Molecular structure of maltoside surfactants controls micelle formation and rheological behavior

The phase behavior and rheological properties of a multi‐component system, made of a zwitterionic surfactant cocoamidopropyl betaine (CAPB), an anionic surfactant sodium lauryl sulfate (SLSS), and mixed salts (tetrasodium pyrophosphate, sodium acid pyrophosphate, sacharrin, and sodium fluoride) in sorbitol/H2O mixed solvent at different mass fraction of SLSS (XSLSS) were systematically investigated by steady and dynamic rheology, dynamic light scattering, and diffusion ordered spectroscopy (DOSY). When fixing the salt concentration and the mass ratio of sorbitol in mixed solvent (R), the zero‐shear viscosity increases first and then decreases showing a maximum with increasing XSLSS, resulting from the formation and entanglement of wormlike micelles. Especially when XSLSS is between 0.33 and 0.80, the mixture is dominated by entangled wormlike micelles coexisting with small micelles and separated wormlike micelles, and shows high viscoelasticity. The maximum of the zero‐shear viscosity is ca. 5 orders of magnitude larger than that of sorbitol/H2O mixed solvent or the CAPB/SLSS aqueous solution. The characteristic structural parameters for the micellar solutions at different XSLSS are also estimated from further analysis of the rheological results, and indicate the stronger network structures of the wormlike micelles are formed in our systems compared with the wormlike micelles formed by a traditional zwitterionic/anionic surfactant aqueous solutions. The great improvements of rheological properties are attributed to the strong screening effects of the mixed salts and the strong solvophobic effect of sorbitol on the electrostatic and hydrophobic interaction between the CAPB and SLSS molecules. The present work has improved our understanding about the aggregation behavior of zwitterionic/anionic mixed surfactants with salts in less polar solvent/H2O mixture, which would be of widely practical importance to optimize the formulation of products for personal care and household cleaning.

Rheological properties of wormlike micellar gels formed by novel bio-based isosorbide surfactants

The effect and enhancement mechanism of hydrophobic interaction and electrostatic interaction on zwitterionic wormlike micelles

Surfactant-based fluids, SB fluids exhibit complex rheological behavior due to substantial structural change caused by the molecules self-assembled colloidal aggregation. Various factors affect their rheological properties. Among these factors, surfactant concentration, shear rate, temperature, and salinity are investigated. One of the most popular surfactants, Aromox® APA-T viscoelastic surfactant (VES) is examined. The study focuses on four different concentrations (1.5%, 2%, 3%, and 4%) over a shear rate ranging from 0.0526 sec−1 to 1944 sec−1 using Bohlin rheometer. For salinity effects, two brine solutions are used; 2 and 4% KCl while for temperature effects, a wide range from ambient temperature of 72°F up to 200°F is covered. The results show that SB fluids exhibit a complex rheological behavior due to its unique nature and the various structures form in the solution. In general, SB fluids at all concentrations exhibit a non-Newtonian pseudo-plastic shear thinning behavior. As the surfactant concentration and/or shear increases, a stronger shear thinning behavior can be seen. Increasing solution salinity promotes formation of rod-like micelles and increases its flexibility. Salinity affects micelles’ growth and their rheological behavior is very sensitive to the nature and structure of the added salt. Different molecular structures are formed; spherical micelles occur first and then increased shear rate and/or salinity promotes the formation of rod-like micelles. Later, rod-like micelles are aligned in the flow direction and form a large super ordered structure of micellar bundles or aggregates called shear induced structure (SIS). Different structures implies different rheological properties. Likewise, rheology improves with increasing temperature up to 100°F. Further increase in temperature reverses the effects and viscosity decreases. However, the effects of temperature and salinity diminish at higher shear rates. Furthermore, a rheology master curve is developed to further understand the rheological behavior of SB fluids and correlate rheological properties to its microscopic structure.

Simulations based on dissipative particle dynamics are performed to investigate the solubilization mechanism of vesicles by surfactants. Surfactants tend to partition themselves between vesicle and the bulk solution. It is found that only surfactants with suitable hydrophobicity are able to solubilize vesicles by forming small mixed micelles. Surfactants with inadequate hydrophobicity tend to stay in the bulk solution and only a few of them enter into the vesicle. Consequently, the vesicle structure remains intact for all surfactant concentrations studied. On the contrary, surfactants with excessive hydrophobicity are inclined to incorporate with the vesicle and thus the vesicle size continues to grow as the surfactant concentration increases. Instead of forming discrete mixed micelles, lipid and surfactant are associated into large aggregates taking the shapes of cylinders, donuts, bilayers, etc. For addition of surfactant with moderate hydrophobicity, perforated vesicles are observed before the formation of mixed micelles and thus the solubilization mechanism is more intricate than the well-known three-stage hypothesis. As the apparent critical micellar concentration (φ(s,v)(a,CMC)) is attained, pure surfactant micelles form and the vesicle deforms because the distribution of surfactant within the bilayer is no longer uniform. When the surfactant concentration reaches φ(s,v)(p), the vesicle perforates. The extent of perforation grows with increasing surfactant concentration. The solubilization process begins at φ(s,v) (sol), and lipids leave the vesicle and join surfactant micelles to form mixed micelles. Eventually, total collapse of the vesicle is observed. In general, one has φ(s,v)(a,CMC)≤φ(s,v)(p)≤φ(s,v)(sol).

Mixed micelles and other structures in the solubilization of bilayer lipid membranes by surfactants

Wormlike micelles (or reverse wormlike micelles) are flexible cylindrical chains that are normally formed in water (or a nonpolar organic solvent) at 25.0 °C or above; the formation of wormlike micelles at lower temperatures is rare. Here, we have reported wormlike micelles formed at low temperature using an ionic liquid surfactant (1-octadecyl-3-nonyl imidazolium bromide) in polar organic solvents (including 1,3-propanediol, 1,2-propylene glycol, N,N-dimethylformamide, and glycerol/1,2-propylene glycol mixture) in the absence of any additives. The viscoelasticity and morphology of the wormlike micelles were studied using rheology, small-angle X-ray scattering, and cryo-transmission electron microscopy. The viscoelastic properties of the wormlike micelles in polar solvents are affected by the solvent type (or the weight ratio of glycerol to 1,2-propylene glycol), surfactant concentration, and temperature. Moreover, the G' and G'' crossover twice in the dynamic curves, which is different from the case in water. The first crossover (at low frequency) corresponds to the relaxation time for the alkyl chains to disentangle from the transient network, and the second crossover (at high frequency) is related to the segmental motion of the chains. Furthermore, the tribological performance of these wormlike micelles is investigated at low temperature. It is found that the protective film (formed by the physical adhesion of the wormlike micelles on the surface of friction disk pair) and the tribochemical reaction together lead to good antifriction and antiwear performance, which indicates the application prospects of these wormlike micelles in low-temperature lubrication.

The research included a study of the viscosity during the formation of worm-like or filamentous micelles of a binary mixture of surfactants and their susceptibility. The mixture of Anionic surfactant Sodium Dodecyl Sulfate (SDS) with cationic surfactant Cetyl Pyridinium Chloride (CPC) was studied at different temperatures (283.15, 303.15, 323.15, and 343.15)K, It was found that the highest viscosity value was at the ratio of 2: 8 of 3% wt of CPC and SDS, respectively, which indicates the formation of filamentous micelles. The research also included a study of the effect of ethyl alcohol with concentrations (0.001, 0.01, 0.1, 1.0)M, as well as at different temperatures on the formation of worm-like micelles at the ratio 2: 8 of CPC and SDS respectively. The thermodynamic functions, as well as the activation energy were calculated by the effect of Ethyl alcohol . The results showed an increase in the formation of worm-like micelles when increasing the concentration of ethyl alcohol as a result of interference with the composition of micelles and the formation of hydrogen bonds with the polar groups of the surfactant molecule . Therefore the mixture viscosity increased at (283.15)K, from 5.3741 Pa.s ×102 before adding (1.0) M, ethyl alcohol to be 5.7077 Pa.s x 102 after alcohol addiction. The research also included preparing titanium dioxide at the nanoscale and diagnosing it with AFM, TEM techniques. The results showed an increase in the formation of micelles with an increase in the concentration of nanoscale titanium dioxide, as the viscosity of the mixture increased at the temperature (283.15)K, from 5.7077 Pa.s ×102) before adding (0.001)M, titanium dioxide and (1.0)M, ethanol to 8.6318 Pa.s×102 after addition.

Wormlike surfactant micelles are widely used in various applications including fracturing technology in oil industry, template synthesis of different nanoobjects, micellar copolymerization of hydrophilic and hydrophobic monomers, and so forth. Most of those applications suggest the solubilization of different additives in the micelles. The present paper is aimed at the comparative study of the effect of the solubilization of hydrophobic (n-decane and 1-phenylhexane) and hydrophilic (N-isopropylacrylamide and acrylamide) substances on the rheological properties and structure of the micelles using several complementary techniques including rheometry, small angle neutron scattering, dynamic light scattering, and diffusion ordered NMR spectroscopy. For these studies, mixed micelles of potassium oleate and n-octyltrimethylammonium bromide containing the excess of either anionic or cationic surfactants were used. It was shown that hydrophobic additives are completely solubilized inside the micelles being localized deep in the core (n-decane, 1-phenylhexane) or near the core/corona interface (1-phenylhexane). At the same time, only a small fraction of hydrophilic additives (14% of N-isopropylacrylamide and 4% of acrylamide) penetrate the micelles being localized at the corona area. Despite different localization of the additives inside the micelles, all of them induce the breaking of wormlike micelles with the formation of either ellipsoidal microemulsion droplets (in the case of hydrophobic additives) or ellipsoidal surfactant micelles (in the case of hydrophilic additives). The breaking of micelles results in the drop of viscosity of the solution up to water value. The main result of this paper consists in the observation of the fact that for all the additives under study, the dependences of the viscosity on the volume fraction of additive lie on the same master curve being shifted along the volume fraction axis by a certain factor depending on the hydrophobicity of the added species. Those data are quite useful for various applications of wormlike surfactant micelles suggesting the solubilization of different additives inside them.

Transition from rigid to flexible structures in micelles of Undecylammonium chloride in aqueous solutions of NaCl

Fundamental understanding of the state of catalyst inks and fabrication process is important to fabricate well-established catalyst layers of proton exchange membrane fuel cells (PEMFCs). The catalyst ink is a slurry which contains platinum-supported carbon and ionomer in a solvent. The state of the catalyst ink affects dynamics of materials during the fabrication process of the catalyst layer(1), and then the resultant structure(2). Additionally, the state of the catalyst ink can have time dependence, which means the procedure of particle agglomeration and sedimentation inside the slurry. Therefore, quantitative evaluation of the catalyst ink is required. In this study, a rheological measurement was conducted to characterize catalyst inks used to fabricate PEMFC catalyst layers. The rheological measurement can apply to opaque and high viscosity slurries like catalyst inks. Ionomer dispersion, pseudo-catalyst ink and catalyst ink with various ionomer content were measured to clarify the relationship between the rheological behavior and the types and amount of particles and interparticle interactions in the slurry. The rheological measurement was conducted by using a rheometer with a cone and plate system. Shear rate range was 200 to 3000 /s. The measurement temperature was 25°C. Downward measurement (from upper to lower shear rate) following the upward measurement was conducted. Ionomer dispersion (DE2020, Sigma-Aldrich), carbon black (Ketjenblack, Lion)–ionomer dispersion (pseudo-catalyst ink), and platinum-supported carbon (Pt/C, TEC10E50E, Tanaka Kikinzoku Kogyo)–ionomer dispersion (catalyst ink) were prepared. The volume fraction of carbon to the solvent (45wt.% 1-propanol aqueous solution) was 2% and ionomer to carbon ratio (I/C) was a parameter from 0.3 to 2.0. The viscosities of the slurries as a function of the shear rate are shown in Figure 1. The three types of the slurries showed quite different rheological properties with each other. The ionomer dispersions are Newtonian fluid in all I/C conditions from 0.3 to 2.0 (Fig. 1 (a)). On the other hand, pseudo-catalyst ink (Fig. 1 (b)) and catalyst ink (Fig. 1 (c)) were non-Newtonian fluid when I/C was low. The pseudo-catalyst ink showed a transition from non-Newtonian to Newtonian as I/C increased from 0.3 to 0.5. The ink also showed the transition from Newtonian to non-Newtonian as I/C increased from 1.0 to 2.0. The catalyst ink slightly showed non-Newtonian properties even in I/C 0.5 and 1.0. Additionally, the catalyst ink with I/C 0.3 and 0.5 showed relatively large hysteresis between the upward and downward measurements. The rheological property of the slurries can be the results of the state difference of the particles in the slurries such as the particle agglomeration and/or ionomer adsorption. The measurements of particle size distribution by dynamic light scattering and zeta potential were applied to the slurries (The results are not shown here). The comprehensive discussion to understand the state of the catalyst ink will be presented. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 16K18028. References T. Suzuki, M. Kobayashi, H. Tanaka, M. Hayase and S. Tsushima, ECS Trans., 69, 465 (2015).W. K. Epting, J. Gelb and S. Litster, Adv. Funct. Mater., 22, 555 (2012). Figure 1

In the present study, the micelle aggregation numbers and hydrodynamic radius distributions of the gemini surfactants (abbreviated as Cm-(EO)n-E-Cm (m = 12–16, n = 1, 2)) were investigated by fluorescence spectroscopy and dynamic light scattering (DLS). The rheological properties of these surfactants were determined using a rheometer. The Surfactants C14-(EO)-E-C14 and C16-(EO)-E-C16 formed worm-like micelles at a concentration of 200 mmol/L. It was found that longer hydrophobic chains or shorter linkage groups promoted the growth of worm-like micelles. Then, the influence of the inorganic salt NaBr and the organic salt sodium salicylate (NaSal) on the formation and properties of wormlike micelles in aqueous C16-(EO)-E-C16 solutions at a surfactant concentration of 20 mmol/L was investigated. The results showed that NaSal could penetrate deeper into the hydrophobic group of the micelle due to its good hydrophobic interaction, which promoted micelle growth and resulted in worm-like micelles. In addition, the tested systems showed a Maxwell behavior at lower frequencies, but deviated from the Maxwell mode at higher frequencies. Finally, the formation of worm-like micelles was also verified by micro polarity measurements.

Micelles are self-assembled aggregates that are formed, in many different structures, from surfactants and have a wide range of applications. The critical micelle concentration (CMC) is the concentration of surfactant above which the micelle formation becomes appreciable. This paper reviews techniques that measure the CMC during the formation of micelles involving metal nanoparticles (NPs) with an emphasis on silver NPs. Specifically this review collects and compares such techniques in terms of their advantages and disadvantages. In doing that, this review identifies the useful experimental data on the CMC that each technique yields, but also points out the limitations of each technique. Furthermore, the authors propose an in situ method by using contact angle determination on a substrate/system to measure the CMC in real time. Thus, the goal of this review is to provide a comprehensive list of the most commonly used techniques for the CMC measurement so that future researchers may make informed decisions on what method(s) to use to best meet their needs.

The objective of this study was to characterize the flow and rheological behaviour of model wax-stabilized water-in-oil (W/O) emulsions consisting of light mineral oil, paraffin wax and glycerol monooleate as the oil phase and water as the dispersed aqueous phase. An[sic] laboratory-scale benchtop flowloop system was used to explore the flow behaviour of the emulsions' oil phase (oil, paraffin wax and surfactant). The key contribution from this work was that the higher initial temperature gradient (40°C compared to 19°C) experienced by the rapidly-cooled oil led to more initial deposition on the flowloop inner wall. The rheological properties of W/O emulsions with different water cuts (10-50wt%) were also studied. Rotational, oscillatory rheology and creep compliance and recovery were characterized on emulsions aged up to 28 days. Overall, the results demonstrated that emulsion composition, and age could significantly influence an emulsion's flow behaviour and rheological properties.

Thermal stability and rheological properties of a novel surfactant–polymer system containing non-ionic ethoxylated fluorocarbon surfactant was evaluated. A copolymer of acrylamide (AM) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) was used. Thermal stability and surfactant structural changes after aging at 100 °C were evaluated using TGA, 1H NMR, 13C NMR, 19F NMR and FTIR. The surfactant was compatible with AM–AMPS copolymer and synthetic sea water. No precipitation of surfactant was observed in sea water. The surfactant was found to be thermally stable at 100 °C and no structural changes were detected after exposure to this temperature. Rheological properties of the surfactant–polymer (SP) system were measured in a high pressure rheometer. The effects of surfactant concentration, temperature, polymer concentration and salinity on rheological properties were studied for several SP solutions. At low temperature (50 °C), the viscosity initially increased slightly with the addition of the surfactant, then decreased at high surfactant concentration. At a high temperature (90 °C), an increase in the viscosity with the increase in surfactant concentration was not observed. Overall, the influence of the fluorocarbon surfactant on the viscosity of SP system was weak particularly at high temperatures and high shear rate. Salts present in sea water reduced the viscosity of the polymer due to a charge shielding effect. However, the surfactant was found to be thermally stable in the presence of salts.