Abstract Number: 234 Working Group: Aerosol Physics
Abstract Highly charged macromolecular ions exhibit various conformations in the gas phase. Intramolecular charge-to-charge interactions induce the transition from the globular structure into the stretched conformation (Larriba and de la Mora, 2012). Such variation of molecular conformation causes complex gas phase dynamics of ions under the electrostatic field. In the present study, the dynamics of multiply-charged molecular clusters were visualized by molecular dynamics (MD) simulation.
The MD simulation was performed by the system consisting of one molecular ion (PEG or tetra-alkyl ammonium ion) with 200 N2 molecules located in a cubic simulation cell (L=20 nm). The temperature was set at 298 K. In order to simulate the electrical drift of the molecules, the electrical field was applied as Ex = Ey = Ez = 10,000 V cm-1.
From the results of visualization by the simulation, the conformation of multiply charged PEG molecules in the gas phase changed from sphere to the non-spherical (string shape) due to the repulsive force between partial charges on the molecule. Such structural transition depends on the number of charges and molecular weight of PEG. In addition, the orientations of the highly charged PEG molecules varied under the electric field. Therefore the transport properties of the highly charged ions were influenced by the molecular structure, charge localization, and orientation.
The electrical mobility of molecular ions was calculated from the change in the position during the specific time interval. Firstly, the MD-calculated mobility of the spherical molecules was validated by the comparison with theoretical values obtained by the Stokes-Einstein's equation. Then, the electrical mobility of the PEG molecules was calculated by the MD simulation with changing molecular weight and the number of charges. It was found that the change in the conformation of the PEG molecules caused a decrease in the calculated mobility, which was also reported in the literature. Thus, the MD simulation is considered to be an effective tool to estimate the electrical mobility of the highly charged macromolecules.