A relatively recent concept in organic reaction engineering is the use of submicron particles to enhance the rate of a reaction. These are usually microparticles of solids, but can also be microdroplets of liquids, or even microbubbles of gases. They can be external agents, participating reactants, or precipitating solids. In this chapter, we cover the role of small particles as a whole, which may be regarded as constituting an additional colloid-like phase normally referred to as the microphase. We begin by classifying the microphase in terms of its mode of action and then proceed to an analysis of the following categories of importance in organic technology: microslurry of (1) catalyst or adsorbing particles in a reactive mixture; (2) solid reactant particles in a continuous phase of the second reactant; and (3) solid particles precipitating from reaction between two dissolved reactants, one of which can be a solid dissolving and reacting simultaneously with the other reactant. The microphase in the first case is externally added, whereas that in the last two cases is a reactant or a product. The field is still developing (with many unproven theories), and hence we restrict the treatment to a simple analysis of selected situations based on reasonable assumptions (thus avoiding often unjustified complexity). A microphase can be described as an assemblage of very small dispersed phase particles with average size (dp) much less than the diffusional length scale of the solute. Usually dp < l0μm, compared to the diffusional length scale which is of the order of 50-60 μm. Although the microphase is a distinct phase, the phase in which it is present is commonly regarded as pseudohomogeneous. In a stricter sense, however, it should be regarded as a microheterogeneous phase. Indeed, several studies have been reported on modeling heterogeneous microphase systems (Holstvoogd et al., 1986, 1988; Yagi and Hikita, 1987). In view of the ability of the particles of such a system, pseudohomogeneous or pseudoheterogeneous, to get inside the fluid film, they can enhance the transport rate of the solute through the film. Experimental observations in typical gas-liquid and slurry systems have clearly demonstrated (see Ramachandran and Sharma, 1969; Uchida et al., 1975; Sada et al., 1977a,b, 1980; Alper et al., 1980; Pal et al., 1982; Bruining et al., 1986; Bhaskarwar et al., 1986; Bhagwat et al., 1987; Mehra et al., 1988; Mehra and Sharma, 1988a; Hagenson et al., 1994) the enhancing role of a microphase made up of fine particles. The case of a second liquid phase acting as a microphase or of a solid product performing a similar function has also been studied and found to enhance the reaction rate (Janakiraman and Sharma, 1985; Mehra and Sharma, 1985, 1988b; Anderson et al., 1998). Mehra et al. (1988) and Mehra (1990a,b, 1996) presented a detailed account of the role of different types of microphases in rate enhancement. In all these cases, either a microphase is separately introduced or one of the reactants or products acts as a microphase.
Pickering emulsions stabilized by colloidal surfactants: Role of solid particles