Aromaticity cannot be measured directly by any physical or chemical experiment because it is not a well-defined magnitude. Its quantification is done indirectly from the measure of different properties that are usually found in aromatic compounds such as bond length equalisation, energetic stabilisation, and particular magnetic behaviour associated with induced ring currents. These properties have been used to set up the myriad of structural-, energetic-, and magnetic-based indices of aromaticity known to date. The cyclic delocalisation of mobile electrons in two or three dimensions is probably one of the key aspects that characterise aromatic compounds. However, it has not been until the last decade that electron delocalisation measures have been widely employed to quantify aromaticity. Some of these new indicators of aromaticity such as the PDI, FLU, ING, and INB were defined in our group. In this paper, we review the different existing descriptors of aromaticity that are based on electron delocalisation properties, we compare their performance with indices based on other properties, and we summarise a number of applications of electronic-based indices for the analysis of aromaticity in interesting chemical problems.

Quantifying aromaticity with electron delocalisation measures

Advances in Clayff Molecular Simulation of Layered and Nanoporous Materials and Their Aqueous Interfaces

The charged surfaces of micaceous minerals, especially illite, regulate the mobility of the major radioisotopes of Cs (134Cs, 135Cs, 137Cs) in the geosphere. Despite the long history of Cs adsorption studies, the nature of the illite surface sites remains incompletely understood. To address this problem, we present atomistic simulations of Cs competition with Na for three candidate illite adsorption sites - edge, basal plane, and interlayer. Our simulation results are broadly consistent with affinities and selectivities that have been inferred from surface complexation models. Cation exchange on the basal planes is thermodynamically ideal, but exchange on edge surfaces and within interlayers shows complex, thermodynamically non-ideal behavior. The basal planes are weakly Cs-selective, while edges and interlayers have much higher affinity for Cs. The dynamics of NaCs exchange are rapid for both cations on the basal planes, but considerably slower for Cs localized on edge surfaces. In addition to new insights into Cs adsorption and exchange with Na on illite, we report the development of a methodology capable of simulating fully-flexible clay mineral nanoparticles with stable edge surfaces using a well-tested interatomic potential model.

https://escholarship.org/content/qt0q69d0fj/qt0q69d0fj.pdf?t=pew7ys

Molecular dynamics simulations of cesium adsorption on illite nanoparticles

XRD and ATR/FTIR investigations of various montmorillonite clays modified by monocationic and dicationic imidazolium ionic liquids

A review on clay chemistry, characterization and shale inhibitors for water-based drilling fluids

Molecular dynamics study of montmorillonite crystalline swelling: Roles of interlayer cation species and water content

Montmorillonite is a clay mineral and the main component in bentonite clay, which is utilized in various applications including its planned use as a buffer material for long-term nuclear waste disposal. In the present paper, a quantum chemical study is presented providing an insight into montmorillonite structure and its surface chemistry, which plays a key role in understanding montmorillonite behavior at the molecular level. A model is first designed by calculating the positions of Mg-substitutions in the octahedral sheet of the layer structure. This model is then used to study (1) charge distribution in the system and (2) the energetics of Na+/Ca2+ cation adsorption on the interlayer surfaces. The results show and verify that the Mg-substitutions are positive charge deficits and the only significant charge defects in the structure. Therefore, the energetics of cation adsorption is found to correlate linearly with the inverse distances between cations and Mg-substitutions in a dry, fully periodic montmorillonite lattice.

Montmorillonite interlayer surface chemistry: effect of magnesium ion substitution on cation adsorption

Phyllosilicates and related clay minerals are of interest due to a variety of technological applications and impact on natural soils. The important properties of these layered minerals arise from their surface chemistry, and therefore understanding the characteristics of their surfaces is desirable. The common focus has been on the basal surfaces, whereas the edge surfaces are little studied. One of the underlying reasons is that the edge surfaces exhibit various possible geometries making it difficult to assume a certain structure with a confidence. The present paper is dedicated to the stability of the edge structures and introduces the largest quantum chemical study on the subject to date. Pyrophyllite was used as a model species. Edge stabilities were determined as cleavage energies, including edge termination by dissociative sorption of water with variable proton configurations. The results show similar stabilities for various edge structures parallel to the (010), (130), (110) and (100) lattice planes, but the edges cleaved by cutting the fewest bonds are suggested to be the most stable on the basis of free energy estimation. The dominant edge is predicted to appear on the (110) crystal face.

Stability of dioctahedral 2:1 phyllosilicate edge structures based on pyrophyllite models

The swelling pressure of montmorillonite is an important factor in designing the bentonite buffer for geo-disposal of nuclear waste. A method to predict the swelling pressure of montmorillonite based on molecular dynamic simulations has been presented. A model structure of a Na-montmorillonite clay was put into contact with pure water, sodium chloride, and calcium chloride solutions. By imitating the experimental determination of the swelling pressure of bentonite clay, a spring sensor was designed to probe the swelling pressure. The modeled swelling pressure and its sensitivity to the dry density of clay were found to be in agreement with experimental data. The model provides an atomic level computational approach for the prediction of swelling pressure of bentonite clay in the context of geo-disposal of nuclear waste.

Estimation of Montmorillonite Swelling Pressure: A Molecular Dynamics Approach

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Papers

Molecular dynamics study of montmorillonite crystalline swelling: Roles of interlayer cation species and water content

Montmorillonite interlayer surface chemistry: effect of magnesium ion substitution on cation adsorption

Stability of dioctahedral 2:1 phyllosilicate edge structures based on pyrophyllite models

Estimation of Montmorillonite Swelling Pressure: A Molecular Dynamics Approach