Mixed emulsifier systems

The results are in accordance with previous results with mixed emulsifier systems of Na hexadecyl sulphate and hexadecanol with K2S20g as initiator Z). Also in the present case with oil soluble initiator and with OPB it is therefore a necessary condition for obtaining a high degree of monomer droplet initiation that not only a fine dispersion of monomer is achieved but that at the same time the concentration of emulsifier left in the aqueous phase is very low. 

Materials and Polymerization. Styrene and methyl methacrylate were obtained from commercial sources and were distilled to remove inhibitor. After distillation, the monomers were stored, under nitrogen, in a refrigerator. For the mixed emulsifier system, Emulphogene BC840(GAF), tridecyloxy-polyethylene-oxyethanol, was used as the nonionic constituent, and sodium lauryl sulfate (K and K Labs) was used as the ionic constituent. The sodium lauryl sulfate was at a concentration below its cms whereas the BD-840 was at a concentration above its cmc. This emulsifier system has been shown to yield mixed micelles (2)/ having a low ionic change (2)/ which produce latlces with rather narrow particle size distributions (2 ) ... 

Influence of the Molar Ratio Lipopeptide/Cetyl Alcohol. As already shown by different authors in the case of classical emulsifiers such as sodium lauryl sulfate (10,14,15), the mixed emulsifier system lipopeptide/cetyl alcohol gives stable miniemulsions for molar ratios LP/C15OH between 2/1 and 1/3. 

As the mixed micelles are very small (10-20 nm) the initial swelling must be very limited. To be able to absorb more of Z, the initial small droplets must be furnished with more emulsifier, and even more importantly, with more fatty alcohol (Z,). This may be achieved by coalescence of initial droplets or by absorbtion of mixed micelles from the surroundings. The assumption that the emulsification takes place by a diffusion process seems to be supported by experiments with mixed systems of ionic emulsifier and fatty alcohol and various dispersed phases, showing that a necessary condition for a rapid emulsification is that the compound to be emulsified have slight water solubility. Furthermore, it has been observed that if even small amounts of Zj are added to Z, before addition to the water-mixed emulsifier system, the extent of emulsification is reduced and the resulting emulsion becomes less stable. 

Oils used in the preparation of pharmaceutical emulsions are of various chemical types, including simple esters, fixed and volatile oils, hydrocarbons, and turpe-noid derivatives. The oil itself may be the medicament, it may function as a carrier for a drug, or even form part of a mixed emulsifier system as in the case of some fixed oils that contain sufficient free fatty acids. 

In the aqueous mixed emulsifier system, the hexadecyltrimethylammonium bromide exists in one of three forms (i) solute... 

Figure 3. Transmission electron micrograph of diluted hexadecyltrimethylammo-nium bromide-cetyl alcohol (1 1 molar ratio) mixed emulsifier system showing rodlike particles. (Reproduced, with permission, from Ref. 2. Copyright 1980, Plenum...

The parameter P(2), the solubilization constant, has a finite value when micelles are present in the mixed emulsifier system. The smaller the value of P(2), the smaller the concentration of micellar hexadecyltrimethylammonium bromide and the better the balance between the hexadecyltrimethylammonium bromide and cetyl alcohol in the crystalline rodlike particles. 

Distribution of Hexadecyltrimethylammonium Bromide in the Mixed Emulsifier System ... 

The formation and stabilization of 0/W emulsions prepared with mixed emulsifier systems has been extensively investigated. However, the mechanisms proposed differ greatly. One of the primary hypotheses attributes the enhanced stability to the formation of a molecular "complex" or layer at the oil/water interface (8-11). The mixture of emulsifier types increases the packing density of the adsorbed interfacial film. Several investigators have shown that more closely packed complexes produce more stable emulsions (9,12-14). Friberg, et al. (15-17) have attributed the enhanced stability of mixed emulsifier emulsions to the formation of liquid crystals at the oil/water interface, which reduce the van der Waals attractive forces. 

The chemical structure of the oil phase itself has a pronounced effect on the formation of an interfacial layer. Spinning drop experiments with oil phases of different chemical structure and the same water solubilities illustrated this effect. Comparison of Figures 1 and 3 shows that interfacial layers having significantly different characteristics form between the same 1 1 SLS/CA solution and different oil phases (styrene and EHA, respectively) with approximately the same water solubilities. This may be the result of different types of specific interactions between the various components of the mixed emulsifier system and either one of the two types of oils. NMR studies will be conducted in order to investigate this point. 

Earlier conductivity measurements have indicated that the most stable miniemulsions are produced with mixed emulsifier molar ratios between 1 1 and 1 3 (22,23). This correlation agrees with a theoretical analysis of mixed emulsifier adsorption onto oil droplets by Lucassen-Reynders (35), who have determined the optimum stability to occur at molar ratios near 1 1. However, the maximum interfacial tensions at these molar ratios were unexpected because, minimum interfacial tensions are usually associated with maximum emulsion stability. In fact, minima values substantially less than 1 dyne/cm have been reported for several oil/mixed emulsifier systems (31,33, 36,37). 

The interfacial tension values increase from A.l dynes/cm for SLS/ decanol to 8.3 dynes/cm for SLS/octadecanol. Conductometric titration results have indicated that all of these mixed emulsifier systems, except the one with decanol, should give a relatively stable emulsion (22,23). Interestingly, the SLS/decanol mixed emulsifier solution was the only case in which the presence of the fatty alcohol reduced the interfacial tension with styrene to below the value measured for SLS alone. Studies are in progress to investigate this phenomenon and to determine the effect of alcohol chain length on miniemulsion stability. 

Apan from the mechanism of droplet formation, the further kinetic treatment of the polymerization of miniemulsions prepared with mixed emulsifier systems is similar to that described in the following section fw miniemulsions formed by diffusion. 

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