Stabilization thin-hquid films

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

First layers of a 10-pm thin monolithic sihca gel structure with 1- to 2- am macropores and 3- to 4-nm mesopores were manufactured by hydrolytic polycondensation of hquid alkoxysiloxane films coated on glass substrates. Separations were performed for steroids, pesticides, dyestuffs, amino acids, pharmaceutically active ingredients, phenols, and plasticizers [3,4]. Besides this commercially available monohthic UTLC product, which resulted from these studies and was used for several hyphenations to MS [5-10], the sol-gel synthesis process was further improved to provide a high homogeneity, reproducibihty, and mechanical stability of the layer material. Optimized synthesis is still used to build up monolithic stationary phases with new characteristics, and synthetic dyes and food dyes were separated on a 100-pm thin layer [11,12]. 

For practical use, it would be inconvenient to use liquid electrodes. It is useful to stabilize the non-aqueous phase. For the classical macroscopic design, different fixation methods are in use (Fig. 7.6). Very common are thin films of soft PVC. The softeners which are traditional additives in PVC technology are highly viscous hquids with a high boiling point. They have proved to be... 

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