Surface activity of mixtures of dodecyl trimethyl ammonium bromide with sodium perfluorooctanoate and sodium octanoate

Abstract

AbstractIn this study, the surface tension of mixtures of cationic surfactant dodecyl trimethylammonium bromide (DTAB) and anionic surfactant sodium perfluorooctanoate (SPFO) was evaluated as a function of DTAB‐SPFO composition and total surfactant concentration, with and without added electrolyte. It was found that the critical micelle concentration (CMC) of these mixtures was reduced by more than two orders of magnitude when compared to the CMC of SPFO and DTAB alone. The surface tension, after CMC, of the mixtures was close to that of SPFO with added electrolyte and even lower than that of SPFO without added electrolyte. Using the appropriate, combination of SPFO and DTAB, surface tensions near 18 mJ/m2 were obtained using a concentration 1/500 of that required for SPFO alone. When these results were compared to those obtained for DTAB‐sodium octanoate (SOCT) mixtures, it was evident that the DTAB‐SPFO system shows a larger degree of synergism than the DTAB‐SOCT system. Contact angle measurements on Teflon® surfaces corroborates that the lower surface tension of DTAB‐SPFO mixtures improves wetting on these hydrophobic surfaces. Using the regular solution theory to interpret the data of CMC and C45mJ/m2 (monolayer formation) versus DTAB‐SPFO composition, it was concluded that even when small concentrations of SPFO are introduced in the mixture, the composition of the micelles (at the CMC) and monolayer adsorbed at the air‐water interface tends to be equimolar in SPFO and DTAB.