The Impact of Thermal History on Water Adsorption in a Synthetic Nanolayered Silicate with Intercalated Li⁺ or Na⁺

Abstract

For applications benefitting from the swelling properties of nanolayered silicates (clay minerals), it is of paramount importance to understand the hysteresis in the clay−water interaction. In this context, the present work investigates how the thermal history of Na + -and Li + -intercalated fluorohectorite affects the hydration process. By combining X-ray diffraction and thermogravimetric analysis, water adsorption of preheated and non-preheated fluorohectorite was measured and analyzed in terms of the characteristic interlayer distance. The number of water molecules per cation was also inferred. We find that some of the hydration states in preheated samples are suppressed, and transitions to higher hydration states are achieved at higher relative humidity values. This could be due to the initial water content that facilities crystalline swelling. However, the data for Li-fluorohectorite do not exclude the possibility of a low temperature Hofmann−Klemen effect at 150 °C. Our study also provides strong hints that the so-called 1.5 water layer state, observed in previous studies on smectites, is a metastable state. In addition, the impact of a hydrogenous structure in the interlayer space of Li-fluorohectorite on the clay's hydration behavior is demonstrated. The results, if generalized, would have strong implications on a wide range of applications, where the thermal history of smectites is important.

Experimental methods to study clay minerals and perspective applications of Fluorohectorite

“…All-in-all the very active ongoing scientific work on natural [140,141] and/or synthetic [142,143] , including FHt [144,145,146,147] , and in particular on FHt-B, should extend dramatically the mentioned applications and hint to new ones. FHt is an important material in clay science that definitely underlines the statement that "clays may be considered the material of the 21 st century" [8] .…”

Effect of Temperature on the Diffusion and Sorption of Cations in Clay Vermiculite

“…Some cations (for example, Li + , Na + , Cs + , Ca 2+ , and Sr 2+ ) show a significant increase in the diffusion rate with increasing temperature. ,, In this case, the addition of ions of small radius between the layers of montmorillonite clay decreases the distance between them, which in turn reduces the diffusion coefficient of interlayer cations . At temperatures above 423 K, this can lead to a decrease in the layer’s charge and some of the interlayer cations can migrate to the octahedral position of the silicate layer, neutralizing the negative charge located there (the Hoffmann–Clemen effect). , It is noted that heating can even cause the penetration of all cations into the clay layers. Therefore, the clay layers are in direct contact with each other at temperatures above 473 K …”

“…2 At temperatures above 423 K, this can lead to a decrease in the layer's charge and some of the interlayer cations can migrate to the octahedral position of the silicate layer, neutralizing the negative charge located there (the Hoffmann−Clemen effect). 36,37 It is noted that heating can even cause the penetration of all cations into the clay layers. Therefore, the clay layers are in direct contact with each other at temperatures above 473 K. 38 In this study, the effect of temperature changes, in the range of 275−425 K, on the diffusion of several ions between clay layers was studied.…”

Unmodified Clay Nanosheets at the Air–Water Interface

“…Clay minerals are widely used materials in the industry and one of the most abundant natural materials on earth. 32 Clays, 30 , 33 − 36 that are at the base for several applications, present the ability of swelling. 32 − 35 , 37 Synthetic sodium fluorohectorite (Na-FHt) can be produced with a very high aspect ratio, high homogeneity, and charge distribution with superior quality compared to other clay minerals.…”

“…Clay minerals are widely used materials in the industry and one of the most abundant natural materials on earth . Clays, ,− that are at the base for several applications, present the ability of swelling. − , Synthetic sodium fluorohectorite (Na-FHt) can be produced with a very high aspect ratio, high homogeneity, and charge distribution with superior quality compared to other clay minerals . It is a 2:1 layered phyllosilicate clay where the structural unit is formed by two inverted silicate tetrahedral sheets, sharing their apical oxygen with one trioctahedral sheet in between. , On the trioctahedral sites, a fraction of Mg 2+ ions is substituted by Li + to generate a structural negative charge, , which is compensated by cations attached to the clay surfaces in the interlayer space prior to delamination, see Figure .…”