Interaction of Unimolecular Micelles of Hydrophobically-Modified Polyelectrolytes with Nonionic/Ionic Mixed Surfactant Micelles

Abstract

Interactions of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and N-dodecylmethacrylamide (DodMAm) with n-dodecyl hexaethylene glycol monoether (C12E6) with and without added n-hexadecyltrimethylammonium chloride (CTAC) in 0.2 M NaCl aqueous solutions were investigated by fluorescence and light-scattering techniques. The polymers with the DodMAm contents (fDod) ranging from 10 to 50 mol % were singly labeled with pyrene (1 mol %) or doubly labeled with pyrene (1 mol %) and naphthalene (4 mol %). These polymers form unimolecular micelles (unimer micelles) owing to intramolecular self-association of polymer-bound dodecyl groups, the compactness of the unimer micelle depending on fDod. The unimer micelles of the polymers with fDod ≥ 30 mol % have a particularly compact nanostructure where polymer chains are highly collapsed. In the presence of C12E6/CTAC mixed micelles, the polymer with fDod = 10 mol % abruptly undergoes bulk phase separation as the mole fraction of CTAC (Y) in the mixed micelle is increased to a certain critical level (Yp) (i.e., Yp ≈ 0.15). With an increase in fDod, the bulk phase separation becomes significantly less abrupt, accompanying a marked increase in Yp. For the polymers with fDod ≥ 40 mol %, no such bulk phase separation was observed in a region of 0 ≤ Y ≤ 0.6. Intrapolymer nonradiative energy transfer indicates that the unimer micelles of the polymers with fDod ≥ 30 mol % are unfolded by interactions with C12E6. The formation of soluble polymer−micelle complexes was manifested by an increase in the hydrodynamic radius (Rh) upon addition of C12E6 to polymer solutions. The Rh for the unimer micelle with fDod = 50 mol % increased from 8.2 to 15.8 nm by the addition of 5.0 mM of C12E6 at a polymer concentration of 1.5 g/L. The Rh at Y = 0 further increased to 21.5 nm as CTAC was added to Y = 0.07, indicating an interplay of hydrophobic and electrostatic interactions in the complexation between the unimer micelle and C12E6/CTAC mixed micelles.