Crude oil emulsification capacity of PAA-PVK latex particles
Since the synthesized oil-soluble brushes feature a hydrophilic core and hydrophobic shell, we further investigated its application potential as a crude oil emulsifier. Crude oil emulsification is of vital importance for petro-chemical industry since it plays an important role in facilitating the collection and transportation process of crude oil. Besides various conventional surfactants, particles represent a promising avenue in realizing oil stabilization by forming Pickering emulsions due to its high stability and various functionalities[15]. After addition of a small amount of PAA-PVK brushes into the mixture of water and oil, both model oil and crude oil could disperse well in water and we could easily observe well defined oil-in-water emulsion droplets in the optical micrographes of Figure 6. The emulsion droplets for crude oil feature small and relatively uniform particles with average size as 3.2 ± 0.7 μm according to size analysis. Furthermore, although PAA core could also facilitate the formation of emulsion due to its partial amphipathy as shown in Figure S4, the PAA-PVK brushes could contribute to smaller latex particles with evidently narrower distribution by additional amphipathic effect compared to those formed in PAA core (4.1 ± 1.1 μm), and the same tendency could also be observed for model oil. Interestingly, after introducing normal water-soluble PS-PSS brushes of similar size (293 nm, the DLS data was shown in Figure S5) under the same procedures, the crude oil could not be dispersed into water as shown in Figure S6, probably due to the strong hydrophilicity of the shell layers. In previous cases[7], the presence of too much polyelectrolyte chains would prevent surfactant from accumulating in oil-water interfaces due to electrostatic repulsion, so using oil-soluble polymer brushes we could ensure sufficient contact of crude oil and reduce the interfacial tension effectively.
To further quantify the emulsification effect of PAA-PVK brushes, we measured the apparent viscosity of emulsion solutions compromising crude oil, water and PAA-PVK brushes of different concentrations. According to rheology tests shown in Figure 7, the apparent viscosity of crude oil was significantly reduced after introducing PAA-PVK brushes and forming Pickering emulsions. After increasing the concentration of PAA core or PAA-PVK brushes, the viscosity of oil-in-water emulsions decreased monotonically and reached a plateau after reaching certain level. To be specific, at room temperature, the viscosity has declined by over 99 % from 12200 mPas for crude oil (Figure S7) to 623, 501.7, 141.2, 131.1, 116.2 mPas after introducing PAA-PVK brushes of 2, 4, 6, 8, 10 mg/mL into the system while the emulsion viscosity under PAA core of the same concentration gradient was 1727, 702.4, 657.6, 438.1, 334.5 mPas. Evidently, the emulsification effect was significantly improved after grafting PVK brushes onto PAA core due to the additional amphiphilic effect, consistent with the observation of smaller and more uniform latex particles in optical micrograph of Figure 6. Moreover, the viscosity of crude oil emulsions formed in PAA-PVK brushes could remain stable within 30 days thanks to the high stability of Pickering emulsions while for PAA cores (Figure 8), phase separation between water and oil gradually happened after storing for 15 days with viscosity increasing dramatically. According to previous research[29-30], the grafted polymers on the nanoparticulate brushes could contribute to higher dilatational modulus and could prevent the emulsions from coalescing and Ostwald ripening. So in this way, compared with bare particles, the nanoparticulate brushes could achieve better stability of Pickering emulsions. Therefore, the formation of stable oil-in-water emulsion facilitated by the oil soluble spherical polymer brushes could endow this novel polymer system with great potential in oil collection and transportation.