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.     

Synthesis and characterization of ordered hexagonal and cubic mesoporous tin oxides via mixed-surfactant templates route

Ordered hexagonal and cubic mesoporous tin oxides were synthesized for the first time in the presence of mixed cationic and neutral surfactants (a mixture of cetyltrimethylammonium bromide cationic surfactant and dodecylamine neutral surfactant) with different alkali and simple inorganic precursors at room temperature. In the synthesis systems, the dodecylamine neutral surfactant may function as a polar organic cosolvent and cosurfactant. The formation of the tin oxide mesostructured material was proposed to be due to the presence of hydrogen-bonding interactions between the supramolecular template and inorganic precursors Sn4+ and OH-, which were assumed to self-assemble around the cationic surfactant molecules. The materials are characterized by X-ray powder diffraction, transmission electron microscopy, thermogravimetric analysis, and N2 adsorption/desorption isotherm. The surface areas of materials evaluated from the N2 sorption isotherms are about 248 m(2)/g for hexagonal mesoporous tin oxide (SnH) and 281 m(2)/g for cubic mesoporous tin oxide (Sn-C) for calcination at 350 degrees C.     

Influence of the phase behavior on the properties of ionic nanoemulsions prepared by the phase inversion composition method

The low-energy emulsification method phase inversion composition (PIC) was used to prepare O/W nanoemulsions in the W/oleylammonium chloride-oleylamine-C12E10/hexadecane ionic system, where the oleylammonium acted as a cationic surfactant. The results obtained, in terms of phase diagrams and emulsion characteristics, were compared with those obtained in the system W/potassium oleate-oleic acid-C12E10/hexadecane [I. Solè, A. Maestro, C. González, C. Solans, J.M. Gutiérrez, Langmuir 22 (2006) 8326], in which the oleate acted as an anionic surfactant. This study was done in order to extend the application range of the ionic nanoemulsions, not only in anionic systems but also in cationic ones, and in order to deep further into the nanoemulsion formation mechanism. The results show again that to obtain small droplet-sized nanoemulsions it is necessary to cross a direct cubic liquid crystal phase along the emulsification path, and it is also crucial to remain in this phase enough time and to use a proper mixing rate to incorporate all the oil into the liquid crystal. Then, when nanoemulsion forms, the oil is already intimately mixed with all the components, and the nanoemulsification is easier. Structural studies made with both cationic and anionic systems confirmed that the size of the "micelles" that form the cubic phase is the same or slightly smaller than the size of the nanoemulsion droplets obtained, depending on the emulsification path, which seems to point out that the nanoemulsions are formed in both cases by a dilution process of this cubic phase. When further watery solution is added to the cubic liquid crystal, these micelles separate, disrupting the cubic structure, and a small fraction of the surfactant migrates to the water. Moreover, due to the change in pH, the spontaneous curvature increases. Then, the phases in equilibrium are an oil-in-water microemulsion (W(m)) and the oil in excess. However, through this emulsification method, the surfactants can be "trapped" in a lower curvature than the spontaneous one, retaining all the oil nanoemulsified.     

The influence of surfactant mixing ratio on nano-emulsion formation by the pit method

The formation of O/W nano-emulsions by the PIT emulsification method in water/mixed nonionic surfactant/oil systems has been studied. The hydrophilic-lipophilic properties of the surfactant were varied by mixing polyoxyethylene 4-lauryl ether (C12E4) and polyoxyethylene 6-lauryl ether (C12E6). Emulsification was performed in samples with constant oil concentration (20 wt%) by fast cooling from the corresponding HLB temperature to 25 degrees C. Nano-emulsions with droplet radius 60-70 nm and 25-30 nm were obtained at total surfactant concentrations of 4 and 8 wt%, respectively. Moreover, droplet size remained practically unchanged, independent of the surfactant mixing ratio, X(C12E6). At 4 wt% surfactant concentration, the polydispersity and instability of nano-emulsions increased with the increase in X(C12E6). However, at 8 wt% surfactant concentration, nano-emulsions with low polydispersity and high stability were obtained in a wide range of surfactant mixing ratios. Phase behavior studies showed that at 4 wt% surfactant concentration, three-liquid phases (W+D+O) coexist at the starting emulsification temperature. Furthermore, the excess oil phase with respect to the microemulsion D-phase increases with the increase in X(C12E6), which could explain the increase in instability. At 8 wt% surfactant concentration, a microemulsion D-phase is present when emulsification starts. The low droplet size and polydispersity and higher stability of these nano-emulsions have been attributed, in addition to the increase in the surface or interfacial activity, to the spontaneous emulsification produced in the microemulsion D-phase.     

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.     

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.     

Solubilization by different-sized surfactant mixtures

The solubilization phenomenon was investigated in mixed surfactant systems. The solubilization power of a mixed surfactant reaches its maximum at a particular temperature at each mixing ratio of surfactants. When the mole fraction of C4E1 in the total surfactant (w1 value) was varied in a water/C12E5/C4E1/decane system, the minimum mole fraction of total surfactant in the system necessary to obtain a single microemulsion phase (xi value) was almost unchanged for w1<0.3, whereas it increased remarkably for w1>0.8. The molar solubilization capacity (Cs=(1-xi)/xi) of the mixed surfactant decreased remarkably for w1<0.3, whereas it decreased gradually for w1>0.8. The result [Formula: see text] is due largely to the characteristic of the function xi(Cs)=1/(1+Cs), specifically, [Formula: see text] , where dxi/dw1=(dxi/dCs)(dCs/dw1). The partial molar solubilization capacity (Cs) of C4E1 was negative at almost all w1, but the Cs value of C12E5 went through a maximum on the addition of C4E1. Propanol (a cosurfactant) has the same effect on the solubilization phenomenon in the water/C12E6/propanol/heptane system. In the water/C12E5/C12E7/decane system, the Cs value of each surfactant did not vary greatly as the mixing ratio of surfactants was varied. The Cs and xi values were close to molar additivity for each mixing ratio.     

Structural Parameters of Mixed Nonionic Surfactants Microemulsions

In this study, we estimated the structural parameters of water/mixed nonionic surfactants/R (+)-limonene microemulsions. The mixed surfactants are sucrose laurate and ethoxylated mono-di-glyceride. U-type microemulsion region was observed in these systems. It was found that changes in the surfactants mixing ratio, surfactants contents and oil/water weight ratio in the microemulsions incite a considerable change in the aggregation number, core radius and interfacial area per mixed surfactants head groups in the formed microemulsions. The interfacial area per mixed surfactant head groups increases while the aggregation number decreases with the increase in the ethoxylated mono-di-glyceride mass fraction in the mixed surfactants. The For an oil/water weight content equals unity, the interfacial area per mixed surfactants head groups is constant for mixed surfactants contents below 35 wt%. For mixed surfactants contents above 35 wt% the interfacial area per mixed surfactants head groups decrease and stabilizes at the lower value. The aggregation number decreases with the increase in the mixed surfactants contents. The aggregation number decreases also with the increase in the oil/water weight ratio at fixed mixed surfactants content.     

Synthesis of highly-ordered mesoporous silica particles using mixed cationic and anionic surfactants as templates

We applied a molecular assembly formed in an aqueous surfactant mixture of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium octylsulfate (SOS) as templates of mesoporous silica materials. The hexagonal pore size can be controlled between 3.22 and 3.66 nm with the mixed surfactant system. In addition, we could observe the lamellar structure of the mixed surfactants with precursor molecules, which strongly shows the possibility of precise control of both the pore size and the structure of pores by changing the mixing ratio of surfactants. Moreover, use of the cationic surfactant having longer hydrophobic chain like stearyltrimethylammonium bromide (STAB) caused the increase in d(100) space and shifted the point of phase transition from hexagonal phase to lamellar phase to lower concentration of SOS.     

Lyotropic liquid crystalline phase behaviour in amphiphile-protic ionic liquid systems

Approximate partial phase diagrams for nine amphiphile-protic ionic liquid (PIL) systems have been determined by synchrotron source small angle X-ray scattering, differential scanning calorimetry and cross polarised optical microscopy. The binary phase diagrams of some common cationic (hexadecyltrimethyl ammonium chloride, CTAC, and hexadecylpyridinium bromide, HDPB) and nonionic (polyoxyethylene (10) oleyl ether, Brij 97, and Pluronic block copolymer, P123) amphiphiles with the PILs, ethylammonium nitrate (EAN), ethanolammonium nitrate (EOAN) and diethanolammonium formate (DEOAF), have been studied. The phase diagrams were constructed for concentrations from 10 wt% to 80 wt% amphiphile, in the temperature range 25 °C to >100 °C. Lyotropic liquid crystalline phases (hexagonal, cubic and lamellar) were formed at high surfactant concentrations (typically >50 wt%), whereas at <40 wt%, only micelles or polydisperse crystals were present. With the exception of Brij 97, the thermal stability of the phases formed by these surfactants persisted to temperatures above 100 °C. The phase behaviour of amphiphile-PIL systems was interpreted by considering the PIL cohesive energy, liquid nanoscale order, polarity and ionicity. For comparison the phase behaviour of the four amphiphiles was also studied in water.     

以混合中性-阳离子表面活性剂为模板合成MCM-48

本文首次将中性和阳离子表面活性剂混和(S^0S^+)形成胶束后,共同作为模板剂合成了立方相中孔分子筛,经此途径得到的中孔分子筛材料既具有MCM-48的立方结构,又能体现出更好的热稳定性。混合不同性质的表面活性剂作为模板剂,将是一种制备不同性能的中孔分子筛的有效易行的途径。     

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.     

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。     

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.     

Interplay between the surface adsorption and solution-phase behavior in dialkyl chain cationic-nonionic surfactant mixtures

Neutron reflectivity, NR, and surface tension have been used to study the adsorption at the air-solution interface of mixtures of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide (DHDAB) and the nonionic surfactants monododecyl triethylene glycol (C12E3), monododecyl hexaethylene glycol (C12E6), and monododecyl dodecaethylene glycol (C12E12). The adsorption behavior of the surfactant mixtures with solution composition shows a marked departure from ideal mixing that is not consistent with current theories of nonideal mixing. For all three binary surfactant mixtures there is a critical composition below which the surface is totally dominated by the cationic surfactant. The onset of nonionic surfactant adsorption (expressed as a mole fraction of the nonionic surfactant) increases in composition as the ethylene oxide chain length of the nonionic cosurfactant increases from E3 to E12. Furthermore, the variation in the adsorption is strongly correlated with the variation in the phase behavior of the solution that is in equilibrium with the surface. The adsorbed amounts of DHDAB and the nonionic cosurfactants have been used to estimate the monomer concentration that is in equilibrium with the surface and are shown to be in reasonable qualitative agreement with the variation in the mixed critical aggregation concentration (cac).     

Synergistic effect of mixed anionic and cationic surfactant systems on the interfacial tension of crude oil-water and enhanced oil recovery

Surfactant based enhanced oil recovery (EOR) is an interesting area of research for several petroleum researchers. In the present work, individual and mixed systems of anionic and cationic surfactants consisting of sodium dodecyl sulphate (SDS) and cetyltrimethylammonium bromide (CTAB) in different molar ratios were tested for their synergistic effect on the crude oil-water interfacial tension (IFT) and enhanced oil recovery performance. The combination of these two surfactant systems showed a higher surface activity as compared to individual surfactants. The effect of mixed surfactant systems on the IFT and critical micellar concentration (CMC) is strongly depends on molar ratios of the two surfactant. Much lower CMC values were observed in case of mixed surfactant systems prepared at different molar ratios as compared to individual surfactant systems. The lowest CMC value was found when the molar concentration of SDS was higher than the CTAB. When the individual and mixed surfacant systems were tested for EOR performance through flooding experiments, higher ultimate oil recovery was obtained from mixed surfactant flooding compared to individual surfactants. Combination of SDS and CTAB or probably other anionic-cationic surfactants show synergism with substantial ability to reduce crude oil water IFT and can be a promising EOR method.     

Lyotropic and interfacial behaviour of an anionic gemini surfactant

The sodium salt of N,N'-hexane-bis (1-dodecen-1-ylsuccinamic acid) is an anionic dimeric (gemini) surfactant. A flooding penetration scan of this surfactant in water demonstrates a sequence of lyotropic phases at room temperature (20 degrees C). Preparation of surfactant-water mixtures has resulted in a phase diagram which shows that the same sequence of phases exists up to 100 degrees C. These phases are tentatively assigned to the sequence: micellar to normal hexagonal (H1) to cubic (V1) to lamellar (Lalpha). The interfacial tension at the n-heptane/water interface has been determined in the presence of this surfactant. The surfactant head group area at the interface is large (2.8+/-0.3 nm2 at 298 K) and the interfacial tension above the critical micelle concentration is low (7 mN m(-1)), but considerably higher than the ultra-low values that have been reported for cationic dimeric surfactants at various hydrocarbon-water interfaces.     

Phase behavior of an extended surfactant in water and a detailed characterization of the concentrated phases

The formation of microemulsions with triglycerides at ambient conditions can be improved by increasing the surfactant-water and surfactant-oil interactions. Therefore, extended surfactants were developed, which contain hydrophilic/lipophilic linkers. They have the ability to stretch further into the oil and water phase and enhance the solubility of oil in water. In this work, the phase behavior of a chosen extended surfactant (C(12-14)-PO(16)-EO(2)-SO(4)Na, X-AES) in H(2)O/D(2)O at high surfactant concentrations (30-100 wt %) and at temperatures between 0 and 90 °C is studied for the first time. The lyotropic liquid crystals formed were determined by optical microscopy, small-angle X-ray scattering (SAXS), and (2)H and (23)Na NMR, and a detailed phase diagram of the concentrated area is given. The obtained mesophases are a hexagonal phase (H(1)), at low temperatures and small concentrations, a lamellar phase (L(α)) at high temperatures or concentrations, a bicontinuous cubic phase (V(2)) as well as a reverse hexagonal phase (H(2)). To our knowledge, this is the first surfactant that forms both H(1) and H(2) phases without the addition of a third compound. From the (2)H NMR quadrupole splittings of D(2)O, we have examined water binding in the L(α) and the H(2) phases. There is no marked difference in the bound water between the two phases. Where sufficient water is present, the number of bound water molecules per X-AES is estimated to be ca. 18 with only small changes at different temperatures. Similar results were obtained from the (23)Na NMR data, which again showed little difference in the ion binding between the L(α) and the H(2) phases. The X-ray diffraction data show that X-AES has a much smaller average length in the L(α) phase compared to the all-trans length than in the case for conventional surfactants. At very high surfactant concentrations an inverse isotropic solution (L(2)), containing a small fraction of solid particles, is formed. This isotropic solution is clearly identified and the size of the reversed micelles was determined using (1)H NMR measurements. Furthermore, the solid particles within the L(2) phase and the neat surfactant were analyzed. The observed results were compared to common conventional surfactants (e.g., sodium dodecyl sulfate, sodium lauryl ether sulfate, and sodium dodecyl-p-benzene sulfonate), and the influence of the hydrophilic/lipophilic linkers on the phase behavior was discussed.     

混合表面活性剂微乳状液的形成和相行为研究进展

讨论了单一表面活性剂,混合表面活性剂,助溶剂等对油／水微乳状液的形成和相行为的影响。对混合表面活性剂微乳状液的形成和相行为研究工作进行了归纳和总结,重点分析了正负离子表面活性剂微乳状液的相行为和表面活性剂微乳状液的相行为和表面活性剂效率,讨论了微乳状液形成的影响因素,并提出了这一研究领域可能的发展前景。     

Phase behavior of polyion-surfactant ion complex salts: effects of surfactant chain length and polyion length

The aqueous phase behavior of a series of complex salts, containing cationic surfactants with polymeric counterions, has been investigated by visual inspection and small-angle X-ray scattering (SAXS). The salts were alkyltrimethylammonium polyacrylates, CxTAPAy, based on all combinations of five surfactant chain lengths (C6, C8, C10, C12, and C16) and two lengths of the polyacrylate chain (30 and 6 000 repeating units). At low water contents, all complex salts except C6TAPA6000 formed hexagonal and/or cubic Pm3n phases, with the hexagonal phase being favored by lower water contents. The aggregate dimensions in the liquid crystalline phases changed with the surfactant chain length. The determined micellar aggregation numbers of the cubic phases indicated that the micelles were only slightly aspherical. At high water contents, the C6TAPAy salts were miscible with water, whereas the other complex salts featured wide miscibility gaps with a concentrated phase in equilibrium with a (sometimes very) dilute aqueous solution. Thus, the attraction between oppositely charged surfactant aggregates and polyions decreases with decreasing surfactant chain length, and with decreasing polyion length, resulting in an increased miscibility with water. The complex salt with the longest surfactant chains and polyions gave the widest miscibility gap, with a concentrated hexagonal phase in equilibrium with almost pure water. A decrease in the attraction led to cubic-micellar and micellar-micellar coexistence in the miscibility gap and to an increasing concentration of the complex salt in the dilute phase. For each polyion length, the mixtures for the various surfactant chain lengths were found to conform to a global phase diagram, where the surfactant chain length played the role of an interaction parameter.