Ion-induced nucleation of dibutyl phthalate vapors on spherical and nonspherical singly and multiply charged polyethylene glycol ions

Dibutyl phthalate vapor nucleation induced by positive polyethylene glycol (PEG) ions with controlled sizes and charges was experimentally studied. The ions were produced by electrospray ionization, classified in a high-resolution differential mobility analyzer, and studied in a nano condensation nucleus counter of the mixing type. Ionic radii of PEG varied from 0.52 to 1.56 nm, including from singly to quadruply charged ions. Some of these ions are fully stretched chains, other are spherical, and others have intermediate forms, all of them having been previously characterized by mobility and mass spectrometry studies. Activation of PEG₁₀₈₀⁺² requires a supersaturation almost as high as that required for small singly charged ions and higher than for PEG₁₀₈₀⁺. This anomaly is explained by the Coulombic stretching of the ion into a long chain, where the two charged centers appear to be relatively decoupled from each other. The critical supersaturation for singly charged spherical ions falls below Thomson's (capillary) theory and even below the already low values seen previously for tetraheptyl ammonium bromide clusters. Spherical PEG₄₁₂₀⁺² falls close to the Thomson curve. The trends observed for slightly nonspherical PEG₄₁₂₀⁺³ and highly nonspherical (but not quite linear) PEG₄₁₂₀⁺⁴ are intermediate between those of multiply charged spheres and small singly charged ions.