Molecular simulation of a reverse osmosis polyamide membrane layer. In silico synthesis using different reactant concentration ratios

International audienceWe use molecular dynamics simulations at the atomistic level to build a model of aromatic polyamide polymer used in reverse osmosis membranes, from a mixture of m-phenylene diamine (MPD) and trimesoyl chloride monomers (TMC). Our purpose is to use different MPD to TMC ratios to reproduce the compositional depth-dependence observed experimentally during interfacial polymerization. MPD to TMC ratios in the range 1:4 to 5:1 have been employed. Reproducibility of the polymerization algorithm is thoroughly studied through the building of several samples under identical conditions. We notice that simulation time of a few microseconds are necessary in order to reach equilibration. We show that the initial monomer ratio has a strong influence onto the final polymer composition and different chemical structures have been created. Large differences are seen concerning cross-linking degree and remaining un-reacted groups. Comparison with available experimental data show that samples built using intermediate values of the MDP to TMC ratio closely resemble aromatic polyamide membranes. We conclude that the simulated samples created under different local concentrations can describe properly the chemical heterogeneities observed experimentally in reverse osmosis membranes

Evaluation of Ca 2+ Binding Sites in Tacrolimus by Infrared Multiple Photon Dissociation Spectroscopy

International audienc

Reduction of Earth Alkaline Metal Salts in THF Solution Studied by Picosecond Pulse Radiolysis

Picosecond pulse radiolysis of tetrahydrofuran (THF) solutions containing earth alkaline metal salt, M^II(ClO₄)₂, at different concentrations are performed using two different supercontinua as probe pulse, one covering the visible and another the near-infrared (NIR) down to the visible. Two types of line scan detectors are used to record the absorption spectra in the range from 400 to 1500 nm. Because of the strong overlap between the spectra of the absorbing species in the present wavelength range, global matrices were built for each M^II system, by delay-wise binding the matrix for pure THF with the available matrices for this cation. The number of absorbers was assessed by Singular Value Decomposition of the global matrix, and a MCR-ALS analysis with the corresponding number of species was performed. The analysis of the results show clearly that solvated electron reacts with the earth alkaline metal molecule and the product has an optical absorption band very different than that of solvated electron in pure THF. So, contrarily to the case of solution containing free Na⁺, in the presence of Mg^II, Ca^II and Sr^II the observed absorption band is not only blueshifted, but its shape is also drastically changed. In fact with Na⁺ solvated electron forms a tight-contact pair but with earth alkaline metal cation solvated electron is scavenged by the undissociated molecule M^II(ClO₄)₂. In order to determine the structure of the absorbing species observed after the electron pulse, Monte Carlo/DFT simulations were performed in the case of Mg^II, based on a classical Monte Carlo code and DFT/PCM calculation of the solute. The UV–visible spectrum of the solute is calculated with the help of the TDDFT method. The calculated spectrum is close to the experimental one. It is due to two species, a contact pair and an anion

Simulating Electron Dynamics in Polarizable Environments

We propose a methodology for simulating attosecond electron dynamics in large molecular systems. Our approach is based on the combination of real time time-dependent-density-functional theory (RT-TDDFT) and polarizable Molecular Mechanics (MMpol) with the point-charge-dipole model of electrostatic induction. We implemented this methodology in the software deMon2k that relies heavily on auxiliary fitted densities. In the context of RT-TDDFT/MMpol simulations, fitted densities allow the cost of the calculations to be reduced drastically on three fronts: (i) the Kohn–Sham potential, (ii) the electric field created by the (fluctuating) electron cloud which is needed in the QM/MM interaction, and (iii) the analysis of the fluctuating electron density on-the-fly. We determine conditions under which fitted densities can be used without jeopardizing the reliability of the simulations. Very encouraging results are found both for stationary and time-dependent calculations. We report absorption spectra of a dye molecule in the gas phase, in nonpolarizable water, and in polarizable water. Finally, we use the method to analyze the distance-dependent response of the environment of a peptide perturbed by an electric field. Different response mechanisms are identified. It is shown that the induction on MM sites allows excess energy to dissipate from the QM region to the environment. In this regard, the first hydration shell plays an essential role in absorbing energy. The methodology presented herein opens the possibility of simulating radiation-induced electronic phenomena in complex and extended molecular systems