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

Hierarchical self-assembly, spongy architecture, liquid crystalline behaviour and phase diagram of Laponite nanoplatelets in alcohol-water binary solvents

Hydrophobicity and solvation of different charged species are among the various key factors that regulate the self-assembly of colloids, and macromolecules in their suspensions. In this paper, we demonstrate a method to tune the interaction potential and the resulting phase behaviour and microstructure of the states that form by using a combination of Laponite nanoplatelets and alcohols in water. This allows us to exquisitely control the self-assembly process of Laponite nanoplatelets. A new class of soft materials, called nanoclay-organogels, is studied systematically for their aging behaviour, microscopic structure and mechanical properties. Real space imaging techniques depicted spongy architecture with nano and micron size pores inside the gel matrix indicating the hierarchical self-assembly of the nanoplatelets in the aqueous solutions of polar organics. We have extensively examined the dispersion stability, aggregation, gelation and liquid crystalline behaviour of Laponite nanoplatelets in different alcohol (methanol, ethanol, 1-proponaol and ethylene glycol, and glycerol)-water binary solvents, thereby proposing a generalized description of nanoclay in alcoholic solutions, which is poorly probed and marginally understood in the literature. A phase diagram of Laponite® in alcohol solutions is proposed, which clearly demarcates regions of isotropic sol, unstable sol, isotropic gel, nematic/birefringent gel, glass, flocculated sedimentation and liquid crystalline structures.

In-situ Observation of Hierarchical Self-Assembly Driven by Bicontinuous Gelation in Mixed Nanodisc Dispersions

The search for new functional soft materials with precise and reconfigurable structures at the nano and meso-scale is a major challenge as well as objective of the current science. Patchy colloids of different shapes and functionalities are considered important new building blocks of a bottom-up approach towards rational design of new soft materials largely governed by anisotropic interactions. Herein, we investigate the self-assembly, growth of hierarchical microstructures and aging dynamics of 2D nano-platelets of two different aspect ratios (Laponite ~25 and Montmorillonite ~250) which form gels with different porosity that is achieved by tuning their mixing ratios. Qualitative in situ real-space studies are carried out, including fluorescent confocal microscopy imaging of the bicontinuous gelation process or final states, which provides dynamic visualization of the self-organization. The bicontinuous gels exhibit a foam-like morphology having pores of a few micrometers in size that can be tuned by varying the mixing ratio of nanoplatelets. It is shown that this new class of clay gels has unique and tunable physical properties that will find potential applications in the development of low cost lithium ion batteries, nanocomposites and nuclear waste management.

Kinetics of anisotropic ordering in Laponite dispersions induced by a water-air interface

In this work, we report the kinetics of ordering occurring at the water-air interface of Laponite dispersions. Propagation of such ordering into the bulk and its relaxation dynamics were systematically studied through light scattering measurements. Depolarization ratio D(p), which accounted for the optical anisotropy, was measured as a function of depth from the interface and aging of the samples. The extent of spatial ordering was found to be several decades larger than the typical particle size. Spatial ordering originated from the interface and percolated into the bulk with aging time t(w). Growth in D(p) with waiting time was found to follow power-law behavior given as D(p)~t(w)(n), with n increasing from 0.1 to 4 as one moved away from the interface into the bulk. D(p) decreased exponentially with depth h given as D(p)~e(-(h/h(0))), where h(0) is the decay length, increasing from 0.4 to 0.75 mm with aging time. Dynamic structure factor measurements performed on the samples at various aging times, depths, and temperatures yielded two distinct relaxation times: one fast mode followed by a slow mode. The fast mode remained invariant while slow mode relaxation time followed an exponential decay with depth. This study indicated that the arrested phase nucleated from the interface and propagated into the bulk, which was not observed when the surface was insulated with a layer of hydrophobic liquid. Dilution of the concentrated samples destroyed the aforesaid ordering and made the dispersion homogeneous implying the ordered state was a glass.