The association of octadecyl-end-capped poly-(sodium 2-acrylamido-2-methyIpropanesulfonates) in water and salt solutions: A study by fluorescence spectroscopy and isothermal titration calorimetry

Steady-state fluorescence spectroscopy and isothermal titration calorimetry (ITC) have been used to study the aggregation in aqueous solutions of poly -(2-acrylamido)-2-methylpropanesulfonic acids, sodium salt mono-endcapped w ith either N,N-di-n-octadecyl or N-4-[(1-pyrenyl)butyl]-N-n-octadecyl which were prepared by free radical polymerization of 2-(acrylamido)-2-methylpro pane sulfonic acid (AMPS) initiated with the azo compounds, 4,4'-azobis{cya no-N,N-di-n-octadecyl}pentanamide and 4,4'-azobis{cyano-N,N-[4-(1-pyrenyl)b utyl] -n-octadecyl} respectively. Both techniques indicate the occurrence o f multimolecular aggregates in solutions of the polymers in water and in 0. 2 M NaCl. The concentration range for aggregation is about 1-14 mmol AMPS 1 (-1) (0.5-2.7 g 1(-1)) in 0.2 1(-1) NaCl and the enthalpy of micellization, estimated from ITC data, is similar to 100 J [mol AMPS](-1). The accessibi lity of the chromophores to neutral molecules and to cationic species was a ssessed by quenching of fluorescence with nitromethane and thallium nitrate , respectively The association of the mono-endcapped polymers is compared t o that of PAMPS derivatives carrying hydrophobic groups randomly attached a long the chain.