On the dependence of electrical mobility on temperature, humidity and structure of alkylammonium ions

Abstract

Insights into the effect of temperature (T) and relative humidity (RH) as well as structure and polarisation on ion mobility help the comparison and interpretation of mobility and mass-based data. We measured alkylammonium ions in air under different T (14 °C, 24 °C, 34 °C and 41 °C) and RH (0 %, 20 %, 40 %) conditions using two individual setups (in both cases a planar differential mobility analyser coupled with a time-of-flight mass spectrometer) and the results are in excellent agreement. Mobility increases with rising T and decreases with water vapour loading. When separating the measurement mobility by structures, clear mass dependence was observed. The measured mobilities exhibited large deviations from theoretically calculated results in dry conditions, which are possibly caused by adduct formation on the monomer ions via clustering (or reactions). This phenomenon seems to be unavoidably associated with light ions under atmospheric pressures, which is worth further exploration and bearing in mind when comparing measurements to calculations. Both methanol and oxygen (occasionally nitrogen or alkyl chain elongation) are possible candidates of the adduct. Under spherical assumption, we used the modified Mason–Schamp's approximation to link the measured mobility to the mobility equivalent diameter. The drag enhancement factor $\xi$ and the effective gas-molecule collision diameter $d_g$ derived from our measurement data are comparable to literature values. Our data also exposed a non-linear dependence on the polarisation parameter ${\epsilon }^{*}$. Polarisation, $\xi$ and $d_g$ were parameterised using linear models against ion structures, T, and RH for primary, secondary and tertiary alkylammonium ions with identical alkyl groups. Our model parametrisations predict mobilities within ±10 % deviation from the measured data. The model also has satisfying predicting power for alkylammonium ions with unidentical alkyl structures.