Abstract:Specular reflectance infrared and electron spin resonance spectroscopies were employed for the analysis of the sites of Li(), Cu() and Cd() cations fixed in the structure of montmorillonite from Jels ˇovy ´Potok (Slovakia) upon heating for 24 h at 300 °C. Li() cations are trapped in two different sites: in the previously vacant octahedra and in the hexagonal holes of the tetrahedral sheet. Cu() cations are fixed deep in the hexagonal holes. They substantially affect the vibration modes of Si-O bonds an… Show more
“…Such sites can occur in the structure or at the inner surface, i.e. interlayers and hexagonal cavities of the hectorite (Clementz et al, 1973;Luca et al, 1991;Karakassides et al, 1999;Spagnuolo et al, 2004). The spectral parameters g || = 2.42 and A || = 10 mT, obtained from low-temperature measurements, are similar to the parameters determined by Luca et al (1991) and assigned to structure-bound Cu(II).…”
Section: Cu In the Modified Starting Materialssupporting
Abstract-The influence of Cu(II) on the hydrothermal and thermal transformations of a synthetic hectorite was investigated by a combined approach using mainly X-ray diffraction, thermal analyses, and electron paramagnetic resonance spectroscopy. The presence of Cu(II) during hydrothermal treatment increased the crystallite size. Copper (II) was both structure-bound and associated with the inner surfaces of the particles. Upon heating, structural destabilization of the hectorite began at~400ºC as indicated by the formation of free radicals. Between 600 and 700ºC, the hectorite converted to enstatite, and in the presence of Cu(II), to enstatite and richterite. The formation of richterite as an additional conversion product is explained by the creation of structural weakness due to structure-bound Cu(II) in F-containing hectorite. Our results suggest that traces of Cu(II), typical of natural environments, may influence the conversion products in high-temperature geochemical systems.
“…Such sites can occur in the structure or at the inner surface, i.e. interlayers and hexagonal cavities of the hectorite (Clementz et al, 1973;Luca et al, 1991;Karakassides et al, 1999;Spagnuolo et al, 2004). The spectral parameters g || = 2.42 and A || = 10 mT, obtained from low-temperature measurements, are similar to the parameters determined by Luca et al (1991) and assigned to structure-bound Cu(II).…”
Section: Cu In the Modified Starting Materialssupporting
Abstract-The influence of Cu(II) on the hydrothermal and thermal transformations of a synthetic hectorite was investigated by a combined approach using mainly X-ray diffraction, thermal analyses, and electron paramagnetic resonance spectroscopy. The presence of Cu(II) during hydrothermal treatment increased the crystallite size. Copper (II) was both structure-bound and associated with the inner surfaces of the particles. Upon heating, structural destabilization of the hectorite began at~400ºC as indicated by the formation of free radicals. Between 600 and 700ºC, the hectorite converted to enstatite, and in the presence of Cu(II), to enstatite and richterite. The formation of richterite as an additional conversion product is explained by the creation of structural weakness due to structure-bound Cu(II) in F-containing hectorite. Our results suggest that traces of Cu(II), typical of natural environments, may influence the conversion products in high-temperature geochemical systems.
“…However, some uncertainty still exists over the precise location of the lithium ions within the clay framework. It has been suggested that the cations migrate from the interlayer (Figure a) into the ditrigonal cavities of the tetrahedral layer (Figure b), − the vacant octahedral sites (Figure c), − or both of these. − 1 Lithium montmorillonite unit cell. Upon thermal treatment, the lithium cations are thought to migrate from the interlayer (a) into the ditrigonal cavities of the tetrahedral layer (b), the vacant octahedral sites (c), or both of these.…”
The Hofmann-Klemen effect describes the observed reduction in expandable character and cation exchange capacity of lithium-saturated montmorillonite upon mild heating. Although it is widely accepted that the observed changes are due to the lithium ions becoming fixated within the layer structure of the clay mineral, some uncertainty still exists as to the precise location of the migrated lithium ions within the clay framework. It has been suggested that the cations reside in the ditrigonal cavities of the tetrahedral layer, the vacant octahedral sites, or both. We employ density functional methods to examine the phenomenon at the electronic structure level. We find that it is energetically preferable for lithium cations to reside in the vacant octahedral sites as opposed to the ditrigonal cavities due to the closer proximity to the negative charge sites in the octahedral layer increasing favorable Coulombic interactions. Occupation of octahedral sites causes structural hydroxyl groups to reorientate perpendicular to the ab plane. Deprotonation and dehydroxylation are not observed during molecular dynamics at experimentally reported temperatures. Dehydroxylation inhibits rather than facilitates cation migration. A comparison of calculated power spectra with experimental infrared spectra suggests that lithium cations do not migrate to ditrigonal cavities when no tetrahedral aluminum is present.
“…The peak broadening of the hybrids compared to the neat clays also suggests that surface modification induces a stacking disorder in the clay particles. The collapsed spacing observed for NAN2SI6 is due to dehydration and reversible migration of the sodium ions to the hexagonal cavities of the tetrahedral silicate sheets upon thermal drying . However, no collapse was observed for the fluorohectorite layers after the same treatment.…”
Section: Resultsmentioning
confidence: 92%
“…In addition, the absorption bands in the low region spectra (1000−500 cm -1 ) become more distinct than those present in the pristine clays. These peaks can be assigned to the Si−O groups of the clay framework and grafted organosiloxane. , No changes are observed above 3500 cm -1 , where typically the −OH stretching vibration appears. The −OH groups that are buried within the layered framework overwhelm the number of edge-located groups.…”
Reaction of bis(trimethoxysilyl)hexane, (CH 3 O) 3 Si(CH 2 ) 6 Si(OCH 3 ) 3 , and the protonated salt of 3-aminopropyltriethoxysilane, (C 2 H 5 O) 3 Si(CH 2 ) 3 NH 3 + Cl -, with different clay aqueous colloidal dispersions produces a network of clay platelets cross-linked on their edges by the corresponding organosiloxanes. In the case of the R,ω-bridging organosiloxane, where the surface modification of the clay by the silane proceeds on its outer surfaces, cross-linking results in a gel network that extends to the whole volume of the initial aqueous dispersion. Drying of this gel and grinding of the as-formed, relatively hard specimens affords fine hybrid powders consisting of cross-linked clay particles that inherit the swelling, intercalation, and ion-exchange properties of the starting clay. Controlling both the composition of the initial suspension (clay type, solvent, and concentration) and the drying process, enables the fabrication of monolithic clay hybrids. In the case of the protonated aminosiloxane, the surface modification of the clay takes place both within its interlayer space and at its edges. The former leads to the formation of silsequioxane pillars within the clay galleries. The latter produces stiff yet ductile monoliths upon drying via condensation of edge-modified adjacent clay layers. When Laponite and (CH 3 O) 3 Si(CH 2 ) 3 N(CH 3 ) 3 + Clare used, optically transparent monoliths (or clay glasses) are obtained.
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