Universal Sol State Behavior and Gelation Kinetics in Mixed Clay Dispersions

Abstract

Sol and gel state behavior, in aqueous salt free dispersions, of clays Laponite (L) and Na montmorillonite (MMT) was studied at various mixing ratios (L:MMT = r = 1:0.5, 1:1, and 1:2). In the sol state, the zeta potential and gelation concentration of L−MMT obeyed the universal relation, X_L−MMT = (rX_L + X_MMT)/(1 + r), where X is zeta potential or gelation concentration (c_g), implying that these properties are linear combinations of the same of their individual components. The low frequency storage modulus (G₀′), relative viscosity (η_r), and apparent cluster size (R) could be universally described by the power-law, G₀′ ∼ ((c/c_g) − 1)^t (c > c_g), and η_r, R ∼ (1 − (c/c_g))^-k,ν (c < c_g), with t = 1.5, k = 1.1, and υ = 0.8 close to the gelation concentration, for r = 1:1 cogel, consistent with the percolation model description of gelation. Interestingly, the hyperscaling relation δ = t/(k + t) yielded δ = 0.56 not too different from the predicted value ∼0.7, while the experimental value of δ obtained from G′′(ω) ∼ ω^δ close to c ≈ c_g yielded δ = 1.5, which was at variance with the hyperscaling result. The experimental data, on hand, mostly supported percolation type gelation mechanism. As the cogels were slowly heated, at a characteristic temperature, T_g, a sharp increase in G′ value was noticed, implying a transition to gel hardening (a new phase state). The temperature-dependent behavior followed the power-law description, G′ ∼ (T_g − T)^−γ (T < T_g), with γ = 0.40 ± 0.05 invariant of composition of the cogel, whereas for MMT and Laponite, γ = 0.25 and 0.55, respectively. It has been shown that the cogel has significantly enhanced mechanical (G₀ increased by 10 times for r = 1:1 cogel) and thermal properties (T_g increased by 13 °C for 1:1 cogel) that can be exploited to design customized soft materials.