An environmental infrared microbalance (EIRM) cell was used to study H₂O sorption on two montmorillonite samples as a function of water content and type of exchangeable cation. The vibrational spectra showed that H₂O sorbed to the clay at low-water content was strongly influenced by the exchangeable cation and by the close proximity to the clay surface. At water contents <6 H₂0 molecules per exchangeable cation, the H-O-H bending mode of H₂O (v₂ mode) shifts to a lower frequency and is characterized by an increase in molar absorptivity. In contrast, the positions of the asymmetric and symmetric OH-stretching modes of sorbed water (v₁ and v₃ modes) shift to higher energies. These observations indicate that H₂O molecules sorbed to the clay surface at low-water content are less hydrogen bonded than in bulk H₂O. In addition, the vibrational-stretching and bending bands of the structural OH groups of the 2:1 layer are also strongly influenced by H₂O content and type of exchangeable cation. By using the EIRM cell, the molar absorptivities of the structural OH-bending vibrations were measured as a function of H₂O content. The position and molar absorptivity of the structural OH-bending bands at 920, 883, and 840 cm^-1 are strongly influenced by H₂O content and type of exchangeable cation. The molar absorptivity of the 920-cm^-1 band, which is assigned to the AlAlOH group, decreased strongly at low-H₂O content. This reduction in intensity is assigned to a dehydration-induced change in orientation of the structural OH groups resulting from the penetration of H₂O molecules into siloxane ditrigonal cavities that are not associated with a net negative charge from isomorphous substitutions.

In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H₂O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.