The Difference Between Miscible and Immiscible Liquids

In any process where we have solvents that are part of the synthetic root to manufacture our main product, we use different kinds of liquids that can be miscible and immiscible.

The strict definition is that a miscible liquid can be absorbed by another liquid; by the end of this process with two materials that are in contact, we will see only one phase again that contains two liquids that are homogeneously distributed in the tank.

So, for instance, if we add some polar solvent in water—for instance, methanol or ethanol—we will see one phase. But if we add some oil to water—for instance, toluene, benzene, or dichloromethane—we will see two different phases. The upper layer will be the lighter one, for instance, toluene and water; the toluene will be in the top, and the heavy layer in this case will be the water. In contrast, when we have dichloromethane, the water will be the upper layer because it is lighter than dichloromethane by the density of the solvent.

Foundational to this concept is clearance, but the real difference when we are using these materials in our synthetic route is critical. When the materials are miscible, we have some mass transfer and diffusion phenomena happening in our media that normally are faster than if we have two liquids that are not miscible. The connection between the materials and the capability to be dissolved or to generate one phase is simple. We have no barrier that prevents the materials from interacting. So, the interaction between those materials will be a function of the circulation flow rate, of the capability to generate by mixing small fluid elements. Inside, we will have diffusivity, and outside we will have distribution of the material into the tank. But there is no barrier between the two phases.

As a consequence, what normally happens in this case is that the competence between the kinetic phenomena that can be a reaction, crystallization, etc., is in competence with the hydrodynamic we’re generating—our capability to distribute the material into the tank. When we have materials that are immiscible—when we have two liquids that are not visible—we have some barrier preventing these two materials from interacting with each other. So, what happens when we start to move these two liquids that are separate is that we generate a continuous phase, and from this first phase, we regenerate drops into a continuous phase. And these drops contain mainly one phase only. It can be the lighter phase or the heavy phase depending on the volume fraction of the material and the interaction between the materials.

The second point is that the capability of the materials to interact with each other is only a function of the area transfer we’re generating, because we’re generating some barrier between these two phases. This is, for instance, the border of the bubble or the bubble of the drop, and in order to transfer one material that is dissolved in the dispersed phase to the continuous phase or opposite, we need to generate enough area—that is, the capability to have contact between these two phases. So, we have non-molecular contact. We have only phase contact, and it reduces dramatically the capability of the materials to connect. As a consequence, we have more mixing intensity, mainly in shear rate than circulation, in order to increase the area transfer between the phases. Consequently, when we have in our root some kinetics of process-like reactions, or two-phase reactions, or mass transfer between two phases to clean one of the phases, it is a little harder to clean the phases because we have only the area transfer. This area transfer is completely a function of the shear rate that we are applying in the tank. So, we do not have the molecular contact that we have in the miscible systems. In many examples of this, you may hear that when we want to progress in processes where we have miscible liquids, some professionals think that we don’t need chemical mixing or a chemical mixing software. Of course, this is not correct. We continue to have some problems of diffusivity and personal materials. But when we come to the two phases, of course the mixing is completely relevant. Normally, with two materials that are not miscible, the mass transfer is the slower step in order to put the materials in contact with each other and progress with the kinetic that can be reaction, crystallization, emulsification, etc. So, this is the difference between miscible and immiscible liquids, which are very fundamental in understanding how to design and operate equipment in the chemical industry.