Theoretical and experimental studies of the aggregation number in the ammonium perfluorooctanoate/water binary mixture

The ammonium perfluorooctanoate/water (APFO/D2O) system was studied by 'ab initio' quantum computational methods employing the 6-31 + G and 6-311 + G basis set at Hartree-Fock level. Infrared and Raman measurements were made in order to clearly understand the nature of the C-O bonds and also to verify the accuracy of the theoretical results. Small angle X-ray scattering experiments were performed for several samples at the desired surfactant weight fraction and from it the average micellar size and aggregation number were calculated using semi-empirical values for the monomer volume. These average aggregation numbers were compared with those obtained from the optimized geometry of the APFO, which showed that, for this fluorinated surfactant, there is some discrepancy between the results achieved by these two approaches. The results show that the average aggregation numbers obtained by semi-empirical methods are lower than those obtained by ab initio calculations.