Chapter 5 - Physical and chemical properties of layered clay mineral particle surfaces

Abstract

Recent years have witnessed a significant increase in interest in surface-active (SA) clay minerals. Distinguishing features of SA clay minerals include their small size, high aspect ratios, and reactive and, in some cases, asymmetric surfaces. Although many different types of clay minerals are found in nature, only a small number have been found to play a role in stabilizing Pickering emulsions or to function as Janus particles and are the focus of this review. Examples of SA clay minerals include kaolinite and halloysite in the kaolin group of 1:1 phyllosilicates (TO structures), and Laponite, montmorillonite, and other members of the 2:1 phyllosilicate group of smectites (TOT structures). In terms of their reactive surfaces, SA clay minerals possess siloxane surfaces with or without permanent charge from isomorphous substitution, alumina hydroxyl surfaces comprised of bridging Al-(OH)-Al groups, and broken edge sites with exposed and reactive aluminol and silanol groups. Each surface has a distinctive hydrophobic/hydrophilic character. Most applications of surface-active clay minerals require some degree of surface modification to “tune” their behavior at the interface between two immiscible fluids. Illustrative examples of surface modification are reviewed. Finally, controlling interfacial behavior (i.e., particle-particle interactions) also requires knowledge about their phase behavior in the continuous phase.