methods

Abstract In the present study, the conductometric and dye-solubilization techniques have been utilized to investigate the interaction between an anionic dye (acid yellow 23 [AY]) and a cationic surfactant (cetyltrimethylammonium bromide [CTAB]) in presence of organic (ethanol)/inorganic (NaCl) additives. From the conductometric method, two critical micelle concentrations (cmc) were found for AY + CTAB mixture in an aqueous system and the cmc values were found to undergo a change with the variation of AY concentrations. The cmc values of AY + CTAB systems were observed higher in the alcoholic medium, while the same was found to be lower in the NaCl solutions. The change in cmc of AY + CTAB systems shows an U-like curve with an increase of temperature. The negative free energy of micellization ( Δ G m o ${\Delta}{G}_{m}^{o}$ ) for the AY + CTAB systems has indicated a spontaneous micelle formation in all of the cases studied. The enthalpy ( Δ H m o ${\Delta}{H}_{m}^{o}$ ), as well as the entropy of micellization ( Δ S m o ${\Delta}{S}_{m}^{o}$ ) for the AY + CTAB systems, were assessed and discussed with proper reasoning. Additionally, the enthalpy-entropy compensation parameters were also investigated and illustrated. The solubility of AY and C D was observed to rise linearly with an increase in the concentration of CTAB/NaCl solution. The solubilization capacity (χ) of AY, the molar partition coefficient (K M) amongst the micellar and the aqueous phase, and free energy of solubilization ( Δ G S o ${\Delta}{G}_{S}^{o}$ ) were evaluated and discussed in detail. The former parameters undergo an increase with an increase of NaCl concentrations.

Novel synthesis route of porous clay heterostructures via mixed surfactant template and their dielectric behavior

Porous clay heterostructures (PCHs) have been developed for a decade now especially for adsorbent and catalytic support applications because of their high specific surface area with uniform small pore size. PCHs have been prepared through the surfactant-directed assembly of organosilicate in the galleries of montmorillonite (MMT), generally, single surfactant was applied. Challenge of using mixed surfactant template derived PCH is proposed to obtain higher surface area aiming for dielectric application. In this study, BC-PCH synthesized from mixed CTAB and CTAC surfactant systems was named and compared with B-PCH and C-PCH that synthesized from CTAB and CTAC surfactant system, respectively. The results revealed that the structure of MMT changed after modification and calcination process, as confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) analyses. Textural properties characterization showed that the BC-PCH samples exhibited higher specific surface area (SBET) 644–738 m2/g than that of B-PCH and C-PCH sample and slightly smaller pore size because the size of micelles in solution system from mixed surfactant decreases, which was confirmed by Dynamic Light Scattering analysis (DLS). Field Emission Scanning Electron Microscopy analysis (FE-SEM) illustrated the rough surface structure of mesoporous BC-PCH samples. The dielectric properties were measured by using the Inductance–Capacitance–Resistance meter (LCR). The dielectric constant (at 103 Hz) increased from 9 and 10 of single CTAB and CTAC surfactant template B-PCH and C-PCH to 15–17 of mixed surfactant series BC-PCHs, almost two-fold was obtained. The improvements of dielectric constant in BC-PCH samples are achieved by the increasing of interfacial polarization. Considering these results, BC-PCH materials can be further applied as candidates for an attractive filler to enhance the dielectric properties of polymer nanocomposite.

Surfactant and Temperature Effects in Tyrosine/Pyridoxine Hydrochloride Systems

AbstractThe fluorescence resonance energy transfer (FRET) from tyrosine to pyridoxine hydrochloride in different surfactant solutions and in deionized water has been investigated by using a fluorescence spectroscopic technique. The Stern–Volmer quenching constant, the distance between donor (tyrosine) and acceptor (pyridoxine hydrochloride), and the energy transfer efficiency were obtained using the Stern–Volmer relation and Forster’s theory of nonradiative energy transfer. The data obtained from the measurement reveals that the FRET occurs more effectively in aqueous SDS micellar solution than in CTAB or linear alcohol ethoxylate micellar systems and deionized water. Moreover, the binding constant, number of binding sites, and thermodynamic parameters ∆G, ∆H, ∆S were obtained by studying the tyrosine‐pyridoxine hydrochloride interactions at three different temperatures. The decrease in binding constant with temperature reflects the existence of weak interaction between tyrosine and pyridoxine hydrochloride at high temperature. The analysis confirms that van der Waals forces and hydrogen bonding are involved in the interaction.

Linking molecular/ion structure, solvent mesostructure, the solvophobic effect and the ability of amphiphiles to self-assemble in non-aqueous liquids

Sixteen non-ionic molecular solvents have been found to exhibit the solvophobic effect and to support the formation of amphiphile self-assembly mesophases. The solvents were low molecular weight polar solvents which contained various combinations of amine, hydroxyl or ether moieties with relatively small proportions of hydrocarbon unit constituents. The studied amphiphiles were hexadecyltrimethylammonium bromide (CTAB), hexadecylpyridinium bromide (C16PyrBr) and tetraethylene glycol monohexadecyl ether (C16E4). Lyotropic liquid crystal mesophases with lamellar, normal hexagonal and normal bicontinuous cubic, with ordered one-, two- and three-dimensional periodic structure respectively, were identified in CTAB and C16PyrBr systems by using cross-polarised optical microscopy (CPOM). Mesophase diversity and thermal stability ranges correlated to the Gordon parameter (G) value, a proxy for the solvent cohesive energy density. Infrared spectroscopy confirmed that all the studied molecular solvents were associative liquids. Solvent mesostructure was studied by synchrotron small angle X-ray scattering. The small sub-set of neat solvents which were mesostructured, with polar and non-polar domain segregation, displayed the lowest G values, and amongst the lowest mesophase diversity and thermal stability ranges. It has been established that the G value is a good indicator of whether or not a molecular solvent is likely to behave as a co-surfactant, residing within the amphiphile-solvent interfacial region of self-assembled objects, thereby influencing specific mesophase structure formation. Structure-property behaviour has been explored and shows that beneficial solvent features for serving as amphiphile-self assembly media, with the potential for rich mesophase diversity, include the presence of hydroxyl > amine > ether moieties, while methyl moieties have an adverse effect larger than that of methylene moieties.

Competitive solubilization of naphthalene and pyrene in various micellar systems

Cosolubilization of polycyclic aromatic hydrocarbons (PAHs) (naphthalene and pyrene) has been studied in surfactant systems of varying nature of their head-group viz. nonionic: Brij30 and Brij56, cationic: DDEAB and CTAB and anionic: SDS. Solubilization capacity of micelles was quantified in terms of molar solubilization ratio, the micelle–water partition coefficient, the first stepwise association constant and average number of solubilizate molecules per micelle determined by employing spectrophotometric and tensiometric techniques. Solubilization capacity of all the surfactant systems was generally higher for naphthalene than pyrene and followed the order: nonionics>cationics>anionic surfactant. Solubility of naphthalene decreased during cosolubilization in all surfactant systems studied while the solubility of pyrene decreased only in Brij30 and Brij56 surfactant systems due to competitive solubilization of PAHs for the same solubilization site. The solubility of pyrene, however, enhanced in presence of naphthalene in CTAB, DDEAB and SDS surfactant systems owing to increase in core volume of the micelles by the palisade layer solubilization of naphthalene. The results of this study can provide valuable information on the selection of particular surfactant systems for selective separation of naphthalene and pyrene from their mixture relevant to surfactant enhanced remediation (SER) technology at the contaminated sites.

Formation of Hybrid Wormlike Micelles upon Mixing Cetyl Trimethylammonium Bromide with Poly(methyl methacrylate-co-sodium styrene sulfonate) Copolymers in Aqueous Solution

The association of cetyltrimethylammonium bromide, CTAB, with a series of P(MMAx-co-SSNa) random copolymers of sodium styrene sulfonate (SSNa) with methyl methacrylate (MMA) was explored in aqueous solution as a function of the MMA molar content, x, of the copolymers. The polyelectrolyte/surfactant complexation in aqueous solution was verified through pyrene fluorescence probing. In addition, turbidimetry studies in dilute or more concentrated aqueous solutions elucidated the phase separation behavior of the P(MMAx-co-SSNa)/CTAB systems as a function of the copolymer composition x and the surfactant to polyelectrolyte mixing charge ratio. It is found that practically phase separation is completely suppressed within the studied mixing range when the MMA content of the copolymers is ∼30-40 mol%. For lower MMA contents the polyelectrolyte/surfactant complex separates out from water, while for higher x values the solubility limits of the copolymers in water are attained. For the intermediate MMA contents, viscoelastic systems are obtained in more concentrated polymer/surfactant solutions provided that the polyelectrolyte is fully complexed with the cationic surfactant ((1)H NMR results). Moreover, the (1)H NMR studies indicate that hybrid P(MMAx-co-SSNa)/CTAB wormlike micelles are formed in water under these conditions. Finally, it is shown that addition of salt prevents syneresis problems and facilitates the rheological investigation.

Effect of Molecular Weight on the Interfacial Dilational Viscoelasticity of Anionic Polyelectrolyte/Surfactant Systems

In this article, the effect of molecular weight on the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water-octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. PSS/cationic surfactant CTAB systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be well explained by electrostatic interaction. Molecular weight of PSS plays a crucial role in the nature of adsorption film. The complex formed by CTAB and higher molecular weight PSS, which has larger dimension and stronger interaction, results in higher dilational modulus at lower surfactant bulk concentration. In the case of PSS/anionic surfactant SDS systems, the co-adsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational modulus at lower surfactant bulk concentration. For PSS/nonionic surfactant T × 100 systems, PSS may form a sublayer contiguous to the aqueous phase, which has little effect on interfacial tension but slightly decreases dilational modulus.

A surfactant type fluorescence probe for detecting micellar growth

We report on the detection of micellar growth in anionic, cationic, and catanionic surfactant systems using a novel surfactant type fluorescence probe, sodium 12-(N-dansyl)amino-dodecanate (12-DAN-ADA). The fluorescent group was incorporated in the tail of the surfactant which tethers the fluorescent group deep inside the apolar micellar cores. The fluorescence anisotropy of 12-DAN-ADA was found to be very sensitive for directly detecting the micellar growth in micelles containing oppositely charged surfactants, including cationic CTAB systems and mixed systems of oppositely charged surfactants (DEAB/SDS); in regard to the like charged SDS micellar systems, the sensitivity can be greatly enhanced by addition of a water soluble quencher which quenches the background fluorescence from the equilibrium population of free 12-DAN-ADA.

Insights into cyclodextrin-modulated interactions between protein and surfactant at specific and nonspecific binding stages

Cyclodextrin (CD) modulated interactions between ionic surfactants with opposite charge and bovine serum albumin (BSA) at specific and nonspecific binding stages have been studied by isothermal titration calorimetry (ITC), fluorescence spectra and circular dichroism spectral measurements. At the specific binding stage with high affinity, the effectiveness of both α- and β-CD for hindering BSA–SDS interactions is quite weak; however, CD is more effective in hindering BSA–CTAB specific interactions. This is due to the cooperative electrostatic and hydrophobic interaction between BSA and SDS, and to the absence of the cooperative interaction between BSA and CTAB at the specific binding stage. For both BSA–SDS and BSA–CTAB systems (especially in the former system), α-CD is more effective in hindering BSA–surfactant interactions than β-CD. At the nonspecific binding stage, the addition of both α- and β-CD can hinder totally both BSA–SDS and BSA–CTAB hydrophobic interactions. This is caused by the more specific hydrophobic interaction between CD and surfactant compared with the hydrophobic interaction between BSA and surfactant. Our results show that the CD effect on the protein–surfactant interaction depends on both the nature of the protein–surfactant interaction and the complexing ability of CD with surfactant.

Effects of Dynamic Surface Tension on Droplet Formation of Surfactant Solution Injected from a Capillary Tube

Effects of dynamic surface tension on the droplet formation of surfactant solutions were studied. Test fluids used were aqueous solutions of CTAB at several surfactant mol concentrations (CD) and CTAB/NaSal aqueous solutions at CD=1.0 mM and at three mol concentrations of NaSal. A droplet formed when a surfactant solution was injected into air from a capillary tube was investigated and the relation between the droplet diameter and the injection velocity V was measured. The size of droplet was evaluated by an equivalent droplet diameter Dexp, which is the diameter of sphere whose volume is the same as that of a droplet injected. For the CTAB systems, Dexp increased with increasing V at relatively low velocities because the dynamic surface tension also increased. However, the diameter decreased with increasing the velocity at relatively high velocities. This phenomenon can be qualitatively predicted from an equation of the force balance at a capillary exit when the effect of surface tension is evaluated using the dynamic surface tension. For the CTAB/NaSal systems, Dexp increased with increasing V at relatively low velocities and reached a constant value. This phenomenon was also predicted qualitatively with the force balance equation.

The Location of Phenolic Antioxidants and Radicals at Interfaces Determines Their Activity

To characterize parameters influencing the antioxidant activity at interfaces a novel ESR approach was developed, which facilitates the investigation of the reaction stoichiometry of antioxidants towards stable radicals. To relate the activity of antioxidants towards the location of radicals at interfaces NMR experiments were conducted. Micellar solutions of SDS, Brij and CTAB were used to model interfaces of different chemical nature. The hydrophilic Fremy's radical was found to be solubilized exclusively in the aqueous phase of SDS micellar solution but partitioned partly into the hydrophilic headgroup area of Brij micelles. In contrast the hydrophobic galvinoxyl was exclusively located in the micellar phase with the increasing depth of intercalation in the order SDS < Brij < CTAB. Gallates revealed a higher stoichiometric factor towards galvinoxyl in CTAB systems, which is accounted to a concentration effect of antioxidant and radical being both solubilized in the palisade layer. In contrast, in SDS solutions hardly any reaction between galvinoxyl and gallates was found. SDS acted as a physical barrier between radical (palisade layer) and antioxidant (stern layer). The influence of the hydrophobic properties of the antioxidant was clearly seen in Brij micelles. Elongation of the alkyl chain in gallate molecule resulted in increasing stoichiometric factors in the presence of galvinoxyl being located in the deeper region of the bulky headgroup area. The reverse trend was found in the presence of Fremy's radical being located in the hydrated area of the micelles.

Dilution method study on the interfacial composition and structural parameters of water/C 12– [formula omitted]–C 12⋅2Br/ n-hexanol/ n-heptane microemulsions: Effect of the oxyethylene groups in the spacer

The interfacial composition and the structure of the water/C 12– EO x –C 12⋅2Br/ n-hexanol/ n-heptane microemulsion have been investigated by the dilution method. The results showed that C 12– EO x –C 12⋅2Br formed a stable water/oil microemulsion with the assistance of n-hexanol. Owing to the relatively large size of the head group, more n-hexanol molecules are populated on the droplet surface than in the C 12–2–C 12⋅2Br system. The radius of the water pool of the C 12–EO 3–C 12⋅2Br system is not as sensitive to W 0 as in the C 12–2–C 12⋅2Br system. Another feature of the present system is that its droplet size is considerably smaller than that of the C 12–2–C 12⋅2Br system, and also those of the CPC and CTAB systems at relative high water content. This provides a potential application for the synthesis of nanoparticles with small size.

Impact of Phenolic Antioxidants on Structural Properties of Micellar Solutions

Antioxidants solubilized in micellar solutions can change micellar properties like the size and shape of micelles, critical micellar concentration (cmc) and viscosity. Interactions arising between antioxidants and the surfactant determine the locations of antioxidants and vice versa. The location and interaction are dependent on the type of both the antioxidant and surfactant. Influences of various antioxidants on the physical and structural properties were tested in micellar systems of cationic CTAB, non-ionic Brij 58 and anionic SDS. The antioxidants used to investigate the effects of gradually increasing lipophilicity were gallic acid (GA) and the gallate esters from methyl to octyl gallate (MG-OG). Hydroxy cinnamic acids (HCAs) like \(p\)-coumaric acid (pC), caffeic acid (CA), ferulic acid (FA) and sinapic acid (SA) were employed to observe effects of functional groups like hydroxyl and methoxy groups. Micellar size and shape determined by small angle neutron scattering (SANS), viscosity and cmc were chosen to characterize the antioxidant influence. In Brij 58 systems propyl gallate (PG) did not affect the cmc or aggregation number but decreased micellar size slightly due to an intercalation of PG into the region of the polyoxyethylene chain and the first adjacent alkyl chain methylene groups. In SDS systems the micellar size and cmc decreased in the presence of PG. This was attributed to PG residing in the Stern layer. However, in CTAB systems micelles swelled at low PG concentration and in the presence of GA, while higher PG concentrations and more lipophilic antioxidants led to a sphere-to-rod transition with a simultaneous increase in viscosity and decrease in cmc. This revealed the intercalation of antioxidants in the palisade layer of CTAB micelles entering into strong interactions of electrostatic and hydrophobic origin. It could be demonstrated that the interactions became stronger the more lipophil an antioxidant is and the more hydroxyl groups are attached to the aromatic ring. Differences in the location and interaction of antioxidant and micelles are proposed as being responsible for the effectiveness of antioxidants.

Effects of added nonionic surfactant and inorganic salt on the rheology of sugar surfactant and CTAB aqueous solutions

The effect of adding tri(oxyethylene) dodecyl ether (C 12EO 3) on the phase and rheological behavior of sucrose hexadecanoate and CTAB aqueous solutions in the presence of added salt (NaBr) was investigated. Viscoelastic solutions are formed in CTAB and C 16SE systems upon addition of lipophilic nonionic surfactant C 12EO 3. The zero-shear viscosity shows a maximum at a certain mixing fraction of C 12EO 3, except in the case of the aqueous CTAB/C 12EO 3 system in the absence of salt. The rheological properties are strongly affected by the addition of salt to the CTAB systems but they remain unaltered in the case of C 16SE systems. In ionic systems, the mixing fraction of C 12EO 3 for the maximum viscosity depends on salt concentration.

Photophysics of norharmane in micellar environments: a fluorometric study

Steady state photophysics of norharmane (NHM) has been studied in different aqueous micellar environments. In aqueous solution at pH 7, excitation of the neutral species promotes a rapid transfer of proton giving rise to the corresponding cationic emission. Aqueous micelles differing in their surface charge characteristics interact with the fluorophore differently. The dependence of the fluorescence of the probe molecule on different micelles has been exploited to determine the critical micellar concentrations (CMCs) of the surfactants. The binding constant ( K) and free energy change (Δ G) for the interaction of norharmane with the micelles have been evaluated from the fluorescence data. The probable location of the probe in the micelles has also been suggested. Polarity of the microenvironment around the probe has been determined for CTAB and TX-100 micellar systems from a comparison of the variation of fluorescence properties of the two prototropic species in water–dioxane mixture with varying composition.

Effect of Nonionic Head Group Size on the Formation of Worm-Like Micelles in Mixed Nonionic/Cationic Surfactant Aqueous Systems

The phase behavior, microstructure and rheological properties of mixtures of polyoxyethylene type nonionic surfactants (C12EOn, n = 0–4) and alkyltrimethylammonium bromide cationic surfactants in water were investigated. Upon addition of C12EO3 to the aqueous cationic system at high surfactant concentration, both the interfacial curvature of aggregates and the effective cross-sectional area per surfactant molecule, aS, decrease, resulting in a hexagonal-lamellar liquid crystal phase transition. This effect becomes more pronounced as the alkyl chain-length of the cationic surfactant increases. At low surfactant concentration, addition of nonionic amphiphile to micellar solution above a particular concentration, C*, induces a rapid unidimensional micellar growth, and particularly in CTAB systems viscoelastic micellar solutions of entangled wormlike micelles are formed. With successive addition of the nonionic amphiphile ultimately a micellar to lamellar phase transformation occurs. Rheological measurements within the Wm-phase show that with decreasing EO chain-length, C* also decreases and viscosity increases more swiftly with increasing concentration of nonionic amphiphile, suggesting a rapid micellar growth. This is attributed to the decrease in aS, as predicted by a simple geometrical model.

Thin-layer chromatographic studies of the mobility of pesticides through soil-containing static flat-beds

The chromatographic behavior of some pesticides has been studied on silica, soil, and mixed layers containing soil, with aqueous ammonium or sodium salt solutions, with or without added N -cetyl- N,N,N -trimethylammonium bromide (CTAB), with pure organic solvents, and with aqueous CTAB systems. One interesting aspect of this study is the migration of pesticides such as phosphamidon and dimethoate through pure soil. Several important binary separations have been achieved, sulfur-containing pesticides have been successfully separated from those with chlorine, phosphorus, or both phosphorus and sulfur in their molecules. Phosphorus-containing pesticides were found to migrate faster than those not containing phosphorus, in a variety of mobile phases, irrespective of the nature of the stationary phase. The trend in the relative mobilities of pesticides on the different stationary phases used is also reported.

Interaction, Stability, and Microenvironmental Properties of Mixed Micelles of Triton X100 and n-Alkyltrimethylammonium Bromides: Influence of Alkyl Chain Length

Micellar properties of binary surfactant systems of Triton X100 (TX100) with three different n-alkyltrimethylammonium bromides (n = 12 (DTAB), 14 (TTAB), and 16 (CTAB)) were investigated by the fluorescence probe technique. The critical micelle concentration (cmc) values of the corresponding mixtures in the whole range of composition were obtained by the pyrene 1:3 ratio method. In order to estimate the interaction between the surfactants in the mixed micelles, the cmc data were treated by using the conventional regular solution approach for mixed micelles. It was found that whereas the interaction parameter values (β12) remained fairly constant for the TX100−TTAB and TX100−CTAB systems, the TX100−DTAB system was not well modeled by using that theory. However, in all cases the results showed deviation from ideal behavior. The stability of the mixed micelles was also discussed in the light of Maeda's treatment (J. Colloid Interface Sci. 1995, 172, 98), and the observed differences between the three systems were justified on the basis of a certain steric factor due mainly to the presence of the phenyl group of TX100 in connection with the length of the monomer of the cationic component. The micropolarity of the mixed aggregates was examined by the pyrene 1:3 ratio index in micellar solutions of concentration well above the cmc. It was observed that the increasing participation of the ionic component induces the formation of more closed micelles with a more dehydrated structure. The polarized fluorescence measurements, by using diphenylbutadiene as a probe, were interpreted by using the wobbling in cone model. It was found that the order parameter decreases as the participation of the cationic surfactant in the micelle increases, indicating the formation of a less ordered structure than that of pure TX100 micelles. Data obtained in this investigation allowed to establish a clear correlation between micellar stability and microenvironmental properties of the mixed aggregates.

Fluorescence Probing Investigations of the Mechanism of Formation of Organized Mesoporous Silica

Spectrofluorometry (pyrene and dipyrenylpropane emission spectra) and time-resolved fluorescence quenching have been used to study the effect of addition of a large excess of sodium hydroxide and sodium silicate on the properties of micelles of cetyltrimethylammonium bromide and chloride (CTAB and CTAC). The systems investigated had overall compositions similar to those used when preparing organized mesoporous silica. Our purpose was to test whether the surfactant counterions are effectively exchanged by silicate ions and micelles grow in the presence of silica. The results show the CTAC or CTAB micelle micropolarity is not affected by the presence of the additives. The pyrene lifetime measurements with the CTAB systems indicate that only a small fraction of micelle-bound bromide ions, about 16%, is exchanged by hydroxyl and silicate ions even when they are largely in excess. The presence of the two additives was found to increase very little the micelle aggregation number in CTAB- and CTAC-containing systems, indicating that there is hardly any micelle growth under the experimental conditions used. Last the micelle microviscosity is also little modified by the effect of the additives. These results call for a reexamination of some of the assumptions underlying models used to describe the synthesis of these extremely interesting materials.