Revealed Properties of Various Self-Assemblies in Two Catanionic Surfactant Systems in Relation to Their Polarity and Molecular Packing State

A catanionic surfactant system is an aqueous solution or dispersion of cationic and anionic surfactants that spontaneously self-assemble into structures such as micelles, vesicles, and coacervates. Their structural diversity varies depending on the ratios of cationic and anionic surfactants (compositions), the chemical structure of the constituent molecules, etc. Herein, two types of catanionic surfactant systems were systematically characterized: (i) cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), both typical ionic surfactants; and (ii) dodecylmethylimidazolium ammonium bromide ([C₁₂mim]Br) and SDS, where the former is an ionic liquid. By observing the sample appearance, turbidity, and particle size, the phase state of each system was analyzed according to the total concentration of surfactants and the molar ratio of cationic surfactants to the total concentration. Especially, for specific compositions of catanionic surfactant vesicles (cataniosome), the closed structure of the vesicles was confirmed through calcein entrapment and release detected with a fluorescence assay. The polarities of the interface of the prepared self-assemblies were evaluated using a fluorescence probe, Laurdan. The packing state of the molecules in the formed self-assembly structure was estimated using Raman spectroscopy. The results clearly indicate consistent phase-transition behavior for the CTAB-SDS (i) and [C₁₂mim]Br-SDS (ii) systems, depending on the total surfactant concentration and composition, while the membrane properties of the two systems differed. The cataniosome formed in the CTAB-SDS system was in a tightly packed membrane state and more hydrophobic than that formed in the [C₁₂mim]Br-SDS system owing to the difference in the structure of the constituting molecule: [C₁₂mim]Br has a larger head group and shorter acyl chain than CTAB. The self-assembly properties evaluated in this study were compared with those of typical lipid membranes, liposomes (lipid vesicles), to determine a possible application of the catanionic systems with various self-assembly formulations.

Interplay between bulk self-assembly, interfacial and foaming properties in a catanionic surfactant mixture of varying composition

The self-aggregation, surface properties and foamability of the catanionic surfactant mixture cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfonate (SOSo) have been investigated to obtain insight on the relation between bulk nanostructures, surfactant packing, and foam stability and aging. Light microscopy, SANS, cryo-TEM, DLS, surface tension, rheometry and direct photography were used to characterize mixtures with varying CTAB molar fraction, x_CTAB. In the bulk, self-assembly is richer in the excess CTAB region than in the excess SOSo one. Starting from neat CTAB micelles and on addition of anionic surfactant, there is a change from small ellipsoidal micelles (1 < x_CTAB ≤ 0.80) to large rodlike micelles (0.65 ≤ x_CTAB ≤ 0.55) and then to vesicles (0 < x_CTAB ≤ 0.50), with coexistence regions in between; SOSo-rich mixtures are thus dominated by vesicles. High size polydispersity for the micelles and vesicles is an intrinsic feature of this system. Foam stability is concomitantly impacted by x_CTAB. SOSo is a small mobile molecule and so it disrupts foam stability, irrespective of the presence of vesicles. Foams are thus only stable in the CTAB-rich regions, and SANS shows that the shape of micelles and vesicles is unchanged inside the foam. Foam drainage is thereby mostly controlled by the presence of the elongated micelles through the solution viscosity, whereas coarsening is influenced by dense surfactant packing at the gas-liquid interfaces.

A facile template route to periodic mesoporous organosilicas nanospheres with tubular structure by using compressed CO2

Periodic mesoporous organosilicas (PMOs) nanospheres with tubular structure were prepared with compressed CO₂ using cationic and anionic mixed surfactant (CTAB/SDS) and triblock copolymer Pluronic P123 as bi-templates. TEM, N₂ adsorption-desorption, solid NMR, and FTIR were employed to characterize the obtained materials. Compressed CO₂ severed as acidic reagent to promote the hydrolysis of organosilicas, and could tune the morphology and structure of the obtained PMOs nanomaterials simple by adjusting the CO₂ pressure during the synthesis process. Rhodamine B (RB) and Ibuprofen (IBU), as the model dye and drug, were loaded into the prepared nanomaterials to reveal its adsorption and desorption ability. Furthermore, different molars of the surfactant (CTAB/SDS) and organosilane precursor (BTEB) were investigated to show the effect of the surfactant concentration on the morphology and structure of the PMOs prepared with compressed CO₂, and some different structures were obtained. A possible mechanism for the synthesis of PMOs with tubular structure using compressed CO₂ was proposed based on the experimental results.

Nanomechanical Characterization of Micellar Surfactant Films via Atomic Force Microscopy at a Graphite Surface

In this work, we study the mechanical properties of sodium dodecyl sulfate (SDS) and dodecylamine hydrochloride (DAH) micellar films at a graphite surface via atomic force microscopy (AFM). Breakthrough forces for these films were measured using silicon nitride cantilevers and were found to be 1.1 ± 0.1 nN for a 10 mM DAH film and 3.0 ± 0.3 nN for a 10 mM SDS film. For 10 mM SDS films, it was found that the addition of 1.5 mM of NaCl, Na₂SO₄, or MgCl₂ produced a 50-70% increase in the measured breakthrough force. Similar results were found for 10 mM DAH films when NaCl and MgCl₂ were added. A model was developed on the basis of previous work on lipid films and CMC data gathered via spectrofluorometry measurements to predict the change in normalized breakthrough forces with added salt concentrations for SDS and DAH films. Using this model, it was found that the activation volume required to initiate the breakthrough was roughly 0.4 nm³ for SDS and 0.3 nm³ for DAH, roughly the volume of a single molecule. Normalized breakthrough force data for SDS films with added MgCl₂ showed an unexpected dip at low added salt concentrations. The model was adapted to account for changing activation volumes, and a curve of activation volume versus magnesium concentration was obtained, showing a minimum volume of 0.21 nm³. The addition of 0.2 mM SDS to a 10 mM DAH solution was found to double the measured breakthrough force of the film. Images taken of the surface showed a phase change from cylindrical hemimicelles to a planar film that may have produced the observed differences. The pH of the bulk solution was varied for both 10 mM SDS and DAH films and was found to have little effect on the breakthrough force.

The Influence of Surfactant's Synergism on the Solubilization of Some Fluorescent Compounds

Aqueous solutions of anionic surfactant sodium dodecylsulfate (SDS) and cationic surfactant cetyltrimethyammonium bromide (CTAB) were prepared at room temperature in order to investigate the influences of surfactants mixing on the solubilization of 2-naphthol-6-sulfonate and toluene. The phase behavior of CTAB/SDS was firstly investigated to obtain the optimum ratio of CTAB/SDS that result in an isotropic solution before reaching the two phase region. From the conductivity and surface tension measurements, the critical micelle concentrations (CMC) of single and mixed surfactant solutions were obtained and the interaction parameter (β) were evaluated. Water solubility enhancement of (0–3 wt.%) toluene by micellar solutions of single surfactant SDS, CTAB and mixed solutions of SDS-CTAB surfactants were then investigated by establishing the phase behavior and measuring the absorption and the emission spectra of these solutions. The toluene was completely solubilized by the micellar solution of single surfactants, whereas turbidity was observed at toluene concentration of about 1.5 wt.% in the micellar solution of SDS-CTAB. The fluorescence behavior of 2-naphthol-6-sulfonate (2NO6S) and toluene in the single and mixed surfactant solutions was also compared. The solubilization of toluene in surfactant solutions was explained in terms of hydrophobic interaction occurring within the surfactant core and the palisade layers.