A novel viscoelastic system from a cationic surfactant and a hydrophobic counterion

The phase behavior of 2-hydroxy-1-naphthoic acid (2,1-HNC) mixed with cetyltrimethylammonium hydroxide (CTAOH) is reported. This novel system is compared with the published one of 3-hydroxy-2-naphthoic acid (3,2-HNC) mixed with CTAOH. We investigated the phase behavior and properties of the phases in aqueous solutions of 100 mM CTAOH with 2,1-HNC. In both systems a multilamellar vesicle phase is formed when the naphthoate/surfactant ratio (r) reaches unity. When an increasing amount of 2,1-HNC is mixed with a micellar solution of 100 mM CTAOH, an isotropic low-viscous micellar solution, a viscoelastic gel (consisting of rodlike micelles), a turbid region (two-phase region), and a viscoelastic liquid crystalline gel (consisting of multilamellar vesicles, MLV) were formed. The vesicular phase is highly viscoelastic and has a yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/2,1-HNC over a two-phase region, where micelles and vesicles coexist. Also it was noticed that 2,1-HNC is dissolved in 100 mM CTAOH until the naphthoate/surfactant ratio reaches approximately 1.5, and the liquid crystalline phases were found to change their color systematically when they were viewed between two crossed polarizers. The vesicles have been characterized by differential interference contrast microscopy, freeze-fracture electron microscopy, and cryo-electron microscopy (cryo-TEM). The vesicles were polydisperse and their diameter ranged from 100 to 1000 nm. The interlamellar spacing between the bilayers was determined with small angle neutron scattering and agrees with the results from different microscopical methods. The complex viscosity rises by six orders of magnitude when rodlike micelles are formed. The complex viscosity decreases again in the turbid region, and then rises approximately six orders of magnitude above the water viscosity. This second rising is due to the formation of the liquid crystalline MLV phase.     

Microemulsion polymerization of styrene using a polymerizable nonionic surfactant and a cationic surfactant

High polymer/surfactant weight ratios (up to about 15:1) of polystyrene microlatexes have been successfully produced by microemulsion polymerization using a small amount of polymerizable surfactant, ω-methoxypoly(ethylene oxide)₄₀ undecyl α-methacrylate macromonomer (PEO-R-MA-40), and cetyltrimethylammonium bromide (CTAB). After generating “seeding particles” in a ternary microemulsion containing only 0.2 wt% CTAB and 0.1 wt% styrene, the additional styrene containing less than 1 wt% PEO-R-MA-40 was added dropwise to the polymerized microemulsion for a period of about 4 h at room temperature. PEO-R-MA-40 copolymerized readily with styrene. The stable microlatexes were bluish-transparent at a lower polymer content and became bluish-opaque at a higher polymer content. Nearly monodisperse latex particles with diameters ranging from 50 to 80 nm and their molar masses ranging from 0.6 to 1.6 × 10⁶ g/mol could be obtained by varying the polymerization conditions. The dependence of the number of particles per milliliter of microlatex, the latex particle size and the copolymer molar mass on the polymerization time is discussed in conjunction with the effect of the macromonomer concentration. Received: 25 October/2000 Accepted: 2 February 2001     

Characterization of fluorescent surfactant aggregates by fluorimetric and viscosimetric techniques

Summary A combination of fluorimetric and viscosimetric methods was used to characterize N,N-dimethyl-N-2-(4-(t-butylphenoxy)ethoxy)ethyl-N-hexadecylammonium chloride (BDHC), a doubletailed surfactant with dissimilar tail groups.BDHC was observed to fluoresce at 315 nm when excited at 274 nm, a feature which could be utilized to determine its critical micelle concentration (CMC). A value of 3.98×10^–5 M was obtained and was observed to be slightly temperature dependent with aT _min of 25.35°C. Fluorescence quenching experiments using 4-nitroaniline as a quencher were performed in order to determine the aggregation number which was found to be 42.0. The hydrodynamic radius of 21.91 Å was obtained using data from viscosimetric experiments. These data, together with theTanford andEinstein-Stokes relationships, were used to determine the micellar structure (spherical) and the diffusion coefficient (D=0.97×10^–6 cm²/s), respectively. The G of micellization forBDHC was determined to be –34.9 kJ/mol.

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Charakterisierung fluoreszierender oberflächenaktiver Aggregate mittels Fluorimetrie und Viskosimetrie

Zusammenfassung Die Verbindung N,N-Dimethyl-N-2-(4-(t-butylphenoxy)ethoxy)ethyl-N-hexadecylammoniumchlorid (BDHC), eine oberflächenaktive Substanz mit zwei unterschiedlichen Seitenketten, wurde mittels einer Kombination von fluorimetrischen und viskosimetrischen Methoden charakterisiert. Anregung bei 274 nm ruft eine Fluoreszenz bei 315 nm hervor; diese Eigenschaft wurde zur Bestimmung der kritischen Micellenkonzentration (CMC) herangezogen. Es wurde ein geringfügig temperaturabhängiger Wert von 3.98×10^–5 M gefunden (T _min=25.35°C). Quenchexperimente mit 4-Nitroanilin ergaben eine Aggregationszahl von 42.0, viskosimetrische Untersuchungen einen hydrodynamischen Radius von 21.91 Å. Die erhaltenen Daten erlauben zusammen mit den Beziehungen nachTanford undEinstein-Stokes die Bestimmung der Micellenstruktur (sphärisch) und des Diffusionskoeffizienten (0.97×10^–6 cm²/s). Das G der Micellenbildung fürBDHC beträgt –34.9 kJ/mol.

     

Asp-Gly based peptides confined at the surface of cationic gemini surfactant aggregates

Cationic gemini surfactants complexed with anionic oligoglycine-aspartate (called gemini peptides hereafter) were synthesized, and their aggregation behaviors were studied. The effects of the hydrophobic chain length (C10-C22) and the length of the oligoglycine (0-4) were investigated, and it was clearly shown by critical micellar concentration, Krafft temperature, and isothermal surface pressure measurements that the hydrophobic effect and interpeptidic interaction influence the aggregation behavior in a cooperative manner. Below their Krafft temperatures, some of them formed both hydro- and organogels with three-dimensional networks and the Fourier transform infrared measurements show the presence of interpeptidic hydrogen bonds.     

Mechanism of copolymerization of acrylamide with a polymerizable surfactant

Associating polymers have been prepared by radical copolymerization in water of acrylamide with a micelleforming cationic polymerizable surfactant. To estimate the locus of initiation, the polymerizations were carried out in the presence of initiators and radical inhibitors of various solubilities (water-soluble or oil-soluble), and the decay of inhibitor concentration has been monitored by electron spin resonance spectroscopy. Conversion–time curves simultaneously determined. The experimental data have been interpreted by taking into account the concentration and lifetime of the different radical species (primary radicals, oligoradicals, inhibitors), their partitioning between the micelles and the aqueous continuous phase, and the dynamics of the micellar system. Analysis of the data provided some insight into the copolymerization mechanism of these micellar systems.     

Disassembly of amyloid fibrils by premicellar and micellar aggregates of a tetrameric cationic surfactant in aqueous solution

We report a finding that not only the micelles but also the premicellar aggregates of a star-like tetrameric quaternary ammonium surfactant PATC can disassemble and clear mature β-amyloid Aβ(1-40) fibrils in aqueous solution. Different from other surfactants, PATC self-assembles into network-like aggregates below its critical micelle concentration (CMC). The strong self-assembly ability of PATC even below its CMC enables PATC to disaggregate the Aβ(1-40) fibrils far below the charge neutralization point of the Aβ(1-40) with PATC. There may be two key features of the fibril disassembly induced by the surfactant. First, the positively charged surfactant molecules bind with the negatively charged Aβ(1-40) fibrils through electrostatic interaction. Second, the self-assembly of the surfactant molecules bound onto the Aβ(1-40) fibrils disaggregate the fibrils, and the surfactant molecules form mixed aggregates with the Aβ(1-40) molecules. The result reveals a structural approach of constructing efficient disassembly agents to mature β-amyloid fibrils.     

Spontaneous aggregate transition in mixtures of a cationic gemini surfactant with a double-chain cationic surfactant

Controllable aggregate transitions were realized by mixing two kinds of cationic surfactants, hexylene-1,6-bis(dodecyldimethylammonium bromide) (C(12)C(6)C(12)Br(2)) and didodecyldimethylammonium bromide (DDAB). It was found that two parameters are the main factors determining the aggregation behavior of the mixed system, the total concentration of DDAB and C(12)C(6)C(12)Br(2) (C(T)), and the mole fraction of DDAB in the mixtures of DDAB and C(12)C(6)C(12)Br(2) (X(DDAB)). How these two parameters act on the aggregate transitions was studied in detail by various measurements including surface tension, turbidity, electrical conductivity, ζ potential, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, and (1)H NMR. When C(T) was constant, spontaneous vesicle-to-micelle transitions were found with decreasing X(DDAB) at high C(T). When X(DDAB) was constant, aggregate transitions were generated by gradually increasing C(T), depending on different X(DDAB) ranges. At X(DDAB) < 0.6, small spherical aggregates formed first and then transferred to vesicles, and finally the vesicles transitioned to micelles. At X(DDAB) ≥ 0.6, the progressive increase in C(T) led to aggregate transitions on the order of the arising of vesicles, the continuous growth of vesicles, the disruption of vesicles into micelles, and the final coexistence of vesicles and micelles. The hydrophobic interaction and electrostatic repulsion between DDAB and C(12)C(6)C(12)Br(2) together with the related degree of ionization and hydration of the surfactants were gradually adjusted by changing the ratio and the total concentration of these two surfactants, which should be responsible for the complicated aggregation behavior.     

Effect of counterion geometry on cationic micelles

The critical micelle concentration, aggregation number, and binding properties of decyltrimethylammonium ion micelles are determined as a function of counterion geometry using the dianions of phthalic, isophthalic, and terephthalic acids. Light-scattering methods were used to secure the CMC and AN data whereas binding data were obtained with a specific ion electrode. It is shown that the differences among the micellar parameters are quite small relative to counterion effects induced by variations in polarizability and hydrophobicity. The data are used as evidence for a disorganized micelle surface containing water-filled grooves and fatty patches which do not discriminate among various geometric dispositions of the carboxylates about the aromatic ring.     

Emulsion copolymerization of styrene with a nonionic styrenic polymerizable surfactant

Among the variety of possible structures for polymerizable surfactants, it seems clear that the most interesting should be those with the reactive group located in the hydrophobic part of the molecule. We report here a study based on such a surfactant. Its general formula is A set of surfactants has been produced with m varying from 23 to 48 and n = 6 or 12. The compounds have been characterised by ¹H-NMR (nuclear magnetic resonance), size exclusion chromatography, surface tension measurements and turbidimetry. These surfactants have been copolymerized with styrene in emulsion polymerization. The coagulum is rather important, except if m is large enough. Although the incorporation of the surfactant in the latex is rather high. Most of the anchored surfactant remains at the surface and is not too buried inside. The particle size decreases with both the amount of surfactant and the length of its hydrophilic part. The use of these polymerizable surfactants leads to an excellent stability of the latex against the addition of electrolytes, and also against freeze-thawing constraints.     

Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface

To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)-dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF(4) was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF(4)(-) ions, but the composition of Br(-) ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr-DTABF(4) mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)-1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixtures, where a stronger hydrogen-bonding exists between BF(4)(-) and HMIM(+) ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions.     

Novel nanotubes from a cationic surfactant and an anionic stiff aromatic counter-ion

A novel class of nanotubes (NT) is reported which consists of two components, a cationic surfactant cetyltrimethylammoniumbromide (CTAB) and an anionic UV absorber, 2-phenylbenzimidazole-5-sulfonic acid (PBSA). When aqueous solutions of the components are mixed, a turbid low-viscosity solution is formed immediately. This solution is stable for several days. Then, within 1 day, the turbid samples are transformed into transparent, isotropic soft gels that contain thin single-walled tubuli (SWNT). The SWNT have been characterized by cryo-TEM micrographs. The NT have an extremely narrow size distribution of the outer (o.d.) and inner (i.d.) diameters. The tubes have an o.d. of 13 +/- 0.5 nm and an i.d. of about 6 nm. Despite their small diameter, the NT are extremely stiff with a persistence length l(p) greater than several micrometers. It is assumed that the tubuli consist of bilayers of the cationic and anionic compounds with a one-to-one ratio. In solution, the tubuli seem to be stabilized by the adsorption of one of the excess amphiphilic compounds.     

Synthesis and characterization of new cationic quaternary ammonium polymerizable surfactants

Two new polymerizable surfactants (surfmers), (11-acryloyloxyundecyl)dimethylethylammonium bromide (ethyl surfmer) and (11-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (hydroxyethyl surfmer), were synthesized and characterized. The binary phase diagrams of both surfmer/water systems are described. Both surfmers can form isotropic solutions and lamellar lyotropic liquid crystalline phases. The hydroxyethyl surfmer/water system forms a lamellar phase for weight concentrations of surfmer between 70 and 90% relative to between 75 and 85% for the ethyl surfmer/water system. The differences in the self assembly of these surfmers were attributed to the ability of hydroxyethyl surfmer to form hydrogen bonds (between two head groups and with water), whereas no such interactions can occur with the ethyl surfmer system.     

Mass spectrometry of surfactant aggregates

In contrast with the enormous amount of literature produced during many decades in the field of surfactant aggregation in liquid, liquid crystalline and solid phases, only a few investigations concerning surfactant self- assembling in the gas phase as charged aggregates have been carried out until now. This lack of interest is disappointing in view of the remarkable theoretical and practical importance of the inherent knowledge. The absence of surfactant-solvent interactions makes it easier to study the role of surfactant-surfactant forces in determining their peculiar self-assembling features as well as the ability of these assemblies to incorporate selected solubilizate molecules. Thus, the study of gas-phase surfactant and surfactant-solubilizate aggregates is a research subject which has exciting potential, including mass and energy transport in the atmosphere, origin of life and simulation of supramolecular aggregation in interstellar space. On the other hand, the structural and dynamic properties of surfactant aggregates in the gas phase could be exploited in a number of interesting applications such as atmospheric cleaning agents, transport and protection of pulmonary drugs or biomolecules and as nanoreactors for specialized chemical reactions in confined space. Spectrometric techniques, together with molecular dynamics simulations, have been the principal investigative tools in this field and appearto be particularly suited to gaining fundamental information on the structure and stability of surfactant-based supramolecular aggregates, charge state effects, entrapment of solubilizate molecules, preferential solubilization sites and chemical reactions localized in a single organized aggregate. The main aim of this review is to present the actual state of the art in this novel and exciting research field underlining the knowledge acquired up to now as well as the aspects needing a more deep understanding. Moreover, intriguing departures of the behavior of surfactant solutions under electrospray ionization conditions from that of ionic, polar and apolar analytes will be discussed.     

Synthesis and characterization of a novel double-tailed cationic surfactant

A novel double-tailed cationic surfactant, N,N′-didodecylacetamidinium bicarbonate, was prepared by reacting dimethylacetamide dimethyl acetal with dodecylamine, followed by reacting with dry ice. MSD, FTIR, ¹H NMR, ¹³C NMR, 2D-NMR were used to confirm its structure. The surfactant reduced the surface tension of pure water to 24.7 mN m⁻¹ and its critical micelle concentration was 8.75 × 10⁻⁵ M. Formation of vesicle was also observed.     

Polymerizable ion-pair amphiphile that has a polymerizable group at cationic ammonium chain

Polymerizable ion-pair amphiphile (PIPA) that has a polymerizable lipoyl group on cationic ammonium chain was prepared to compare the advantage of ion-pair form with the conventional polymerizable ammonium surfactant in the points of stability of vesicle and permeation properties. PIPA introduced in this paper, compared to previously reported one with polymerizable group on anionic chain side, showed similar vesicle stability in addition to high encapsulation efficiency and resistance to disruption by SDS or Triton X-100. Reasons for such high encapsulation and low permeability of the PIPA vesicle are investigated and the possible applications are briefly discussed.     

Phase and rheological behavior of the polymerizable surfactant CTAVB and water

The phase and rheological behaviors of the polymerizable surfactant, cetyltrimethylammonium benzoate (CTAVB), and water as a function of surfactant concentration and temperature are investigated here. The critical micelle concentration (cmc) and the (cmc(2)), as well as the Krafft temperature (T(K)), are reported. A large highly viscous micellar solution region and hexagonal- and lamellar-phase regions were identified. The micellar solutions exhibit shear thickening in the dilute regime, below the overlapping or entanglement concentration. At higher concentrations, wormlike micelles form and the solutions show strong viscoelasticity and Maxwell behavior in the linear regime and shear banding flow in the nonlinear regime. The linear viscoelastic regime is analyzed with the Granek-Cates model, showing that the relaxation is controlled by the kinetics of reformation and scission of the micelles. The steady and unsteady responses in the nonlinear regime are compared with the predictions of the Bautista-Manero-Puig (BMP) model. Model predictions follow the experimental data closely.     

Surface complexation of DNA with a cationic surfactant

We have studied the surface complexation of DNA with a cationic surfactant (DTAB) using a combination of methods: dynamic surface tension, ellipsometry and Brewster angle microscopy. Below the surfactant critical aggregation concentration (cac), complexation occurs only at the surface, and the results are consistent with neutralization of the surfactant charges by the free polymer ions. Above the cac, surfactant starts to bind cooperatively to DNA in the bulk, and adsorption of the preformed hydrophobic surfactant DNA aggregate is now possible, leading to thick surface layers. At still higher concentrations of surfactant (still below saturation of binding in the bulk), there is decrease in adsorption due to competition with bulk aggregates. Finally, as surfactant concentration is increased still further, bulk aggregates become less soluble and large amounts are adsorbed, forming a surface layer, which is solid-like and brittle.     

Phase behavior of mixed solution of a glycerin-modified cationic surfactant and an anionic surfactant

The phase behavior of mixed solution of newly synthesized monoglycerylcetyldimethylammonium chloride (MGCA) and sodium octyl sulfate (SOS) in water was investigated by cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), differential scanning calorimetry (DSC), and fluorescence polarizing for evaluation of the microviscosity of bilayers. No precipitate was observed in the mixed solution except at concentrations below 20 mM over all mixing ratios, and stable vesicles were formed in a considerably wide range of mixing ratio, even at the equimolar ratio. Vesicles formed in aqueous 1/1 MGCA/SOS mixture were found to exhibit no phase transition, and fluorescence polarizing measurements showed that the vesicle bilayers have a high fluidity. This flexibility allows the bilayers to have a spontaneous curvature, and thus vesicles rather than flat lamellae can be stabilized in the mixture even at the equimolar ratio. In addition, because the glycerin group of MGCA interacts strongly with water, the hydration repulsion contributes to prevent the bilayers consisting of MGCA and SOS from adhering and flocculating even though the charge neutralization between MGCA and SOS occurs at the equimolar ratio.     

Aqueous two-phase system of an anionic gemini surfactant and a cationic conventional surfactant mixture

The aqueous two-phase system formed by the mixture of dodecyltrimethylammonium bromide (DTAB) with a gemini surfactant O,O′-bis(sodium 2-lauricate)-p-benzenediol (C₁₁pPHCNa) has been studied. Two two-phase regions were observed, one was a wide region in the cationic surfactant-rich side and the other in the vicinity of R = 1:1, where R is the mixing mole ratio of DTAB to C₁₁pPHCNa in global solution. Multi-lamellar vesicles are formed in the concentrated upper phase of cationic surfactant-rich systems and spherical aggregates in the concentrated bottom phase at R = 1:1. The microstructure of the solution and the phase behavior of the aqueous two-phase system strongly depended on the total concentration and the composition of the system.