Novel Pickering Emulsifiers Based on pH-Responsive Poly(2-(diethylamino)ethyl methacrylate) Latexes

The emulsion copolymerization of 2-(diethylamino)­ethyl methacrylate (DEA) with a divinylbenzene cross-linker in the presence of monomethoxy-capped poly­(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded near-monodisperse, sterically stabilized PEGMA-PDEA latexes at 10% solids. Dynamic light scattering studies indicated intensity-average diameters of 190 to 240 nm for these latexes at pH 9. A latex-to-microgel transition occurred on lowering the solution pH to below the latex p<i>K</i><sub>a</sub> of 6.9. When dilute HCl/KOH was used to adjust the aqueous pH, a systematic reduction in the cationic microgel hydrodynamic diameter of 80 nm was observed over ten pH cycles as a result of the gradual buildup of background salt. However, no such size reduction was observed when using CO<sub>2</sub>/N<sub>2</sub> gases to regulate the aqueous pH because this protocol does not generate background salt. Thus, the latter approach offers better reversibility, albeit at the cost of slower response times. PEGMA-PDEA <i>microgel</i> does not stabilize Pickering emulsions when homogenized at pH 3 with <i>n</i>-dodecane, sunflower oil, isononyl isononanoate, or isopropyl myristate. In contrast, PEGMA-PDEA <i>latex</i> proved to be a ubiquitous Pickering emulsifier at pH 10, forming stable oil-in-water emulsions with each of these four model oils. Lowering the solution pH from 10 to 3 resulted in demulsification within seconds. This is because these pH-responsive particles undergo a latex-to-microgel transition, which leads to their interfacial desorption. Six successive demulsification/emulsification cycles were performed on these Pickering emulsions using HCl/KOH to adjust the solution pH. Demulsification could also be achieved by purging the emulsion solution with CO<sub>2</sub> gas to lower the aqueous pH to 4.8. However, complete phase separation required CO<sub>2</sub> purging for 4 h at 20 °C. A subsequent N<sub>2</sub> purge raised the aqueous pH sufficiently to induce a microgel-to-latex transition, but rehomogenization did not produce a stable Pickering emulsion. Presumably, a higher pH is required, which cannot be achieved by a N<sub>2</sub> purge alone.