Co-adsorption of carboxymethyl-cellulose and cationic surfactants at the air-water interface

We investigated the interaction between an anionic polyelectrolyte (carboxymethylcellulose) and cationic surfactants (DTAB, TTAB, and CTAB) at the air/water interface, using surface tension, ellipsometry, and Brewster angle microscopy techniques. At low surfactant concentration, a synergistic phenomenon is observed due to the co-adsorption of polyelectrolyte/surfactant complexes at the interface, which decreases the surface tension. When the surfactant critical aggregation concentration (cac) is reached, the adsorption saturates and the thickness of the adsorbed monolayer remains constant until another characteristic surfactant concentration, C-0, is reached, at which all the polymer charges are bound to surfactant in bulk. Above C-0, the absorbed monolayer becomes much thicker, suggesting adsorption of bulk aggregates, which have become more hydrophobic due to charge neutralization.