Adsorption, droplet size effects interfacial

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

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

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. 

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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