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.

We studied the effect of neutral polymer poly­(vinyl alcohol) on the rheological properties and microstructure of highly charged mixed wormlike micelles of anionic and cationic surfactants, potassium oleate and n-octyl­trimethyl­ammonium bromide, without adding salt. It was shown that the polymer induces a hundredfold increase of viscosity and of longest relaxation time and the appearance of well-defined plateau modulus, which was assigned to interlacing of polymer and micellar chains. When the amount of added polymer exceeds 2 wt %, the rheological characteristics (the viscosity, the longest relaxation time, and the plateau modulus) level off because of microphase separation appearing as a result of the interplay of the segregation on the microscopic scale triggered by the energetic repulsion between polymer and surfactant components, on the one hand, and the translational entropy of counterions preventing the macroscopic phase separation, on the other hand. The formation of surfactant-rich and polymer-rich microphases was evidenced by small-angle neutron scattering and cryogenic transmission electron microscopy data. The results obtained open a new way to modify the rheological properties and the microstructure of wormlike micellar solutions.

We study the rheological properties of wormlike micellar aqueous solutions of an anionic surfactant potassium oleate containing solubilized 1-phenyldodecane. We show that upon increasing the amount of absorbed hydrocarbon the rheological behavior of semidilute micellar solutions changes drastically, showing a sequence of different regimes: (i) a "fast-breaking" entangled regime, when very long micellar chains form a network; (ii) an "unbreakable" entangled regime, when the shortening of the micelles leads to the decrease of their reptation time up to the values close to the breaking time; (iii) an unentangled regime (for the first time evidenced for wormlike micelles), where the micelles are so short that they cannot interlace. Within the entangled regime, an unusual rheological behavior has been discovered, probably characterized by the dominant role of end or bond interchange reactions or "breathing" modes, which leads to a novel hypothesis that hydrocarbon is distributed nonuniformly along the micellar length, thus increasing the probability of micellar breakage at certain points.

The rheology and the structural evolution of xanthan solutions and gels upon increasing polymer concentration were monitored in the absence and in the presence of cross-linker (chromium chloride). Direct visualization of the unperturbed structure of these systems by freeze-fracture transmission electron microscopy revealed the microphase-separated morphology with percolated polymer skeleton composed of aggregated chains. The thickness of the skeleton increases with increasing polymer concentration and at the addition of cross-linker, indicating enhancement of microphase separation. Upon cross-linking, the networks acquire higher plateau modulus G′, but it becomes less dependent on polymer concentration C than in the un-cross-linked state (G′ ∼ C 1.35 and G′ ∼ C 2.1, respectively). The molecular imaging also unraveled the presence of backfolded xanthan segments and many sharp kinks at distances smaller than the persistence length, suggesting the presence of flexible hinges between double-helical fragments of the polysaccharide, which is in agreement with small-angle X-ray scattering data.

No abstract

No abstract

Cross-linking of polysaccharides by metal ions provides polymer gels highly required by industrial applications. In this article, we study the rheological properties and microstructure of solutions of a stiff anionic polysaccharide xanthan cross-linked by chromium (III) ions, and we demonstrate that their properties are highly sensitive to the preparation pH. Stable gels are obtained in a wide range of pH from 2.4 to 7.8. The maximum elastic modulus is observed for the gels made at pH 6.3, and by freeze-fracture transmission electron microscopy it is shown that they are characterized by the most dense network structure. However, out of this pH interval, no gelation is observed. At low pH (< 2.4) it is due to high protonation of carboxylic groups of xanthan preventing their interaction with chromium ions, and to the disappearance of oligomeric ions, which are effective in cross-linking. At high pH (> 7.8) the absence of gelation is caused by the transformation of reactive chromium ions into insoluble chromium hydroxide. At the same time, for the gels initially formed at pH 6.3, subsequent change of pH to strongly acidic (1.4) or basic (8.9) medium does not affect appreciably their properties, meaning that chromium cross-links are stable once they are formed. These observations open a reliable route to produce polysaccharide gels with required mechanical properties in a wide pH range where they initially cannot be prepared. It is also shown that the increase of pH to 6.3 of the initially ungelled solution prepared at pH 1.5 results in gelation. This effect offers a facile way for delayed gelation of polysaccharides, which is especially required by oil industry.

No abstract