Effect of hydrophobic modification of a nonionic cellulose derivative on the interaction with surfactants. Rheology

The interactions between various surfactants (anionic, cationic, and nonionic) and a nonionic ethyl(hydroxyethyl)cellulose (EHEC) polymer or a hydrophobically modified analogue (HM-EHEC) have been examined. The study has been performed as a comparative investigation between the hydrophobically modified polymer and the unmodified parent polymer using theology. The rheological results have been analyzed with the aid of a simple modified Maxwell model. Information about the low-frequency behavior was extracted from this model, and the rheological features were presented in terms of the complex viscosity and a characteristic relaxation time. In the presence of an ionic surfactant, the rhelogical measurements revealed significant polymer/surfactant interaction for both EHEC and HM-EHEC, but the interaction peak, observed in the different rheological quantities, was more pronounced and located at a lower surfactant concentration for the hydrophobically modified polymer. In the presence of a nonionic surfactant, the polymer/surfactant interaction was weaker. By considering the temperature effect, an important difference in the polymer dynamics between the hydrophobically modified polymer and the unmodified analogue was established. The hydrophobically modified polymer shows a normal temperature dependency, while the motion of the unmodified analogue is slowed down with increasing temperature. The finding for the unmodified polymer is attributed to increased polymer/polymer attractions with an increased temperature. It has also been shown that the hydrophobically modified polymer has features regarding the formation and breakdown of the polymer/surfactant complex in common with other hydrophobically modified polymers (both nonionic and ionic). At a certain surfactant concentration the network starts to lose its connectivity, however, because of an increased surfactant aggregation number of the mixed micelles, the breakdown process is postponed when the surfactant chain length is increased.