Small-angle neutron scattering studies of porous media: Assessment of liquid and gas as contrast-matching fluids using carbon aerogel

Abstract

Contrast-matching (CM) small-angle neutron scattering (SANS) experiments have gained popularity due to their ability to assess and characterise porous media, as well as the interactions between penetrating fluids and solid matrix. This study evaluates the differences in performance between two commonly used fluids—CD₄ and H₂O/D₂O mixtures—in a CM SANS study using carbon aerogel, a material known for its homogeneous, inert, and tunable structure. For CM SANS experiments employing H₂O/D₂O mixtures, the sample preparation process is critical to achieving thorough pore saturation. At the scale of ∼5 nm, corresponding to the dominant pore size in the aerogel, CM SANS data for both CD₄ and H₂O/D₂O exhibit near-ideal two-phase solid-fluid behaviour, indicating good accessibility of pores and minimal fluid condensation in confinement. At larger scales (around 125 nm), however, CM SANS results using pressurised CD₄ deviate from the two-phase model. This deviation is consistent with an increase in porosity under pressure, attributed to elastic deformation of the solid matrix at length scales beyond the range investigated by SANS. Notably, this effect is absent if H₂O/D₂O mixtures are used as CM fluid, and should, therefore, be considered when conducting CM SANS experiments with high-pressure CD₄. These findings offer valuable insights into the suitability of penetrating fluids in CM SANS experiments and highlight the remarkable elasticity of the carbon aerogel matrix, which reliably returns to its original state even after undergoing significant structural deformation.