First Direct Imaging of Electrolyte-Induced Deswelling Behavior of pH-Responsive Microgels in Aqueous Media Using Scanning Transmission X-ray Microscopy

Lightly cross-linked sterically stabilized poly(2-vinylpyridine) latexes exhibit pH-responsive behavior, undergoing a latex-to-microgel transition below pH 4.1 as a result of protonation of the pyridine pendent groups. We have examined both the latex and microgel states of such particles directly in aqueous solution using scanning transmission X-ray microscopy (STXM). Moreover, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy studies confirm that the nitrogen atoms of the microgel particles are fully protonated at low pH. The addition of salt causes partial deswelling of these microgel particles, but spectroscopic analysis confirms the retention of their cationic character. This is the first direct visualization of the effect of electrolyte screening on microgel dimensions in aqueous solution. In each case, the observed particle dimensions are consistent with dynamic light scattering characterization, especially when polydispersity effects are taken into consideration.