Temperature effects on the diffusion of water and monovalent counterions in the hydrated montmorillonite

We investigate here the effect of temperature on the diffusion of water and cations in the Wyoming-type montmorillonite clay. The considered cations are monovalent compensating ions, such as Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺ in one-, two- and three-hydration states. For this purpose, molecular dynamics simulations have been performed to obtain the dynamic behaviour regarding the interlayer ions and water molecules under a temperature range between 260 and 400 K. The diffusion coefficient of water and cations in different hydrated clays increases with temperature. The influence of temperature on the diffusion of water is much greater than that of cations in one-, two- or three-hydrated clay. The degree of hydration plays an important role on the diffusion behaviour of water and counterions. We found that the effect of temperature is negligible in weakly hydrated clay, whereas it became significant in highly hydrated one. Besides, the size and mass of cations’ hydrate also affect the diffusion behaviour of water and cations in the interlayer space of hydrated clay.     

How water and counterions diffuse into the hydrated montmorillonite

Molecular dynamics simulations have been performed to study the diffusive transport properties of water and counterions in 1-, 2- and 3-layer hydrated Wyoming-type montmorillonite (MMT). The considered counterions included in the clay are monovalent cations, such as Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and divalent cations, such as Ca²⁺, Ni²⁺, Zn²⁺ and Pb²⁺. The diffusion simulation results of Li-, Na-, K-, Cs-MMT, based on NVE ensemble and SPC/E water model, compared well to available experimental quantities and previous simulations, which permit us to study the diffusion behaviour of heavy metal counterion including in the different hydrated montmorillonite. Our simulation results show that the diffusion coefficients of both water molecule and counterions increase versus the quantity of water content. At the same hydrated state, the diffusion of cations strongly depends on their size, weight and capacity of attracting water. Monovalent cations diffuse always much faster than divalent ones. Although the increasing of hydrate degree can raise the diffusion coefficient of divalent heavy cations, the activity of these heavy counterions is still very weak. The diffusion of cations in the interlayer space of clay is much weaker than that in bulk water. That means the presence of charged clay has great influence on the diffusion of cations, which change totally the diffusion behaviour of cations.     

Temperature effect in a montmorillonite clay at low hydration—microscopic simulation

The effect of temperature in the range 0–150°C was studied for homo-ionic montmorillonite clays with Na⁺ and Cs⁺ compensating ions in low hydration states. Monte Carlo and molecular dynamics simulations were employed to provide both static and dynamic information concerning the interlayer ions and water molecules, and emphasis was laid on the temperature activation of the diffusion coefficients. Principal structural changes were limited to the interlayer water phase. In the monohydrated systems, neither of the cations was seen to enter into the hexagonal cavities of the clay. Cs⁺ exhibited clear site-to-site diffusion between sites allowing coordination to six oxygen atoms of the clay sheets, this behaviour persisting to high temperatures. Preferential sites for the Na⁺ counterion were much less well-defined, even at low temperatures. The behaviour of the water phase in the monohydrated states was similar for the two ions. A rapid approach to bulk dynamics was seen in the transition from monohydrated to bihydrated Na-montmorillonite. A detailed quantitative comparison of the temperature activation of diffusion for a two-dimensional water phase and three-dimensional bulk water is presented for the first time.     

Swelling of K+, Na+ and Ca2+-montmorillonites and hydration of interlayer cations: a molecular dynamics simulation

This paper performs molecular dynamics simulations to investigate the role of the monovalent cations K, Na and the divalent cation Ca on the stability and swelling of montmorillonite. The recently developed CLAYFF force field is used to predict the basal spacing as a function of the water content in the interlayer. The simulations reproduced the swelling pattern of these montmorillonites, suggesting a mechanism of their hydration different (K+ 相似文献   

Molecular dynamics simulations of carbon dioxide hydrate growth in electrolyte solutions of NaCl and MgCl2

Molecular dynamics simulations are performed to study the growth of carbon dioxide (CO₂) hydrate in electrolyte solutions of NaCl and MgCl₂. The kinetic behaviour of the hydrate growth is examined in terms of cage content, density profile, and mobility of ions and water molecules, and how these properties are influenced by added NaCl and MgCl₂. Our simulation results show that both NaCl and MgCl₂ inhibit the CO₂ hydrate growth. With a same mole concentration or ion density, MgCl₂ exhibits stronger inhibition on the growth of CO₂ hydrate than NaCl does. The growth rate of the CO₂ hydrate in NaCl and MgCl₂ solutions decreases slightly with increasing pressure. During the simulations, the Na⁺, Mg²⁺, and Cl^? ions are mostly excluded by the growing interface front. We find that these ions decrease the mobility of their surrounding water molecules, and thus reduce the opportunity for these water molecules to form cage-like clusters toward hydrate formation. We also note that during the growth processes, several 5¹²6³ cages appear at the hydrate/solution interface, although they are finally transformed to tetrakaidecahedral (5¹²6²) cages. Structural defects consisting of one water molecule trapped in a cage with its hydrogen atoms being attracted by two Cl^? ions have also been observed.     

Hofmeister series and ionic effects of alkali metal ions on DNA conformation transition in normal and less polarised water solvent

Normal and less polarised water models are used as the solvent to investigate Hofmeister effects and alkali metal ionic effects on dodecamer d(CGCGAATTCGCG) B-DNA with atomic dynamics simulations. As normal water solvent is replaced by less polarised water, the Hofmeister series of alkali metal ions is changed from Li⁺ > Na⁺ ? K⁺ ? Cs⁺ ? Rb⁺ to Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺. In less polarised water, DNA experiences the B→A conformational transition for the lighter alkali metal counterions (Li⁺, Na⁺ and K⁺). However, it keeps B form for the heavier ions (Rb⁺ and Cs⁺). We find that the underlying cause of the conformation transition for these alkali metal ions except K⁺ is the competition between water molecules and counterions coupling to the free oxygen atoms of the phosphate groups. For K⁺ ions, the ‘economics’ of phosphate hydration and ‘spine of hydration’ are both concerned with the DNA helixes changing.     

Diffusion of ions in supercritical water: Dependence on ion size and solvent density and roles of voids and necks

We have performed molecular dynamics simulations of alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and halide (F⁻, Cl⁻, Br⁻, I⁻) ions in supercritical water at 673 K. The calculations were done for water at three different densities of 1.0, 0.7 and 0.35 g cm⁻³ to investigate the effects of solute size on the diffusion of ions in supercritical water. On increase of ion size, we observe a maximum for diffusion of ions in supercritical water of higher densities (1.0 and 0.7 g cm⁻³). However, no such maximum is found for ion diffusion in the supercritical water of low density (0.35 g cm⁻³) or for diffusion of neutral solutes at all densities. These results are analyzed in terms of passage through voids and necks present in supercritical water. Correlations of the observed diffusion behavior with the sizes of ions and voids present in the systems are discussed.     

Energy of migration of monovalent ions in NaCl: An experimental and theoretical study

Experimental diffusion measurements show that migration enthalpies of Cl^?, Br^? and I^? in NaCl are comparable, while that of F^? is considerably lower. Earlier studies had shown that migration enthalpies of Na⁺, K⁺, Rb⁺ and Cs⁺ in NaCl were similar. The polarised point ion model predicts migration energies of ions (by vacancy mechanism) to monotonically increase with ion size, contrary to experiment. Inversely, the shell model calculations rightly predict the variation of migration energies with ionic size. Thus, migration energies by vacancy mechanism do not vary significantly for ions larger than the host ions. However, in the case of the small ions, Li⁺ and F^?, the migration energies by vacancy mechanism are much lower and in good agreement with experiment for F^?.     

Nano-sized local magnetic field induced by circular motion of ions and molecules in a nanotorus under gigahertz rotating electric fields

Recently, we reported molecular dynamics simulations of stable cyclotron motions of ions and water molecules in a carbon nanotorus, induced by different rotating electric fields (EFs). This study is devoted to the calculation and characterisation of the magnetic field (MF) induced by these cyclotron motions. Results show that carbon nanotorus containing ions or water molecules acts as an EF-to-MF transducer. Components of the instantaneous induced MF show large-scale oscillations superimposed by strong fluctuations arising respectively from overall circular motion and random collisions of moving species. Analysis of the space-dependencies of the induced MF components shows that the induced MF is maximum at the centre of the nanotorus. The MF induced by cyclotron motion of ions follows the orders B(Ca²⁺)?>?B(Na⁺)?≈?B(K⁺) at E?=?1.0?V/nm and B(E?=?1.0?V/nm)?>?B(E?=?0.5?V/nm)?>?B(E?=?0.1?V/nm). The time-averaged induced MF of the cyclotron motion of 81 water molecules is almost 10² times stronger than that of ions. The induced MF strength is decreased with increasing distance from nanotorus and decays effectively at about 17.3–18.1 and 15.9–18.2?nm along the z-axis of the nanotorus for ions and water molecules, respectively. The magnitude of the MF induced by cyclotron motions of water molecules and ions, respectively, decreases and increases in the nanotorus with freed carbon atoms.