Structure and Dynamics of Benzene Confined in Silica Nanopores

Abstract

The structure and dynamics of benzene confined at 293 K in silica nanopores of different diameters (D = 2.0 nm and D = 3.6 nm) are investigated by means of grand canonical Monte Carlo and molecular dynamics simulations. In order to account in a realistic way for the interactions between benzene and the silica surface, we consider a recent model that accounts for the π-electrons of the aromatic cycle in the benzene molecule. Confined benzene exhibits significant layering and orientational ordering in the vicinity of the silica surface (up to two adsorbed layers) and tends to recover its bulk properties in the pore center. Using suitable order parameters, we show that benzene molecules close to the pore surface tend to have their ring lying flat on the silica surface (and hence perpendicular to the pore axis). Such a preferential parallel orientation with respect to the silica surface suggests that a proper description of the π-electrons of the benzene aromatic ring and its specific Coulombic interaction with the partial charges carried by the silica atoms is crucial. The dynamics of benzene confined in the silica nanopores is always slower than in the bulk. Both the translational and rotational dynamics of confined benzene can be described as a bulklike contribution in the pore center that depends on the pore size and a surface contribution that is nearly insensitive to the pore size. These simulation results are discussed in the light of available experimental data on the structure and dynamics of benzene confined in nanoporous silicas.

Hard and soft confinement effects on the glass transition of polymers confined to nanopores

“…In addition, close to the hard wall with strong interaction, we can observe that r(R) displays obvious density oscillations, which are characteristic of confined fluids. Such density oscillations of fluids in the vicinity of confining surface has been observed in different geometries [11,25]. We also can observe the influence of interaction strength on the density profile: Increasing interaction strength will induce more pronounced density oscillations and give rise to an overall long range decay of r(R) as compared to the weak interaction.…”

“…The equilibrated configuration has the dimension 40 Â 40 Â 40, which is also the dimension of the simulation box used in all following studies. We define a function h(x,y,z) that equals 1 if (x,y,z) belongs to the wall and 0 if (x,y,z) belongs to the void [11]. The nanopores used in this work can be obtained by removing the void corresponding to h(x,y,z) ¼ 0 out of the generated equilibrated configuration.…”

“…The geometries that have been considered include thin films [3e8,12e15], narrow pores [1,2,10,11] and spherical cavities [9,16].…”

Surface of glassyGeS2: A model based on a first-principles approach

“…We first determined the structural properties of g-GeS 2 (s) by calculating the partial pair correlation functions g αβ (r) with α, β = Ge or S. In order to compare the partial pair correlation functions for g-GeS 2 (b) and g-GeS 2 (s), g αβ (r) for g-GeS 2 (s) have been corrected for the finite size of the sample [65][66][67]:…”

Molecular Modeling of Glassy Surfaces

“…We first determined the structural properties of g-GeS 2 (s) by calculating the partial pair correlation functions g αβ (r) with α, β = Ge or S. In order to compare the partial pair correlation functions for g-GeS 2 (b) and g-GeS 2 (s), g αβ (r) for g-GeS 2 (s) have been corrected for the finite size of the sample: [83] g αβ (r) * = g αβ (r)/f (r) with f (r) = 1 − r/(2h) (13.2) where h is the thickness of the slab. While this correction is not needed to compare the position of the peaks in the g αβ (r) functions, it allows correcting the peak amplitudes for the finite size of the sample.…”