Slow dynamics, hydration and heterogeneity in Laponite dispersions

Abstract

Age dependent hydration, heterogeneity and network rigidity of aqueous Laponite dispersions in their sol, gel and glass phases have been studied systematically in the clay concentration range c = 0.3–3.5 % (w/v) which revealed the following interesting features: (i) both in the sol and gel regime (c < 1.8% (w/v)), the intensity of light scattered I(q,c) scaled with concentration c as, I(q,c) ∼ c^α with α = 0.95 at t = 0, and 0.63 after t = 6 months implying that this temporal growth resulted from the formation of a colloidal gel; (ii) in the glass phase (c ≥ 2% (w/v)), scattered intensity from samples remained constant (α = 0), meaning that there was disorder in the system at higher solid contents, and the clay particles were randomly arranged, and this intensity profile did not change with the aging; (iii) the dynamic structure factor data showed low concentration dispersions (gels) having a single mode relaxation time (fast mode) which shifted to longer relaxation time with aging. In the glass phase, in addition to the fast mode that remained invariant of aging, a slow mode which shifted to longer relaxation time with aging was noticed; (iv) rigidity modulus (G′) and yield stress (σ) measurements showed that there are two universal power-law relations governing their behavior as a function of clay concentration; G′, σ ∼ c^δ with δ = 2.3 for gels and 2.8 for glass. Both these parameters revealed similar logarithmic time dependence with aging as σ, G′ = βln(t_w/t_m), where β is a constant and t_m is the characteristic time scale for the development of micro structures; (v) Raman spectroscopy data revealed three distinct OH bands characteristic of the structure of water which revealed change in the structured water content with the aging, and the heterogeneity of the samples was established using a Cole–Cole plot. We have proposed a time-dependent phase diagram for the salt free suspensions of Laponite clay using the abovementioned results.