Spontaneous evolution of self-assembled phases from anisotropic colloidal dispersions

Abstract

We investigate the spontaneous evolution of various self-assembled phase states from a homogeneous aqueous dispersion of high-aspect ratio Montmorillonite (Na Cloisite) nanoclay platelets grounded on the observations made over a period of 3.5 years. We have established the t w -c phase diagram for this system for the first time in salt-free suspensions under normal pH conditions using rheology experiments and have detected that these suspensions do undergo nontrivial phase evolution and aging dynamics. Distinctive phase separation, equilibrium fluid and equilibrium gels in the t w -c phase space are discovered. Cole-Cole plots derived from rheology measurements suggested the presence of inter connected network-like structures for c > c g, c g being the gelation concentration. All dispersions formed stable sols during the initial time, and with aging networklike structures were found to form via two routes: one for c < c g , by phase separation and another for c > c g , through equilibrium gelation. This has invoked and called for a revisit of the phase diagram of aging MMT dispersions.Emergence of anisotropic interaction between patchy colloids give rise to an array of unusual and novel soft materials like empty liquids with vanishing density, arrested networks and equilibrium gels which do not necessarily follow an underlying phase separation process to be borne. Recent research in patchy colloids has thrown up possibilities for intelligent design and development of structured nano-assemblies following a bottomup approach. These systems are inherently rich in dynamics, and a hierarchy of time and length scales adequately defines the properties of the soft matter so formed. Phase stability of dispersions of anisotropic particles has been of much scientific debate in the recent times. Many phase diagrams have been conceived and proposed for colloidal particles having different degree of geometrical anisotropy, namely rods, platelets, disks etc in their dispersion states [1][2][3] . Clays are discotic platelets with varying aspect ratio and surface charge density, and their dispersions exhibit an array of soft matter phases that evolve with waiting time t w as well as solid concentration c. Thus, the t w -c phase diagram is replete with non-trivial and equilibrium soft matter phases continuously evolving with time. From the application point of view, suspensions of clays offer unique properties, including their ability to form arrested states like gels, glasses and liquid crystalline structures under ambient conditions. Remarkably, compared to spherical colloids, clay platelets exhibit gel and glass phases at very low concentrations, which owe their origin to their unique geometrical structure and surface charge heterogeneity.In this work we demonstrate the spontaneous evolution of various self-assembled phases from a homogeneous aqueous dispersion of Montmorillonite (MMT, platelet diameter 250 nm and thickness 1 nm) nanoclay platelets based on the observations made over a period of 3.5 years. We have established ...

Equilibrium gels of limited valence colloids

“…X-rays scattering experiments [58] have confirmed that for c < 1% wt the scattering intensity progressively increases with waiting time, a signature of a very slow ongoing phase-separation, while for c > 1% wt it approaches a time-independent value, an expected signature of an equilibrium gel. Similar conclusions have resulted from the experimental investigation of other clays, including high aspect ratio Montmorillonite [59]. In all cases, a region of thermodynamic instability has been detected at low densities, consistent with the theoretical expectation by invoking an effective (although unknown) limited valence of the clay particles.…”

Spatially uniform dynamics in equilibrium colloidal gels

“…For higher concentrations, the samples remained arrested showing no aging. Similar results were also found with other compounds ( 30 – 33 ), some of which were of biological interest ( 34 , 35 ). More recently, the flexibility offered by DNA nanotechnology ( 36 ), exported to soft matter ( 37 ), has been mastered to generate an experimental system with tunable valence and interaction strength ( 38 ).…”

Clay–graphene oxide liquid crystals and their aerogels: synthesis, characterization and properties

“…It has the capability for soaking up a large amount of water with dramatic osmotic swelling. Hence, it is spontaneously exfoliated into aluminosilicate monolayers and forms stable colloidal dispersion without any physico-chemical treatment or exfoliating agent [8]. Moreover, it is interesting that for the dispersion with high clay concentration, the sol/gel transition occurs and LCs can be observed in the gel.…”