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.