INTRODUCTION
Nowadays, crude oil serves as one of the most important natural resources for chemical engineering and energy supply. Due to extensive exploration, most of the current crude oil reserve remains as heavy oil with viscosity exceeding 10000 mPas and the high viscosity of crude oil could pose severe challenges to its exploration, refinery and transportation process by forming conglomeration and congestion[1-2]. Therefore, it is of vital importance to reduce the viscosity of crude oil and improve its flowability for current chemical industries. Among various viscosity reduction practices, chemical additives serve as a promising solution due to its stable performance and cost efficiency. For example, researchers have widely used amphipathic polymers, such as α-olefins-maleic anhydride copolymers[3-5], ethylene-vinyl acetate copolymers[6], and poly[N,N-(dimethylamino) ethyl methacrylate] (PDMA)-poly(lauryl methacrylate) (PLMA) copolymers[7] to stabilize crude oil in water by forming emulsions and the effect of internal and external parameters such as chain lengths, hydrophilicity/hydrophobicity balance as well as pH and ionic strength on emulsification process has been extensively investigated.
As an alternative to traditional emulsifiers, micro- or nanoparticles have been utilized to form Pickering emulsions due to its unparalleled strengths such as stability, tunability and stimuli-responsiveness[8-11]. As a result, it has been widely applied to advanced industrial and biomedical applications such as conducting biphasic catalysis[12-13] and building compartmentalized artificial cells[14]. Moreover, researchers have relied on the unique properties of Pickering emulsions to promote crude oil emulsification process. For example, Saha et al. utilized carbon black (CB) particles with proper hydrophilic/hydrophobic balance to emulsify model oil and crude oil in water[15]. Compared with commercial surfactant, the CB particles are stable and biodegradable with their hydrophobicity easily tuned by adding acid or salt. However, nanoparticles are rarely used to stabilize crude oil and quantitative study of its emulsification effect on crude oil, as represented by viscosity or interfacial tension change, is lacked somehow in previous cases[15-16]. Due to the larger specific area as well as delicate and tunable structure, nanoparticles, especially amphipathic core-shell or Janus nanoparticles have great potential to improve crude oil flowability by forming Pickering emulsions. For example, poly(N-isopropylacrylamide) (PNIPAM) modified magnetic nanoparticles have been used to form oil-in-water Pickering emulsions for targeted oil separation[16]. After being exposed to an external magnetic, the latex particles could be efficiently separated from water and oil recovery could be achieved by elevating the temperature to dissemble the assembly. Recently, our group has conducted research on utilizing water-soluble spherical polymer brushes (SPB) to stabilize model oil, i.e., decane or hexadecane in water[17]. After introducing water-soluble polystyrene (PS)-poly(sodium p-styrenesulfonate) (PSS) brushes into oil/water mixtures, Pickering emulsions with well-tuned performance could be prepared successfully. However, in their cases, model oil including toluene and dodecane was used and the emulsification stability of crude oil in water still remains a challenge due to its high viscosity and complicated composition.
Herein, we present a novel practice to significantly enhance crude oil flowability and emulsion stability by forming Pickering emulsions in water via monodispersed oil-soluble spherical polymer brushes with high stability, which were prepared by inverse phase emulsion polymerization. The core and brush layer consisted of poly(acrylic acid) (PAA) and poly(N-Vinylcarbazole) (PVK) respectively and the length as well as photo-luminescence of the PVK chains could be tailored by the dose of N-Vinylcarbazole (VCz) with its nanostructure analyzed by small-angle X-ray scattering (SAXS). More importantly, utilizing the hydrophobicity of the brush layer and hydrophilicity of the core, efficient crude oil stabilization could be achieved by forming Pickering emulsion with viscosity lowered by 99% for over a month, which enables the nanoparticulate Pickering system to be a potential alternative of traditional dispersants in crude oil transportation and oil spillage remediation.