A.V. Shibaev

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.

Effect of polymer on the arrangement of mixed anionic/cationic wormlike surfactant micelles revealed by SANS

We report the results of small-angle neutron scattering (SANS) study of the effect of neutral water-soluble polymer poly(vinyl alcohol) on the structure of salt-free aqueous solutions of charged cylindrical micelles formed by the mixture of anionic (potassium oleate) and cationic (n-octyltrimethylammonium bromide (C8TAB)) surfactants at a fixed molar ratio [C8TAB]/[potassium oleate] = 0.46. The dependences of the scattering on surfactant concentration were examined both in the presence and in the absence of polymer. It was observed that at high q range, the scattering function is independent of surfactant concentration and added polymer indicating that the local cylindrical structure of the micelles remains unchanged. At intermediate q range, with growing surfactant concentration the scattering function becomes increasingly dominated by a structure peak that reveals enhanced interactions between neighboring micelles. The peak position q* scales with the surfactant concentration Csurf as Csurf 0.6 and Csurf 0.53 in the absence and in the presence of polymer, respectively. The most important observation concerns the fact that added polymer decreases significantly the average intermicellar distance, which may be due to microsegregation of polymer and surfactant components typical for microphase separation.We report the results of small-angle neutron scattering (SANS) study of the effect of neutral water-soluble polymer poly(vinyl alcohol) on the structure of salt-free aqueous solutions of charged cylindrical micelles formed by the mixture of anionic (potassium oleate) and cationic (n-octyltrimethylammonium bromide (C8TAB)) surfactants at a fixed molar ratio [C8TAB]/[potassium oleate] = 0.46. The dependences of the scattering on surfactant concentration were examined both in the presence and in the absence of polymer. It was observed that at high q range, the scattering function is independent of surfactant concentration and added polymer indicating that the local cylindrical structure of the micelles remains unchanged. At intermediate q range, with growing surfactant concentration the scattering function becomes increasingly dominated by a structure peak that reveals enhanced interactions between neighboring micelles. The peak position q* scales with the surfactant concentration Csurf as Csurf 0.6 and Csurf 0.53 in the absence and in the presence of polymer, respectively. The most important observation concerns the fact that added polymer decreases significantly the average intermicellar distance, which may be due to microsegregation of polymer and surfactant components typical for microphase separation.