Interactions between cationically modified hydroxyethyl cellulose and oppositely charged surfactants studied by gel swelling experiments — effects of surfactant type, hydrophobic modification and added salt

Interactions between cationically modified hydroxyethyl cellulose (cat-HEC; JR400) and oppositely charged surfactants have been studied by equilibrium and time-dependent gel swelling experiments (measurements of the volume of a covalently cross-linked gel immersed in a quasi-infinite aqueous surfactant solution). It is demonstrated that many important characteristics — such as the onset and cooperativity of the surfactant binding, the water content of a polymer–surfactant complex, and the response to added salt — are obtained rather more easily from the gel swelling experiment than from conventional experiments on linear polymers. Responsive gels based on cat-HEC show many interesting features, such as a very large swelling in pure water and a sharp collapse, followed by a sharp reswelling, as the concentration of anionic surfactant is increased in the external solution. The latter behavior is due to the binding of surfactant in two discrete and cooperative steps (two-step binding) at the critical surfactant concentrations cac and cac(2), respectively. Both binding steps were affected by added NaCl, and the cac was found to go through a maximum with increasing salt content, in accordance with previously unconfirmed predictions for oppositely charged polymer–surfactant pairs. A gel collapse with added sodium dodecylsulfate was found even in the presence of 1 M NaCl; this was attributed to a binding of the surfactant also to the HEC backbone. Comparisons with gels based on hydrophobically modified cat-HEC (cat-HMHEC; LM200) show that a hydrophobic modification has strong effects both on the extent of the swelling and on the details of the collapse/reswelling behavior. The approach of cat-HEC gels to their equilibrium volumes in surfactant solutions were studied, and a transient shrinking/reswelling was observed under certain circumstances.