Adsorbed water phyllosilicates

The definition of adsorbed water adopted in Sec. 2.3 requires an arrangement of water molecules that differs significantly from that in an appropriate reference aqueous phase. For water on the surfaces of kaolinite group minerals the reference phase is bulk liquid water, whereas for water on vermiculite and smectite surfaces the reference phase is an aqueous solution because of the presence of exchangeable cations on the 2 1 layer silicates. On the ba,si.s of this definition, the consensus developed in Sec. 2.3 is that the spatial extent of adsorbed water on a phyllosilicate... 

G. W. Brindley and G. Brown, Crystal Structures of Clay Minerals and Their X ray Identification. Mineralogical Society, London, 1980. Chapter 3 of this standard reference contains an excellent discussion of X-ray diffraction studies of adsorbed water on phyllosilicates. 

J. W. Stuck and W. L. Banwart, Advanced Chemical Methods for Soil and Clay Minerals Research. Reidel, Dordrecht, The Netherlands, 1980. This book provides an excellent introduction to the use of NMR and INS techniques for the investigation of adsorbed water structure. Many experimental results for adsorbed water on phyllosilicates are presented. 

J. Texter, K. Klier, and A. C. Zettlemoyer, Water at surfaces, Prog. Surface Membrane Sci. 12 327 (1978). This review gives an account of the available data concerning the properties of water on oxide and organic solid surfaces. The general conclusions drawn are similar to those stated in the present chapter for the structure of adsorbed water on phyllosilicates, except that hydrogen bonding of the water to the surface plays a more prominent role. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Application of microwave irradiation in organic reactions has added a new dimension to solid phase synthesis. By the use of this technique, it is now possible to carry out reactions without the use of toxic or other solvents, which is one of the main problems associated with green synthesis. In these, the reactants are dissolved in a suitable solvent like water, alcohol, methylene chloride etc. and the solution stirred with a suitable adsorbent or solid support like silica gel, alumina or phyllosilicate (M" "-montomorillonite). After stirring. 

---