Interlayer water structure

Heat Capacity Measurements and Interlayer Water Structure. The heat capacity of the interlayer water has been measured for the 10A,... 

Figure 3.11 The coordination of water molecules abotit the Na atom situated above the basal tetrahedral oxygen atoms. The left view shows an oblique view of the interlayer water structure and the right is a projection onto the layer of the same Na-water coordination arrangement showing the placement of the Na atoms above the tetrahedra of the adjacent silicate layer. In these illustrations, only the ions and the coordinating water oxygen atoms are shown hydrogen atoms have been omitted.

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

Calcination of LDHs removes the interlayer water, interlayer anions and the hydroxyl groups, resulting in a mixed metal oxide that cannot be achieved by mechanical means. It is especially interesting that the calcined LDH is able to regenerate the layered structure when it is exposed to water and an-... 

Fig. 2.14 The structure of halloysite. Hydrogen bonded interlayer water is shown in this 10 A or 0.01 nm. form.

The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. 

The type phengite suggested by Foster (1956) has a composition almost identical to that of the Belt illite (No.8). When the H20 content is appreciably higher and the K20 content lower than muscovite, the minerals have been called hydromicas or hydromuscovites. The excess H20 in some instances is present as interlayer water, particularly in the trioctahedral hydrobiotites. Table XII contains a selection of sericite and hydromuscovite analyses and Table XIII the structural formulas. The H20 and K20 values of these minerals are similar to those reported for the illite minerals however, the MgO content of the sericites and hydromuscovites is lower and the NazO contents higher than for the illites (Table XIV). 

Yet further, it should be emphasized that the listed site types may not act either solely, as reactions typically proceed through multiple steps, nor independently, as sites may interact. Site interaction is strongly indicated in the case of kaolinites, where spectroscopic properties and/or populations of certain catalytic entities, i.e. structural iron, and O -centers have been shown to be simultaneously modified in the presence of synthetically introduced interlayer water (141). 

The experimental studies of water interactions with clay minerals are very extensive. The structure, dynamics and interactions of interlayer water with the surface of clay minerals were reviewed in several papers [32, 33] and described in a number of books [15, 34, 35]. Therefore, we will review only the most important studies concerning experimental investigations of the structure and interactions of water molecules on clays. 

Considerable advances in the understanding of the structure of interlayer water in both smectites and vermiculites have been also achieved by means of computational simulations [61]. 

Sposito and co-workers employed a rigid framework for the clay lattice that has been used successfully to predict rf(001) layer spacing, interlayer structure, and water self-diffusion coefficients [62-80], Calculated layer spacings and thermodynamic properties, as well as interlayer water configurations and interlayer-species self-diffusion coefficients are in agreement with available experimental data. 

In conclusion, definite information on the density of adsorbed interlayer water cannot be derived from the bulk density measurements. However, such measurements supplement adsorption isotherm data, and both will be helpful in the interpretation of x-ray structure analysis of the configuration of water molecules and exchange cations in the interlayer space. 

---