## 1 Introduction  
Deliveries of functional hydrophobic materials using polymeric nanoparticles have been of great interest in biomedical sciences [1; 2; 3; 4; 5; 6]. Furthermore, emulsions containing polymers can be applied to modify the surface of porous materials, endowing hydrophobic characteristics [7; 8; 9].  
There are many methodologies for preparing nanoparticles including miniemulsion polymerization [10; 11; 12; 13], interfacial polymerization, solvent evaporation, solvent deposition, nonoprecipitation, desolvation of natural polymers [14], and emulsification-diffusion [15; 16; 17]. Among these methods, miniemulsion polymerization is conducted by adopting cosurfactants such as hexadecane, cetyl alcohol, and so on. The existence of surfactant and cosurfactant ensures the formation of stable o/w miniemulsion (or nanoemulsion) under high shear without coalescence and Ostwald ripening [18]. Unfortunately, however, resulting nanoparticles and submicroparticles show insufficient dispersibility because of particle coagulation due to a lack of surface charge.  
Zeta potential is the potential difference between the dispersion medium (water, in this case) and the stationary layer of fluid attached to the dispersed particle [19]. Therefore, the zeta potential can be used as a key indicator of the stability of colloidal dispersions because the magnitude of the zeta potential indicates the degree of electrostatic repulsion between particles in a dispersion. For small particles, high zeta potential will confirm the electrostatic stability, preventing aggregation in the dispersion. If the zeta potential is small, this electrostatic repulsion may not be implemented by exceeding attractive forces, resulting in the flocculation of particles. [PERSON] et al. [20] reported that reagglomeration of the TiO\({}_{2}\) nanoparticle dispersion could be prevented by electrostatic stabilization when its zeta potential value was less than \(-\)30 mV or greater than +30 mV. Many other researchers have investigated the effects of zeta potential on particle stability for metallic powders including zirconia and titania [21; 22; 23; 24].  
Polystyrene nanocapsules have been adopted as vehicles for photochromophores [25], phase change materials [26], and so on, by virtue of their ease of preparation and thermal stability. However, chronic particle aggregation of polystyrene nanoparticles has constrained their practical usage. Even though several researchers have investigated the miniemulsion polymerization of styrene (St) and acrylic acid (AA) to enhance the colloidal stability of polystyrene nanocapsules, copolymerization of AA and St with over 20 wt % of AA has rarely been considered for water compatibility of AA [27; 28; 29]. In this work, we adopted AA to miniemulsion polymerization of St to increase the zeta potential and resulting electrostatic stability of St/AA submicroapsules containing phytoncide oil. To this aim, St/AA submicroapsules were prepared at over 20 wt % of AA content. Mean particle size and zeta potential of St/AA submicroapsules were then analyzed by dynamic light scattering (DSL). Dispersability of submicroapsules was also evaluated by TurbiscanTM Lab to confirm the sink stability over time.