Water vapor nucleation

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Nucleation in a cloud chamber is an important experimental tool to understand nucleation processes. Such nucleation by ions can arise in atmospheric physics theoretical analysis has been made [62, 63] and there are interesting differences in the nucleating ability of positive and negative ions [64]. In water vapor, it appears that the full heat of solvation of an ion is approached after only 5-10 water molecules have associated with... 

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. 

Homogeneous Nucleation a process by which gases interact and combine with droplets made up of their own kind. For instance, the collision and subsequent adherence of water vapor to a water droplet is a homogeneous nucleation. See nucleation. 

The effect of physical aging on the crystallization state and water vapor sorption behavior of amorphous non-solvated trehalose was studied [91]. It was found that annealing the amorphous substance at temperatures below the glass transition temperature caused nucleation in the sample that served to decrease the onset temperature of crystallization upon subsequent heating. Physical aging caused a decrease in the rate and extent of water vapor adsorption at low relative humidities, but water sorption could serve to remove the effects of physical aging due to a volume expansion that took place in conjunction with the adsorption process. 

The nucleation of these decomposition processes was studied by means of thermomicroscopy on single crystal cleavage plates of calcite, magnesite, dolomite and smithsonite (Fig. 59). The shape of the nuclei was found to be different for these carbonates, which might be also of importance for the decomposition mechanism. The partial pressure of water vapor has a pronounced effect on the decomposition of transition metal carbonates such as ZnC03 and CdC03. The evolution of C02 is probably catalyzed in the presence of water vapor and shifted to considerably... 

A flow stream produced from boiling water appears white in color. Similar to cloud in the sky, condensed water vapor shows a white color in the atmosphere. Humid air leads to condensation when nucleating materials are present in the atmosphere, producing a white-colored fog. However, condensed water vapor and fog appear as black smoke when the background is brighter than the foreground. 

Nucleation is necessary for the new phase to form, and is often the most difficult step. Because the new phase and old phase have the same composition, mass transport is not necessary. However, for very rapid interface reaction rate, heat transport may play a role. The growth rate may be controlled either by interface reaction or heat transport. Because diffusivity of heat is much greater than chemical diffusivity, crystal growth controlled by heat transport is expected to be much more rapid than crystal growth controlled by mass transport. For vaporization of liquid (e.g., water vapor) in air, because the gas phase is already present (air), nucleation is not necessary except for vaporization (bubbling) beginning in the interior. Similarly, for ice melting (ice water) in nature, nucleation does not seem to be difficult. 

Although Eq. 27 appears to be the most likely initiation reaction, we cannot rule out a process in which water vapor and DMTC react, based on the ab initio results described in Sect. 4.6. If this does occur, however, it apparently does not lead to homogeneous nucleation of particles, since anecdotal evidence from the glass industry indicates that DMTC and water vapor can be premixed prior APCVD of tin oxide without substantial buildup of solids in delivery lines. Perhaps this is due to significant kinetic barriers to the decomposition of the tin-water complexes that initially form, so that further gas-phase reaction does not occur until the reactants enter the heated boundary layer above the substrate. 

Figure 12.22 shows the composition in terms of the weight percent HNO, and H2S04 as a function of temperature as solid SAT is cooled from 194 K under conditions corresponding to a pressure of 50 rnbar in an atmosphere containing 5 ppm HzO and an HNO, concentration of 10 ppb (Koop and Carslaw, 1996). Under these particular conditions, as the temperature falls below 192 K, the SAT is in equilibrium with a liquid film on the particle containing both HN03 and H20. The particular temperature at which SAT deliquesces is a function of the water vapor and gaseous nitric acid concentrations as shown in Fig. 12.23. As the temperature falls further and more HNO, and HzO are taken up into the liquid, the solid SAT dissolves completely, forming a ternary solution of the two acids and water. This solution can then act again to nucleate PSCs.

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

A particular, and unusual, atmospheric application of such data involves the formation of noctilucent clouds (NLC s) in the vicinity of the mesopause (at 82 km, in the summer hemisphere, where temperatures can fall as low as 130 K, and ice can exist even at the miniscule ambient water vapor concentrations found there). The presence of laige water-aggregated hydronium ions led to the suggestion [e.g., 63-65] that these provide condensation sites for ice particles. Detailed simulation studies bore out the likely relationship between positive ion nucleation and the behavior of some NLC s [66], notwithstanding a strong possibility that meteoritic dust and smoke also had a dominant role [67], ITie contribution to NLC formation of hydronium-ion/electron... 

Arnold, F Ion-induced nucleation of atmospheric water vapor at the mesopause, Planet. Space Set, 28,... 

Besides small gas bubbles, other nucleation sites (e.g., at minute dust particles) may give rise to the cavitation phenomenon. Normally, the surface tension of water is too high to allow the formation of water vapor bubbles at the relatively small negative pressures created by the sonic field. However, at the surface of the dust particles the surface tension of water may be sufficiently low to create a water vapor bubble in the sonic field and thus start the cavitation process. 

If a glass is held for a long period at an elevated temperature it may start to crystallize or devitrijy. Devitrification of fused quartz (silica glass) to cristabolite is slow. Nucleation is usually at a free surface and is often stimulated by contamination from alkali ions such as sodium. The rate of growth of cristabolite is increased by oxygen and water vapor. With surface contamination, devitrification of fused quartz may occur at temperatures as low as 1000 °C. However, if the surface is clean it rarely occurs below 1150 °C. 

Figure 2.29 Scanning electron micrographs at approximately the same magnification of four microporous membranes having approximately the same particle retention, (a) Nuclepore (polycarbonate) nucleation track membrane (b) Celgard (polyethylene) expanded film membrane (c) Millipore cellulose acetate/cellulose nitrate phase separation membrane made by water vapor imbibition (Courtesy of Millipore Corporation, Billerica, MA) (d) anisotropic polysulfone membrane made by the Loeb-Sourirajan phase separation process...

If particles (or ions) are already present in a supersaturated vapor, nucleation will take place preferentially on these particles at supersaturations far smaller than for the homogeneous vapor. In this case, nucleation takes place heterogeneously on the existing nuclei at a rate dependent on the free energy of a condensate cap forming on or around the nucleus. Heterogeneous nuclei always occur in the earth s atmosphere. They are crucial to the formation of water clouds and to the formation of ice particles in supercooled clouds. 

It was found by Wilson (1897) that when air at 20°C, initially saturated with water vapor and free of any condensation nuclei, was expanded with an expansion ratio in excess of 1.37, homogeneous nucleation occurred. What is the value of S implied by this expansion ratio ... 

Figure 5.1 Gibbs energy of nucleation of water drops at different water vapor oversaturations of at 25°C. Curve 1 is plotted for oversaturation p/poo = 8.1 = 0.5...

As a particular example, one can consider the homogeneous nucleation in the pure water vapor at 25° C. The surface tension coefficient of water is a = 71.96 N/m at this temperature. Table 5.1 shows some characteristics of the new phase. When the oversaturation is p/p =8.1, the critical nucleus of 0.5 nm radius is seen to comprise 18 water molecules. The equihbrium pressure of such nuclei is not high (approximately 10 bar). Since the water vapor pressure in real clouds is usually no more than 0.1% over that of the saturated vapor, it is unrealistic to expect in the rea sonable time scale the homogeneous formation of water drops in Earth s atmosphere. 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un-... 

In a number of other cases, however, notably in the exothermic decompositions of solids which can become explosive and in the endothermic transformation of hydrates of salts and of carbonates (to oxides), the slow processes seem to be nucleation. The rate laws for such nucleation-con-trolled processes can be very complex, and the rate studies are difficult to make and to reproduce. In many of these cases smj amounts of impurities play an important role in governing the de fed t nff and growth of nuclei, and there are a number of instances of sm bA ounts of water vapor having a significant catalytic effect. ... 

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