Ionic liquids desolvation effects

Cationic micelles, such as those formed from cetyltrimethylammonium bromide, may also accelerate reaction. Studies involving reactions with alkoxide ions indicate that factors involved include the concentration of reagents within the micelle, desolvation of the alkoxide ions, and stabilization of the transition state [92]. There have also been studies of the effects of oil-water emulsions [93] and vesicles [94] on reactivity, and ionic liquids have been shown to accelerate the ethanolysis of l-fluoro-2,4-dinitrobenzene [95]. 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

---