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

Interactions of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and N -dodecylmethacrylamide (DodMAm) with n -dodecyl hexaethylene glycol monoether (C 12 E 6 ) 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 ( f Dod ) 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 f Dod . The unimer micelles of the polymers with f Dod ≥ 30 mol % have a particularly compact nanostructure where polymer chains are highly collapsed. In the presence of C 12 E 6 /CTAC mixed micelles, the polymer with f Dod = 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 ( Y p ) (i.e., Y p ≈ 0.15). With an increase in f Dod, the bulk phase separation becomes significantly less abrupt, accompanying a marked increase in Y p . For the polymers with f Dod ≥ 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 f Dod ≥ 30 mol % are unfolded by interactions with C 12 E 6 . The formation of soluble polymer−micelle complexes was manifested by an increase in the hydrodynamic radius ( R h ) upon addition of C 12 E 6 to polymer solutions. The R h for the unimer micelle with f Dod = 50 mol % increased from 8.2 to 15.8 nm by the addition of 5.0 mM of C 12 E 6 at a polymer concentration of 1.5 g/L. The R h 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 C 12 E 6 /CTAC mixed micelles.