Mobility of Intermediate Sized Aqueous Ions in a Neutral Gas

A mobility versus size relationship is calculated for aqueous ions of intermediate size in an argon gas. The method is based on the Chapman-Enskog transport theory. It involves the microscopic interaction potential between the aqueous ion and the neutral gas molecules. A model potential is used that accounts for the water molecules in the cluster-ion as well as its finite size. The results agree with the Langevin small ion theory at the small end of the size spectrum, give a lower mobility for intermediate sized ions, and then agree with the Stokes viscosity theory at the large end of the size spectrum. An explicit function containing adjustable parameters is fitted to the numerical results in the intermediate size region. The results are applied to data from a Wilson cloud chamber ion mobility experiment.