Comparison of mobility equivalent diameter with Kelvin-Thomson diameter using ion mobility data

Abstract

Mobility distributions of both positive and negative cluster ions were studied in nitrogen gas enriched by ethanol, isopropanol, n-butanol and acetone vapours, respectively. The ions were generated into the system by a radioactive source normally used as a bipolar aerosol charger. The ion mobility distributions were measured with a time-of-flight mobility spectrometer (TOF). The measured ion mobility can be used to estimate the mobility equivalent diameter via conventional Millikan–Fuchs formula based on Stokes law corrected by molecule slip effects. When the vapour concentration is increased the cluster ions will get larger due to enhanced molecule attachment onto ions. Here the vapour concentration is assumed to be high enough so that the chemical tracer effects would become of minor importance. Therefore the Kelvin–Thomson equation for ion induced nucleation is first assumed to be valid. The Kelvin–Thomson diameter of the ions in all cases was found to be systematically smaller than the corresponding mobility equivalent diameter according to Millikan–Fuchs, indicating of inconsistencies between the formulas based on macroscopic quantities at 1 nm size range. Thereafter, other available mobility vs diameter relationships were surveyed. The closest agreement between mobility equivalent and Kelvin–Thomson diameters at this size was obtained using the relationship given by Tammet [J. Aerosol Sci. 26, 459 (1995)].