Fluorescence study of chromophore labeled strong polyelectrolyte bound with oppositely charged surfactant

Four strong polyelectrolyte samples of 2-(acrylamido)-2-methylpropanesulfonic acid (AMPS) and N,N-dimethylacrylamide (DMAA) were radically copolymerized with a single label of naphthalene or pyrene, with both labels and without label, containing about 40 mol % AMPS. Fluorescence nonradiative energy transfer (NRET) I_Py/I_Np, anisotropy r, I₁/I₃ and excimer emission I_E/I_M of pyrene labels were observed in dilute aqueous solutions with and without cationic surfactant of cetyltrimethylammonium bromide (CTAB). The overlap concentration was determined as 3 g/L from the appearance of intermolecular excimer. The variation of intra- and intermolecular NRET with total polyelectrolyte concentration showed that the charged chains preferentially interpenetrated each other rather than reduce their coil volume as their concentration beyond the overlap threshold. By binding with CTAB, the polyelectrolyte chain became more coiled as known from the reduced viscosity. The intramolecular NRET was dominant when CTAB]Д᎒^-5 M and then the intermolecular NRET occurred at higher CTAB concentrations with hydrophobic aggregation between CTAB tails bound on different polyelectrolyte chains. The CTAB concentration corresponding to the maxima of I_Py/I_Np just is equal to the AMPS monomer concentration, indicating the formation of 1:1 binding between surfactant and polyelectrolyte in very dilute solutions. Added salt of NaCl up to 0.1 M hardly affected the intramolecular NRET but affected the I_Py/I_Np value for the intermolecular NRET.