Pyrene Micropartitioning and Solubilization by Sodium Dodecyl Sulfate Complexes with Poly(ethylene glycol)

We examine the effect of poly(ethylene glycol) (PEG) on pyrene solubilization behaviors in aqueous sodium dodecyl sulfate (SDS) solutions. These solutions display strong polymer−surfactant complexation. Following the definitions of Ikeda and Maruyama (J. Colloid Interface Sci. 1994, 166, 1) we distinguish between the macroscopic solubilizing power and the microscopic solubilization capacity. With pyrene as a model solubilizate, we use ultraviolet absorbance spectrophotometry to measure solubilizing powers. We use excimer fluorescence spectroscopy to identify polymer−surfactant binding transitions and to measure the aggregation numbers of free SDS micelles and of PEG-bound SDS aggregates that contain solubilized pyrene in order to calculate solubilization capacities. The solubilization capacity and solubilizing power of free SDS micelles both increase with increasing aggregation number, when the aggregation number is increased by increasing ionic strength. The solubilization capacity is approximately 3 times more sensitive than the solubilizing power to a change in aggregation number. For a given value of the ionic strength, the aggregation number of a PEG-bound SDS aggregate is approximately 50−60% smaller than that of a free micelle, while its solubilization capacity is within approximately 20% of that of a free micelle. As a result, PEG increases the macroscopic solubilizing power at all SDS concentrations above the critical aggregation concentration by virtue of the greater number of distinct surfactant aggregates formed for a given SDS concentration in the presence of PEG. Compared to free micelles, the solubilizing power of PEG-bound SDS aggregates is significantly more sensitive to ionic strength.