Abstract
AbstractIn 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.
Highlights
AB ST R ACT : In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals
We have focused our attention on H bonding interactions of structural OH groups which are contained within the clay structure
Considering the spatial length scales associated with H bonding, clayÀwater and clayÀorganic interactions provide a useful framework to better understand these complex materials
Summary
Soil and Environmental Sciences, Purdue University, 915 W. It is the size domain slightly larger than Angstroms that continues to present experimental, theoretical and numerical challenges It is at the scale of nanometres, which defines active sites on clay surfaces, where both clayÀwater surface interactions and particleÀparticle interactions occur, to name two important areas. These span from the sub-Angstrom scale interactions of hydroxyl groups to the approximate size of particles (>1000 nm) and will rely mainly on spectroscopic observations. The scale to be considered is that associated with clayÀwater and clayÀorganic interactions (Fig. 1) At these length scales, multiple molecular mechanisms are operative (including H bonding) and span from the size of the H2O molecule and small polar organic solutes (0.3 nm) to that of large biological molecules (e.g. proteins) and the limit of clay–water swelling (10 nm). This review will conclude with one illustrative example of clay particle-particle interactions at this ‘particle’ scale (Fig. 1)
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