The influence of the interfacial resistance on the liquid—liquid mass transfer of carboxylic acids

The instantaneous solute concentration in freely suspended single droplets were measured as function of the exchange time by means of a radionuclide technique with and without chemical reaction in liquid—liquid systems in several drop sizes covering the range of Reynolds numbers 114–443.The transfer rates of benzoic acid and/or caprylic acid from toluene drop into water as well as of caprylic acid into aq. NaOH solution were investigated. Benzoic and caprylic acids prevail in the toluene phase as dimers and in the water phase as monomers. In case of the mass transfer of the benzoic acid the chemical reaction of the dimer to the monomer causes an interfacial resistance which has the same order of magnitude as the mass transfer resistance in the continuous phase.In case of the mass transfer of the caprylic acid the solute forms a monolayer at the interface. The rate determining step is the desorption of the solute from this monolayer.In case of the mass transfer of the caprylic acid to aq. NaOH solution the mass transfer resistance of the continuous phase can be neglected due to the enhancement of the mass transfer by the instantaneous reaction in this phase. The mass transfer rate is very high at the beginning of the mass exchange due to the interfacial turbulence caused by the chemical reaction. Also at longer contact times the mass exchange rate is high probably due to the reduction of the free desorption energy (increase of the desorption constant) caused by electrically charged monolayer.