Rheology of branched wormlike micelles

Wormlike Micellar Glycerol Solutions Formed from a Double-Tailed Surfactant with Two Quaternary Ammonium Head Groups

Herein, a quaternary ammonium surfactant with dual heads and tails, N1,N1,N1,N3,N3-pentamethyl-N3-(3-(2-tetradecylhexadecanamido)propyl)propane-1,3-diaminium dibromide, abbreviated as Di-C14-N2, was synthesized. For the first time, clear observation of aggregate structures formed by surfactants in pure glycerol systems was achieved using cryogenic transmission electron microscopy (cryo-TEM). The system's rheological properties were analyzed using both steady-state shear and oscillatory rheological measurements. The lubricating efficiency of the Di-C14-N2 glycerol solution was assessed for its tribological properties using a tribological wear tester, white light interferometer, and scanning electron microscope. In glycerol, Di-C14-N2 formed long wormlike micelles, which resulted in a glycerol solution with the zero-shear viscosity of 1013 Pa·s at 90 mM, which is the most viscous glycerol system up to now. The system displayed distinct rheological properties from the aqueous system, as evidenced by two intersections in the loss and storage moduli. The formed wormlike micelles in glycerol lead to a significant alteration in the viscoelasticity of the system, thus endowing the Di-C14-N2 glycerol solution with potential as an eco-friendly lubricant. The friction coefficient of the system was found to be 23% lower and the wear rate was 83% lower than that of pure glycerol after the addition of Di-C14-N2. This demonstrates that the addition of Di-C14-N2 greatly improves the frictional properties of pure glycerol. This study offers the possibility of directly observing the aggregate structures formed by surfactants in pure glycerol systems. It contributes to the exploration of the self-assembly behavior of surfactants in nonaqueous polar media, thereby aiding in a deeper understanding of the correlation between molecular structure, mesoscale structure, and macroscopic properties.

Small-angle scattering of complex fluids in flow

Complex fluids encompass a significant proportion of the materials that we use today from feedstocks such as cellulose fibre dispersions, materials undergoing processing or formulation, through to consumer end products such as shampoo. Such systems exhibit intricate behaviour due to their composition and microstructure, particularly when analysing their texture and response to flow (rheology). In particular, these fluids when flowing may undergo transitions in their nano- to microstructure, potentially aligning with flow fields, breaking and reassembling or reforming, or entirely changing phase. This manifests as macroscopic changes in material properties, such as core-annular flow of concentrated emulsions in pipelines or the favourable texture of liquid soaps. Small-angle scattering provides a unique method for probing underlying changes in fluid nano- to microstructure, from a few angströms to several microns, of complex fluids under flow. In particular, the alignment of rigid components or shape changes of soft components can be explored, along with local inter-particle ordering and global alignment with macroscopic flow fields. This review highlights recent important developments in the study of such complex fluid systems that couple flow or shear conditions with small-angle scattering measurements, and highlights the physical insight obtained by these experiments. Recent results from neutron scattering measurements made using a simple flow cell are presented, offering a facile method to explore alignment of complex fluids in an easily accessible geometry, and contextualised within existing and potential future research questions.

Coacervate Droplets for Synthetic Cells

The design and construction of synthetic cells - human-made microcompartments that mimic features of living cells - have experienced a real boom in the past decade. While many efforts have been geared toward assembling membrane-bounded compartments, coacervate droplets produced by liquid-liquid phase separation have emerged as an alternative membrane-free compartmentalization paradigm. Here, the dual role of coacervate droplets in synthetic cell research is discussed: encapsulated within membrane-enclosed compartments, coacervates act as surrogates of membraneless organelles ubiquitously found in living cells; alternatively, they can be viewed as crowded cytosol-like chassis for constructing integrated synthetic cells. After introducing key concepts of coacervation and illustrating the chemical diversity of coacervate systems, their physicochemical properties and resulting bioinspired functions are emphasized. Moving from suspensions of free floating coacervates, the two nascent roles of these droplets in synthetic cell research are highlighted: organelle-like modules and cytosol-like templates. Building the discussion on recent studies from the literature, the potential of coacervate droplets to assemble integrated synthetic cells capable of multiple life-inspired functions is showcased. Future challenges that are still to be tackled in the field are finally discussed.

Stimuli-responsive viscosity modifiers

Stimuli responsive viscosity modifiers entail an important class of materials which allow for smart material formation utilizing various stimuli for switching such as pH, temperature, light and salinity. They have seen applications in the biomedical space including tissue engineering and drug delivery, wherein stimuli responsive hydrogels and polymeric vessels have been extensively applied. Applications have also been seen in other domains like the energy sector and automobile industry, in technologies such as enhanced oil recovery. The chemistry and microstructural arrangements of the aqueous morphologies of dissolved materials are usually sensitive to the aforementioned stimuli which subsequently results in rheological sensitivity as well. Herein, we overview different structures capable of viscosity modification as well as go over the rheological theory associated with classical systems studied in literature. A detailed analysis allows us to explore correlations between commonly discussed models such as molecular packing parameter, tube reptation and stress relaxation with structural and rheological changes. We then present five primary mechanisms corresponding to stimuli responsive viscosity modification: (i) packing parameter modification via functional group conditioning and (ii) via dynamic bond formation, (iii) mesh formation by interlinking of network nodes, (iv) viscosity modification by chain conformation changes and (v) viscosity modification by particle jamming. We also overview several recent examples from literature that employ the concepts discussed to create novel classes of intriguing stimuli responsive structures and their corresponding rheological properties. Furthermore, we also explore systems that are responsive to multiple stimuli which can provide enhanced functionality and versatility by providing multi-level and precise actuation. Such systems have been used for programmed site-specific drug delivery.

Parabolic Viscosity Behavior of NaCl-Thickened Surfactant Systems upon Temperature Change

The viscosity of household care products plays an important role in pleasant delivery using consumer experience at home. A novel solution to mitigate the sharp rising of viscosities at low temperatures of detergents was proposed. By designing the formulation of the surfactant blend, formulators can achieve acceptable viscosity profiles in the temperature range encountered in daily life. The verification and modulation of formulas bearing parabolic viscosity-temperature behavior were systematically studied, including in single, binary, and ternary systems, based on the modulation of sodium ethoxylated alkyl sulfate (AES) by other anions, zwitterions, and nonions. The R ratio theory was used to have a better understanding of the molecular assembly of surfactants behind the parabolic behavior exhibited in rheology analyses. One of the key findings is that the parabolic viscosity-temperature phenomenon could be easily observed in the highly hydrated ethoxylated anionic systems like AES-based systems. For those anions lacking ethoxylation, especially sodium linear alkylbenzene sulfonate (LAS), the monotonic variation of hydration affinity with temperature led to the disappearance of parabola in the observed temperature window (>0 °C). Moreover, salinity played an important role in the hydration affinity of the polar group and the interaction between the hydrophilic headgroups. A balanced salinity should be optimized to modulate the hydration affinity in a desired range so that the parabola could be easily tuned within the target temperature region. These findings provide opportunities for the formulators in the household care industry to design products with better pourability through carefully selecting a combination of surfactants and fine-tuning their ratios to improve consumer use experience, especially in winter.

Rheological dynamics and structural characteristics of supramolecular assemblies of β-cyclodextrin and sulfonic surfactants

Cyclodextrins are highly functional compounds with a hydrophobic cavity capable of forming supramolecular inclusion complexes with various classes of molecules including surfactants. The resultant rich nanostructures and their dynamics are an interesting research problem in the area of soft condensed matter and related applications. Herein, we report novel dynamical supramolecular assemblies based on the complexation of β-cyclodextrin with 3 different sulfonic surfactants, which are sodium hexadecylsulfate, sodium dodecylbenzenesulfonate, and myristyl sulfobetaine. It was observed that a β-cyclodextrin : surfactant/2 : 1 molar ratio was ideal for inducing axial growth and imparting large viscosities in the suspensions. Such complexation processes were accompanied by intriguing nanostructural phase behaviors and rheological properties that were very sensitive to the molecular architecture of sulfonic surfactants. The presence of an amino group in the head group of the surfactant allowed for large viscosities that reached 2.4 × 10⁴ Pa s which exhibited gel-like behavior. In contrast, smaller viscosity values with a lower consistency index were observed when a bulky aromatic ring was present instead. DIC microscopy was used to visually probe the microstructure of the systems with respect to sulfonate molecular architecture. Additionally, surface tension measurements, and FTIR and NMR spectroscopies were used to gain insights into the nature of interactions that lead to the complexation and nanostructural characteristics. Finally, mechanics correlating the supramolecular morphologies to the rheological properties were proposed.

Impacts of iron rust particle and weak alkalinity on surfactant micelle structure and drag reduction ability

The surfactant drag reduction effect fades away in practical pipe flow mainly due to the weakly alkaline environment and existence of iron rust in solution. Based on our previous work, this phenomenon is further investigated in the present study. Our results show that both impacts of weak alkalinity and iron rust on micelle structure cause a decrease in the surfactant drag reduction ability. The impact of weak alkalinity on the micelle structure is much more direct and powerful compared with that of iron rust, and consequently, there is a big difference in the destroying degrees of the two impacts on the surfactant drag reduction ability. When the factors of weak alkalinity and iron rust coexist in solution, their impacts on the surfactant micelle structure and drag reduction are mutually independent due to their different impact mechanisms. Finally, the methods for eliminating the impact of weak alkalinity and iron rust on surfactant drag reduction performance are provided.

Transferring Micellar Changes to Bulk Properties via Tunable Self-Assembly and Hierarchical Ordering

Hierarchical self-assembly is an effective means of preparing useful materials. However, control over assembly across length scales is a difficult challenge, often confounded by the perceived need to redesign the molecular building blocks when new material properties are needed. Here, we show that we can treat a simple dipeptide building block as a polyelectrolyte and use polymer physics approaches to explain the self-assembly over a wide concentration range. This allows us to determine how entangled the system is and therefore how it might be best processed, enabling us to prepare interesting analogues to threads and webs, as well as films that lose order on heating and "noodles" which change dimensions on heating, showing that we can transfer micellar-level changes to bulk properties all from a single building block.

pH and light dual stimuli-responses of mixed system of 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecyl ammonium bromide) and trans-ortho-hydroxyl cinnamic acid

Stimuli-responsive smart supramolecular self-assembly with controllable morphology and adjustable rheological property has attracted widespread concern of scientists in recent years due to the great potential application in microfluidics, controlled release, biosensors and so on. In this study, a pH and UV light dual stimuli-responsive system was constructed by combining Gemini surfactant 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecyl ammonium bromide) (12-3(OH)-12·2Br^-) with trans-ortho-hydroxyl cinnamic acid (trans-OHCA) in aqueous solution. The phase behavior and stimuli-responsive behavior of the system including the microstructural transition, rheological property, intermolecular interaction, and isomerization reaction were explored by various experiment techniques such as rheometer, UV-vis spectrum, polarized optical microscopy (POM), transmission electron microscopy (TEM), dynamic light scattering (DLS) as well as theoretical calculation. The system displays abundant phase behaviors that supramolecular self-assemblies of different morphologies in different states such as spherical micelle, wormlike micelle, lamellar liquid crystal, and aqueous two phase system (ATPS) were observed even at lower concentration, which provide the research basis on the abundant stimuli-responsiveness of the system. The results prove that the multiple ionization and the photo-isomerization of trans-OHCA endow the system with plentiful responses to pH and UV light stimuli. It is expected that this study on the dual stimuli-responsive system with abundant self-assembly behaviors and adjustable rheological behaviors would be of theoretical and practical importance, which would provide essential guidance in designing and constructing smart materials with multiple stimuli-responses.

Topological Dynamics of Micelles Formed by Geometrically Varied Surfactants

The molecular architecture of sugar-based surfactants strongly affects their self-assembled structure, i.e., the type of micelles they form, which in turn controls both the dynamics and rheological properties of the system. Here, we report the segmental and mesoscopic structure and dynamics of a series of C16 maltosides with differences in the anomeric configuration and degree of tail unsaturation. Neutron spin-echo measurements showed that the segmental dynamics can be modeled as a one-dimensional array of segments where the dynamics increase with inefficient monomer packing. The network dynamics as characterized by dynamic light scattering show different relaxation modes that can be associated with the micelle structure. Hindered dynamics are observed for arrested networks of worm-like micelles, connected to their shear-thinning rheology, while nonentangled diffusing rods relate to Newtonian rheological behavior. While the design of novel surfactants with controlled properties poses a challenge for synthetic chemistry, we demonstrate how simple variations in the monomer structure can significantly influence the behavior of surfactants.

Wormlike micelles of CTAB with phenols and with the corresponding phenolate derivatives - When hydrophobicity and charge drive the coacervation

HYPOTHESIS

Hydrophobicity and the presence or absence of charge in phenol derivatives are relevant on the rheology and phase behavior when they are assembled with a cationic surfactant, forming wormlike micelles. The incorporation of phenols with a greater number of rings into the micellar palisade is entropically favored, but a solubilization limit or coacervation are two paths followed by the solutions, depending on the electrical nature of the aromatic co-solutes.

EXPERIMENTS

The investigations were carried out with systems formed by a fixed concentration of hexadecyltrimethylammonium bromide (CTAB) and increasing concentrations of neutral phenols (1-naphthol, 2-naphthol, 2,3-dihydroxynaphthalene and R and S-binol) and with their corresponding phenolate derivatives. The monophasic limits of the systems were established, as well as their linear and non-linear rheology. The structural investigation of the coacervates formed with the phenolates were done using SAXS and Cryo-TEM.

FINDINGS

The zero-shear viscosity of the solutions reaches maxima values close to the solubility limit of the aromatics, which depends on the numbers of rings and hydroxyl groups (position and number). However, when the correspondent ionized phenols were investigated, beyond the maxima values for the zero-shear viscosity, liquid-liquid biphasic systems are formed, in which the upper phase contains a coacervate, associated with branched wormlike micelles. However, when the ratio between phenolate and CTAB is around 3:1 the coacervate evolves to a lamellar structure.

Scission energy and topology of micelles controlled by the molecular structure of additives

We employ coarse-grained (CG) molecular dynamics simulations (MD) to investigate the effects of the molecular structure of additives on the scission energy and morphology of charged micelles. Considering sodium dodecyl sulfate (SDS) as a representative charged surfactant and taking trimethylphenylammonium chloride (TMPAC) and octyltrimethylammonium bromide (OTAB) as oppositely charged additives, we show that the scission energy and topology of micelles vary significantly depending on the molecular structure of the hydrophobic part of the additives. The cyclic aromatic tail of the TMPAC disrupts the core structure of the SDS micelle and hence decreases the micelle scission energy, whereas the linear alkyl tail of the OTAB packs very well with the micelle core and increases the scission energy. Although both the additives have similar head structures, they lead to very different micelle morphologies because of the difference in the shape of their tail structures; ring-like or toroidal shaped micelles are formed in SDS/TMPAC solution whereas bicelle-like structures are formed in SDS/OTAB solution when the additive to surfactant ratio is higher than a certain value.

The preparation of hydroxyapatite nanowires and nanorods via aliphatic micelles as soft templates

Hydroxyapatite nanoparticles were tunably synthesized via the use of an aliphatic–ethanol–water three-phase mixture system using micelles as soft templates via an emulsion–hydrothermal synergistic method.

Strong Viscosity Increase in Aqueous Solutions of Cationic C22-Tailed Surfactant Wormlike Micelles

The viscoelastic properties and structure parameters have been investigated for aqueous solutions of wormlike micelles of cationic surfactant erucyl bis(hydroxyethyl) methylammonium chloride with long C22 tail in the presence inorganic salt KCl. The salt content has been varied to estimate linear to branched transition conditions due to screening of the electrostatic interaction in the networks. The local cylindrical structure and low electrostatic repulsion was obtained by SANS data. The drastic power law dependencies of rheological properties on surfactant concentrations were obtained at intermediate salt content. Two power law regions of viscosity dependence were detected in semi-dilute solutions related to “unbreakable” and “living” micellar chains. The fast contour length growth with surfactant concentration demonstrated that is in good agreement with theoretical predictions.

Photoresponsive Viscoelastic Solutions Based on Chiral Wormlike Micelles in Mixed Solutions Containing an Amphiphile Derived from Rosin

A novel rosin-based photoresponsive anionic amphiphile, sodium N-azophenyl maleopimaric acid imide carboxylate (AzoMPCOONa), has been successfully synthesized. Its molecular structure was characterized by ¹H and ¹³C NMR and mass spectrometry (MS). The photoisomerization of AzoMPCOONa was evaluated by ultraviolet (UV)-visible spectrometry and ¹H NMR. The structure of AzoMPCOONa could be converted between the trans and cis isomers by irradiation with UV/visible light. Importantly, a fascinating photoresponsive viscoelastic solution was prepared by mixing AzoMPCOONa and cetyltrimethylammonium bromide (CTAB). The properties of the photoresponsive viscoelastic solution were further investigated by rheology, circular dichroism (CD), and cryogenic transmission electron microscopy (cryo-TEM). Initially, the AzoMPCOONa/CTAB system was a gel-like solution composed of entangled wormlike micelles possessing the right-handed chiral structure. After UV irradiation for 10 min, the gel-like solution transformed into a slightly viscous solution, its zero-shear viscosity dramatically reduced by 2 orders of magnitude, and the aggregates were converted into rod-like micelles and spherical micelles. In addition, the right-handed chiral structure of the aggregates disappeared. These dramatic changes in the viscosity and the aggregate structure can be attributed to the photoisomerization of the azobenzene group in AzoMPCOONa, which led to changes in the molecular geometry and the packing parameter of the AzoMPCOONa/CTAB system. Interestingly, the right-handed chiral structure of wormlike micelles also is photoresponsive. The results reveal the superiority of forest resources for preparing viscoelastic solutions.

Cryo-TEM and rheological study on shear-thickening wormlike micelles of zwitterionic/anionic (AHSB/SDS) surfactants

HYPOTHESIS

Shear-thickening micelles were mostly made of cationic surfactants, but shear-thickening was rarely reported in the zwitterionic/anionic surfactants. Since wormlike micelles were essential in shear-thickening systems, it should be common for the hybrid wormlike micelles formed by zwitterionic/anionic surfactants, and their fundamental features need to be clarified.

EXPERIMENTS

The micellization of zwitterionic surfactant homologies alkyl dimethyl amidopropyl hydroxyl sulfobetaine (AHSB) and sodium dodecyl sulfate (SDS) in brine was studied, and various environmental factors were considered systematically. Light scattering, rheology, zeta potential, ¹H NMR and cryo-TEM techniques were employed to characterize the AHSB/SDS wormlike micelles.

FINDINGS

AHSB/SDS hybrid wormlike micelles were formed in a wide x_SDS region to endow them with apparent viscosities, in which the electrostatic and hydrophobic interactions between AHSB and SDS molecules were critical. AHSB with the longer tail, the higher c_AHSB and c_NaCl were advantageous to enhance the viscosity because of the longitudinal growth of wormlike micelles. The shear-thickening AHSB/SDS samples were commonly composed of unbranched wormlike micelles with various length, and the shear-induced alignment of wormlike micelles was the major cause as verified by cryo-TEM. Moreover, the quantitative relationships on the critical shear rate ɣ̇_c were established, and the activation energies were obtained from the temperature-dependent ɣ̇_c.

Networks of Micellar Chains with Nanoplates

Abstract

Nanocomposite networks of surfactant micellar chains and natural bentonite clay nanoplates are studied by rheometry, small-angle neutron scattering, and cryogenic transmission electron microscopy. It is shown that, in an aqueous medium in the presence of a small part of an anionic surfactant, sodium dodecyl sulfate, the molecules of a biodegradable zwitterionic surfactant, oleyl amidopropyl dimethyl carboxybetaine, form micron-length living micellar chains which entangle and form a network possessing well-defined viscoelastic properties. It is found that addition of negatively charged clay nanoplates leads to an increase in viscosity and relaxation time by an order of magnitude. This is explained by the incorporation of the nanoplates into the network as physical multifunctional crosslinks. The incorporation occurs via the attachment of semispherical end-caps of the micelles to the surface of the particles covered with a surfactant layer, as visualized by cryogenic transmission electron microscopy. As the amount of nanoplates is increased, the rheological properties reach plateau; this is associated with the attachment of all end parts of micelles to nanoplates. The developed nanocomposite soft networks based on safe and eco-friendly components are promising for various practical applications.

Effect of the Substituent Position on the Phase Behavior and Photoresponsive Dynamic Behavior of Mixed Systems of a Gemini Surfactant and trans-Methoxy Sodium Cinnamates

Mixed systems of the Gemini cationic surfactant trimethylene-1,3-bis (dodecyldimethylammonium bromide) (12-3-12·2Br^-) and the photosensitive additives trans-methoxy sodium cinnamates with different substituent positions (trans-ortho-methoxy cinnamate, trans-OMCA; trans-meta-methoxy cinnamate, trans-MMCA; and trans-para-methoxy cinnamate, trans-PMCA) were selected for investigating the effects of the substituting position of methoxy on the system phase diagram and UV light-responsive behavior of the wormlike micelles. The differences in phase behaviors of the selected systems were analyzed by calculating the potential distribution, molecular volume, and free energy of solvation of cinnamates and the binding energies between photosensitive additives and the surfactant. The photoresponsive behaviors of wormlike micelle solutions formed in the selected systems were studied by the rheological method and UV-vis and H nuclear magnetic resonance (¹H NMR) spectroscopy; the kinetics of photoisomerization of trans-OMCA, trans-MMCA, and trans-PMCA were studied by first-order derivative spectrophotometry. The results reveal that the methoxy substituent position has a great influence on the phase behavior and photosensitivity of the studied systems. In addition, the photoisomerization of the studied cinnamates follows the first-order opposite reaction laws; the different reaction rates play the decisive role in the photosensitivity of the wormlike micelles. This paper would afford a deeper understanding of the UV light-responsive mechanism at the molecular level and provide essential guidance in preparing smart materials with adjustable light sensitivity.

Disruption of Cationic/Anionic Viscoelastic Surfactant Micellar Networks by Hydrocarbon as a Basis of Enhanced Fracturing Fluids Clean-Up

Studies of the effects produced by the solubilization of hydrophobic substances by micellar aggregates in water medium are quite important for applications of viscoelastic surfactant solutions for enhanced oil recovery (EOR), especially in hydraulic fracturing technology. The present paper aims at the investigation of the structural transformations produced by the absorption of an aliphatic hydrocarbon (n-decane) by mixed wormlike micelles of cationic (n-octyltrimethylammonium bromide, C8TAB) and anionic (potassium oleate) surfactants enriched by C8TAB. As a result of contact with a small amount (0.5 wt%) of oil, a highly viscoelastic fluid is transformed to a water-like liquid. By small-angle neutron scattering (SANS) combined with cryo-TEM, it was shown that this is due to the transition of long wormlike micelles with elliptical cross-sections to ellipsoidal microemulsion droplets. The non-spherical shape was attributed to partial segregation of longer- and shorter-tail surfactant molecules inside the surfactant monolayer, providing an optimum curvature for both of them. As a result, the long-chain surfactant could preferably be located in the flatter part of the aggregates and the short-chain surfactant—at the ellipsoid edges with higher curvature. It is proven that the transition proceeds via a co-existence of microemulsion droplets and wormlike micelles, and upon the increase of hydrocarbon content, the size and volume fraction of ellipsoidal microemulsion droplets increase. The internal structure of the droplets was revealed by contrast variation SANS, and it was shown that, despite the excess of the cationic surfactant, the radius of surfactant shell is controlled by the anionic surfactant with longer tail. These findings open a way for optimizing the performance of viscoelastic surfactant fluids by regulating both the mechanical properties of the fluids and their clean-up from the fracture induced by contact with hydrocarbons.

Dirhamnolipid ester - formation of reverse wormlike micelles in a binary (primerless) system

We report new dirhamnolipid ester forming reverse wormlike micelles in nonpolar solvents without the addition of any primer. Therefore, these compounds represent a rare case of a binary system showing this gel-like behavior. In this study, the influence of the concentration of the rhamnolipid ester and the ester alkyl chain length on the rheological properties of the reverse wormlike micelles in toluene was investigated in detail. Highly viscoelastic solutions were obtained even at a relatively low concentration of less than 1 wt %. The phase transition temperatures indicate that the formation of reverse wormlike micelles is favored for dirhamnolipid esters with shorter alkyl chain lengths. Oscillatory shear measurements for the viscoelastic samples reveal that the storage modulus (G') and the loss modulus (G'') cross each other and fit the Maxwell model very well in the low-ω region. As is typical for wormlike micelle systems, the normalized Cole-Cole plot of G''/G'' max against G'/G'' max was obtained as a semicircle centered at G'/G'' max = 1. The formation of network structures was also verified by polarized light microscopy. The sample was birefringent at ambient temperature and anisotropic at an elevated temperature. Differential scanning calorimetry analysis yielded a transition enthalpy of about ΔH SG/GS = ±7.2 kJ/mol. This value corresponds to a strong dispersion energy and explains the formation of the highly viscous gels by the entanglement of wormlike micelles through the interaction of the alkyl chains.

Molecular Dynamics Simulations and Density Functional Theory on Unraveling Photoresponsive Behavior of Wormlike Micelles Constructed by 12-2-12·2Br- and trans-ortho-Methoxy Cinnamate

Photoresponsive systems with controllable self-assembly morphologies and adjustable rheological properties have attracted widespread interest by researchers in the past few years. Among them, the photoresponsive systems consisting of ortho-methoxycinnamic (OMCA) and Gemini surfactants are endowed with rich self-assemblies with different states and in different scales including spherical micelles, wormlike micelles, vesicles, aqueous two-phase system (ATPS), etc. All these self-assemblies display excellent photoresponsive behavior. However, the mechanism of these photoresponsive behaviors has not been unraveled systematically so far. In this study, molecular dynamics (MD) simulations, density functional theory (DFT) calculations, transmission electron microscopy, and rheology are employed to investigate the photoresponsive behaviors of wormlike micelles caused by photoisomerization of trans-OMCA in 12-2-12·2Br^-/trans-OMCA solutions and to unravel the underlying mechanisms of these photoresponsive behaviors. The experimental results show that 12-2-12·2Br^-/trans-OMCA micelles display photoresponsiveness after UV-light irradiation, with the transformation of micellar morphologies from wormlike micelle to spherical micelles. In MD simulations, certain micelle morphologies in experiments and the specific packing between 12-2-12·2Br^-/OMCA were successfully captured. The larger three-dimensional structure and steric hindrance of cis-OMCA disturb the interior structure of micelles. The stronger hydrophilicity of cis-OMCA induces the escape of cis-OMCA from the interval of micelles to the solution. The energy results prove that trans-OMCA associates more strongly with 12-2-12·2Br^- than cis-OMCA. These causes lead to the fission and repacking of wormlike micelles.

Density Field Thermodynamic Integration (DFTI): A "Soft" Approach to Calculate the Free Energy of Surfactant Self-Assemblies

Thermodynamic integration is one of the most established methods to quantify excess free energies between different metastable states. Excess intermolecular interactions in surfactant assemblies are on the scale of the energy of thermal fluctuations. Therefore, these materials can be deformed and topologically altered via relatively small mechanical stresses. It is thus intuitive to design reaction paths and associated order parameters that exploit the "soft" nature of these materials to mechanically rather than alchemically morph surfactant assemblies from state to state. Here, we propose a novel method coined "density field thermodynamic integration" (DFTI) that adopts the universality and transferability of alchemical methods while simultaneously exploiting the soft excess interactions between surfactant molecules. DFTI was designed for a rapid quantification of the free energy differences between different metastable structures in soft fluid materials. The DFTI method uses an external field coupled to the local density to mechanically morph the system between metastable states of interest. Here, we explored the capability of the DFTI method to swiftly and accurately calculate free energy differences between states. To this aim, we studied two different coarse-grained lipidic surfactant systems: (i) a fusion stalk and (ii) a worm-like micelle. Our results illustrate that DFTI can provide an efficient, versatile, and rather reliable method to calculate the free energy differences between surfactant assemblies.

Effect of residual chemicals on wormlike micelles assembled from a C22-tailed cationic surfactant

HYPOTHESIS

Ultra-long-chain surfactants, particularly C₂₂-tailed ones, have attracted considerable attention because of their ease of self-assembly into wormlike micelles (WLMs). Commercial C₂₂-tailed surfactants often contain non-negligible amounts of chemical residues introduced during their production. Since the noncovalent driving force of wormlike self-assembly can be greatly affected by the composition, we hypothesized that the residual chemicals could play a significant role in tuning the micelle microstructure and macroscopic properties of the surfactants.

EXPERIMENTS

To confirm this hypothesis, a highly pure (>99%) C₂₂-tailed cationic surfactant, N-erucylamidopropyl-N,N,N-trimethylammonium iodide (EDAI) was synthesized, and various amounts of corresponding reactants (iodomethane or N-erucamidopropyl-N,N-dimethylamine) or solvents (acetone) commonly used in surfactant synthesis were introduced as residues. The impact of each individual residue on the macroscopic appearances, rheological properties, and micelle morphology of the surfactant solution were investigated.

FINDINGS

Increasing the residue fraction in the EDAI solution resulted in an initial increase, followed by a dramatic drop in solution viscosity. This behavior was described in terms of micellar structural transformations based on analysis of cryo-TEM observations and surface tension measurements. These findings are of crucial importance in understanding the sophisticated behaviors of WLMs and will benefit the industrial preparation of ultra-long-chain surfactants for commercial use.

Oil-Induced Viscoelasticity in Micellar Solutions of Alkoxy Sulfate

Rheological properties of the solution of an extended surfactant, sodium alkoxy sulfate (C₈-(PO)₄-(EO)₁-SO₄Na), are investigated as a function of the presence of various paraffinic oils over a range of salt conditions in the Winsor III microemulsion region at oil fractions where the microemulsion is "oil-starved". The addition of as small as 3 vol % alkane to 2 wt % surfactant solutions at salt concentrations where the oil-water interfacial tension is minimized induces a sudden shift in the rheological behavior. The solution viscosity increases by 5 orders of magnitude, with solid-like behaviors (G' > G″) being observed in the entire frequency region investigated (0.01-100 rad/s). Commonly, in the cases where wormlike micelles are present in the solution, alkanes are believed to be solubilized in the core of micelles, leading to a radial growth of the cylindrical part of the wormlike micelle, resulting in a drop of end-cap energy (E_C) and micelle length and a reduction in viscosity. In this study, however, the addition of oil causes the formation of wormlike micelles. The viscosity of solubilized-oil samples does, however, decrease with an increase in incorporated oil volume. We hypothesize that this "abnormal oleo-responsive" viscoelastic behavior is related to a spacer of intermediate hydrophilicity, that is, polypropylene oxide (PO) segment of the alkoxy sulfate, being inserted between the hydrophobic tail and hydrophilic head (the ethoxylated sulfate segment) of the extended surfactant. The addition of a small amount of oil likely extends the PO moiety and increases the tail length of the surfactant in the aggregates as well as reducing the headgroup size, driving the formation of wormlike micelles from a solution that initially had a viscosity consistent with the absence of such structures.

Wormlike Micellar Solutions, Beyond the Chemical Enhanced Oil Recovery Restrictions

While traditional oil recovery methods are limited in terms of meeting the overall oil demands, enhanced oil recovery (EOR) techniques are being continually developed to provide a principal portion of our energy demands. Chemical EOR (cEOR) is one of the EOR techniques that shows an efficient oil recovery factor in a number of oilfields with low salinity and temperature ranges. However, the application of cEOR under the harsh conditions of reservoirs where most of today’s crude oils come from remains a challenge. High temperatures, the presence of ions, divalent ions, and heterogeneous rock structures in such reservoirs restrict the application of cEOR. Polymer solutions, surfactants, alkaline-based solutions, and complex multi-components of them are common chemical displacing fluids that failed to show successful recovery results in hostile conditions for various reasons. Wormlike micellar solutions (WMS) are viscoelastic surfactants that possess advantageous characteristics for overcoming current cEOR challenges. In this study, we first review the major approaches and challenges of commonly used chemical agents for cEOR applications. Subsequently, we review special characteristics of WMS that make them promising materials for the future of cEOR.

Thermodynamic insights and molecular environments into catanionic surfactant systems: Influence of chain length and molar ratio

HYPOTHESIS

Imidazolium-based Ionic liquids as new generation cationic surfactants can provide designable alkyl chain length. In the catanionic surfactant systems, the alkyl chain lengths and molar ratios can greatly influence the interactions such as electrostatic and hydrophobic interaction. The variation in these interactions has a significant effect on the molecular environments of the self-assembly structure, and this process is always accompanied by the transition of aggregates and release or consumption of heat. Hence, it is of interest to study the relationship between intermolecular interactions, molecular environments, self-assembly structure and the change in energy of system in the catanionic surfactant mixed systems.

EXPERIMENTS

The enthalpy change ΔH of titrations the imidazolium-based into SDS micelle solution was studied to characterize the heat by using isothermal titration calorimetry (ITC) during the transitions of the aggregate structures. The corresponding self-assembly structure was monitored via cryogenic transmission electron microscopy (cryo-TEM). Employing proton magnetic resonance (¹H NMR), we focus on the interactions between imidazolium-based ILs and SDS based on the variations in the molecular environments of aggregates.

FINDINGS

Of these imidazolium-based ionic liquids/SDS system, the 1-octyl-3-methylimidazolium ([OMIM]Cl)/SDS system shows several features such as intense energy absorption and releasing processes, which indicate the formation of high entanglement wormlike micelles and vesicles. This is related to the formation of self-adjusting state between the SDS and [OMIM]Cl molecules due to the balance between the electrostatic interaction and hydrophobic interaction. Varying the alkyl chain length appears to cause significant differences to the molecular environments. From the molecular environments, three different models about the polarity of the catanionic surfactant molecules are used to explain the balance of the intermolecular interactions.

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.

Enhanced stability and high temperature-tolerance of CO2 foam based on a long-chain viscoelastic surfactant for CO2 foam flooding

CO₂ switchable foams have gained increasing attention recently for their smart properties. However, their performance at high temperature and high pressure has been less documented. In this study, a long-chain viscoelastic surfactant, N1-(3-aminopropyl)-N3-octadecylpropane-1,3-diamine bicarbonate (ODPTA) has been studied as a CO₂ foam agent for its application in CO₂ flooding in complex and harsh reservoir conditions, and the foam performance under static and dynamic conditions was tested up to 160 °C and 10.5 MPa using a visualized foam-meter and in sand-pack flooding experiments. The viscosity of the ODPTA and conventional surfactant solutions saturated with dissolved CO₂ was measured using a long coiled-tube viscometer at HTHP, and its effect on the high temperature-tolerance of CO₂ foams has been analyzed. The experimental results show that CO₂ foam generated using ODPTA is much more stable than the conventional surfactants (such as SDS and alkylphenol ethoxylates) and has high temperature-tolerance up to 160 °C, and has also exhibited excellent mobility control in CO₂ flooding experiments. The viscosity of the ODPTA–CO₂ bulk phase can be maintained as high as 12 mPa s under 160 °C and 10.5 MPa, which is much higher than that of the conventional surfactant solutions (similar to water). ODPTA's good foam performance with extremely high temperature-tolerance can be attributed to its high bulk phase viscosity in the brine water saturated with CO₂.

CO₂ switchable foams have gained increasing attention recently for their smart properties.