Charge evaporation from nanometer polystyrene aerosols

The charge distribution of polystyrene nanoparticles electrosprayed from l-methyl-2-pyrrolidone (NMP) dimethylammonium formate (10/1 vol) is studied by a continuous tandem differential mobility analyzer (CTDMA) technique. Two DMAs operated in series as narrow band mobility filters are scanned in a quasi-continuous fashion providing highly informative two-dimensional (2D) spectra. The first DMA selects naturally charged particles, which are then partially neutralized by a radioactive source and subsequently analyzed in the second DMA. The mobility pairs found in both DMAs finally yield the charge and size distribution. A 1 mM solution of relatively monodisperse polystyrene with an average mass of 34.5 kDa forms polymer aggregates, including from 1 to 4 single molecules, leading to discrete particle sizes of 4.7, 5.95, 6.80 and 7.5 nm in diameter. Observed charge states are unusually low relative to previously studied water-soluble polymers such as polyethylene glycol. Large particles are originally charged in such a fashion as to give an almost constant mobility (in air) of 0.23 cm2/V/s, implying a critical electric field for ion evaporation from a polystyrene sphere of some 0.7 V/nm. This is the first measurement of this critical field for nonpolar nanoparticles, and results in a value which is three times smaller than those previously measured for polar materials.