Deliquescence point

At point B, the steps disappear due to surface dissolution. This is the deliquescence point. Source Ref. 78. 

Finally, upon reaching the deliquescence point, the surface step stracture collapses and a thick liquid film is formed. 

In addition, the high concentrations of ions in solutions of high ionic strength such as sea salt particles (especially near their deliquescence point) can alter gas solubility. In this case, the Henry s law constants must be modified using Setchenow coefficients to take this effect into account (e.g., Kolb et al., 1997). 

Chlorine nitrate is formed from the reaction of CIO with N02.) These reactions also occur when NaCl is in the aqueous phase, in competition with the hydrolysis of N205 and C10N02, i.e., above the deliquescence point of NaCl in sea salt (e.g., Behnke et al., 1997). Photolysis then generates chlorine atoms, e.g.,... 

Figure 7.14 shows the calculated ratio of S(IV) oxidation with the uptake and reaction of N03 to that without the NO, contribution as a function of the chloride concentration in particles (Rudich et al., 1998). For reference, the saturation concentration of Cl- in sea salt particles (i.e., at the deliquescence point) is 6 M at room temperature. Under the assumptions of these particular calculations, the rate of aqueous-phase oxidation of S(IV) is estimated to increase by as much as 25% when N03 chemistry is taken into account. This uptake and reaction of NO, also decrease its gas-phase concentrations. 

The ammonium nitrate formed in reaction (54) can exist either as a solid particle or in solution, and since this reaction is an equilibrium, it can redissociate to form the reactants. The deliquescence point for NH4N03 at 25°C is 62% RH i.e., at a water vapor concentration corresponding to 62% RH, the solid particle dissolves to form a concentrated liquid solution. 

At water vapor concentrations above the deliquescence point, the equilibrium is that between the reactant gases and aqueous ammonium nitrate. As treated in detail by Mozurkewich, the equilibrium constant, K 4-54, then depends on the solution concentrations or activities ... 

If the relative humidity is above the deliquescence point of NaCl (75% at 25°C), the sea salt particles are... 

Atmospheric aerosols are hygroscopic, taking up and releasing water as the RH changes (see also Section C.l) because some of the chemical components are themselves deliquescent in pure form. For example, sodium chloride, the major component of sea salt, deliquesces at 298 K at an RH of 75%, whereas ammonium sulfate, (NH4)2S04, and ammonium nitrate, NH4N03, deliquesce at 80 and 62% RH, respectively. (See Table 9.16 for the deliquescence points of some common constituents of atmospheric particles.) De-... 

TABLE 9.16 Deliquescence Points of Some Salts Commonly Found in Ambient Air at 25°C ... 

It should be noted that hysteresis occurs as the particles are dried out i.e., the liquid solution does not form a solid particle as the water evaporates at the same RH as it went through the solid - liquid transition. Typically one must reach RHs 20-30% or more below the deliquescence point in order to dry the particle. Figure 9.41, for example, shows the uptake of water by solid (NH4)2S04 and its subsequent dehydration (Tang et al., 1995). At 80% RH the solid deliquesces but does not solidify (effluoresce) on drying until an RH of 37% is reached. (The presence of other species has been shown to increase this effluorescence... 

It should be noted that as with all analytical techniques that involve subjecting the sample to vacuum conditions before and/or during the analysis, separation of components via selective crystallization is expected (e.g., Ge et al., 1998a). Hence these particles may not have actually existed in these crystalline forms at relative humidities above their deliquescence points in the atmosphere, although the various constituents observed were clearly present. 

For example, Neubauer et al. (1998) have shown that the spectra can be very sensitive to the amount of water present and whether the particle is aqueous, i.e., above the deliquescence point, or solid (but holding adsorbed water on the surface). Figure 11.72 shows the... 

Take as an example, a small dry particle of NaCl of a given mass (mu) that is introduced into air at a water vapor pressure corresponding to SA in Fig. 14.38a. Assuming that the RH is above the deliquescence point of NaCl, 75% at 25°C, the particle will take up water, dissolve, and form a stable droplet of radius rA. Similarly, if the air saturation ratio increases to Su, the particle will, under equilibrium conditions, take up water and grow to radius ru. 

Hygroscopic behavior has been well characterized in laboratory studies for a variety of materials, for example, ammonium sulfate (Figure 14), an important atmospheric material. When an initially dry particle is exposed to increasing RH it rapidly accretes water at the deliquescence point. If the RH increases further the particle continues to accrete water, consistent with the vapor pressure of water in equilibrium with the solution. The behavior of the solution at RH above the deliquescence point is consistent with the bulk thermodynamic properties of the solution. However, when the RH is lowered below the deliquescence point, rather than crystallize as would a bulk solution, the material in the particle remains as a supersaturated solution to RH well below the deliquescence point. The particle may or may not undergo a phase transition (efflorescence) to give up some or all of the water that has been taken up. For instance, crystalline ammonium sulfate deliquesces at 79.5% RH at 298 K, but it effloresces at a much lower RH, 35% (Tang and Munkelwitz, 1977). This behavior is termed a hysteresis effect, and it can be repeated over many cycles. 

Instability attributable to excipient-mediated water distribution in solids and powders has been explained by excipient physical properties. " Crystalline materials will not uptake moisture until the deliquescent point is reached. In contrast, amorphous excipients will absorb water until their glass transition temperatures fall below the ambient temperature when the mobility of the molecules has increased so much that excipient crystallization will occur to expel the absorbed water from the crystal lattice. Before crystal-... 

Recent work (LM Russell, unpublished results) has shown that for soluble salts that adsorb water below their deliquescence point (including NaCl), the DRH values increase... 

Seawater contains sea salt to about 3.5% by weight, of which 85% is sodium chloride. It can be safely assumed that the sea-salt content of jet and film drops is similar. As the drops enter the atmosphere they experience lower relative humidities and dry up until their water content is in equilibrium with the environment. The particle radius then is about one-quarter of the parent drop s radius. Although parts of sodium chloride may crystallize, one should not expect the particles to dry up completely, because the deliquescence point of magnesium chloride, which is also present in sea salt, lies at 31% r.h. (see Table 7-6). Such low relative humidities are not reached in marine air. 

Stelson et al. (1979, 1982) directed their attention to the opposite end of the relative humidity scale and suggested from the knowledge of partial pressures above crystalline NH4N03 that this material tends to evaporate when the relative humidity falls to values below the deliquescence point of NH4NO3 (62%). Accordingly, the partitioning of nitrate between gas phase and particulate matter will be quite variable. 

Multicomponent aerosol particles exhibit behavior similar to that of single-component salts. As the ambient RH increases the salt mixture is solid, until the ambient RH reaches the deliquescence point of the mixture, at which the aerosol absorbs atmospheric moisture and produces a saturated solution. A typical set of data of multicomponent particle deliquescence, growth, evaporation, and then crystallization is shown in Figure 10.7 fora KCl-NaCl particle. Note that the DRH for the mixed-salt particle occurs at 72.7% RH, which is lower than the DRH of either NaCI (75.3%) or KC1 (84.2%). 

The preceding analysis can be extended to aerosols containing more than two salts. Thus one can prove that water activity reaches a minimum at the deliquescence point of the aerosol. Another consequence of this analysis is that the DRH of a mixed salt is always lower than the DRH of the individual salts in the particle. Wexler and Seinfeld (1991) solved (10.76) for the case of the system containing NH4NO3 and NH4C1. Their calculations at 303 K are depicted in Figure 10.8. 

The mutual deliquescence points of a series of pairs are given in Table 10.4. 

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