Microviscosity in clusters of ethyl hydroxyethyl cellulose and sodium dodecyl sulfate formed in dilute aqueous solutions as determined with fluorescence probe techniques

The microviscosity has been measured in dilute aqueous solutions of ethyl hydroxyethyl cellulose (EHEC) and sodium dodecyl sulfate (SDS) by utilizing three steady-state fluorescence probe techniques: intramolecular excimer formation by 1,3-di(1-pyrenyl)propane (P3P), fluorescence depolarization of perylene, and intramolecular rotational relaxation about bonds with (p-(dimethylamino)benzylidene)malononitrile (BMN). Results obtained by the three techniques are compared. They all detect qualitatively the same behavior with a well-developed maximum in microviscosity and rigidity of the EHEC/SDS clusters formed at a surfactant concentration close to or slightly higher than the critical surfactant concentration where adsorption to the polymer starts. The EHEC/SDS clusters have, independent of composition, higher microviscosities than ordinary SDS micelles. The microviscosity is also compared with other EHEC/SDS/water system features such as the bulk viscosity, the actual adsorption isotherm, the average aggregation numbers, and the micropolarity as sensed by pyrene of the EHEC/SDS clusters formed. The maximum in microviscosity corresponds to a rather low degree of SDS adsorption to EHEC (approximate to 0.5 mmol of SDS per gram of EHEC) and a low aggregation number (approximate to 10) where the polymer content of each polymer-bound surfactant cluster is high. It coincides (according to the surfactant concentration) with a maximum in bulk viscosity for polymer concentrations higher than the critical overlap concentration (c*). It is suggested that the maximum in bulk viscosity is due to a three-dimensional network of polymer and cluster tie points while the maximum in microviscosity is related to a high content of hydrophobic polymer segments which stabilizes the surfactant clusters.