Phase separation studies have been carried out concerning the addition of different surfactants to systems containing either ethyl(hydroxyethyl)cellulose (EHEC) or the hydrophobically modified analogue (HM-EHEC). The polymer concentration was kept constant at 1 g polymer/100 g of water (1 w/w%), while the surfactant concentration was varied. In the polymer/sodium dodecyl sulfate (SDS) systems, which were studied in more detail, phase diagrams were obtained in the presence of various concentrations of inert salt. Moreover, the phase behavior of the HM-EHEC/SDS system showed no marked changes by purification of the surfactant or by contamination with decanol. It was shown that the SDS binding to EHEC was cooperative and could be described in the framework of a closed association model, while for HM-EHEC a two-step binding model had to be used to get a proper description of the binding. In the noncooperative part of the HM-EHEC binding isotherm, the binding showed similarities to the adsorption of SDS on a hydrophobic surface and could be described by a Langmuir adsorption model. From the binding isotherms for SDS, binding isotherms for ionic surfactants with varying chain length could be calculated from simple assumptions. These isotherms give a basis for the interpretation of the phase diagrams. For the ionic surfactants, the phase behavior could be rationalized by considering the polymer/surfactant complex to possess polyelectrolyte characteristics. The observations were discussed in terms of an attractive hydrophobic interaction and a repulsive electrostatic force. Addition of nonionic surfactants was discussed by observing that generally a pair of a nonionic polymer and a nonionic surfactant segregates. However, with HM-EHEC there is an extra attractive hydrophobic interaction due to the presence of polymer hydrophobic tails which can serve as nucleation sites for the surfactants.
