# Emulsions Stabilized with an Electrostatic Complex of
## 1 Introduction  
Emulsions can be prepared with emulsifiers that reduce the interfacial tension and stabilize droplets of discontinuous phase [1; 2; 3; 4]. Emulsions are required to be stable in terms of their droplet size during storage for their further use. Emulsions are destabilized following the order of droplet agglomeration, droplet coalescence, and phase separation [5; 6; 7; 8]. The agglomeration is prevented electrostatically by choosing a charged emulsifier and sterically by selecting a flexible polymeric emulsifier, in order to achieve stable emulsions [9; 10; 11; 12]. Even if the droplets are agglomerated, they may be stable and seldom subjected to coalescence if they are completely coated with insoluble matters (e.g., microcapsule wall) because the insoluble matters hardly allow for direct droplet\(-\)to\(-\)droplet contact [13; 14; 15; 16]. When stimuli such as pH value change, temperature change, and lightirradiation are applied to the emulsions, the physicochemical properties of the emulsifiers (e.g., the conformation and the solubility) can be changed, causing the droplets to be destabilized [17; 18; 19; 20]. Poly(N\(-\)isopropylacrylamide\(-\)_co\(-\)_methacrylic acid\(-\)_co\(-\)_octadecyl acrylate)(P(NIPAM\(-\)MAA\(-\)ODA)) was prepared for its use as an emulsifier for the preparation of temperature\(-\) and pH\(-\)responsive emulsions. The copolymer chain could be contracted in response to heat and acidification and cause the droplets to be coalescenced and destabilized [21]. If the chain contraction takes place, the surface \(-\)activity of the copolymer and the interfacial area where a copolymer chain can stabilize may decrease. In another related study, poly(2\(-\)hydroxyethyl acrylate\(-\)_co\(-\)_counamyl acrylate\(-\)_co\(-\)_2\(-\)ethylhexyl acrylate)(P(HEA\(-\)CA\(-\)EHA)) was used to stabilize oil droplets in water to prepare a temperature\(-\) and UV\(-\)responsive emulsion. The copolymer chain could also shrink in response to heat and the light irradiation, leading to the destabilization of the droplets [22]. Recently, a proteinoid comprising Asp, Leu, and cystamine was prepared as a disulfide emulsifier for the preparation of a reduction\(-\)responsive emulsion [23]. When the emulsion was put under a reductive condition, the droplet was destabilized because the disulfide bond of the proteinoid was broken down and lost its emulsifying ability. More recently, poly(2\(-\)hydroxyethyl acrylate\(-\)_co\(-\)_propyl methacrylate) (P(HEA/PMA)) was prepared as a thermo\(-\)sensitive emulsifier for the preparation of Near Infrared (NIR)\(-\)responsive emulsion [24]. Upon the light irradiation, gold nanoparticles contained in the emulsion generated heat due to the surface plasmon resonance, the heat rendered the thermos\(-\)sensitive polymers shrunk, and the polymer lost its droplet\(-\)stabilizing ability. In this study, a complex of quarternized cellulose nanofiber (QCNF) and octanoyl gelatin (OC\(-\)Gel) was used as an emulsifier for the preparation of oil/water (O/W) emulsions, and the effect of the pH value on the stability of the oil droplets was investigated. OC\(-\)Gel is negatively charged, and it can form an electrostatic complex with the positively charged QCNF when the pH value is greater than the isoelectric point. The insoluble complex can be formed on the surface of oil droplets because OC\(-\)Gel is interface\(-\)active, and it would isolate a droplet from others and prevent coalescence. When the pH value decreases below the isoelectric point (e.g., an acidic condition), OC\(-\)Gel changes to be positively charged, the complex becomes solubilized, and the droplets are subjected to coalescence (Figure 1). The pH\(-\)responsive emulsion developed in the present study would be applicable to a drug carrier that can release its cargo under an acidic condition (e.g., stomach). Figure 1: Schematic representation of O/W emulsion prepared using the complex of QCNF and OC\(-\)Gel as emulsifier.