Binding on strong polyelectrolytes of mixed ionic and nonionic surfactants below their critical micelle concentration observed by fluorescence

The binding of mixed surfactants of cationic cetyltrimethylammonium bromide (CTAB) and nonionic octaethylene glycol monododecyl ether (C12E8) on anionic polyelectrolyte poly[2-acrylamido-2-methylpropanesulfonic acid (PAMPS)] and fluorophore-labeled copolymers containing about 40 mol% of AMPS was investigated at different mole fractions, Y , of CTAB in the surfactant mixture. The excimer emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA.HCl), nonradiative energy transfer (NRET) between pyrene and naphthalene labels and I-1/ I-3 of the pyrene label were determined by varying the total surfactant concentration, c(Surf). The I-E/ I-M value of PyMeA.HCl firstly increases and then decreases to 0 with c(Surf), showing a maximum on every curve. The critical aggregation concentration of the mixed surfactants determined from the I-E/ I-M maximum decreased from 5x10(-5) to 1x10(-5) mol/l as Y increased from 0.1 to 0.50, and then leveled off as Y increased up to unity. And at least 5x10(-6) mol/l CTAB was required for the mixed surfactants to bind on the PAMPS cooperatively. Equimolar binding of CTAB on AMPS was formed at I-E/ I-M=0 when Y greater than or equal to0.25, while at Y =0.1 some CTAB molecules in the mixed micelle were directed to the water phase without binding with AMPS. Both the intramolecular and the intermolecular NRET increased and then decreased with c(Surf), having a maximum on each curve corresponding to the equimolar binding of CTAB and AMPS so long as Y >0, indicating the coiling of the chain and interchain aggregation upon bound surfactants. The I-Py/ I-Np value at the maximum decreased with decreasing Y because more nonionic surfactant C12E8 participated into the polyelectrolyte-mixed surfactant complexes together with bound CTAB.