Electric Field Induced Polarization Effects Measured by in Situ Neutron Spectroscopy

Abstract

Despite the success of electrical stimulation in many areas, including clay or sludge dewatering, extraction of juices from fruit pulp, fracture healing, and targeted drug delivery, the induced transport mechanisms are controlled by unknown factors. While electroosmotic dewatering of clays particles <10 μm is well-known, understanding of how tightly bound water molecules are removed from within the clay interlayers is still incomplete. By performing quasielastic neutron scattering experiments with in situ electric field stimuli on calcium montmorillonite (Ca-Mt) prehydrated at relative humidity (RH) of 58 and 85%, we observed an increase in the water mean residence time, suggesting that hydrogen bonding lifetimes are prolonged under electric field. Assuming that at these RH most all water present resides within the interlayer space of the Ca-Mt, this result indicates strong polarization of the water by the interlayer cation. This electroosmotic induced reorganization has important implications in terms of ion mobility and aqueous chemical reaction mechanisms.

Assessing Diffusion Relaxation of Interlayer Water in Clay Minerals Using a Minimalist Three-Parameter Model

“…However, before we analyze in detail the results presented in Figure 4, it is important to recall that in clay minerals different populations of water molecules, classified according to their characteristic dynamics, coexist in the interlayer. 8,14,15 With this in mind, we can argue that the evolution of α(Q) vs Q corroborates the assumption that long-range processes are strongly coupled to local structural rearrangements (or structural relaxation), 8,17 where the presence of restricted diffusion is accounted for the observation that as Q → 0, α < 1 as depicted in Figure 4(a). 8,18 Of more interest is the consistent difference between α(Q) for Ht and Mt, suggesting different hydrogen relaxation processes in the two clay minerals.…”

“…On the other hand, at low Q values, similarly to observations for human acetylcholinesterase with and without inhibitor, , the lower energy energy barrier and narrower distributions observed for Mt can be related to the reduced EISF, higher dehydration temperature, but lower enthaply, i.e., this barrier behavior reflects that more electrons are shared elsewhere and not in the water molecule. This can be explained by Mt polarizing the water molecules more in the interlayer than Ht, by making the ensemble of water molecules in the interlayer space less capable of hopping between the interlayer cation and the surface of its interlayer, hence not only distorting the water molecule tetragonal symmetry but also holding it longer. , This dynamical difference is fully corroborated by the thermal analysis results, Figure .…”

“…Notably, and as clearly observed in the inserted plot in Figure (b), for Q = 0.4 Å –1 , the proposed model captures the slight difference observed in the spectra of Mt and Ht. However, before we analyze in detail the results presented in Figure , it is important to recall that in clay minerals different populations of water molecules, classified according to their characteristic dynamics, coexist in the interlayer. ,, With this in mind, we can argue that the evolution of α­(Q) vs Q corroborates the assumption that long-range processes are strongly coupled to local structural rearrangements (or structural relaxation), , where the presence of restricted diffusion is accounted for the observation that as Q → 0, α < 1 as depicted in Figure (a). , Of more interest is the consistent difference between α­(Q) for Ht and Mt, suggesting different hydrogen relaxation processes in the two clay minerals. This is better understood by the analysis of the EISF, Figure (b), where we again observe almost the same Q-dependence for both samples, with slightly different magnitudes.…”

“…The larger net negative charge of the Mt layers stems from the fact that two of its three octahedral sites are occupied predominantly by trivalent cations (Al 3+ ) with partial isomorphic substitution of Mg 2+ as well as partial substitution of Al 3+ for Si 4+ in the tetrahedral sheet. On the other hand, for Ht, all three octahedral sites are occupied predominantly by divalent cations (e.g., Mg 2+ ) with some isomorphic substitution of Li + , but nil substitution in the tetrahedral sheet. , …”

“…On the other hand, for Ht, all three octahedral sites are occupied predominantly by divalent cations (e.g., Mg 2+ ) with some isomorphic substitution of Li + , but nil substitution in the tetrahedral sheet. 14,15 QENS is an ideal tool for the study of confined water due to the extraordinarily high incoherent scattering cross-section of the hydrogen atoms, allowing study of the self-diffusion of water, i.e., how the average position of these scattering centers autocorrelate in time, from 10 −9 to 10 −13 s, and space, from 1 to 100 Å. 6,16 Such information is vital for poorly ordered heterogeneous materials such as clay minerals, because competing short-and long-range interactions result in frustration, while complex coupling between different variables leads to non-Markovian, scale-invariant multiple relaxation processes.…”