The influence of hydrophobic counterions on micellar growth of ionic surfactants

Molecular insights: How counterions determine surfactant aggregation

The morphology of surfactant aggregates inherently determines their properties and functions. Hence, the modulation of morphology holds significant importance in their applications. Opting for an appropriate counterion emerges as an efficient and convenient method to shape the aggregate morphology of ionic surfactants. Both inorganic and organic counterions exert influence on surfactant aggregation by neutralizing the repulsive headgroup charges and synergistically adjusting the hydrophobic association among the alkyl tails. In this process, the charge property, polarizability, hydrophobicity, configuration, and functional groups of counterions are of pivotal influencing factors. A delicate adjustment of counterion structures can yield dramatic differences in the aggregation behavior of ionic surfactants. This review provides a comprehensive overview of the fundamental principles governing the impact of inorganic and organic counterions on the formation of diverse surfactant aggregates, encompassing spherical micelles, wormlike micelles, vesicles, coacervates, and other aggregates. It aims at providing insightful understanding on how to select counterions for surfactants to achieve desired aggregate structures and properties.

Temperature-Derived Purification of Gold Nano-Bipyramids for Colorimetric Detection of Tannic Acid

Gold nanostructures have attracted broad attention. Among various nanostructures, gold nanobipyramids have shown great potential in sensing, biomedicine, environmental protection, chemical catalysis, and optics due to their unique physical and optical properties and ease of chemical functionalization. Compared with other plasmonic nanostructures, gold nanobipyramids possess narrow optical resonances, stronger plasmonic local field enhancement, and size- and shape-dependent surface plasmon resonance. However, the synthesis and purification of homogeneous gold nanobipyramids are very challenging. The gold nanobipyramids synthesized via the commonly used seed-mediated growth method have low yields and are often coproduced with spherical nanoparticles. In this study, we reported a temperature-derived purification method for the isolation of gold bipyramids. In the presence of salt, by altering the temperature of the solution, large gold bipyramids can be separated from small spherical nanoparticles. As a result, a yield of as high as 97% gold nanobipyramids can be achieved through a single round of purification, and correspondingly, the ratio between the longitudinal surface plasmon resonance (LSPR) and transverse SPR intensity significantly increases to as high as 6.7. The purified gold nanobipyramids can be used as a colorimetric probe in the detection of tannic acid with a detection limit of 0.86 μM and a linear detection range from 1.25 to 37.5 μM.

Tensiometric and rheological investigations of single and mixed systems consisting of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions

The surface tension (σ), critical micelle concentration (CMC), surface excess (Γ), minimum area occupied by a surfactant molecule (A min), the viscosity and oscillatory rheological studies of aqueous solutions containing cocamidopropyl betaine (CAPB) and sodium dodecylbenzene sulfonate (SDBS) at molar fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 and 25 °C were presented. CAPB and SDBS were not found to interact synergistically in water at any of the molar fractions studied. This is due to the fact that the critical micelle concentrations of these mixtures were higher than those predicted by Clint’s equation, indicating an antagonism that rarely occurs in mixed amphoteric/anionic surfactant systems. The minimum area occupied by a surfactant molecule (A min) was reduced in CAPB/SDBS mixtures compared to unmixed surfactants. In contrast, the viscosity of the mixed CAPB/SDBS system increased significantly from 1.0 mPa s to a maximum of 36.0 Pa s at higher CAPB mole fractions (0.5–0.8). The rheograms obtained from the oscillation measurements of the viscous CAPB/SDBS solutions are characteristic of wormlike micelles (WLMs) according to the Maxwell model. The results of this surprising binary CAPB/SDBS surfactant mixing system are presented and discussed.

Dilute hydrochloric acid induced switching from antagonism to synergism in the binary mixed systems of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS)

A synergism is reported in the binary mixtures of cocamidopropyl betaine (CAPB) and sodium dodecyl benzene sulfonate (SDBS) at mole fractions of 0.25, 0.50, and 0.75, in dilute hydrochloric acid solutions (∼0.05 M) HCl, at 25 °C. Such synergism is confirmed by surface tension values, critical micelle concentration (CMC), and interaction parameter values according to Holland and Rubingh’s model and the satisfaction of Hua and Rosen conditions. Surface excess (Γ) at air/water interface and the minimum area occupied by surfactant molecule (A min) also indicated a reduction in the A min of mixed CAPB/SDBS compared to single surfactants that is due to the strong electrostatic interaction between surfactants heads. The composition of CAPB/SDBS mixed micelles, the interaction parameter (β) and the micellar activity coefficients (f 1 and f 2) were evaluated from the regular solution model for these binary amphoteric/anionic mixed systems. It is clear the CAPB surfactants are converted to the cationic form by charging in the presence of HCl, which enhances the interaction between the oppositely charged CAPB and SDBS heads and overcomes the unfavourable packing between the tails predicted in pure water. Such strong interaction was also reflected on the phase behavior of CAPB/SDBS binary mixed system. Accordingly, a switching from antagonism to synergism is reported for CAPB/SDBS binary mixed system in the presence of dilute hydrochloric acid.

Mixed micellar solubilization for procion blue MxR entrapment and optimization of necessary parameters for micellar enhanced ultrafiltration

In this study, micellar enhanced ultrafiltration, MEUF, being an active methodology, has been employed to remove Procion Blue MxR (PBM) from synthetic effluent. MEUF is being applied to reduce the toxicity level of aqueous system using the micellar media of cationic surfactants i.e. Cetyl trimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC). Subsequently, the effect of addition of nonionic surfactant, Triton X-100 (TX-100), on solubilizing power of cationic surfactants is investigated. The values of partition coefficient and free energy of partition reflect the extent of interaction of the dye with the surfactants. Initially molecules of pollutants i.e. dye form ion pairs with ionic surfactants and, later on, the same is incorporated within micelle. Maximum value of free energy of partition ΔG_p has been found to be -55.49 kJmol^-1 and -50.43 kJmol in the presence of CPC and CTAB, respectively. The size of pollutant, thus, increases and, consequently, can be easily filtered. The effect of various factors i.e. concentration of surfactant, concentration of electrolyte (NaCl), transmembrane pressure, revolutions per minute (RPM) and pH, have been investigated to find the optimum conditions for maximum removal of PBM from aqueous system. The efficiency of MEUF has been assessed by calculating the values of rejection percentage and permeate flux. Both the surfactants were observed as strong candidates for PBM encapsulation but overall, maximum rejection percentage (R%) of 96.90% was attained by CPC.

Ion-Specific Effects on Vesicle-to-Micelle Transitions of an Amino Acid Surfactant Probed by Chemical Trapping

Ion-specific effects widely exist in biological and chemical systems and cannot be explained by classical theories. The complexity of ion-specific effects in protein systems at the molecular level necessitates the use of mimetic models involving smaller molecules, such as amino acids, oligopeptides, and other organic molecules bearing amide bonds. Therefore, it is of theoretical value to determine the effect of additional salts on the aggregation transitions of acyl amino acid surfactants. Herein, the effects of specific tetraalkylammonium ions (TAA⁺) on sodium lauroyl glycinate (SLG) aggregation were studied by dynamic light scattering (DLS) and transmission electron microscopy. Although previous studies have shown that the kosmotropic TAA⁺ ions tend to induce micellar growth or micelle-to-vesicle transitions of some anionic surfactants, TAA⁺ addition in the present study induced partial vesicle-to-micelle transitions in SLG solutions. The chemical trapping (CT) method was employed to estimate changes in the interfacial molarities of water, amide bonds, and carboxylate groups during such transitions. The vesicle-to-micelle transitions were accompanied by a marked rise in interfacial water molarity and a decline in interfacial amide bonds molarity, suggesting that the hydrated TAA⁺ entered the interfacial region and disrupted hydrogen bonding, thus preventing the SLG monomers from packing tightly. Molecular dynamic simulation was also performed to demonstrate the salt-induced cleavage of amide-amide bonds between SLG headgroups. Furthermore, both CT and DLS results show that the ability of tetraalkylammonium cations to induce such transitions increased with increasing size and hydrophobicity of the cation, which follows the Hofmeister series. The current study offers critical molecular-level evidence for understanding the specific effects of tetraalkylammonium ions on the aggregation transitions of an acyl amino acid surfactant.

Sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine oxide (DDAO) in single and mixed systems as corrosion inhibitors of zinc in hydrochloric acid

The influence of surfactant synergism between sodium dodecyl benzene sulfonate (SDBS) and N,N-dimethyldodecan-1-amine Oxide (DDAO) on zinc corrosion in 0.05 M hydrochloric acid (HCl) solutions at 25 °C was investigated. Firstly, solutions of SDBS and DDAO with mole fractions of 0.00, 0.25, 0.50, 0.75 and 1.00 were prepared and their surface tension and critical micelle concentration (CMC) values in water and in 0.05 M HCl were measured as a function of total surfactant concentration. The SDBS/DDAO mixed system exhibited a strong synergism in 0.05 M HCl with a highly negative interaction parameter β (average β = −23.46), according to regular solution theory. Secondly, the adsorption of single surfactants SDBS and DDAO and SDBS/DDAO surfactant mixture on 2.0% zinc powder was investigated by the depletion method to find out the role of synergism in the adsorption tendency of these surfactants on the zinc surface and thus their corrosion inhibiting effect. The adsorption tendency of single surfactant and the mixed surfactant systems onto 2.0% zinc powder followed the order: SDBS > 0.75 SDBS/0.25 DDAO ≈ 0.25 SDBS/0.75 DDAO > DDAO > 0.50 SDBS/0.50 DDAO. Finally, the corrosion of zinc was investigated using the potentiodynamic polarization technique. It was found that SDBS and DDAO act as efficient corrosion inhibitors for zinc in 0.05 M HCl solution with increasing corrosion inhibition efficiency when they are mixed. Additionally, images of scanning electron microscopy were obtained for zinc sheets in solutions containing single and mixed SDBS/DDAO surfactants in the presence and absence of 0.05 M HCl. The microscopic images show an improvement in the protection of the zinc surface against acid attack in the presence of single and mixed SDBS/DDAO surfactants.

pH-Responsive Rheological Properties and Microstructure Transition in Mixture of Anionic Gemini/Cationic Monomeric Surfactants

Surfactant aggregates have long been considered as a tool to improve drug delivery and have been widely used in medical products. The pH-responsive aggregation behavior in anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C12C3C12(SO3)2) and its mixture with a cationic monomeric surfactant cetyltrimethylammonium bromide (CTAB) have been investigated. The spherical-to-wormlike micelle transition was successfully realized in C12C3C12(SO3)2 through decreasing the pH, while the rheological properties were perfectly enhanced for the formation of wormlike micelles. Especially at 140 mM and pH 6.7, the mixture showed high viscoelasticity, and the maximum of the zero-shear viscosity reached 1530 Pa·s. Acting as a sulfobetaine zwitterionic gemini surfactant, the electrostatic attraction, the hydrogen bond and the short spacer of C12C3C12(SO3)2 molecules were all responsible for the significant micellar growth. Upon adding CTAB, the similar transition could also be realized at a low pH, and the further transformation to branched micelles occurred by adjusting the total concentration. Although the mixtures did not approach the viscosity maximum appearing in the C12C3C12(SO3)2 solution, CTAB addition is more favorable for viscosity enhancement in the wormlike-micelle region. The weakened charges of the headgroups in a catanionic mixed system minimizes the micellar spontaneous curvature and enhances the intermolecular hydrogen-bonding interaction between C12C3C12(SO3)2, facilitating the formation of a viscous solution, which would greatly induce entanglement and even the fusion of wormlike micelles, thus resulting in branched microstructures and a decline of viscosity.

Self-Assembled Quaternary Ammonium-Containing Comb-Like Polyelectrolytes for the Hydrolysis of Organophosphorous Esters: Effect of Head Groups and Counter-Ions

The aim of this work was to increase the efficiency of catalytic systems for the hydrolytic cleavage of 4-nitrophenyl esters of phosphonic acids. Quaternary ammonium-containing comb-like polyelectrolytes («polymerized micelles») with ester cleavable fragments and a low aggregation threshold were used as catalysts. The synthesis of poly(11-acryloyloxyundecylammonium) surfactants with different counterions (Br^- , NO₃ ^- , CH₃ C₆ H₄ SO₃ ^- ) and head groups was realized by micellar free-radical polymerization. Molecular weight, critical association concentration, particle sizes and solubilization properties toward Orange OT were determined. Self-assemblies organized by poly(11-acryloyloxyundecyltrimethyl ammonium) bromide successfully catalyze the hydrolysis of 4-nitrophenyl butylchloromethylphosphonate up to two orders of magnitude compared to aqueous alkaline hydrolysis. The development of these catalysts is promising for industrial applications and organophosphorus compound detoxification.

Effect of Added Tetraalkylammonium Counterions on the Dilational Rheological Behaviors of N-Cocoyl Glycinate

Ion specific effect, which is also known as Hofmeister effect, has been reported in numerous systems including ionic surfactant aggregates. Acyl amino acid surfactants have attracted growing attentions in the field of novel surfactants research due to their environmentally benign characteristics. The objective of this study was to investigate the effect of different salts containing NH₄⁺ and tetraalkylammonium (TAA⁺), where alkyl = methyl (TMA⁺), ethyl (TEA⁺), and propyl (TPA⁺), cations on the dilational rheological properties of interfacial film are stabilized by potassium N-cocoyl glycinate (KCGl). The interfacial behaviors were studied using oscillating drop shape analysis method. The interfacial tensions (IFTs) and dilational rheological parameters results illustrate that KCGl in the presence of salts has better interfacial activity and stronger intermolecular interaction, indicating that added cations contribute to denser molecular packing at oil-water interface. Ion specific effects were observed in the system. Among the cations, KCGl shows highest dilational modulus in the presence of NH₄⁺. The overall interaction between cations and headgroups of KCGl decreases in the sequence NH₄⁺ >TMA⁺ >TEA⁺ ≈TPA⁺, which follows Hofmeister series. The increasing hydrophobicity of TAA⁺ prevents the interaction between cations and KCGl's headgroup, and therefore prevent amphiphiles from packing closely at interface. The results present a theoretical origin for useful application of KCGl in cosmetics, petroleum and daily chemical industries.

Influence of Electrostatic Interactions During the Resorcinol-Formaldehyde Polymerization on the Characteristics of Mo-Doped Carbon Gels

The resorcinol (R)-formaldehyde (F) polymerization was carried out in different experimental conditions to obtain RF/Mo doped carbon xerogels with different morphology, porosity and nature and dispersion of metal. Attractive or repulsive electrostatic interactions were forced in the starting aqueous solution of RF-monomers using different synthesis conditions, namely, combinations of cationic or anionic surfactants, Mo-precursors and pH values. The results showed that when both cationic surfactant and Mo-precursor were used at neutral pH, attractive interactions with the anionic RF-macromolecules are favored during polymerization and the final carbon xerogel exhibited the most developed porosity and the strongest Mo-organic phase interaction, leading to deeper Mo-phase reduction during carbonization and the formation of highly-dispersed crystalline nanoparticles of Mo2C. On the contrary, the use of both anionic surfactant and Mo-precursor leads to repulsive interactions, which generates less porous carbon gels with a Mo-phase formed by large MoO3 platelet structures and low Mo-surface contents. RF/Mo-doped gels with intermediate properties were obtained by combining cationic and anionic surfactants, metal precursors or both. After carbonization, the obtained materials would be suitable to be used directly as catalysts with different physicochemical properties and active phases.

Kinetics and Mechanism of Cationic Micelle/Flexible Nanoparticle Catalysis: A Review

The aqueous surfactant (Surf) solution at [Surf] > cmc (critical micelle concentration) contains flexible micelles/nanoparticles. These particles form a pseudophase of different shapes and sizes where the medium polarity decreases as the distance increases from the exterior region of the interface of the Surf/H2O particle towards its furthest interior region. Flexible nanoparticles (FNs) catalyse a variety of chemical and biochemical reactions. FN catalysis involves both positive catalysis ( i.e. rate increase) and negative catalysis ( i.e. rate decrease). This article describes the mechanistic details of these catalyses at the molecular level, which reveals the molecular origin of these catalyses. Effects of inert counterionic salts (MX) on the rates of bimolecular reactions (with one of the reactants as reactive counterion) in the presence of ionic FNs/micelles may result in either positive or negative catalysis. The kinetics of cationic FN (Surf/MX/H2O)-catalysed bimolecular reactions (with nonionic and anionic reactants) provide kinetic parameters which can be used to determine an ion exchange constant or the ratio of the binding constants of counterions.

Kinetics and Mechanism of Counterionic Salt-Catalysed Piperidinolysis of Anionic Phenyl Salicylate in the Presence of Cationic–Nonionic Mixed Micelles

The quantitative correlation of counterion-affinity to aqueous hexadecyltrimethylammonium bromide (HDAB, cationic micelles/nanoparticles) and the counterion-induced HDAB micellar growth, in the presence of different amounts of poly(ethylene glycol hexadecyl ether) (C16E20, nonionic surfactant), was achieved by the use of a semi-empirical kinetic (SEK) method. The values of the ratio of cationic HDAB, as well as mixed HDAB–C16E20, micellar binding constants of X and Br, KX/ KBr (= KXBr or RXBr) for X = 4-ClC6H4CO2-, were obtained by the SEK method. The concentration range (0.006–0.015 M) of pure HDAB was found to have no influence on the values of KXBr or RXBr. These observations were also recorded upon addition of a nonionic surfactant, C16E20, in an aqueous solution of HDAB. The mean value of KXBr or RXBr obtained in the presence of pure HDAB ( KXBr or RXBr = 50.3) is 2.3 times larger than that in the presence of mixed HDAB–C16E20 (m KXBr or m RXBr = 21.7). From rheometric measurements of aqueous HDA+/4-ClC6H4CO2- with 0.015 M HDAB, single symmetric maxima (at both 25 and 35 °C) were obtained at [4-ClC6H4CO2Na] = 0.03 M. This is evidence for the existence of wormlike micelles/nanoparticles. However, the absence of a maximum in rheometric data for aqueous HDA+/C16E20/4-ClC6H4CO2- with 0.015 M HDAB and 0.006 M C16E20 at various [4-ClC6H4CO2Na] revealed the existence of spherical micelles.

Study of Cationic and Nonionic Mixed Micelles with NaBr and 3,5-Cl2C6H3CO2Na by Use of Probe Nucleophilic Reaction of Piperidine with Ionized Phenyl Salicylate

Behaviors of cationic and nonionic mixed micelles in the form of hexadecyltrimethylammonium bromide (HDABr) and hexadecyltrimethylammonium bromide-Polyethylene glycol hexadecyl ether (C₁₆E₂₀), in the presence of inert salts (NaBr and 3,5-dichlorosodium benzoate), by the use of reaction probe between Pp and ionized PhSH (Pp = piperidine and PhSH = phenyl salicylate), has been reported in this work. The values of R_X^Br (R_X^Br denotes ion exchange constants obtained in the presence of micelles of different structural features) or K_X^Br (K_X^Br denotes ion exchange constants obtained in the presence of micelles of the same structural features) for 3,5-Cl₂C₆H₃CO₂^- were almost the same at three different [HDABr]_T (0.006, 0.010 and 0.015 M). The average value of R_X^Br or K_X^Br determined, in the presence of pure HDABr micelles, using semi empirical kinetic (SEK) method appeared to be almost 2½-fold larger (R_X^Br or K_X^Br = 198) than that in the presence of mixed HDABr-C₁₆E₂₀ micelles (R_X^Br or K_X^Br = 78). Rheological measurements indicated the existence of wormlike/twisted micelles and vesicle at 0.015 M pure HDABr, various [3,5-Cl₂C₆H₃CO₂Na], and 25 and 35℃ whereas there were evidence of only spherical micelles in the presence of mixed HDABr-C₁₆E₂₀ ([HDABr]_T = 0.015 M and [C₁₆E₂₀]_T = 0.006 M) at both temperatures.

Composition and Concentration Gradient Induced Structural Transition from Micelles to Vesicles in the Mixed System of Ionic Liquid-Diclofenac Sodium

Catanionic surfactant-hydrotrope mixtures have proven to be a striking alternative to tune microstructures over a wide range of compositions and also to minimize precipitation that is normally observed in catanionic mixtures at an equimolar ratio. These mixtures are supposed to be of great relevance in biological systems when a hydrotrope is a "drug". Keeping this in view, here we report composition- and dilution-induced structural changes in a catanionic mixture comprising ionic liquids (ILs), such as 1-dodecyl-3-methylimidazolium bromide (C12mimBr)/1-tetradecyl-3-methylimidazolium bromide (C14mimBr), and a drug, diclofenac sodium (DFNa), in aqueous solution. The structural changes are probed by small-angle neutron scattering (SANS), dynamic light scattering (DLS), and zeta-potential measurements. SANS data and size distribution curves clearly depict the formation of low curvature structures on going from the cation-rich to anion-rich composition up to a 0.7 mole fraction of DFNa (XDFNa). The amphiphilic nature of DFNa is supposed to alter the surface charge density, which is provoked by its incorporation into resulting aggregates, as confirmed by modified zeta-potential values. The modification of the average packing parameter resulting from the IL and DFNa complexation equilibrium seems to play a vital role in bringing out structural transitions of mixed aggregates. We also focused our attention to study the effect of dilution in concentrations ranging from 100 to 25 mM. At XDFNa = 0.0 and 0.1, the size of prolate ellipsoids decreases on dilution, mimicking classic behavior, but an opposite trend is observed at other XDFNa values. Dilution-induced transformation to larger aggregates is thought to be driven by the release of DFNa molecules from the mixed micelles on account of the critical micelle concentration (cmc) (solubility) mismatch between the two components. The role of other interactions such as cation-π and π-π in stabilizing the mixed aggregates in addition to hydrophobic interactions is probed by (1)H NMR.

Tetraalkylammonium interactions with dodecyl sulfate micelles: a molecular dynamics study

2D number-density maps for S atoms of the polar head in the x–y plane.

Microstructural morphologies of CTAB micelles modulated by aromatic acids

pH and temperature-dependent aggregation behaviour of CTAB with solubilized aromatic acids.

Crystalline fibrillar gel formation in aqueous surfactant-antioxidant system

Cetyltrimethylammonium bromide (CTAB) is a well-known cationic surfactant capable to micellize into diverse morphologies in aqueous medium. We observed the formation of an opaque gel state from aqueous CTAB solution in the presence of the aromatic additive, para-coumaric acid (PCA). Optical microscopic images revealed the presence of large fibrils in the system at room temperature. Gel nature of the fibrils was confirmed by rheological measurements. Presence of interstitial water in the fibrils was recognized with Raman spectroscopy. On heating the sample above 30 (°) C, the fibrillar gel state changes to a transparent liquid state with Newtonian flow properties. Dynamic light scattering study hinted the presence of small micelles in the solution above 30 (°) C. Thus the system showed a temperature-dependent structural transition from opaque water-swollen gel to transparent micellar liquid. The formation of water-swollen fibrillar network is attributed to surfactant-additive intermolecular interactions in aqueous medium. Transition to micelle phase above 30 (°) C is related to Kraft transition which is observed at significantly lower temperature for CTAB in the absence of PCA. The structural features of PCA play a key role in promoting fibrillar network formation and elevating the Kraft transition in aqueous solution of CTAB.

Microstructural transition of aqueous CTAB micelles in the presence of long chain alcohols

The effect of long chain alcohols (C₉OH–C₁₂OH) on the micellar properties of CTAB in the presence of an inorganic salt, KBr, has been systematically studied by viscometry, rheology, DLS and the direct imaging technique,i.e.cryo-TEM.

Influence of hydrotropic coions on the shape transitions of sodium dioctylsulfosuccinate aggregates in an aqueous medium

The slope change of the CH plot is a manifestation of the shape change of the ionic micelles. However, binding of the salicylate coion to ionic micelles is an exception to this inference.

Turning up the heat on wormlike micelles with a hydrotopic salt in microfluidics

In equilibrium, wormlike micelles can transition from entangled to branched structures with increasing surfactant concentrations and ionic strength. Under flow conditions, structural transition of micellar solutions can follow very different trajectories. In this study we consider the flow of a semi-dilute wormlike micellar solution through an array of microposts, with focus on its rheological and microstructural evolutions. Specifically, the micellar solution (precursor) contains cationic surfactant cetyltrimethylammonium bromide (CTAB) and hydrotropic organic salt 3-hydroxynaphthalene-2-carboxylate (SHNC). We report the formation of a flow induced structured phase (FISP), with entangled, branched, and multi-connected micellar bundles, evidenced by electron microscopy and small-angle neutron scattering (SANS). By integrating gold-etched microheaters with the micropost design in a microfluidic device, we investigate the localized temperature effect on both the precursor and FISP, with complementary investigations from SANS. We observe that the FISP does not completely disintegrate at high temperatures, whereas, the precursor exhibits shortening of wormlike micelles as temperature increases. We also correlate the microstructure of both FISP and precursor with two point passive microrheology and bulk rheology characterizations.

Advanced molecular self-assemblies facilitated by simple molecules

Advanced materials are often based on smart molecular self-assemblies that either respond to external stimuli or have hierarchical structures. Approaches to this goal usually stem from complicated molecular design and difficult organic synthesis. In this invited feature article, we demonstrate that desired molecular self-assemblies can be made conveniently by introducing simple functional molecules into amphiphilic systems. We show that upon introducing specific small molecules which serve as responders, modulators, or even building blocks, smart supramolecular architectures can be achieved which avoid complicated organic synthesis. We expect that this could be a general and economical way to produce advanced materials in the near future.

Microfluidic flows of wormlike micellar solutions

The widespread use of wormlike micellar solutions is commonly found in household items such as cosmetic products, industrial fluids used in enhanced oil recovery and as drag reducing agents, and in biological applications such as drug delivery and biosensors. Despite their extensive use, there are still many details about the microscopic micellar structure and the mechanisms by which wormlike micelles form under flow that are not clearly understood. Microfluidic devices provide a versatile platform to study wormlike micellar solutions under various flow conditions and confined geometries. A review of recent investigations using microfluidics to study the flow of wormlike micelles is presented here with an emphasis on three different flow types: shear, elongation, and complex flow fields. In particular, we focus on the use of shear flows to study shear banding, elastic instabilities of wormlike micellar solutions in extensional flow (including stagnation and contraction flow field), and the use of contraction geometries to measure the elongational viscosity of wormlike micellar solutions. Finally, we showcase the use of complex flow fields in microfluidics to generate a stable and nanoporous flow-induced structured phase (FISP) from wormlike micellar solutions. This review shows that the influence of spatial confinement and moderate hydrodynamic forces present in the microfluidic device can give rise to a host of possibilities of microstructural rearrangements and interesting flow phenomena.

Nonaromatic hydrotropic cationic ammonium salts as a rheology modifier for an anionic/zwitterionic surfactant mixture

In this article, we report additive-induced micellar growth and rheology modification for mixtures of anionic (sodium dodecyl sulfate, SDS) and zwitterionic (N-alkylated glycine derivative, Empigen BB or EBB) surfactants. Two nonaromatic hydrotropic salts (hexyltrimethylammonium bromide, C6TAB, and/or dibutylenebis(dimethylbutylammonium bromide), 4-4-4) are used as novel additives to induce micellar growth in these systems. Nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), transmission electron microscopy (TEM), the Weissenberg effect, and rheology measurements were employed to assess mixed micelle formation, micellar growth, and rheology modifications. Finally, the manner in which the surfactants and hydrotropes self-assemble into aggregates has also been deduced from 2D NMR NOESY measurements. In this study, both hydrotropic ions have been found to contribute to similar structural modifications in the mixed micelles of the anionic and zwitterionic surfactants. However, the extent of the rheology modification in solution is found to be quite different when the gemini hydrotrope (4-4-4) versus the monomeric hydrotrope (C6TAB) is employed.