H₂O/D₂O contrast variation for ultra-small-angle neutron scattering to minimize multiple scattering effects of colloidal particle suspensions

This study investigated the use of solvent contrast (H₂O/D₂O ratio) as a means to optimize the ultra-small-angle neutron scattering (USANS) signal. By optimizing the signal, it was possible to reduce the undesirable effects of coherent multiple scattering while still maintaining a measurable scattered intensity. This result will further enable the use of USANS as a probe of the interactions between colloidal particles and their structures within concentrated suspensions as well as particle dispersion/aggregation. As a model system, we prepared silica colloidal particle suspensions at different solid concentrations. USANS curves were measured using the classical Bonse–Hart double crystal diffractometer while varying the scattering length density of the aqueous phase, thus varying the contrast to the silica particles. As a means of assessing the impact of multiple scattering effects on different q-values, we analyzed the scattered intensity at different contrasts at three different q values. The data were then used to determine the match point of the silica particle suspensions from the expected square root dependence of the scattered intensity with solvent composition, to analyze any differences associated with the solid concentration change, and to determine the optimum H₂O/D₂O ratio in terms of high transmission (T_SAS > 80%) and high enough scattering intensity associated with the contrast of the system. Through this investigation series, we confirmed that adjusting the contrast of the solvent (H₂O/D₂O) is a good methodology to reduce multiple scattering while maintaining a strong enough scattering signal from a concentrated suspension of silica particles for both USANS and rheometric USANS (rheo-USANS) experiments.