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

Abstract

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 ± 0.5Å and the SDS/solvent separation is 6.5 ± 0.5Å. 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.