A look at the thermodynamics of the association of amphiphilic polyelectrolytes in aqueous solutions: Strengths and limitations of isothermal titration calorimetry

The dissolution in water of the sodium salts of poly(2-acrylamido)-2-methyl propanesulfonic acid (PAMPS), N,N-di-n-octadecyl-endcapped-poly(2-acrylamid o)-2-methylpropanesulfonic acid [(C-18)(2)-PAMPS], cholesteryl-endcapped-po ly( 2-acrylamido)-2-methylpropanesulfonic acid (Chol-PAMPS), and a random c opolymer of AMPS and N-dodecylmethacrylamide (PAMPS-Dod(20)) was studied by isothermal titration calorimetry (ITC). The endcapped polymers form multim olecular aggregates in aqueous solutions. The concentration range for aggre gation and the enthalpy of micellization were determined as functions of th e electrolyte concentration (0.01 M < [NaCl] < 0.3 M) and temperature (288 K < T < 308 K) for solutions of (C-18)(2)-PAMPS. At 298 K and 0.2 M NaCl, a ggregation of this amphiphilic polyelectrolyte occurs in solutions with con centrations ranging from about 1 to 14 mmol of AMPS L-1 (0.5-2.7 gL(-1))wit h an enthalpy of micellization of similar to 100 J (mol of AMPS)1. The effe ct of the experimental conditions, such as the method of preparation of the polymer stock solution and its concentration, on the ITC results is descri bed in the case of Chol-PAMPS. Titrations performed with poly( sodium-2-acr ylamido-2-methylpropane sulfonate) (PAMPS) confirmed that PAMPS does not ag gregate in aqueous NaCl, whereas PAMPS-Dod(20) forms predominantly unimolec ular micelles. The advantages and limitations of ITC for studying the micel lization thermodynamics of amphiphilic polyelectrolytes are discussed.