Melt Synthesis, Structural, Characterization and Scaling of Swelling 2:1-Layer Silicate Materials

Melt synthesis, characterization, and refinement of single crystal structures of swelling 2:1-layer silicates were the main fundamental topics of the presented thesis. In particular, large scale syntheses of both lithium and sodium fluorohectorite were successfully achieved. Furthermore, the crystal structure of one-, and two-layer hydrate of sodium fluorohectorite and the one-layer hydrate of sodium brittle mica were thoroughly investigated and characterized in detail. Swelling sodium fluorohectorite with good crystallinity in an ideal composition of Na0.85[Mg2.15Li0.85]Si4O10F2 was synthesized for investigating the hydrated structure. Melt synthesis was done in closed molybdenum crucibles using pure reagents (glass with composition Na2O-2SiO2, Li2SiO3 MgF2, MgO, SiO2). The crystal structures of one- and two-layer hydrate of sodium fluorohectorite were studied. The one-layer hydrate of sodium fluorohectorite (at relative humidity 45 %) showed two planes of interlayer sodium along [100]. The two-layer hydrate of sodium fluorohectorite showed sodium interlayer cations being located in the middle of the interlayer. In addition, sodium brittle mica with a target composition Na4[Mg6]Si4Al4O20F4 was successfully synthesized via melt synthesis in a gas tight molybdenum crucible and the refinement of the one-layer hydrate of sodium brittle mica was done. The synthetic sodium brittle mica swells only to the one-layer hydrate and could not be further hydrated to the two-layer hydrate. Generally, natural swelling layer silicates (smectites) usually contain impurities such as iron oxide (pigmentation material), quartz, and carbonate. However, these impurities hinder the employment of swelling layer silicates in industry for cutting edge and advanced applications. In addition, they suffer from small particle size under 5 µm limiting their aspect ratio. For industrial applications, pure synthetic swelling layer silicates with superior properties are highly desirable. Therefore, a large scale synthesis of sodium fluorohectorite Na0.6[Mg2.4Li0.6]Si4O10F2 was carried out in three steps. (i) Synthesis of glass, glass was used as precursor and low melting agent, the amorphous glass with composition Na2O-Li2O-6SiO2 was synthesized from sodium carbonate Na2CO3, lithium carbonate Li2CO3, and silicic acid SiO2∙nH2O via melt synthesis in an open glassy carbon crucible at 1075 °C under flowing argon in a high frequency induction furnace, where the temperature was increased with a constant rate of 300°C/hr. (ii) dehydration and decarboxylation of silicic acid SiO2∙nH2O and magnesium basic carbonate MgCO3∙Mg(OH) respectively at 900 °C for one hour in a corundum crucible in a chamber furnace. (iii) Mixing and melting the glass, the material obtained by dehydration and decarboxylation of SiO2∙nH2O and MgCO3∙Mg(OH)2 together with magnesium fluoride to achieve a composition of Na0.6[Mg2.4Li0.6]Si4O10F2. The total mixture was transferred into a glassy carbon crucible and melted at 1265 °C under argon for 15 min. The synthetic sodium fluorohectorite showed uniform and high intracrystalline reactivity, represented a pure phase, which was colorless and of good crystallinity. High aspect ratio layer silicates would be an optimum functional material for future application in polymer layered silicate nanocomposites. Delamination via osmotic swelling is known in laponite-type clays. High hydration energy of the interlayer cation, such as lithium can force layer silicates to swell infinitely and delaminate. Consequently, the lithium fluorohectorite with variable layer charge was synthesized via melt synthesis in an open glassy carbon crucible in a high frequency induction furnace. The same procedure used for sodium fluorohectorite was applied for lithium fluorohectorite, where the glass with composition Li2O-2SiO2 was prepared via reaction of lithium carbonate with silicic acid at 1200 °C for 1hr. Due to the high fugacity of lithium fluoride, excess of one mole Li and F was added via lithium silicate and magnesium fluoride respectively. The raw material of lithium fluorohectorite was melted at 1350 °C for 10 min. The synthetic lithium fluorohectorite showed uniform intracrystalline reactivity, came in large well crystalline tactoids and completely delaminated to a single silicate layers in water. The lithium fluorohectorite behavior reveals that these materials have high potential for barrier application and flame retardancy. Furthermore, the lithium fluorohectorite was synthesized in large scale

Tailoring the Pore Sizes of Microporous Pillared Interlayered Clays through Layer Charge Reduction

A F-rich potassium hectorite, [K_0.48(2)]^inter[Mg_2.54(8)Li_0.43]^oct[Si₄]^tetO₁₀F₂, with a layer charge of <i>x</i> = 0.48 per formula unit (pfu) was synthesized by high temperature melt synthesis. After Mg-exchange, the layer charge could be reduced significantly post synthesis by annealing (250 °C) as confirmed by alkylammonium exchange and cation exchange capacity. By pillaring this new low charge material with Me₂DABCO²⁺ (<i>N</i>,<i>N</i>-dimethyl-1,1-diazabicyclo [2.2.2]­octane dication) and Rh­(bpy)₃³⁺ (rhodium-tris-2,2′-bipyridin trication), we observed a remarkable increase in micropore volume and pore diameter by Ar/Ar­(l) physisorption measurements. This method allows the tailoring of pore sizes of pillared clays by reducing the layer charge and consequently the pillar density

Onset of Osmotic Swelling in Highly Charged Clay Minerals

Delamination by osmotic swelling of layered materials is generally thought to become increasingly difficult, if not impossible, with increasing layer charge density because of strong Coulomb interactions. Nevertheless, for the class of 2:1 layered silicates, very few examples of delaminating organo-vermiculites were reported in literature. We propose a mechanism for this repulsive osmotic swelling of highly charged vermiculites based on repulsive counterion translational entropy that dominates the interaction of adjacent layers above a certain threshold separation. Based on this mechanistic insight, we were able to identify several organic interlayer cations appropriate to delaminate highly charged, vermiculite-type clay minerals. These findings suggest that the osmotic swelling of highly charged organoclays is a generally applicable phenomenon rather than the odd exemption

Nanoplatelets of Sodium Hectorite Showing Aspect Ratios of ≈20 000 and Superior Purity

Applying a combination of melt synthesis followed by long-term annealing a fluorohectorite is obtained which is unique with respect to homogeneity, purity, and particle size. Counterintuitively, the hectorite undergoes a disorder-to-order transition upon swelling to the level of the bilayer hydrate. Alkylammonium-exchanged samples show at any chain length only a single basal spacing corroborating a nicely homogeneous layer charge density. Its intracrystalline reactivity improves greatly upon annealing, making it capable to spontaneously and completely disintegrate into single clay lamellae of 1 nm thickness. Realizing exceptional aspect ratios of around 20 000 upon delamination, this synthetic clay will offer unprecedented potential as functional filler in highly transparent nanocomposites with superior gas barrier and mechanical properties

Two-Step Delamination of Highly Charged, Vermiculite-like Layered Silicates via Ordered Heterostructures

Because of strong Coulomb interactions, the delamination of charged layered materials becomes progressively more difficult with increasing charge density. For instance, highly charged sodium fluorohectorite (Na_0.6Mg_2.4Li_0.6Si₄O₁₀F₂, Na-Hec) cannot be delaminated directly by osmotic swelling in water because its layer charge exceeds the established limit for osmotic swelling of 0.55 per formula unit Si₄O₁₀F₂. Quite surprisingly, we found that this hectorite at the border of the smectite and vermiculite group can, however, be utterly delaminated into 1-nm-thick platelets with a high aspect ratio (24 000) in a two-step process. The hectorite is first converted by partial ion exchange into a one-dimensionally ordered, interstratified heterostructure with strictly alternating Na⁺ and <i>n</i>-butylammonium (C4) interlayers. This heterostructure then spontaneously delaminates into uniform single layers upon immersion in water whereas neither of the homoionic phases (Na-Hec and C4-Hec) swells osmotically. The delamination of more highly charged synthetic layered silicates is a key step to push the aspect ratio beyond the current limits

The nature of laponite: pure hectorite or a mixture of different trioctahedral phases?

Summarization: A series of laponites and synthetic OH-and fluorinated hectorites prepared from hydrothermal and melting experiments at both industrial and laboratory scale were examined with XRD and FTIR (MIR and NIR) to determine their mineralogical composition and possible compositional heterogeneity. The end materials contained both Li-and Na-bearing phases. The industrial hydrothermal OH-smectites prepared at low temperatures consist of random mixed layer hectorite-stevensite-kerolite with about 40–50% hectorite layers, the remaining being stevensite and kerolite at roughly equal proportions. The FTIR spectra of these smectites contain, besides the main Mg3 OH stretching/overtone bands at 3695–3690 and 7225–7214 cm−1, respectively, additional OH overtone bands at ~3716 and 7265 cm−1 (hydrated state). These bands might be linked to Mg2 LiOH stretching modes. The melt-derived smectites are kerolite-free but still contain stevensite layers, although the preparation methods involved heating in the excess of 1000◦ C. In these smectites Li might be partitioned to both octahedral and interlayer sites. Subsequent annealing of the melt-derived Mg-Li smectites caused migration of the exchangeable Li to the vacant octahedral due to the Hofmann-Klemen effect and thus decrease of the layer charge, as was indicated by the νO-D method. Hydrothermal synthesis of Mg-Li smectites at high temperature (400◦ C) and pressure (1 kbar), yielded pure hectorite without stevensite or kerolite domains.Presented on: Mineral

Large Scale Self-Assembly of Smectic Nanocomposite Films by Doctor Blading versus Spray Coating: Impact of Crystal Quality on Barrier Properties

Flexible transparent barrier films are required in various fields of application ranging from flexible, transparent food packaging to display encapsulation. Environmentally friendly, waterborne polymer–clay nanocomposites would be preferred but fail to meet in particular requirements for ultra high water vapor barriers. Here we show that self-assembly of nanocomposite films into one-dimensional crystalline (smectic) polymer–clay domains is a so-far overlooked key-factor capable of suppressing water vapor diffusivity despite appreciable swelling at elevated temperatures and relative humidity (R.H.). Moreover, barrier performance was shown to improve with quality of the crystalline order. In this respect, spray coating is superior to doctor blading because it yields significantly better ordered structures. For spray-coated waterborne nanocomposite films (21.4 μm) ultra high barrier specifications are met at 23 °C and 50% R.H. with oxygen transmission rates (OTR) < 0.0005 cm³ m^–2 day^–1 bar^–1 and water vapor transmissions rates (WVTR) of 0.0007 g m^–2 day^–1. Even in the most challenging environments (38 °C and 90% R.H.), values as low as 0.24 cm³ m^–2 day^–1 bar^–1 and 0.003 g m^–2 day^–1 were found for OTR and WVTR, respectively