MICROVISCOSITY IN CLUSTERS OF ETHYL HYDROXYETHYL CELLULOSE AND SODIUMDODECYL-SULFATE FORMED IN DILUTE AQUEOUS-SOLUTIONS AS DETERMINED WITHFLUORESCENCE PROBE TECHNIQUES

The microviscosity has been measured in dilute aqueous solutions of et hyl hydroxyethyl cellulose (EHEC) and sodium dodecyl sulfate (SDS) by utilizing three steady-state fluorescence probe techniques: intramolec ular 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). Re sults obtained by the three techniques are compared. They all detect q ualitatively the same behavior with a well-developed maximum in microv iscosity 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 cl usters 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 micropol arity as sensed by pyrene of the EHEC/SDS clusters formed. The maximum in microviscosity corresponds to a rather low degree of SDS adsorptio n 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 ea ch polymer-bound surfactant cluster is high. It coincides (according t o the surfactant concentration) with a maximum in bulk viscosity for p olymer concentrations higher than the critical overlap concentration ( c). It is suggested that the maximum in bulk viscosity is due to a th ree-dimensional network of polymer and cluster tie points while the ma ximum in microviscosity is related to a high content of hydrophobic po lymer segments which stabilizes the surfactant clusters.