Uncertainties in the static dielectric constants computed from molecular dynamics simulations

Electrical noise in electrolytes: a theoretical perspective

Seemingly unrelated experiments such as electrolyte transport through nanotubes, nano-scale electrochemistry, NMR relaxometry and surface force balance measurements, all probe electrical fluctuations: of the electric current, the charge and polarization, the field gradient (for quadrupolar nuclei) and the coupled mass/charge densities. The fluctuations of such various observables arise from the same underlying microscopic dynamics of the ions and solvent molecules. In principle, the relevant length and time scales of these dynamics are encoded in the dynamic structure factors. However, modelling the latter for frequencies and wavevectors spanning many orders of magnitude remains a great challenge to interpret the experiments in terms of physical processes such as solvation dynamics, diffusion, electrostatic and hydrodynamic interactions between ions, interactions with solid surfaces, etc. Here, we highlight the central role of the charge-charge dynamic structure factor in the fluctuations of electrical observables in electrolytes and offer a unifying perspective over a variety of complementary experiments. We further analyze this quantity in the special case of an aqueous NaCl electrolyte, using simulations with explicit ions and an explicit or implicit solvent. We discuss the ability of the standard Poisson-Nernst-Planck theory to capture the simulation results, and how the predictions can be improved. We finally discuss the contributions of ions and water to the total charge fluctuations. This work illustrates an ongoing effort towards a comprehensive understanding of electrical fluctuations in bulk and confined electrolytes, in order to enable experimentalists to decipher the microscopic properties encoded in the measured electrical noise.

Parallel-slipped π-π electron-donor-acceptor in adsorption process: Molecular dynamics simulation

Molecular dynamics simulation was used to study the adsorption of single wall carbon nanotubes (SCNT) in levofloxacin (LEV) solutions of different concentrations by Radial distribution function, mean square displacement and interaction energy. The results showed that levofloxacin molecules were adsorbed around the carbon nanotubes. The adsorption effect of large concentration solution was not as good as that of low concentration solution because of agglomeration. LEV molecules with different concentration were free diffusion within 15ns, and gradually agglomerated under the influence of adsorption. The energy change is proportional to the concentration of the molecule. The distance between benzene rings corresponding to the agglomeration effect of levofloxacin molecules was 0.4 nm, which should be the effect of parallel-slipped π-π electron-donor-acceptor (EDA) interactions. The simulation results are valuable to study the adsorption and removal of benzenes by adsorbent.