Size of the Sodium Dodecylsulfate Aggregate Bound to Polyethylene Glycol: Effect of Different Cations

Abstract

Dynamic fluorescence spectroscopy of micelle-solubilized pyrene was used to measure the size of the sodium dodecylsulfate (SDS) aggregate bound to polyethylene glycol (PEG). The measurements were carried out with very dilute PEG and SDS concentrations, to ensure that there were no free surfactant aggregates in solution and that all aggregates present were bound to the polymer. Effect of salts of sodium, cesium, lithium, and magnesium of varying concentrations on the size of the bound aggregate was determined. The affinity of the cation toward PEG controlled the size of the SDS aggregate bound to the polymer. The stronger the binding between the cation and PEG, the smaller was the bound SDS aggregate. There was no effect of either the anion or the salt concentration for a particular cation on the size of the bound SDS aggregate. As the ionic strength increased, the only parameter affected was the number of bound surfactant aggregates per polymer chain. Results are discussed in terms of simultaneous affinities of the cations toward the polymer and the micelle.