Cloud point curves and conjugate coexistence curves for the quasi-binary water systems of ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HMEHEC) have been determined. The cloud point curves are remarkably independent of the polymer concentration up to as high as 20 wt % polymer, and they are clearly different from the coexistence curves. This is interpreted as an effect of the multicomponent nature of the polymer. Hydrophobic modification of EHEC lowers the cloud point by approximately 15 °C. On heating above the cloud point, EHEC redistributes to the concentrated phase, but, however, a substantial amount of the polymer remains in the dilute phase even far above the cloud point. For HMEHEC, essentially all polymer is found throughout in the concentrated phase. Addition of the anionic surfactant, sodium octylbenzenesulfonate (SOBS), affects the phase behavior. Small amounts of SOBS cause a swelling of the concentrated phase for both EHEC and HMEHEC, due to an electrostatic repulsion between polymer−surfactant aggregates. On further addition of SOBS, a dissolution of polymer from the concentrated phase is observed for EHEC. For HMEHEC, the surfactant binding swells the concentrated phase until the one-phase region is reached.

Responsive gels made by chemically cross-linking two modified cellulose ethers, separately and in mixtures, have been investigated with respect to their swelling in aqueous solutions containing anionic or cationic surfactants and/or added salt. One of the polymers (catHM-HEC) is a salt-sensitive and strongly hydrophobically associating (hydroxyethyl)cellulose modified with cationic hydrophobic side chains. The other polymer, ethyl(hydroxyethyl)cellulose (EHEC), is a temperature-sensitive, weakly self-associating polymer. Both polymers bind ionic surfactants, and it was found that the response of the cross-linked gels to the content of surfactant or salt in the swelling medium reflected the expected surfactant binding isotherms. A maximum in swelling was generally observed when the surfactant concentration in the swelling medium was close to the critical micelle concentration (cmc). The collapse/swelling behavior of pure catHM-HEC gels on addition of cationic and anionic surfactants showed a close correspondence to the previously studied phase behavior of non-cross-linked catHM-HEC in mixtures with the same surfactants. Mixed gels of EHEC and catHM-HEC in different proportions showed a behavior intermediate between those of the pure polymer gels. It was thus possible to customize gels with respect to their responses to different influences.

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The swelling in water and in ionic surfactant solutions of a polymer gel based on poly(acrylamide) (PAm) with pendent poly(ethylene oxide) (PEO) chains (0−2.6 mol %) was investigated. In pure water, the swelling increased linearly with the content of PEO side chains. The anionic surfactants sodium dodecyl sulfate (SDS) and sodium octylbenzenesulfonate (SOBS) both bound to the PEO side chains above a critical association concentration (cac). For SOBS, binding isotherms to the gels were obtained, and the cac values for the surfactant in PEO solutions were determined by NMR, both at varying concentrations of added NaCl. Both surfactants affected the swelling of the copolymer gels similarly. When the surfactant concentration in the swelling medium was increased at low concentrations of added NaCl, a substantial swelling occurred at the cac, and the volume continued to increase up to a concentration just above the critical micellization concentration (cmc) for the free surfactant in the swelling medium. At higher concentrations of surfactant the gels started to deswell. At high contents of added NaCl (ca. 0.5 M and above), the swelling isotherm changed:  The gels instead began to shrink at the cac, indicating a cross-linking of the PEO chains by the surfactant micelles. The gel volume went through a minimum when the ratio of PEO chains to bound micelles was approximately 2. At higher concentrations, the gel continued to swell until it was saturated with surfactant. The amount of bound surfactant at saturation increased with large amounts of added salt. The experiments in the presence of salt clearly showed that the binding of surfactant to the gels continues even at surfactant concentrations exceeding the cmc in the swelling medium.