Counterions clays

The solids were used as catalysts in the benchmark cyclopropanation reaction between styrene and ethyl diazoacetate (Scheme 7). As far as the nature of the clay is concerned, laponite was foimd to be the best support for the catalytic complexes. The best enantioselectivity results (Table 7) were obtained with ligand 6b (69% ee in trans cyclopropanes and 64% ee in cis cyclopropanes) but the recovered solid showed a lower activity and enantioselectivity, which was attributed to partial loss of the chiral ligand from the support. In general, the use of the three chiral ligands led to enantioselectivity results that were intermediate between those obtained in homogeneous phase with CuCl2 and Cu(OTf)2 as catalyst precursors. This seemed to indicate that the sohd behaved as a counterion with an intermediate coordinating abihty to the copper centers. 

The interaction of clay crystals within a domain depends upon the DLVO repulsive pressure in the slit-shaped pores and the balance between repulsive pressure [Pr] from counterion hydration and the attractive pressure [Pa] generated by van der Waals forces and the recently discovered ion-ion correlation attraction between the counterions in the confined space of the overlap pores [see Kjellander et al., 1988a, b]. When Ca Is the counterion, the attractive pressure dominates and the overlap pores are stabilized In a primary potential minimum. However, when the crystal... 

Another important feature of this type of sorption is that the nature of the cations serving as the counterions on the faces of clays affects the intensity of NAC sorption. These minerals bear a net negative charge in their interior due to iso-morphic substitutions (e.g., inclusion of a Al3+ in place of a Si4+ in the crystal lattice). As a result, the clay s exterior is covered by cations like potassium and sodium. NAC sorption is much greater when the clay has adsorbed potassium rather than calcium or sodium. This result has been interpreted to mean that the large hydrated ions of sodium and calcium can block NAC access to sites on a clay s surface, while the much less hydrated potassium ions can serve as counterions without blocking NAC... 

At high levels of i (i.e., [M+] )ex[/]), Eq. 11-55 indicates that the organic counterion concentrations must asymptotically approach a constant value set by the total surface charge density (the cation exchange capacity of the clay) as long as CEC Fvic[co-ion], ex ... 

Frequently substantially more than catalytic amounts of a Lewis acid metal halide are required to effect Friedel-Crafts alkylation. This is due partly to complex formation between the metal halide and the reagents or products, especially if they contain oxygen or other donor atoms. Another reason is the formation of red oils. Red oils consist of protonated (alkylated) aromatics (i.e., arenium ions) containing metal halides in the counterions or complexed with olefin oligomers. This considerable drawback, however, can be eliminated when using solid acids such as clays,97 98 zeolites (H-ZSM-5),99,100 acidic cation-exchange resins, and perfluoro-alkanesulfonic acid resins (Nafion-H).101-104... 

Abstract We use Nuclear Magnetic Resonance relaxometry (i.e. the frequency variation of the NMR relaxation rates) of quadrupolar nucleus ( Na) and H Pulsed Gradient Spin Echo NMR to determine the mobility of the counterions and the water molecules within aqueous dispersions of clays. The local ordering of isotropic dilute clay dispersions is investigated by NMR relaxometry. In contrast, the NMR spectra of the quadrupolar nucleus and the anisotropy of the water self-diffusion tensor clearly exhibit the occurrence of nematic ordering in dense aqueous dispersions. Multi-scale numerical models exploiting molecular orbital quantum calculations, Grand Canonical Monte Carlo simulations, Molecular and Brownian Dynamics are used to interpret the measured water mobility and the ionic quadrupolar relaxation measurements. 

NMR measurements is performed by numerical simulations with a multi-scale modelling [11] of the structure of the clay dispersions and the diffusion of the water molecules or the sodium counterions, by using Brownian Dynamics in order to bridge the gap between the time scale accessible by Molecular Dynamics (typically a few ps) and that explored by the NMR measurements (from ns to ms). 

Counterions are the ions required to maintain electroneutrality of the charged clay particles. 

Another descriptor of the mobility of water molecules in contact with the clay layers is the water self-diffusion coefficient. A fine recent review summarizes the theoretical and practical aspects of measurement by spin-echo nmr methods of this parameter (36) The plot of the decrease in the water self-diffusion coefficient as a function of C, the amount of suspended clay, for the same samples, is again a straight line going through the origin. By resorting once more to a similar analysis in terms of a two-state model (bound and "free water), one comes up (25) with a self-diffusion coefficient, for those water molecules pinched in-between counterions and the clay surface, of 1.6 10 15 m2.s 1,... 

MC and MD studies of hydrated smectites with monovalent counterions Li+, Na+, K+, Cs+ were also performed [62, 63, 69, 70, 72, 77-80], An increase of the simulation cell size of 2 1 Na-saturated clay or alternation of its shape from rectangular did not have a significant effect on the calculated interlayer properties [70]. It has been revealed that the mechanism of swelling and hydration depends upon the interlayer ion charge. Also the greater role of the clay mineral surface in organizing interlayer water in the case of K-montmorillonite with a weakly solvating counterion was concluded [64, 68]. 

Using MC simulations Delville and co-workers have investigated the clay-water interface [83-87], The number of hydration layers (2-3) increases suddenly during the swelling process [85]. For hydrated montmorillonite with interlayer sodium counterions it was determined that the water content of the pore is a function of the interlamellar distance. Water molecules are layered in successive shells, whose number (1-4) depends on the available interlayer space [87]. The MD study of structure of water in kaolinite [88] has indicated two types of adsorbed water molecules according to different orientations with respect to the structure of clay sheets with HH vector parallel or perpendicular to the surface. 

In addition to MC and MD simulations, several quantum-chemical studies of hydrated clay minerals with or without exchangeable cations in the interlayer space have been performed at the ab initio and semiempirical level of theory. The total energy of hydrated layered silicates (talc and pyrophyllite) without exchangeable counterions, the position and interactions of interlayer water... 

FIG. 3 Cation surface species on the basal planes of 2 1 layer type clay minerals. Inset indicates the spectroscopic methods used to quantify counterion surface species, their intrinsic time scales over which molecular structure is probed, and the residence time of surface species. 

FIG. 9 Representation of counterion approach to the clay mineral surface in the modified Gouy-Chapman (MGQ model. 

The n-butylammonium vermiculite system is an example of a three-component system of a monodisperse colloid, electrolyte and solvent. There are four constituents in the macroionic solution — the negatively charged clay plates, n-butylammonium ions (counterions), chloride ions (co-ions), and water — but these may not vary independently because they are subject to the restriction that... 

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