The partition coefficient of various alcohols between water and alkaliperfluorooctanoate micelles

Abstract

The partition coefficient P of 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, benzyl alcohol, and phenol has been determined between water and the micellar phase of two ionic surfactants, sodium and potassium perfluorooctanoate (NaPFO and KPFO), using thermodynamic methods based on Krafft point and critical micelle concentration determinations. The standard free energy of transfer of a methylene group from water to both perfluoro micelles is equal to −440 cal/mole; it is about −600 and −660 cal/mole, respectively, in the cases of the transfer to sodium dodecyl sulfate (SDS) and to trimethyldodecylammonium bromide (nDOTMABr). This difference due to the phobicity of the CH 2 CF 2 interaction is counterbalanced for the lower alcohol homologs by a strong ion-dipole interaction between the hydroxyl group and the carboxylate head-group whose increased acidity is induced by the electron-withdrawing ability of the CF 2 groups of the surfactant anion. Thus the values for the lower alcohol homologs ( n = 4) are larger with NaPFO than with SDS despite the phobicity effect and the longer carbon chain of SDS. The effect of counterion on P values is also discussed. The nonideality of the solute + micelle interaction which seems essentially dependent on the surfactant properties is shown to decrease in the order nDOTMABr > SDS > KPFO. Using a log/log plot of P values in the aqueous micellar solutions and in the water + octanol system, it is shown that phenol is more solubilized in SDS solutions than could be deduced from the water + octanol system, but the opposite is true with KPFO micelles. The salting constant of some polar molecules was determined in pre-micellar aqueous NaPFO solutions using a precise vapor pressure method and the results were compared to those previously obtained with nDOTMABr solutions. For molecules of smaller size (acetone, acetonitrile) the salting-in constants are almost identical in both pre-micellar solutions. For bulkier molecules (ethers), the salting-in effect is less for NaPFO compared to nDOTMABr solutions; these results are interpreted in terms of phobicity effects between CH 2 and CF 2 groups which intervene predominantly only when large numbers of CH 2 groups are involved.