Quantifying the Influence of Poly(Ethylene glycol) on the Micelle Formation of Nonionic Detergents

The influence of poly(ethylene glycol) (PEG with molecular weights between 400 and 4000) on the critical micelle concentration (CMC) of nonionic detergents with maltose as well as oligo(ethylene glycol) head groups is determined by using 8-anilinonaphthalene-1-sulfonate (ANS) as fluorescence probe. The CMC is found to increase with the concentration of PEG (0%-30% (w/v)) in a way that is determined by the molar concentration of oxyethylene (OE) units and independent of the molecular weight of the added polymer. The effect is explained with the extended conformation of PEG in aqueous solution allowing for an interaction of detergent monomers with individual OE units via their alkyl tails. The fluorescence spectra of ANS are found to exhibit two major emission peaks that are affected in position and intensity by binding to micelles as well as PEG. A model with two conformations of ANS combined with two binding sites in the micelles is used to explain the spectra and their correlation with detergent properties. The shapes of the titration curves are shown to depend on the aggregation number and the equilibrium constant describing binding of ANS to micelles and are analyzed to find that PEG competes with micelles for binding of ANS.