Interaction of cetyltrimethylammonium bromide and poly(2-(acrylamido)-2-methylpropanesulfonic acid) in aqueous solutions determined by excimer fluorescence

Abstract

The binding interaction of the cationic surfactant cetyltrimethylammonium bromide (CTAB) and anionic poly(2-(acrylamido)-2-methylpropanesulfonic acid) (PAMPS) in dilute aqueous solutions was studied using the excimer fluorescent emission of the cationic probe 1-pyrenemethylamine hydrochloride (PyMeA·HCl). In the absence of CTAB, the saturation binding of PyMeAH+ on PAMPS is about 2.4 AMPS repeat units for one probe cation as determined by the relative emission intensity, IE/IM, of the excimer to monomer. With increasing CTAB concentration, IE/IM firstly increases, reaches a maximum, then decreases to zero. The IE/IM maximum indicates a critical aggregation concentration (cac) of 10−5 mol/l for CTAB in PAMPS solutions. The CTAB concentration at which IE/IM is zero is exactly equal to the PAMPS concentration, indicating that the probe cation is thoroughly excluded from the binding site of PAMPS by the CTAB cation and the equivalent stoichiometric aggregation is formed between CTAB and PAMPS. The blueshift of the excimer emission and the excitation spectra shows that the decrease of IE/IM with increasing CTAB concentration above the cac is caused mainly by the decrease of the static excimer.