Effects of Surfactants on the Swelling Transition of pNIPAM Microgels

We investigate the effects of anionic and cationic surfactants on the volume phase transition of thermo-responsive poly(N-Isopropilacrylamide), pNIPAM, microgels. The influence of both surfactants, SDS and DTAB respectively, is totally different despite their hydrophobic tail is identical. Whereas the cationic surfactant DTAB does not change appreciably the swelling behavior, the addition of the anionic surfactant SDS significantly swells the microgel and shifts the transition to higher temperatures. In addition, the swelling transition broadens and exhibits a two-step swelling process that can be understood by considering that SDS molecules bounds to pNIPAM segments hydrophobically and repel each other via electrostatic interaction. The microgel-surfactant mixture then behaves as an ionic network in which the first step of de-swelling arises from the change in the polymer solubility with temperature whereas the second step is attributed to its competition with the ionic contribution. The experimental results are quantitatively interpreted in terms of the classical Flory-Huggins theory with important modifications to account for the free energy of association of the surfactant molecules with the polymer network and for a swelling-dependent ionic contribution.