Mobility of Multiply Protonated Poly(ethylene oxide)s in Helium at Different Electric Field Strengths. Molecular Dynamics Simulation of Ion Drift

Abstract

The drift of multiply protonated poly(ethylene oxide) chains in helium in electrostatic fields of various strengths is simulated using the molecular dynamics method. The simulation results are compared with the predictions of the kinetic theory of ion mobility, which relates the effect of increasing field strength to increasing ion temperature. As would be expected, the internal temperature of the ion T_ion increases with increasing random kinetic energy received by the ion from the field. However, it grows more slowly than expected in the two-temperature theory. Ion mobility is calculated as a function of the field strength E at constant gas temperature T (300 K) and as a function of T at low E. The results of these two series of calculations are compared at the same internal ion temperatures. The results coincide at T_ion close to T. At high ion temperatures, they diverge somewhat (by about 8% at T_ion = 600 K), which does not agree with the theory. Conformations and sizes of drifting ions, as well as their collision cross sections, calculated from the mobility, indicate that an increase in the number of attached protons leads to unfolding of the polymer chain. This effect is in satisfactory agreement with the Rayleigh criterion for the stability of a charged drop. An increase in field strength affects the collision cross section for several reasons. They include an increase in ion temperature leading to larger ion sizes, a decrease in the influence of long-range attractive interactions, and dipole alignment that is more pronounced with fewer protons attached.