Sulfuric acid nucleation

The role of ammonia in water-sulfuric acid nucleation... 

As mentioned in Section IX-2A, binary systems are more complicated since the composition of the nuclei differ from that of the bulk. In the case of sulfuric acid and water vapor mixtures only some 10 ° molecules of sulfuric acid are needed for water oplet nucleation that may occur at less than 100% relative humidity [38]. A rather different effect is that of passivation of water nuclei by long-chain alcohols [66] (which would inhibit condensation note Section IV-6). A recent theoretical treatment by Bar-Ziv and Safran [67] of the effect of surface active monolayers, such as alcohols, on surface nucleation of ice shows the link between the inhibition of subcooling (enhanced nucleation) and the strength of the interaction between the monolayer and water. 

The reaction is sustained by addition of iron metal which reacts with the sulfuric acid formed, regenerating Fe(n) in solution. To ensure that the desired crystal form precipitates, a seed of a-FeO(OH) is added. However, with appropriate choice of conditions, for example of pH and temperature and by ensuring the presence of appropriate nucleating particles, the precipitation process may be adapted to prepare either the orange-brown y-FeO(OH), the red a-Fe203 or the black Fe304. 

The rotating hemisphere electrode has been used to investigate the effect of AC on the electrodissolution and deposition reactions of zinc in zinc chloride [25] and copper in acid copper sulfate solutions [55], AC was found to increase the rate of nucleation and produce more uniform deposit on the zinc electrode. The corrosion of an iron rotating hemisphere in dilute sulfuric acid was investigated by Haili [31] using the AC impedance measurement. 

Fig. 23. Significant coarsening of the domain structure did not occur, at least for the time scale investigated. Thus, charge-induced reconstruction in sulfuric acid results in a much smaller domain size than that associated with a freshly flame-annealed crystal. The domain boundaries act as preferred nucleation sites for subsequent (hex) (1 X 1) transformation, which occurs much more rapidly than for a freshly flame-annealed sample. Studies of this nature have provided keen insight into aging effects that would be difficult to assess by other means.

Herrero and Abruna [25] have also studied the kinetics and mechanism of Hg UPD on Au(lll) electrodes in the presence and absence of bisulfate, chloride, and acetate ions. In the absence of the interacting anions (in perchloric acid), the Hg UPD was significantly controlled by gold-mercury surface interactions. In sulfuric acid solutions, the kinetics of the initial and final stages of mercury deposi-tion/dissolution was altered. The presence of two well-ordered structures at potentials below and above mercury deposition led to the formation of two pairs of sharp spikes in cyclic voltammograms. In the chloride medium, the voltammetric profile exhibited two sharp peaks and thus it was very similar to that obtained in sulfuric acid solution. Neither nucleation, nor growth kinetics mechanism was found to be linked to the process of formation/disruption of the mercury chloride adlayer. The transients obviously deviated from the ideal Langmuir behavior. 

The vapor pressure of H2S04 above solutions with water depends on the solution composition and the temperature. For example, the vapor pressure at 25°C varies from 2.6 X 10-9 Pa for a 54.1 wt% H2S04-H20 solution to 5.9 X 10 6 Pa for a 76.0 wt% solution (Marti et al., 1997). The vapor pressures above solutions partially neutralized with ammonia are also reported by Marti et al. (1997) as discussed in Chapter 9.B.1, the vapor pressures of the partially neutralized solutions are orders of magnitude smaller than those of the acid. As a result, ammonia may play an important role in nucleation of gaseous sulfuric acid in the atmosphere to form new particles. 

Kreidenweis, S. M., and J. H. Seinfeld, Nucleation of Sulfuric Acid-Water and Methanesulfonic Acid-Water Solution Particles Implications for the Atmospheric Chemistry of Organosulfur Species, Atmos. Environ., 22, 283-296 (1988a). 

Roedel, W., Measurement of Sulfuric Acid Saturation Vapor Pressure Implications for Aerosol Formation by Heteromolecular Nucleation, J. Aerosol Sci., 10, 375-386 (1979). 

The term binary homogeneous nucleation is used to describe the formation of particles from two different gas-phase compounds such as sulfuric acid and water such nucleation can occur when their individual concentrations are significantly smaller than the saturation concentrations needed for nucleation of the pure compounds. It is believed that in the atmosphere, formation of particles from low-volatility gases occurs not by condensation of a single species but rather by the formation and growth of molecular clusters involving at least two, and as described shortly, probably three or more different species. 

FIGURE 9.30 Theoretically predicted and experimentally measured concentrations of H2S04 required for homogeneous nucleation of sulfuric acid at a rate of 1 particle cm 3 s 1 (adapted from Hoppel et al., 1994 based on theoretical calculations of Jaecker-Voirol and Mirabel (1989) and experimental data of Wyslouzil et al. (1991). 

As discussed in detail by Pandis et al. (1995), nucleation theory shows there is a critical sulfuric acid concentration above which binary nucleation of sulfuric acid and water should occur. Based on the work of Jaecker-Voirol and Mirabel (1989), they parameterize the theoretical dependence of this critical concentration needed to generate nuclei at a rate of 1 cm 3 s 1 on the relative humidity (RH, expressed in this case as a fraction, i.e., between 0 and 1) and temperature, T (in K) ... 

Jaecker-Voirol, A., and P. Mirabel, Heteroniolecular Nucleation in the Sulfuric Acid-Water System," Atmos. Environ., 23, 2053-2057 (1989). 

Kulmala, M., A. Laaksonen, and L. Pirjola, Parameterizations for Sulfuric Acid Water Nucleation Rates, J. Geophys. Res., 103, 8301-8307 (1998). 

Wyslouzil, B. E., J. H. Seinfeld, R. C. Flagan, and K. Okuyama, Binary Nucleation in Acid-Water Systems. II. Sulfuric Acid-Water and a Comparison with Methanesulfonic Acid-Water, J. Chem. Phys., 94, 6842-6850 (1991). 

These sulfuric acid particles become less concentrated as the temperature decreases or the water vapour increases. Under very cold stratospheric conditions, these liquid aerosols may take up water and HNO, forming ternary solutions H,S0/HN0,/H,0, which eventually freeze [19,24,26], Below 192 K, HNO, becomes the dominant condensed acid, and H,S04 drops to below 3 wt %. The thermodynamics and freezing nucleation of ice and H,S04 or HNO, hydrates from such solutions are however not well understood [27,28]. Other types of solid particles, such as the less stable nitric acid dihydrate (NAD, HN0,.2H,0) [29], sulfriric acid tetrahydrate (SAT, H S04.4H,0) [18,30], sulphuric acid hemihexahydrate (SAH, H2S04.6.5H20) [18], nitric acid penta-hydrate (NAP, HN03.5H,0) [31] and more complex sulfuric acid/nitric acid mixed hydrates [32] may also be a key to understanding Type IPSC nucleation and evolution [28],... 

---