# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 1 Introduction
On the other hand, the size of electrospray droplets ranges from hundreds of micrometers down to several tens of nanometers, and the size distribution of electrospray droplets is nearly monodispersed [32; 33]. Notably, the structure and morphology of electrospray particles are effectively controlled and carefully designed by adjusting the operating parameters, solution properties, and experimental device of the electrospray process [34; 35]. The latter is not only critical for achieving improvement in the properties of electrospray particles and diversifying of the structure as well as morphology, but it also promotes the efficiency of the process and broadens the applications of electrospray particles. Besides, when PCL is used as shell material to encapsulate active substance via electrospraying, high encapsulation efficiency of the fragile active substance, as well as its protection, and avoiding any particle aggregation attributed to the Coulomb repulsions among charged droplets as well as rapid solvent evaporation during their flight toward the collector [36; 37]. Although flashing nanoprecipitation, as a solvent displacement technology, also fabricates polymeric particles in simple and rapid way, the high requirements in the state of working streams (strong turbulence), special micro-mixers and the lipophilicity of active substances make it less feasible compared with electrospraying. For solvent evaporation, on the one hand, the use and removal of surfactant will deteriorate the final structure as well as the properties of particles. Meanwhile, the removal of the organic solvent also takes a lot of effort and time. On the other hand, the size, as well as the particle size distribution, are difficult to design and control. In most cases, particles with broad size distribution are obtained. Therefore, electrospraying is regarded as a green, rapidity and efficient technology to design polymeric microparticles. During the process of electrospraying, there are lot of interrelated parameters which influence the realization and the outcomes. By controlling these parameters, different structures and morphologies of electrospray particles are obtained [38; 39; 40]. According to their inherent characteristics, these parameters divide into two categories, solution formation parameters, and operating parameters. Solution formation parameters mainly include solvent systems, polymer concentration, polymer molecular weight, and active substance concentration. These solution formulation parameters mainly determine the physico-chemical properties of electrospraying solution (including viscosity, surface tension, vapor pressure, dielectric constant, electrical conductivity, and chains entanglement). Process parameters mainly include applied voltage, flow rate, working distance, spraying mode, nozzle gauge,collection method, environmental temperature, environmental pressure, and environmental humidity. Among these interrelated parameters, the selection of a suitable solvent system as the carrier of PCL is fundamental and a prerequisite for the optimization of electrospray and controlling the morphology as well as the structure of electrospray particles. Firstly, the main solution properties including the surface tension, electrical conductivity, viscosity and vapor pressure depend mainly on the used solvent system [41]. These properties influence the process stability and the formation of Taylor cone during the electrospraying, but also the working range values of operating parameters, i.e., the applied voltage, the flow rate, and the working distance. The formation of Taylor cone during electrospraying is indispensable for stabilizing experimental process and achieving the micromization, nanonization, as well as homogenization of electrospray particles [42; 43]. It is essential to understand that only in the stable cone-jet mode the size and morphology of electrosprayed particles can be controlled and designed by carefully changing other parameters.