Poly(acrylic acid) microgels (carbopol® 934)/surfactant interactions in aqueous media Part II: Ionic surfactants

The interaction of sodium dodecylsulfate (SDS) or benzalkonium chloride (BkCl) with carbopol microgels (0.25% (w/w)) in water was studied through pH, trasmittance, viscosity, surface tension, conductivity, fluorescence, oil solubilization, and microcalorimetry measurements. In the case of the anionic surfactant, enthalpy-driven hydrophobic absorption of SDS into carbopol microgels began when SDS concentration reached 0.05-0.08% and ended around 0.6%. These concentrations were estimated as the critical aggregation (cac) and saturation binding concentration, respectively. The hydrophobic absorption of the surfactant accompanied by its counter-ion caused carbopol microgels to swell and promoted the occurrence of bridges among several carbopol microgels. As a consequence, the consistency of the dispersions increased significantly. Above binding saturation, further addition of SDS produced a shielding effect among the anionic charges of carbopol and its dehydration, which was shown as a decrease in the viscosity of the dispersions. At low shear rates, the dispersions behaved as pseudoplastic owing to orientation of carbopol/SDS aggregates in the flow direction. Increasing shear rates caused the inter-microgel bridges to break, the water layer surrounding them to diminish, and the system to show a shear-thickening behavior. In contrast, carbopol/BkCl aggregates showed shear-thickening flow in the whole range of shear rates analyzed. Electrostatic interactions between BkCl and carbopol carboxylic groups release protons to the medium and decrease the internal osmotic pressure of the microgels. This may favor the establishment of hydrophobic interactions among surfactant tails, and induces carbopol microgels to collapse. The cac was approximately 0.01% BkCl. Saturation binding occurred at 0.3-0.5%, indicating that only 25-40mol% carboxylic groups were neutralized with BkCl. The shrinking of carbopol microgels as BkCl is absorbed prevents additional surfactant molecules from interacting with the remaining carboxylic groups. Microcalorimetry assays revealed that the aggregation process occurred with a strong gain in enthalpy.