Formation and rheology of viscoelastic "double networks" in wormlike micelle-nanoparticle mixtures

Association in Like-Charged Surfactant-Nanoparticle Systems: Interfacial and Bulk Effects

The association of similarly charged surfactant molecules and nanoparticles in an aqueous solution remains unresolved, and the understandings reported in the literature are conflicting. To address this issue, we undertake a fundamental study to investigate bulk and interfacial phenomena in binary mixtures of (i) positively charged nanoparticles and cationic surfactants and (ii) negatively charged nanoparticles and anionic surfactants. We find that the surfactant molecules adsorb on the surface of the nanoparticle despite similar charge, leading to supercharging of particles and simultaneously driving more surfactant molecules to the air-dispersion interface. Hence, the properties of the dispersed species, such as the size and zeta potential, and the interfacial properties, such as the surface tension and surface excess concentration, change significantly. This effect is more pronounced at a low surfactant concentration and is observed irrespective of the size of nanoparticles and surfactant-particle combination. Further, we elucidate the important role of electrostatic interactions in the surfactant-particle complexation process by varying the pH of the dispersions. Contrary to changes in the properties of the dispersed species and interface, the presence of particles does not appreciably change the bulk property, such as the critical micelle concentration.

Improvement of the Rheological Behavior of Viscoelastic Surfactant Fracturing Fluids by Metallic-Type Nanoparticles

The use of nanotechnology in the field of acidizing, particularly in fracturing fluids, has garnered significant attention over the past decade. Viscoelastic surfactants (VESs) are utilized as one of the most effective fracturing fluids, possessing both elasticity and viscosity properties. These fluids are crucial additives in acidizing packages, enhancing their performance. However, various factors, such as salinity, temperature, pressure, and concentration, can sometimes weaken the efficacy of these fluids. To address this, the integration of nanoparticles has been explored to improve fluid retention in reservoirs and enhance the efficiency. This study focuses on investigating the impact of the main metallic-type nanoparticles on the rheological behavior of VES fluids. Iron oxide, magnesium oxide, and zinc oxide nanoparticles were utilized, and the microscopic-scale rheological behavior of the fluids was thoroughly evaluated. The highest performance for enhancing fluid gelation, stability, and rheological characteristics of VES fluids was found for Fe2O3 nanoparticles at an optimum concentration of 500 ppm. At this concentration and shear rate of 100 s-1, the viscosity of the fluid reached 169.61 cP. For iron oxide nanoparticles at a concentration of 500 ppm, by increasing the temperature from 25 to 85 °C, the gelation state of the fluid increased from 7 h and 50 min to 17 h and 45 min. This improvement is attributed to their high surface area and the increased density of entanglement points within the micelles, leading to a more interconnected structure with enhanced viscoelastic properties. Furthermore, iron oxide nanoparticles significantly enhance gelation by physically connecting the micelles, thereby improving stability and structure. The absorption of surfactant molecules by the nanoparticles additionally contributes to micelle reconstruction and shape alteration. The presence of iron oxide nanoparticles helps maintain the gel structure even at elevated temperatures, preventing rapid viscosity decrease. Our findings may provide new insights for development of high-performance, economical, and environment-friendly fracturing fluids used in well stimulation operations.

Performance Evaluation and Formation Mechanism of Viscoelastic Surfactant Fracturing Fluids with Moderate Interfacial Activity Enhanced by Janus-SiO2 Nanoparticles

Nanoparticles (NPs) exhibit great potential to improve various properties of viscoelastic surfactant (VES) fracturing fluids in the development of low-permeability reservoirs. In the present study, the amphiphilic Janus NPs (JANPs) were fabricated via the Pickering emulsion method and employed to construct the novel JA12C (JANPs with dodecyl hydrophobic carbon chains)-assisted VES fracturing fluid (JAVES). The successful fabrication of JANPs was confirmed via Fourier transform infrared spectroscopy (FTIR) measurements and water contact angle tests. The rheology behavior of the VES fracturing fluid incorporating various SiO2 NPs including hydrophilic SiO2 NPs (HLNPs), JA8C (JANPs with octyl hydrophobic carbon chains), and JA12C was systematically investigated. It was revealed that the additional JA12C significantly improved the tolerance and proppant suspension properties. To explore the subsequent oil recovery performance of various gel breaking liquids, the formation wettability and the oil-water interfacial tension (IFT) were studied after the evaluation of breaking properties and formation damage properties of various fracturing fluids. The results suggested that the JAVES gel breaking liquid showed remarkable wettability alternation capability and moderate oil-water IFT reduction ability, which can partially reduce the impact on reservoir permeability. Moreover, the formation mechanism of the JAVES was proposed by molecular dynamics simulations at the molecular level, which was further visually verified via the cryo-TEM images. The improved viscoelasticity of developed the JAVES with moderate interfacial activity is advantageous to enhance subsequent oil recovery.

Development of a Novel Surfactant-Based Viscoelastic Fluid System as an Alternative Nonpolymeric Fracturing Fluid and Comparative Analysis with Traditional Guar Gum Gel Fluid

Surfactant-based viscoelastic (SBVE) fluids have recently gained interest from many oil industry researchers due to their polymer-like viscoelastic behaviour and ability to mitigate problems of polymeric fluids by replacing them during various operations. This study investigates an alternative SBVE fluid system for hydraulic fracturing with comparable rheological characteristics to conventional polymeric guar gum fluid. In this study, low and high surfactant concentration SBVE fluid and nanofluid systems were synthesized, optimized, and compared. Cetyltrimethylammonium bromide and counterion inorganic sodium nitrate salt, with and without 1 wt% ZnO nano-dispersion additives, were used; these are entangled wormlike micellar solutions of cationic surfactant. The fluids were divided into the categories of type 1, type 2, type 3, and type 4, and were optimized by comparing the rheological characteristics of different concentration fluids in each category at 25 °C. The authors have reported recently that ZnO NPs can improve the rheological characteristics of fluids with a low surfactant concentration of 0.1 M cetyltrimethylammonium bromide by proposing fluids and nanofluids of type 1 and type 2. In addition, conventional polymeric guar gum gel fluid is prepared in this study and analyzed for its rheological characteristics. The rheology of all SBVE fluids and the guar gum fluid was analyzed using a rotational rheometer at varying shear rate conditions from 0.1 to 500 s^-1 under 25 °C, 35 °C, 45 °C, 55 °C, 65 °C, and 75 °C temperature conditions. The comparative analysis section compares the rheology of the optimal SBVE fluids and nanofluids in each category to the rheology of polymeric guar gum fluid for the entire range of shear rates and temperature conditions. The type 3 optimum fluid with high surfactant concentration of 0.2 M cetyltrimethylammonium bromide and 1.2 M sodium nitrate was the best of all the optimum fluids and nanofluids. This fluid shows comparative rheology to guar gum fluid even at elevated shear rate and temperature conditions. The comparison of average viscosity values under a different group of shear rate conditions suggests that the overall optimum SBVE fluid prepared in this study is a potential nonpolymeric viscoelastic fluid candidate for hydraulic fracturing operation that could replace polymeric guar gum fluids.

Experimental Study on the Stability of a Novel Nanocomposite-Enhanced Viscoelastic Surfactant Solution as a Fracturing Fluid under Unconventional Reservoir Stimulation

Fe3O4@ZnO nanocomposites (NCs) were synthesized to improve the stability of the wormlike micelle (WLM) network structure of viscoelastic surfactant (VES) fracturing fluid and were characterized by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Then, an NC-enhanced viscoelastic surfactant solution as a fracturing fluid (NC-VES) was prepared, and its properties, including settlement stability, interactions between NCs and WLMs, proppant-transporting performance and gel-breaking properties, were systematically studied. More importantly, the influences of the NC concentration, shear rate, temperature and pH level on the stability of NC-VES were systematically investigated. The experimental results show that the NC-VES with a suitable content of NCs (0.1 wt.%) shows superior stability at 95 °C or at a high shear rate. Meanwhile, the NC-VES has an acceptable wide pH stability range of 6-9. In addition, the NC-VES possesses good sand-carrying performance and gel-breaking properties, while the NCs can be easily separated and recycled by applying a magnetic field. The temperature-resistant, stable and environmentally friendly fracturing fluid opens an opportunity for the future hydraulic fracturing of unconventional reservoirs.

Novel Trends in the Development of Surfactant-Based Hydraulic Fracturing Fluids: A Review

Viscoelastic surfactants (VES) are amphiphilic molecules which self-assemble into long polymer-like aggregates-wormlike micelles. Such micellar chains form an entangled network, imparting high viscosity and viscoelasticity to aqueous solutions. VES are currently attracting great attention as the main components of clean hydraulic fracturing fluids used for enhanced oil recovery (EOR). Fracturing fluids consist of proppant particles suspended in a viscoelastic medium. They are pumped into a wellbore under high pressure to create fractures, through which the oil can flow into the well. Polymer gels have been used most often for fracturing operations; however, VES solutions are advantageous as they usually require no breakers other than reservoir hydrocarbons to be cleaned from the well. Many attempts have recently been made to improve the viscoelastic properties, temperature, and salt resistance of VES fluids to make them a cost-effective alternative to polymer gels. This review aims at describing the novel concepts and advancements in the fundamental science of VES-based fracturing fluids reported in the last few years, which have not yet been widely industrially implemented, but are significant for prospective future applications. Recent achievements, reviewed in this paper, include the use of oligomeric surfactants, surfactant mixtures, hybrid nanoparticle/VES, or polymer/VES fluids. The advantages and limitations of the different VES fluids are discussed. The fundamental reasons for the different ways of improvement of VES performance for fracturing are described.

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.

Design and synthesis of an azobenzene-betaine surfactant for photo-rheological fluids

HYPOTHESIS

Morphology of surfactant self-assemblies are governed by the intermolecular interactions and packing constraints of the constituent molecules. Therefore, rational design of surfactant structure should allow targeting of the specific self-assembly modes, such as wormlike micelles (WLMs). By inclusion of an appropriate photo-responsive functionality to a surfactant molecule, light-based control of formulation properties without the need for additives can be achieved.

EXPERIMENTS

A novel azobenzene-containing surfactant was synthesised with the intention of producing photo-responsive wormlike micelles. Aggregation of the molecule in its cis and trans isomers, and its concomitant flow properties, were characterised using UV-vis spectroscopy, small-angle neutron scattering, and rheological measurements. Finally, the fluids capacity for mediating particle diffusion was assessed using dynamic light scattering.

FINDINGS

The trans isomer of the novel azo-surfactant was found to form a viscoelastic WLM network, which transitioned to inviscid ellipsoidal aggregates upon photo-switching to the cis isomer. This was accompanied by changes in zero-shear viscosity up to 16,000×. UV-vis spectroscopic and rheo-SANS analysis revealed π-π interactions of the trans azobenzene chromophore within the micelles, influencing aggregate structure and contributing to micellar rigidity. Particles dispersed in a 1 wt% surfactant solution showed a fivefold increase in apparent diffusion coefficient after UV-irradiation of the mixture.

Self-assembly and viscosity changes of binary surfactant solutions: A molecular dynamics study

HYPOTHESIS

Structure and self-assembly of surfactants in the solution shows a fundamental influence on its viscosity. Through molecular simulations using Martini force field, synergistic effects in aggregation as well as the viscosity changes of a binary ionic surfactant system can be modelled. Simulations: Coarse-grained molecular dynamics simulations are performed to model the SDS/CAPB binary surfactant solution, and both equilibrium and non-equilibrium methods are used to calculate the viscosity of the equilibrated micellar systems.

FINDINGS

Our simulation results indicate that the new version of the Martini force field can provide more reasonable self-assembly of surfactant, both single and binary system. Synergistic effects in micelle formation for SDS/CAPB have been successfully reproduced, that is, the formation of cylindrical micelles or even wormlike micelles at a lower concentration when compared with the pure system. Meanwhile, both equilibrium and non-equilibrium methods provide quantitatively comparable viscosity for each system. For pure micellar system, the viscosity linearly increases with the total concentration. Nevertheless, our simulation fails to capture the viscosity enhancement of the solution in corresponding with the formation of rodlike or wormlike micelles, and a full parameter optimization of force field is still necessary.

Development of Nanofluids for the Inhibition of Formation Damage Caused by Fines Migration: Effect of the interaction of Quaternary Amine (CTAB) and MgO Nanoparticles

Fines migration is a common problem in the oil and gas industry that causes a decrease in productivity. In this sense, the main objective of this study is to develop nanocomposites based on the interaction of quaternary amine (hexadecyltrimethylammonium bromide—CTAB) and MgO to enhance the capacity of retention of fine particles in the porous medium. MgO nanoparticles were synthesized by the sol–gel method using Mg(NO₃)₂·6H₂O as a precursor. Nanoparticles were characterized by dynamic light scattering (DLS), the point of zero charge (pH_pzc), thermogravimetric analysis, and Fourier transform infrared spectroscopy (FT-IR). Different nanoparticle sizes of 11.4, 42.8, and 86.2 nm were obtained, which were used for preparing two system nanofluids. These systems were evaluated in the inhibition of fines migration: in the system I MgO nanoparticles were dispersed in a CTAB-containing aqueous solution, and system II consists of a nanocomposite of CTAB adsorbed onto MgO nanoparticles. The fines retention tests were performed using Ottawa sand 20/40 packed beds and fine particles suspensions at concentrations of 0.2% in a mass fraction in deionized water. Individual and combined effects of nanoparticles and CTAB were evaluated in different treatment dosages. The analysis of the interactions between the CTAB and the MgO nanoparticles was carried out through batch-mode adsorption and desorption tests. The best treatment in the system I was selected according to the fines retention capacity and optimized through a simplex-centroid mixture design for mass fractions from 0.0% to 2.0% of both CTAB and MgO nanoparticles. This statistical analysis shows that the optimal concentration of these components is reached for a mass fraction of 0.73% of MgO nanoparticles and 0.74% in mass fraction of CTAB, where the retention capacity of the porous medium increases from 0.02 to 0.39 mg·L⁻¹. Based on the experimental results, the nanofluids combining both components showed higher retention of fines than the systems treated only with CTAB or with MgO nanoparticles, with efficiencies up to 400% higher in the system I and higher up to 600% in the system II. To evaluate the best performance treatment under reservoir conditions, there were developed core flooding tests at fixed overburden pressure of 34.5 MPa, pore pressure at 6.9 MPa and system temperature at 93 °C. Obtaining critical rate increases in 142.8%, and 144.4% for water and oil flow in the presence of the nanofluid. In this sense, this work offers a new alternative for the injection of nanocomposites as a treatment for the problem of fines migration to optimize the productivity of oil and gas wells.

Hierarchical and synergistic self-assembly in composites of model wormlike micellar-polymers and nanoparticles results in nanostructures with diverse morphologies

Using Monte Carlo simulations, we investigate the self-assembly of model nanoparticles inside a matrix of model equilibrium polymers (or matrix of wormlike micelles) as a function of the polymeric matrix density and the excluded volume parameter between polymers and nanoparticles. In this paper, we show morphological transitions in the system architecture via synergistic self-assembly of nanoparticles and the equilibrium polymers. In a synergistic self-assembly, the resulting morphology of the system is a result of the interaction between the nanoparticles and the polymers and corresponding re-organization of both the assemblies. This is different from the polymer templating method. We report the morphological transition of nanoparticle aggregates from percolating network-like structures to non-percolating clusters as a result of the change in the excluded volume parameter between nanoparticles and polymeric chains. Corresponding to the change in the self-assembled structures of nanoparticles, the matrix of equilibrium polymers also simultaneously shows a transition from a dispersed state to a percolating network-like structure formed by the clusters of polymeric chains. We show that the shape anisotropy of the nanoparticle clusters formed is governed by the polymeric density resulting in rod-like, sheet-like or other anisotropic nanoclusters. It is also shown that the pore shape and the pore size of the porous network of nanoparticles can be changed by changing the minimum approaching distance between nanoparticles and polymers. We provide a theoretical understanding of why various nanostructures with very different morphologies are obtained.

Experimental Study of Sulfonate Gemini Surfactants as Thickeners for Clean Fracturing Fluids

Hydraulic fracturing is one of the important methods to improve oil and gas production. The performance of the fracturing fluid directly affects the success of hydraulic fracturing. The traditional cross-linked polymer fracturing fluid can cause secondary damage to oil and gas reservoirs due to the poor flow-back of the fracturing fluid, and existing conventional cleaning fracturing fluids have poor performance in high temperature. Therefore, this paper has carried out research on novel sulfonate Gemini surfactant cleaning fracturing fluids. The rheological properties of a series of sulfonate Gemini surfactant (DSm-s-m) solutions at different temperatures and constant shear rate (170 s−1) were tested for optimizing the temperature-resistance and thickening properties of anionic Gemini surfactants in clean fracturing fluid. At the same time, the microstructures of solutions were investigated by scanning electron microscope (SEM). The experimental results showed that the viscosity of the sulfonate Gemini surfactant solution varied with the spacer group and the hydrophobic chain at 65 °C and 170 s−1, wherein DS18-3-18 had excellent viscosity-increasing properties. Furthermore, the microstructure of 4 wt.% DS18-3-18 solution demonstrated that DS18-3-18 self-assembled into dense layered micelles, and the micelles intertwined with each other to form the network structure, promoting the increase in solution viscosity. Adding nano-MgO can increase the temperature-resistance of 4 wt.% DS18-3-18 solution, which indicated that the rod-like and close-packed layered micelles were beneficial to the improvement of the temperature-resistance and thickening performances of the DS18-3-18 solution. DS18-3-18 was not only easy to formulate, but also stable in all aspects. Due to its low molecular weight, the damage to the formation was close to zero and the insoluble residue was almost zero because of the absence of breaker, so it could be used as a thickener for clean fracturing fluids in tight reservoirs.

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.

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.

The Study of a Novel Nanoparticle-Enhanced Wormlike Micellar System

In this work, a novel nanoparticle-enhanced wormlike micellar system (NEWMS) was proposed based on the typical wormlike micelles composed of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). In order to strengthen the structure of wormlike micelles, silica nanoparticles are used to design the novel nanoparticle-enhanced wormlike micelle. The stability and morphologies of silica nanoparticles were studied by dynamic light scattering (DLS) and transmission electron microscopy (TEM) at first. After the formation of NEWMS, the rheological properties were discussed in detail. The zero-shear viscosity of NEWMS increases with the addition of silica nanoparticles. Dynamic oscillatory measurements show the viscoelastic properties of NEWMS. Through comparison with the original wormlike micelles, the entanglement length and mesh size of NEWMS are nearly unchanged, while the contour length increases with the increase of silica concentration. These phenomena confirm the enhanced influence of silica nanoparticles on wormlike micelles. The formation mechanism of NEWMS, especially the interactions between wormlike micelles and nanoparticles, is proposed. This work can deepen the understanding of the novel NEWMS and widen their applications.

Stabilization of spherical nanoparticles of iron(III) hydroxides in aqueous solution by wormlike micelles

HYPOTHESIS

The low K_sp value of Fe(OH)₃ (3 × 10^-38 at 298 K) explain the immediate coagulation when the pH of a solution of Fe(III) is adjusted to 7. However, stable dispersions of Fe(OH)₃ can be formed when the pH is adjusted to 7 in the presence of wormlike micelles formed by cetyltrimethylammonium bromide and sodium salicylate. The formation of a structure containing Fe(OH)₃ nanoparticles decorating wormlike micelles is responsible for the high stability of the dispersions.

EXPERIMENTS

Fe(OH)₃ nanoparticles were obtained by increasing the pH of solutions of cetyltrimethylammonium bromide and Fe(III), previously complexed with salicylate at pH 3. The interaction between nanoparticles and the chains of wormlike micelles was investigated by DLS, SAXS, TEM and Cryo-TEM.

FINDINGS

DLS revealed higher scattering contrast and slower diffusion for wormlike micelles in the presence of nanoparticles. These results were interpreted as the decoration of the chains of wormlike micelles by nanoparticles of Fe(OH)₃. A pearl-necklace model was successfully used to adjust SAXS curves, revealing nanoparticles with ∼3 nm of diameter, spaced ∼2 nm apart along the string. This result agrees with TEM and Cryo-TEM images. The formed structure prevents the coagulation of nanoparticles, assuring high stability to the dispersion.

Can More Nanoparticles Induce Larger Viscosities of Nanoparticle-Enhanced Wormlike Micellar System (NEWMS)?

There have been many reports about the thickening ability of nanoparticles on the wormlike micelles in the recent years. Through the addition of nanoparticles, the viscosity of wormlike micelles can be increased. There still exists a doubt: can viscosity be increased further by adding more nanoparticles? To answer this issue, in this work, the effects of silica nanoparticles and temperature on the nanoparticles-enhanced wormlike micellar system (NEWMS) were studied. The typical wormlike micelles (wormlike micelles) are prepared by 50 mM cetyltrimethyl ammonium bromide (CTAB) and 60 mM sodium salicylate (NaSal). The rheological results show the increase of viscoelasticity in NEWMS by adding nanoparticles, with the increase of zero-shear viscosity and relaxation time. However, with the further increase of nanoparticles, an interesting phenomenon appears. The zero-shear viscosity and relaxation time reach the maximum and begin to decrease. The results show a slight increasing trend for the contour length of wormlike micelles by adding nanoparticles, while no obvious effect on the entanglement and mesh size. In addition, with the increase of temperature, remarkable reduction of contour length and relaxation time can be observed from the calculation. NEWMS constantly retain better viscoelasticity compared with conventional wormlike micelles without silica nanoparticles. According to the Arrhenius equation, the activation energy E_a shows the same increase trend of NEWMS. Finally, a mechanism is proposed to explain this interesting phenomenon.

Structural Evolution of Wormlike Micellar Fluids Formed by Erucyl Amidopropyl Betaine with Oil, Salts, and Surfactants

Solutions of extended, flexible cylindrical micelles, often known as wormlike micelles, have great potential as the base for viscoelastic complex fluids in oil recovery, drilling, and lubrication. Here, we study the morphology and nanostructural characteristics of a model wormlike micellar fluid formed from erucyl amidopropyl betaine (EAPB) in water as a function of a diverse range of additives relevant to complex fluid formulation. The wormlike micellar dispersions are extremely oleo-responsive, with even as little as 0.1% hydrocarbon oil causing a significant disruption of the network and a decrease in zero-shear viscosity of around 100-fold. Simple salts have little effect on the local structure of the wormlike micelles but result in the formation of fractal networks at larger length scales, whereas even tiny amounts of small organic species such as phenol can cause unexpected phase transitions. When forming mixtures with other surfactants, a vast array of self-assembled structures are formed, from spheres to ellipsoids, lamellae, and vesicles, offering the ultimate sensitivity in designing formulations with specific nanostructural characteristics.

Self-Assembly of Nanoparticle-Surfactant Complexes with Rodlike Micelles: A Molecular Dynamics Study

The self-assembly of nanoparticles (NPs) with cationic micelles of cetyltrimethylammonium chloride (CTAC) is known to produce stable nanogels with rich rheological and optical properties. Coarse-grained molecular dynamics (MD) simulations are performed to explore the molecular mechanisms underlying this self-assembly process. In an aqueous solution of CTAC surfactants, a negatively charged NP with a zeta potential of less than -45 mV is observed to form a stable vesicular structure in which the particle surface is almost entirely covered with a double layer of surfactants. In comparison, surfactants form a monolayer, or a corona, around an uncharged hydrophobic NP with the tailgroups physically adsorbed onto the particle. In the presence of sodium salicylate salt, such NP-surfactant complexes (NPSCs) interact with rodlike CTAC micelles, resulting in the formation of stable junctions through the opening up of the micelle end-cap followed by surfactant exchange, which is diffusion-limited. The diffusive regime spans several hundred nanoseconds, thereby necessitating MD simulations over microsecond time scales. The energetics of NPSC-micelle complexation is analyzed from the variation in the total pair-potential energy of the structures.