Phase behavior, rheological property, and transmutation of vesicles in fluorocarbon and hydrocarbon surfactant mixtures

We present a detailed study of a salt-free cationic/anionic (catanionic) surfactant system where a strongly alkaline cationic surfactant (tetradecyltrimethylammonium hydroxide, TTAOH) was mixed with a single-chain fluorocarbon acid (nonadecafluorodecanoic acid, NFDA) and a hyperbranched hydrocarbon acid [di-(2-ethylhexyl)phosphoric acid, DEHPA] in water. Typically the concentration of TTAOH is fixed while the total concentration and mixing molar ratio of NFDA and DEHPA is varied. In the absence of DEHPA and at a TTAOH concentration of 80 mmol·L(-1), an isotropic L(1) phase, an L(1)/L(α) two-phase region, and a single L(α) phase were observed successively with increasing mixing molar ratio of NFDA to TTAOH (n(NFDA)/n(TTAOH)). In the NFDA-rich region (n(NFDA)/n(TTAOH) > 1), a small amount of excess NFDA can be solubilized into the L(α) phase while a large excess of NFDA eventually leads to phase separation. When NFDA is replaced gradually by DEHPA, the mixed system of TTAOH/NFDA/DEHPA/H(2)O follows the same phase sequence as that of the TTAOH/NFDA/H(2)O system and the phase boundaries remain almost unchanged. However, the viscoelasticity of the samples in the single L(α) phase region becomes higher at the same total surfactant concentration as characterized by rheological measurements. Cryo-transmission electron microscopic (cryo-TEM) observations revealed a microstructural evolution from unilamellar vesicles to multilamellar ones and finally to gaint onions. The size of the vesicle and number of lamella can be controlled by adjusting the molar ratio of NFDA to DEHPA. The dynamic properties of the vesicular solutions have also been investigated. It is found that the yield stress and the storage modulus are time-dependent after a static mixing process between the two different types of vesicle solutions, indicating the occurrence of a dynamic fusion between the two types of vesicles. The microenvironmental changes induced by aggregate transitions were probed by (19)F NMR as well as (31)P NMR measurements. Upon replacement of NFDA by DEHPA, the signal from the (19)F atoms adjacent to the hydrophilic headgroup disappears and that from the (19)F atoms on the main chain becomes sharper. This could be interpreted as an increase of microfluidity in the mixed vesicle bilayers at higher content of DEHPA, whose alkyl chains are expected to have a lower chain melting point. Our results provide basic knowledge on vesicle formation and their structural evolution in salt-free catanionic surfactant systems containing mixed ion pairs, which may contribute to a deeper understanding of the rules governing the formation and properties of surfactant self-assembly.     

Phase behavior and properties of reverse vesicles in salt-free catanionic surfactant mixtures

Salt-free 1:1 cationic/anionic (catanionic) surfactant mixture tetradecyltrimethylammonium laurate (TTAL) could be prepared by mixing equimolar tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA) in water. Given the condition of suitable range of weight fraction of TTAL in total surfactant, rho=WTTAL/(WTTAL+WLA), and at existence of a small amount of water, it was found that the mixtures of so-obtained TTAL and LA could spontaneously form stable reverse vesicles in various organic solvents including toluene, tert-butylbenzene, and cyclohexane. The reverse vesicle phase shows a blue color against room light and exhibits strong birefringence under polarized microscope. The reverse vesicles are very sensitive to temperature change. Increasing temperature could make the rho values within which reverse vesicles were constructed move to higher values. In organic solvents of alkanes such as n-heptane, reverse vesicles could still form but become unstable upon time and centrifugation. Increasing temperature could accelerate phase separation, and finally a gel-like bottom phase was usually observed. Interestingly, the stable reverse vesicles formed by so-called salt-free catanionic surfactant mixtures still show some resistance against adding inorganic salts. They can trap inorganic ions such as Zn2+ and S2- into their hydrophilic layers. This opens the door for template applications of reverse vesicles to prepare inorganic nanoparticles.     

A salt-free zero-charged aqueous onion-phase enhances the solubility of fullerene C60 in water

An onion-phase (multilamellar vesicular phase or Lalpha-phase) was prepared from salt-free zero-charged cationic and anionic (catanionic) surfactant mixtures of tetradecyltrimethylammonium hydroxide (TTAOH)/lauric acid (LA)/H2O. The H+ and OH- counterions form water (TTAOH + LA --> TTAL + H2O), leaving the solution salt free. The onion-phase solution has novel properties including low conductivity, low osmotic pressure and unscreened electrostatic repulsions between cationic and anionic surfactants because of the absence of salt. The spherical multilamellar vesicles have an average 250 nm radius as measured by freeze-fracture transmission electron microscopy (FF-TEM) and the maximum interlayer distance, i.e., the thickness of the hydrophobic bilayer and the water layer, was calculated to be around 52 nm by small-angle X-ray scattering (SAXS). Extremely hydrophobic C60 fullerene can be solubilized in this salt-free zero-charged aqueous onion-phase. As a typical result, 0.588 mg.mL(-1) (approximately 0.82 mmol.L(-1)) C60 has been successfully solubilized into a 50 mmol.L(-1) catanionic surfactant onion-phase aqueous solution. The weight ratio of fullerene to TTAL is calculated to be around 1:40. Solubilization of C60 in the salt-free catanionic onion-phase solution was investigated by using different sample preparation routes, and a variety of techniques were used to characterize these vesicular systems with or without encapsulated C60. The onion-phase solution changed color from slightly bluish to yellow or brown after C60 was solubilized. 1H and 13C NMR measurements indicated that the C60 molecules are located in the hydrophobic layers, i.e., in the central positions [omega-CH3 and delta-(CH2)x] of the hydrophobic layers of the TTAL onion-phase. Salt-free zero-charged catanionic vesicular aqueous solutions are good candidates for enhancing the solubility of C60 in aqueous solutions and may broaden the functionality of fullerenes to new potential applications in biology, medicine, and materials. Hopefully, our method can also be extended to solubilize functionalized carbon nanotubes in aqueous solutions.     

Phase behavior of salt-free catanionic surfactant aqueous solutions with fullerene C60 solubilized

A salt-free catanionic surfactant system, tetradecyltrimethylammonium laurate (TTAL), was constructed by mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA). The H+ and OH- counterions form water (TTAOH+LA-->TTAL+H2O), leaving the solution salt-free. The phase behaviors at fixing the total surfactant concentration (cTTAL) to be 33.0 and 55.0 mmol L(-1), respectively, were studied through varying the molar ratio of r=nLA/nTTAOH from 0.70 to 1.20. With an increasing value of r, one observed an L1-region, an Lalpha/L1 two-phase region with a birefringent Lalpha-phase at the top, and finally a single Lalpha-phase. The ability to solubilize a fullerene mixture of C60 and C70 of different phases in different regions was tested. The colloidal stability and phase behavior of different phases with embedded fullerenes were investigated as a function of r, cTTAL, and weight ratio of fullerene to surfactant (WF/WTTAL). The 33.0 or 55.0 mmol L(-1) zero-charged vesicle-phase at r=1.00 could solubilize a considerable amount of fullerenes without macroscopic phase separation and obvious vesicular structure breakage. However, these colloidal solutions became unstable at lower concentrations of surfactants, and a precipitate would be observed at the bottom. The micellar (L1-phase) solubilization at the TTAOH-rich side was less pronounced compared to the vesicular solubilization of the zero-charged vesicle-phase, and the solubilizing ability decreased at higher r values. In the Lalpha/L1 two-phase region, a brown or dark-brown Lalpha-phase was usually found at the top of a colorless or yellowish L1-phase, indicating that most of the fullerenes were embedded in the upper Lalpha-phase. The influence of fullerene incorporation on the property of the zero-charged TTAL vesicle-phase was also investigated, and evidence has been found that the system tended to be more fluid after fullerenes were incorporated into the hydrophobic microdomains of aggregates.     

Phase behavior and rheological properties of salt-free catanionic TTAOH/DA/H2O system in the presence of hydrophilic and hydrophobic salts

In the cationic and anionic (catanionic) surfactant mixed system, tetradecyltrimethylammonium hydroxide (TTAOH)/decanoic acid (DA)/H(2)O, abundant phase behaviors were obtained in the presence of hydrophilic and hydrophobic salts. The microstructures of typical L(α) phases with the different compositions were characterized by the transmission electron microscope (TEM) images. Aqueous double-phase transition induced by addition of hydrophilic salts was observed when the cationic surfactant was in excess. Salt-induced reversible vesicle phases could be obtained when the anionic surfactant was excess, whereas the vesicle phase at lower salinity behaves highly viscoelastic but is much less viscoelastic with high salinity which was demonstrated by measuring their rheological properties. The L(α) phase with the positive membrane charges can be finally transferred into an L(1) phase with added salts. The ion specificity of hydrophilic and hydrophobic salts is discussed, and the order of cations is summarized, which is significant for the further study of the Hofmeister effects on catanionic surfactant mixed systems.     

Aggregation behavior and adsorption properties of salt-free catanionic surfactant mixtures containing tetradecyltrimethyl ammonium salts

The aggregation behavior of salt-free catanionic surfactants, tetradecyltrimethyl ammonium hydroxide (TTAOH)/fatty acid (FA) including octanoic acid (OA), decylic acid (DA) and lauric acid (LA) in aqueous solutions were studied. The critical micelle concentration(cmc), surface tension at cmc (γ_cmc), surface excess (Г_max), mean molecular surface area (A_min), adsorption efficiency (pc₂₀) and surface tension reduction effectiveness (π_cmc) were obtained from surface tension isotherms. The influence of temperature on the surface tension of salt-free TTAOH/FA (TTAOF) systems was investigated. Data of adsorption dynamics indicated that at fixed adsorption time, the order of adsorption capacity was TTAOH?相似文献   

Two routes to vesicle formation: metal-ligand complexation and ionic interactions

Two routes to vesicle formation were designed to prepare uni- and multilamellar vesicles in salt-free aqueous solutions of surfactants. The formation of a surfactant complex between a double-chain anionic surfactant with a divalent-metal ion as the counterion and a single-chain zwitterionic surfactant with the polar group of amine-oxide group is described for the first time as a powerful driving force for vesicle-phases constructed from salt-free mixtures of aqueous surfactant solutions. As a typical example, a Zn(2+)-induced charged complex fluid, vesicle-phase has been studied in aqueous mixtures of tetradecyldimethylamine oxide (C(14)DMAO) and zinc 2,2-dihydroperfluorooctanoate [Zn(OOCCH(2)C(6)F(13))(2)]. This ionically charged vesicle-phase formed due to surfactant complexation has interesting rheological properties and is not shielded by excess salts because there are no counterions in the solution. Such a vesicle-phase of surfactant complex is important for many applications; for example, the vesicle-phase was further used to produce in situ the vesicle-phase of the salt-free cationic/anionic (catanionic) surfactants, C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13). The salt-free catanionic vesicle-phase could be produced through injecting H(2)S gas into the C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) vesicle-phase, because the zwitterionic surfactant C(14)DMAO can be charged by the H(+) released from H(2)S to become a cationic surfactant and Zn(2+) was precipitated as ZnS. After the ZnS precipitates were removed from C(14)DMAO/Zn(OOCCH(2)C(6)F(13))(2) solutions, the final mixed solution does not contain excess salts as do other cationic/anionic surfactant systems. Both the C(14)DMAO-Zn(OOCCH(2)C(6)F(13))(2) complex and the resulting catanionic C(14)DMAOH(+)-(-)OOCCH(2)C(6)F(13) solution are birefringent Lalpha-phase solutions that consist of uni- and multilamellar vesicles. Ring-shaped semiconductor ZnS materials with encapsulated ZnS precipitates and regular spherical ZnS particles were prepared, which resulted in a transition from vesicles composed of metal-ligand complexes to vesicles held together by ionic interactions in the salt-free aqueous systems. This strategy should provide a new method to prepare inorganic materials. The present routes to form vesicles solve a problem: how to prepare nanomaterials using surfactant self-assembly, with structure controlled not by the growing material, but by the phase behavior of the surfactants.     

Self-assembled aggregates originated from the balance of hydrogen-bonding, electrostatic, and hydrophobic interactions

Rich phase behavior was observed in salt-free cationic and anionic (catanionic) mixtures of a double-tailed surfactant, di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA), and tetradecyldimethylamine oxide (C(14)DMAO) in water. At a fixed C(14)DMAO concentration, phase transition from L(1) phase to L(α) phase occurs with increasing amounts of DEHPA. Moreover, in the L(α) phase, with the increase in DEHPA concentration, a gradual transition process from vesicle phase (L(αv)) to stacked lamellar phase (L(αl)) was determined by cryo- and FF-TEM observations combining with (2)H NMR measurements. The rheological data show that the viscosity increases with DEHPA amounts for L(αv) phase samples because of the increase in vesicle density. At a certain molar ratio of DEHPA to C(14)DMAO, i.e., 80:250, the samples are with the highest viscoelasticity, indicating the existence of densely packed vesicles. While for L(αl) phase samples, with increasing DEHPA amount, a decrease of bilayer curvature was induced, leading to a decrease of viscosity obviously. Compared with general catanionic surfactant mxitures, in addition to the electrostatic interaction of ion pairs, the transition of the microstructures is also ascribed to the formation of the hydrogen bonding (-N(+)-O-H···O-N-) between C(14)DMAO molecules and protonated C(14)DMAOH(+), which induces the growth of aggregates and the decrease of aggregate curvatures.     

DNA encapsulation by biocompatible catanionic vesicles

The encapsulation of DNA by catanionic vesicles has been investigated; the vesicles are composed of one cationic surfactant, in excess, and one anionic. Since cationic systems are often toxic, we introduced a novel divalent cationic amino-acid-based amphiphile, which may enhance transfection and appears to be nontoxic, in our catanionic vesicle mixtures. The cationic amphiphile is arginine-N-lauroyl amide dihydrochloride (ALA), while the anionic one is sodium cetylsulfate (SCS). Vesicles formed spontaneously in aqueous mixtures of the two surfactants and were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM); the vesicles are markedly polydisperse. The results are compared with a study of an analogous system based on a short-chained anionic surfactant, sodium octylsulfate (SOS). Addition of DNA to catanionic vesicles resulted in associative phase separation at very low DNA concentrations; there is a separation into a precipitate and a supernatant solution; the latter is first bluish but becomes clearer as more DNA is added. From studies using cryo-TEM and small angle X-ray scattering (SAXS) it is demonstrated that there is a lamellar structure with DNA arranged between the amphiphile bilayers. Comparing the SOS containing DNA-vesicle complexes with the SCS ones, an increase in the repeat distance is perceived for SCS. Regarding the phase-separating DNA-amphiphile particles, cryo-TEM demonstrates a large and nonmonotonic variation of particle size as the DNA-amphiphile ratio is varied, with the largest particles obtained in the vicinity of overall charge neutrality. No major differences in phase behavior were noticed for the systems here presented as compared with those based on classical cationic surfactants. However, the prospect of using these systems in real biological applications offers a great advantage.     

Electric conductivities of 1:1 electrolytes in liquid methanol along the liquid-vapor coexistence curve up to the critical temperature. II. KBr and KI solutions

The molar conductivities Lambda of KBr and KI in dilute methanol solutions were measured along the liquid-vapor coexistence curve up to the critical temperature (240 degrees C). The concentration dependence of Lambda in each condition was analyzed by the Fuoss-Chen-Justice equation to obtain the limiting molar conductivities and the molar association constants. Using the present data together with the literature ones, the validity of the Hubbard-Onsager (HO) dielectric friction theory based on the sphere-in-continuum model was examined for the translational friction coefficients zeta of the halide ions (the Cl(-), Br(-), and I(-) ions) in methanol in the density range of 2.989rho(c)> or =rho> or =1.506rho(c), where rho(c)=0.2756 g cm(-3) is the critical density of methanol. For all the halide ions studied, the friction coefficient decreased with decreasing density at rho>2.0rho(c), while the nonviscous contribution Deltazeta/zeta increased; Deltazeta was defined as the difference between zeta and the friction coefficient estimated by the Stokes law. The density dependence of zeta and Deltazeta/zeta were well reproduced by the HO theory at rho>2.0rho(c). The HO theory also explained the ion-size dependence of Deltazeta/zeta which decreased with ion-size at rho>2.0rho(c). At rho<2.0rho(c), on the other hand, the HO theory could not explain the density and the ion-size dependences of zeta and Deltazeta/zeta. These results indicated that the application limit of the HO theory lied about rho=2.0rho(c) which is the same as the application limit observed for the alkali metal ions. The present results were also compared with the results in subcritical aqueous solutions.     

Densely stacked multilamellar and oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles of a salt-free catanionic extractant and surfactant system

In the phase diagram of an excellent extractant of rare earth metal ions, di(2-ethylhexyl) phosphate (HDEHP, commercial name P204), mixing with a cationic trimethyltetradecylammonium hydroxide (TTAOH) in water, a birefringent Lalpha phase was found, which consists of densely stacked multilamellar vesicles. The densely stacked multilamellar vesicles are remarkably deformed, as observed by means of cryotransmission electron microscopy (cryo-TEM). Further, self-assembled structures-oligovesicular vesicles, bilayer cylinders, and tubes joining with vesicles-were also observed. The self-assembled phase is transparent, anisotropic, and highly viscous, possessing elastic properties determined by rheological measurements. This is the first time that birefringent Lalpha phase with remarkably deformed amphiphilic bilayer membranes has been constructed through combining a hydrophobic organic extractant having double chains with a water-soluble surfactant having a single chain, which may direct primarily toward acquiring an understanding of the mechanism of salt-free catanionic vesicles and secondarily to determine if vesicle-extraction technology utilizing extractants is possible.     

Investigations into the bending constant and edge energy of bilayers of salt-free catanionic vesicles

Using molecular dynamics simulation, we performed theoretical calculations on the curvature constant and edge energy of bilayers of salt-free, zero-charged, cationic and anionic (catanionic) surfactant vesicles composed of alkylammonium cations (C(m)(+)) and fatty acid anions (C(n)(-)). Both the minimum size and edge energy of vesicles were calculated to examine the relation between the length of the surfactant molecules and the mechanical properties of the catanionic bilayers. Our simulation results clearly demonstrate that, when the chain lengths of the cationic and anionic surfactants are equal, both the edge energy and the rigidity of the catanionic bilayers increase dramatically, changing from around 0.36 to 2.77 kBT·nm(-1) and around 0.86 to 6.51 kBT·nm(-1), respectively. For the smallest catanionic vesicles, the curvature is not uniform and the surfactant molecules adopt a multicurvature arrangement in the vesicle bilayers. We suspect that the multicurvature bending of bilayers of catanionic vesicles is a common phenomenon in rigid bilayer systems, which could aid understanding of ion transport through bilayer membranes.     

Molecular dynamics investigation of a density-driven glass transition in a liquid crystal system

Molecular dynamics simulations are carried out to address the density-driven glass transition in a system of rodlike particles that interact with the Gay-Berne potential. Since crystallization occurs in this system on the time scale of the simulations, direct simulation of the glass transition is not possible. Instead, glasses with isotropic orientational order are heated to a temperature T, and the relaxation times by which nematic orientational order develops are determined. These relaxation times appear to diverge at a critical density rho(c); i.e., the system can equilibrate at rhorho(c) (at the temperature T). The relaxation times follow a power-law scaling as the critical density is approached, suggesting that this density-driven glass transition concurs with mode coupling theory.     

LA/C14DMAO/H2O体系多结构自组装体及其模板金纳米材料制备与性能

月桂酸(LA)与十四烷基二甲基氧化胺(C₁₄DMAO)形成的无盐阴/阳离子表面活性剂混合体系表现出丰富的相行为。运用冷冻蚀刻透射电子显微镜(FF-TEM)和偏光显微镜(POM)、差示扫描量热(DSC)、流变和[2]H NMR测定对体系相行为和微观结构进行了研究,发现水溶液中可自聚集形成胶束(L₁)、层状(L_αl)、囊泡(L_αν)和凝胶相。以胶束相和层状相为软模板制备了金纳米材料,运用透射电子显微镜(TEM)和能谱仪(EDS)表征了金纳米材料。与用传统阳离子表面活性剂溶液制备金纳米材料相比,该体系由于具有自身还原性而不需要加入还原剂NaBH₄。实验证明：还原过程不会破坏模板溶液原有微观结构,且可通过调控聚集体结构实现控制制备金纳米材料形貌的目的。HK-2细胞的噻唑蓝(MTT)比色法实验进一步证明,本体系制备的球形金纳米材料作为基因载体具有高效和低毒的特点,在基因治疗中具有潜在的实际应用价值,可为寻求安全可靠的基因治疗途径提供实验数据和理论参考。     

Vesicles of a new salt-free cat-anionic fluoro/hydrocarbon surfactant system

Weakly basic tetradecyldimethylaminoxide (C14DMAO) molecules can be protonated to form a cationic surfactant, C14DMAOH+, by an acidic fluorocarbon surfactant, an 8-2-fluorotelomer unsaturated acid (C7F15CF==CHCOOH), to form a salt-free cationic and anionic (cat-anionic) fluoro/hydrocarbon surfactant system in aqueous solution. The high Krafft point of C7F15CF==CHCOOH was largely reduced as a result of being mixed with a C14DMAO micelle solution. A study of the phase behavior of the new salt-free cat-anionic fluoro/hydrocarbon surfactant system clearly indicates the existence of a birefringent Lalpha-phase region at (25.0+/-0.1) degrees C. The birefringent Lalpha phase consists of vesicles, which include uni- and multilamellar vesicles with one to dozens of shells, and oligovesicular vesicles, as demonstrated by freeze-fracture and cryo-transmission electron microscopy (FF- and cryo-TEM) images. The size distribution and structural transitions in the salt-free cat-anionic fluoro/hydrocarbon surfactant system were studied by dynamic light scattering (DLS) and 1H and 19F NMR spectroscopy. The formation of a salt-free cat-anionic vesicle phase could be induced by the strong electrostatic interaction between the cationic hydrocarbon C14DMAOH+ and the anionic fluorocarbon C7F15CF==CHCOO-, which provided evidence that the electrostatic interaction between the cationic and anionic surfactants is larger than the nonsynergistic interaction between the stiff fluorocarbon and the soft hydrocarbon chains of the surfactants.     

正负离子表面活性剂在短链脂肪醇/水中沉淀-囊泡转化和双水相的形成

首次报道在短链脂肪醇/水溶剂中十二烷基硫酸钠和辛基三甲基溴化铵混合体系由沉淀转化为囊泡，并出现表面活性剂双水相的新现象，以期探索正负离子表面活性剂混合体系研究的新途径。     

Influence of ionic charges on the bilayers of lamellar phases

The influence of ionic charges on the mesophases in the ternary system of C(12-16)E(6) (LA 070), ethylhexylglycerid (EHG), and water was studied. The charge was introduced by adding the ionic surfactant SDS (sodium dodecyl sulfate). The single lamellar phase (5 wt % LA 070 and 240 mM EHG in water) yields a bluish homogeneous solution. With the addition of SDS, the samples become more and more clear. Rheology measurements indicate that increased charge density increases the storage modulus G', and the lamellar phases show typical behavior of a viscoelastic fluid with a yield stress at higher SDS concentration. SAXS measurements show that the interlamellar distance D decreases with SDS concentration. The addition of ionic surfactants suppresses the Helfrich undulations, flattens the bilayers, and decreases interbilayer spacing due to electrostatic repulsions of the ionic surfactant head groups. Furthermore, the L(alpha) phase transforms into vesicle phases as the SDS concentration is increased. Second, it is shown that with added NaCl electrolyte the phase with charged surfactant behaves again in the same way as the initial uncharged system. The addition of salt screens the electrostatic interaction, which leads to a higher flexibility of the bilayers and a decrease of the storage modulus G'. Theoretical calculations show that the shear moduli of the L(alpha) phases are much smaller than the osmotic pressure of the systems. Several models are proposed for the explanation of the shear moduli. The model due to Lekkerkerker for the electric contribution of the bending constant of the bilayer seems to yield good results for the transition to vesicles.     

Charge-induced unilamellar vesicle formation and phase separation in solutions of Di-n-decylmethylamine oxide

A double-tail amine oxide surfactant, di-n-decylmethylamine oxide (2C10MAO), was prepared, and the effects of protonation on aggregate structure were examined by small-angle neutron scattering (SANS), cryo-transmission electron microscopy (cryo-TEM), turbidity, electric conductivity, and solubilization of an oil-soluble dye at various degrees of neutralization, X, defined as the mole ratio of HCl/2C10MAO. The surfactant makes an L(2) phase in the nonprotonated state (X = 0) in water. The L(2) phase is in equilibrium with an aqueous L(1) phase. On protonation, unilamellar vesicles (ULVs) are formed over a wide range of compositions (0.05 < X< 0.4-0.5 at C = 10 mM) as observed by cryo-TEM. At X = 0.2, the ULV is stable over a wide concentration range (3 mM < or = C < 0.1 M), but an L(alpha) phase replaces the vesicle phase at C > 0.1 M. SANS results show that the mean radius of the ULV is about 25 nm and the bilayer thickness is about 2 nm, consistent with the extended configuration of the alkyl chains of the surfactant. An important contribution to the enhanced stability of the bilayer structures over the L(2) phase is suggested to be the translational entropy of the counterions. The enhanced stability of the bilayers diminishes as the counterion concentration increases either by an increase of X or by the addition of a salt. When the counterion concentration exceeds a critical value, the ULV solutions transform into the L(2) phase (or L(2)/L(1) two-phase system at low surfactant concentrations). The critical composition X is about 0.4-0.5 in water, but it is below 0.4 in D(2)O. The critical NaCl concentration is below 5 mM at X = 0.2. The stability of ULVs against multilamellar vesicles is ascribed partly to undulation forces and partly to the adjustable nature of the spontaneous curvature of amine oxide monolayers. The characteristics of the ULV of the surfactant remain the same within a temperature range 25-50 degrees C at X = 0.2. An iridescent lamellar phase and possibly an L(3) phase were observed in a very narrow X range (0 < X < 0.02) prior to the vesicle phase.     

Temperature-controlled vesicle aggregation in the mixed system of sodium n-dodecyl sulfate/n-dodecyltributylammonium bromide

Temperature-controlled vesicle aggregation was investigated in a catanionic surfactant system of sodium n-dodecyl sulfate/n-dodecyltributylammonium bromide. Vesicle aggregation took place as the temperature reached the critical value (Tc). Tc can be adjusted by the variations of the total surfactant concentration and the mixed molar ratio. It was also found that the temperature variation above Tc can greatly influence the vesicle aggregation rate. The vesicle aggregation process was irreversible as long as T >/= Tc, whereas the vesicle disaggregation process occurred only below Tc.     

Zero spontaneous curvature and its effects on lamellar phase morphology and vesicle size distributions

Equimolar mixtures of dodecyltrimethylammonium chloride (DTAC) and sodium octyl sulfonate (SOSo) show a vesicle phase at >99 wt % water and a single, fluid lamellar phase for water fractions below 80 wt %. This combination is consistent with the bilayer bending elasticity kappa approximately k(B)T and zero bilayer spontaneous curvature. Caillé line shape analysis of the small-angle X-ray scattering from the lamellar phase shows that the effective kappa depends on the lamellar d spacing consistent with a logarithmic renormalization of kappa, with kappa(o) = (0.8 +/- 0.1)k(B)T. The vesicle size distribution determined by cryogenic transmission electron microscopy is well fit by models with zero spontaneous curvature to give (kappa + (kappa/2)) = (1.7 +/- 0.1)k(B)T, resulting in kappa = (1.8 +/- 0.2)k(B)T. The positive value of kappa and the lack of spontaneous curvature act to eliminate the spherulite defects found in the lamellar gel phases found in other catanionic mixtures. Current theories of spontaneous bilayer curvature require an excess of one or more components on opposite sides of the bilayer; the absence of such an excess at equimolar surfactant ratios explains the zero spontaneous curvature.