Non-Arrhenian Ionic dc Conductivity of Homoionic Alkali Exchanged Montmorillonites with Low Water Loadings

Abstract

Dc conductivity of the whole series of homoionic alkali exchanged montmorillonites is investigated by means of Complex Impedance Spectroscopy. Conductivity of the samples is measured at the dry state and at various water loadings between 0 and 6 absorbed water molecules per cation. Dc conductivity of all the dehydrated samples follows an Arrhenius behavior. In contrast, the hydrated samples exhibit a non-Arrhenius temperature dependence of dc conductivity that is fruitfully fitted by using the VTF's empirical law. It is then shown that the critical temperature, T VTF , increases with water loading until the later equals approximately 3 for the Li + and Na + samples and higher values for the K + , Rb + , and Cs + samples. So, it appears that the departure from the Arrhenius behavior is directly related to the number of water molecules in interaction with the alkali extra-framework cations. For water loadings higher than 3, activation energy for dc conductivity tends to values of about 0.2-0.3 eV for all samples independently on the alkali extra-framework cations. In contrast, activation energy appears to be very sensitive to the considered alkali sample for water loadings lower than 1 and, in that case, closely related to the dehydration enthalpy of the samples.

Ion exchange selectivity in clay is controlled by nanoscale chemical–mechanical coupling

“…The initial, transient regime of collapse from a uniform population of 3-water layer Na-MMT immediately following mixing provides crucial insight into the dynamic equilibration process. Both the rate and extent of collapse are largely invariant with temperature (SI Appendix), which is consistent with the non-Arrhenian cation diffusion previously observed for MMT (13,35), but inconsistent with other processes that could be proposed to limit collapse. We propose that diffusional demixing within the interlayer is the first step in the process of collapse, both in the initial transient regime and during the subsequent period of dynamic equilibrium, and is promoted through positive feedback (36) between the local chemical composition and the interlayer spacing (Fig.…”

Cooperative chemical-mechanical interactions during ion exchange promote rotational ordering in hydrated montmorillonite

“…51 Regions that become enriched with potassium exhibit weaker hydration that promotes the recruitment of more potassium, and the opposite is true of sodium. 11,12 Both f and a are largely invariant with temperature, which is consistent with the non-Arrhenian cation diffusion previously observed for MMT, 29,52 but inconsistent with most other processes likely to limit collapse. Thus, diffusional separation of sodium and potassium within the interlayer are required to initiate collapse.…”

Atomic scale characterization and surface chemistry of metal modified titanate nanotubes and nanowires

“…However, several different trends were described already. The activation energy of conduction decreases with increasing surface coverage in certain oxides [107,133,163], clay materials [163,164], zeolites [154,160,163], layered zirconium phosphonates [140], amine-intercalated titanium(IV)-phosphate [139] and proteins [117,161]. It increases (or stays almost constant) in collagene [159] and in zeolite [169] and silica/ alumina catalyst systems [155], and it changes as a maximum function in certain textiles [116] and Vycor-glass [172].…”