Polymerization of anionic wormlike micelles

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.     

Viscoelastic wormlike micellar solutions made from nonionic surfactants: structural investigations by SANS and DLS

A highly viscoelastic micellar solution of nonionic surfactants in a dilute region was recently reported. A transient network of wormlike micelles formed with the addition of short-EO-chain poly(oxyethylene) dodecyl ether surfactants (C12EO(j), j = 1-4) to poly(oxyethylene) cholesteryl ethers (ChEO(m), m = 10 and 15). A gradual increase in micellar length with an increasing C12EO(j) concentration was assumed from the results of model calculations and rheological measurements. We report in this study the results of structural investigations with small-angle neutron scattering (SANS) to confirm this assumption. Tuning from spherical to wormlike and to locally flat structures can be achieved by way of three methods. One can either increase the C12EO(j) concentration or decrease j (smaller headgroup size) at a fixed concentration of C12EO(j). The third possibility is to increase the temperature at a fixed composition. All three methods result in the same structural transition. The formation of a transient network of wormlike micelles analogous to polymer solutions can be observed with dynamic light scattering (DLS). A stretched exponential approach was applied to fit the correlation functions.     

Wormlike micelles and microemulsions in aqueous mixtures of sucrose esters and nonionic cosurfactants

A study of the phase and rheological behavior of sucrose hexadecanoate (C16SE)/cosurfactant/water systems in the presence of solubilized oil, using complementary techniques such as dynamic light scattering and small angle X-ray scattering, is presented. Viscoelastic wormlike micellar solutions are found when a nonionic lipophilic cosurfactant is added to C16SE aqueous systems. Contrary to previous reports, the effect of oil solubilization on these wormlike micelles is not unique and depends on several factors. Linear alkyl chain oils that tend to solubilize in the micellar core have a disrupting effect, decreasing the relaxation time and the viscosity of the systems. This effect is larger as the molecular volume of oil increases and as the solubility of the cosurfactant in oil increases. On the other hand, oils that penetrate in the palisade layer, such as p-xylene, induce micellar growth and have a thickening effect at a given micellar composition. Thermodynamic considerations are used to explain the experimental results.     

Small-angle neutron scattering from a hexagonal phase under shear

Summary The shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.     

Wormlike micelle formation and flow alignment of a pluronic block copolymer in aqueous solution

The self-assembly into wormlike micelles of a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer Pluronic P84 in aqueous salt solution (2 M NaCl) has been studied by rheology, small-angle X-ray and neutron scattering (SAXS/SANS), and light scattering. Measurements of the flow curves by controlled stress rheometry indicated phase separation under flow. SAXS on solutions subjected to capillary flow showed alignment of micelles at intermediate shear rates, although loss of alignment was observed for high shear rates. For dilute solutions, SAXS and static light scattering data on unaligned samples could be superposed over three decades in scattering vector, providing unique information on the wormlike micelle structure over several length scales. SANS data provided information on even shorter length scales, in particular, concerning "blob" scattering from the micelle corona. The data could be modeled based on a system of semiflexible self-avoiding cylinders with a circular cross-section, as described by the wormlike chain model with excluded volume interactions. The micelle structure was compared at two temperatures close to the cloud point (47 degrees C). The micellar radius was found not to vary with temperature in this region, although the contour length increased with increasing temperature, whereas the Kuhn length decreased. These variations result in an increase of the low-concentration radius of gyration with increasing temperature. This was consistent with dynamic light scattering results, and, applying theoretical results from the literature, this is in agreement with an increase in endcap energy due to changes in hydration of the poly(ethylene oxide) blocks as the temperature is increased.     

Temperature-induced growth of wormlike copolymer micelles

We study the temperature-induced growth of polymer micelles based on Pluronic P84 in brine (2 M NaCl) using small-angle neutron scattering, static and dynamic light scattering, and viscometry as a function of temperature and polymer concentration. Spherical micelles below 30 degrees C are shown to grow between about 30 and 40 degrees C into wormlike micelles long enough to enter the semidilute regime for polymer volume fraction larger than 0.005. The entanglements in this regime are responsible for a huge increase in the viscosity. Above about 41 degrees C, the micellar aggregates become denser as the cloud point is approached and the viscosity drops.     

A phenol-induced structural transition in aqueous cetyltrimethylammonium bromide solution

The effect of phenol on the structure of micellar solution of a cationic surfactant, cetyltrimethylammonium bromide (CTAB) was investigated using viscosity, dynamic light scattering (DLS), small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) techniques. The relative viscosity and apparent hydrodynamic diameters of the micelles in CTAB solution increase initially and then decrease with addition of phenol. SANS studies indicate a prolate ellipsoidal structure of the micelles. The axial ratio of the prolate ellipsoidal micelles increases and then decreases with addition of phenol, consistently with DLS and viscosity measurements. NMR studies confirm the solubilization of phenol to the palisade layer and growth of the micelles at high concentration of phenol as revealed from the broadening of peaks.     

Dynamic light scattering from non-entangled wormlike micellar solutions

The fuzzy cylinder theory, originally proposed for conventional polymer solutions, was applied to wormlike micellar solutions to take into account effects of the intermicellar collision and hydrodynamic interaction on the self-diffusion of wormlike micelles in solution at finite concentrations. Previously reported apparent hydrodynamic radius data obtained by dynamic light scattering for non-entangled wormlike micelles formed in aqueous solution by non-ionic surfactants, polyoxyethylene monoalkyl ethers C(i)E(j), were analyzed by this theory to estimate the persistence length q of the wormlike micelles. The results of q estimated were consistent with those obtained from radius of gyration data obtained by static light scattering.     

Influence of a hydrophobic diol on the micellar transitions of Pluronic P85 in aqueous solution

Micellization behavior of an amphiphilic ethylene oxide-propylene oxide-ethylene oxide tri-block copolymer Pluronic P85 [(EO)(26)(PO)(39)-(EO)(26)] in aqueous solution and in the presence of a hydrophobic C(14)diol (also known as Surfynol104) was examined by physico-chemical methods such as viscometry, cloud point (CP) and scattering techniques viz. dynamic light scattering (DLS) and small angle neutron scattering (SANS). The addition of diol decreases the cloud point and gelation temperature of aqueous Pluronic P85 copolymer solution. DLS and SANS measurements of the polymer in aqueous solution indicated micellar growth and sphere to rod transition in the presence of diol. Surfynol 104 is a sparingly water soluble diol surfactant with a solubility of approximately 0.1 wt%. However, up on addition to Pluronic solution, diol gets incorporated in the block copolymer micelles and leads to structural transition of the micelles. An increase in the temperature and the presence of added sodium chloride in the solution further enhances this effect. The addition of hydrophobic C(14)diol increases the hydrodynamic size and aggregation numbers of the micellar system. The micellar parameters for the copolymer in the presence of C(14)diol are reported at different temperatures and added sodium chloride concentrations.     

Wormlike micelles mediated by polyelectrolyte

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt: a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.     

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.     

Shape transformation of poly(butadiene)‐b‐poly(ethyleneoxide) plus DTAB compound micelles in aqueous solutions

The micellar shape of Poly(butadiene)‐b‐poly(ethyleneoxide) (PB‐PEO) plus Dodecyltrimethylammoniumbromide (DTAB) compound micelles was investigated by light scattering, small‐angle X‐ray scattering and small‐angle neutron scattering in dependence of the molar ratio between block copolymer and surfactant. The given block copolymer forms cylindrical micelles in binary aqueous solution, which transform to spherical aggregates upon the addition of a sufficiently high amount of DTAB. It is interesting to note that the micellar shape seems to be a bistable feature, in the sense that it depends not only on the molar ratio of BCP and DTAB but also in a predictable manner on the preparation procedure of the solution.     

Formation of intermediate micellar phase between hexagonal and discontinuous cubic liquid crystals in brine/N-acylamino acid surfactant/N-acylamino acid oil system

The phase behavior in the brine/sodium N-dodecanoyl sarcosinate (Sar)/isopropyl N-dodecanoyl sarcosinate (SLIP) system has been investigated by means of phase study, static light scattering, and small-angle X-ray scattering. The liquid crystal phases, hexagonal (H(1)) and discontinuous cubic (I(1)), melt upon the addition of NaCl, which shows the similar effect to the increasing of temperature. The addition of SLIP to the brine/Sar solution at high Sar concentration induces the phase transition from H(1) to I(1) via the isotropic micellar solution (W(m2)). The micellar structure in the W(m2) phase also changes from the wormlike to the globular micelle with SLIP concentration. Adding NaCl reduces the repulsive force between the Sar head groups and simultaneously the space of the solubilized SLIP in the palisade layer, leading SLIP to shift their location further into the micelle core. As a consequence, the hexagonal symmetry breaks into the micelle solution and the liquid crystal order is destabilized entropically.     

TODGA based w/o microemulsion in dodecane: an insight into the micellar aggregation characteristics by dynamic light scattering and viscometry

N,N,N',N'-tetraoctyl diglycolamide abbreviated as TODGA, is one of the most promising extractant for actinide partitioning from high level nuclear waste. It forms reverse micelles in non polar solvents on equilibration with aqueous HNO(3) solutions. This reverse micellar system undergoes phase separation into dilute and concentrated reverse micellar solutions at high aqueous acid concentration. Small angle neutron scattering (SANS) studies reported in the literature explained this phenomenon based on gas-liquid type phase transition in the framework of Baxter adhesive hard sphere theory in the presence of a strong inter-micellar attractive interaction. The present investigation attempts to throw further light on this system by carrying out systematic dynamic light scattering (DLS) and viscometry studies, and their modeling on the TODGA reverse micellar solutions in the dodecane medium. The variation of the diffusion coefficient with the micellar volume fraction observed from the DLS studies is suggestive of the presence of an attractive interaction between the TODGA reverse micelles, which weakens at the high micellar volume fraction due to the increased dominance of the excluded volume effect. It is suggested that this weakened interaction is responsible for the absence of phase separation in this system at high TODGA concentration. The results thus highlight the importance of the presence of an attractive interaction between the TODGA micelles in determining the observed phase separation in the TODGA reverse micellar systems. The modeling of the DLS and viscosity data, however, suggest that the characteristic stickiness parameter of this system could be smaller than the critical value required for inducing a gas-liquid type phase transition.     

Static and dynamic structures of spherical nonionic surfactant micelles during the disorder-order transition

We have investigated the static and dynamic structures of nonionic surfactant micelles, a C(12)E(8)/water binary system, during the disorder-order transition using small angle x-ray scattering, static light scattering, and dynamic light scattering techniques. In the disordered phase, the micelles have spherical shape and intermicellar interactions are governed by the hard core and weak long ranged attractive potentials. With increase of the micellar concentration, the disordered micelles transform to the three characteristic ordered micellar phases, a hexagonally close packed lattice, a body centered cubic lattice, and an A15 lattice having area-minimizing structure. The stability of these phases is well explained by balance of a close packing rule and a minimal-area rule proposed by Ziherl and Kamien [Phys. Rev. Lett. 85, 3528 (2000)]. The role of hydrodynamic interactions in surfactant micellar solutions was compared with that in hard sphere colloidal particle suspensions.     

Viscoelastic micellar water/CTAB/NaNO(3) solutions: rheology, SANS and cryo-TEM analysis

Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.     

Effect of ionic strength on the rheological behavior of aqueous cetyltrimethylammonium p-toluene sulfonate solutions

The influence of ionic environment on the rheological properties of aqueous cetyltrimethylammonium p-toluene sulfonate (CTAT) solutions has been studied under three different flow fields: simple shear, opposed-jets flow and porous media flow. Emphasis was placed in the experiments on a range of CTAT concentration in which wormlike micelles were formed. It is known that these solutions exhibit shear thickening in the semi-dilute regime, which has been explained in terms of the formation of shear-induced, cooperative structures involving wormlike micelles. In simple shear flow, the zero shear viscosity exhibits first an increase with salt addition followed by a decrease, while the critical shear rate for shear thickening increases sharply at low salt contents and tends to saturate at relatively high ionic strengths. The results are explained in terms of a competition between micellar growth induced by salt addition and changes in micellar flexibility caused by ionic screening effects. Dynamic light scattering results indicate that micelles grow rapidly upon salt addition but eventually achieve a constant size under static conditions. These observations suggest that the wormlike micelles continuously grow with salt addition, but, as they become more flexible due to electrostatic screening, the wormlike coils tend to adopt a more compact conformation. The trends observed in the apparent viscosities measured in porous media flows seem to confirm these hypotheses-but viscosity increases in the shear thickening region-and are magnified by micelle deformation induced by the elongational nature of the local flow in the pores. In opposed-jets flow, the solutions have a behavior that is close to Newtonian, which suggests that the range of strain rates employed makes the flow strong enough to destroy or prevent the formation of cooperative micellar structures.     

Microrheology of wormlike micellar fluids from the diffusion of colloidal probes

The microrheology of cationic micellar solutions has been investigated as a function of added organic salts using quasielastic light scattering (QELS). Two organic salts, sodium p-toluene sulfonate and sodium salicylate, were used to induce microstructural changes in cetyl trimethylammonium bromide (CTAB) micelles. The mean-squared displacement (MSD) of polystyrene probe particles embedded in CTAB micellar solutions was monitored by QELS in the single-scattering regime. Through the use of the generalized Stokes-Einstein relationship, the frequency-dependent complex shear moduli of each fluid were estimated from the Laplace transform of the corresponding MSD. The salt-induced transition from nearly spherical to elongated wormlike micelles and consequent changes in fluid response from viscous to viscoelastic are clearly captured by microrheology.     

Thermoresponsive core-shell microgels with silica nanoparticle cores: size, structure, and volume phase transition of the polymer shell

Core-shell microgels made of the thermoresponsive polymer poly(N-isopropylacrylamide) (PNIPAM) and silica nanoparticles as inorganic cores were investigated by dynamic light scattering (DLS) and small angle neutron scattering (SANS). In order to study the response of the particles upon changes of temperature, experiments were done in a temperature interval close to the volume phase transition temperature of the PNIPAM shell. While DLS probes the hydrodynamic dimensions of the particles, determining their centre of mass diffusion, SANS provides the correlation length xi of the PNIPAM network. Additionally, the composite particles were characterised by electron microscopy as well as atomic force microscopy to reveal the core-shell structure and at the same time the approximate dimensions and the shape of the microgels.     

Mixed spherical and wormlike micelles: a contrast-matching study by small-angle neutron scattering

Small-angle neutron scattering studies were used to investigate the effect of adding an alcohol ethoxylate nonionic surfactant (d-C12E20) to aqueous solutions of a cationic surfactant, erucyl bis(hydroxyethyl) methylammonium chloride (EHAC), with and without salt (KCl). The systematic use of contrast-matching, by alternately highlighting or hiding one of the surfactants, confirms that mixed micelles are formed. In salt-free solutions, mixed spherical micelles are formed and a core-shell model combined with a Hayter-Penfold potential was used to describe the data. The core radius is dominated by the EHAC tails and the outer radius determined by the ethoxylate headgroups of the nonionic surfactant. Addition of KCl promotes micellar growth; however, results of varying the solvent contrast revealed that when the nonionic surfactant is incorporated into the wormlike structure micellar breaking is promoted. Thus, mixed wormlike micelles with shorter contour lengths compared to the pure EHAC worms are formed.