Mario R. Rojas

Shear rheology and porous media flow of wormlike micelle solutions formed by mixtures of surfactants of opposite charge

The rheology of solutions of wormlike micelles formed by oppositely charged surfactant mixtures (cationic cetyl trimethylammonium p-toluene sulfonate, CTAT, and anionic sodium dodecyl sulfate, SDS), in the dilute and semi-dilute regimes, were studied under simple shear and porous media flows. Aqueous mixtures of CTAT and SDS formed homogeneous solutions for SDS/CTAT molar ratios below 0.12. Solutions of mixtures exhibited a strong synergistic effect in shear viscosity, especially in the semi-dilute regime with respect to wormlike micelles, reaching a four order of magnitude increase in the zero-shear rate viscosity for solutions with 20 mM CTAT. Oscillatory shear results demonstrated that the microstructure of CTAT wormlike micelles is sensitive to SDS addition. The cross-over relaxation times of wormlike micelles of 20 mM CTAT solutions increased by three orders of magnitude with the addition of up to 2 mM of SDS, and the solutions became increasingly elastic. The shear thickening process observed in shear rheology became more pronounced in porous media flow due to the formation of stronger cooperative structures induced by the extensional component of the flow.

Effect of ionic environment on the rheology of wormlike micelle solutions of mixtures of surfactants with opposite charge

This work reports on the rheological characterization of aqueous solutions of mixtures of a cationic surfactant (hexadecyltrimethylammonium p-toluenesulfonate, CTAT), capable of forming wormlike micelles, and an anionic surfactant (sodium dodecylsulfate, SDS) when the ionic environment of the solution is altered by the addition of electrolytes. Previous work showed that mixtures CTAT/SDS exhibit a strong rheological synergy in shear flows caused by promotion of interaction between wormlike micelles by the presence of the dodecylsulfate anion. In this work, we explore the CTAT/SDS synergy in the presence of electrolytes. The effect of the added electrolyte varies with wormlike micelle concentration range and type of flow. In simple shear flows and relatively low wormlike micelle concentrations (dilute regime), electrolyte addition inhibits the shear thickening effect observed in CTAT/SDS solution. In porous media flows, which have an important elongational component, electrolyte addition results in an appreciable increase in apparent viscosity at low electrolyte concentrations. In the semi-dilute regime, electrolyte addition at low concentrations strengthens the synergy between CTAT and SDS, leading to higher shear viscosities, especially at low-shear rates. An important consequence of these results is the potential for manipulation of the rheology of solutions of wormlike micelles by induction of intermicelle associations and/or promotion of conformational changes by electrolyte addition.

Assessment of the Effectiveness of Secondary Wastewater Treatment Technologies to Remove Trace Chemicals of Emerging Concern

This work presents the results of a literature review and statistical analysis of removals of chemicals of emerging concern (CECs) during conventional wastewater treatment. Process-dependent attenuations are examined for the 42 most frequently measured and reported CECs. Biological treatment processes contributing to the review include conventional activated sludge, membrane bioreactors, trickling filters, sequencing batch reactors, and lagoons. Also summarized are compound-specific physical characteristics and biodegradability data that are potential determinants of removal. As anticipated, results of the statistical analysis point to biodegradability and hydrophobicity as the most important contributing factors for removal. Supplemental materials are available for this article. Go to the publishers online edition of Critical Reviews in Environmental Science and Technology to view the supplemental file.

Synergistic effects in flows of mixtures of wormlike micelles and hydroxyethyl celluloses with or without hydrophobic modifications.

This work presents experimental results on simple shear and porous media flow of aqueous solutions of two hydroxyethyl celluloses (HEC) and two hydrophobically modified hydroxyethyl celluloses (HMHEC) with different molecular weights. Mixtures of these polymers with a cationic surfactant, cetyltrimethylammonium p-toluenesulfonate (CTAT) were also studied. Emphasis was given to the range of surfactant concentrations in which wormlike micelles are formed. The presence of hydrophobic groups, the effect of the molecular weight of the polymers, the surfactant and polymer concentrations, and the effect of the flow field type (simple shear versus porous media flow) were the most important variables studied. The results show that the shear viscosity of HEC/CTAT solutions is higher than the viscosities of surfactant and polymer solutions at the same concentrations, but surface tension measurements indicate that no complex formation occurs between CTAT and HEC. On the other hand, a complex driven by hydrophobic interactions was detected by surface tension measurements between CTAT and HMHEC. In this case, the viscosity of the mixture increases significantly more (up to four orders of magnitude at high CTAT concentrations) in comparison with HEC/CTAT aqueous solutions. Increments in the molecular weight of the polymers increase the interaction with CTAT and the shear viscosity of the solution, but make phase separation more feasible. In porous media flow, the polymer/CTAT mixtures exhibited higher apparent viscosities than in simple shear flows. This result suggests that the extensional component of the flow field in porous media flows leads to a stronger interaction between the polymer and the wormlike micelles, probably as a consequence of change of conformation and growth of the micelles.

Synthesis of amphiphilic dendrons and their interactions in aqueous solutions with cetyltrimethylammonium p-toluenesulfonate (CTAT)

In this work, we report synthesis and rheology of an interesting structured fluid based on the self-assembly of amphiphilic dendrons and wormlike micelles. Two amphiphilic dendrons were synthesized by the combination of aliphatic chains and polar dendritic heads. They showed different degrees of hydrophobicity and formed micelles in aqueous solution at critical micelle concentrations (CMC) of 25 and 125 ppm. The dendrons were soluble in water up to a concentration of approximately 1200 ppm, and produced no measurable increase in the viscosity of the solvent. The rheology of solutions of mixtures of each dendron with cetyltrimethylammonium p-toluenesulfonate (CTAT, a cationic surfactant) was characterized in simple shear flow. In the concentration range in which CTAT forms semidilute solutions of wormlike micelles, dendron addition produced a substantial synergy in zero-shear rate viscosity. Parallel-plate oscillatory shear measurements demonstrated that the CTAT/dendron mixtures are significantly more elastic than CTAT solutions. The viscosity synergy occurs at dendron concentrations lower than their CMC, and it is stronger for the more hydrophobic dendron. This suggests that the interactions between dendrons and wormlike micelles are basically hydrophobic, which implies attachment of dendron micelles to wormlike CTAT micelles in a manner similar to micellization of surfactants on polyelectrolytes.