Binding of Cationic Mixed Micelles to Pyrene-Labeled Poly((acrylamido)-2-methylpropanesulfonate)

The interaction between pyrene-labeled poly(acrylamido)-2-methylpropane sulfonate) (PyPAMPS) and mixed micelles of n-dodecylhexaoxyethylene glycol monoether/n-hexadecyltrimethylammonium chloride (C12E6/CTAC) was studied by turbidimetry, quasielastic light scattering (QELS), fluorescence quenching, and UV spectroscopy. The present report focuses on the effect of the pyrene label on the polymer-micelle interaction. With nonlabeled PAMPS, we observe by turbidity and QELS a critical mole fraction of ionic surfactant (Yc) corresponding to the onset of polyelectrolyte-micelle interaction. The same Yc is observed by turbidity and QELS for PyPAMPS. However, PyPAMPS shows a lower additional transition by the same methods, which we refer to as “Yc1” to distinguish it from “Yc2” which is seen for both PAMPS and PyPAMPS. Steady-state fluorescence, in the presence of a hydrophobic quencher (N,N-dimethylaniline) solubilized in the micelles, also shows a discontinuity at Yc1. Therefore, we conclude that Yc1 and Yc2 correspond to the binding of micelles to polymeric pyrene sites and AMPS sites, respectively. Analysis of UV spectra at varying Y demonstrates that the polymer-bound pyrene penetrates inside micelles and resides at or near the hydrophobic core. These results indicate preferential binding of micelles to pyrene binding sites; nevertheless, both Yc1 and Yc2 show a linear dependence on the square root of the ionic strength. This dependence suggests the dominant role of electrostatic forces, consistent with the observation that nonionic micelles will not bind to PyPAMPS. We conclude that conjoint hydrophobic and electrostatic effects determine the interaction between PyPAMPS and C12E6/ CTAC mixed micelles.