Interactions between anionically modified hydroxyethyl cellulose and cationic surfactants

Macroscopic properties of aqueous solutions of several modified hydroxyethy l cellulose (HEC) samples and their interactions with cationic surfactants are studied by solubility, light scattering, electric birefringence, rheolo gy, and surface tension measurements. Modified HEC samples carry anionic gr oups (an-HEC D0) and anionic and hydrophobic groups in random distribution (HM-an-HEC D1-D4). The molar substitution of anionic (an) groups is about 0 .07 in all samples while that of the hydrophobic (HM) groups ranges from 0 in an-HEC D0 to 0.012 in HM-an-HEC D4. In a 1 wt% solution this corresponds to 2.7 mM anionic and 0 to 0.46 mM hydrophobic groups. In the dilute conce ntration range the polymers behave like typical polyelectrolytes whereas in the semi-dilute range they resemble uncharged polymers. On addition of opp ositely charged surfactants the phase behavior of all polyelectrolytes is s imilar. With increasing surfactant concentrations the transparent solutions become turbid and the phases separate. Finally, resolubilization takes pla ce with excess surfactant concentrations. With the HM-an-HEC compounds visc oelastic solutions are formed with cationic surfactants. The intermolecular interaction between hydrophobic parts of the polymers and the surfactants and interactions of oppositely charged ionic groups of the two components l ead to formation of a temporary network with gel-like properties. With an-H EC the interaction can only take place via charges. Viscosity enhancement w ith increasing surfactant concentration is therefore lower with an-HEC than with HM-an-HEC compounds.