Formation of Cubic-Phase Microemulsions with Anionic and Cationic Surfactants at Equal Amounts of Oil and Water

The formation and microstructure of cubic phases were investigated in anionic and cationic surfactant-containing systems at 25 degrees C. In the system sodium dodecyl sulfate(SDS)-dodecyltrimethylammonium bromide(DTAB)-water, mixing of two surfactants shows the phase transition hexagonal phase (H(1))-->surfactant precipitate, accompanied by an obvious decrease in the cross-sectional area per surfactant in the rod micelles of the hexagonal liquid crystal. In the mixed systems brine(A)-dodecane(B)-SDS(C)-DTAB(D)-hexanol(E), the isotropic discontinuous cubic phase is formed from the H(1) phase at a low cationic surfactant weight fraction, Y=D/(C+D), and from the lamellar phase at high Y upon dilution with equal amounts of oil and brine, respectively. The minimum surfactant concentration to form the cubic phase decreases with increases both in cationic surfactant weight fraction Y from 0 to 0.30 and in hexanol weight fraction, W(1)=E/(C+D+E), accordingly. The maximum solubilization for oil of the cubic phase reaches 43 wt% at 14 wt% of mixed surfactants and alcohol. Copyright 2000 Academic Press.     

正、负离子表面活性剂混合体系溶致液晶生成的相行为

研究了烷基(C8,C12,C14)三甲基溴化铵、烷基(C12,C14)溴化吡啶与烷基(C8,C12)硫酸钠混合体系溶致液晶形成的条件与结构的变化.在高浓度的水溶液中,随着正、负离子表面活性剂摩尔比接近于1,液晶结构由六角相过渡为层状相.表面活性剂非极性链长改变,对相行为影响显著,短碳链的正、负离子表面活性剂混合体系,在等摩尔比时,体系为层状液晶或立方液晶为主,夹杂少许沉淀.随碳链增长,两类表面活性剂间的静电吸引效果表现为生成沉淀的摩尔比例范围变宽,沉淀量增多,共存的液晶相减少,甚至消失.若只改变正离子的极性头基,季胺盐比吡啶盐与烷基硫酸盐的作用要强,形成不溶物的混合摩尔比例范围更宽.     

Elongated silica nanoparticles with a mesh phase mesopore structure by fluorosurfactant templating

Mesoporous silica materials with pore structures such as 2D hexagonal close packed, bicontinuous cubic, lamellar, sponge, wormhole-like, and rectangular have been made by using surfactant templating sol-gel processes. However, there are still some "intermediate" phases, in particular mesh phases, that are formed by surfactants but which have not been made into analogous silica pore structures. Here, we describe the one-step synthesis of mesoporous silica with a mesh phase pore structure. The cationic fluorinated surfactant 1,1,2,2-tetrahydroperfluorodecylpyridinium chloride (HFDePC) is used as the template. Like many fluorinated surfactants, HFDePC forms intermediate phases in water (including a mesh phase) over a wider range of compositions than do hydrocarbon surfactants. The materials produced by this technique are novel elongated particles in which the layers of the mesh phase are oriented orthogonal to the main axis of the particles.     

Use of the ternary phase diagram of a mixed cationic/glucopyranoside surfactant system to predict mesostructured silica synthesis

Mixed surfactant systems have the potential to impart controlled combinations of functionality and pore structure to mesoporous metal oxides. Here, we combine a functional glucopyranoside surfactant with a cationic surfactant that readily forms liquid crystalline mesophases. The phase diagram for the ternary system CTAB/H(2)O/n-octyl-beta-D-glucopyranoside (C(8)G(1)) at 50 degrees C is measured using polarized optical microscopy. At this temperature, the binary C(8)G(1)/H(2)O system forms disordered micellar solutions up to 72 wt% C(8)G(1), and there is no hexagonal phase. With the addition of CTAB, we identify a large area of hexagonal phase, as well as cubic, lamellar and solid surfactant phases. The ternary phase diagram is used to predict the synthesis of thick mesoporous silica films via a direct liquid crystal templating technique. By changing the relative concentration of mixed surfactants as well as inorganic precursor species, surfactant/silica mesostructured thick films can be synthesized with variable glucopyranoside content, and with 2D hexagonal, cubic and lamellar structures. The domains over which different mesophases are prepared correspond well with those of the ternary phase diagram if the hydrophilic inorganic species is assumed to act as an equivalent volume of water.     

阳离子与非离子混合表面活性剂模板合成介孔SiO2

利用各种两亲分子有序组合体构成超分子模板,合成从介观到宏观尺度不同形态的无机材料成为材料科学新崛起的研究方向[1].介孔SiO2在催化、吸附、分离介质及化学传感器等方面有广阔的应用前景.     

Phase behavior of concentrated aqueous solutions of cetyltrimethylammonium bromide (CTAB) and sodium hydroxy naphthoate (SHN)

We have characterized the phase behavior of mixtures of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the organic salt 3-sodium-2-hydroxy naphthoate (SHN) over a wide range of surfactant concentrations using polarizing optical microscopy and X-ray diffraction. A variety of liquid crystalline phases, such as hexagonal, lamellar with and without curvature defects, and nematic, are observed in these mixtures. At high temperatures the curvature defects in the lamellar phase are annealed gradually on decreasing the water content. However, at lower temperatures these two lamellar structures are separated by an intermediate phase, where the bilayer defects appear to order into a lattice. The ternary phase diagram shows a high degree of symmetry about the line corresponding to equimolar CTAB/SHN composition, as in the case of mixtures of cationic and anionic surfactants.     

Phase Behavior in Mixtures of Cationic Dimeric and Anionic Monomeric Surfactants

The formation of mixed aggregates has been investigated in the mixture of oppositely charged surfactants vastly differing in molecular geometry and size. The systems considered is mixture of the cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide), and anionic surfactant, sodium dodecyl sulfate. Various mixed nano- and microaggregates (micelles, vesicles, thin lamellar sheets, and tubules) were formed depending on bulk composition and total surfactant concentration. Two types of aggregates were found in precipitate, the tubules as prevailing aggregates on the gemini-rich side, and vesicles as prevailing aggregates on the SDS-rich side. The tubules formation was ascribed to mutual influence of specific structure of cationic dimeric surfactant and electrostatic interactions at the bilayer/solution interface. The proposed mechanism involved the formation of lamellar sheets, which rolled-up into tubules.     

混合超分子液晶模板法合成六方介孔相含钛氧化硅

采用混合十六烷基三甲基溴化铵（CTAB）与不同碳链的脂肪胺（CnNH2n＋3, n=8,10,12,14,16）作模板,在四甲基氢氧化胺为碱源的条件下,合成了具有六方介孔结构的含钛氧化硅Ti MCM 41分子筛材料. XRD和TEM测试表明所合成材料具有高度的长程有序结构,样品的N2吸附/脱附等温线表明,高度有序的Ti MCM 41材料展示了毛细凝聚的陡峭台阶和狭窄的介孔孔径分布.对反应物配比中Ti/Si比、脂肪胺碳链长度n对六方介孔相结构的影响进行了研究,实验发现当Ti/Si< 0.15和n< 16时,均可获得具有六方介孔结构的含钛氧化硅Ti MCM 41;而当Ti/Si≥0.15或n >16时,产物将分别发生从六方向无定形态或从六方向层状介孔相结构的转移,从混合表面活性剂的堆积参数对这种相转移现象进行了分析.     

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.     

Interaction Parameters of Anhydrous Cationic Surfactant Mixtures by Inverse Gas Chromatography

The miscibility of anhydrous cationic surfactant dodecylpyridinium chloride (DPC) and hexadecylpyridinium bromide (cetylpyridinium bromide (CPB)) mixtures has been studied by using them as stationary phases in Inverse Gas Chromatography (IGC). The temperature zone of work was determined by IGC and Differential Scanning Calorimetry (DSC) techniques. Values of the interaction parameter between the surfactants obtained at four different compositions and at four temperatures showed that the miscibility depends on the overall composition and suggested that the interactions are more favorable near the center of the composition range. Results are compared with other anhydrous cationic surfactant mixtures studied by IGC, the system didodecyldimethylammonium bromide (DDAB) and dioctadecyldimethylammonium bromide (DODAB), two twin-tailed surfactants, and are interpreted in terms of the structure of the anhydrous lamellar liquid crystals compared with that of aqueous lamellar mesophases.     

辛基三甲基溴化铵与辛基硫酸钠混合水溶液的相行为

对辛基三甲基溴化铵(OTAB)与辛基硫酸钠(SOS)正、负离子混合表面活性剂水溶液的相行为进行了研究.在高浓度的溶液中,混合表面活性剂形成液晶相,随着混合摩尔比OTAB/SOS接近于1,液晶结构由六角相转层状相,同时夹杂少量沉淀物;在中等浓度时,任意混合摩尔比例下皆为均相透明溶液;在低浓度下,在很宽的OTAB/SOS混合摩尔比的范围,出现双水相,其中的表面活性剂稀薄相,为不同大小的胶团与囊泡组成的稀溶液,另一表面活性剂富集相中则为数密度很大的囊泡聚集体,富集相对油溶性染料的增溶作用比非富集相高得多.     

Spontaneous formation of vesicles and dispersed cubic and hexagonal particles in amino acid-based catanionic surfactant systems

Mixed catanionic surfactant systems based on amino acids were investigated with respect to the formation of liquid crystal dispersions and the stability of the dispersions. The surfactants used were arginine-N-lauroyl amide dihydrochloride (ALA) and N(alpha)-lauroyl-arginine-methyl ester hydrochloride (LAM), which are arginine-based cationic surfactants; sodium hydrogenated tallow glutamate (HS), a glutamic-based anionic surfactant; and the anionic surfactants sodium octyl sulfate (SOS) and sodium cetyl sulfate (SCS). It is demonstrated that in certain ranges of composition there is a spontaneous formation of vesicular, cubic, and hexagonal structures. The solutions were characterized with respect to internal structure and size by cryogenic transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS), and turbidity measurements. Vesicles formed spontaneously and were found for all systems studied; their size distribution is presented for the systems ALA/SCS/W and ALA/SOS/W; they are all markedly polydisperse. The aging process for the system ALA/SOS/W was monitored both by turbidity and by cryo-TEM imaging; the size distribution profile for the system becomes narrower and the number average radius decreases with time. The presence of dispersed particles with internal cubic structure (cubosomes) and internal hexagonal structure (hexosomes) was documented for the systems containing ALA and HS. The particles formed spontaneously and remained stably dispersed in solution; no stabilizer was required. (Cubosome and hexosome are USPTO registered trademarks of Camurus AB, Sweden.) The spontaneous formation of particles and their stability, together with favorable biological responses, suggests a number of applications.     

The microstructure of di-alkyl chain cationic/nonionic surfactant mixtures: observation of coexisting lamellar and micellar phases and depletion induced phase separation

The evolution of the microstructure and composition occurring in the aqueous solutions of di-alkyl chain cationic/nonionic surfactant mixtures has been studied in detail using small angle neutron scattering, SANS. For all the systems studied we observe an evolution from a predominantly lamellar phase, for solutions rich in di-alkyl chain cationic surfactant, to mixed cationic/nonionic micelles, for solutions rich in the nonionic surfactant. At intermediate solution compositions there is a region of coexistence of lamellar and micellar phases, where the relative amounts change with solution composition. A number of different di-alkyl chain cationic surfactants, DHDAB, 2HT, DHTAC, DHTA methyl sulfate, and DISDA methyl sulfate, and nonionic surfactants, C12E12 and C12E23, are investigated. For these systems the differences in phase behavior is discussed, and for the mixture DHDAB/C12E12 a direct comparison with theoretical predictions of phase behavior is made. It is shown that the phase separation that can occur in these mixed systems is induced by a depletion force arising from the micellar component, and that the size and volume fraction of the micelles are critical factors.     

Spontaneously formed nonequilibrium vesicles of cetyltrimethylammonium bromide and sodium octyl sulfate in aqueous dispersions

It is well-known that vesicles form in mixtures of cationic and anionic surfactants. We have investigated mixtures of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) with the latter in excess over a long time, about 500 days. We have followed the growth of the aggregates by light scattering and checked the morphologies by cryogenic transmission electron microscopy (cryoTEM). All samples showed a monotonic growth with decreasing rate (the change of size was about linear on a logarithmic time scale). In series of samples with weight ratio 30:70 of CTAB/SOS and total surfactant concentration between 0.5 and 3 wt %, the size increased with the surfactant concentration up to 2 wt % and decreased thereafter; cryoTEM examination revealed that the samples contained a majority of open bilayer structures at the highest concentrations. Part of the sample at 2 wt % was diluted to 0.5 wt % after 60 days. The size measured after dilution was slightly smaller than before but well above that found in the directly prepared 0.5 wt % sample, and the particle size in the three samples continued to grow in parallel. Structures other than unilamellar vesicles were observed also in samples at 2 wt % total surfactant concentration at CTAB/SOS ratios close to the borders of the vesicle lobe in the (quasi) ternary phase diagram as published (Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski, J. A. J. Phys. Chem. 1996, 100, 5874). The results clearly show that the spontaneous vesicle populations do not represent equilibrium populations. They also suggest that the vesicle lobes in the phase diagram mainly represent areas where a lamellar phase is easily dispersed in the form of vesicles in an aqueous solution.     

Transfection mediated by gemini surfactants: engineered escape from the endosomal compartment

The structure of the lipoplex formed from DNA and the sugar-based cationic gemini surfactant 1, which exhibits excellent transfection efficiency, has been investigated in the pH range 8.8-3.0 utilizing small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-TEM). Uniquely, three well-defined morphologies of the lipoplex were observed upon gradual acidification: a lamellar phase, a condensed lamellar phase, and an inverted hexagonal (H(II)) columnar phase. Using molecular modeling, we link the observed lipoplex morphologies and physical behavior to specific structural features in the individual surfactant, illuminating key factors in future surfactant design, viz., a spacer of six methylene groups, the presence of two nitrogens that can be protonated in the physiological pH range, two unsaturated alkyl tails, and hydrophilic sugar headgroups. Assuming that the mechanism of transfection by synthetic cationic surfactants involves endocytosis, we contend that the efficacy of gemini surfactant 1 as a gene delivery vehicle can be explained by the unprecedented observation of a pH-induced formation of the inverted hexagonal phase of the lipoplex in the endosomal pH range. This change in morphology leads to destabilization of the endosome through fusion of the lipoplex with the endosomal wall, resulting in release of DNA into the cytoplasm.     

Effect of alkyl chain asymmetry on catanionic mixtures of hydrogenated and fluorinated surfactants

In this work we studied and compared the physicochemical properties of the catanionic mixtures cetyltrimethyl-ammonium bromide–sodium dodecanoate, cetyltrimethyl-ammonium bromide–sodium perfluorodacanoate, octyltrimethylammonium bromide–sodium perfluorodacanoate and cetyltrimethyl-ammonium bromide–sodium octanoate by a combination of rheological, transmission electron microscopy (TEM) and polarized optical microscopy measurements. The binary mixtures of the surfactants have been analyzed at different mixed ratios and total concentration of the mixture. Mixtures containing a perfluorinated surfactant are able to form lamellar liquid crystals and stable spontaneous vesicles. Meanwhile, system containing just hydrogenated surfactants form hexagonal phases or they are arranged in elongated aggregates.     

X-ray diffraction study of the structure of carboxymethylcellulose-cationic surfactant complexes.

We have investigated dilute aqueous solutions of an anionic polymer (carboxymethylcellulose) mixed with cationic surfactants of different chain lengths (dodecyl to octadecyl trimethylammonium bromides: DTAB, TTAB, CTAB and OTAB). The structures of the concentrated phases formed above the precipitation threshold were studied by X-ray diffraction. Different body-centred cubic structures with space groups Pm3n were observed in the presence of surfactant with a short aliphatic chain (DTAB), despite the fact that the polymer persistence length is comparable to the repeat distance of the structure (5 nm). For larger surfactant chain lengths (TTAB and CTAB), the structure of the precipitates can be either cubic (Pm3n) or 2D hexagonal depending on the initial surfactant and polymer concentrations. For still larger chain length (OTAB), the structure becomes lamellar. This structural evolution from micellar cubic towards 2D hexagonal and lamellar is attributed to the decrease of the local curvature of the surfactant aggregates, as observed for flexible synthetic polymers and short DNA fragments under similar conditions. Furthermore, the structure of the bulk complexes formed just below the precipitation threshold anticipates the structure seen in the precipitated phases.     

Interaction of cationic surfactants with carboxymethylcellulose in aqueous media

We have examined the polymer-surfactant interaction in mixed solutions of the cationic surfactants, i.e., dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, tetradecyltriphenylphosphonium bromide, and tetradecylpyridinium bromide and a semiflexible anionic polyelectrolyte carboxymethylcellulose in water and aqueous salt solutions by various techniques: tensiometry, viscosimetry or ion-selective electrode method, and dynamic light scattering. We have investigated the effect of varying surfactant chain length, head group size, counterion, and ionic strength on the critical aggregation concentration (CAC) of mixed polymer surfactant systems and the collapse of the polymer molecule under different solution conditions. The CAC decreases with increasing alkyl chain length. Above a certain surfactant concentration, mixed aggregates start growing until their macroscopic phase separation. The growth is more rapid with greater surfactant tail length and with increasing head group size. This is attributed in both cases to the increasing hydrophobic interaction between polymer and surfactant. Among surfactants with monovalent halide counterions, iodide induces the strongest binding, reflected by the onset of growth of the mixed aggregates at low surfactant concentration. This is perhaps related to the decreasing hydration of the counterion from chloride to iodide. The surfactant concentration at which the viscosity of the solution starts to decrease sharply is smaller than the CAC, and probably reflects polymer chain shrinkage due to noncooperative binding.     

阳离子和两性混合表面活性剂为模板合成三维六方介孔二氧化硅

用阳离子表面活性剂CnTAB(n=12,14,16,18)和两性生物表面活性剂SDG以8∶2的摩尔比混合作为模板剂,在酸性条件下晶化,碱性条件下老化合成了三维有序介孔二氧化硅。合成的产物用XRD、SEM、TEM和N2吸附进行表征。结果表明,在不同链长的表面活性剂CnTAB(n=12,14,16,18)中,C14TAB与SDG混合所得样品C14DG的有序度最好。而C16TAB和C18TAB与SDG混合所得样品的孔径约为9 nm。两性表面活性剂SDG对产物的形貌和三维六方结构都有影响。     

In situ GISAXS study of the formation of mesostructured phases within the pores of anodic alumina membranes

The formation and subsequent transformations of mesostructured silica within the confined tubular environment of anodic alumina membrane (AAM) channels [porous alumina membrane (PAM) channels] were investigated for the first time in situ with grazing incidence small-angle X-ray scattering (GISAXS) techniques, in combination with ex situ transmission electron microscopy (TEM) of the same samples. A better understanding of the mesostructure formation mechanism within the confined space of the AAM pores is a direct result of this study. Three different surfactants were used as the structure-directing agents in acid-catalyzed silica synthesis solutions. With ionic cetyltrimethylammonium bromide acting as the structure-directing agent, a columnar hexagonal structure with mesopores oriented parallel to the AAM channels was observed to form directly from the beginning of the synthesis. In samples synthesized with the nonionic surfactants Brij 56 and Pluronic P123, a circular hexagonal structure was found to form first; here, the mesopores are aligned around the circumference of the AAM channels. The circular structure subsequently transforms directly into a columnar hexagonal (P123 surfactant), or a mixture of columnar hexagonal and a new curved lamellar phase with lamellae oriented parallel to the walls of the AAM channels (Brij 56 surfactant). These transformations occur after complete solvent evaporation and therefore differ from a simple evaporation-induced phase formation. The existence of a previously postulated lamellar phase could be proven by GISAXS and TEM investigations.