Simulation of the Drift of a Macromolecular Ion in a Gas under the Action of an Electric Field

A new molecular dynamics method is proposed for simulating the drift of a macromolecular ion in a gas under the action of a uniform electrostatic field. The method explicitly takes into account all atoms of the ion-gas system and maintains a constant gas temperature using a collisional thermostat. The method is applied to the drift of a protonated poly(ethylene oxide) chain in helium. The drift velocity, temperature, size, and degree of alignment of this macromolecular ion are determined over a wide range of field strengths. The mobility in weak fields is in agreement with the available experimental data. The dependence of the mobility on the field strength in the strongest fields is close to the dependence predicted by the theory of ion transport. The calculated and theoretical dependences of the ion temperature on the drift velocity are also close. In strong fields, the heating of the ion and the stretching caused by the gas flow force the polymer chain of the ion to assume unfolded conformations. The field orients the ion dipole along the field strength vector. The ion collision cross section is calculated based on the drag coefficient of a sphere moving in a rarefied gas. It is influenced by both the unfolding and dipole alignment of the ion.