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 pKa 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 CO2/N2 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 microgel does not stabilize Pickering emulsions when homogenized at pH 3 with n-dodecane, sunflower oil, isononyl isononanoate, or isopropyl myristate. In contrast, PEGMA-PDEA latex 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 CO2 gas to lower the aqueous pH to 4.8. However, complete phase separation required CO2 purging for 4 h at 20 °C. A subsequent N2 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 N2 purge alone.