Equilibrium solvation in quadrupolar solvents

We present a microscopic theory of equilibrium solvation in solvents with zero dipole moment and non-zero quadrupole moment (quadrupolar solvents). The theory is formulated in terms of autocorrelation functions of the quadrupolar polarization (structure factors). It can be therefore applied to an arbitrary dense quadrupolar solvent for which the structure factors are defined. We formulate a simple analytical perturbation treatment for the structure factors. The solute is described by coordinates, radii, and partial charges of constituent atoms. The theory is tested on Monte Carlo simulations of solvation in model quadrupolar solvents. It is also applied to the calculation of the activation barrier of electron transfer reactions in a cleft-shaped donor-acceptor complex dissolved in benzene with the structure factors of quadrupolar polarization obtained from Molecular Dynamics simulations.Comment: Submitted to J. Chem. Phys., 20 pages and 13 figure

Polarizability Anisotropy Relaxation in Nanoconfinement: Molecular Simulation Study of Acetonitrile in Silica Pores

We present the results of a molecular simulation study of polarizability anisotropy relaxation of liquid acetonitrile confined in approximately cylindrical silica pores of diameters in the range of 20–40 Å. Grand Canonical Monte Carlo simulation is used to determine the density of acetonitrile in pores in equilibrium with the bulk liquid, and canonical-ensemble molecular dynamics is then used to calculate the trajectories of the filled pores prepared in this way. We find that the pores are wetting, partially due to hydrogen bonding between acetonitrile nitrogen and pore silanol groups and that acetonitrile molecules have preferential orientations relative to the interface. The mobility of molecules in interfacial regions is considerably reduced and dependent mainly on their proximity to the interface. We include the contributions of molecular and interaction-induced polarizabilities to the collective polarizability anisotropy relaxation. We find that this relaxation includes a slowly relaxing component absent from the corresponding process in bulk acetonitrile and that the amplitude of this component increases as the pore diameter decreases. These results are in agreement with optical Kerr effect experiments on acetonitrile in silica pores in a similar diameter range. Further analysis of our data indicates that collective reorientation and predominantly translational “collision-induced” polarizability dynamics both contribute to the slowly relaxing portion of polarizability anisotropy decay. We further find that pore anisotropy plays a role, giving rise to different relaxation rates of polarizability anisotropy components with a different mix of axial and radial character and that collective reorientation contributing to polarizability anisotropy relaxation is somewhat faster at long times than single-molecule orientational relaxation

Nanoconfined Electrolyte Solutions in porous Hydrophilic Silica Membranes.

International audienceManipulating of electrolyte solutions is ubiquitous in numerous fields ranging from condensed to soft matter. The fundamental understanding of the molecular mechanism ruling the macroscopic properties is inescapable to well rationalize the engineering processes. In confined situations such as nanofluidic or nanofiltration, interactions between the fluids and the surface drastically modify the usual macroscopic properties. In this work we addressed a structural and dielectric analysis of confined NaCl, NaI, MgCl2, and Na2SO4 in nanoporous hydrophilic silica membranes. We highlight a dielectric anisotropy of confined water and an unusual increase in the radial component of permittivity (ε) of confined solutions with respect to the decrease in ε in the bulk solution phase. Structurally, we investigate the size, the polarizability, and the salt effect on water confinement and ions translocation through a cylindrical silica nanopore