We propose and develop a classical density functional theory for the description of a minor amount of water dissolved in ionic liquid in the vicinity of an electrode. In addition to the electrostatic energy and lattice-gas mixing entropy terms, the utilised grand canonical potential contains several phenomenological terms/parameters that describe short-range interactions between ions of ionic liquid, water molecules and the electrode. Some of these have been earlier introduced in the theory of electrical double layer in pure ionic liquids. Based on this, we investigate the role of the remaining ’specific interaction’ parameters e those that characterize possible (i) specific interaction of ions and molecules with the electrode, which are responsible for their specific adsorption; and (ii) hydrophilicity/hydrophobicity of ions. As a result we obtain water electrosorption isotherms as a function of the potential drop across the electrical double layer, investigate its asymmetry with respect to the sign of electrode potential, and establish the relationship between the sign of this asymmetry and hydrophobicity/hydrophilicity of cations and anions. We also calculate the effect of water electrosorption on the double layer differential capacitance which brings clear new features to its voltage dependence, some of which have been already experimentally observed.
Here, we present a new model of adsorption-induced deformation of mesoporous solids. The model is based on a simplified version of local density functional theory in the framework of solvation free energy. Instead of density, which is treated as constant here, we used film thickness and pore radius as order parameters. This allows us to obtain a self-consistent system of equations describing simultaneously the processes of gas adsorption and adsorbent deformation, as well as conditions for capillary condensation and evaporation. In the limit of infinitely rigid pore walls, when the film becomes several monolayers thick, the model reduces to the well-known Derjaguin–Broekhoff–de Boer theory for pores with cylindrical geometry. We have investigated the effects of enhanced flexibility of the solid as well as the influence of pore size distribution on the adsorption/deformation process. The formulation of the theory allows to determine the average pore size and its width from the desorption branch of the strain isotherm only. The model reproduces the nonmonotonic behavior of the strain isotherm at low relative pressure. Furthermore, we discuss the effect of rigidity of the adsorbent on the pore size distribution, showing qualitatively different results of the adsorption isotherms for rigid and highly flexible materials, in particular, the shift of evaporation pressure to lower values and the absence of a limiting value of the loading at high relative pressure. We also discuss the results of the theory with respect to experimental data obtained from the literature.
In situ XRD and NMR experiments combined with molecular dynamics simulations using the grand canonical ensemble (GCMD) show that cation size, charge and solvation energy play critical roles in determining the interlayer expansion of smectite clay minerals when exposed to dry supercritical CO2 under conditions relevant to the earthâ€™s upper crust, petroleum reservoirs, and geological CO2 sequestration conditions (323 K and 90 bar). The GCMD results show that the smectite mineral, hectorite, containing interlayer alkali and alkaline earth cations with relatively small ionic radii and high solvation and hydration energies (e.g., Li+, Na+ Mg2+, and Ca2+) does not intercalate dry CO2 and that the fully collapsed interlayer structure is the energetically most stable configuration. With increasing cation size and decreasing cation solvation energy, the energy barrier to CO2 intercalation decreases. With K+, Rb+, Cs+, Sr2+, and Ba2+ the monolayer structure is the stable configuration, and CO2 should spontaneously enter the interlayer. With Cs+ there is not even an energy barrier for CO2 intercalation, in agreement with the experimental XRD and NMR results that show clay layer expansion and CO2 incorporation. The number of intercalated CO2 molecules decreases with increasing size of the alkali cation but does not vary with ion size for the alkaline earth cations. 13C NMR spectroscopy and the GCMD simulations show that the average orientation of the intercalated CO2 molecules is with their O-C-O axes parallel to the basal clay surface and that they undergo a combination of rapid rotation about an axis perpendicular to the main molecular axis and wobbling motion with respect to the basal surface. Incorporation of CO2 in the interlayer decreases the coordination of Cs+ by the oxygen atoms of the basal surfaces, which is compensated by CO2 molecules entering their solvation shell, as predicted based on previously published NMR results. The GCMD simulations show that the strength of the interaction between the exchangeable cation and the clay structure dominates the intercalation energetics in dry scCO2. With relatively small cations, the cation-clay interactions outcompete cation solvation by CO2 molecules. The computed residence times for coordination among of interlayer species are consistent with the computed energetics.
The effect of an interplay between electrostatic and excluded volume interactions on the conformational behavior of a dipolar chain has been studied theoretically and by means of molecular dynamics simulations. Every monomer unit of the dipolar chain comprises a dipole formed by a charged group of the chain and an oppositely charged counterion. The counterion is assumed to freelymove around the chain but keeping the distance between oppositely charged ions (the dipole length) fixed. The novelty of the developed mean-field theory is that variations of the dipole parameters (the dipole length and the counterion size) have been accounted for in both electrostatic and excluded volume contributions to the total free energy of the dipolar chain. It has been shown that conformational transitions between swollen and collapsed states of the chain can be induced by fine-tuning the balance between electrostatic and excluded volume interactions. In particular, in lowpolar media not only globule but also extended coil conformations can be realized even under strong electrostatic attraction. The results ofMD simulations of a dipolar chain with variable dipolar length support theoretical conclusions.