Oil-in-Water Nanosized Emulsions for Drug Delivery and Targeting. Tamilvanan Shunmugaperumal. Читать онлайн. Newlib. NEWLIB.NET

Автор: Tamilvanan Shunmugaperumal
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Химия
Год издания: 0
isbn: 9781119585251
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and consequently, due to a lower electrostatic repulsion between the colloidal droplets (Goldstein et al. 2007b). On the other hand, immunoemulsions stabilized by both anionic and cationic emulsifiers exhibited a multifold increase in cell binding in contrast to the emulsions without antibodies.

      Anionic phospholipids are also commonly utilized for the stabilization of API‐carrying nanosized emulsion droplets both individually and in binary mixtures (Trotta et al. 2002). Soybean lecithin and modified phospholipid, n‐hexanoyl lysolecithin (6‐PC), alone and as 1 : 1 mixtures were used as stabilizers of MCT droplets in water (Trotta et al. 2002). Although individual uncharged phospholipids provide emulsion droplets, a moderate negative charge for stabilization, mixed phospholipids produce much more stable emulsions and a large negative zeta‐potential value. A possible explanation for this phenomenon is related to the increased incorporation of polar compounds from the soya lecithin into the mixed interfacial film when 6‐PC is present. This interfacial film acts as a stabilizer by forming a high energy barrier that repels adjacent droplets and leads to the formation of stabilized emulsified droplets. The stability of the emulsion did not noticeably change, even in the presence of the model destabilizing API, indomethacin, demonstrating the high potential for such mixed emulsifiers for the formulation of colloidal API delivery systems (Trotta et al. 2002). Lysolecithin has one fatty acid ester chain removed from the glycerol backbone, in addition, lysolecithin is toxic (destroys RBC cell membranes). Furthermore, although the role of phospholipids is essential for the stability of the emulsions, possible cataractogenic effects due to the phosphatidyl choline (PC) and, basically, to a derivative of the same, lysophosphatidyl, have been described by different authors (Cotlier et al. 1975; Kador and Kinoshita 1978).

      A new class of surface‐active dialkyl maleates can be utilized for emulsion polymerization (Abele et al. 1997). Here, the emulsion droplets of monomeric maleates are self‐stabilized and simultaneously serve as liquid “reactive storage carriers.” Three types of head group in the dialkyl maleates were studied—nonionic, cationic, and zwitterionic with different lengths of hydrophobic alkyl chain. Cationic and zwitterionic dialkyl maleates with the longest alkyl chains ‐C16H33 and ‐C17H35 provided the best stability for o/w nanosized emulsions. When compared with the data obtained for the well‐known nonionic surfactant nonylphenol‐poly (ethylene oxide) (NPEO10) and the cationic cetyltrimethyl ammonium bromide (CTAB), an excellent stabilizing capacity especially for the cationic maleates can be stated. Whereas nonionic dialkyl maleates show almost the same emulsifying ability and stability as NPEO10, the cationic derivatives of these novel surfactants are more effective in stabilization than the traditional CTAB.

      In many cases, however, greater emulsion stability can be achieved without imparting a significant surface charge to the emulsion droplets, by means of steric stabilization (Capek 2004). Nonionic surfactants possessing bulky hydrophilic groups like PEO protruding into the dispersion media decrease coalescence arising from droplet collisions. Another contribution to the steric stabilization of emulsions by nonionic surfactants is provided by the close packing of PEO chains at the droplet surface. The compact packing of PEO chains at the droplet surface creates steric stabilization because little or no interpenetration of PEO chains on different droplet surfaces occurs due to entropic repulsion (Dale et al. 2006). Large head groups carrying simultaneously charges of opposite signs, such as in zwitterionic surfactants, can cause similar effects. In polar dispersion media of low‐to‐medium ionic strength, these groups are, as a rule, strongly solvated (hydrated in the most common case of H2O) (Yaseen et al. 2006). Voluminous and on an average almost non‐charged hydration shells, surrounding the emulsion droplet, possess a significant steric rigidity and can also effectively stabilize emulsions. There are, however, only a few examples in the literature that use zwitterionic surfactants as effective emulsion stabilizers. For example, lecithin was used for the stabilization of perfluorooctyl bromide (PFOB) in water emulsions, to be used as oxygen‐carrying system in a bio‐artificial liver device (Moolman et al. 2004). The Sauter mean diameter of 0.2 μm PFOB emulsion droplet in water was obtained by high‐pressure homogenization. The emulsion was stable for several months even at a volume fraction of 20%. Nonionic surfactants are more often used for emulsion stabilization than zwitterionic phospholipids because they are synthetically manufactured, can be well defined analytically, and have significantly less batch‐to‐batch variation than naturally occurring (egg yolk, soybean) lecithins.

      In general, fulfilling both stabilization mechanisms (smaller droplet size and lesser susceptibility of surfactant toward chemical degradation) simultaneously leads not only to the highest emulsion stability but also to lesser sensitivity to changes in the external conditions such as pH, ionic strength, and temperature. Therefore, the use of mixtures of different classes of surfactants for emulsion stabilization is frequently the most effective solution in many practical cases.