Surface deposition and phase behavior of oppositely charged polyion/surfactant ion complexes. 1. Cationic guar versus cationic hydroxyethylcellulose in mixtures with anionic surfactants.

Mixtures of cationic guar (cat-guar) or cationic hydroxyethylcellulose (cat-HEC) with the anionic surfactants sodium dodecyl sulfate or sodium lauryl ether-3 sulfate have been investigated by a wide range of complementary techniques (phase studies, turbidity measurements, dynamic light scattering, gel-swelling experiments, and in situ null ellipsometry), with the following objectives in mind: (1) to establish the relationship between the bulk phase behavior (precipitation and redissolution) of the polyion/surfactant ion complexes and formation/deposition of such complexes at silica surfaces and (2) to obtain molecular interpretations of the large, previously unresolved, quantitative differences between the various investigated mixtures. There were clear similarities, for each studied system, between the bulk phase behavior, gel swelling, and surface deposition on increasing surfactant concentration. This is because all phenomena reflect the polyion/surfactant ion binding isotherm: an initial binding step at a low critical association concentration (cac) of the surfactant and a second more-or-less cooperative binding step beginning at a second cac, the cac(2). The details of the interactions are system-specific, however, and cat-guar/surfactant mixtures generally had larger precipitation regions and gave rise to larger adsorbed amounts on silica compared to mixtures with cat-HEC of a similar charge density. The observed quantitative differences are attributed to a difference in the hydrophobicity of the polyions. For cat-guar, the comparatively weak hydrophobic polyion/surfactant attraction is seen as a very gradual binding commencing at the cac(2) and continuing past the bulk critical micelle concentration of the surfactant, resulting in an unusually large phase-separation region. For cat-HEC, the dissolution of the precipitate takes place at lower surfactant concentrations because of a stronger hydrophobic interaction between the surfactant and the polyion. The results have implications for the successful design of oppositely charged polyelectrolyte/surfactant formulations for surface deposition applications.