Anionic Polyelectrolyte Binding to Mixed Cationic−Zwitterionic Surfactant Micelles: A Molecular Perspective from 2H NMR Spectroscopy

Abstract

Complex formation between an anionic polyelectrolyte poly(sodium styrenesulfonate) (PSSS) and mixed zwitterionic−cationic surfactant micelles was monitored using deuterium NMR spectroscopy of the specifically deuterium-labeled surfactants hexadecylphosphocholine (HDPC-γ-d6) and cetyltrimethylammonium bromide (CTAB-γ-d9). The deuterium signal from both surfactants decreased in direct proportion to the amount of added PSSS, reflecting the phase separation of the charge-neutralized PSSS−surfactant complex. The insoluble complex was preferentially enriched with CTAB over HDPC. The PSSS/CTAB charge ratio was the primary determinant of the degree of complex formation. Deuterium NMR T1 and T2 relaxation time measurements indicated that soluble PSSS−surfactant complexes exist prior to their phase separation. Within these soluble complexes motions of the cationic surfactant CTAB, but not the zwitterionic surfactant HDPC, were hindered. Quadrupolar echo deuterium NMR spectra of the phase-separated complexes showed that the flocculated material is inhomogeneous in composition and that surfactant mobility within the insoluble complex varies according to the CTAB/HDPC ratio.