Interactions between quaternary ammonium surfactant oligomers and water-soluble modified guars

The interaction between hydroxypropylguar (HPG) and its dodecyl-modified de rivative (HMHPG) and cationic surfactant oligomers has been investigated by measurements of the solution viscosity at constant shear rate, microviscos ity of the aggregates (dipyrenylpropane fluorescence emission spectra), and aggregation number of the polymer hydrophobe and of the surfactant (time-r esolved fluorescence quenching). The surfactants are dodecyltrimethylammoni um bromide (DTAB, monomeric surfactant) and some of its dimers and trimers which differed by the carbon number s of the polymethylene spacer connectin g the surfactant moieties (2 less than or equal to s less than or equal to 20), Most results refer to a polymer concentration of 1 wt%. Only a weak in teraction was evidenced between HPG and these surfactants, whereas strong i nteractions occurred between HMHPG and the surfactant oligomers, The intera ction became stronger as the degree of oligomerization of the surfactant in creased. 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 H MHPG + surfactant systems goes through a maximum, as usually found for asso ciating polymers. The second range extends from the cmc to about 10 cmc, Pr ecipitation of a polymer/surfactant complex occurs in this range with all s urfactants forming threadlike micelles, For the other surfactants the visco sity goes through a minimum. In the third range, which corresponds to surfa ctant concentrations above 10 cmc, resolubilization of the precipitated HMH PG/surfactant complexes occurs and a solution-to-gel transition is observed for the surfactants which form threadlike micelles or vesicles, The concen tration corresponding to this transition is about the same as that for pure surfactant solutions, Some polymer hydrophobes may contribute to the forma tion of additional bridges between surfactant micelles. (C) 1999 Academic P ress.