Theoretical analysis of adsorption of various compounds onto hydrophilic and hydrophobic silicas compared to activated carbons

Abstract

The aim of this study was to analyze various theoretical models (clusters, systems with periodic boundary conditions) and methods, which could be applied to investigate the adsorption phenomena and for better interpretation of the experimental data. The density functional theory (DFT) and semiempirical (PM7) methods were used to model the adsorption phenomena at a surface of fumed nanooxides, silica gels, activated carbons, etc. The main idea is that appropriate theoretical analysis allows a deeper insight into interfacial phenomena related to the structure & properties of the adsorption layers vs. the textural and other characteristics of adsorbents. Comparison of the theoretically calculated characteristics with experimental ones can allow more accurate interpretation of the effects observed in various experiments on the adsorption phenomena. It was established that polarization of nonpolar and polar molecules adsorbed onto a polar surface and charge (& proton) transfer play an important role, as well as confined space effects. It enhances the interaction energy of adsorbed molecules bound to a solid surface and affects the surface orientation of adsorbed molecules, as well the behavior of the adsorption layer vs. temperature, pressure or concentration, as well other conditions. Surface hydrophobization reduces the interaction energy for both polar and nonpolar adsorbates. Adsorbates clusterization reduces the average energy of interaction of the adsorption layer with a surface per a molecule. The charge transfer is observed for both polar and nonpolar molecules interacting with polar surface functionalities. The mostly strong interfacial effects changing the behavior of the adsorption layer are observed upon proton transfer to the adsorbed molecules or vice versa. Variation in orientation of adsorbed molecules results in overestimation of the specific surface area estimated using a fixed value of surface area occupied by a probe molecule (e.g. 0.162 nm 2 for N 2).

Atomic Structure and Morphology of Fumed Silica

“…Gas-phase polymerization leads to the formation of small oligomeric molecules (SiO 2 ) x (x  6 -10) that form protoparticles (Fig. 1) [15,16] as a kinetically unstable cluster of SiO 2 molecules [17]. The primary particle formation at the next stage is a result of spatial optimization of [SiO 4/2 ] species coordination with the unification of separate oligomeric chains.…”

Textural characteristics of composite adsorbents analyzed with density functional theory and self-consistent regularization procedure

“…where E is the interaction energy of an adsorbate molecule with the pore walls and neighboring molecules calculated with the LD potentials. The integral adsorption equation may be written as a sum of several equation 4for various components of complex adsorbents with different pore size distribution functions describing pores of different shapes [43][44][45] ( ) ( )…”

“…where R max and R min are the maximal and minimal pore radii, respectively, may be used as a criterion of the reliability of the pore model, since S BET is a conventional parameter independent on the pore shape and material type. The differential PSDs (calculated using the SCV/SCR model) with respect to pore volume [43][44][45]. This recalculation provides better view of the PSD in the large range (from nanopores to broad mesopores and macropores) since the differential f V (R) is much lower in the range of broad mesopores and macropores than that in the range of nanopores and narrow mesopores.…”

“…Note that there is a variety of developments ] not yet implemented in the firm software which, however, can be used for more accurate description of complex and hybrid materials. In previous papers [43][44][45], a self-consistent regularization (SCR) procedure applied to complex models of pores in complex materials (nanosilica + silica gel + carbons (activated carbon and carbon black)) was described. This approach was based on a blend of Kelvin and modified BET equations [46][47][48] (MND method) that was used in the SCV/SCR method with models of slit-shaped and cylindrical pores and voids between spherical nanoparticles (SCV model) [43][44][45].…”

“…In previous papers [43][44][45], a self-consistent regularization (SCR) procedure applied to complex models of pores in complex materials (nanosilica + silica gel + carbons (activated carbon and carbon black)) was described. This approach was based on a blend of Kelvin and modified BET equations [46][47][48] (MND method) that was used in the SCV/SCR method with models of slit-shaped and cylindrical pores and voids between spherical nanoparticles (SCV model) [43][44][45]. Note that the SCR procedure was also successfully applied to analyze the SAXS data to compute the particle size distributions with models of spherical, lamellar, and cylindrical particles [49][50][51].…”