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.
2.2.6. “Stealth” Property of Nanosized Emulsions: In Vitro Demonstrations
Anionic emulsion formulations capture apolipoproteins (apo) along with other plasma proteins within minutes after an infusion in human blood, facilitating their fast elimination. In contrast, cationic emulsions reveal a much longer retention time in the plasma. Moreover, cationic colloidal carriers can promote the penetration of therapeutic agents into cell surfaces possibly via an endocytotic mechanism (Calvo et al. 1997). To improve the API targeting efficacy of colloidal carriers of anionic emulsions and to further prolong the circulating effect of the cationic emulsions, a mixed stabilizer film at the oil–water droplet interface composed of nonionic Poloxamer 188 and ionic lipoid E80 and stearylamine/oleylamine was created by combining the effects of electrostatic and steric barriers at the oil–water interface (Tamilvanan et al. 2005). In order to prove this concept, surface (charge)‐modified o/w nanosized emulsions (cationic and anionic) were prepared following the well‐established combined emulsification techniques (de novo) and these two emulsions were characterized for their droplet size distribution and surface charge. Marketed lipofundin MCT 10%, deoxycholic acid‐based anionic emulsions, oleylamine‐/stearylamine‐based cationic emulsions, and oleic acid‐based anionic emulsions were selected in this study. The effect of these emulsions on in vitro adsorption of plasma proteins was investigated by means of two‐dimensional polyacrylamide gel electrophoresis (2D PAGE).
Competition between steric repulsion by poloxamer and electrostatic attraction by ionic components led to the sensitive adsorption of small molecular weight proteins like apo, albumins by the surface of colloidal carriers whereas all emulsion droplets were effectively shielded from the adsorption of larger proteins like immunoglobulins, fibrinogen, etc., enhancing the shelf life of emulsion formulations in the blood (Fig. 2.2). The apoA‐I along with apoA‐IV have been suggested to modulate the distribution of apoE between the different lipoprotein particles in the blood and thereby affect their clearance (Tamilvanan et al. 2005). In addition, the attachment of apoE would greatly alter the in vivo distribution of fat emulsions since this protein is a ligand for the apoE‐specific receptors on the liver parenchymal cells. The higher the preferential adsorption of apoA‐I onto the cationic emulsion droplets, the more intensified the displacement/redistribution of apoE would, therefore, be expected to occur on these types of cationic emulsion formulations in the blood (Tamilvanan et al. 2005). Indeed, the ratio of apoA‐I to apoA‐IV was very close to 1 for Lipofundin® MCT 10% whereas it was about 0.26 for deoxycholic acid‐based anionic emulsion and above 5 for oleic acid‐based anionic emulsion and both cationic emulsions. This indicates that emulsions having similar surface/interfacial charge imparted by different anion‐forming stabilizers (oleic or deoxycholic acids) exhibited markedly different protein adsorption patterns.
Figure 2.2. Amount of major proteins on the 2‐D gels of plasma proteins adsorbed on emulsions with negative or positive surface charge in comparison with Lipofundin MCT 10%.
[Taken with permission from Elsevier (Tamilvanan et al. 2005).]
2.2.7. Advantages of Stabilizers in Nanosized Emulsions
In some practical cases where emulsions are applied under real, sometimes quite harsh conditions, such as high temperature or shear stresses, the stabilization of colloidal carriers by conventional surfactants (ionic as well as nonionic) can appear to be insufficient to keep the initially acceptable emulsion properties intact. For ionic surfactants as stabilizers, the stabilization mechanism based on the electrical double layer fails at high amounts of electrolyte. The situation for nonionic surfactants as stabilizers is only slightly better because their molecules are typically not strongly adsorbed (Tadros et al. 2004; Tadros 2006). Therefore, the most effective way to stabilize emulsions by creation of a protective adsorption layer is the use of amphiphilic macromolecular compounds or polymeric surfactants (Tadros et al. 2004). In contrast to the commonly used monomolecular surfactants, polymeric surfactants do not only adsorb much stronger but also retain this ability under high electrolyte concentration or/and high temperature (Tadros et al. 2004; Tadros 2006). This is made possible due to a special molecular design of surface active polymers that have in their structure anchor groups responsible for strong adsorption at the interface and stabilizing groups protruding from the interface into the dispersion medium and forming a bulky layer with thicknesses of several nanometers.
Most often used stabilizers for the preparation of emulsions, in the fields of agrochemicals, pharmaceuticals, and personal care products, are either block or graft copolymers. In block copolymers, the hydrophobic blocks reside at the surface or even partly penetrate in the oil droplet, making trains or short loops whereas the hydrophilic blocks protrude in the dispersion medium as loops or tails providing steric stabilization (Benichou et al. 2004). As examples, PEO‐PPO‐PEO triblock copolymer (commercially available as “Pluronics”) or PPO‐PEO‐PPO can be mentioned. Triblock copolymers are, however, not the most efficient stabilizers because the PPO chain is not hydrophobic enough to attach strongly at the o/w interface (Benichou et al. 2004). The surface activity of these polymeric surfactants is rather the result of a rejective anchoring or negative enthalpic energy change of the PPO group because of its low solubility in water and most oils. Alternative and more efficient graft copolymers consist of a polymeric backbone attached to the interface and several chains dangling into the continuous phase and forming at the interface a “brush” structure.
A typical example of commercial graft was described (Jumaa and Müller 2002). Here, mixtures of polyoxyethylene‐660‐12‐hydroxystearate (Solutol HS15) with the anionic lipid composition. Lipoid S75 was employed to enhance the long term as well as accelerated (by freezing and centrifugation) stability of o/w nanosized emulsions. Emulsion stabilized by phospholipids displayed a stable behavior after autoclaving and centrifugation but de‐emulsified after freezing. In contrast, emulsions prepared only with Solutol HS15 demonstrated a significant change in particle size after autoclaving. The best results were obtained using a stabilizer mixture revealing a combination of electrostatic stabilization mechanism typical for the anionic phospholipids and the steric stabilization mechanism originating from nonionic