Structural studies of agar–gelatin complex coacervates by small angle neutron scattering, rheology and differential scanning calorimetry

Agar–gelatin complex coacervates are studied by small angle neutron scattering (SANS), rheology (in both flow and temperature scan modes) and differential scanning calorimetry (DSC) in order to probe the microscopic structure of this dense protein–polysaccharide-rich phase. DSC and isochronal temperature sweep (rheology) experiments yielded a characteristic temperature at ≈35 ± 2 °C. Rheology data revealed a second characteristic temperature at ≈75 ± 5 °C which was absent in DSC thermograms. In the flow mode, shear viscosity (η  ) was found to scale with (Carreau model) applied shear rate (γ˙) as: η  (γ˙)∼(γ˙)−k with k = 1.2 ± 0.2 indicating non-Newtonian and shear-thinning features independent of ionic strength. The static structure factor S(q) deduced from SANS data in the low wave vector (0.018 Å−1 < q < 0.072 Å−1) region was fitted to Debye–Bueche function, S(q) ∼ 1/(1 + ζ2q2)2 that yielded a size ζ ≈ 220 ± 20 Å identified with the size of the inhomogeneities present. In the high-q region, called the Ornstein–Zernike regime, S(q) ∼ 1/(1 + ξ2q2) gave correlation length ξ ≈ 12 ± 2 Å. The results taken together imply the existence of a weakly interconnected and heterogeneous network structure inside the coacervate phase. Structural features of this material are compared with those of agar and gelatin gel, and gelatin coacervate.