# Direction of mass transfer in two film theory

In an extraction process compound A is transferred from one solvent to another solvent. The two solvent may be, for example, water and dichloromethane. These two solvents are immiscible and when mixing them together, one of the solvents, say solvent 1, forms droplets that are dispersed in solvent 2, as depicted [...].

Fig. 4 shows the concentration profile across the interface between the two solvents. For this concentration profile, answer the following questions:

(a) In which direction is the mass transfer, from solvent 1 to solvent 2 or vice versa? Motivate your answer by a short explanation (2 pt).

How can you tell which direction the mass transfer will go?

I would assume that because solvent 1 forms droplets in solvent 2, then solvent 1 should be the organic phase. Would the mass transfer be from solvent 2 to solvent 1? Because the aqueous phase would contain compound A.

Edit

Just to see if I have understood it correctly, if I am given the concentration profile below. Then mass transfer would occur from the aqueous phase to the organic phase because the aqueous phase has a higher concentration.

Just keep in mind: the driving force of mass transfer is a concentration gradient. Nature tries to 'equilibriate' a difference in concentration between two phases.

As such, the mass transfer in that sense will be from the phase of high concentration (S2) to the phase of lower concentration (S1).

Two film theory is only a simplification of the mass transport directly at the interface, in which we assume there are two boundary layers on each side of the interface in which the concentration gradient is constant, giving us linear concentration profiles, and the concentration directly at the interface is determined by Henrys Law. This makes dealing with such problems much easier.

Mass and Heat Transfer are often taught in one course as they have many similiarities, but Heat Transfer offers a more intuitive introduction to the principles of transport phenomena in my view:

In heat transfer the driving force is the temperature gradient. We intuitively know that heat transfer will be from the hotter system to the colder system, and formally know that from the 2nd law of thermodynamics. Can you see the similarity to mass transport?

• So whenever I am given a concentration profile and asked to determine the direction the mass transfer, I should look at which side has the higher concentration at the interface and then I know that it will be directed from that layer to the one with the lower concentration? – Johan Mar 11 '19 at 18:07
• If you don't mind, could you have a look at the edit I made and tell me if I have understood the concept? Thank you! – Johan Mar 11 '19 at 18:10
• yea that should be fine, as long as you are not talking about osmosis. – LoschmidtsSchnitzel Mar 11 '19 at 18:14