Coadsorption of sodium dodecyl sulfate with hydrophobically modified nonionic cellulose polymers. 1. Role of polymer hydrophobic modification

We examined coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC). Coadsorption to nonselective, hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for the polymers. Optical reflectometry provided the total extent of adsorption as well as the adsorption kinetics. We referenced SDS concentrations against the critical association concentration (cac) and saturation concentration (C-sat) for solution-phase polymer/surfactant binding as determined by pyrene solubilization and fluorescence spectroscopy methods. Depending on the bulk SDS concentration, the total adsorbed mass could be either increased or decreased compared to the adsorbed mass attained from SDS-free polymer solutions. Above the cac, the total adsorbed amount decreased with increasing SDS concentration, for either polymer on either the silica or the PDMS surface. At high SDS concentrations with silica surfaces, adsorption of either polymer was completely prevented, leaving the surface bare, but the threshold SDS concentrations for this effect were well above the C-sat for the particular polymer. On the hydrophobic PDMS surface, only SDS adsorbed when its concentration exceeded the ordinary critical micelle concentration (measured in the absence of polymer). On either surface and at any SDS concentration where some polymer did adsorb, the total extent of adsorption was greater with the more hydrophobic hmHEC polymer than with the HPC polymer.