Landau theory description of observed isotropic to anisotropic phase transition in mixed clay gels.

Abstract

A characteristic new cooperative dehydration transition, in 1:1 Laponite-MMT cogel, was observed at T(c) ≈ 60 °C, a temperature at which the storage modulus (G(')) and depolarization ratio (D(p)) showed sharp increase, and the isotropic cogel turned into an anisotropic one. The dehydration dynamics could be described through power-law relations: G(') ∼ (T(c)-T)(-γ) and D(p) ∼ (T(c)-T)(-β) with γ ≈ β = 0.40 ± 0.05. The x-ray diffraction data revealed that the crystallite size decreased from 17 nm (at 20 °C) to 10 nm (at 80 °C) implying loss of free and inter-planar water. When this cogel was spontaneously cooled below T(c), it exhibited much larger storage modulii values which implied the existence of several metastable states in this system. This phase transition could be modeled through Landau theory, where the depolarization ratio was used as experimental order parameter (ψ). This parameter was found to scale with temperature, as ψ ∼ (T(c)-T)(-α), with power-law exponent α = 0.40 ± 0.05; interestingly, we found α ≈ β ≈ γ.

• Authors: Ravi Kumar Pujala, Nisha Pawar, H B Bohidar
• Journal: The Journal of chemical physics (J Chem Phys) Vol. 134 Issue 19 Pg. 194904 (May 21 2011) ISSN: 1089-7690 [Electronic] United States
• PMID: 21599085 (Publication Type: Journal Article)
• Copyright: © 2011 American Institute of Physics.