Poly(acrylic acid) microgels (carbopol 934)/surfactant interactions in aqueous media. Part I: nonionic surfactants.

The interaction between Tween 80 and Pluronic F-127 with carbopol in water was studied as a function of surfactant concentration. 0.25% carbopol microgels dispersions showed a continuous decrease in transmittance, viscosity and conductivity when surfactant concentration ranged from 0.01-0.02% to 0.50% Tween 80 or from 0.03-0.06% to 0.30% Pluronic F-127. These limit values can be considered as the critical association concentration and the saturation binding concentration, respectively. In this concentration range, a strong rise in pH (from 3.18 to 3.50) suggested that surfactant-polymer binding occurred mainly through a stoichiometric hydrogen-bonding interaction between the oxyethylene and carboxylic groups. In the presence of carbopol, the concentration of Tween 80 at the air/water interface decreases as the surfactant is adsorbed onto the polymer and drawn into the bulk solution. In contrast, the interaction with the polymer seems to change the conformation of the expanded chains of Pluronic F-127, making it easier for more molecules of surfactant to be at the interface and increasing the thickness of the interfacial surfactant layer. Fluorescence probes indicated that the carbopol network presents a more apolar medium than pure water, and the differences in the hydrophile-lipophile balance (HLB) of each surfactant were responsible for the lower I(I)/I(III) values obtained with Tween 80/carbopol systems. Microcalorimetry titration data made it possible to conclude that Tween 80/carbopol interaction, at 298K, is an enthalpy-driven process due to stabilization of Tween 80 units inside the polymer network. In contrast, Pluronic F-127/carbopol association (endothermic process) occurs owing to a gain in entropy when polymer-surfactant interaction allows the restoration of free water hydrogen-bonding structure, resembling the micellization process.