Interactions between Quaternary Ammonium Surfactant Oligomers and Water-Soluble Modified Guars.

The interaction between hydroxypropylguar (HPG) and its dodecyl-modified derivative (HMHPG) and cationic surfactant oligomers has been investigated by measurements of the solution viscosity at constant shear rate, microviscosity of the aggregates (dipyrenylpropane fluorescence emission spectra), and aggregation number of the polymer hydrophobe and of the surfactant (time-resolved fluorescence quenching). The surfactants are dodecyltrimethylammonium bromide (DTAB, monomeric surfactant) and some of its dimers and trimers which differed by the carbon number s of the polymethylene spacer connecting the surfactant moieties (2 </= s </= 20). Most results refer to a polymer concentration of 1 wt%. Only a weak interaction was evidenced between HPG and these surfactants, whereas strong interactions occurred between HMHPG and the surfactant oligomers. The interaction became stronger as the degree of oligomerization of the surfactant increased. The results led us to distinguish three ranges of concentration of added surfactant. The first range corresponds to surfactant concentrations below the surfactant cmc in water. In this range mixed aggregation occurs between polymer hydrophobes and surfactant ions, and the viscosity of the HMHPG + surfactant systems goes through a maximum, as usually found for associating polymers. The second range extends from the cmc to about 10 cmc. Precipitation of a polymer/surfactant complex occurs in this range with all surfactants forming threadlike micelles. For the other surfactants the viscosity goes through a minimum. In the third range, which corresponds to surfactant concentrations above 10 cmc, resolubilization of the precipitated HMHPG/surfactant complexes occurs and a solution-to-gel transition is observed for the surfactants which form threadlike micelles or vesicles. The concentration corresponding to this transition is about the same as that for pure surfactant solutions. Some polymer hydrophobes may contribute to the formation of additional bridges between surfactant micelles. Copyright 1999 Academic Press.