Isothermal calorimetric investigation of the interaction of poly(N-isopropylacrylamide) and ionic surfactants

The interaction of poly(N-isopropylacrylamide) (PNIPAM) with an anionic (SDS) and three cationic (dodecyl and hexadecyltrimethylammonium bromides, DTAB and CTAB, and dodecylammonium chloride, DAC) surfactants has been studied using isothermal titration calorimetry between 17 and 29 degreesC. The high sensitivity of this technique allows precise determination of the parameters that characterize polymer-surfactant interactions. Based on these results, it is possible to conclude that the interaction of PNIPAM with ionic surfactants is significantly affected by temperature. This is attributed to changes in polymer hydration, which become more important near the lower critical solution temperature (approximate to32 degreesC). Interaction of PNIPAM with SDS is more favorable than with cationic surfactants. Surfactant hydrophobicity is also an important factor for this interaction, with CTAB displaying a more intense interaction than DTAB. The contribution from the size of the headgroup was also investigated: DAC interacts more strongly than DTAB with PNIPAM. Thus, for the same degree of hydrophobicity, a smaller headgroup favors interaction.