Characteristics of the “Hypercoordination” of hydroxide (OH−) in water: A comparative study of HF/MM and B3LYP/MM MD simulations

Investigating water/oil interfaces with opto-thermophoresis

Charging of interfaces between water and hydrophobic media is a mysterious feature whose nature and origin have been under debate. Here, we investigate the fundamentals of the interfacial behaviors of water by employing opto-thermophoretic tweezers to study temperature-gradient-induced perturbation of dipole arrangement at water/oil interfaces. With surfactant-free perfluoropentane-in-water emulsions as a model interface, additional polar organic solvents are introduced to systematically modify the structural aspects of the interface. Through our experimental measurements on the thermophoretic behaviors of oil droplets under a light-generated temperature gradient, in combination with theoretical analysis, we propose that water molecules and mobile negative charges are present at the water/oil interfaces with specific dipole arrangement to hydrate oil droplets, and that this arrangement is highly susceptible to the thermal perturbation due to the mobility of the negative charges. These findings suggest a potential of opto-thermophoresis in probing aqueous interfaces and could enrich understanding of the interfacial behaviors of water.

Investigation of the Structure of Concentrated NaOH Aqueous Solutions by Combining Molecular Dynamics and Wide-Angle X-ray Scattering

Classical molecular dynamics has been performed with explicit polarization on NaOH aqueous solutions from 0.5 mol L^-1 up to 9.7 mol L^-1. We adapted a force field of OH^- for polarizable simulation in order to reproduce the NaOH structural and thermodynamics properties in aqueous solutions. A good agreement between theoretical and experimental results has been found. Wide-angle X-ray scattering (WAXS) intensities issued from molecular dynamics are compared to experimental ones measured on Synchrotron facilities. The structure of the first coordination shell of Na⁺ has been studied to determine the variation of the oxygen number and hydroxide oxygen around the cation. In addition, Na⁺-OH^- McMillan-Mayer potential issued from molecular dynamics simulations has been calculated and allows for calculating Na⁺-OH^- pair association constant of 0.1 L mol^-1, which is in good agreement with the experiments.