V.S. Molchanov

How a viscoelastic solution of wormlike micelles transforms into a microemulsion upon absorption of hydrocarbon: new insight

In this article, we investigate the effect of hydrocarbon addition on the rheological properties and structure of wormlike micellar solutions of potassium oleate. We show that a viscoelastic solution of entangled micellar chains is extremely responsive to hydrocarbons-the addition of only 0.5 wt % n-dodecane results in a drastic drop in viscosity by up to 5 orders of magnitude, which is due to the complete disruption of micelles and the formation of microemulsion droplets. We study the whole range of the transition of wormlike micelles into microemulsion droplets and discover that it can be divided into three regions: (i) in the first region, the solutions retain a high viscosity (∼10-350 Pa·s), the micelles are entangled but their length is reduced by the solubilization of hydrocarbons; (ii) in the second region, the system transitions to the unentangled regime and the viscosity sharply decreases as a result of further micelle shortening and the appearance of microemulsion droplets; (iii) in the third region, the viscosity is low (∼0.001 Pa·s) and only microemulsion droplets remain in the solution. The experimental studies were accompanied by theoretical considerations, which allowed us to reveal for the first time that (i) one of the leading mechanisms of micelle shortening is the preferential accumulation of the solubilized hydrocarbon in the spherical end caps of wormlike micelles, which makes the end caps thermodynamically more favorable; (ii) the onset of the sharp drop in viscosity is correlated with the crossover from the entangled to unentangled regime of the wormlike micellar solution taking place upon the shortening of micellar chains; and (iii) in the unentangled regime short cylindrical micelles coexist with microemulsion droplets.

Viscoelasticity of Smart Fluids Based on Wormlike Surfactant Micelles and Oppositely Charged Magnetic Particles

Novel viscoelastic smart suspensions based on cationic wormlike micelles (WLMs) of erucylbis(hydroxyethyl)methylammonium chloride and oppositely charged submicron magnetite particles in the presence of added low molecular weight salt were prepared and investigated. The suspensions demonstrate remarkable stability against sedimentation, which can be due to the incorporation of particles into the network of entangled WLMs by linking to energetically unfavorable micellar end-caps. Added particles enhance significantly the viscosity, the plateau modulus, and the relaxation time of the system, acting as additional multifunctional physical cross-links in the micellar network. The increase of plateau modulus stops when the concentration of particles reaches ca. 1.5 wt %, indicating that all micellar end-caps available in the system are linked to the particles. Further addition of particles may lead just to the redistribution of micellar ends between the particles without creation of new elastically active chains. The increase of rheological characteristics by added particles is more pronounced in suspensions with a smaller content of low molecular weight salt KCl when the WLMs are shorter in length and therefore contain a larger amount of end-caps responsible for the interaction with the particles. Magnetite particles not only enhance the rheological characteristics but also impart magnetoresponsive properties to the suspension. Upon application of magnetic field, the liquidlike system transforms into a solidlike one, demonstrating a constant value of storage modulus in all frequency range and the appearance of yield stress, which is due to the formation of field-aligned chainlike aggregates of particles opposing the flow. A combination of responsive properties inherent to both the matrix and the particles makes these smart fluids very competitive with other magnetic soft matter materials for various applications.

Effects of concentration and temperature on viscoelastic properties of aqueous potassium oleate solutions

The rheological properties of semidilute aqueous solutions containing long cylindrical micelles of an anionic surfactant, potassium oleate, are studied. It is shown that, at surfactant concentrations above 1 wt %, the rheological properties of the solutions are adequately described in terms of the simple Maxwell model of a viscoelastic liquid characterized by a single relaxation time. Based on the analysis of normalized dependences of the loss modulus on the storage modulus, the characteristic times of the processes governing the rheological properties of the above systems, i.e., the average breaking time and reptation time of micelles, are estimated. It is found that the breaking time of micelles decreases and relaxation time increases with increasing surfactant concentration due to lengthening of micellar chains.

Dominant role of wormlike micelles in temperature-responsive viscoelastic properties of their mixtures with polymeric chains

Temperature effects on the rheological properties of viscoelastic solutions containing entangled wormlike micelles of potassium oleate and hydrophobically modified polyacrylamide were studied in a wide range of polymer concentrations. A very pronounced drop of viscosity by four orders of magnitude was observed at heating from 20 to 78 °C both in the presence and in the absence of polymer indicating that the wormlike micelles are mainly responsible for this effect. The highly thermosensitive behavior was attributed to the shortening of micellar chains induced by heating. Although the decrease in viscosity is almost the same for both surfactant and surfactant/polymer systems, the absolute values of the viscosity in the presence of polymer are by few orders of magnitude higher, which is due to the formation of a common network of entangled polymer and micellar chains. As a result, the added polymer allows one to get highly temperature responsive system that keeps viscoelastic properties in a much wider range of temperatures, which makes it very promising for various practical applications.

Temperature effect on the viscoelastic properties of solutions of cylindrical mixed micelles of zwitterionic and anionic surfactants

The viscoelastic properties of semidilute mixed aqueous solutions of oleyl amidopropylbetaine and sodium dodecylbenzene sulfonate are studied in the temperature range of 20–40°C. It is shown that, at 20°C, the solution viscosity can be as high as 390 (Pa s), which is related to the formation of a network of entanglements of long cylindrical micelles of surfactants. It is revealed that, upon heating, the viscosity and relaxation time of the system decrease, while the contour length of cylindrical micelles decreases consider-ably. It is shown that this susceptibility of viscoelastic surfactant solutions to temperature is due to the low energy of break, which is much lower than the energy of covalent bonds.

A study of n -dodecylammonium acrylamido-2-methylpropanesulfonate association in aqueous solutions

Association of a micelle-forming monomer, n-dodecylammonium 2-acrylamido-2-methylpropanesulfonate, in aqueous solutions has been studied in a concentration range of 1.5 × 10−3–2.5 × 10−1 M with the use of conductometry, capillary and rotational viscometry, isothermal translational diffusion, and atomic force microscopy. Two critical micelle concentrations have been determined, and the shape of n-dodecylammonium 2-acrylamido-2-methylpropanesulfonate micelles has been proposed.

Effect of polymer on rheological behavior of heated solutions of potassium oleate cylindrical micelles

The rheological characteristics of aqueous solutions of potassium oleate cylindrical micelles and their mixtures with hydrophobized polyacrylamide are studied at different temperatures and polymer concentrations no higher than the concentration of overlapping of coils. It is shown that, at all temperatures, the viscosities of surfactant-polymer solutions appear to be noticeably higher than the viscosities of individual surfactant solutions; however, the presence of the polymer has no effect on the viscous flow activation energy.

Colloidal properties of polymerizable counterion surfmers solutions based on alkylamino 2-acrylamido-2-methylpropanesulfonates in different solvents

Abstract Three polymerizable surfactants (surfmers) bearing 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as polymerizable group in the counterion have been designed, synthesized, and their micelle-forming properties have been investigated in different solvents. Solutions of dodecylammonium (DDA-AMPS), cetylammonium (CA-AMPS), and cetyltrimethylammonium (CTA-AMPS) 2-acrylamido-2-methylpropanesulfonates were studied in water, water–dioxane mixtures in a whole range of components ratios, and xylene. The solutions were investigated by capillary and rotational viscometry, conductometry, and small-angle neutron scattering; aggregates adsorbed from the solutions on as-split mica were studied by atomic force microscopy (AFM). The surfmers follow general regularities of surfactant behavior in aqueous solutions. Decrease in polarity of the media with an increase in dioxane content in water–dioxane mixtures leads to disappearance of direct micelles. At low water content (below 5%) the surfmers form reverse micelles in the mixtures. The surfmers form reverse micelles in low polarity solvents, such as xylene, which should affect the polymerization results.

Preparation of Magnetic Fluids Based on Associated Polymers

This paper describes two methods of preparation of magnetic fluids based on associated polymers. The first method is ultrasonic (US) dispersing of magnetite microparticles (MP) in aqueous solutions of associated polymers such as hydrophobically modified polyacrylamides (HM PAAm) – terpolymers of acrylamide (AAm), sodium acrylate (SA) and N-dodecylacrylamide (DDAA). According to the second method the magnetic fluids were formed by addition of magnetic polymers, synthesized by means of a micellar copolymerization of AA, SA and DDAA in the presence of MP, into water. The investigation of the rheological properties has shown that magnetic fluids obtained can be reversibly transformed in physical gels in magnetic field (MF).

Soft magnetic nanocomposites based on adaptive matrix of wormlike surfactant micelles

The paper describes a new type of soft magnetic nanocomposite (SMN) based on a transient network of wormlike surfactant micelles with embedded oppositely charged submicron particles of magnetite acting as cross-linking agents. We study the change of the rheological properties of the SMNs with different contents of particles in response to magnetic field. We show that even at low field strengths the system acquires solid-like behavior, which can be attributed to the aggregation of particles into chain-like/column structures. A solid-like behavior appears at a rather small volume fraction of particles (0.002–0.04) indicating weak restrictions imposed by the matrix to the reorganization of particles under magnetic field, which can be due to the self-assembled structure of the micellar network. In the oscillatory rheological measurements, SMNs show a linear viscoelastic response in an unusually wide region of values of strain, magnetic field strength and content of particles, which is caused by the viscoelastic contribution of the micellar network. Upon gradual increase of magnetic field strength H, the dynamic moduli G′ and G′′ demonstrate slow growth followed by a sharp rise with a scaling law H3.0 and reach a plateau at 0.15 T. The highest values of the storage modulus G′ in SMNs are close to those in magnetorheological fluids with liquid Newtonian carrier, where particles move freely and the G′ value is defined by the interactions of magnetized particles and chain-like/columns structures. SMNs have a yield stress, which grows with the increase of magnetic field strength and finally levels off just at the same magnetic field strength at which the G′ and G′′ values reach a plateau indicating the saturation of the particles magnetization. The concentration dependencies of the elastic modulus and yield stress suggest the transition from chain-like to columnar structures of the particles. The new SMNs possessing the features of both magnetic fluids and magnetic gels have promising potential in a wide range of applications requiring responsiveness to magnetic field.