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.     

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.     

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.     

双烃链正,负离子表面活性剂复合物水溶液的表面化学性质研究

本文研究了具有双烃链的正、负离子表面活性剂混合水溶液的表面和液相性质、。负离子表面活性剂是琥珀酸二己酯磺酸钠[简写为(C6)2SNa],正离子表面活性剂是氯化二正辛基羟乙基甲基铵[(C8)2NCl]和氯化辛基羟乙基二甲基铵[C8NCl]。为了增加复合物的溶解度,在铵基上引入了羟乙基。测定了表面张力-浓度关系,用GIBBS公式计算表面吸附量和吸附分子面积。结果表明,由于正、负表面活性离子之间的强烈相互作用,所研究的两种混合物体系的表面活性远高于单独的表面活性剂。在等摩尔混合和离子强度0.1mol/kg情况下,(C6)2SNa-(C8)2NCl体系的吸附层组成是对称的(摩尔比为1:1),且在临界胶团浓度(cmc)以上析出新相,表明此cmc实质上是复合物的溶解度;而(C6)2SNa-C8NCl体系的吸附层为不对称组成(摩尔比非1:1),在cmc以上可能形成相当大的胶团,两种体系混合溶液的起泡性有极大差异。     

Self-assembly of mixed anionic and nonionic surfactants in aqueous solution

We present the phase diagram and the microstructure of the binary surfactant mixture of AOT and C(12)E(4) in D(2)O as characterized by surface tension and small angle neutron scattering. The micellar region is considerably extended in composition and concentration compared to that observed for the pure surfactant systems, and two types of aggregates are formed. Spherical micelles are present for AOT-rich composition, whereas cylindrical micelles with a mean length between 80 and 300 ? are present in the nonionic-rich region. The size of the micelles depends on both concentration and molar ratio of the surfactant mixtures. At higher concentration, a swollen lamellar phase is formed, where electrostatic repulsions dominate over the Helfrich interaction in the mixed bilayers. At intermediate concentrations, a mixed micellar/lamellar phase exists.     

Conductometric and fluorometric investigations on the mixed micellar systems of cationic surfactants in aqueous media

Micellar properties of binary mixtures of hexadecyldiethylethanolammonium bromide surfactant with tetradecyldimethylammonium, trimethylammonium, triphenylphosphonium, diethylethanolammonium, and pyridinium bromide surfactants have been characterized employing conductometric and fluorescence techniques. The critical micelle concentration (cmc*) and the degree of counter-ion binding values (delta) of the binary systems were determined from the conductivity measurements. The results were analyzed in light of various existing theories to calculate micellar composition, activity coefficients, and the interaction parameter (beta). Partial contribution of each surfactant, cmc1*, cmc2*, to the overall cmc* value was also evaluated. Aggregation numbers and micropolarity of the mixed micelles were determined from fluorescence measurements. The results were discussed in terms of synergetic interactions in these systems on the basis of the head group/head group and tail/tail interactions and the counter-ion binding.     

Phase behavior in mixtures of cationic and anionic surfactants in aqueous solutions

Phase behavior of cationic/anionic surfactant mixtures of the same chain length (n=10, 12 or 14) strongly depends on the molar ratio and actual concentration of the surfactants. Precipitation of catanionic surfactant and mixed micelles formation are observed over the concentration range investigated. Coacervate and liquid crystals are found to coexist in the transition region from crystalline catanionic surfactant to mixed micelles.The addition of oppositely charged surfactant diminishes the surface charge density at the mixed micelle/solution interface and enhances the apparent degree of counterion dissociation from mixed micelles. Cationic surfactants have a greater tendency to be incorporated in mixed micelles than anionic ones.     

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.     

芳香族小分子物质在表面活性剂溶液中的状态

阴、阳离子表面活性剂在水溶液中混合后很容易形成沉淀,因而以往在应用上将这两种表面活性剂视为配伍禁忌。但是最近几年,混合阴、阳离子表面活性剂水溶液理论性质的研究受到了一种程度的重视。研究发现,该混合体系具有与单纯的离子型表面活性剂或离子与非离子型表面活性剂混合物十分不同的性质,     

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.     

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.     

Interactions in mixed cationic surfactants and dextran sulfate aqueous solutions

The interactions between a hydrophilic anionic polysaccharide, dextran sulfate, and oppositely charged surfactants, n-alkylammonium chlorides (the number of carbon atoms per chain being 10, 12, and 14), were investigated by optical microscopy, X-ray diffraction, microelectrophoretic mobility, conductivity, surface tension, and light-scattering measurements at 303 K. The increase of surfactant alkyl chain length shifts both the critical aggregation (cac) and the critical micelle concentrations (cmc) toward lower surfactant concentration. Light-scattering and microelectrophoretic data revealed the coexistence of differently structured complexes beyond the cac. The presence of giant vesicles indicates that at least one type of species is ordered in bilayers. X-ray analysis of dry n-alkylammonium dextran sulfates exhibited mesomorphous ordering and interplanar spacings typical for lamellar structures; i.e., n-alkylammonium molecules form more or less disordered bilayers interconnected with dextran sulfate chains, thus forming multilamellar stacks. The average basic lamellar thickness increased linearly with the increase of surfactant chain length, whereas the average number of lamellar bilayers in the stack of lamellae decreases.     

Interaction between anionic and cationic gemini surfactants at air/water interface and in aqueous bulk solution

The mixture of the anionic O,O′-bis(sodium 2-lauricate)-p-benzenediol (C₁₁pPHCNa) and cationic (oligoona)alkanediyl-α, ω-bis(dimethyldodecylammonium bromide) (C₁₂-2-E_x-C₁₂·2Br) gemini surfactants has been investigated by surface tension and pyrene fluorescence. The results show that the surface tension γ drops faster with total surfactant concentration C_T for α₁ = 0.1 or 0.3 than for α₁ = 0.7 or 0.9, where α₁ is the mole fraction of C₁₁pPHCNa in the bulk solution on a surfactant-only basis. The fast drop in γ for α₁ < 0.5 indicates strong adsorption at the air/water interface owing to the interaction between oppositely charged components, resulting in the formation of the adsorption double layers in the subsurface. The slow descent in γ for α₁ > 0.5 is attributed to the pre-aggregation in the solution before the critical micelle concentration cmc. A possible mechanism is proposed.     

混合表面活性剂水溶液的浊点性质

正负离子表面活性剂混合体系在水溶液中很容易形成沉淀,这曾在很大程度上限制了该体系理论性质的研究及其应用。近年来的研究发现,该体系在吸附和胶团形成等方面存在很强的协同效应,对其相关性质的研究越来越多。与单一离子性表面活性剂性质不同,后者的溶解     

Interaction of cationic surfactant and anionic polyelectrolytes in mixed aqueous solutions

Surfactant–polymer interactions in aqueous solutions have been studied using dynamic surface tension, polyelectrolyte titration, nephelometric turbidity, and dynamic light scattering. For the preparation of complexes, a technical cationic surfactant was used in combination with two poly(maleic acid-co-polymers) of similar structure but different hydrophobicity. The dynamic surface tensions of mixed solutions as functions of surfactant concentration at constant polyelectrolyte content, as well as changes in the surface activity due to the influence of polyanion at constant surfactant concentration are discussed in terms of a complex or aggregate formation in the bulk phase. The interaction of the surfactant with poly(maleic acid-alt-propene) (P-MS-P) and poly(maleic acid-alt--methylstyrene) (P-MS-MeSty), respectively, is strong in both cases and results in the formation of nanoparticles with properties depending on the composition of the corresponding mixture.     

Rheology of wormlike micelles in aqueous systems of a mixed amino acid-based anionic surfactant and cationic surfactant

Amino acid-based anionic surfactant, N-dodecanoylglutamic acid, after neutralizing by 2, 2′, 2″-nitrilotriethanol forms micellar solution at 25 °C. Addition of cationic cosurfactants hexadecyltrimethylammonium chloride (CTAC), hexadecylpyridinium chloride (CPC), and hexadecylpyridinium bromide (CPB) to the semi-dilute solution of anionic surfactant micellar solutions favor the micellar growth and after a certain concentration, entangled rigid network of wormlike micelles are formed. Viscosity increases enormously ～4th order of magnitude compared with water. With further addition of the cosurfactants, viscosity declines and phase separation to liquid crystal occurs. The wormlike micelles showed a viscoelastic behavior and described by Maxwell model with a single stress-relaxation mode. The position of viscosity maximum in the zero-shear viscosity curve shifts towards lower concentration upon changing cosurfactant from CPB to CTAC via CPC; however, the maximum viscosity is highest in the CPB system showing the formation of highly rigid network structure of wormlike micelles. In all the systems, viscosity decays exponentially with temperature following Arrhenius type behavior.     

Determination of Etofenprox in environmental samples by HPLC after anionic surfactant micelle-mediated extraction (coacervation extraction)

A method is described for determination of residues of the insecticide Etofenprox in environmental samples. Anionic surfactant micelle-mediated extraction (coacervation extraction) was evaluated for isolation of Etofenprox before HPLC. The optimum conditions used for extraction included: 0.09 g sodium dodecanesulfonate (SDoS), 3.1 mL (3.3, for concentrations below 0.04 mg L⁻¹) 12 mol L⁻¹ HCl, 5 min vortex stirring, 5 min centrifugation at 4000 rpm, 2 h equilibration time. The limits of quantification (LOQ) and detection (LOD) were 0.01 and 0.004 mg L⁻¹, respectively, and recoveries obtained from five real samples ranged from 94.33±2.48 to 100.13±2.71%. The precision of the method was good; relative standard deviations (RSD) were less than 7%.      

Hydrophobic effect for anionic dye - cationic surfactant interaction in aqueous and mixed solvent

Abstract

The interaction between anionic dyes [Reactive Orange 122 (R.O 122), Reactive Blue 19 (R.B 19), Reactive Violet 5 (R.V 5) and Acid Green 20 (A.G 20)] with cationic surfactant cetyltrimethylammoniun bromide (CTAB) has been investigated by spectrophotometry and conductance technique. The used dyes are characterized by tautomeric behavior which affects the mechanism of the interaction. Various parameters such as dye structure, surfactant composition, solvent composition, temperature and pH of the medium were studied. The spectral data were applied for calculating the binding constant between dye and surfactant (K_b), fraction of micellization (?_mic), and standard free energy change of binding (ΔG°_b) in 0,10,20 and 30 v/v % acetonitile (AN). Conductance technique was constructed to estimate the ion pairing constant (K_a) at different temperatures and v/v % AN. Thermodynamic parameters (ΔG°, ΔH° and ΔS°) for ion pair formation were evaluated. The role of hydrophobic and electrostatic effect on dye-surfactant interaction was discussed.     

Aqueous surfactant two-phase systems in a mixture of cationic gemini and anionic surfactants

The phase behavior as well as the microstructures of the cationic gemini surfactant and anionic conventional surfactant aqueous two-phase system (ASTP) have been studied. The ASTP formation can be attributed to the coexistence of different kinds of aggregates in the upper and lower phases. The effects of temperature, shearing, surfactant concentration and mixing molar ratio on the phase separation of the ASTP-forming systems are systematically investigated. The ASTP can be destroyed by applying shear and increasing temperature. In this process, the lamellar structures (flat bilayers) in the ASTP are transformed into vesicles. Variation of surfactant structure also affects the phase behavior and the aggregates transformation. Appropriate molecular packing is crucial for the formation of ASTP.     

The aggregation of branched polyethylenimine and cationic surfactants in aqueous systems

The aggregation of binary mixtures of branched polyethylenimine with a molecular mass of 30 000 and cationic surfactants, such as cetyltrimethylammonium bromide, cetyldimethylethylammonium bromide, or cetyldimethyl(2-hydroxyethyl)ammonium bromide, in aqueous solutions is studied. The critical micellization concentrations, as well as the sizes and shapes of aggregates, are determined by conductometry, small-angle neutron scattering, and dynamic light scattering methods. Catalytic activity of cationic surfactant-polyethylenimine compositions in the hydrolysis of tetracoordinated phosphorus acid 4-nitrophenyl esters is studied via spectrophotometry. The influence of spatial characteristics of aggregates on the catalytic activity and selectivity of nanostructured polymeric systems is revealed.