Preferential solvation and self-association in alcohol-acetonitrile mixtures observed through mass spectrometric analysis of clusters: Influence of alkyl chain length

Molecular clusters formed by fragmentation of liquid droplets of alcohol (methanol or n-butanol)-acetonitrile mixtures have been detected and analyzed by means of a specially designed mass spectrometer. In the methanol-acetonitrile mixture, methanol clusters retain a sizable magnitude through most of the composition range, whereas acetonitrile clusters decrease in intensity upon increasing the concentration of methanol. Hydrogen bonding among methanol molecules controls the clustering. On the other hand, in n-butanol-acetonitrile mixtures, self-association of n-butanol through hydrogen bonding is remarkably promoted by the mixing with acetonitrile. With decreasing the acetonitrile contents, however, n-butanol self-associated clusters disintegrate completely. The interaction among n-butanol molecules changes from hydrogen bonding to dispersive, depending on the mixing ratio. When phenol is added as a solute to these binary mixtures, the solvation of phenol is found to be controlled by the solvent molecular clustering.