Ellen M. Lee

Adsorption of ammonium perfluorooctanoate and ammonium decanoate at the air/solution interface

The composition and structure of layers of ammonium decanoate (AMDEC) and ammonium perfluorooctanoate (APFO) adsorbed at the air/liquid interface have been studied by a combination of surface tension measurements and neutron reflectivity.At the critical micelle concentration (c.m.c.) and in solutions of pH = 8.8 and ionic strength 0.1, the areas occupied per surfactant anion were found to be 41 ± 2.5 A2 for APFO and 35 ± 3 A2 for AMDEC. In each case it was necessary to include fugacity corrections and to fit the surface pressure-In c curve by least squares for there to be agreement with the surface excess as measured by neutron reflection.The structure of the surfactant layers was determined at the c.m.c. using neutron reflectivity measurements on three different isotopic combinations of water and surfactant. The thickness of the APFO layer was found to be 20 ± 2 A, ca. 30% larger than the fully extended chain length and the thickness of the AMDEC layer was 17 ± 2 A, only ca. 15% greater than the fully extended chain length. Both model fitting and a more direct method of analysing the separation between surfactant chains and water suggest that the chains are ca. 25–30% immersed in the aqueous phase, with the APFO being slightly more immersed than the AMDEC.

The Determination of the Structure of a Mixed Surfactant Monolayer by Specular Neutron Reflection

Neutron reflection in combination with isotopic substitution has been used to determine the surface structure of a mixed monolayer of sodium dodecyl sulphate (SDS) and dodecanol at the air/solution interface. Two different methods of analysis have been applied to the data. The first fits a single structural model to different reflectivity profiles arising from manipulaton of the refractive index distribution by isotopic substitution. The second, based on the kinematic approximation, is more direct, and in particular allows the distances between the centres of the distributions of the two surfactants to that of the solvent to be obtained without recourse to modelling. At a SDS concentration of 0.0067M and at a SDS/dodecanol ratio of 0.005 the dodecanol/solvent separation is 9.0 ± 0.5A and the SDS/solvent separation is 6.5 ± 0.5A. The two methods of analysis provide a consistent structural model, which for concentrations less than the critical micellar concentration (cmc) the SDS is more immersed in the solvent than the dodecanol, and that the SDS molecules are less fully extended than the dodecanol molecules.