Monomeric Surfactant Stabilization

Further evidence for the theory of stabilisation due to enhanced intermolecu-lar surfactant interactions was presented recently [112]. Two different surfactants were employed to stabilise w/o HIPEs sorbitan monooleate, a monomeric surfactant, and a polymeric surfactant. Salt addition enhanced the stability of the HIPEs, but more so in the presence of the polymeric surfactant. Again, the interactions between the salt and the surfactant were held responsible. The polymer contained ionic groups, which enabled it to interact strongly with the salt sorbitan monooleate, however, possesses groups which can only participate in hydrogen-bonding. 

Therefore, micelle-forming surfactant molecules (e.g., SDS) will be present in three different forms, namely, on the lipid surface, as micelles, and as monomeric surfactant molecules in solution. Lecithin will form liposomes, which have also been detected in nanoemulsions for parenteral nutrition [77], Mixed micelles have to be considered in glycocholate/lecithin-stabilized and -related systems. Micelles, mixed micelles, and liposomes are known to solubilize drugs, and are therefore attractive alternative drug-incorporation sites (especially with respect to the low incorporation capacity of lipid crystals). 

The last major class of emulsifiers and stabilizers is that of the monomeric surfactants which adsorb at interfaces, lower the interfacial tension, and, hopefully, impose a stabilizing barrier between emulsion drops. Surfactants are the most widely studied and perhaps best understood class of emulsifiers and stabilizers. Perhaps because they are more amenable to both experimental and theoretical analysis, they have been used to probe the finer points of emulsified systems. They will therefore be discussed in more detail than polymers and sols. 

Monomeric Surfactants. These are low molecular weight amphiphilic molecules (ionic or nonionic) that can diffuse to the interface quickly to provide stability during emulsion formation. They are mobile and move in and out of the interface in a dynamic way. 

During years of investigation to improve stability and to control, sustain, and/ or prolong the release of active materials, monomeric surfactants have been progressively replaced by polymeric emulsifiers as outer or inner interface stabilizers. 

Omotosho et al. (1986) were the first to study the influence of BSA with a nonionic surfactant in the inner aqueous phase to stabilize W/O/W emulsions. They concluded that interfacial complexation between BSA and the non-ionic surfactant occurs at the inner W/O interface. This complex membrane has been found to enhance the stability of multiple emulsions and to slow down release of solute entrapped within the emulsion droplets. BSA has been investigated as a replacement for some of the monomeric surfactants in the inner phase and found to provide good stabilization for W/O/W multiple emulsions (Fredrokumbaradzi and Simov, 1992 Evison et al., 1995). 

Multiple emulsions are much more stable, with smaller globule size and smaller inner droplet size, more monodispersed and more viscous compare to multiple emulsions stabilized with monomeric surfactant. Today s multiple emulsions can retain the solute for longer periods of time on the shelf. 

The last major class of emulsifiers and stabilizers is that of the monomeric surfactants that adsorb at interfaces and produce electrical, mechanical, and steric barriers to drop coalescence, in addition to their role in lowering the interfacial free energy between the dispersed and continuous phases. Since these materials are the central concern of this work, they are addressed in detail below. 

In abroad sense, the model developed for the cobaloxime(II)-catalyzed reactions seems to be valid also for the autoxidation of the alkyl mercaptan to disulfides in the presence of cobalt(II) phthalocyanine tetra-sodium sulfonate in reverse micelles (142). It was assumed that the rate-determining electron transfer within the catalyst-substrate-dioxygen complex leads to the formation of the final products via the RS and O - radicals. The yield of the disulfide product was higher in water-oil microemulsions prepared from a cationic surfactant than in the presence of an anionic surfactant. This difference is probably due to the stabilization of the monomeric form of the catalyst in the former environment. 

SANS studies and fluorescence correlation spectroscopy have been successfully combined to study the size of water-in-oil (heptane) microemulsions stabilized by sodium Z A-2-ethylhexyl sulpho succinate (AOT) with and without a fluorescently labeled peptide (phalloidin, a fungal toxin of mass 789 Da) and protein (ot-chymotrypsin, a serine protease of mass 25 kDa). In incorporation of the small peptide, phalloidin did not increase the size of the microemulsion droplets whereas the presence of ot-chymotrypsin significantly increased the size of the microemulsion droplets. Furthermore, the studies suggested that while all the phalloidin was in the disperse water phase, the a-chymotrypsin appears to be dispersed in the oil phase in monomeric form and protected from contact with the oil by a shell of surfactant. 

Both explanations are reasonable. Critical micelle concentrations are decreased by addition of both electrolytes and hydrophobic non-ionic solutes to water [15]. But submicellar aggregates could coexist in solution with monomeric and micellized surfactant, although their concentration is probably low [2,23]. They could interact with, and be stabilized by, hydrophobic substrates. 

Polymerized surfactant assemblies exhibit enhanced stabilities, and decreased permeabilities as compared with the corresponding monomeric ones. Potential applications as drug carriers and devices for solar energy conversions have been discussed Moreover, previous studies indicated their suitability as synthetic models for bio-membranes, able to mimick simple cell membrane functions and cell-cell interactions. 

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