Hygroscopic particle-vapor

Panicle huental Pressure Laplace s Formula 257 Limit of Applicability of Kelvin Relation 258 Hygroscopic Particle-Vapor Equilibrium 259 Charged Particle-Vapor Equilibria 263 Solid-Particle-Vapor Equilibrium 265... 

The life persistency of a smoke cloud is deterrnined chiefly by wind and convection currents in the air. Ambient temperature also plays a part in the continuance or disappearance of fog oil smokes. Water vapor in the air has an important role in the formation of most chemically generated smokes, and high relative humidity improves the performance of these smokes. The water vapor not only exerts effects through hydrolysis, but it also assists the growth of hygroscopic (deliquescent) smoke particles to an effective size by a process of hydration. Smoke may be generated by mechanical, thermal, or chemical means, or by a combination of these processes (7). 

Water Surface Burst. When the entire platform of a water surface detonation is vaporized, the primary particle population is exactly like that described under airburst. However, the particles of the primary population act as condensation nucleii for the late-time condensation of sea salts. The salt particles are hygroscopic and eventually dissolve and leave the primary population behind. However, particle transport is affected by the sea salt particle growth which temporarily, at least, produces larger particles than does an airburst. 

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. 

The second reason for the difference between the deposition of stable aerosols and many therapeutic aerosols is hygroscopic growth and evaporation. For drugs to be effective, they must show an appreciable aqueous solubility. Solid drug particles may pick up water vapor and dissolve, and droplets of drug solutions can exchange water with the environment to equalize vapor pressure. The relative humidity profile in the respiratory tract depends on the ambient conditions and the breathing pattern [67]. 

This analysis can be applied to a small dry salt particle exposed to increasing relative humidity. The particle remains solid until, if it is hygroscopic, a characteristic relative humidity less than 100% at which it absorbs water and dissolves, forming a saturated solution. The relative humidities at which this occurs for saturated solutions of various salts are shown in Table 9.3. These values will vary with crystal size because of the Kelvin effect. For sodium chloride,. solution takes place at a relative humidity of 75% at which the diameter about doubles. With increasing relative humidity, the equilibrium relationship between drop size and vapor pressure is determined by the interaction of the Kelvin effect and vapor pressure lowering. 

The intensity of exposure variable is partially affected by the physical state and properties of the toxicant. Heavier-than-air gases are particularly affected by environmental conditions for example, warm environments increase the vaporization of some substances (such as mustard), making inhalational toxicity more likely. Increased humidity increases particle size by hygroscopic effects. Increased particle size may decrease the respiratory exposure to a toxicant because larger particles may precipitate prior to inhalation, or they may be collected preferentially in the upper airways, which have better clearance mechanisms. 

A film (membrane) is mounted on a sealed cup containing calcium chloride dried before-hand at 200 °C (or another hygroscopic powder). The anhydrous calcium chloride should have a particle size between 600 rm and 2.36 mm. The cup should be made of a Ught material that is impermeable to water vapor. The opening of the cup, over which the film is mounted, should be on the order of 3000 mm. The distance between the top of the anhydrous CaCl2 and the film should be approximately 6 mm. (It is important to avoid touching the film with the powder.) The assembly is placed in an environmental chamber where the temperature and relative humidity should be checked frequently (temperature from 21 °C to 23 1 °C and relative humidity at 50% 2%). Air must circulate continuously at a velocity of up to 0.3 m s. ... 

As seen from the Kohler equation (4.223), the condensation of water vapor into preexisting droplets is only possible when the saturation ratio is larger than one. However, the mass transfer of water molecules is possible for 6 < 1 when the solid (aqueous particles are not existing) particle surface provides a large enough affinity to H2O. This property is called hygroscopicity. Each crystalline water-soluble surface dissolves or deliquesces at a certain RH (Table 4.15). 

In the immediate vicinity of the interface between free water and vapor, the vapor pressure at equilibrium is the saturated vapor pressure. Very moist products have a vapor pressure at the interface almost equal to the saturation vapor pressure. If the concentration of solids is increased by the removal of water, then the dissolved hygroscopic solids produce a fall in the vapor pressure due to osmotic forces. Further removal of water finally results in the surface of the product dried. Water now exists only in the interior in very small capillaries, between small particles, between large molecules, and bound to the molecules themselves. This binding produces a considerable lowering of vapor pressure. Such a product can therefore be in equilibrium only with an external atmosphere in which the vapor pressure is considerably decreased. 

Tl. Mikhailov, E., Vlasenko, S., Niessner, R., and P oschl, U. Interaction of aerosol particles composed of protein and saltswith water vapor hygroscopic growth and microstructural rearrangement, Atmos. Chem. Phys., 4, 323—350, 2004, http //www.atmos-chem-phys.net/4/323/2004/. 

The kinetics of chemical interactions between fission product vapor and aerosols is mainly controlled by gas phase mass transport, by the kinetics of the chemical reaction, and by mass transport in the condensed phase. Another factor potentially influencing the kinetics of vapor deposition is that the heat liberated by condensation or by chemical reaction of vapor with aerosol must be disposed of. Because of their small masses, aerosol particles have only limited capacity for conducting away this heat, compared with the structures within the reactor coolant system. This problem may arise particularly in the deposition of water vapor onto aerosol particles which have been previously covered by hygroscopic or water-soluble compounds such as CsOH. 

---