Liquid–crystalline aqueous clay suspensions

Abstract

This article demonstrates the occurrence of a true isotropic͞nematic transition in colloidal Brownian aqueous suspensions of natural nontronite clay. The liquid-crystalline character is further evidenced by polarized light microscopy and small-angle x-ray scattering experiments in the presence and absence of modest external magnetic fields. The complete phase diagram ionic strength͞volume fraction then exhibits a clear biphasic domain in the sol region just before the gel transition in contrast with the situation observed for other swelling clays in which the sol͞gel transition hinders the isotropic͞ nematic transition. Small-angle x-ray scattering measurements of gel samples reveal strong positional and orientational orders of the particles, proving unambiguously the nematic character of the gel and, thus, clearly refuting the still prevalent ''house of cards'' model, which explains the gel structure by means of attractive interactions between clay platelets. Such order also is observed in various other swelling clay minerals; therefore, this very general behavior must be taken into account to reach a better understanding of the rheological properties and phase behavior of these systems.colloids ͉ liquid crystal ͉ phase transitions S welling clay minerals are layered compounds that bear a negative layer charge compensated by interlayer exchangeable cations whose valence and hydration properties control both swelling and colloidal behavior. One of the most important properties of swelling clay minerals is their ability to form yield stress materials when dispersed in water. This feature, extensively used in various industrial applications (drilling fluids, food industry, cosmetic industry, etc.), also plays a major role in many fundamental processes occurring at the Earth's surface, such as slipping processes in plate-boundary faults (1-4) or landslide triggering (5-9). For these reasons, numerous studies have focused on the rheology of aqueous clay suspensions with particular emphasis on yield stress, thixotropy, and aging (10-15). However, most studies neglect a key feature of clay minerals, i.e., their anisotropic shape. Actually, due to their high aspect ratio typically ranging between 25 and 1,000, these materials should very likely form liquid-crystalline phases (16), such as those observed for rod-like clay particles, such as imogolite in aqueous media (17), or organophilic sepiolite clay particles in nonaqueous solvents (18). A phase transition was indeed observed by Langmuir (19, ** ) as early as 1938 in suspensions of natural hectorite swelling clay. However, all subsequent studies failed to reproduce this crucial observation and give evidence of a clear thermodynamic liquid-crystalline order but instead revealed a dominant gel formation (20). Such behavior is observed for both highly polydisperse natural samples (21) and synthetic monodisperse ones (22). The structure and formation mechanisms of the gel are still under debate. Indeed, although some of the gel features indicate nematic ordering (23,24)...

In-Depth Insights into the Key Steps of Delamination of Charged 2D Nanomaterials

“…1a) of synthetic Na 0.5 -hectorite (19 (26)(27)(28). All structures consist of equidistant layers with long-range 1D ordering within a stack which is referred to as a tactoid.…”

Interplay of anisotropy in shape and interactions in charged platelet suspensions

“…Charged platelet suspensions, such as swelling clays [1][2][3][4][5][6][7][8], disk-like mineral crystallites [9][10][11][12][13][14][15][16] or exfoliated nanosheets [17][18][19][20][21][22] abound in natural as well as industrial environments. Such minerals can be found in various particle sizes varying between 10 nm up to a few micrometers and the majority of them have a platelet-like shape with high aspect ratio, typically ranging between 20 and 1000.…”

Cluster, Glass, and Gel Formation and Viscoelastic Phase Separation in Aqueous Clay Suspensions

“…Small-angle X-ray scattering (SAXS), [23][24][25][26] small-angle neutron scattering (SANS), [27][28][29][30] static and dynamic light scattering (SLS and DLS), 8,11,31-38 rheology, 18,30,32,39,40 scanning and transmission electron microscopy (SEM and TEM), 41 and nuclear magnetic resonance (NMR) 42 have been applied to clay suspensions to investigate Van Olphen's house-of-cards structure in Laponite dispersions.…”