Liquid aerosol-phase

Assuming that the system is open and c remains constant during the condensation of A, then we can calculate the timescale xa from (12.145). This timescale corresponds to the establishment of equilibrium between the gas and liquid aerosol phases as a result of changes in the aqueous-phase concentration of A. Following the nondimensionalization procedure of the previous section, we find... 

Colloid chemistry investigates substance mixtures. These substance mixtures can be heterogenous, such as emulsions (in which tiny droplets of one liquid are dispersed in another), suspensions (consisting of a fine dispersion of solid particles in a liquid volume phase), and aerosols (in which liquid droplets are dispersed in the gas phase). However, there are also homogenous mixtures in which the solute is present in larger, supermolecular aggregates. These homogenous mixtures include micellar solutions and liquid crystalline... 

Dispersed System Disperse Phase Dispersing Agents and Dispersion Medium. Dispersed system is an apparently homogeneous, system which consists of a microscopically heterogeneous mixture of two or more finely divided phases, eg liquid-liquid (emulsions such as milk) liquid-solid (suspension, such as clay in water) gas-liquid (aerosols, such as fogs, clouds, mists) and gas-solid (such as smoke, dusts)... 

Nanoparticles are frequently used as a suspension in some kind of solvent. This is a two phase mixture of suspended solid and liquid solvent and is thus an example of a colloid. The solid doesn t separate out as a precipitate partially because the nanoparticles are so small and partially because they are stabilised by coating groups that prevent their aggregation into a precipitate and enhance their solubility. Colloidal gold, which has a typical red colour for particles of less than 100 nm, has been known since ancient times as a means of staining glass. Colloid science is a mature discipline that is much wider than the relatively recent field of nanoparticle research. Strictly a colloid can be defined as a stable system of small particles dispersed in a different medium. It represents a multi-phase system in which one dimension of a dispersed phase is of colloidal size. Thus, for example, a foam is a gas dispersed in a liquid or solid. A liquid aerosol is a liquid dispersed in gas, whereas a solid aerosol (or smoke) is a solid dispersed in a gas. An emulsion is a liquid dispersed in a liquid, a gel is liquid dispersed in a solid and a soils a solid dispersed in a liquid or solid. We saw in Section 14.7 the distinction between sol and gel in the sol gel process. 

These calculations (Fig. 5.8) assumed chemical equilibria at each temperature. It is well known that atmospheric liquid aerosols are often supercooled (Carslaw et al. 1997 Sattler et al. 2001 Buseck and Schwartz, 2004). If these simulations were run without water ice as a solid phase in the minerals database (in essence, supercooling), then the predicted pH values up to 12 km would be just an extension of the lower data points (temperature = 0 to 25 °C) in Fig. 5.8. Given that stratospheric aerosols are concentrated acidic solutions (Carslaw et al. 1997), clearly the equilibrium calculations... 

The particular chemical form in which an element exists in water is its speciation. For example, an element can be present as a simple hydrated ion, as a molecule, as a complex with another ion or molecule, and so forth. From what was said previously, bare ions or bare polar molecules do not exist in water. At the least, they would be solvated species. Species of an element are distinguishable from one another stoichibmetrically, structurally, and energetically. In addition to aqueous species, one can distinguish elements in different phases, for example, as gaseous species, as solid phases, or in adsorbed states, and on the basis of particle sizes. In the atmosphere, for example, speciation extends over liquid, gas, and aerosol phases (Seinfeld, 1986). The notion of chemical speciation is central to equilibrium and kinetic aspects of aquatic chemistry, as will be evident throughout this book. 

The chemical composition of stratospheric particles at a given altitude is governed by local thermodynamic conditions, and specifically is a complex function of temperature and water vapor concentration. The solubility of gas-phase compounds into liquid aerosol particles... 

Aerosol particles in the atmosphere contain a variety of volatile compounds (ammonium, nitrate, chloride, volatile organic compounds) that can exist either in the particulate or in the gas phase. We estimate in this section the timescales for achieving thermodynamic equilibrium between these two phases and apply them to typical atmospheric conditions. The problem is rather different compared to the equilibration between the gas and aqueous phases in a cloud discussed in the previous section. Aerosol particles are solid or concentrated aqueous solutions (cloud droplets are dilute aqueous solutions), they are relatively small, and aqueous-phase reactions in the aerosol phase can be neglected to a first approximation because of the small liquid water content. 

In the electrostatic encapsulation process, a core material and an immiscible liquid coating phase are converted into oppositely charged aerosols so that the core phase (having higher surface tension) is surrounded by a shell of the coating phase, which is then hardened by a suitable means, forming small microcapsules. 

Any of these substances can be a solid, a liquid, a gas, a vapour (which is the gaseous phase of a liquid or solid material at ambient temperature and pressure), an aerosol (a dispersion of microscopic solid or liquid particles in a gaseous medium), a smoke (an aerosol usually of carbon particles with less than 0.1 [xm diameter), or a fog (a visible liquid aerosol). 

Liquid aerosols may be formed by one of two processes, depending on whether the dispersed system begins as a liquid or undergoes a phase change... 

The gas-phase mixture is considered an ideal gas, and in this case Dalton s law states that concentrations are equal to partial pressures divided by the overall pressure p (N m" ). According to HEA, these partial pressures are equal to the saturation pressures of the liquid aerosols. The appropriate description of such saturation pressures depends on the circumstances (see Table 18.2). A hydrocarbon gas does not readily dissolve in water, and therefore two sets of immiscible aerosols will exist in independent equilibrium with the gas phase. Raoult s law describes equilibrium over dilute mixtures, whereas equilibrium over nonideal binary solution requires contaminant-specific empirical models. An example of the latter is Wheatley s model, which states that ... 

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