Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Photoresponsive Behavior of Wormlike Micelles Constructed by Gemini Surfactant 12-3-12·2Br- and Different Cinnamate Derivatives

The photoresponsive wormlike micelles constructed by Gemini surfactants and cinnamate derivatives play a great role in the field of smart materials. However, how the structure of cinnamate derivatives affects the photoresponsive behavior of micelles is still a hotspot for scientists to research. Here, three kinds of aromatic salts with different ortho-substituted groups including trans- o-methoxy cinnamate ( trans-OMCA), trans- o-hydroxy cinnamate ( trans-OHCA), and trans-cinnamate ( trans-CA) were introduced into Gemini surfactant 12-3-12·2Br^- aqueous solutions to construct photoresponsive wormlike micelles through their noncovalent interactions. Their properties were researched using the rheological method, cryo-transmission electron microscopy, and ¹H NMR and two-dimensional nuclear Overhauser effect spectra. The results show that these cinnamate derivatives could well construct wormlike micelles with 12-3-12·2Br^-. Furthermore, subtle differences in the ortho substituents' structure have a significant effect on the photoresponsive behavior of formed wormlike micelles. Specifically, the zero viscosity (η₀) of 40 mM 12-3-12·2Br^-/24 mM trans-OHCA mixed solution decreases from 26.72 to 2.6 Pa·s with the shortening of the length of wormlike micelles after UV irradiation. Correspondingly, the η₀ for the same ratio of 12-3-12·2Br^-/ trans-OMCA decreases from 2.42 to 0.06 Pa·s and the wormlike micelles are transited into rodlike micelles and even spherical micelles after the same UV irradiation time. However, the variation of wormlike micelles in the 12-3-12·2Br^-/ trans-CA system induced by UV light is not obvious with η₀ being maintained at around 2.89 Pa·s. This study will help us better understand the effects of chemical groups on macrophenomena and microinteraction for micellar systems. It provides a theoretical basis for the construction of photoresponsive micelles, thus widening their application in the field of soft materials.

Analytical modeling of micelle growth. 1. Chain-conformation free energy of binary mixed spherical, wormlike and lamellar micelles

HYPOTHESES

A quantitative molecular-thermodynamic theory of the growth of giant wormlike micelles of nonionic surfactants can be developed on the basis of a generalized model, which includes the classical "phase separation" and "mass action" models as special cases. The generalized model describes spherocylindrical micelles, which are simultaneously multicomponent and polydisperse in size.

THEORY

By analytical minimization of the free-energy functional we derived explicit expressions for the chain-extension and chain-end distribution functions in the hydrocarbon core of mixed micelles from two surfactants of different chainlengths.

FINDINGS

The hydrocarbon core of a two-component micelle is divided in two regions, outer and inner, where the ends of the shorter and longer chains are located. The derived analytical expression for the chain-conformation free energy implies that the mixing of surfactants with different chainlengths is always nonideal and synergistic, i.e. it leads to decrease of the micellar free energy and to enhancement of micellization and micelle growth. The derived expressions are applicable to surfactants with different headgroups (nonionic, ionic, zwitterionic) and to micelles of different shapes (spherical, wormlike, lamellar). The results can be incorporated in a quantitative theory of the growth of giant mixed micelles in formulations with practical applications in detergency.

Rheological Behavior of Environmentally Friendly Viscoelastic Solutions Formed by a Rosin-Based Anionic Surfactant

It is of great significance to explore novel applications of renewable resources. In this study, a rosin-based anionic surfactant (abbreviated R-11-2-Na), which contains a large hydrophobic group of 30 carbon atoms, was synthesized. R-11-2-Na forms wormlike micelles in the presence of the equimolar organic salt choline chloride, endowing solutions with strong viscoelasticity. The wormlike micellar solutions were investigated using rheology, small-angle X-ray scattering, and freeze-fracture transmission electron microscopy (FF-TEM) methods at 25 °C. Due to the strong van der Waals interactions caused by the large hydrophobic group contained in R-11-2-Na, the zero-shear viscosity (η₀) of solutions showed extremely strong dependence on the concentration with an exponent of 23.4. The cross-sectional diameter of the wormlike micelles in the present system was significantly larger than that of the wormlike micelles formed by surfactants containing conventional alkyl tails. This finding may be attributed to the steric hindrance brought by the bulky and rigid dehydroabietic acid unit in the hydrophobic part. The wormlike micelles also showed high tolerance to the organic salt concentration. The present study reveals the notable qualities of rosin-based derivatives in forming complex fluids and facilitates new utilizations of forest resources.

Getting into Shape: Reflections on a New Generation of Cylindrical Nanostructures' Self-Assembly Using Polymer Building Blocks

Cylinders are fascinating structures with uniquely high surface area, internal volume, and rigidity. On the nanoscale, a broad range of applications have demonstrated advantageous behavior of cylindrical micelles or bottlebrush polymers over traditional spherical nano-objects. In the past, obtaining pure samples of cylindrical nanostructures using polymer building blocks via conventional self-assembly strategies was challenging. However, in recent years, the development of advanced methods including polymerization-induced self-assembly, crystallization-driven self-assembly, and bottlebrush polymer synthesis has facilitated the easy synthesis of cylindrical nano-objects at industrially relevant scales. In this Perspective, we discuss these techniques in detail, highlighting the advantages and disadvantages of each strategy and considering how the cylindrical nanostructures that are obtained differ in their chemical structure, physical properties, colloidal stability, and reactivity. In addition, we propose future challenges to address in this rapidly expanding field.

Probing the dynamic self-assembly behaviour of photoswitchable wormlike micelles in real-time

Understanding the dynamic self-assembly behaviour of azobenzene photosurfactants (AzoPS) is crucial to advance their use in controlled release applications such as drug delivery and micellar catalysis. Currently, their behaviour in the equilibrium cis- and trans-photostationary states is more widely understood than during the photoisomerisation process itself. Here, we investigate the time-dependent self-assembly of the different photoisomers of a model neutral AzoPS, tetraethylene glycol mono(4',4-octyloxy,octyl-azobenzene) (C8AzoOC8E4) using small-angle neutron scattering (SANS). We show that the incorporation of in situ UV-Vis absorption spectroscopy with SANS allows the scattering profile, and hence micelle shape, to be correlated with the extent of photoisomerisation in real-time. It was observed that C8AzoOC8E4 could switch between wormlike micelles (trans native state) and fractal aggregates (under UV light), with changes in the self-assembled structure arising concurrently with changes in the absorption spectrum. Wormlike micelles could be recovered within 60 seconds of blue light illumination. To the best of our knowledge, this is the first time the degree of AzoPS photoisomerisation has been tracked in situ through combined UV-Vis absorption spectroscopy-SANS measurements. This technique could be widely used to gain mechanistic and kinetic insights into light-dependent processes that are reliant on self-assembly.

Viscoelastic micellar solution formed by a Se-based ionic liquid surfactant and its response to redox changes

The incorporation of a Se atom endows the surfactants with redox-sensitivity, and that its site plays a crucial role both in single surfactant solution and viscoelastic micellar solution formed by surfactant and NaSal.

Coherent states field theory in supramolecular polymer physics

In 1970, Edwards and Freed presented an elegant representation of interacting branched polymers that resembles the coherent states (CS) formulation of second-quantized field theory. This CS polymer field theory has been largely overlooked during the intervening period in favor of more conventional "auxiliary field" (AF) interacting polymer representations that form the basis of modern self-consistent field theory (SCFT) and field-theoretic simulation approaches. Here we argue that the CS representation provides a simpler and computationally more efficient framework than the AF approach for broad classes of reversibly bonding polymers encountered in supramolecular polymer science. The CS formalism is reviewed, initially for a simple homopolymer solution, and then extended to supramolecular polymers capable of forming reversible linkages and networks. In the context of the Edwards model of a non-reacting homopolymer solution and one and two-component models of telechelic reacting polymers, we discuss the structure of CS mean-field theory, including the equivalence to SCFT, and show how weak-amplitude expansions (random phase approximations) can be readily developed without explicit enumeration of all reaction products in a mixture. We further illustrate how to analyze CS field theories beyond SCFT at the level of Gaussian field fluctuations and provide a perspective on direct numerical simulations using a recently developed complex Langevin technique.

Added-Value Surfactants

Surfactants are ubiquitous in cellular membranes, detergents or as emulsification agents. Due to their amphiphilic properties, they cannot only mediate between two domains of very different solvent compatibility like water and organic but also show fascinating self-assembly features resulting in micelles, vesicles, or lyotropic liquid crystals. The current review article highlights some approaches towards the next generation surfactants, for example, those with catalytically active heads. Furthermore, it is shown that amphiphilic properties can be obtained beyond the classical hydrophobic-hydrophilic interplay, for instance with surfactants containing one molecular block with a special shape. Whereas, classical surfactants are static, researchers have become more interested in species that are able to change their properties depending on external triggers. The article discusses examples for surfactants sensitive to chemical (e.g., pH value) or physical triggers (temperature, electric and magnetic fields).

Incorporating physiologically relevant mobile phases in micellar liquid chromatography for the prediction of human intestinal absorption

Micellar liquid chromatography is a popular method used in the determination of a compound's lipophilicity. This study describes the use of the obtained micelle-water partition coefficient (log Pmw ) by such a method in the prediction of human intestinal absorption (HIA). As a result of the close resemblance of the novel composition of the micellar mobile phase to that of physiological intestinal fluid, prediction was deemed to be highly successful. The unique micellar mobile phase consisted of a mixed micellar mixture of lecithin and six bile salts, i.e. a composition matching that found in the human intestinal environment, prepared in ratios resembling those in the intestine. This is considered to be the first method to use a physiological mixture of biosurfactants in the prediction of HIA. As a result, a mathematical model with high predictive ability (R2 PRED  = 81%) was obtained using multiple linear regression. The micelle-water partition coefficient (log Pmw ) obtained from micellar liquid chromatography was found to be a successful tool for prediction where the final optimum model included log Pmw and polar surface area as key descriptors with high statistical significance for the prediction of HIA. This can be attributed to the nature of the mobile phase used in this study which contains the lecithin-bile salt complex, thus forming a bilayer system and therefore mimicking absorption across the intestinal membrane.

Enthalpy and Entropy of Scission in Wormlike Micelles

The free scission energy is the thermodynamic parameter that governs the contour length of wormlike micelles (WLMs). It is the contour length and the propensity to coil and entangle that determine the viscoelastic properties of this commercially important substance class. The free scission energy Δ F_sc and the associated change in enthalpy Δ H_sc and entropy Δ S_sc on scission have been determined for a mixed anionic/zwitterionic surfactant system (sodium laureth sulfate and cocamidopropyl betaine) at various salt concentrations (3-5 wt % NaCl). Both enthalpy Δ H_sc and entropy Δ S_sc changes decrease linearly with increasing NaCl concentration. At NaCl concentrations above 4 wt %, Δ S_sc even adopts negative values. The term TΔ S_sc decreases more rapidly than Δ H_sc around room temperature and causes the observed elongation of WLMs upon addition of NaCl. It is suggested that Δ S_sc is initially positive due to fewer bound counterions per surfactant molecule at end caps compared to the intact, cylindrical parts before scission, leading to a net release of ions upon scission. Negative values of Δ S_sc are attributed to hydrophobic hydration occurring at the end caps at high salt concentrations. ²³Na NMR measurements indicate the presence of immobilized ions, supporting a previously proposed ion-cloud model based on neutron scattering results.

Investigation of Active-Inactive Material Interdigitated Aggregates Formed by Wormlike Micelles and Cellulose Nanofiber

In this work, a novel active-inactive material interdigitated aggregates (AIMIAs) structure was constructed by self-assembled wormlike micelles (WLMs) and one-dimensional cellulose nanofiber (CNF). The rheological behaviors and microstructures of the AIMIA systems with different CNF concentrations were investigated by rheometer, cryogenic transmission electron microscopy, and environmental scanning electron microscope. Some key parameters, including zero-shear viscosity (η₀), relaxing time (τ_R), and contour length ( L), were calculated to analyze the changes in the properties of different systems. Meanwhile, a proper mechanism describing the interaction between CNF and WLMs was proposed. Through this work, we expect to deepen the understanding of the AIMIAs structure and widen its application.

Flow fluctuations in wormlike micelle fluids

We investigate the unstable flow of wormlike micelle solutions in pressure driven capillary flow, with a focus on the effect of entrance geometry on the fluid fluctuations. The flow is measured at different points in the capillary using particle image velocimetry while simultaneously measuring the pressure drop across the entire capillary. The fluctuations are characterized by rapid flow rate jumps that correspond with a decrease in the pressure drop followed by a longer recovery period. Velocimetry measurements in the entrance region show a transition to unstable flow above a critical flow rate, where large flow circulations are observed in the tapered geometry and localized jets are observed in an abrupt contraction. The transition to this unstable flow is shown to occur at a similar dimensionless extension rate normalized by the micelle relaxation time. A rapid breakdown in micelle alignment is observed in polarized light microscopy at the onset of the flow rate jump, indicating the importance of rapid micelle structural changes on the fluctuations. We characterize the system by analyzing the power spectral densities and develop a dynamical systems model to describe the relationship between pressure and flow rate. These developments provide understanding to control flow fluctuations and motivation for more detailed study of the coupling of fluid microstructure transitions and flow fluctuations.

Structure-Property Relationships of Oligonucleotide Polyelectrolyte Complex Micelles

Polyelectrolyte complex micelles (PCMs), nanoparticles formed by electrostatic self-assembly of charged polymers with charged-neutral hydrophilic block copolymers, offer a potential solution to the challenging problem of delivering therapeutic nucleic acids into cells and organisms. Promising results have been reported in vitro and in animal models but basic structure-property relationships are largely lacking, and some reports have suggested that double-stranded nucleic acids cannot form PCMs due to their high bending rigidity. This letter reports a study of PCMs formed by DNA oligonucleotides of varied length and hybridization state and poly(l)lysine-poly(ethylene glycol) block copolymers with varying block lengths. We employ a multimodal characterization strategy combining small-angle X-ray scattering (SAXS), multiangle light scattering (MALS), and cryo-electron microscopy (cryo-TEM) to simultaneously probe the morphology and internal structure of the micelles. Over a wide range of parameters, we find that nanoparticle shape is controlled primarily by the hybridization state of the oligonucleotides with single-stranded oligonucleotides forming spheroidal micelles and double-stranded oligonucleotides forming wormlike micelles. The length of the charged block controls the radius of the nanoparticle, while oligonucleotide length appears to have little impact on either size or shape. At smaller length scales, we observe parallel packing of DNA helices inside the double-stranded nanoparticles, consistent with results from condensed genomic DNA. We also describe salt- and thermal-annealing protocols for preparing PCMs with high repeatability and low polydispersity. Together, these results provide a capability to rationally design PCMs with desired sizes and shapes that should greatly assist development of this promising delivery technology.

Oil-induced formation of branched wormlike micelles in an alcohol propoxysulfate extended surfactant system

The addition of oil to an extended surfactant-water system (sodium tetrapropylene glycol (2-ethyl)octyl ether sulfate, C₁₀PO₄SO₄Na) induces the elongation of spherical micelles into oil-swollen branched wormlike micelles (WLMs) near the phase inversion point of the surfactant-oil-water (SOW) system. The hydrophilic-lipophilic-difference (HLD) framework, which has been associated with surfactant curvature, was successfully used to predict the conditions under which WLMs are produced for both polar and non-polar oils. At HLD = 0, the formation of low-curvature surfactant structures including WLMs and liquid crystals are favored in water-rich systems. Micellar growth begins around HLD = -0.5, and reaches a plateau upon the formation of a branched WLM network at HLD = 0. Above the entanglement concentration, the branched WLMs exhibit Maxwell and shear thinning behavior which is suitable for the suspension of nanoparticles, among others.

Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging

The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0-7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice.

Can percolation theory explain the gelation behavior of diblock copolymer worms?

It is well known that polymerization-induced self-assembly (PISA) offers an efficient synthetic route for the production of highly anisotropic diblock copolymer worms. When prepared in aqueous media, such worms form thermoresponsive free-standing hydrogels that are (i) readily sterilizable, (ii) can act as a 3D matrix for the culture of normal mammalian cells and (iii) can induce stasis in human stem cell colonies. Herein we critically examine the gelation behavior of two types of diblock copolymer worms in terms of recent advances in percolation theory for rigid rods, which explicitly account for the effect of rod length polydispersity. More specifically, we use small-angle X-ray scattering (SAXS) to determine the weight-average worm contour length, L w, and the mean worm cross-sectional radius, R. This approach enables a direct comparison to be made between the theoretical critical worm volume fraction, φ c, required for gelation and the experimental values indicated by rheological measurements and tube inversion experiments. Given that these diblock copolymer worms are relatively flexible rather than truly rod-like, reasonably good agreement between these two parameters is observed, particularly for shorter, relatively stiff worms. For longer, more flexible worms a proportionality constant of approximately two is required to reconcile theory with experimental values for φ c. These findings are expected to have important implications for the aqueous gelation behavior exhibited by various other anisotropic nanoparticles, such as cellulose nanocrystals and semicrystalline block copolymer rods, and also fibril-forming small molecule (e.g. dipeptide) gelators.

Structure, rheology, and microrheology of wormlike micelles made of PB-PEO diblock copolymers

A diblock copolymer made of poly(1,4-butadiene)-block-polyethylene oxide, with a degree of polymerization of the polybutadiene and polyethylene oxide blocks of 37 and 57, respectively, self-assembles in water as worm-like micelles determined by small angle neutron scattering with an average diameter of ∼12.7 nm, a core radius of ∼2.7 nm, a shell radius of ∼3 nm, and an estimated persistence length of >225 nm. Worm-like micelles of almost the same diblock copolymer, but with a smaller polyethylene oxide block (degree of polymerization 45) were also measured. The worm-like micelles were also observed with negative staining using low energy electron microscopy. The boundary between dilute and semidilute regimes was estimated to be ∼0.8 wt%. The viscoelastic spectra at low and intermediate frequencies do not follow the Maxwell model. These micelles do not present the same rheological behavior of worm-like micelle solutions of conventional surfactants. The slow dynamics of the self-assembly explains this uncommon behavior for the system. Any micellar rearrangement is impeded due to the extremely high hydrophobicity of the polybutadiene block; stress mainly relaxes by the reptation mechanism. Using diffusive wave spectroscopy, we measured the mean square displacement of particles in the micellar solution. From the mean square displacement, we obtained G'(ω) and G''(ω) at high frequencies. |G*| exhibits a power law behavior showing the stress relaxation changes as frequency increases, first dominated by the Rouse-Zimm modes and then by the bending modes of the Kuhn segments. This allowed us to estimate the worm-like micelle persistence lengths that depend on the copolymer concentration.

Unlocking Structure-Self-Assembly Relationships in Cationic Azobenzene Photosurfactants

Azobenzene photosurfactants are light-responsive amphiphiles that have garnered significant attention for diverse applications including delivery and sorting systems, phase transfer catalysis, and foam drainage. The azobenzene chromophore changes both its polarity and conformation (trans-cis isomerization) in response to UV light, while the amphiphilic structure drives self-assembly. Detailed understanding of the inherent relationship between the molecular structure, physicochemical behavior, and micellar arrangement of azobenzene photosurfactants is critical to their usefulness. Here, we investigate the key structure-function-assembly relationships in the popular cationic alkylazobenzene trimethylammonium bromide (AzoTAB) family of photosurfactants. We show that subtle changes in the surfactant structure (alkyl tail, spacer length) can lead to large variations in the critical micelle concentration, particularly in response to light, as determined by surface tensiometry and dynamic light scattering. Small-angle neutron scattering studies also reveal the formation of more diverse micellar aggregate structures (ellipsoids, cylinders, spheres) than predicted based on simple packing parameters. The results suggest that whereas the azobenzene core resides in the effective hydrophobic segment in the trans-isomer, it forms part of the effective hydrophilic segment in the cis-isomer because of the dramatic conformational and polarity changes induced by photoisomerization. The extent of this shift in the hydrophobic-hydrophilic balance is determined by the separation between the azobenzene core and the polar head group in the molecular structure. Our findings show that judicious design of the AzoTAB structure enables selective tailoring of the surfactant properties in response to light, such that they can be exploited and controlled in a reliable fashion.

Effects of additives on the viscoelastic responses of cationic gemini surfactant solutions

Several additives, including inorganic (NaCl) and organic salts (derivatives of benzoate), were added into aqueous solutions of a gemini cationic surfactant, 2-hydroxypropyl-1,3-bis (myristyldimethylammonium chloride) (abbreviated as 14-3(OH)-14(2Cl)). The mixed systems were investigated using rheological measurement, cryo-TEM and 1H NMR analysis. The results showed that addition of salts induced rich aggregate morphologies in the 14-3(OH)-14(2Cl)/salt systems. The influence of an inorganic salt on the viscoelasticity of 14-3(OH)-14(2Cl) solutions is much weaker than that of organic salts. Furthermore, the ability of three organic salts in enhancing the viscoelasticity of 14-3(OH)-14(2Cl) solutions is in the order sodium m-hydroxybenzoate > sodium o-hydroxybenzoate > sodium p-hydroxybenzoate. The different roles of these organic salt isomers arise from the different types of hydrogen bonding formed between 14-3(OH)-14(2Cl) and the organic counter ions.

Formation of Well-Defined Vesicles by Styrene Addition to a Nonionic Surfactant and Their Polymerization Leading to Viscous Hybrid Systems

Self-assembled structures in aqueous solutions can be fixed by polymerization after adding hydrophobic monomers and can thereby be used as templates which allow to substantially alter the properties of these systems. In this work, we started from a self-assembled micellar system consisting of the nonionic surfactants tetradecyldimethylamine oxid and Pluronic L35 to which styrene was added as a polymerizable monomer. Interestingly, it was observed that styrene induces a transition from micelles to well-defined vesicles in a similar manner as a typical cosurfactant. The structural transition of the aggregates upon styrene addition as well as the structures formed after initiating a polymerization reaction were investigated by means of turbidity, dynamic and static light scattering, small-angle neutron scattering, and rheology measurements. Especially the scattering results confirmed the interesting effect of styrene on the mesoscopic structure and showed a structural evolution from rod-like micelles for low styrene concentrations to vesicles at intermediate styrene amounts, and then finally the formation of microemulsion droplets for high styrene content. Their polymerization of the vesicles again leads to a shape change to wormlike, polymerized aggregates, whose presence then results in rather viscous systems. In contrast, the microemulsions with higher styrene content then are templated and retain their size after polymerization, thereby leading to nanolattices.

Multilamellar Vesicle Formation Probed by Rheo-NMR and Rheo-SALS under Large Amplitude Oscillatory Shear

The formation of multilamellar vesicles (MLVs) in the lyotropic lamellar phase of the system triethylene glycol mono n-decyl ether (C₁₀E₃)/water is investigated under large amplitude oscillatory shear (LAOS) using spatially resolved rheo-NMR spectroscopy and a combination of rheo-small angle light scattering (rheo-SALS) and conventional rheology. Recent advances in rheo-NMR hardware development facilitated the application of LAOS deformations in high-field NMR magnets. For the range of investigated strain amplitudes (10-50) and frequencies (1 and 2 rad s^-1), MLV formation is observed in all NMR and most SALS experiments. It is found that the MLV size depends on the applied frequency in contrast to previous steady shear experiments where the shear rate is the controlling parameter. The onset of MLV formation, however, is found to vary with the shear amplitude. The LAOS measurements bear no indication of the intermediate structures resembling aligned multilamellar cylinders observed in steady shear experiments. Lissajous curves of stress vs strain reveal a transition from a viscoelastic solid material to a pseudoplastic material.

Branching and alignment in reverse worm-like micelles studied with simultaneous dielectric spectroscopy and RheoSANS

Topology and branching play an important but poorly understood role in controlling the mechanical and flow properties of worm-like micelles (WLMs). To address the challenge of characterizing branching during flow of WLMs, dielectric spectroscopy, rheology, and small-angle neutron scattering (dielectric RheoSANS) experiments are performed simultaneously to measure the concurrent evolution of conductivity, permittivity, stress, and segmental anisotropy of reverse WLMs under steady-shear flow. Reverse WLMs are microemulsions comprised of the phospholipid surfactant lecithin dispersed in oil with water solubilized in the micelle core. Their electrical properties are independently sensitive to the WLM topology and dynamics. To isolate the effects of branching, dielectric RheoSANS is performed on WLMs in n-decane, which show fast breakage times and exhibit a continuous branching transition for water-to-surfactant ratios above the corresponding maximum in zero-shear viscosity. The unbranched WLMs in n-decane exhibit only subtle decreases in their electrical properties under flow that are driven by chain alignment and structural anisotropy in the plane perpendicular to the electric field and incident neutron beam. These results are in qualitative agreement with additional measurements on a purely linear WLM system in cyclohexane despite differences in breakage kinetics and a stronger tendency for the latter to shear band. In contrast, the branched micelles in n-decane (higher water content) undergo non-monotonic changes in permittivity and more pronounced decreases in conductivity under flow. The combined steady-shear electrical and microstructural measurements are capable, for the first time, of resolving branch breaking at low shear rates prior to alignment-driven anisotropy at higher shear rates.

Asymmetric cationic gemini surfactants: an improved synthetic procedure and the micellar and surface properties of a homologous series in the presence of simple salts

The micellar and morphological properties of symmetric, cationic gemini surfactants have been well studied in the literature as a function of nature and type of the spacer group and the length and type of hydrophobic chain. In this paper, we have examined the effects of tail asymmetry on the properties of a series of cationic surfactants, the N-alkyl-1-N′-alkyl-2-N,N,N′,N′-tetramethyldiammonium dibromide. A novel synthetic method is used to prepare a series of these surfactants and the consequences of asymmetry on micellar properties are presented. This new method has been shown to be more efficient, with higher yields of the asymmetric surfactants than the yields of the accepted literature method. The critical micelle concentration values and the micelle sizes of the asymmetric gemini surfactants, 12-4-12, 12-4-10, 12-4-8, and 12-4-6 gemini surfactants, were obtained from conductivity and dynamic light scattering. With increasing chain asymmetry, the size of the micelle increased due to the formation of loose micelles. The addition of NaCl and Na2SO4 to the surfactant solutions increased the aggregate size, and this effect was more pronounced with increasing salt concentrations. These results are interpreted in terms of the effect these ions have on the “compactness” of the micelle structure.

Phase Engineering of Hydrophobic Meso-Environments in Silica Particles for Technical Performance Enrichment

Hexadecyltrimethylammonium bromide (CTAB) was utilized to template the growth of mesoporous silica particles via ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in the reaction solutions with varied volume fractions of ethanol ( f_R). The use of 9,10-bis(phenylethynyl) anthracene (BPEA) as a fluorescence probe unraveled a clear difference in interior structure between the CTAB micelles confined at different f_R. At f_R of 0.3, the confined CTAB micelles consisting of regularly and densely packed alkane chains, which created crystalline interiors, in which the doped BPEA molecules were effectively isolated in the monomeric form and well protected against aggressive attack from the surrounding environment. At f_R of 0.4 or 0.5, the confined CTAB micelles consisting of less regularly but densely packed alkane chains created glassy interiors, which enabled reversible aggregation of the doped BPEA in response to the surrounding environmental change, for instance, the ethanol content in the particle dispersion. At f_R of 0.6 or 0.7, the confined CTAB micelles consisting of loosely packed alkane chains created amorphous interiors, which offered sufficiently large free spaces to facilitate the material exchange with the surrounding environment, as evidenced by noticeable intake of the Pyronin Y molecules present in the particle dispersion. The revealed phase modulation of the interiors of surfactant micelles, confined in the pores of mesoporous particles, from crystalline to glassy and amorphous structures, which were scarcely reported in literature, will inspire rational design of mesoporous silica particles with desired technical performance according to the purposes of the practical application.

A responsive anionic wormlike micelle using pH-directed release of stored sodium based on polybasic acids

Responsive wormlike micelles are very useful in a number of applications, whereas it is still challenging to create dramatic viscosity changes in anionic surfactant systems. Here a differential pH-responsive wormlike micelle based on sulfonic surfactants was developed, which is formed by mixing sodium dodecyl trioxyethylene sulphate (SDES) and ethylenediaminetetraacetic acid tetrasodium (EDTA4-·4Na+) at the molar ratio of 1 : 1. The phase behavior, aggregate microstructure and viscoelasticity of the SDES/EDTA4-·4Na+ solution were investigated via macroscopic observation, cryo-TEM and rheological measurements. It was found that the phase behavior of the SDES/EDTA4-·4Na+ solution undergoes transitions from a water-like fluid to viscoelastic upon decreasing the pH. On decreasing the pH from 12.01 to 3.27 by adding HCl, the viscosity of the transparent solutions with wormlike micelles was increased rapidly and reached ∼3100 mPa s. Furthermore, on increasing the pH by adding NaOH, the viscosity was slightly increased due to the addition of Na+. However, the increase in the concentration of Na+ is much smaller than the theoretical addition. The same phenomenon was noted in the sodium citrate solution, but does not exist in the sodium formate system. The viscosity of the micellar solution has a sensitive response to inorganic acids and tolerance to inorganic bases due to the characteristics of polybasic acids.

TEMPO-oxidised cellulose nanofibrils; probing the mechanisms of gelation via small angle X-ray scattering

The structure of dispersions of TEMPO-oxidised cellulose nanofibrils (OCNF), at various concentrations, in water and in NaCl aqueous solutions, was probed using small angle X-ray scattering (SAXS). OCNF are modelled as rod-like particles with an elliptical cross-section of 10 nm and a length greater than 100 nm. As OCNF concentration increases above 1.5 wt%, repulsive interactions between fibrils are evidenced, modelled by the interaction parameter νRPA > 0. This corresponds to gel-like behaviour, where G' > G'' and the storage modulus, G', shows weak frequency dependence. Hydrogels can also be formed at OCNF concentration of 1 wt% in 0.1 M NaCl(aq). SAXS patterns shows an increase of the intensity at low angle that is modelled by attractive interactions (νRPA < 0) between OCNF, arising from the screening of the surface charge of the fibrils. Results are supported by ζ potential and cryo-TEM measurements.

Growth of wormlike micelles of surfactant induced by embedded polymer: role of polymer chain length

Incorporation of polymer chains into wormlike surfactant micelles, which find a large range of applications, offers the opportunity to modify their structure and properties. In this paper, using spectroscopic, scattering and rheological techniques and computer simulations, we study the incorporation of poly(4-vinylpyridine) of two different molecular weights (MWs) into entangled networks of wormlike surfactant micelles of potassium oleate. Using NMR-spectroscopy we show that, independent of its MW, the polymer incorporates into the core-corona interface of the surfactant micelles. According to SANS data, the polymer does not alter the micelle structure or the micelle radius, but diminishes the packing density of the surfactant. At the same time, rheology reveals a stark difference between the surfactant networks with embedded polymers of different MWs. Networks with the higher-MW polymer possess larger viscosity and a longer relaxation time, which we attribute to the larger length of the hybrid micelles. Moreover, we demonstrate that in an intermediate concentration range the higher-MW polymer is able to link neighbouring surfactant micelles together, which has never been previously observed. However, with a further increase in polymer content the micelles become smaller due to the high breaking susceptibility of the boundaries of polymer-containing sections, leading to the stabilization of micellar end-caps by the embedded macromolecules. This process is more prominent in the case of the shorter polymer. Our finding that an increased MW of macromolecules permits the formation of longer hybrid micelles and enhances their rheological properties is of obvious importance for the fundamental understanding of polymer-surfactant interactions and the development of new industrial formulations based on hybrid polymer-wormlike surfactant micelles.

Rheological behavior and mechanism of pH-responsive wormlike micelle variations induced by isomers of phthalic acid

Responsive wormlike micelles (WLMs) constructed by different carboxylic acids are fascinating. However, it is unknown how the position of the carboxylic groups alters the stimuli-response of wormlike micellar systems. Herein, three pH-responsive WLMs based on Gemini-like surfactants (named o-EAPA, m-EAPA, and p-EAPA) were formed and studied through the complexation of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and o-phthalic acid (o-PA), m-phthalic acid (m-PA), or p-phthalic acid (p-PA) at the molar ratio of 2 : 1. The viscoelasticity, phase behavior and aggregate microstructure were separately explored by rheological, appearance observation and cryo-TEM methods. The results show that all phthalic acids can protonate UC22AMPM, thereby forming WLMs. However, with the shorter spacer distance between two carboxyl groups in phthalic acid, o-EAPA exhibits the longer length scale of aggregates and a more efficient thickening ability compared to the other two systems. Similar results in the N,N-dimethyl oleoaminde-propylamine (DOAPA) and o-PA, m-PA, and p-PA systems further verify the applicability of this mechanism. Furthermore, the phthalic acid based WLMs are found to exhibit intriguing reversible pH-responsive behaviors, which include promptly switching between a high elastic system and a low viscosity fluid by pH control. The o-EAPA system possesses a larger viscosity maximum, which produces more precipitous viscosity changes as the pH varies. This study is beneficial for the formation of pH-responsive WLMs and to determine their advantages for applications.