JOURNAL OF COLLOID AND INTERFACE SCIENCE 182, 501–510 (1996) ARTICLE NO. 0494 Unusual Thixotropic Properties of Aqueous Dispersions of Laponite RD NORBERT WILLENBACHER 1 BASF AG, Polymer Research Division, 67056 Ludwigshafen, Germany Received November 10, 1995; accepted April 17, 1996 exchangeable and in aqueous dispersions these ions diffuse The rheological properties of aqueous dispersions of the syn- into the water and plate-like particles with negatively thetic hectorite clay Laponite RD that form gel-like structures charged faces are formed. The charge on the oxide edge of have been investigated. Special emphasis has been laid on the the platelets is less negative or even positive depending upon phenomenon of thixotropy. Structural recovery at rest aftersteady the pH of the dispersing medium. The average size of these shear is considered a fundamental thixotropic process and has particles is 30 nm in diameter and 1–4 nm in height, as been characterized by small amplitude oscillatory shearmeasure- revealed by electron microscopy (5, 6) and independently ments. A special sealing technique was used to prevent the evapo- confirmed by various scattering techniques (6–8) and ultra- ration of water during these experiments. After cessation of steady centrifuge analysis (8). Thus these particles are an order of shear Éh*É increases monotonically with time and even after 16 days no equilibrium viscosity value is reached. A single powerlaw magnitude smaller than those in refined natural clays. Éh*É Ç t n holds within the time regime from 10 to 10 6 s. The Generally, two different mechanisms for gelation in clay exponent n Å 0.13 { 0.02 is independent of clay concentration mineral dispersions are proposed, leading to different types and mechanical pre-treatment of the material. This type of kinetics of three-dimensional ordering. In the first case the formation has not been reported so far for the thixotropic recovery of any of an equilibrium structure is induced by long-range electro- clay dispersion and it does not fit the common definition of thixot- static repulsion between interacting double-layers. This ropy. The reorganization of the gel structure determining this mechanism, which has been proposed by Norrish (9) and thixotropic phenomenon is interpreted as a cooperative self de- supported by experiments of Callaghan and Ottewill (10), laying process similar to the aging of glassy polymers or precipita- accounts for gel formation at low electrolyte concentration tion from supersaturated solid solutions.  1996 Academic Press, Inc. ( õ10 03 mol dm 03 , 1:1 electrolyte ) . At higher ionic strength Key Words: clay; flocculated dispersion; thixotropy. another mechanism suggested by van Olphen (1, 11) is fa- vored: the double layers around the faces of the platelets are INTRODUCTION compressed and the electrostatic attraction between oppo- sitely charged faces and edges, together with the attractive Clay minerals of the montmorillonite or hectorite type van der Waals forces between the particles, gives rise to are able to form three-dimensional gel-like structures when edge-to-face as well as edge-to-edge associations, leading to dispersed in aqueous media. Therefore, these materials are a linked, three-dimensional network (house of cards). commonly used as thickening, plasticizing, or emulsifying Nevertheless, particle interaction and structure formation agents in various industrial products such as paints, plasters, in gelled clay dispersions are still a matter of debate. adhesives or cosmetical formulations. Consequently exten- Due to its high purity and narrow particle size distribution sive studies have been reported on their colloidal and rheo- Laponite has served as a model clay system and has been logical properties (Refs. 1–4). investigated under various conditions of electrolyte content, Laponite RD (Laporte Inc., UK) is a fully synthetic clay pH range, and clay concentration. Static and dynamic light similar in structure and composition to natural hectorite of scattering data support the formation of tactoids consisting the smectic group. Each layer comprises three sheets, two of several platelets (7). A spatially heterogeneous structure outer tetrahedral silica sheets and a central octahedral mag- of isolated particles with local orientational correlation has nesia sheet. Part of the magnesium in the central sheet is been suggested on the basis of small angle X-ray and neutron replaced by lithium, resulting in a net negative charge of the scattering experiments (12, 13) and has been evidenced by layer, which is balanced by sodium ions located between cryofracture electron microscopy ( 14 ) . In addition, correla- adjacent layers in a stack. The idealized chemical formula tions between macroscopic electrical ( 15 ) and rheological is [(Si 8 (Mg 5.34 Li 0.66 )O 20 (OH) 4 ]Na 0.66 . The sodium ions are properties ( 16 ) and microstructure have been attempted. Furthermore, a phase diagram has been established (14) 1 E-mail: norbert.willenbacher @zkm.x400.basf-ag.de. and fundamental colloidal and surface properties of swelling 501 0021-9797/96 $18.00 Copyright  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.