The Solvent Properties of Adsorbed Water

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 

Another important chemical property of adsorbed water on vermiculite and smectite surfaces is its Br nsted acidity. This property should refer principally to the acidity of the solvated exchangeable cations, as described by the reaction 

The equilibrium constant of this reaction has been measured for a variety of metal cations in aqueous solutions and is known to correlate positively with both ionic potential and Lewis acid softness. As the ionic potential increases, the intensity of the positive coulomb field of the cation increases and repulsion of a solvating water proton becomes more likely. As the Lewis acid softness increases, the covalency of the M—O bond in a solvation complex increases and electron density is withdrawn from the O—H bond, thereby promoting the loss of the proton. 

The reaction in Eq. 2.3 has been investigated extensively on siloxane surfaces by coupling it with a Br nsted base protonation reaction, e.g., 

Exchangeable cation Ionic potential, nm Misono softness, nm Water activity nh , mol kg mol  

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