Cationic exchange, layered silicates interactions

In addition to MC and MD simulations, several quantum-chemical studies of hydrated clay minerals with or without exchangeable cations in the interlayer space have been performed at the ab initio and semiempirical level of theory. The total energy of hydrated layered silicates (talc and pyrophyllite) without exchangeable counterions, the position and interactions of interlayer water... 

The layered silicate nanoparticles are usually hydrophilic and their interactions with nonpolar polymers are not favorable. Thus, whereas hydrophilic polymers are likely to intercalate within Na-activated montmorillonite clays [24-29], hydrophobic polymers can lead to intercalated [23,30-32] or exfoliated [33] structures only with organophilized clays, i.e., with materials where the hydrated Na+ within the galleries has been replaced by proper cationic surfactants (e.g., alkylammonium) by a cation exchange reaction. The thermodynamics of intercalation or exfoliation have been discussed [34-37] in terms of both enthalpic and entropic contributions to the free energy. It has been recognized that the entropy loss because of chain confinement is compensated by the entropy gain associated with the increased conformational freedom of the surfactant tails as the interlayer distance increases with polymer intercalation [34,38], whereas the favorable enthalpic interactions are extremely critical in determining the nanocomposite structure [39]. 

All polar surface sites are well hydrated in aqueous suspensions the hydrophilic nature is characteristic of clay minerals in general. The exposed siloxane surface, however, is considered to have a predominantly hydrophobic character for neutral 2 1 layer silicates, where isomorphic substitution has not occurred, and for the siloxane side of 1 1 layer silicates [20]. Spectroscopic investigations of clay-water interactions suggest that (1) water molecules can coordinate directly to exchangeable metal cations and (2) physisorbed water molecules occupy interstitial pores, interlamellar spaces between exchangeable metal cations, or polar sites on external surfaces [20]. The presence of hydrated exchangeable cations in the interlayer of expandable 2 1 phyllosili-... 

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