INTERACTIONS BETWEEN MODIFIED HYDROXYETHYL CELLULOSE (HEC) AND SURFACTANTS

The macroscopic properties of aqueous solutions of several modified hy droxyethyl cellulose (HEC) samples and their interactions with anionic and cationic surfactants are studied by solubility, electric birefrin gence, rheology, and surface tension measurements. The modified HEC sa mples carry cationic groups (cat-HEC, A), cationic and hydrophobic gro ups in random distribution (HM-cat-HEC, B), or both cationic and hydro phobic groups at the same substituent (cat-HMHEC, C). The cat-HEC poly mers behave like typical polyelectrolytes, while the additionally hydr ophobic samples B and C combine the solution properties of both charge d and hydrophobic polymers. On the addition of an oppositely charged s urfactant all polymer solutions show similar phase behavior of associa tive phase separation at certain amounts of surfactant, followed by re solubilization with excess surfactant concentrations. The hydrophobic and cationic parts of samples B and C interact synergistically with th e anionic surfactant molecules, resulting in stronger viscoelastic pro perties than for only cationic HEC. Furthermore, the hydrophobic parts also allow cooperative interactions with surfactants of the same char ge. Nevertheless, differences are found for these hydrophobic and cati onic derivatives depending on whether the charges and the hydrophobic parts are separately attached at the polymer backbone (sample B) or be long to one substituent (sample C). On the addition of anionic surfact ants sample B forms a strong gel in the pre-precipitation area and a d ilute 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 co ncentration is observed only for sample B.