Microfluidic-SANS Study of Micellar Shape and Interactions in Mixed Solvents*

We report a microflow approach to perform small angle neutron scattering (SANS) of contrast variation and phase mapping of multicomponent liquid mixtures, focussing on surfactant systems. We integrate a low scattering microfluidic device in a SANS beamline to yield a reconfigurable liquid flow cell. By contrast with conventional techniques, our approach continually varies solution composition during SANS acquisition. We demonstrate our contrast variation approach with model systems, including sodium dodecyl sulphate (SDS) and a block-copolymer (F127) in isotopic aqueous mixtures during linear composition ramps with 5 s time resolution. The form and structure factor are quantitatively determined with a global fit of the dataset, leading to unprecedented robustness and precision. Ternary and quaternary phase mapping of surfactant systems is then demonstrated by dynamically scanning across the phase space. We first validate the approach with a diluting of micellar 10% SDS in D₂O, then study the effect of 1-butanol addition in the shape and interactions of micelles. The generality of the microflow-SANS approach, ensuring component mixing yet minimal composition dispersion is then demonstrated with a quaternary system of two surfactants, solvents and polymer.