Water-in-oil nanoemulsion

Wang L., Li, X., Zhang, G., Dong, J. and Eastoe, J. (2007) Oil-in-water nanoemulsions for pesticide formulations. Journal of Colloid and Interface Science, 314, 230-235. 

Yang, H.J., Cho, W.G. and Park, S.N. (2009) Stability of oil-in-water nanoemulsions prepared using the phase inversion composition method. Journal of Industrial and Engineering Chemistry,... 

TEMPO has been structurally modified to bring about new selectivities. Highly efficient anionic water-soluble TEME<), oil-in-water nanoemulsion containing TEME for oxidation of alcohols and a waste-free system were developed. Especially, the sterically less crowded azabicyclo-Af-oxyls oxidized /-menthol to Z-menthone with much higher efficiencies than TEME O (equation 23). ... 

An oil-in-water nanoemulsion made with flaxseed oil and saquinavir resulted in a significant increase in bioavailability of saquinavir in mice after oral or intravenous administration (Vyas et al. 2008). 

Figure 15.13 Silicone oil in water nanoemulsions stabilized with HMI (top curve 1.6% w/w HMI bottom curve 2.4% wjw HMI).

Oil-in-water (OAV) nanoemulsion Oil droplets distributed in the aqueous phase. 

To achieve the above criteria complex multipheise systems are formulated (i) Oil-in-Water (0/W) emulsions (ii) Water-in-Oil (W/0) emulsions (iii) solid/liquid dispersions (suspensions) (iv) emulsions-suspension mixtures (suspoemulsions) (v) nanoemulsions (vi) nanosuspensions (vii) multiple emulsions. All these disperse systems require fundamental understanding of the interfacial phenomena involved, such as the adsorption and conformation of the various surfactants and polymers used for their preparation. This will determine the physical stability/instability of these systems, their application and shelf-life. 

Many lipophilic drugs are formulated as oil-in-water (0/W) nanoemulsions. The drug may be an oil with low viscosity which can be directly emulsified in water using a surfactant such as lethicin or castor oil ethoxylate. For viscous drug oils, the latter... 

An emulsion is a mixture of two immiscible liquids, one of which is uniformly dispersed within the other as small droplets (i.e., droplet diameter in the range O.l-lOOpm) (McClements, 2005). In this chapter, the concern is exclusively with dispersions of oil in water, that is, oil-in-water emulsions. Advances in homogenizer technology have allowed the development of nanoemulsions with droplets far smaller and with more uniform size distributions than commonly seen in manufactured foods (Weiss et al, 2008). There is not a commonly accepted size cut-off for nanoemulsions and different researchers have used different definitions below lOOOnm (Muller et al, 2000), 500mn (Anton et al, 2008), 200nm (Higami et al, 2003 Solans et al, 2005), and lOOmn (Luykx et al, 2008). Emulsions with nano-scale crystalline droplets are sometimes referred to as solid lipid nanoparticles (SLN). 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

These are transparent or translucent systems covering the size range from 5 to 50nm. Unlike emulsions and nanoemulsions (which are only kinetically stable), microemulsions are thermodynamically stable as the free energy of their formation is either zero or negative. Microemulsions are better considered as swollen micelles normal micelles can be swollen by some oil in the core of the micelle to form O/W microemulsions. Reverse micelles can be swollen by water in the core to form W/O microemulsions. 

Water-in-oil (W/O) nanoemulsion Water droplets distributed in the oil phase. 

Methods of nanoemulsion preparation have been described in detail. A schematic illustration of the overall process is depicted in Figure 12.2. Three different approaches can be used to incorporate the drug and/or the various components in the aqueous or oil phase. The most common approach is to dissolve the water-soluble ingredients in the aqueous phase and the oil-soluble ingredients in the oil phase. The second approach, which is used in fat emulsion preparations involves the dissolution of an aqueous-insoluble emulsifier in alcohol and then the dispersion of the alcohol solution in water followed by evaporation and total removal of the alcohol until a fine dispersion of the emulsifier in the aqueous phase is reached. The third approach, which is mainly used for hydrophobic drug... 

Alginate-coated chitosan core nanoparticles loaded with rabeprazole, an antiulcer agent which is chemically instable in the stomach, were developed using water-in-oil (W/O) nanoemulsion technique. The drug permeation from the prepared NP was significantly higher and controlled RP release compared to the pure drug [106],... 

The change in interfacial free energy AG, is always positive in the formation of a microemulsion or a nanoemulsion as both y and AA are positive positive AA is due to an increase in the interfacial area between oil and water when droplets are formed. The entropy contribution -TAS is always negative as both T and AS are positive positive AS is due to an increase in the disorder of the... 

The rate of Ostwald ripening is 1.1 x 10 and 2.4 x 10 m /s at 1.6 and 2.4% HMI, respectively. These rates are 3 orders of magnitude lower than those obtained using a nonionic surfactant. Addition of 5% glycerol was found to decrease the rate of Ostwald ripening in some nanoemulsions, which may be due to the lower oil solubility in the water-glycerol mixture. 

---