Interactions between modified hydroxyethyl cellulose (HEC) and surfactants

The macroscopic properties of aqueous solutions of several modified hydroxyethyl cellulose (HEC) samples and their interactions with anionic and cationic surfactants are studied by solubility, electric birefringence, rheology, and surface tension measurements. The modified HEC samples carry cationic groups (cat-HEC, A), cationic and hydrophobic groups in random distribution (HM-cat-HEC, B), or both cationic and hydrophobic groups at the same substituent (cat-HMHEC, C). The cat-HEC polymers behave like typical polyelectrolytes, while the additionally hydrophobic samples B and C combine the solution properties of both charged and hydrophobic polymers. On the addition of an oppositely charged surfactant all polymer solutions show similar phase behavior of associative phase separation at certain amounts of surfactant, followed by resolubilization with excess surfactant concentrations. The hydrophobic and cationic parts of samples B and C interact synergistically with the anionic surfactant molecules, resulting in stronger viscoelastic properties than for only cationic HEC. Furthermore, the hydrophobic parts also allow cooperative interactions with surfactants of the same charge. Nevertheless, differences are found for these hydrophobic and cationic derivatives depending on whether the charges and the hydrophobic parts are separately attached at the polymer backbone (sample B) or belong to one substituent (sample C). On the addition of anionic surfactants sample B forms a strong gel in the pre-precipitation area and a dilute sol in the post-precipitation area, while sample C forms highly viscoelastic gels in both the pre-precipitation and post-precipitation areas. A viscosity enhancement with increasing cationic surfactant concentration is observed only for sample B.