Poly(DEAEMa-co-PEGMa): a new pH-responsive comb copolymer stabilizer for emulsions and dispersions

Stimulus responsive copolymers are an important class of surfactants that are attracting growing attention in the literature. When used to stabilize colloids, they confer responsiveness to an otherwise nonresponsive system. In this work, a new pH-responsive comb copolymer surfactant, poly(DEAEMa-co-PEGMa), where DEAEMa and PEGMa are diethylaminoethyl methacrylate and poly(ethylene glycol) methacrylate, is introduced and used to stabilize emulsions and particulate dispersions. The copolymer contained 70 mol % of DEAEMa. Turbidity versus pH measurements and photon correlation spectroscopy of the copolymer solutions revealed pH-triggered collapse of the chains above the pK(a). The surface activity of the copolymer increased with pH. The minimum surface tension measured was 33.6 mN/m at pH = 10. These data enabled identification of the pK(a) for poly(DEAEMa-co-PEGMa) as 6.8. The emulsions consisted of tetradecane-in-water and had a droplet size in the range 5-11 mum. They were slightly flocculated when the pH value was close to the isoelectric point. The emulsions phase separated at low pH values. The particulate dispersions were based on carbendazim, which is a fungacide, and had an average size of 1.8 mum. The data for the emulsions and carbendazim dispersions show that the extent of flocculation decreases with decreasing size of the dispersed phase. Analysis of the data suggest that optimum emulsion stability occurs in the pH region of 5.3-6.8 as judged by turbidity measurements. Electrophoretic mobility measurements as a function of pH for the emulsions and carbendazim dispersions reveal a similar isoelectric point in the range of 8.5-9.0, which is about two pH units higher than the pK(a) of the copolymer. A mechanism that explains the pH-responsive stability of the emulsions and dispersions is presented and discussed.