Cluster formation of 1-butanol-water mixture leading to phase separation

Microscopic structures of 1-butanol (1-BuOH) - water mixtures in the presence and absence of salts are studied through the mass spectrometric analysis of clusters generated from the fragmentation of liquid droplets. The analysis of cluster structures provides information on the phase separation of 1-BuOH - water mixtures from the microscopic viewpoint. In a saturated solution of 1- BuOH in water, 1- BuOH exists as hydrated 1-BuOH clusters and self-associated 1- BuOH clusters. With further addition of 1- BuOH, a 1- BuOH rich phase is generated. When the salt (LiCl, NaCl, MgCl2, etc.) coexists in the 1- BuOH - water mixtures, the cation is preferentially solvated by the 1- BuOH to form M+(1-BuOH)(m) or M2+(1-BuOH)(m) clusters: M+ = Li+, Na+, Mg2+ = Mg2+, etc., m = 1, 2, 3,... Since the formation of M+(1-BuOH)(m) corresponds to an increase of the self-associated 1-BuOH clusters in water, the presence of the salt induces the phase separation at lower 1- BuOH concentrations.