Section 6.1. In situ characterization of electrosprayed CsH 2 PO 4 solid acid nanoparticles
6.1.6. Discussion
5 10 15 20 25 30 10
20 30 40 50 60
Mean particle diameter (nm)
PVP concentration (g/L)
20 40 60 80 100 120
10 20 30 40 50 60 70 80
Mean particle diameter (nm)
Temperature (C)
200 400 600 800 1000 1200
10 20 30 40
Mean particle diameter (nm)
Nitrogen flow rate (cm3 min-1)
Figure 6.9. Mean particle size of aerosol vs. (a) PVP concentration, (b) electrospray chamber temperature, and (c) sheath nitrogen flow rate through electrospray capillary with standard deviation of the distributions indicated.
the liquid before droplets form when the cone jet breaks up. The formation of aerosol droplets is more facile and occurs more frequently for high conductivity and low surface tension solutions, resulting in smaller initial droplet size.
Similarly, highly charged liquid aerosol droplets experience both repulsive Coulombic forces and stabilizing forces due to surface tension. As the solvent evaporates without carrying a significant amount of charge away from the primary droplet, the charge concentration increases up to the Rayleigh limit, when Coulomb fission or oscillatory instabilities occur according to the charge residue model. The Rayleigh limit is reached faster when the surface tension is lower and the droplet charge higher, leading to more frequent droplet fission events. Solvent evaporation rate is higher from smaller droplets, also favoring more frequent fission events. Fission events can occur multiple times during droplet flight, leading to significantly smaller aerosol droplets. When the aerosol droplets reach the neutralizer, the charge per droplet is reduced so that fission events become impossible. Beyond the neutralizer droplet size reduction can only occur via solvent evaporation. At this stage, the final electrolyte particle size is dependent on the droplet size and the solute concentration. As the solvent evaporates, the solute concentration of the droplets increases. The solute precipitates when critical supersaturation is reached, forming either solid particles or hollow shells, depending mainly on the solvent evaporation rate. Volume precipitation is favored at low evaporation rates, while surface precipitation is favored at high evaporation rates.
Considering the results of this paper, the decrease in the mean particle size with increasing CsH2PO4 concentration observed in Fig. 5 is consistent with the increased solution conductivity.
A higher conductivity results in smaller initial aerosol droplets and a higher evaporation rate. In addition, higher solution conductivity permits more charge to be transferred on the aerosol droplets at the Taylor cone. Stronger repulsive Coulomb forces cause less stable aerosol droplets leading to more frequent disintegration events and, hence, smaller aerosol droplets. The leveling off of the particle size is plausibly a result of the higher amount of solute per droplet as the CsH2PO4
concentration continues to increase.
If we take this measured particle size to reflect solid rather than liquid particles, we can estimate, based on the solubility limit, the droplet sizes prior to solid precipitation. This provides an indication of the liquid content of the particles as they impinge onto the substrate. In the case of 25 nm and 45 nm solid CsH2PO4 particles, the droplet sizes, beyond which no further Coulomb fission events are possible, are 86 nm and 150 nm, respectively, in a 53 wt% methanol-water
solution. We also observe a decrease in the standard deviation, i.e., a more monodisperse particle size distribution with increasing CsH2PO4 concentration, consistent with a more stable Taylor cone. A CsH2PO4 concentration larger than 5 g/L leads to frequent clogging of the capillary and hence is not suitable for long-term operation (>1 hour).
The slight decrease of the mean particle diameter with increasing methanol concentration is consistent with the decreasing surface tension of the solution; however, the effect is moderated by the simultaneous drop in solution conductivity, which reduces the droplet charge. In addition to smaller size initial aerosol droplets due to lower surface tension, methanol has a lower boiling point than water. Decreasing the water-methanol ratio should allow faster evaporation of the solution and, with that, more frequent droplet disintegration events before the aerosol enters the neutralizer. This effect is less dramatic because of the decrease of the solution conductivity with increasing methanol concentration, so that the droplets are charged to a smaller fraction of the Rayleigh limit, driving fewer fission events.
Similarly, the slightly lower surface tension with increasing PVP content does not impact particle size. The added volume of the surfactant as a skin on the electrolyte particles probably influences the evaporation rate adversely to counter the surface tension effect.176
When the applied capillary voltage is increased, the droplets are charged to a higher fraction of the Rayleigh limit, facilitating droplet disintegration.177 Since the range of the voltage variation is very limited, the particle size reduction at the high end of the applied voltage range is also barely detectable.
When the electrospray temperature is varied, the reduction of the mean particle diameter can be explained by the higher evaporation rate of the solvent and the reduction of the surface tension. A higher solvent evaporation rate allows more frequent fission events leading to smaller electrolyte particles. At very high evaporation rates, hollow particles with larger diameters are generated as in other salt containing liquid systems, explaining the marked increase in the mean particle diameter and the width of the particle size distribution, without the increase of the solute supply rate.
Finally, an increase in the nitrogen flow rate effectively reduces the partial pressure of the solvents and hence increases the evaporation rate in addition to possible convective effects, leading to more frequent fission events and smaller electrolyte particles.
Methanol (mol %)
CsH2PO4
(g/L)
PVP (g/L)
Temperature (°C)
N2 flow rate (cm3/min)
Liq flow rate*
(ml/h)
Voltage*
(kV)
Range 22 – 53 0.5 - 10 1 – 30 22 – 120 200 - 1200 0.5 – 2.5 4.9 – 5.75
Default 53 5 or 10 1 100 1000 0.5 5
Table 6.1. Parameters varied in the electrospray deposition process. The sprayed solution consists of CsH2PO4 dissolved in a water-methanol mixture, in some cases with PVP added as a surfactant.
*The voltage and liquid flow rate settings cannot be varied entirely independently as they are restricted to values that yield a stable Taylor cone, and constant particle concentration.