Читать книгу Oil-in-Water Nanosized Emulsions for Drug Delivery and Targeting - Tamilvanan Shunmugaperumal - Страница 32

Sometimes mixtures of natural zwitterionic surfactants used for emulsion stabilization contain small amounts of polar compounds that can be incorporated into the adsorption layer and lead to a modest droplet charge, which additionally stabilizes the emulsion (Tamilvanan 2008). Surface layer with charged natural admixtures reported by Trotta et al. (2002) is only a particular case of a very large class of emulsion‐stabilizing systems based on a tailored application of ionic–zwitterionic surfactant mixtures. Mixtures of dipalmitoylphosphatidylcholine (DPPC) and homologues and dimyristoylphosphatidylethanolamine (DMPE) phospholipids were utilized by Ishii and Nii (2005) for stabilizing model API‐carrying o/w nanosized emulsions. In contrast to the data, the main stability factor was found to be the optimal average hydrophilic–lipophilic balance (HLB) value of the stabilizers’ mixture, defined similarly for nonionic surfactants (Trotta et al. 2002). For example, emulsions prepared with mixtures of dimyristoylphosphatidylcholine (DMPC, zwitterionic) and DMPE behaved similarly to emulsions prepared by DMPC alone. This fact was explained by the equivalence of HLB values for both surfactants used, regardless of their ionic nature. However, the ionic character of a surfactant like DMPE (and therefore the charge of respective emulsion droplets) can be affected by the pH of the dispersion medium.

The o/w nanosized emulsions stabilized by mixed ionic/nonionic surfactants revealed very high physical stability and were found to be most appropriate for dermatological applications as a ceramide‐carrying colloidal system (Yilmaz and Borchert 2005). Greater emulsion stability was achieved by the combination of the nonionic steric stabilizer Tween 80 and the phospholipid co‐stabilizers phytosphingosine and phosphatidylethanolamine.