Sodium dodecyl sulfate micelles: Accurate analysis of small-angle X-ray scattering data through form factor and atomistic molecular dynamics modelling
Ian W. Hamley, Valeria Castelletto
Abstract
The structure of micelles of sodium dodecyl sulfate (SDS) is probed via analysis of small-angle x-ray scattering (SAXS) data with the aim to fit the data over an extended wavenumber q range. This provides detailed information on the micelle shape, which can be described as a polydisperse triaxial ellipsoidal core-shell structure according to model form factor fitting. This model was necessary to fit the data over a wide q range, which is not accurately represented by simpler models such as biaxial ellipsoidal core-shell structures. Data for SDS (at fixed concentration) in a NaCl concentration series revealed increasing structure factor effects with decreasing salt concentration. This reflects decreased charge screening on the headgroups. The structure factor could be modelled using a simple hard sphere structure factor. The analysis of form factor was complemented by atomistic molecular dynamics (MD) simulations, starting from an unbiased initial configuration of a defined number of molecules in a box. The MD configurations were used to calculate the form factor using the software CRYSOL (for SAS analysis of solution scattering, traditionally for proteins here for micelles) accounting for the boundary layer hydration effects. Good agreement with experimental data was found for systems with association numbers close to p = 60. This association number is consistent with that obtained from analysis of the form factor (in the case where structure factor effects could be neglected) and from model-free analysis of the forward scattering intensity. It is also in agreement with prior literature and our findings in regard to form factor parameters are also compared to previous reports.