The interaction between the hydrophobic, nonionic cellulose derivative ethyl hydroxyethyl cellulose (EHEC; fraction CST-103) and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied as a function of temperature from 20 to 50 °C in dilute aqueous solutions, i.e. a polymer concentration slightly below the critical overlap concentration (c*) and a surfactant concentration up to three times the normal critical micelle concentration (cmc). Methods utilized in this investigation include equilibrium dialysis and steady-state fluorescence quenching. The results show that the average aggregation numbers of the polymer-bound SDS clusters decrease with an increase in temperature although the magnitude of the effect is composition dependent and is most pronounced for compositions which give the largest cluster sizes. The adsorption of SDS to EHEC shows a break-point at an intermediate value of the adsorption isotherm above which the cooperativity increases. This break-point diminishes and disappears, i.e. the cooperativity decreases, as the temperature increases up to 50 °C. It is suggested that the mechanism responsible for these two steps in the adsorption process is at first adsorption of SDS to aggregated EHEC chains and then to a mainly deaggregated state of EHEC. The critical surfactant concentration where the adsorption to the polymer starts seems to be slightly shifted toward lower values as the temperature is raised from 20 to 50 °C. To summarize the results, the interaction between EHEC and SDS gets more intensive as the temperature is raised. Two fluorophore/quencher pairs, which previously have been used for determination of average aggregation numbers in aqueous surfactant and polymer−surfactant systems utilizing the steady-state fluorescence quenching technique, are compared. A good agreement between the two pairs is reported. Reference measurements of average aggregation numbers and adsorption isotherms for the PEO/SDS/water system are also given.

The aqueous interaction of the sodium salt of ibuprofen with the cellulose ethers ethyl hydroxyethyl cellulose, EHEC, and hydroxypropyl methyl cellulose, HPMC, has been investigated in the concentration range 0-500 mM ibuprofen and 0.1-1% (w/w) polymer, by cloud point, capillary viscometry, equilibrium dialysis, and fluorescence probe techniques. Ibuprofen forms micelles in pure water, with the critical micelle concentration, cmc, at 180 mM. A combination of time-resolved and static fluorescence quenching shows that micelle-like ibuprofen aggregates are formed in the solution. The average aggregation number of pure ibuprofen micelles in water is about 40. In the presence of EHEC or HPMC the aggregation numbers decrease. The interaction of ibuprofen with cellulose ethers is similar to the normally accepted model for polymer-surfactant interaction, although more complex. Ibuprofen adsorbs to the polymer in the form of mixed polymer-drug micelles, noncooperatively up to cmc and cooperatively when cmc is passed. The interaction starts below 50 mM ibuprofen as monitored by the fluorescent probes pyrene and 1,3-di(1-pyrenyl)propane, P3P, with a maximum in microviscosity below cmc, corresponding to polymer-dense mixed micelles. The study illustrates the importance of a precise apprehension of the aggregation behavior as a background for transport studies in drug-polymer systems.

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 (≈0.5 mmol of SDS per gram of EHEC) and a low aggregation number (≈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.

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