# Emulsions Stabilized with an Electrostatic Complex of
## 3 Results and Discussion
### Scanning Electron Microscopy  
Figure 7 shows the SEM micrographs of OC\(-\)Gel, QCNF, and the QCNF/OC\(-\)Gel complex. HpGel formed a smooth surface. It can aggregate at the molecular level to form a compact and smooth surface. In fact, gelatin is known to have good film\(-\)forming properties [39; 40; 41]. On the contrary, QCNF formed a rough surface. It is a cellulose nanofiber (i.e., a bundle of cellulose) and can be entangled to form a broken surface. On the other hand, the QCNF/OC\(-\)Gel complex built a rough surface, having smooth leaves. HpGel complexed with QCNF and formed smooth leaf\(-\)like films around the nanofibers, forming an uneven surface. Figure 6: (a) X\(-\)ray diffraction spectra of QCNF, (b) OC\(-\)Gel, and (c) QCNF/OC\(-\)Gel complex. Figure 7: _Cont._Measurement of Size and Stability of Oil Droplets of Em(OC\(-\)Gel), Em(QCNF), and Em(QCNF/OC\(-\)Gel) by Microscopic Observation  
Figure 8a shows the optical micrographs of Em(OC\(-\)Gel), Em(QCNF), and Em(QCNF/OC\(-\)Gel). Oil droplets whose diameter are a few to tens of nanometers were found in all the emulsion samples, suggesting that OC\(-\)Gel and QCNF were able to stabilize oil droplets. OC\(-\)Gel was surface\(-\)active (Figure 3); thus, it would reduce O/W interfacial tension and stabilize the oil droplets effectively. QCNF is a cellulose nanofiber whose surface is quaternized. Since the quaternized domain would be more polar than the unquaternized one, it would be amphiphilic and thus be able to stabilize oil droplets. In addition, QCNF is a kind of particle, and it might reduce O/W interfacial tension and act as an emulsifier, just like Pickering emulsions can be prepared using nanoparticles as an emulsifier [42; 43; 44; 45]. Figure 8b shows the size distributions of the oil droplets of Em(OC\(-\)Gel), Em(QCNF), and Em(QCNF/OC\(-\)Gel). Em(OC\(-\)Gel) exhibited a unimodal distribution; most of the droplets were found in the range of 5 to 25 \(\upmu\)m, and the mean diameter was calculated to be 14.1 \(\pm\) 6.12 nm. Em(QCNF) showed a distribution somewhat deviating from a unimodal distribution; most droplets fell within the same size range as those of Em(OC\(-\)Gel), and the  
Figure 7: (a) SEM micrographs of OC\(-\)Gel, (b) QCNF, and (c) QCNF/OC\(-\)Gel complex. mean diameter was 11.2 \(\pm\) 6.39 nm. Em(QCNF/OC\(-\)Gel) also showed a non\(-\)unimodal distribution; most droplets were in the same size range as those of the other emulsions, and the mean diameter was 12.3 \(\pm\) 5.66 nm. Considering the mode of the size distribution, it could be said that Em(OC\(-\)Gel) was more homogeneous in size than Em(QCNF) and Em(QCNF/OC\(-\)Gel). Figure 9 shows the time\(-\)dependent droplet size change of Em(OC\(-\)Gel), Em(QCNF), and Em(QCNF/OC\(-\)Gel) at pH 3, 5, 7.4 and 9. The droplet size of Em(OC\(-\)Gel) began to increase 3 days after preparation, and the mean diameter changed from 14.1 nm to larger than 35 nm at all the pH values tested. The droplets can undergo agglomeration when the surface potential of the droplets (e.g., Zeta potential) and the steric hindrance arisen by the emulsifier are not high enough to prevent the droplets from coming in contact with one another. In fact, OC\(-\)Gel is a kind of protein, and it has a positive or negative charge value, depending on the pH value of the medium. In nature, however, its charge is less than 20 mV and insufficient to prevent the aggregation of oil droplets [46,47]. In addition, since apolar and polar amino acid residues are in gelatin in a random sequence, there are no hydrophilic segments which are long enough to push away approaching droplets. Thus, OC\(-\)Gel would hardly exhibit the steric hindrance to prevent agglomeration. Agglomerated oil droplets can be subjected to coalescence when the O/W interfacial energy increases due to the surface migration of the emulsifier.