Ionization of carboxylic acid clusters in the gas phase and on free ArN and (H2O)N nanoparticles: valeric acid as a model for small carboxylic acids.

We investigate ionization of valeric (n-pentanoic) acid clusters both in the gas phase and on argon and water nanoparticles using mass spectrometry. Compared to the ionization of a single valeric acid molecule, new reaction channels are observed in clusters, mostly attributed to proton transfer between two valeric acid molecules and formation of valeric anhydride. These reactions are also observed when valeric acid molecules are deposited and generate clusters on ArN, and are independent of the ionization method, whether electron ionization or photoionization is used. Valeric acid clusters exhibit a high water affinity, both in neutral clusters and after ionization. When valeric acid is adsorbed on (H2O)M ice nanoparticles, no new specific reactions with water are observed. However, in this case, electron ionization yields mostly protonated water clusters while the photoionization spectrum does not differ significantly from free and ArN-deposited valeric acid clusters. Based on quantum chemical calculations, we extrapolate our results to carboxylic acids with 1-8 carbon atoms. The calculations show that the high affinity to water can be expected in the whole investigated size range while the highest probability of anhydride formation is predicted for n = 3-6. The observed reaction patterns in the ionization of valeric acid are thus prototypical for ionization of clusters of short-chain fatty acids.