Isothermal titration calorimetry and fluorescence spectroscopy studies of the interactions between surfactants and a phosphorylcholine-based polybetaine

Abstract

The interactions between a polybetaine and anionic, cationic, zwitterionic and neutral surfactants were studied by a fluorescence experiment with pyrene as a probe, isothermal titration calorimetry, and ¹H NMR spectroscopy. The polybetaine was a phosphorylcholine-based polymer (PNIPAM-PC) consisting of equimolar amounts of N-isopro- pylacrylamide and N-phosphoryl- choline-N′-ethylenedioxybis (ethyl)acrylamide. Strong association took place between PNIPAM-PC and the anionic surfactants sodium n-dodecyl sulfate (SDS) and sodium n-hexadecyl sulfate (SHS) via a cooperative mechanism driven by electrostatic interactions between the surfactant headgroup and the trimethylammonium group of the PC moiety linked to the polymer. The onset of binding between PNIPAM-PC and SDS or SHS takes place for surfactant concentrations of 2.0 and 0.028 mmo11^-1, respectively, which are lower than their respective critical micelle concentrations (8.3 and 0.058 mmo11^-1). No interactions were detected between PNIPAM-PC and zwitterionic surfactants bearing either a phosphoryl-choline headgroup (n-hexadecanoyl lysophosphocholine) or a dimethyl-3-ammonio-1-propanesulfonate group, cationic surfactants bearing a trimethylammonium headgroup, or neutral surfactants bearing a hepta(ethyleneglycol) headgroup. The mechanism of binding is discussed in terms of surfactant charge and chain length and is compared to the association of surfactants to polyampholytes and polyelectrolytes.