Surfactant Counterion Research Articles

Ultralow interfacial tension achieved by extended anionic surfactants with a short hydrophobic chain

Extended surfactant is a new type of oil displacement agent with excellent performance in enhanced oil recovery (EOR). In this paper, the novel extended surfactants with short hydrophobe (ethylhexyl) have been studied. The dynamic interfacial tensions (IFTs) between extended surfactant (C8PxEyC) solutions and n-alkanes were measured by the spinning drop method. The effect of surfactant concentration, polyoxypropylene (PO) numbers, polyoxyethylene (EO) numbers and electrolyte on the IFTs were investigated. The experimental results show that the interfacial activity of the extended surfactants mainly depends on the PO groups, and the short hydrophobe extended anionic surfactant shows the similar behavior of reducing IFT to that of long alkyl chain one. The IFTs decrease with an increase of the PO numbers against n-alkanes, and an increase of the EO numbers leads to the destruction of the tight arrangement of the adsorption film because the size of EO group is smaller than that of PO group, which results in an increase of IFT. The addition of electrolyte can reduce the IFTs by weakening the electrostatic repulsion between surfactant molecules at the interface, but it has little effect on its HLB. The long EO chain of extended surfactant prevents divalent cation from binding two surfactant counterions, which results in the little effect on reducing IFT. This study is of great significance for the insight of extended surfactants and their application in high-salt reservoirs.

Removal of ionic dyes with different charges by foam separation

Substances with an opposite charge to that of the ionic surfactant are preferentially removed during foam separation. The effect of charge on foam separation was studied using cationic surfactant dodecyltrimethylammonium chloride (DTAC) and several anionic dyes with different numbers of anionic groups: Acid Blue 119 (mono-anion), Ethyl Orange (mono-anion), Xylene Fast Yellow 2G (di-anion), and Acid Yellow 23 (tri-anion). The negatively charged surfactant counterion chloride (Cl-) and the anionic dye (Acid Blue 119) were competitively removed during foam separation. The first-order rate constant (k) for the removal of the two types of anions was estimated to be 8 × 10-4 min−1 for Cl- and 2 × 10-3 min−1 for Acid Blue 119. The results indicated that the target material Acid Blue 119 was removed 2.5 times faster than the surfactant counterion Cl-. The k values of these dyes were determined using the DTAC at 298 K. The results showed that the removal rate increased with increasing number of anionic groups among the investigated anionic dyes: Acid Blue 119 < Ethyl Orange < Xylene Fast Yellow 2G < Acid Yellow 23. The increase in the number of anionic groups in the dye provided binding at multiple points to the adsorption monolayer of the surfactant and became stronger in the foam. In foam separation using an ionic surfactant, it was found that the number of ionic groups in the substance significantly affected the removal rate.

Effects of Selected Metal Chlorides on the Micellization Parameters of Cetrimonium Bromide at Different Temperatures

The pre-modification of surfactant counterions using transition metal salts provides a means to activate the interface using corresponding properties of metal ions. In this work, a cationic surfactant cetyltrimethylammonium bromide (CTAB) was modified by pre-mixing metal chlorides (FeCl3, CuCl2 and ZnCl2) and CTAB in 1:1 molar ratio using methanol as solvent. As a result, FeCl3, CuCl2 and ZnCl2 were added as [FeCl3Br]-1, [CuCl2Br]-1 and [ZnCl2Br]-1 respectively acting as counter anions (CAs) instead of bromide anion. Hence CTAB was called CTAF, CTAC and CTAZ in order of added CAs i.e., [FeCl3Br]-1, [CuCl2Br]-1 and [ZnCl2Br]-1 respectively. The effect of these metal chlorides on the micellar behaviour of CTAB was investigated using conductivities of their different solutions. Furthermore, the critical micelle concentration (CMC) as well as the degree of counterion binding were considered at 25.0, 30.0, 35.0, 40.0 and 45.0 and#176;C 0.1 and#176;C. The analysis of results revealed that incorporation of metal cations reduces the CMC values in the order CTAF CTAZ CTAC CTAB. Change in entropy (S), enthalpy (H) and free energy (G) were calculated to understand the thermodynamics of micellization under the circumstances. The aim of this study is to improve the understanding regarding the presence of transition metals in micellization process as well as the possibility of their interactions with Bromide counterions and possibility of the formation of complex counter-anions.

Very low surface tensions with “Hedgehog” surfactants

Previous studies [Langmuir, 2014, 30, 21, 6057–6063, Phys. Chem. Chem. Phys., 2017,19, 23869–23877] have shown that surfactants bearing highly-methylated alkyl tails (so-called “hedgehog” groups) are able to reduce the limiting surface tension at the aqueous critical micelle concentration (CMC) to γCMC ~24 mN m−1, which is considerably lower than for common n-alkyl tail surfactants (30–40 mN m−1). In the quest to develop even more effective and efficient non-fluorinated surfactants, this study introduces new amphiphiles having double and triple hedgehog tails and examines relationships between surfactant structure and aqueous solution properties. Of particular interest are links between γCMC, the effective hydrophobic-tail layer density (ρlayer) and total number of carbon and silicon atoms in the hydrophobic tails (NC+Si). Interestingly, γCMC is seen to depend on ρlayer rather than NC+Si, and ρlayer ~0.63 g cm−3 appears to be an optimal surface layer density for promoting low γCMC. For a surfactant bearing trimethysilyl (TMS) chain tips, exchanging the surfactant counterions from Na+ to Mg2+ reduced γCMC from 23.8 mN m−1 to 21.5 mN m−1, which is very low for a hydrocarbon surfactant, and comparable to typical fluorinated surfactants. A new measure of the ability of different surfactants to lower surface tension is proposed, which is helpful for targeting low surface energy (tension) non-fluorinated surfactants. In terms of both γCMC and CMC TMS-terminal tips are shown to be key groups for promoting hydrophobicity and/or low surface tensions.

Molecular Assembly in Block Copolymer-Surfactant Nanoparticle Dispersions: Information on Molecular Exchange and Apparent Solubility from High-Resolution and PFG NMR.

Internally structured block copolymer-surfactant particles are formed when the complex salts of ionic-neutral block copolymers neutralized by surfactant counterions are dispersed in aqueous media. Here, we report the 1H NMR signal intensities and self-diffusion coefficients (D, from pulsed field gradient nuclear magnetic resonance, PFG NMR) of trimethyl alkylammonium surfactant ions and the poly(acrylamide)-block-poly(acrylate) (PAAm-b-PA) polyions forming such particles. The results reveal the presence of an “NMR-invisible” (slowly exchanging) fraction of aggregated surfactant ions in the particle core and an “NMR-visible” fraction consisting of surface surfactant ions in rapid exchange with the surfactant ions dissociated into the aqueous domain. They also confirm that the neutral PAAm blocks are exposed to water at the particle surface, while the PA blocks are buried in the particle core. The self-diffusion of the polyions closely agree with the self-diffusion of a hydrophobic probe molecule solubilized in the particles, showing that essentially all copolymer chains are incorporated in the aggregates. Through centrifugation, we prepared macroscopically phase-separated systems with a phase concentrated in particles separated from a clear dilute phase. D values for the surfactant and block copolymer indicated that the dilute phase contained small aggregates (ca. 5 nm) of surfactant ions and a few anionic-neutral block copolymer chains. Regardless of the overall concentration of the sample, the fraction of block copolymer found in the dilute phase was nearly constant. This indicates that the dilute fraction represented a tail of small particles created by the dispersion process rather than a true thermodynamic solubility of the complex salts.

Modifications in the nanoparticle-protein interactions for tuning the protein adsorption and controlling the stability of complexes

We report the pathways to suppress or enhance the protein adsorption on nanoparticles and thereby control the stability of the nanoparticle-protein complexes with the help of selective additives. This has been achieved by tuning the electrostatic interaction between the nanoparticles and proteins, in the presence of surfactant and multivalent counterions. The preferential binding of the proteins with the surfactant and multivalent ions induced charge reversibility of nanoparticles can lead to adsorption of an otherwise non-adsorbing protein and vice versa. The findings are demonstrated for anionic silica nanoparticles and two globular proteins [lysozyme (cationic) and bovine serum albumin (BSA) (anionic)] as model systems, in the presence of two ionic surfactants [anionic sodium dodecyl sulfate (SDS) and cationic dodecyltrimethylammonium bromide (DTAB)], and ZrCl4 as multivalent salt. Small-angle neutron scattering with the unique advantage of contrast variation has been used to probe the role of individual components in the multi-component system. It is shown that the non-adsorbing behavior of BSA with silica nanoparticles changes into adsorbing in the presence of oppositely charged DTAB surfactant, whereas the strong adsorbing behavior of lysozyme on nanoparticles modifies to be non-adsorbing in the presence of oppositely charged SDS surfactant. The presence of multivalent counterions (ZrCl4) leads the charge reversal of the nanoparticles, transforming the lysozyme from adsorbing to non-adsorbing, and no significant change in the behavior of BSA. The results presented can find potential applications in the field of nanobiotechnology.

The removal selectivity of heavy metal cations in micellar-enhanced ultrafiltration: A study based on critical micelle concentration

Micellar enhanced ultrafiltration (MEUF) is a process to remove heavy metal by complexing it with the counterionic surfactant micelle, and the complex formed was subsequently removed using ultrafiltration. The surfactant is usually added in excess, which renders high rejection (low selectivity) for all metal cations in the mixture. This work emphasises MEUF conducted at surfactant dosage around the critical micelle concentration (CMC). Few studies revealed that the presence of counterions could lower the CMC of a charged surfactant. The magnitude of reduction depends on the strength of electrostatic interaction between the counterions and surfactant. Such observation suggests that the MEUF process's selectivity could be altered based on the dosage of surfactant and the types of cations presented in the solution. This work started with determining CMC of sodium dodecyl sulfate (SDS) in the presence of single or binary metal cations at varying pH (pH 3, pH 10 and without pH adjustment). The lowest CMC (4.00 mM) was obtained in the solution containing 100 ppm copper (II) at pH 3, about 42% lower compared to CMC of SDS obtained in pure water without pH adjustment (7.00 mM). Then, the filtration was performed using 7 mM of SDS, under varying pH (pH 3, pH 10 and no pH adjustment), initial metal concentration of 100 mg/L, pressure of 2.5 bar using polyethersulfone (PES) membrane of 10 k Da molecular weight cut-off (MWCO). Overall, the highest metal rejection was obtained at pH 10, with 66% of copper and 26% of chromium (III) was rejected. The highest selectivity of copper (II) over chromium (III) was observed at neutral pH. Such findings could be due to the higher diffusivity of copper cations over chromium cations in aqueous since copper cations can reach the micelle surface and form metal-micelle complex faster than chromium cations.

Fluorescence Study of the Influence of the Structure and Hydrophobicity of Fluorescent Dyes and Cationic Surfactants on their Association in Aqueous Solutions

The statistical characteristics of the dependences of the fluorescence signal of analytical systems as a function of the integral parameters of the structure of fluorescent reagents and cationic surfactants on their association in aqueous solutions has been investigated. Molecular weight, surface area, and their first-order molecular connectivity index have been taken as parameters of the structure of the reagents and cationic surfactants. The influence of the hydrophobicity of the reagent and cationic surfactants, such as the octanol–water distribution constant and octanol–water partition coefficient, on the fluorescence signal of the reagent–cationic surfactant associates have also been investigated. It is shown that the associates of anionic reagents with cationic surfactant counter ions are characterised by high stability and a higher analytical signal compared with associates in which there is no electrostatic attraction between the reagent and the surfactant ion. The effect of hydrophobicity of the reagent and cationic surfactant in the absence of electrostatic attraction between the interacting particles is similar. The increase in the role of the influence of the structure of cationic reagents in their association with cationic surfactants, when the electrostatic attraction is absent and the stability of the associates is due mainly to hydrophobic interactions, is noticeable. The regularities of the influence of the colloid-chemical state on the analytical signal of associated cationic surfactants in solutions have been investigated. The study made it possible to formulate a rational basis for the search and design of analytical systems for the determination of large cations by the fluorescence method.

Phosphonium‐based polythiophene conjugated polyelectrolytes with different surfactant counterions: thermal properties, self‐assembly and photovoltaic performances

AbstractPhosphonium‐based polythiophene conjugated polyelectrolytes (CPEs) with three different counterions (dodecylsulfate, octylsulfate and perfluorooctane sulfonate) are synthesized to determine how the nature of the counterion affects the thermal properties, the self‐assembly in thin films and the performance as the cathode interfacial layer in polymer solar cells (PSCs). The counterion has a significant effect on the thermal properties of the CPEs, affecting both their glass transition and crystalline behaviour. Grazing‐incidence wide‐angle X‐ray scattering studies also indicate that changing the nature of the counterion influences the microstructural organization in thin films (face‐on versus edge‐on orientation). The affinity of the CPEs with the underlying photoactive layer in PSCs is highly correlated with the counterion species. Finally, in addition to an increase of the power conversion efficiency of ca 15% when using these CPEs as cathode interfacial layers in PSCs, a higher device stability is noted, compared to a reference device with a calcium interlayer. © 2020 Society of Industrial Chemistry

A Water-Dispersible Quinoid-Resonant Conducting Polymer for Organic Electronics

Developing stable and solution-processable highly conductive polymers has been the research goal in organic electronics since the first demonstration of metallic conductive polyacetylene. Here, we used a unique quinoid-resonant building block thieno[3,4-b]thiophene (TbT) to develop a new water-dispersible conducting polymer, PTbT-Me:PSS. Linear polymerization and large surfactant counterion, poly(styrenesulfonate) (PSS−), were introduced, which enabled a high electrical conductivity of 68 S cm−1 and exhibited water-dispersible property. Interchain bipolaron was found in PTbT-Me:PSS when compared with polaron in PEDOT:PSS in their conducting mechanism. Moreover, we applied this highly conductive PTbT-Me:PSS as the solution-processed polymer thermoelectric material and a decent power factor of 3.1 μW m−1 K−2 was achieved.

Impact of Spacer Nature and Counter Ions on Rheological Behavior of Novel Polymer-Cationic Gemini Surfactant Systems at High Temperature.

Compatible surfactant-polymer (SP) hybrid systems at high temperature are in great demand due to the necessity of chemical flooding in high-temperature oil reservoirs. The rheological properties of novel SP systems were studied. The SP system used in this study consists of a commercial polymer and four in-house synthesized polyoxyethylene cationic gemini surfactants with various spacers (mono phenyl and biphenyl ring) and different counterions (bromide and chloride). The impact of surfactant concentration, spacer nature, counterions, and temperature on the rheological features of SP solutions was examined using oscillation and shear measurements. The results were compared with a pure commercial polymer. All surfactants exhibited good thermal stability in seawater with no precipitation. Shear viscosity and storage modulus were measured as a function of shear rate and angular frequency, respectively. The experimental results revealed that the novel SP solution with a mono phenyl and chloride counterions produces a better performance in comparison with the SP solution, which contains mono phenyl and bromide counterions. Moreover, the effect is enhanced when the mono phenyl ring is replaced with a biphenyl ring. Shear viscosity and storage modulus decrease by increasing surfactant concentration at the same temperature, due to the charge screening effect. Storage modulus and complex viscosity reduce by increasing the temperature at a constant angular frequency of 10 rad/s. Among all studied SP systems, a surfactant containing a biphenyl ring in the spacer with chloride as a counterion has the least effect on the shear viscosity of the polymer. This study improves the understanding of tuning the surfactant composition in making SP solutions with better rheological properties.

How the Influence of Different Salts on Interfacial Properties of Surfactant–Oil–Water Systems at Optimum Formulation Matches the Hofmeister Series Ranking

AbstractIn this work, we present the effects of salts on sodium dodecyl benzene sulfonate micellization and on the interfacial performance of a sodium dodecyl benzene sulfonate–heptane–brine system at optimum formulation, i.e., hydrophilic–lipophilic deviation (HLD) = 0. In order to do that, interfacial tension and dilational interfacial rheology properties of surfactant–heptane–water systems at optimum formulation are measured using an interfacial spinning drop tensiometer with an oscillating velocity, which can accurately measure interfacial rheology properties at both low and ultralow interfacial tensions. The brines used contain one of the following salts: MgCl2, CaCl2, NaCl, NH4Cl, NaNO3, CH3COONa, or Na2SO4. We performed a one‐dimensional salinity scan with each of these salts to achieve an optimum formulation. In relation to the Hofmeister series, we found that, at optimum formulation, systems with chaotropic ions (NH4+, NO3−) present interfaces with ultralow interfacial tensions, very low dilational modulus, and a low phase angle, whereas kosmotropic ions (Mg2+, Ca2+, SO4−2) generate high interfacial tension and high rigidity monolayers. Intermediate ions in the Hofmeister series (Na+, CH3COO−, Cl−) present interfaces with intermediate properties. Furthermore, according to the Hofmeister series, interfaces can be respectively ordered from higher to lower rigidity for surfactant counterions Mg2+ &gt; Ca2+ &gt; Na+ &gt; NH4+ and coions SO42− &gt; CH3COO– &gt; Cl− &gt; NO3−, which correspond to a salting‐out (highest rigidity) and salting‐in (lowest rigidity) effect. We observed that counterions have a more significant effect on surfactant–oil–water system properties than those that act as coions.

Influence of surfactant chain length, counterion and OrMoSil precursors on reversibility and working interval of pH colorimetric sensors

An in-depth study was conducted to determine the influence of some important parameters to produce a pH colorimetric sensor characterized by long-term stability, lack of leaching, high reversibility, repeatability and extended working range. Bromothymol Blue (BB) was used as pH sensitive molecule. Its performance was monitored while tailoring the concentration of some added cationic surfactants, together with the organic fraction of the sol-gel composition to avoid leaching phenomena, low homogeneity and to increase reversibility. Five cationic surfactants with different linear alkyl chain length (from C8 to C18) were added to a sol-gel containing BB. The C16 was chosen as optimal. The effect of the surfactant counterion was also investigated. The C16 surfactant had a pH working interval of 3 pH unit by using chloride, 1.5 with bromide and 1 pH unit with p-toluenesulfonate as counterions, respectively. Chloride was characterized by a larger inhomogeneity while p-toluenesulfonate gave homogeneity and high repeatability. The hexadecyltrimethylammonium p-toluenesulfonate was therefore chosen as the best surfactant. Leaching in alkaline medium was avoided by using the dodecyl moiety among three organic groups linked to the OrMoSil matrix (dodecyl, phenyl and methyl). Colorimetric sensor arrays made with these specifications remained stable and reversible for months.

Interactions of cationic surfactants with polyvinylpyrrolidone (PVP): Effects of counter ions and temperature

The interactions between the polymer polyvinylpyrrolidone (PVP) and conventional surfactants such as cetyltrimethylammonium chloride (CTACl) and cetyltrimethylammonium bromide (CTABr) were investigated using a conductometric method. The critical micelle concentrations (CMCs), the critical aggregation concentrations (CACs) and degrees of ionization (α) of aqueous solutions of the surfactants and PVP were determined via electrical conductivity measurements at various temperatures. The thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of the surfactants were calculated in the absence and presence of the polymer, and the results were compared to determine the effects of counter ion on the micellization of the surfactants and the energetics of the process. The main goal of this work is to elucidate the various driving forces that lead to complexation between the surfactants and polymer. Electrostatic interactions play a major role in the binding of surfactants to polymers or surfaces in aqueous systems. The strength of the interaction is found to depend on the nature of the surfactant counter ion type.

Application of foam separation method for removal of alkaline earth metal ions from solution

Foam separation was used to separate dissolved alkaline earth metal ions from aqueous solution. The mechanism is based on anionic surfactant adsorption onto the bubbles generated in the aqueous alkaline earth metal solution. The negatively charged bubbles rise while adsorbing the positive metal ions and change to foam at the surface. The foam in the glass tube drains excess water as it rises and is finally turned into dry foam. The foam separation method with sodium decyl sulfate (SDeS) (8 mmol L−1, below the critical micelle concentration (CMC)) was used to remove alkaline earth metal (Mg, Ca, and Sr) chlorides in the respective systems. The initial concentrations of the surfactant and alkaline earth metal chloride were 8 mmol L−1 (below the CMC) and 2.5 mmol L−1 in 250 mL of solution, respectively. The results showed that the removal rate in the one-component alkaline earth metal system increased with increasing atomic number: Mg (46%) < Ca (58%) < Sr (77%) in 5 h of foam separation. A similar trend was observed in the foam separation in a mixed system of alkaline earth metals. However, the concentration of the surfactant counterion (Na+) remained almost constant after the operation, indicating that the divalent ions, rather than the monovalent surfactant counterions, were preferentially adsorbed onto the sulfate group of the anionic surfactant. The changes in the concentration of the alkaline earth metal ions were analyzed using kinetics. The plot of the first-order rate constant against the ionic radius of each metal suggested a linear relationship in each series of alkaline earth metals or alkali metals. The present study indicates that foam separation using SDeS can be used to effectively remove large alkaline earth metal ions from contaminated water.

Anionic surfactant-stabilized hydrophobic ionic liquid-based bicontinuous microemulsion: Formulation, microstructure and laccase kinetics

The composition-dependent phase behavior of the [Cmmim][PF6]/buffer/[Cnmim][AOT]/n-butanol pseudoternary microemulsion system was investigated by the method of T-γ fishlike phase diagram. It is found that the phase inversion temperature and the surfactant efficiency decrease with the increase of the alkyl chain length (n) of the surfactant counterion and that (m) of the cation of the hydrophobic ionic liquid (HIL). The [C2mim][AOT]-stabilized [Omim][PF6]-based pseudoternary microemulsion system was found to have high surfactant efficiency and moderate phase inversion temperature. The microstructure of bicontinuous microemulsion formed in the pseudoternary system was characterized by SAXS technique, and the influence of microstructure on the catalytic performance of solubilized laccase was also explored. As the mass ratio of HIL to water increases, the microdomain size of the system decreases, and the catalytic efficiency (kcat/Km) of laccase also decreases. When the surfactant content gradually increases, the microdomain size of the system changes little, but its interfacial rigidity increases, and also the interaction between the interface and the solubilized enzyme increases accordingly, resulting in a decrease in the kcat/Km of the solubilized laccase. The variation of temperature over 35 °C–55 °C has little effect on the microdomain size of the system, but it has marked effect on the kcat/Km of the solubilized laccase. The optimal catalytic efficiency of laccase in the present microemulsion is estimated to be ca. 50 °C, which is significantly higher than that (40 °C) in the aqueous medium system, indicating that the bicontinuous microstructure in the system is helpful to improve the thermal stability of laccase. The present study is helpful for the rational formulation of laccase-compatible anionic surfactant-stabilized hydrophobic ionic liquid-based bicontinuous microemulsions, which is suitable for template-assisted biosynthesis of conducting polymers.

The influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces

The objective of this research is to examine the influence of surfactant and solution composition on PFAS adsorption at fluid-fluid interfaces. Surface tensions were measured for select PFAS, as well as a representative hydrocarbon surfactant. These data are supplemented with data sets collected from the literature. The influence of surfactant headgroup charge, specifically for zwitterionic PFAS, was investigated. The impacts of surfactant counterion for ionic PFAS and the influence of headgroup size for nonionic PFAS were also investigated. In addition, the influence of solution ion composition, ionic strength, and pH was examined. The impact of co-solutes, specifically ethanol, humic acid, and trichloroethene, was also examined, as well as the behavior of PFAS mixtures and fluorocarbon-hydrocarbon surfactant mixtures. The data were interpreted within the framework of a QSPR model recently developed to predict fluid-fluid interfacial adsorption coefficients (Ki) of PFAS. The results demonstrate that all of the factors investigated have some degree of impact on Ki values. Thus, the composition of soil-pore water and groundwater is likely to affect the magnitude of PFAS adsorption at air-water and organic liquid-water interfaces. However, the influence on Ki of most of the factors investigated is small for lower PFAS concentrations (less than ∼1–10 mg/L). Hence, their impacts on fluid-fluid interfacial adsorption are likely to be relatively minor at the low PFAS concentrations representative of many environmental systems, especially compared to the impact of other factors such as fluid saturations, porous-medium properties, and PFAS molecular structure. The results of this study indicate that the revised QSPR model provides reasonable first-order predictions of Ki for a wide range of PFAS in environmental systems.

Molecular dynamics simulation of spherical-to -threadlike micelle transition in a cationic surfactant solution

ABSTRACTThe effect of sodium salicylate (NaSal) on the spherical-to-threadlike micelle shape transition in 3-hexadecyloxy-2-hydroxy-propyl trimethyl ammonium bromide (R16HTAB) solution was studied using molecular dynamics simulation. The simulations were started from a preassembled infinitely long threadlike micelle of R16HTAB. By analyzing the aggregation morphologies and structural details, we find that the preassembled threadlike micelle in the absence of NaSal was unstable and assembled into a spherical micelle. While in the presence of NaSal, the threadlike micelle exhibited fluctuations but remained the threadlike shape during the long simulation run. The Sal− ions were found to penetrate inside the micelle, which promoted the junction between the surfactant and salicylate counterion. The aromatic Sal− ions located in the surfactant headgroup region with their phenyl groups pointing toward the interior core region of the micelle. From another simulation started with two individual spherical micelles, we found that the Sal− ions can link the two spherical micelles into a long threadlike micelle, in accordance with a mode proposed by experimental studies. Our studies showed that the H-bonds and electrostatic interactions between the Sal− ions and the surfactants played an important role in micellar growth and stabilising the threadlike micelle.

Modified method of conductometric data analysis to calculate the conductivity of surfactant ions

Methodology for simple analytical refinement of the equivalent electrical conductivities of surfactant ions and counterions was proposed in the framework of the Debye – Hückel – Onsager theory as applied to surfactant dispersions at various concentrations. The developed methodology is based on the use of the mathematical form for the concentration dependencies of the specific conductivity in the premicellar region and makes it possible to calculate the equivalent conductivities of surfactant ions both under infinite dilution conditions and near the CMC. One of the advantages of the described method is the possibility of calculating the ion conductivities in the presence of a minimum number of experimental points (formally, a straight line can be constructed and its tangent of the angle of inclination can be determined even by two points corresponding to region 0.2 CMC — 0.8 CMC). Using the values of the equivalent conductivities of surfactant ions and counterions calculated for the required concentrations, allows to determine the parameters of the solution more accurately, including the contribution of micelles to the total conductivity of the solution.

Phase-dependent shear-induced order of nanorods in isotropic and nematic wormlike micelle solutions.

Small angle X-ray scattering with in situ shear was employed to study the assembly and ordering of dispersions of gold nanorods within wormlike micelle solutions formed by the surfactant cetylpyridinium chloride (CPyCl) and counter-ion sodium salicylate (NaSal). Above a threshold CPyCl concentration but below the isotropic-to-nematic transition of the micelles, the nanorods self-assembled under quiescent conditions into isotropically oriented domains with hexagonal order. Under steady shear at rates between 0.5 and 7.5 s-1, the nanorod assemblies acquired macroscopic orientational order in which the hexagonal planes were coincident with the flow-vorticity plane. The nanorods could be re-dispersed by strong shear but re-assembled following cessation of the shear. In the nematic phase of the micelles at higher surfactant concentration, the nanorods did not acquire hexagonal order but instead formed smectic-like layers in the gradient-vorticity plane under shear. Finally, at still higher surfactant concentration, where the micelles form a hexagonal phase, the nanorods showed no translational ordering but did acquire nematic-like order under shear due to alignment in the flow. Depletion forces mediated by the wormlike micelles are identified as the driving mechanism for this sequence of nanorod ordering behaviors, suggesting a novel mechanism for controlled, reconfigurable assembly of nanoparticles in solution.