Spinodal decomposition and phase separation kinetics in nanoclay–biopolymer solutions

Intensity of light, I ( q , t ), scattered from homogeneous aqueous solutions, of nanoclay (Laponite) and protein (gelatin‐A), was studied to monitor the temporal and spatial evolution of the solution into a phase‐separated nanoclay–protein‐rich dense phase, when the sample temperature was quenched below spinodal temperature, T s (=311 ± 3 K). The zeta potential data revealed that the dense phase comprised charge‐neutralized intermolecular complexes of nanoclay and protein chains of low surface charge. The early stage, t < 500 s, of phase separation could be described adequately through Cahn‐Hilliard theory of spinodal decomposition where the intensity grows exponentially, I ( q, t ) = I 0 exp.(2 R ( q ) t ). The wave vector, q dependence of the growth parameter, R ( q ) exhibited a maxima independent of time. Corresponding correlation length, 1/ q c = ξ c was found to be ≈75 ± 5 nm independent of quench depth. In the intermediate regime, anomalous growth described by I ( q , t ) ∼ t α with α = 0.1 ± 0.02 independent of q was observed. Rheological studies established that there was a propensity of network structures inside the dense phase. Isochronal temperature sweep studies of the dense phase determined the melting temperature, T m = 312 ± 4 K, which was comparable with the spinodal temperature. The stress‐diffusion coupling prevailing in the dense phase when analyzed in the Doi‐Onuki model yielded a viscoelastic correlation length, ξ v determined from low‐frequency storage modulus, G ′ 0 ≈ k B T /ξ , which was ξ v ≈ 35 ± 3 nm indicating 2ξ v ≈ ξ c . It is concluded that the early stage of phase separation in this system was sufficiently described by linear Cahn‐Hilliard theory, but the same was not true in the intermediate stage.