Résumé
We study the effect of isopropyl alcohol (IPA) and various salts (sodium chloride, sodium thiocyanate, sodium carbonate, and urea) on the rheological behavior and the conformation of aqueous guar solutions. Ultralow angle light scattering, conventional light scattering, and neutron scattering are used to probe the structure of guar over length scales spanning 5 decades, from a few angstroms to several tens of microns. Although both IPA and salts worsen solvent conditions, their effect is very different. Isopropyl alcohol promotes the formation of a network of large-scale structures via intermolecular associations, thus increasing dramatically the elastic response of guar solutions. Salts, on the contrary, affects guar on a local scale, leading to a more collapsed chain configuration, thus to a lower effective volume fraction and to reduced viscosity.