A Look at the Thermodynamics of the Association of Amphiphilic Polyelectrolytes in Aqueous Solutions: Strengths and Limitations of Isothermal Titration Calorimetry

Abstract

The dissolution in water of the sodium salts of poly(2-acrylamido)-2-methylpropanesulfonic acid (PAMPS), N,N-di-n-octadecyl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid [(C18)2-PAMPS], cholesteryl-endcapped-poly(2-acrylamido)-2-methylpropanesulfonic acid (Chol-PAMPS), and a random copolymer of AMPS and N-dodecylmethacrylamide (PAMPS-Dod20) was studied by isothermal titration calorimetry (ITC). The endcapped polymers form multimolecular aggregates in aqueous solutions. The concentration range for aggregation and the enthalpy of micellization were determined as functions of the electrolyte concentration (0.01 M < [NaCl] < 0.3 M) and temperature (288 K < T < 308 K) for solutions of (C18)2-PAMPS. At 298 K and 0.2 M NaCl, aggregation of this amphiphilic polyelectrolyte occurs in solutions with concentrations ranging from about 1 to 14 mmol of AMPS L-1 (0.5−2.7 g L-1) with an enthalpy of micellization of ∼100 J (mol of AMPS)-1. The effect of the experimental conditions, such as the method of preparation of the polymer stock solution and its concentration, on the ITC results is described in the case of Chol-PAMPS. Titrations performed with poly(sodium-2-acrylamido-2-methylpropane sulfonate) (PAMPS) confirmed that PAMPS does not aggregate in aqueous NaCl, whereas PAMPS-Dod20 forms predominantly unimolecular micelles. The advantages and limitations of ITC for studying the micellization thermodynamics of amphiphilic polyelectrolytes are discussed.