Charge of 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 ... 

Figure 4.8 The motion of the oil droplets within Millikan s apparatus depends on the charge of droplets and on the electric field. Millikan observed the droplets with the telescope. He could make the droplets fall more slowly, rise, or pause as he varied the strength of the electric field. From his observations, he calculated the charge on each droplet. 

Some of the droplets cany an excess of positive electric charge and others an excess of negative electric charge. The spray or stream of droplets is passed along a tube that is usually heated. 

There is no clear explanation of lightning. However, the most popular theory is the charging of clouds at high voltages, up to 20 million volts and a charging current, i-100 kA or so, due to the movement of hot air upwards and big droplets of water downwards. This process tends to make the tops of the clouds positive and the bottom... 

We saw in Chapter 5 that there is a driving force tending to make dispersions of precipitates in alloys coarsen and we would expect a dispersion of droplets in water vapour to do the same. Water droplets in clouds, however, carry electrostatic charges and this gives a different result for the driving force. 

Electrostatic Precipitation—the electrical charging of the liquid droplets may come about by the interaction of the gas and liquid streams. Not much known of this action. 

In trne thermospray, charging of the droplets is dne to the presence of a bnffer in the mobile phase. Both positively and negatively charged droplets are formed dne to the statistical flnctnation in anion and cation density occnrring when the liqnid stream is disrnpted. As with the interfaces previonsly described, involatile bnffers are not recommended as blocking of the capillary is more likely to occnr if temperatnre control is not carefnlly monitored and for this reason ammoninm acetate is often nsed. 

A limited number of polyanion-polycation systems were tested using a droplet/falling annulus method (Fig. 4). This technique, which has been described elsewhere [64] reduces the net impact velocity between the droplet with the oppositely charged counterion fluid. A stream of droplets was directed into a collapsing annular liquid sheet. By matching the velocities of the droplet and sheets, the impact conditions can be moderated. It has been shown to produce monodisperse spherical capsules, though it requires several days of calibration for each new system and is obviously not practical for a massive screening such as was carried out herein. 

Charging by transport When charged liquid droplets or solid particles settle on an isolated object, the object is charged. The transferred charge is a function of the object s capacitance and of the conductivities of the droplet, particle, and interface. 

---