Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

Abstract

Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous medium. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. In particular, an aqueous dispersion of Laponite undergoes a liquid-to-solid transition at about 1 vol % concentration. The evolution of the physical properties as the dispersion transforms to the solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article, we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behavior. Furthermore, the continuously developing microstructure that is responsible for the eventual formation of a soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. In particularly, extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or a repulsive glass-like structure. We carefully examine and critically analyze the literature and propose a state (phase) diagram that suggests an aqueous Laponite dispersion to be present in an attractive gel state.

Effect of sodium pyrophosphate and understanding microstructure of aqueous LAPONITE®dispersion using dissolution study

“…Lastly, we also obtain a cryo-TEM image of a high concentration LAPONITE ® dispersion so as to directly relate a microstructure to the rheological findings. (13). A particle of LAPONITE ® has a disc-like shape with a diameter in the range of 25-30 nm and a thickness of about 1 nm (13).…”

“…However, Suman and Joshi (13) analyzed the SAXS results and, based on the findings of Greene et al (33), observed that decoupling of intensity into a product of form factor and structure factor is a poor approximation for a disk-like particle having aspect ratio around 25, which also possess dissimilar charges. The very fact that Greene et al (33) suggest that, owing to high aspect ratio and high concentration of the particles, decoupling approximation leads to spurious peaks in structure factor, Suman and Joshi (13) The introduction of salt in clay dispersion greatly influences the aggregation dynamics. This has resulted in numerous studies exploring salt-clay interactions (35)(36)(37).…”

“…(13). A particle of LAPONITE ® has a disc-like shape with a diameter in the range of 25-30 nm and a thickness of about 1 nm (13). An idealized unit cell of the hectorite clay crystal is illustrated in Fig.…”

“…In a LAPONITE ® particle, unit cell shown in Fig. 1 repeats itself in two directions such that a single disc having 25 nm diameter contains around 1100 unit cells with the net charge on the same to be around 770 e (13). In aqueous medium, sodium ions undergo dissociation giving a permanent negative charge to the faces of a clay particle.…”

Gel and glass transition in fragile colloidal clays

“…The investigated system is a synthetic clay that dispersed in water originates a charged colloidal system characterized by a strong anisotropy due to the shape of the nano-disc particles (with a 25:1 ratio) and due to the inhomogeneity of the charges (negative and positive respectively distributed on the faces and rims [10]) that determines a microscopic competition between directional attractive and repulsive interactions. Laponite ® peculiar phase diagram has been largely investigated both experimentally [10][11][12][13][14][15][16][17][18] and theoretically [19][20][21][22][23][24][25][26]. The system spontaneously evolves towards different arrested states depending on the clay concentration and ionic strength.…”