Ion-Pair Evaporation from Ionic Liquid Clusters

A differential mobility analyzer (DMA) is used in atmospheric pressure N2 to select a narrow range of electrical mobilities from a complex mix of cluster ions of composition (CA)n(C+)z. The clusters are introduced into the N2 gas by electrospraying concentrated (∼20 mM) acetonitrile solutions of ionic liquids (molten salts) of composition CA (C+ = cation, A– = anion). Mass analysis of these mobility-selected ions reveals the occurrence of individual neutral ion-pair evaporation events from the smallest singly charged clusters: (CA)nC+→(CA)n-1C++CA. Although bulk ionic liquids are effectively involatile at room temperature, up to six sequential evaporation events are observed. Because this requires far more internal energy than available in the original clusters, substantial heating (∼10 eV) must take place in the ion guides leading to the mass analyzer. The observed increase in IL evaporation rate with decreasing size is drastic, in qualitative agreement with the exponential vapor pressure dependence predicted by Kelvin's formula. A single evaporation event is barely detectable at n = 13, while two or more are prominent for n ≤ 9. Magic number clusters (CA)4C+ with singularly low volatilities are found in three of the four ionic liquids studied. Like their recently reported liquid phase prenucleation cluster analogs, these magic number clusters could play a key role as gas-phase nucleation seeds. All the singularly involatile clusters seen are cations, which may help understand commonly observed sign effects in ion-induced nucleation. No other charge-sign asymmetry is seen on cluster evaporation.

Graphical AbstractA differential mobility analyzer-mass spectrometer is used to detect ion-pair evaporation events from ionic liquid clusters.Figure optionsDownload full-size imageDownload high-quality image (135 K)Download as PowerPoint slide