# Emulsions Stabilized with an Electrostatic Complex of
## 4 Conclusions  
OC\(-\)Gel, QCNF, and QCNF/OC\(-\)Gel were used as emulsifiers for the preparation of O/W emulsions, and the emulsion stability was investigated in terms of droplet size at different pH values (pH 3, 5, 7.4, 9). A hydrophobically modified gelatin (i.e., OC\(-\)Gel) was more surface\(-\)active than unmodified gelatin. When the medium pH value was 7.4 (greater than the isoelectric point of OC\(-\)Gel), the maximum complexation took place at a QCNF/OC\(-\)Gel mass ratio of 1/8. The complexation degree increased rapidly when the medium pH value increased to 6, and thereafter, it increased slowly. Therefore, it could be said that a QCNF/OC\(-\)Gel mass ratio of 1/8 and a pH value of 7.4 is the near\(-\)optimum condition for the formation of this complex. FT\(-\)IR and X\(-\)ray diffraction spectroscopy revealed that QCNF was included in the electrostatic complex. According to SEM microscopy,  
Figure 9: (a) Time\(-\)dependent droplet size change of Em(OC\(-\)Gel), (b) Em(QCNF), and (c) Em(QCNF/OC\(-\)Gel) at pH 3 (\(\bullet\)), 5 (\(\Circle\)), 7.4 (\(\blacktriangledown\)), and 9 (\(\triangle\)).  
the surface of the QCNF/OC\(-\)Gel complex reflected both the characteristic of the surface of OC\(-\)Gel aggregation (i.e., smooth) and that of the surface of QCNF (i.e., rough).  
OC\(-\)Gel, QCNF, and the QCNF/OC\(-\)Gel complex could all emulsify mineral oil in water successfully. However, they showed different stability in oil droplet size. The droplet size of Em(OC\(-\)Gel) increased more than two times in 20 days at all pH vales tested (pH 3, 5, 7.4, 9), probably because the electrostatic and steric stabilization potency of OC\(-\)Gel was relatively weak, whereas the droplet size of Em(QCNF) remained almost constant during the same period at all the pH values, suggesting that QCNF was more potent than OC\(-\)Gel. Since QCNF is strongly positively charged and is long (tens of micrometer in length), it would be able to stabilize the droplets electrostatically and sterically. On the other hand, the emulsion stabilized with the QCNF/OC\(-\)Gel complex was stable in the droplet size at all pH values except at pH 3.0, where the complex could be dissolved; thus, the droplets were subjected to coalescence. The complex seemed to play a role as a capsule wall, isolating droplets and preventing their coalescence. The pH\(-\)responsive emulsion developed in the present study would be able to be used as a drug carrier which can release its payload under an acidic condition (e.g., in the stomach).  
Methodology, S.C.P.; Investigation, H.K.S. and S.C.P.; Visualization, H.K.S. and S.C.P.; Supervision, J.-C.K. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2018R1A6A1A03025582). This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2022R1A2C2003353). This research was supported by \"Regional Innovation Strategy (RIS)\" through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (2022 RIS\(-\)005).  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflict of interest.