It was recently found by Carner4) (with nitrobenzene-water system) and Terjesen6-8) (withCCl4-water system) that an addition of a very small amount of surface active agents decreased the extraction of acetic acid from droplets of organic solvents.
The authors carried out the experiment with 8 organic solvents to determine what surface active agents worked as poisonous substances for them. The results revealed that the range of poisonous substances for these organic solvents covered not only the surface active agents as shown by Carner and Terjesen but also other high molecular substances (cf. Tables 1 and 2). Moreover these poisonus substances were found to possess elective affinity, which was particularly obvious in polyethylenglycol (PEG mol wt.≅4000). Generally speaking, benzene, toluene, hexane, heptane and cyclohexane were easily contaminated with the poisonous substances, while butylacetate, isopropylether and methyl-isobutylketone were relatively not so.
Photographs were taken by schlieren method, with the apparatus shown in Fig. 2, to make clear the mechanism of solute transfer. Fig. 7-I-a, II-a, III-a and IV-a show acetic acid transfer to pure water and Fig. 7-I-c, II-b, III-b and IV-b, to additives-containing water, In the latter group, but not in the former one, vortex ring motions were clearly observed behind the drops. Generally such a vortex ring was observed when the rigid sphere moved in a fluid at Re>25, so it was naturally supposed that the contaminated droplets were of stagnant sphere. Values of C2/C1 observed of the most contaminated drops of several solvents were compared with those calculated of rigid sphere by means of Ep. (3) as shown in Fig. 8 and Table 3. They showed good agreement in butylic acid (where m is small) transfer as well as acetic acid transfer, revealing that k_c is very large in comparison with kd in the period of free rising.
It was also found that the mechanism of transfer was effected even by the most purified solute, as well Fig. 7-V is a schlieren photograph of heat transfer from the pure benzene drop (none solute) to pure water, which is different from solute transfer shown in Fig. 7-I-a. Consequently we may safely conclude that the mass transfer from droplets in liquid-1iquid system where solute is contained is different, in mechanism, from the heat transfer from droplets in liquid-liquid system where no solute is contained, and that the method of Colburn and Welsh, in which, in order to obtain the data on individual coefficients, two pure liquids of limited solubility are contacted in the abscnce of a third solute, cannot be applied to the study of mass transfer from droplets.
It is proposed that, in practice, when the contamination is unavoidable, PEG aqueous solution (0.2 -1%) should be added to water phase for the purpose of increasing the extraction rate, whichever side of the phases might be contaminated with poisonous substances.