Reactants immiscible layers

Loss of agitation causing stratification of immiscible layers. Insufficient mixing of reactants results in unwanted accumulation of unreacted reactants. Possibility of runaway reaction upon resumption of agitation. 

The reactants occupy separate immiscible layers. Hexamethylene-diamine is dissolved in water (bottom layer), and adipoyl chloride is dissolved in hexane (top layer). 

As with polyesters, the amidation reaction of acid chlorides may be carried out in solution because of the enhanced reactivity of acid chlorides compared with carboxylic acids. A technique known as interfacial polymerization has been employed for the formation of polyamides and other step-growth polymers, including polyesters, polyurethanes, and polycarbonates. In this method the polymerization is carried out at the interface between two immiscible solutions, one of which contains one of the dissolved reactants, while the second monomer is dissolved in the other. Figure 5.7 shows a polyamide film forming at the interface between an aqueous solution of a diamine layered on a solution of a diacid chloride in an organic solvent. In this form interfacial polymerization is part of the standard repertoire of chemical demonstrations. It is sometimes called the nylon rope trick because of the filament of nylon produced by withdrawing the collapsed film. 

Interfdci l Composite Membra.nes, A method of making asymmetric membranes involving interfacial polymerization was developed in the 1960s. This technique was used to produce reverse osmosis membranes with dramatically improved salt rejections and water fluxes compared to those prepared by the Loeb-Sourirajan process (28). In the interfacial polymerization method, an aqueous solution of a reactive prepolymer, such as polyamine, is first deposited in the pores of a microporous support membrane, typically a polysulfone ultrafUtration membrane. The amine-loaded support is then immersed in a water-immiscible solvent solution containing a reactant, for example, a diacid chloride in hexane. The amine and acid chloride then react at the interface of the two solutions to form a densely cross-linked, extremely thin membrane layer. This preparation method is shown schematically in Figure 15. The first membrane made was based on polyethylenimine cross-linked with toluene-2,4-diisocyanate (28). The process was later refined at FilmTec Corporation (29,30) and at UOP (31) in the United States, and at Nitto (32) in Japan. 

Room temperature ionic liquids are air stable, non-flammable, nonexplosive, immiscible with many Diels-Alder components and adducts, do not evaporate easily and act as support for the catalyst. They are useful solvents, especially for moisture and oxygen-sensitive reactants and products. In addition they are easy to handle, can be used in a large thermal range (typically —40 °C to 200 °C) and can be recovered and reused. This last point is particularly important when ionic liquids are used for catalytic reactions. The reactions are carried out under biphasic conditions and the products can be isolated by decanting the organic layer. 

When the reactants involved in a step growth polymerization process are mutually immiscible, we can employ an interfacial polymerization method. Two solutions, each containing one of the monomers, are layered one on top of the other. This creates a phase boundary that forms wth the least dense liquid on top. The different monomers can then meet and polymerize at the interface. A commonly demonstrated example of this is the manufacture of nylon 610 by the interfacial reaction between an aqueous solution of hexamethylenediamine with sebacoyl chloride dissolved in carbon tetrachloride. Because the reaction only occurs at the interface, it is possible to pull the products from this interface to isolate the final product. 

For a triphasic reaction to work, reactants from a solid phase and two immiscible liquid phases must come together. The rates of reactions conducted under triphasic conditions are therefore very sensitive to mass transport effects. Fast mixing reduces the thickness of the thin, slow moving liquid layer at the surface of the solid (known as the quiet film or Nemst layer), so there is little difference in the concentration between the bulk liquid and the catalyst surface. When the intrinsic reaction rate is so high (or diffusion so slow) that the reaction is mass transport limited, the reaction will occur only at the catalyst surface, and the rate of diffusion into the polymeric matrix becomes irrelevant. Figure 5.17 shows schematic representations of the effect of mixing on the substrate concentration. 

Interfacial polymerization provides another method for depositing a thin layer upon a porous support [47,49]. In this case polymerization occurs between the two reactive monomers at the interface of the two-immiscible solvents. Heat treatment is often applied to complete the interfacial polymerization, to cross-link water-soluble monomer or prepolymer. The advantage of this technique is that the reaction is inhibited by the passage of a limited supply of reactants through the already formed polymer layer, resulting in extremely thin film of thickness in the range of 1-2 nm. Such membranes are therefore referred to as thin film composites. TFCs of various poly(amides) are very popular in RO apphcations. 

Only for reactions that are usually homogeneously catalyzed in the liquid phase, and carried out in the absence of a second or even third phase, i.e., a gas or an immiscible liquid, are the procedures known required for kinetic analysis (e.g. [17— 20]). In two-phase systems in which the catalytic reaction takes place in the liquid phase between a liquid reactant and gaseous reactants the quantitative analysis can be more complicated because the gaseous reactions have to be transferred over the gas-liquid boundary layer into the liquid phase. In this situation the reaction engineering prediction of the reactor performance can be performed easily as long as the rate of transfer of the gasous reactants into the liquid phase is fast compared with the intrinsic catalytic reaction according to Eq. (1) [21]. 

Interfacial polymerization has become a very important and useful technique for the synthesis of thin-film composite RO and NF membranes [5, 13]. Polymerization occurs at the interface between two immiscible solvents that contain the reactants (Fig. 3.6-8). For instance, a UF membrane is immersed in an aqueous diamine solution. The excess of water is removed, and the saturated support is put in contact with an organic phase that contains an acyl chloride. As a consequence, the two monomers react to form a thin layer (1 to 0.1 pm) of PA on top of the U F membrane. 

A particularly promising concept for designing catalysts, which combine the advantages of homogeneous and heterogeneous catalysis, involves coating a solid support with a thin layer of an IL, whereby the IL forms a second phase or is immiscible with the bulk fluids phase containing the reactants and products. Such a concept may be applicable to batch operation in the slurry phase as well as continuous operation in fixed-bed reactors (Scheme 10.2) [25, 35]. The chemical... 

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