Highly viscoelastic reverse worm-like micelles formed in a lecithin/urea/oil system

While lecithin alone can form spherical or ellipsoidal reverse micelles in oil, we found that urea can promote the growth of lecithin reverse worm-like micelles in oil. In a mixed system of urea and lecithin, the urea binds to the phosphate group of lecithin, thus reducing the interface curvature of the molecular assembly and inducing the formation of reverse worm-like micelles. The regions in which these micelles form increased with lecithin concentration. In addition, the zero-shear viscosity (η ₀) of the reverse worm-like micelles rapidly increased upon the addition of urea, reaching a maximum of 2 million times the viscosity of n-decane. We examined the change in η ₀ in detail by performing dynamic viscoelasticity measurements. Values for η ₀ increased with urea concentration because the disentanglement time of reverse worm-like micelles increased with micellar growth.     

A novel reverse worm-like micelle from a lecithin/sucrose fatty acid ester/oil system

We report a novel method for forming reverse worm-like micelles in nonpolar organic solvents. This method requires the addition of trace amounts of a sucrose fatty acid ester (SFE), in addition to lecithin and a nonpolar organic solvent. The region in which these micelles formed increased with lecithin concentration and hydrophobicity of SFE. In addition, zero-shear viscosity (η ₀) of the reverse worm-like micelles increased rapidly upon addition of SFE, reaching 1–3 million times the viscosity of n-decane. Furthermore, the change in η ₀ was examined in detail by performing dynamic viscoelasticity measurements. Results showed that the η ₀ of solution increased upon addition of SFE because both the length and number of reverse worm-like micelles increased along with SFE concentration.     

Rheology and microstructure studies of SDS/CTAB/H2O system

Phase behavior of mixed sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) aqueous solution was studied. The rheological properties and microstructure were investigated using a rheostat and freeze-fracture technique and are shown to be closely related to the phase behavior. Experimental investigations reveal two symmetrical aqueous two-phase systems (ATPS) in the ternary phase diagram of SDS/CTAB/H₂O system. In the surfactant rich phase of ATPS or in the adjacent stoichiometric state of ATPS, the system has high viscosity because of its long range ordered structure. Lamellar phase was found in the high viscosity samples in which the cationic and anionic surfactant are in 1: 3 or 3: 1 stoichiometry. In addition, the viscosity has a tendency to increase when salt was added to the solution. The viscosity increase is due to the salt can screen the repulsion between different charged headgroups and thus reduces the effective size of surfactants and facilitates the spherical or rod likes micelles to be transformed to worm-like micelles which can form hexagonal or liquid crystal phases. Large-size salt ions like sodium sulfate (especially organic salt ions) have more significant effect on the surfactant solution viscosity. The text was submitted by the authors in English.     

Viscoelasticity and mass transfer in phenol–CTAB aqueous systems

The rheological and mass transport properties of phenol in micellar solutions of hexadecyltrimethylammonium bromide (CTAB) were studied by rheometry and spectrophotometry. The presence of phenol located between headgroups of the CTAB diminishes the repulsive forces between the cationic groups and induces a sharp increase in viscosity that is attributed to the one-dimensional micellar growth favoring the formation of worm-like micelles. It is found that the mass transfer of phenol between two immiscible phases is significantly retarded by the presence of CTAB. The transfer is particularly slow when the diffusion takes place from a surfactant solution phase to an organic phase. This behavior is attributed to the phenol–surfactant interaction that leads to micellar growth and viscoelastic behavior. However, at elevated temperature, viscosity decreases and mass transfer increases. This particular rheological behavior offers the possibility of regulating the mass transfer, which might be interesting for applications.     

部分水解聚丙烯酰胺与蠕虫状胶束在微米级毛细管中的驱替粘度

部分水解聚丙烯酰胺(HPAMs)被大量地用作三次采油中驱替液的增稠剂,表面活性剂在一定的条件下可以通过自组装形成蠕虫状胶束,具有与高分子相似的增稠的作用。本文在半径为1–10 μm的毛细管中,分别考察了HPAMs与蠕虫状胶束的微观驱替行为,研究结果表示毛细管内腔的尺寸限制了这些非牛顿流体的增稠作用。随着毛细管半径的减小,聚合物溶液的剪切变稀越剧烈,甚至从非牛顿流体转变为牛顿流体的流体行为。结合驱替研究和超滤、电镜的结果,证明了高分子的缠绕结构在毛细管中已被破坏。通过对比驱替数据,蠕虫状胶束在毛细管中能够更大程度地保留宏观的粘度,我们提出表面活性剂能够通过自组装修复被破坏的缠绕结构,比高分子聚合物在微观有限空间中有更好的增稠能力。     

Cryo-TEM studies of worm-like micellar solutions

Solutions of worm-like micelles display a rich rheological behavior that makes them useful as, for example, drag-reducing agents or viscosity enhancers. The properties of these solutions depend on the morphology and interactions between the micelles, both of which can be tuned by changing solution conditions. Although there has been extensive theoretical study of these solutions, there are often conflicting explanations, or no explanation, of an observed trend. Application of cryogenic transmission electron microscopy (cryo-TEM) for the direct visualization of the micelles can help correlate microstructure to rheology. Of particular interest is the cause of a maximum in viscosity as a function of increasing surfactant or salt concentration. Several studies support the theory of a transition from linear to branched micelles, while other studies report no change in microstructure or no connection between structure evolution and changes in viscosity. More systematic and thorough studies that combine cryo-TEM with other experimental techniques are needed.     

Morphological transitions during the formation of templated mesoporous materials: theoretical modeling

We put forward a theoretical model for the morphological transitions of templated mesoporous materials. These materials consist of a mixture of surfactant molecules and inorganic compounds which evolve dynamically upon mixing to form different morphologies depending on the composition and conditions at which mixing occurs. Our theoretical analysis is based on the assumption that adsorption of the inorganic compounds onto mesoscopic assemblies of surfactant molecules changes the effective interactions between the surfactant molecules, consequently lowering the spontaneous curvature of the surfactant layer and inducing morphological changes in the system. On the basis of a mean field phase diagram, we are able to follow the trajectories of the system starting with different initial conditions, and predict the final morphology of the product. In a typical scenario, the reduction in the spontaneous curvature leads first to a smooth transition from compact spherical micelles to elongated worm-like micelles. In the second stage, the layer of inorganic material coating the micelles gives rise to attractive inter-micellar interactions that eventually induce a collapse of the system into a closely packed hexagonal array of coated cylinders. Other pathways may lead to different structures including disordered bicontinuous and ordered cubic phases. The model is in good qualitative agreement with experimental observations.     

Bola反离子诱导的高粘弹十六烷基硫酸钠蠕虫胶束溶液

利用稳态和频率扫描研究了十六烷基硫酸钠(SHS)与Bola盐(N,N’-双乙基二甲基-α,ω-乙烷溴化铵(Bola2Et)或N,N’-三甲基-α,ω-丁烷溴化铵(Bola4))混合体系的流变行为.实验结果表明两个体系均在45℃时形成了长蠕虫胶束,溶液呈现出高粘弹性,尤其是SHS/Bola2Et溶液表现出很高的弹性,零剪接粘度高达2520Pa’s,表观上呈现胶状.这些结果归因于在Bola反离子诱导下,表面活性剂SHS与Bola反离子形成了静电吸引的2:1结构复合物.由于这两个Bola反离子的联接链长度均短于季铵离子的静电平衡距离,因此形成的复合物在形状上有利于蠕虫胶束生成.作为对比,添加简单的四甲基铵反离子无法诱导SHS形成蠕虫胶束,溶液仅呈现低粘度.     

Intrinsic parameters for the structure control of nonionic reverse micelles in styrene: SAXS and rheometry studies

Shape, size, and internal structure of nonionic reverse micelle in styrene depending on surfactant chain length, concentration, temperature, and water addition have been investigated using a small-angle X-ray scattering (SAXS) technique. The generalized indirect Fourier transformation (GIFT) method has been employed to deduce real-space structural information. The consistency of the GIFT method has been tested by the geometrical model fittings, and the micellar aggregation number (N(agg)) has been determined. It was found that diglycerol monocaprate (C(10)G(2)), diglycerol monolaurate (C(12)G(2)), and diglycerol monomyristate (C(14)G(2)), spontaneously self-assemble into reverse micelles in organic solvent styrene under ambient conditions. The micellar size and the N(agg) decrease with an increase in surfactant chain length, a scenario that could be understood from the modification of the critical packing parameter (cpp). A clear picture of one-dimensional (1-D) micellar growth was observed with an increase in surfactant weight fraction (W(s)) in the C(10)G(2) system, which eventually formed rodlike micelles at W(s) ≥ 15%. On the other hand, micelles shrunk favoring a rod-to-sphere type transition upon heating. Reverse micelles swelled with water, forming a water pool at the micellar core; the size of water-incorporated reverse micelles was much bigger than that of the empty micelles. Model fittings showed that water addition not only increase the micellar size but also increase the N(agg). Zero-shear viscosity was found to decrease with surfactant chain but increase with W(s), supporting the results derived from SAXS.     

Structure and rheology of reverse micelles in dipentaerythrityl tri-(12-hydroxystearate)/oil systems

We report the formation of reverse rod-like micelles and their rheological properties in novel nonionic surfactant, dipentaerythrityl tri-(12-hydroxystearate) (designated as WO-6)/oil systems without external water addition. Small-angle X-ray scattering (SAXS) was used to investigate the structure of the micelles and their flow properties were studied by rheological measurements. We found that WO-6 spontaneously self-assembles into reverse micelles in a variety of organic solvents at ambient conditions, their structure depending on solvent molecular architecture, surfactant concentration, and temperature. Rod-like micelles with a maximum length of ca. 12 nm and a cross section diameter of ca. 2 nm were observed in cyclohexane. When cyclohexane was replaced with a linear chain octane, the length and the cross section diameter were simultaneously increased. With a further increase of hydrocarbon chain length of solvent oils from octane to hexadecane, the rod-like micelles grew axially, keeping the cross section diameter (ca. 3 nm) virtually constant. Increasing surfactant concentration also favored one-dimensional micellar growth. On the other hand, micelles shrunk with the rise of temperature, which is similar to a rod-to-sphere transition, and is essentially the opposite temperature dependence to that often observed in aqueous micellar systems. A structural picture drawn by SAXS is well supported by rheology; the relative (zero-shear) viscosity of the WO-6/oil systems was found to be markedly greater than that expected for a dispersion of spherical particles due to the elongated micellar structure, despite quantitative inconsistency with semi-empirically predicted values for rigid rod-like particles.     

Shape and Size of Highly Concentrated Micelles in CTAB/NaSal Solutions by Small Angle Neutron Scattering (SANS)

Highly concentrated micelles in CTAB/NaSal solutions with a fixed salt/surfactant ratio of 0.6 have been studied using Small Angle Neutron Scattering (SANS) as a function of temperature and concentration. A worm-like chain model analysis of the SANS data using a combination of a cylindrical form factors for the polydisperse micellar length, circular cross-sectional radius with Gaussian polydispersity, and the structure factor based on a random phase approximation (RPA) suggests that these micelle solutions have a worm-like micellar structure that is independent of the concentration and temperature. The size of the micelle decreases monotonically with increasing temperature and increases with concentration. These observations indicate that large micelles are formed at low temperature and begin to break up to form smaller micelles with increasing temperature.     

The molecular weight dependence of intrinsic viscosity of rod-like micelles of dodecyldimethylammonium chloride

It is shown that the non-linear logarithmic dependence of the intrinsic viscosity on the molecular weight for rod-like micelles of dodecyldimethylammonium chloride (as reported by Ozeki and Ikeda [1]) can be interpreted in terms of the Yamakawa-Fujii theory of worm-like chains. Characteristic parameters of the micelles are estimated: persistence length (a=14 nm), linear mass density (M _L=4800 nm^–1), diameter (d=3 nm), molecular pitch (b=0.052), and the number of surfactant chains in a layer of rod-like micellen=12. The results are compared with those derived from light-scattering measurements.     

A dually effective inorganic salt at inducing obvious viscoelastic behavior of both cationic and anionic surfactant solutions

Hydrazine nitrate (HN), an inorganic salt, was first found to have dual effects on inducing obvious viscoelasticity of both cationic and anionic surfactant solutions. It was interesting that the surfactant solutions exhibited characteristic wormlike micelle features with strong viscoelastic properties upon the addition of this inorganic salt. The rheological properties of the surfactant solutions have been measured and discussed. The apparent viscosity of the solutions showed a volcano change with an increase of the HN concentration. Correspondingly, the microstructures of the micelles in the solutions changed with the apparent viscosity. First, wormlike micelles began to form and grew with an increase of the HN concentration. Subsequently, the systems exhibited linear viscoelasticity with characteristics of a Maxwell fluid in the intermediate mass fraction range, which originated from a 3D entangled network of wormlike micelles. Finally, a transition from linear micelles to branched ones probably took place at higher HN contents. In addition, the origin of the dual effects brought by HN addition on inducing viscoelasticity in both cationic and anionic surfactant solutions was investigated.     

Quantitative model for prediction of hydrodynamic size of nonionic reverse micelles

The sizes of nonionic reverse micelles were investigated as a function of the molecular structure of the surfactant, the type of oil, the total concentration of surfactant [NP], the ratio of surfactant to total surfactant (r), the water to surfactant molar ratio (omega), temperature, salt concentration, and polar phase. The basis of our investigation was a mixture of nonylphenol polyethoxylates--NP4 and NP7, various polar phases, and several oils. Micelle sizes were determined using dynamic light scattering (DLS). A central composite experimental design was used to quantitatively model micelle size as a function of omega, surfactant concentration, and r. The model has demonstrated the capability of predicting the mean diameter of micelles from 4 to 13 with a precision of +/-2 nm as measured by DLS. This quantitative correlation between the size of reverse micelles and the synthetic variables provides the foundation for choosing experimental conditions to control reverse micelle size. In turn, this allows control of the size of nanoparticles synthesized within them.     

pH-dependent phase behavior of carbohydrate-based gemini surfactants. Effect of the length of the hydrophobic spacer

The phase behavior of a series of carbohydrate-based gemini surfactants with varying spacer lengths was studied using static and dynamic light scattering between pH 2 and 12. Cryo-electron microscopy pictures provide evidence for the different morphologies present in solution. The spacer length of the gemini surfactants was varied from two to 12 methylene units. At near neutral pH, spherical vesicles were obtained for gemini surfactants with a spacer shorter than 10 methylene units, whereas nonspherical vesicles were obtained for spacer lengths of 10 and 12. Upon decreasing the pH, the vesicles underwent transitions toward worm-like micelles and spherical micelles for a spacer length of six and larger, whereas for shorter spacers, these transitions are not observed. For the shortest spacer at low pH, perforated vesicles are observed, and vesicles built from the gemini surfactant with a spacer of four methylene units only underwent a transition toward worm-like micelles. Upon increasing the pH to slightly basic values, flocculation followed by redispersion upon charge reversal was observed up to a spacer length of eight methylene units. The redispersal is explained by hydroxide-ion binding to the uncharged vesicular surface. By contrast, vesicles formed from the gemini surfactants with 10 and 12 methylene units only undergo a transition toward inverted phases. The observations can be understood in terms of the packing parameter.     

Use of reverse micelles to make either spherical or worm-like palladium nanocrystals: influence of stabilizing agent on nanocrystal shape

Depending on the water content, reverse micelles induce the formation of fcc metallic palladium worm-like nanocrystals made of spheres. After extraction from the nanoreactor, either spheres or worm-like nanocrystals are obtained, and it was found that the binding energy between the coating agent and the Pd surface is a key parameter in shape control (i.e., in the surface reconstruction).     

A new reverse wormlike micellar system: mixtures of bile salt and lecithin in organic liquids

We report a new route for forming reverse wormlike micelles (i.e., long, flexible micellar chains) in nonpolar organic liquids such as cyclohexane and n-decane. This route involves the addition of a bile salt (e.g., sodium deoxycholate) in trace amounts to solutions of the phospholipid lecithin. Previous recipes for reverse wormlike micelles have usually required the addition of water to induce reverse micellar growth; here, we show that bile salts, due to their unique "facially amphiphilic" structure, can play a role analogous to that of water and promote the longitudinal aggregation of lecithin molecules into reverse micellar chains. The formation of transient entangled networks of these reverse micelles transforms low-viscosity lecithin organosols into strongly viscoelastic fluids. The zero-shear viscosity increases by more than 5 orders of magnitude, and it is the molar ratio of bile salt to lecithin that controls the viscosity enhancement. The growth of reverse wormlike micelles is also confirmed by small-angle neutron scattering (SANS) experiments on these fluids.     

Viscometric study of the sodium hyaluronate-sodium chloride-alkyl-(n)-ammonium surfactant system

Viscosity and rheological properties of the system sodium hyaluronate (NaHA)-NaCl-alkyl-(n)-ammonium surfactant were studied. The system with monomeric (n=1) surfactant dodecyltrimethylammonium bromide DTAB separated into two phases below the 200 mM NaCl concentration. At high NaCl concentration (>0.3 M), the hyaluronate–surfactant interaction is screened. At low NaCl concentration (0.2 M), the hyaluronate–surfactant interaction appears at higher concentration of micelles. In the system with dimeric surfactant (n=2) alkanediyl-,ω-bis(dimethyldodecylammonium bromide) referred to as 12-s-12, the hyaluronate–surfactant interaction was observed at low surfactant concentration, opposite to the case of NaHA-NaCl-monomeric surfactant. In certain range of surfactant concentration, viscosity of the system can be fairly good described by the empirical Jones–Dole equation. However, the fit fails at high surfactant concentration. From the rheological measurements follows that NaHA aqueous solution shows non-Newtonian behaviour. When adding NaCl and shearing, viscosity increases as a result of affecting the ion atmosphere which surrounds micelles, by shearing. The same effect was observed in the system NaHA-NaCl-ammonium surfactant (both monomeric and dimeric). The hyaluronate–surfactant interaction is assumed at low shearing. The influence of surfactant structure on viscosity at the same NaCl concentration shows that the increasing value of the carbon number in spacer of dimeric surfactant increases viscosity and stretches the hyaluronate coils.     

Recent advances in particle-based simulation of surfactants

We review the recent literature on particle-based simulation of surfactants, focusing on key methodological developments in the areas of surfactant self-assembly, micelle formation, micelle kinetics, the properties of worm-like micelles, interfacial adsorption and surface tension. We pay particular attention to dissipative particle dynamics where a considerable amount of work has been performed recently to improve parametrisation and apply the method quantitatively to surfactant systems. We discuss highlights and outstanding challenges and make suggestions for priority areas for future research directions.     

Effects of surfactant micelles on viscosity and conductivity of poly(ethylene glycol) solutions

The neutral polymer-micelle interaction is investigated for various surfactants by viscometry and electrical conductometry. In order to exclude the well-known necklace scenario, we consider aqueous solutions of low molecular weight poly(ethylene glycol) (2-20)x10(3), whose radial size is comparable to or smaller than micelles. The single-tail surfactants consist of anionic, cationic, and nonionic head groups. It is found that the viscosity of the polymer solution may be increased several times by micelles if weak attraction between a polymer segment and a surfactant exists, epsilon相似文献