# Emulsions Stabilized with an Electrostatic Complex of
## 3 Results and Discussion
### Measurement of QCNF/HpGel Complexation Degree  
Figure 4a shows the optical density at 600 nm of the QCNF/OC\(-\)Gel mixture suspension (pH 7.4) at different mass ratios. The suspension was somewhat turbid when the mass ratio was 1:0 (i.e., QCNF suspension). QCNF is a fiber of cellulose whose surface is quarternized; it is not soluble but disperses well in water due to its polar surface, and it can scatter visible light, giving rise to some turbidity. The optical density increased with increasing the ratio: it showed its maximum value (ca. 1.15) at the ratio of 1:8 and then decreased to 0.74 when the ratio increased to 1:20. Gelatin (Type B) has its isoelectric point around pH 4.8 [32; 33; 34]. The acid chloride of octanoyl chloride can react with the amino groups of gelatin to form amide bonds. Accordingly, the number of amino groups in  
Figure 3: Air/water interfacial tensions at different concentrations of QCNF (\(\bullet\)), Gel (\(\Circle\)), HC\(-\)Gel (\(\blacktriangledown\)), and OC\(-\)Gel (\(\triangle\)) solution. OC\(-\)Gel would be less than that in Gelatin; thus, the isoelectric point of the former one would be lower than that of the latter one. OC\(-\)Gel would be positively charged in the suspension whose pH value is 7.4 (much higher than the isoelectric point) and it would readily electrostatically interact with the positively charged QCNF to form complexes. This can explain why the optical density increased when the ratio increased to 1:8. The surface of QCNF will become saturated with OC\(-\)Gel as the ratio increases because the positive charge point and the surface area to accommodate OC\(-\)Gel is limited. Not to increase further in the optical density in the mass range of 1:8 to 1:20 could be ascribed to the surface saturation. The optical density rather decreased in the higher mass ratio range. OC\(-\)Gel is a kind of amphiphilic and surface\(-\)active molecule, and it would be able to disperse the QCNF/OC\(-\)Gel complex and stabilize it, accounting for a decrease in the optical density in the higher ratio range. Figure 4b shows the optical density at 600 nm of the QCNF/OC\(-\)Gel mixture suspension at different pH values with the QCNF/OC\(-\)Gel mass ratio kept at 1:8. The QCNF suspension (i.e., control suspension) showed a low optical density (ca. 0.135) at pH 3.0, and it exhibited no significant change in the optical density in the full range of pH values tested. QCNF has no titrable and ionizable groups, the surface charge would hardly be affected by pH value change, and it would hardly aggregate regardless of the pH value due to its strong positive charge caused by the tertiary amino groups. In fact, QCNF was well dispersed in buffer solutions, and its suspension was homogeneous and stable at all the pH values tested. The QCNF/OC\(-\)Gel mixture suspension showed no appreciable change in the optical density either when the pH value was changed from 3 to 4. OC\(-\)Gel would be positively charged in the range of pH 3-4 because its isoelectric point was close to 4.8; thus, there would be no electrostatic interaction between QCNF and OC\(-\)Gel. The QCNF/OC\(-\)Gel mixture suspension markedly increased in optical density when the pH value was changed from 4 to 5. OC\(-\)Gel became negatively charged during the pH change because its isoelectric point fell within pH 4-5. Accordingly, a strong electrostatic interaction between QCNF and OC\(-\)Gel would take a place in pH 4-5 to form complexes, accounting for the rapid increase in optical density [35; 36]. The optical density of the mixture suspension increased slowly when the pH value increased from 5 to 7, and thereafter it remained almost constant in range of pH 7 to 9. After the isoelectric point, the negative charge of a protein (e.g., OC\(-\)Gel) increases with increasing pH value in a saturated manner; thus, the amount of OC\(-\)Gel complexed with QCNF would increase in the same manner.