Adsorption, droplet size effects

F. Droplet-size effects on the interfacial adsorption process 204... 

F. Droplet-Size Effects on the Interfacial Adsorption Process... 

It is generally accepted that one of the attractive features of the microemulsion environment for materials synthesis is the stabilization of the produced particles by the microemulsion surfactants. However, in the specific case of alkoxide/ microemulsion systems, there have been no investigations into the manner in which this stabilization is effected. For example, when the particle size exceeds the microemulsion droplet size, are the particles expelled from the water pools, or do the particles rather induce the enlargement of the microemulsion water droplets There have been no investigations into the role of surfactant adsorption in the colloidal... 

The final performance characteristic of any formulation, including SEDDS, is the effect produced physiologically in terms of the drug absorption. The extent of adsorption might be determined by the metabolism of the oil once ingested, the droplet size and charge and the resultant partition of the drug between the oil droplets and the excess of the aqueous phase. 

Naturally, the concentration of surfactant c becomes smaller during emulsification due to depletion the total interfacial area markedly increases due to the decrease in droplet size, the more so for a higher (p value. Thus more surfactant becomes adsorbed. This also happens during foam formation, but the increase in area is far smaller. A decreased c value gives a longer adsorption time. Consequently, the effective y value will increase, often strongly, during the emulsification process. 

Figure 16. Droplet size dependence of adsorption rate constant (a) and effective area factors (b) for ferrocene derivatives on the water-NB interface. (Reprinted with permission from Ref. 121 copyright 1998, American Chemical Society.)...

In a spraying process, a liquid is forced through an orifice (the spray nozzle) to form droplets by the application of hydrostatic pressure. The effect of surfactants and/or polymers on the droplet size spectrum of a spray can be described in terms of their effects on the surface tension. Since surfactants lower the surface tension of the liquid, one would expect that their presence in the spray solution would result in the formation of smaller droplets. However, when considering the role of surfactants in droplet formation, one should consider the dynamics of surfactant adsorption at the air/liquid interface. In a spraying process, a fresh liquid surface is continuously being formed. The surface tension of this... 

As mentioned above, the faster the rate of adsorption of surfactant molecules, the greater the effect of reducing the droplet size. However, with liquid jets there is an important factor that may enhance surfactant adsorption. Addition of surfactants reduce the surface velocity (which is generally lower than the mean velocity of flow of the jet) below that obtained with pure water. This results from surface... 

These data proof the need of using surfactants at C CMC. The excessive bulk concentration of surfactants is required not only for an effective decrease of y but also for the formation of a protective adsorption layer after the abrupt increase in interfacial area caused by the growing number of small size droplets. Thus, emulsions formed with typical surfactants behave towards sedimentation stability similar to those stabilized by natural surfactants. In this respect the advantage becomes better understandable of the so-called true , i.e. micelle-forming, surfactants as compared to the surfactants unable to form aggregate. True surfactants not... 

Of the possible emulsifiers, most are what are considered true surfactants, in that they are effective at lowering significantly the interfacial tension between the two hquid phases. Other additives such as polymers and sols function primarily as stabihzers, rather than emulsifiers. Most polymers are not sufficiently effective at lowering interfacial tensions to act in that regard. In addition, because of their molecular size, the adsorption process for polymers is generally very slow relative to the timescale of the emulsification process. The same applies to stabilizing colloids, in which their action requires the wetting of the particles by the two hquid phases to facihtate their location at the interface. The primary function of polymers and sols in emulsions is in the retardation of droplet flocculation and coalescence. 

The emulsifying effects of a small quantity of a block copolymer, A-B, added to immiscible blend of homopolymers A and B, were examined by Leibler (1988). The theory predicted the reduction of the interfacial tension coefficient, Vi2, caused by equilibrium adsorption of a copolymer at the interface. For well-chosen compositions and molecular weights of the copolymer, low values of Vj2 are to be expected. This suggests a possible existence of thermodynamically controlled stable droplet phase, in which the minor phase homopolymer drops are protected by an interfacial film of the copolymer, interfacing the matrix polymer. The size distribution of the droplets is expected to depend on the rigidity and spontaneous radius of curvature of the interfacial film that can be controlled by molecular structure of the copolymer. 

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