# Stabilization of Pickering Emulsions by Hairy Nanoparticles Bearing Polyanions
## 1 Introduction  
Conventional emulsions used in daily life are commonly stabilized by low molecular weight surfactants that are thermodynamically stable. However, this kind of emulsion increases the costs and environmental impact [1]. Pickering emulsions are among the most well-studied alternative strategies; these were first proposed by [PERSON] [2] and are applied in various fields such as drug delivery [3, 4], cosmetics [5, 6], food science [7], manufacturing of microcapsules [8, 9], and porous materials [10, 11, 12, 13]. Pickering emulsions not only have outstanding stability, but also satisfy the demand for intelligent stimuli-responsive emulsions because of functional nanoparticles anchored at the water/oil interface such as spherical nanoparticles [14], nanocrystals [15, 16], and nanotubes [17]. [PERSON] et al. [18] reported that thermally-responsive emulsions could be created with the SiO\({}_{2}\)-PDMAEMA particles such that stable emulsions prepared at low temperature were rapidly broken by increasing the temperature above the critical flocculation temperature (CFT). [PERSON] et al. prepared a new class ofdonor-acceptor Stenhouse adduct (DASA)-functionalized silica microspheres to formulate visible light-controlled Pickering emulsions. This unique inversion behavior was applied to control the encapsulation and release of fluorescein sodium salt [19]. In recent years, studies have focused on the functional applications of Pickering emulsions stabilized by creative synthetic polymeric nanoparticles, but few have evaluated the stabilization of Pickering emulsions--this is significant for their practical applications.  
For Pickering emulsion stabilization, several factors must be considered including particles (types, size, aspect ratio, and grafting density), salt concentration, and oil phase. For instance, [PERSON] et al. [20] investigated the effect of particle aspect ratio on the stability of Pickering emulsions. They reported spindle-type hematite particles with higher aspect ratio particles deposited more readily at the water/oil interface. This demonstrated that destabilization can be achieved simply by shape changes. [PERSON] et al. [21] found that fumed silica particles could easily form stable volume-filling networks under high salt concentration where the interparticle interactions were attractive. [PERSON] et al. [22] prepared oil-in-water (O/W)-type Pickering emulsions stabilized by PS@PNIPAM hairy particles from various oils (eg. heptane, hexadecane, and toluene), but emulsions could not be formed when 1-undecanol was used as the oil phase because the wettability of hairy particles was higher for 1-undecanol than for water.  
Nevertheless, due to complicated operation processes and repeated experimental procedures which cause vast time expenditure and material waste [23], there are relatively few systematic studies on the stability of Pickering emulsions to satisfy the growing application demands in multiple fields with long-term conservation. Hairy nanoparticles bearing polyelectrolytes, composed of a core and a layer of polymer chains densely grafted via covalent bonds on the core surface [24], are the most used candidates for preparation of Pickering emulsion [25; 26; 27; 28] and further applied to achieve the tunability of emulsification and demulsification. Polyanion-modified nanoparticles are becoming an appealing choice due to many advantages like feasibility, uniform size, and outstanding stability of products in a practical environment. In this work, poly (sodium \(p\)-styrenesulfonate)-modified polystyrene nanoparticles (PS@PSS) were synthesized via surface-initiated photoemulsion polymerization [29] and were used as the solid surfactant to prepare Pickering emulsions under various conditions. The effects of ultrasonic power, ultrasonic time, standing time, oil phases, salt concentration, and water:oil ratio on the formation and stabilization of Pickering emulsions were systematically investigated to establish the optimum condition for the formation of the desired Pickering emulsions.