## 3 Results and Discussion
### Role of Sodium Chloride  
Two possible roles for inorganic salts during the preparation of the microcapsules have been put forward. One possible role is the synergistic effect. Salt and surfactant work together to decrease the critical micelles concentration, which helps increase the dispersibility and stability of emulsion. The other role is ionic strength. The addition of salt can increase the ionic strength of emulsion and help the colloidal form double layer as a stabilizer at a low concentration. The repulsive force between particles with the same kind of charge is strengthened, which helps the formation of larger droplets and easily keeps it not aggregate [43]. To verify the role of sodium chloride, paraffin emulsion with different amounts of sodium chloride were prepared and the emulsion size changing behaviors are shown in Figure 6. For each sample, emulsion size and the distribution width increased over time. Without the addition of sodium chloride, the average size of paraffin droplets was 0.49 (0.21) \(\upmu\)m in the first hour, then it gradually increased and reached 1.72 (0.54) \(\upmu\)m after stirring for 4 h. The numbers in brackets indicate the distribution degrees of the droplet sizes. The situation was almost the same for emulsion with the addition of 1 wt% sodium chloride. However, the average size of emulsion visibly increased after adding 2 wt% sodium chloride. The emulsion size changed from 1.17 (0.18) \(\upmu\)m to 2.15 (0.70) \(\upmu\)m as the stirring time increased. Moreover, the size distribution became wider than the first two samples. With the addition of sodium chloride 4 wt%, the emulsion size and size distribution further increased. The emulsion size increased from 2.79 (0.92) \(\upmu\)m at 1 hour to 3.35 (1.79) \(\upmu\)m at 4 hours. Therefore, we argue that sodium chloride possesses a strong effect on the morphology of paraffin emulsion, the enlargement of droplets, and the widening size of distribution, which should be attributed to the enhanced electric double layer strength. A parameter of D/D\({}_{0}\) was performed and used to evaluate the stability of emulsion. D and D\({}_{0}\) were the average sizes of emulsion droplets stirring at needed time and 1 hour, respectively. The larger D/D\({}_{0}\) value reflected the lower stability of emulsion. Interestingly, the D/D\({}_{0}\) value of paraffin emulsion without sodium chloride increased significantly with increasing stirring time and reached 3.51 at 4 hours. Therefore, we argue that the emulsion stability is rather poor and aggregation occurs remarkably, resulting in microcapsules with poor morphology. With the incorporation of sodium chloride, the increase level of D/D\({}_{0}\) declined and achieved a minimum state with 4 wt% sodium chloride. The D/D\({}_{0}\) value only increased to 1.2 after being stirred for 4 hours. Thus, we argue that sodium chloride improves the stability of paraffin emulsion, which benefits the obtainment of microcapsules with good morphology. When 8 wt% sodium chloride was introduced, the decrease of emulsion size and increase of D/D\({}_{0}\) value might be due to the hydration effect being too strong. The electrostatic properties are important for emulsion stability as they can reduce or eliminate the influence of flocculation by increasing the repulsive forces between droplets [44; 45]. The Zeta potentials of paraffin emulsion and prepolymer solution with different content of sodium chloride are shown in Figure 7. Despite a low value variation, the absolute Zeta potential of paraffin emulsion system increased and reached the peak at the sodium chloride content of 4 wt% with the increasing amount of sodium chloride. The repulsive forces effectively prevented the aggregation of droplets and improved their stability, leading to the rather steady size distribution of droplets (Figure 6D). Meanwhile, the Zeta potential of prepolymer reduced.