A Femtosecond Study of Solvation Dynamics and Anisotropy Decay in a Catanionic Vesicle: Excitation‐Wavelength Dependence

The structure and dynamics of a catanionic vesicle are studied by means of femtosecond up‐conversion and dynamic light scattering (DLS). The catanionic vesicle is composed of dodecyl‐trimethyl‐ammonium bromide (DTAB) and sodium dodecyl sulphate (SDS). The DLS data suggest that 90 % of the vesicles have a diameter of about 400 nm, whereas the diameter of the other 10 % is about 50 nm. The dynamics in the catanionic vesicle are compared with those in pure SDS and DTAB micelles. We also study the dynamics in different regions of the micelle/vesicle by varying the excitation wavelength (λ_ex) from 375 to 435 nm. The catanionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the λ_ex‐dependent variation of the solvation dynamics is more prominent in the first case. The solvation dynamics in the vesicle and the micelles display an ultraslow component (2 and 300 ps, respectively), which arises from the quasibound, confined water inside the micelle, and an ultrafast component (<0.3 ps), which is due to quasifree water at the surface/exposed region. With an increase in λ_ex, the solvation dynamics become faster. This is manifested in a decrease in the total dynamic solvent shift and an increase in the contribution of the ultrafast component (<0.3 ps). At a long λ_ex (435 nm), the surface (exposed region) of a micelle/vesicle is probed, where the solvation dynamics of the water molecules are faster than those in a buried location of the vesicle and the micelles. The time constant of anisotropy decay becomes longer with increasing λ_ex, in both the catanionic vesicle and the ordinary micelles (SDS and DTAB). The slow rotational dynamics (anisotropy decay) in the polar region (at long λ_ex) may be due to the presence of ionic head groups and counter ions.     

Micellar Behavior of Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition,Temperature, Salt,and Surfactants

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examined by surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). The critical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%; micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studies show that the copolymers form ellipsoidal micelles with semi major axis ～23 nm and semi minor axis ～8 nm. No significant change in the structure was found with temperature and presence of salt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.     

Vesicle formation induced by layered double hydroxides in the catanionic surfactant solution composed of sodium dodecyl sulfate and dodecyltrimethylammonium bromide

In this paper, it is reported that positively charged Mg₃Al layered double hydroxide (LDH) nanoparticles can induce the spontaneous formation of vesicles in micelle solution of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with a mass ratio of 8:2. The formation of vesicles was demonstrated by negative-staining transmission electron microscopy observations. The size of the vesicles increased with the increase in the concentration of Mg₃Al-LDH nanoparticles. A composite of LDH nanoparticles encapsulated in vesicles was formed. A possible mechanism of LDH-induced vesicle formation was suggested. The positively charged LDH surface attracts negatively charged micelles or free amphiphilic molecules, which facilitates their aggregation into bilayer patches. These bilayer patches connect to each other and finally close to form vesicles. It was also found that an adsorbed compound layer of SDS and DTAB micelles or molecules on the LDHs surface played a key role in vesicle formation.     

DSC Investigations of DDAB,DTAB and DHAB Vesicle Aqueous Solutions in Presence of SDS

Interactions of anionic surfactant sodium dodecyl sulphate (SDS) with vesicles formed by synthetic dialkyldimethylammonium bromides i.e. didodecyldimethylammonium bromide (DDAB), ditetradecyldimethylammonium bromide (DTAB), dihexadecyldimethylammonium bromide (DHAB) have been examined by using differential scanning microcalorimetry and electron transmission microscopy. The temperatures and standard enthalpies characterising gel to liquid-crystal transitions increase significantly with increase of SDS concentration for all investigated systems. It means that incorporation of SDS monomers in these vesicular bilayers significantly stabilises their gel states. Moreover, in case of DDAB and DTAB vesicle systems added SDS limits kinetic features of recorded isobaric heat capacity dependencies on temperature observed earlier for the pure vesicular solutions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hexafluoroisopropanol-induced coacervation in aqueous mixed systems of cationic and anionic surfactants for the extraction of sulfonamides in water samples

Hexafluoroisopropanol (HFIP)-induced coacervation in aqueous mixed systems of catanionic surfactants of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) was described in detail, and its application in the extraction of strongly polar sulfonamides (SAs) was investigated. With 10 % (v/v) HFIP inclusion, coacervation formation and two-phase separation occur in a wide range of SDS/DTAB mole ratios (88:12～0:100 mol/mol) and total surfactant concentrations (10～200 mmol/L). The interactions between HFIP and DTAB play an important role in coacervation formation. The HFIP-induced SDS–DTAB coacervation extraction proves to be an efficient method for the extraction and preconcentration of SAs. Both hydrophobic interaction and polar interactions (hydrogen–bond, electrostatic, and π-cation) contribute to the distribution of SAs into coacervate phase. The proposed HFIP-induced SDS–DTAB coacervation extraction combined with HPLC–UV was employed for the extraction and quantitative determination of SAs in environmental water samples. Limits of detection were 1.4～2.5 ng mL^?1. Excellent linearity with correlation coefficients from 0.9990 to 0.9995 was obtained in the concentration of 0.01～10 μg mL^?1. Relative recoveries were in the range of 93.4～105.9 % for analysis of the lake, underground, and tap water samples spiked with SAs at 0.01, 1.0, and 10 μg/mL, respectively. Relative standard deviations were 0.7～3.2 % for intraday precision and 1.3～4.6 % for interday precision (n?=?3). Concentration factors were 17～49 for three water samples spiked with 0.01 μg/mL SAs. The results demonstrate that the proposed extraction method is feasible for the preconcentration and determination of trace SAs in real water samples. Graphical abstract

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Blastulae aggregates: new intermediate structures in the micelle-to-vesicle transition of catanionic systems

A new type of intermediate structure was found in the salt-induced micelle-to-vesicle transition in a catanionic system composed of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) in aqueous solution with an excess of anionic surfactant. The appearance of symmetrically shaped hollow structures, which we named blastulae vesicles, is presented.     

Role of the surfactant headgroup on the counterion specificity in the micelle-to-vesicle transition through salt addition

A transition from micelles to vesicles is reported when salts are added to a catanionic micellar solution composed of sodium dodecylcarboxylate (SL) and dodecyltrimethylammonium bromide (DTAB), with an excess of SL. The counterion binding and increase in aggregate size was monitored by mass spectrometry, rheology and dynamic light scattering measurements, whereas the vesicles were characterized by freeze-fracture and cryo-transmission microscopy experiments. The effect of counterions on the formation of vesicles was studied and compared to a previously studied catanionic system with a sulfate head group, SDS/DTAB. As in the latter case, no anion specificity was found, while large differences in the hydrodynamic radii of the formed objects were observed, when the cation of the added salt was varied. A classification of the cations could be made according to their ability to increase the measured hydrodynamic radii. It is observed that, if the sulfate headgroup of the anionic surfactant is replaced by a carboxylic group, the order of the ions is reversed, i.e. it follows the reversed Hofmeister series. Different morphologies are observed as the ionic strength of the system is increased. The aggregates are analogous to those found in the SDS/DTAB system.     

4-二氰乙烯基-N,N-二甲基苯胺与不同表面活性剂的相互作用

用光谱法研究了荧光分子4-二氰乙烯基-N,N-二甲基苯胺(DCVA)与十二烷基硫酸钠(SDS)、聚氧乙烯(23)月桂醚(BRIJ35)、十二烷基三甲溴化铵(DTAB)胶束间的相互作用.与在水中相比,在上述3类表面活性剂溶液中探针的荧光强度分别增加到3.3、5.4和5.1倍;最大发射波长随表面活性剂浓度的增加分别蓝移了5、11和9nm.由此可知,DCVA在DTAB和BRIJ35胶束中所处微环境的极性相似,而在SDS胶束中DCVA所处微环境的极性较大.通过DCVA在3种表面活性剂溶液中的荧光强度,计算出了DCVA与其胶束的结合常数分别为1.7×10³、1.4×10³和8.8×10².     

Specific alkali cation effects in the transition from micelles to vesicles through salt addition

The transition of ionic micelles to vesicles with added salts is explored in this paper. The catanionic surfactant solution was comprised of sodium dodecylsulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) with an excess of SDS. The micellar size increased with concentration for all salts. No anion specificity was found, probably because of the excess of SDS. However, when the cation of the added salt was varied, large differences were observed in the hydrodynamic radii of the aggregates. A classification of the cations according to their ability to increase the measured hydrodynamic radii follows a Hofmeister series. The change in aggregate size can be explained by modified counterion binding and dehydration of the surfactant headgroups.     

Silicone nanocapsules templated inside the membranes of catanionic vesicles

A simple and effective way to synthesize hollow silicone resin particles of controlled diameter is presented. The synthesis utilizes catanionic vesicles as templates for the polycondensation/polymerization processes of 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) within their membranes. Two different surfactant systems were used to form the vesicular templates: mixtures of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylbenzenesulfonate (SDBS) in the cationic (the DTAB/SDBS system) or anionic (the SDBS/DTAB system) rich region of the phase diagram. The templates obtained from these surfactant mixtures form spontaneously unilamellar vesicles in aqueous solution. The vesicular templates swell upon addition of D4H, thus increasing their size. The silicone resin was obtained in acid- or base-catalyzed polycondensation and ring-opening polymerization processes of D4H. In the case of the DTAB/SDBS system the formation of a densely cross-linked silicone material with SiO3/2 units allowed the nanocapsules to retain the vesicular shape after removal of the template, whereas in the SDBS/DTAB system, the polymer produces capsules which are too smooth to support surfactant lysis. The morphology of the silicone nanocapsules was analyzed using transmission electron microscopy (TEM) and, in some cases, atomic force microscopy (AFM). TEM and AFM reveal discrete hollow particles with a small amount of linked or aggregated hollow silica shells.     

Characterization of surfactant and phospholipid vesicles for use as pseudostationary phases in electrokinetic chromatography

The physical, electrophoretic and chromatographic properties (mean diameter, electroosmotic flow, electrophoretic mobility, elution range, efficiency, retention, and hydrophobic, shape, and chemical selectivity) of three surfactant vesicles and one phospholipid vesicle were investigated and compared to a conventional micellar pseudostationary phase comprised of sodium dodecyl sulfate (SDS). Chemical selectivity (solute-pseudostationary phase interactions) was discussed from the perspective of linear solvation energy relationship (LSER) analysis. Two of the surfactant vesicles were formulated from nonstoichiometric aqueous mixtures of oppositely charged, single-tailed surfactants, either cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) in a 3:7 mole ratio or octyltrimethylammonium bromide (OTAB) and SDS in a 7:3 mole ratio. The remaining surfactant vesicle was comprised solely of bis(2-ethylhexyl)sodium sulfosuccinate (AOT) in 10% v/v methanol, and the phospholipid vesicle consisted of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and phosphatidyl serine (PS) in 8:2 mole ratio. The mean diameters of the vesicles were 76.3 nm (AOT), 86.9 nm (CTAB/SOS), 90.1 nm (OTAB/SDS), and 108 nm (POPC/PS). Whereas the coefficient of electroosmotic flow (10(-4) cm2 V(-1) s(-1)) varied considerably (1.72 (OTAB/SDS), 3.77 (CTAB/SOS), 4.05 (AOT), 5.26 (POPC/PS), 5.31 (SDS)), the electrophoretic mobility was fairly consistent (-3.33 to -3.87 x 10(-4) cm2 V(-1) s(-1)), except for the OTAB/SDS vesicles (-1.68). This resulted in elution ranges that were slightly to significantly larger than that observed for SDS (3.12): 3.85 (POPC/PS), 8.6 (CTAB/SOS), 10.1 (AOT), 15.2 (OTAB/SDS). Significant differences were also noted in the efficiency (using propiophenone) and hydrophobic selectivity; the plate counts were lower with the OTAB/SDS and POPC/PS vesicles than the other pseudostationary phases (< or = 75,000/m vs. > 105,000/m), and the methylene selectivity was considerably higher with the CTAB/SOS and OTAB/SDS vesicles compared to the others (ca. 3.10 vs. < or = 2.6). In terms of shape selectivity, only the CTAB/SOS vesicles were able to separate all three positional isomers of nitrotoluene with near-baseline resolution. Finally, through LSER analysis, it was determined that the cohesiveness and hydrogen bond acidity of these pseudostationary phases have the greatest effect on solute retention and selectivity.     

Self-assembly in a catanionic mixture with an aminoacid-derived surfactant: from mixed micelles to spontaneous vesicles

The aqueous self-assembly of a novel lysine-derived surfactant with a gemini-like architecture, designated here as 12-Lys-12, has been experimentally investigated for the amphiphile alone in water and in a mixture with dodecyltrimethylammonium bromide (DTAB). The neat surfactant forms interesting micrometer-sized rigid tubules in the dilute region, resulting in very viscous solutions. For the catanionic mixture with DTAB, various single and multiphase regions were identified (up to a total surfactant concentration of 1.5 wt %) by means of combined polarizing light microscopy, cryo-TEM, and NMR. In the DTAB-rich side, for a mixing molar ratio in the range 2 < DTAB/12-Lys-12 < 4, a region of stable, unilamellar vesicles can be found. Furthermore, it was found that upon addition of 12-Lys-12 to pure DTAB solutions, the mixed micelles grow and beyond a given mixing ratio, vesicles assemble and coexist with small micelles. The transition is not continuous, since there is a narrow mixing range where phase separation occurs. Self-diffusion measurements and cryo-TEM imaging show that the average vesicle radius is on the order of 30-40 nm.     

Kinetics of thermo-induced micelle-to-vesicle transitions in a catanionic surfactant system investigated by stopped-flow temperature jump

The kinetics of thermo-induced micelle-to-vesicle transitions in a catanionic surfactant system consisting of sodium dodecyl sulfate (SDS) and dodecyltriethylammonium bromide (DEAB) were investigated by the stopped-flow temperature jump technique, which can achieve T-jumps within ～2-3 ms. SDS/DEAB aqueous mixtures ([SDS]/[DEAB] = 2/1, 10 mM) undergo microstructural transitions from cylindrical micelles to vesicles when heated above 33 °C. Upon T-jumps from 20 °C to final temperatures in the range of 25-31 °C, relaxation processes associated with negative amplitudes can be ascribed to the dilution-induced structural rearrangement of cylindrical micelles and to the dissolution of non-equilibrium mixed aggregates. In the final temperature range of 33-43 °C the obtained dynamic traces can be fitted by single exponential functions, revealing one relaxation time (τ) in the range of 82-440 s, which decreases with increasing temperature. This may be ascribed to the transformation of floppy bilayer structures into precursor vesicles followed by further growth into final equilibrium vesicles via the exchange and insertion/expulsion of surfactant monomers. In the final temperature range of 45-55 °C, vesicles are predominant. Here T-jump relaxations revealed a distinctly different kinetic behavior. All dynamic traces can only be fitted with double exponential functions, yielding two relaxation times (τ(1) and τ(2)), exhibiting a considerable decrease with increasing final temperatures. The fast process (τ(1)～ 5.2-28.5 s) should be assigned to the formation of non-equilibrium precursor vesicles, and the slow process (τ(2)～ 188-694 s) should be ascribed to their further growth into final equilibrium vesicles via the fusion/fission of precursor vesicles. In contrast, the reverse vesicle-to-micelle transition process induced by a negative T-jump from elevated temperatures to 20 °C occurs quite fast and almost completes within the stopped-flow dead time (～2-3 ms).     

Encapsulation of curcumin in cationic micelles suppresses alkaline hydrolysis

The alkaline hydrolysis of curcumin was studied in three types of micelles composed of the cationic surfactants cetyl trimethylammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB) and the anionic surfactant sodium dodecyl sulfate (SDS). At pH 13, curcumin undergoes rapid degradation by alkaline hydrolysis in the SDS micellar solution. In contrast, alkaline hydrolysis of curcumin is greatly suppressed in the presence of either CTAB or DTAB micelles, with a yield of suppression close to 90%. The results from fluorescence spectroscopic studies reveal that while curcumin remains encapsulated in CTAB and DTAB micelles at pH 13, curcumin is dissociated from the SDS micelles to the aqueous phase at this pH. The absence of encapsulation and stabilization in the SDS micellar solution results in rapid hydrolysis of curcumin.     

Horseradish peroxidase activity in a reverse catanionic microemulsion

In this paper we present the first results of enzymatic activities in a reverse microemulsion medium based on a mixture of an anionic and a cationic surfactant, called catanionic microemulsion. The studied system is composed of sodium dodecyl sulfate (SDS)/dodecyltrimethylammonium bromide (DTAB)/n-hexanol/citrate buffer/n-dodecane, with high SDS/(SDS + DTAB) weight fractions. It turns out that the results are similar to those obtained in classical reverse microemulsions, except that the presence of DTAB exerts an inhibiting effect on the enzyme. Nevertheless, enzymatic superactivities are found even at a DTAB to total surfactant ratio of 15%, corresponding to 3% weight fraction of cationic surfactant in the microemulsion. The influence of pH and hexanol content on the enzymatic activities is also studied.     

Optimization of interactions between a cationic conjugated polymer and chromophore-labeled DNA for optical amplification of fluorescent sensors

Cationic conjugated polymers (CCPs) have been widely utilized as signal amplifiers in biosensors to improve the detection sensitivity through fluorescence resonance energy transfer (FRET) from CCPs to dye-labeled probes or targets. This paper investigates the effect of sodium dodecyl sulfate (SDS) on energy transfer between a cationic polyfluoreneethynylene copolymer (P1) and Texas Red labeled single-stranded DNA (ssDNA-TR). The presence of SDS in solution affects both the optical properties of P1 and TR emission within P1/ssDNA-TR complexes, which provides basic information on the role of SDS in FRET between P1 and ssDNA-TR. Although the quantum yield of P1 decreases in the presence of low concentrations of SDS, the presence of SDS reduces TR fluorescence quenching within P1/ssDNA-TR complexes and increases the number of optically active polymer repeat units within the proximity of TR, which are beneficial to P1-sensitized TR emission. In the absence of SDS, FRET from P1 to ssDNA-TR provides a 2.6-fold enhancement in TR emission intensity as compared to that upon direct excitation of TR at 595 nm. At the optimum SDS concentration (5 microM), P1-sensitized TR signal output increases to 11.3-fold relative to direct excitation of TR. This study highlights the importance of modulation of the CCP/ssDNA-dye interaction in improving the signal output of dye-labeled DNA by CCP through FRET.     

Fluorescence studies of interactions of ionic surfactants with poly(amidoamine) dendrimers

The pyrene fluorescence measurements have been carried out for the micelle formation of sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dimethylene bis(dodecyldimethylammonium bromide) (12-2-12) in the presence of fixed different amounts of various generations of poly(amidoamine) (PAMAM). The critical micelle concentration (cmc) of SDS decreases with an increase in the fixed amount of PAMAM, suggesting the facilitation of micellization due to the participation of SDS-PAMAM complex in the micelle formation. This behavior has not been observed for DTAB/12-2-12 in the presence of various generations of PAMAM. The results indicate that SDS always has stronger interactions with all the generations of PAMAM in comparison to those of DTAB and 12-2-12.     

Effect of class I and II organic modifiers on retention and selectivity in vesicle electrokinetic chromatography

Vesicles are large aggregates of surfactant monomers consisting of a spherical bilayer surrounding an internal cavity of solvent. The bilayer structure allows vesicles to be attractive models for the study of various transmembrane and binding processes. The use of thermodynamically stable vesicles (TSV) formed from oppositely charged surfactants for use as a pseudostationary phase in electrokinetic chromatography (EKC) was first accomplished using dodecyltrimethylammonium bromide and sodium dodecyl sulfate (DTAB/SDS). Surfactant vesicles have demonstrated enhanced separation characteristics compared to conventional micelles in EKC, although only investigated in aqueous media. Organic modifiers have been widely studied and used in EKC to enhance separation conditions. In this study, vesicles formed from cetyltrimethylammonium bromide and sodium octyl sulfate (CTAB/SOS) were investigated in the presence of "class I and II" organic modifiers. Electrophoretic and chromatographic parameters were examined as well as linear solvation energy relationship analysis (LSER) to characterize the effects of the modifiers on retention and selectivity in EKC. LSER analysis is a useful way to quantitatively investigate solute/solvent interactions responsible for retention and selectivity.     

A biologically friendly single step method for gold nanoparticle formation

There has been a keen interest for developing a biologically friendly approach for the preparation of gold nanoparticles for their application reasons. A biocompatible, quick and single step method is established for the preparation of gold nanoparticles in lecithin (Egg phosphatidylcholine)/water systems where lecithin itself acts as a reductant for hydrogen tetrachloro aurate (HAuCl(4)) to form the gold nanoparticles. Small gold nanoparticles (5-7 nm in diameter) were prepared in lamellar phases formed by lecithin within 6-7h of HAuCl(4) addition. Sonication of aqueous mixture of lecithin/HAuCl(4) reduces the time of reduction process to seconds when a sonicator with probe (100 W) is used. Most of the particles are found attached to lecithin structures and are comparatively large in size. Some 10nm particles are found attached to small lecithin vesicles (～100 nm) formed during sonication. The nanoparticles formed were stabilized by an anionic surfactant sodium dodecylsulfate (SDS) which proved to be a good stabilizer, the nanoparticles being stable up to six months. To the best of our knowledge, this is the first report where a biological surfactant lecithin itself has acted as a reductant and no other chemical reductants were required for the gold nanoparticle formation. Particles were characterized by Uv-vis spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). Lamellar phases were characterized by a polarizing microscope.     

Study of Fuchsin Acid Fading in Micellar Media

The rate constant of alkaline fading of fuchsin acid (FA^2?) was measured in the presence of nonionic (TX‐100), cationic (dodecltrimethylammonium bromide, DTAB), and anionic (sodium dodecyl sulfate, SDS) surfactants. FA^2? has three negatively charged substituents and one positive charge, and this makes the behavior of FA^2– different from dyes such as bromophenol blue. It was observed that the reaction rate constant decreased in the presence of TX‐100, DTAB, and SDS. Binding constants of FA^2? to TX‐100, DTAB, and SDS and the related thermodynamic parameters were calculated by the stoichiometric (classical) model. The results show that the binding of FA^2? to SDS is endothermic in both regions, and the binding of FA^2? to DTAB and TX‐100 is exothermic in one region and endothermic in another region of the used concentration range of these surfactants. Also, the binding constants of FA^2? to surfactant molecules of SDS/TX‐100 and DTAB/TX‐100 mixed micelles were obtained.