Microreactors in Organic Chemistry and Catalysis, Second Edition (2013)

8.2. Background

In liquid–liquid systems, molecules at the region of contact of the two phases have a different molecular environment than those in the bulk of both phases. There are equal cohesive forces in all directions between molecules inside the phase, while those at the region of contact have unbalanced cohesive forces because they are not wholly surrounded by the same molecules. Consequently, they are strongly attracted toward the direction of the bulk phase. As a result, a boundary between the two phases is formed known as the interface or surface area making it more difficult for one phase mixing with the other. The force at the surface or interface is defined as the surface or interfacial tension. The term interface is used when both phases are liquids, whereas surface is usually used for gas–liquid or solid–liquid systems. An example of boundary formation in nature is the liquid–air surface in which the unbalanced attractive forces result in liquid surface contraction by pulling molecules at the surface to the bulk of the liquid. This contraction is the reason for the formation of spherical-shaped liquid droplets in nature. The stronger the cohesive forces in a phase, the higher is the interfacial tension. Any decrease in the strength of the interaction will lead to a weaker tension, which consequently increases the miscibility of the system. There are many ways of increasing the miscibility, for example, addition of surfactants, mechanical stirring, or applying high temperatures. In liquid–liquid systems, the surface in each phase is subject to forces from the other which makes the surfaces tension weaker than the gas–liquid surface. The formation of a boundary in a liquid–liquid system does not only depend on the differences in the molecular environment between the phase and the interface area, but also on the degree of phase saturation in certain solvent systems. This is especially the case for partially miscible liquid–liquid systems. Such systems consist of a phase α and phase β. The addition of a small amount of phase α to a large amount of phase β leads to the formation of a miscible system. When phase α is continuously added to the system, a single phase is retained until phase β becomes fully saturated with phase α, and then two separated phases are formed. If addition of phase α continues until phase β becomes the minor and completely soluble phase, a miscible system will form again [1, 9, 10].