Singularly narrow size distributions of 200 nm polystyrene latex spheres determined by high resolution mobility analysis

Abstract

A high-resolution differential mobility analyzer (DMA) is used for the first time to study the mobility distribution of Polystyrene Latex (PSL) particles. We investigate in particular some unique, nominally 200 nm and 100 nm PSL particles, known by other methods to be unusually monodisperse. The suspensions are electrosprayed after adding ammonium acetate to a concentration of 10 mmol/L. Mobility analysis is carried out after charge reduction with a second electrospray of the opposite polarity. The 200 nm particles typically produce two narrow peaks, with relative intensities that depend on electrospray parameters. Under the most favorable spraying conditions achieved, the more mobile peak becomes dominant, is narrowest, and has a repeatable mobility; we attribute it provisionally to the unpolluted suspended particles. The less mobile peaks are substantially more variable but never entirely removed. We initially attributed them to attachment of involatile solutes in the suspension to the atomized PSL particles; however, this hypothesis is put in doubt by later observations. The measured relative width of the mobility distribution for the most mobile peak is FWHMZ = (1.65 ± 0.08) %. This translates into a width for the diameter distribution FWHMd = (1.09 ± 0.05) % or standard deviation σd = (0.46 ± 0.02) %, by far the most monodisperse polymeric submicron particle suspension so far reported. Concentrations of ammonium acetate of 100 mmol/L and 40 mmol/L were also tested in order to reduce residue attachment by forming smaller initial electrosprayed drops. These higher salinities, however, led to capillary clogging, apparently because the narrow electrospraying jet precludes the passage of the PSL particles. The 100 nm particles gave a wider variation in mean diameter and size distribution depending on electrospraying parameters, perhaps because of the suspension's considerably larger content of involatile solutes. Unlike the 200 nm particles, no spraying condition was found under which a precisely repeatable size distribution could be achieved for the 100 nm particles. However, the data do point to an estimated upper bound on the size distribution width of FWHMd ≈ 3 % (σd ≈ 1.3 %), which is uniquely narrow for particles of this size.