Droplet charge density

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

The fractional ionization, a, of ionic micelles is increased by hydrophobic non-ionic solutes which decrease the charge density at the micellar surface and the binding of counterions (Larsen and Tepley, 1974 Zana, 1980 Bunton and de Buzzaccarini, 1982). Consistently, microemulsion droplets are less effective at binding counterions than otherwise similar micelles. 

The alternative mechanism, the ion-evaporation model, was proposed initially by Iribarne and Thomson13 (Fig. 4) and involves desolvation of the droplets, producing an increase in charge density over the droplet surface that causes coulombic explosion and eventually leads to ejection of individual ions. 

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