Halloysite, adsorbed water

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

Given the structure in Fig. 2.3 as a prototype of the monolayer arrangement of adsorbed water on kaolinite group minerals, there remains only the question of its single-particle properties. Proton and deuteron NMR measurements are instructive in this respect.As indicated in Table 2.1, NMR line shapes show whether a preferred orientation of water molecules exists on the NMR time scale, and NMR relaxation data can yield a value for the rotational correlation time, In the case of 10-A halloysite, the NMR signal from water protons or deuterons in the... 

S. Yariv and S. Shoval, The natiure of the interaction between water molecules and kaolin-like layers in hydrated halloysite, Clays and Clay Minerals 23 473 (1975). M. I. Cruz, M. Letellier, and J. J. Fripiat, NMR study of adsorbed water. II Molecular motions in the monolayer hydrate of halloysite, J. Chem. Phys. 69 2018 (1978). 

Experimentally, the conditions for the study of the infrared spectra of adsorbed water are particularly favorable in expanding lattice clays, /.e., montmorillonites, vermiculites, and halloysite because of the relatively large amount of water present with respect to the amount of solid. 

Halloysite-10A represents a structure with few if any interlayer cations, allowing one to investigate the relatively simple case of water interacting with a clay surface. Similarly, ice-like models have been proposed for water adsorbed on smectite and vermie-ulite surfaces (2, 12, 12). These represent cases of charged clay layers with adsorbed exchangeable cations. 

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