How a Viscoelastic Solution of Wormlike Micelles Transforms into a Microemulsion upon Absorption of Hydrocarbon: New Insight

Abstract

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.

Understanding viscosity reduction of a long-tail sulfobetaine viscoelastic surfactant by organic compounds

“…These organic compounds reduce the viscosity of the surfactant solutions by solubilizing in the core of cylindrical micelles. 5,6,8,[28][29][30] Solubilization disrupts the cylindrical micelle structure, changing longer cylindrical micelles into shorter cylindrical micelles, Kralchvesky and coworkers 31,32 determined the solubilization kinetics of non-polar oils in mixed cylindrical micelles of a nonionic surfactant and a triblock copolymer. The micelles adsorb on the oil-aqueous solution interface before solubilization takes place.…”

“…The micelles adsorb on the oil-aqueous solution interface before solubilization takes place. 31, 32 Shibaev and co-workers 30 showed there are three regimes of micelle change in viscoelastic solutions of potassium oleate at 20 C: (i) the micellar network regime (ii) the transition regime, and (iii) the microemulsion regime. The existence of micellar network regime was attributed to the reduction in length of the cylindrical micelles, disrupting the micelle network.…”

“…For rheological measurements, the surfactant solutions were mixed with the organic compounds and equilibrated for one week at 30 C and 60 C. In case there was phase separation in the solution, the surfactant solution phase was tested.…”

Magnetically tunable viscoelastic response of soft magnetic nanocomposites with wormlike surfactant micellar matrix

“…Linear viscoelastic properties of SMNs allow investigation of the dynamic moduli of the system. Also, the WLMs are "living" objects, they can reversibly break under stress [18,20,21] that makes them promising as a matrix where particles can move freely enough even at low particle concentrations. This shows that WLMs are perfect for the stabilization of particle suspensions, because they are able to link to the particles by their end-caps.…”

Viscoelastic properties of new mixed wormlike micelles formed by a fatty acid salt and alkylpyridinium surfactant

“…Long entangled wormlike micelles of surfactants can impart to aqueous solutions pronounced viscoelastic properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], which are quite sensitive to external stimuli: ionic strength [2,8], type of salt [11,15], temperature [10], various additives including polymers [6,7] and hydrocarbons [12][13][14]. Such responsiveness is provided by the self-assembled character of micellar chains formed by weak non-covalent interactions [1,9].…”