Small-angle X-ray Scattering (SAXS) and Small-Angle Neutron Scattering (SANS) Study on the Structure of Sodium Caseinate in Dispersions and at the Oil-Water Interface: Effect of Calcium Ions

The structure of sodium caseinate particles, as affected by the presence of calcium ions (Ca2+), in aqueous solution and in oil (toluene)-in-water emulsion, was investigated by small-angle X-ray and neutron scattering (SAXS and SANS). SAXS analyses indicated that the sodium caseinate was small particles with a hydrodynamic diameter (Dh) of ~ 12 nm. In the presence of Ca2+, the caseinate particles aggregated as large particles with Dh > 100 nm as determined by dynamic light scattering. The networks within the large particles were self-assembled from the small Ca2+-cross-linked particles (Rg ~ 6.5‒8.0 nm), as probed by SAXS. The fractal dimension increased from 2.5 to 3.4 with increasing protein and CaCl2 concentrations, suggesting a denser structure. The integrity of the caseinate particles at the oil-water interface was enhanced by Ca2+ cross-linking, as observed by transmission electron microscopy. The oil‒water interface stabilised by Ca2+-cross-linked caseinate particles was ~ 30 nm thick, six times thicker than that stabilised by sodium caseinate (~ 5 nm) as analysed by SANS with contrast variation technique. Quantifying the structure of sodium caseinate in an aqueous solution and at the oil-water interface provides valuable insights for designing new casein-based functional materials.