Facile Scalable Synthesis of Rectorites

Abstract

Using a synthetic Na-fluorohectorite ([Na0.5]inter[Mg2.5Li0.5]oct[Si4]tetO10F2), mixed-cation heterostructures with an alternating interstratification pattern are formed under appropriate conditions via simple cation-exchange reactions. The most important requirement for the spontaneous formation of these ordered mixed-layer structures is a high degree of charge homogeneity of the smectite. The partial exchange of cations with largely different hydration enthalpies (Na+ by K+) results in the regular interstratification of hydrated (d = 12.4 Å) and nonhydrated (d = 10.0 Å) interlayers at 40% relative humidity (RH). By combining analytical methods that are sensitive to either hydrated or collapsed interlayers (selective cation exchange, hydrosorption isotherms, and 23Na MAS NMR spectroscopy), we proposed a novel mechanism that is centered at the interlayers and does not require polar lamellae, as suggested in the literature for rectorite formation. Upon formation of the regular interstratification, the charge density of the interlayers changes from homogeneous to alternating between interlayers in the stacking direction. This simple redistribution of exchangeable interlayer cations is facile and rapid. The cation exchange capacity (CEC) of the collapsed interlayers is higher than the average CEC, while the CEC of the hydrated interlayers will be correspondingly lower. The local CEC of these interlayers deviates by ∼18%−28% from the average CEC, as calculated independently from ion-exchange experiments and hydrosorption measurements. Moreover, this alternating differentiation of the interlayer cation concentration seems to be driven by thermodynamics. Even a mixture of homocationic Na-hectorite and K-hectorite slowly converts to a regularly interstratified material when immersed in water. After the interlayers are differentiated, they may be selectively manipulated, creating dual functional materials, where distinct nanoreactors are separated by a 1-nm-thick insulating lamellae and are arranged in a regular manner.