Phase separation of aqueous solutions of cellulose derivatives: Influence of surfactants

In this paper we examine the thermal behaviour of two non-ionic cellulose derivatives: ethyl(hydroxyethyl)cellulose (EHEC) and methylcellulose (MC) in dilute aqueous solutions. These polymers exhibit a lower critical solution temperature, i.e. demixing occurs by raising the temperature. The interaction with a cationic surfactant is investigated: various amounts of cetyltrimethylammonium bromide (CTAB) were added to the dilute polymer solutions corresponding to surfactant concentrations below and above the CMC. Dynamic light scattering (DLS), viscosimetric and theological measurements were performed. By increasing the temperature we show that these two polymers behave in a very different way: a micellisation, followed by a collapse is observed for EHEC, while a macroscopic phase separation is seen for MC. The surfactant interacts strongly with EHEC, which reduces the aggregation and swells the coils, while only a fraction of the MC sample is sensitive to CTAB. The results are discussed and compared to what is already known about thermal gelation and phase separation of EHEC and MC aqueous solutions.