Charge and momentum transfer processes between aerosol particles and gas molecules proceed via collisions. There is a need to better understand such collision driven phenomena in order to predict particle dynamics and growth processes. This dissertation consists of two research projects focusing on charge and momentum transfer processes for aerosol particles in the sub 20 nm size range. The research on particle charging was aimed at bridging the gap between diffusion charging theory (traditionally used in aerosol physics) and chemical ionization, both of which are collision based reactions. In diffusion charging theory, the occurrence of a back charging reaction is neglected, i.e. once an ion collides with a particle, charge is transferred to the particle, and the charged particle does not lose charge on colliding with uncharged vapor molecules. However, in chemical ionization, charge transfer can occur in both directions - from charge-donating ion to vapor molecule and back from charged vapor molecule to the original charge-donating species. We examined the occurrence and the rate of this back charging reaction using a combination of experimental methods (electrospray ionization- mass spectrometry) and collision kinetics models. Studies conducted with charged amino acid clusters (~0.5 nm in size) and trimethylamine vapor molecules illustrated that charge transfer could occur from aerosol cluster ions to neutral vapor molecules upon collision.
The second set of studies conducted involved ion mobility measurements of protein ions generated via electrospray ionization (ESI) with charge reduction from both non-denaturing and denaturing ESI solutions. A critical examination of the ESI process on gas-phase protein structure was carried out. Mobility diameters (dp) and collision cross sections (Ω) were inferred from the mobility spectra. An effective gas molecule diameter of 0.3 nm was accounted for in these calculations. While calculating collision cross sections, a non unity momentum scattering coefficient (ξ = 1.36) was considered to account for diffuse collisions between gas molecules and protein ions. The effective density of protein ions electrosprayed from non-denaturing solution was 0.96 g cm-3 while that from denaturing was 0.98 g cm-3.