# Influence of Solvent Selection in the Electrospraying Process of Polycaprolactone
## 3 Selection of Solvent Systems Used in PCL Electrospraying
### Hansen Solubility Parameter  
The Hansen Solubility Parameters (HSPs) methodology is one of the approaches used to design the formulation system with PCL. [PERSON] et al. have determined the three partial solubility parameters of PCL by the group contribution method, swelling experiments, and turbidimetric titration [99], whereas [PERSON] et al. have used [100] the experimental results obtained by [PERSON] and [PERSON] by inverse gas chromatography [101]. The values change according to the molecular weight of the PCL used, its concentration, and the working temperature (Table 1). Thus, for fixed molecular weight or concentration, the dispersive component decreases, whereas its polar and hydrogen ones increase with increasing the concentration or molecular weight. The value of RS, which depends on the number of appropriate solvent systems to dissolve the PCL, decreases with increasing molecular weight by keeping the concentration at a constant value. Nevertheless, the changes of RS are relatively low with the increase in concentration for fixed molecular weight. Thus, the solubility of PCL in a solvent system depends mainly on its molecular weight and the working temperature, since its partial solubility parameters decrease with its increase. The PCL Hansen solubility parameters of common solvents are given in Table 2. Good solvents are those which have partial solubility parameters close to PCL ones and a distance lower than the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **Temperature \({}^{\circ}\)C** & **Method** & \(R_{S}\) & \(\varepsilon_{D}\) & \(\delta_{P}\) & \(\delta_{H}\) & **Reference** \\ \hline PCL & - & Group contribution & - & 17.0 & 4.8 & 8.3 & [99] \\ PCL-14,000-0.5 g/5 mL \({}^{\circ}\) & 25 & Swelling tests & 7.1 & 17.8 & 6.1 & 7.8 & [99] \\ PCL-14,000-0.5 g/5 mL & - & Turbidimetric titration & \(4.5<\mathrm{R}<7.0\) & 16.2 & 3.3 & 9.1 & [99] \\ PCL-14,000-0.5 g/5 mL & - & Turbidimetric titration & & & & & \\ PCL-14,000-0.5 g/5 mL & - & (hexane-butanol) & & & & & \\ PCL-14,000-2.5 g/5 mL & 25 & Swelling tests & 7.1 & 17.6 & 6.2 & 8.0 & [99] \\ PCL-65,000-0.5 g/5 mL & 25 & Swelling tests & 5.5 & 17.8 & 6.2 & 7.7 & [99] \\ PCL-65,000-0.5 g/5 mL & - & Turbidimetric titration & & & & & \\ PCL-65,000-0.5 g/5 mL & - & (heptane/butanol) & & & & & \\ PCL-65,000-0.5 g/5 mL & - & Turbidimetric titration & & & & & \\ PCL-65,000-0.5 g/5 mL & - & (hexane/butanol) & & & & & \\ PCL-65,000-2.5 g/5 mL & 25 & Swelling tests & 5.0 & 17.0 & 7.7 & 8.3 & [99] \\ PCL-7\% by mass & 70 & Inverse gas chromatography & - & 15.5 & 2.4 & 7.4 & [100] \\ PCL-7\% by mass & 80 & Inverse gas chromatography & - & 14.9 & 2.6 & 7.3 & [100] \\ PCL-7\% by mass & 90 & Inverse gas chromatography & - & 14.5 & 2.3 & 7.4 & [100] \\ PCL-7\% by mass & 100 & Inverse gas chromatography & - & 14.2 & 2.2 & 7.3 & [100] \\ PCL-7\% by mass & 110 & Inverse gas chromatography & - & 13.8 & 2.2 & 7.3 & [100] \\ \hline \hline \end{tabular}  
* PCL-14000-0.5 g/5 mL represents that the molecular weight of PCL is 14,000 g/mol and the concentration of PCL is 0.5 g/5 mL.  
\end{table} 
Table 1: Hansen solubility parameters of PCL under different conditions and different measurement methods. RS value. The reading of the table global approach needs to be confirmed from an experimental point of view. Indeed, there are also two types of exceptions, i.e., (i) solvents, such as acetic acid, benzyl alcohol and 2,2,2-Trifluoroethanol, have a higher distance from PCL is than the RS of solubility sphere, but solubilize PCL, and (ii) several solvents with a distance from PCL less than the RS, cannot solubilize PCL. Also, the distance between solvent and PCL also characterizes the compatibility as well as interactions between them. A small distance indicates that the compatibility and interactions between solvent and PCL are strong. Thus, the entanglements between the PCL molecular chains, are suppressed, due to the changes in PCL-PCL interactions by PCL-solvent ones.