Gels from modified hydroxyethyl cellulose and ionic surfactants

The macroscopic properties of aqueous solutions of hydroxyethyl cellulose (HEC) and of modified HEC samples which carry cationic groups (cat-HEC) or a cationic and hydrophobic group (cat-HMHEC) and their interactions with ionic surfactants (SDS and STS) are investigated by rheological, electric birefringence and surface tension measurements. The gel formation of the surfactant free polymer solutions strongly depends on the polymer concentration and the constitution of the polymer. HEC forms thermoreversible gels for T-c < 50 degrees C and c greater than or equal to 5 wt% while cat-HEC reaches already the gel stare at 4 wt%. The hydrophobic poly-electrolytes are not water soluble for c > 3 wt%. HEC does not interact with ionic surfactants. The I wt% viscoelastic solutions of cat-HEC and cat-HMHEC can be transformed into physically crosslinked gels by the addition of oppositely charged surfactants. The polymer solutions are first transparent and show an increase in viscosity and elasticity with increasing surfactant concentration (pre-precipitation area). For higher surfactant concentrations associative phase separation occurs because of charge neutralization and then a second single phase area is observed. In this post-precipitation area the viscosity continuously decreases by further addition of surfactants. The concentration dependence of the gel stare formed by cat-HMHEC and STS is strongly affected by the history of the complexes: A 1 wt% polymer-STS-gel retains a yield stress value when diluted down to 0.7 wt% polymer concentration while the addition of STS to the 0.7 wt% polymer solution does not result in a gel state. Network formation is also observed for the hydrophobic polyelectrolytes by the addition of anionic surfactants below the critical overlap concentration of the surfactant free cat-HMHEC solutions. Copyright (C) 1996 Elsevier Science Ltd.