Clays surface property changes

Rengasamy, P. and Oades, J.M., Interaction of monomeric and polymeric species of metal ions with clay surfaces. 11. Changes in surface properties of clays after addition of iron(in), Aust. J. Soil. Res., 15, 235, 1977. 

Layered aluminosilicates are the most important secondary minerals in the clay fraction of soils. When layer silicate minerals are clay or colloidal size (<2 gm effective diameter), their large surface area greatly influences soil properties. Most of the important clay minerals have similar silicate structures. Inasmuch as clay minerals are such important clay components, and as different clay minerals can change sail properties greatly, an understanding of soil properties begins with an understanding of silicate structures. 

The reinforcement of polypropylene and other thermoplastics with inorganic particles such as talc and glass is a common method of material property enhancement. Polymer clay nanocomposites extend this strategy to the nanoscale. The anisometric shape and approximately 1 nm width of the clay platelets dramatically increase the amount of interfacial contact between the clay and the polymer matrix. Thus the clay surface can mediate changes in matrix polymer conformation, crystal structure, and crystal morphology through interfacial mechanisms that are absent in classical polymer composite materials. For these reasons, it is believed that nanocomposite materials with the clay platelets dispersed as isolated, exfoliated platelets are optimal for end-use properties. 

Changes in the surface properties of cationic complexes, in which the surface metal cations of natural clays have been exchanged with an ammonium cationic surfactant, could be simply confirmed by mixing them with polar solvents and observing their dispersibility. It should be noted that quantitative measurements of surface properties may be obtained by contact-angle measurements [4, 10]. This method allowed measuring the change of surface polarity of Cloisite 30B (a dimethylbenzyl tallow ammonium chloride-modified montmorillonite) subjected to elevated temperatures for the duration of typical... 

The in situ intercalative polymerization associated with the UV curing is a technique which was successfully employed in the synthesis of hybrid films, when fast polymerization of liquid monomers yielded in solid materials with designed properties. It was proved to have high eflftciency for epoxy oligomers, vinyl ethers, oxetanes in the presence of onium salts as photoinitiators [232]. The literature is not abundant in reports on the in situ UV-initiated polymerization of epoxides in the presence of layered silicates [233-237], as compared to data on thermally cured or melt compounded nanocomposites. In some studies, the clays were used either unmodified [235, 237] or organically modified [233, 236, 238] or treated with various reagents able to change their surface properties [239-243]. 

For radioactive waste, which still is capable of generating heat, a problem exists it has been found that exposure of swelling clays to steam causes the smectite to change from being hydrophilic, to become hydrophobic (Couture, 1985 Bish et at., 1998). In the study of Bish et at. (1998) two Na-smectites were examined. Their 7 values decreased from about 33-34 mJ/m, to 21-24 mJ/m after steam treatment, and concomittintly, their -potentials decrease by about 20%. As an example, the surface properties of the smectite SWy-1, at 7 = 41.1, 7 = 1.9 and 7 = 33.2 mJ/m, changes after steam treatment with steam, to = 42.1, 7 = 1.6 and 7 = 24.2 mJ/m. This means... 

Organic material can be positioned on a clay mineral surface in two ways either by the excheinge of the native inorganic cations by an organic cation (e.g., a quaternary ammonium cation) or by surface adsorption of a neutral organic molecule (e.g., an amine) on the external sinface of the clay peirticles. Tables 9.2 and 9.3 illustrate the changes in surface thermodynamic properties as the coverage of the clay surfaces varies. It is clear that modest amounts... 

In soils, the surface properties and reactivity of the clay fraction are of greater importance than its bulk composition, and infrared spectroscopy has a pecular contribution to make in this field. In materials of high surface area, the vibrations of surface groups can be directly observed. Studies of changes in these vibrations when organic and inorganic molecules are adsorbed on the surface provide information on the mechanism of adsorption. Further information can be obtained from changes in the spectrum of the adsorbed molecule. 

Infrared spectra can provide direct evidence on the chemical and physical processes that lead to adsorption on surfaces, and these reactions may, in turn, serve to differentiate the various types of surface present in such heterogenous systems as soil clays. Changes in the concentration or structure of the adsorbed species on aging or other treatments are readily followed in a single small specimen without the necessity for destructive analyses. The surface properties of clay minerals and other colloids of importance in soil have been studied particularly by Fripat and his colleagues, who have made considerable use of infrared spectroscopy. A review (Fripiat [1964]) of their work in this field has appeared. Studies of adsorbed species on silica, silica-alumina, alumina, and zeolites have been principally concerned with the highly dehydrated systems of interest in catalytic applications. This field has been reviewed by Little [1966] and Hair [1967]. 

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