Reactions in the Aqueous Phase

We have seen in Chapter 8 that reactions in the aqueous phase present in the atmosphere in the form of clouds and fogs play a central role in the formation of sulfuric acid. Thus, an additional mechanism of particle formation and growth involves the oxidation of SOz (and other species as well) in such airborne aqueous media, followed by evaporation of the water to leave a suspended particle. 

FIGURE 9.33 Size distribution of particles in clouds (solid line) and below the clouds (dashed line), showing two modes (adapted from Hoppel et al., 1994). 

Relative Importance of Various Aerosol Growth Mechanisms 

Different mechanisms of aerosol growth give rise to different so-called growth laws, which are expressions relating the change in particle size (e.g., volume or diameter) with time to the particle diameter. Because different mechanisms of particle formation give rise to different growth laws, one can test experimental data to see which mechanism or combination of mechanisms is consistent with the observations. For a more detailed discussion of this approach, see Friedlander (1977), Heisler and Friedlander (1977), McMurry and Wilson (1982), Pandis et al. (1995), and Kerminen and Wexler (1995). 

As we have seen in our earlier discussion of the size distribution of tropospheric particles, the chemical components are not generally distributed equally among all sizes but, rather, tend to be found in specific size ranges characteristic of their source. Generally, the smallest ultrafine particles are produced by homogeneous nucleation and hence tend to contain secondary species such as sulfate and likely organics (see Section A.2). Particles in the Aitken nuclei range are produced 

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Assuming complete binding of the dienophile to the micelle and making use of the pseudophase model, an expression can be derived relating the observed pseudo-first-order rate constant koi . to the concentration of surfactant, [S]. Assumirg a negligible contribution of the reaction in the aqueous phase to the overall rate, the second-order rate constant in the micellar pseudophase lq is given by ... 

At the same time, the extraction rate was accelerated by the reactions in the aqueous phase [23],... 

As discussed in Chapters 7, 8, and 9, there are a number of free radical species whose reactions in the aqueous phase drive the chemistry of clouds and fogs. These include OH, HOz, NO-, halogen radicals such as Cl2, sulfur oxide radicals, and R02. Generation of these radicals in the liquid phase for use in kinetic... 

As already discussed, -ynul is a net probability normalized to the number of gas-surface collisions and is the parameter actually measured in experiments (and hence also often referred to as ymcas). In Eq. (QQ), each conductance represents one of the processes involved i.e., Tg involves the conductance for gas-phase diffusion, rran that for reaction in the aqueous phase, and rsol that for solubility and diffusion into the bulk. Each of the terms has been normalized and made unitless by dividing by the rate of gas-surface collisions, Eq. (PP), except for a, which by definition is already normalized to this parameter. 

Nitrate and nitrite photochemistry might also play a role in atmospheric hydrometeors. Nitrite photolysis has been shown to account for the majority of hydroxyl photoformation in irradiated fog water from a polluted site [ 14]. In addition, the generation of mutagenic and carcinogenic compounds from amino acids and amines dissolved in fog water [147] is a process that can be linked with nitrite photochemistry [20,141]. Furthermore, the formation of atmospheric nitrophenols partially takes place in aqueous solution. Reactions in the aqueous phase can account for about 30% of the atmospheric sources of mononitrophenols and for the vast majority of the dinitrophenol ones [ 148], and irradiation of nitrate and nitrite can possibly play a role in the process (see Sect. 3.2). Mono- and dinitrophenols are toxic compounds, and their occurrence in rainwater is thought to be a contributory factor in forest decline [149-151]. 

Maxwell et al. [ 11 ] proposed a radical entry model for the initiator-derived radicals on the basis of the following scheme and assumptions. The major assumptions made in this model are as follows An aqueous-phase free radical will irreversibly enter a polymer particle only when it adds a critical number z of monomer units. The entrance rate is so rapid that the z-mer radicals can survive the termination reaction with any other free radicals in the aqueous phase, and so the generation of z-mer radicals from (z-l)-mer radicals by the propagation reaction is the rate-controlling step for radical entry. Therefore, based on the generation rate of z-mer radicals from (z-l)-mer radicals by propagation reaction in the aqueous phase, they considered that the radical entry rate per polymer particle, p p=pJNp) is given by... 

Water insolubility and resistance to hydrolysis water solubility could lead to more reaction in the aqueous phase and wall fouling. Other expedients to reduce aqueous phase reactions include use of a water-soluble free-radical scavenger or a chelating agent to minimize redox reactions in the aqueous phase. (Such water-soluble chelating agents include salts of oxalic acid and ethylene diamine tetraacetic acid.)... 

To follow the reaction in the aqueous phase a toluene solution with a definite diol concentration was agitated with an equal volume of water. After measuring the equilibrium concentration of the distribution the reaction was started by addition of the catalyst (I0 -10" m HCl or NaOH, respectively). At definite reaction times probes of the toluene phase were analyzed by GC or (and) IR. The reaction of the cyclosiloxanediol D3D 2 was investiated in water directly by HPLC. 

Hydrazine is generally obtained by reaction in the aqueous phase. It is subsequently concentrated by successive evaporations and rectifications and then, in order to eliminate the water to the maximum extent, one of the methods most recently employed consists in using the dehydrating power of alkaline or alkaline-earth substances, such as caustic soda, potash, or alkaline-earth oxides (quicklime of barium oxide). It is then separated from the dehydrating agent by distillation. 

The notion that chemical reactions in the aqueous phase may be important to atmospheric chemistry dates back at least 30 yr, when Junge and Ryan (1958) called attention to the great potential of cloud water for the oxidation of dissolved S02 by heavy-metal catalysis. At that time the process appeared to be the only viable oxidation mechanism for atmospheric S02. Later, when the concept of OH radical reactions gained ground, the gas-phase oxidation of S02 by OH was recognized to be equally important. The recent revival of interest in aqueous phase reactions is connected with efforts to achieve a better understanding of the origins of rainwater acidity. An oxidation of N02 to nitric acid also takes place in cloud water. Contrary to previous ideas, however, this process was recently shown to have little influence on atmospheric reactions of N02. 

The absorption and IR spectra in both the aqueous and the organic phase were consistent with the extraction equilibrium (A.67). There is no quantitative information on the equilibrium constant for Reaction (A.67) or for the complex formation reactions in the aqueous phase. 

The reaction of triphasc catalysis is carried out in a three-phase liquid (organic) - solid (catalyst) - liquid (aqueous) condition. In general, the reaction mechanism of the triphasc catalysis is (i) mass transfer of reactants form the bulk solution to the surface of the catalyst pellet, (ii) diffusion of reactants to the interior of the catalyst pellet (active sites) through pores, and (iii) surface or intrinsic reaction of reactants with active sites. For step (iii). the substitution reaction in the organic phase and ion exchange reaction in the aqueous phase occurred. 

A typical LLPTC cycle involves a nucleophilic substitution reaction, as shown in Eq. (8). A difficult problem in the kinetics of PT-catalyzed reactions is to sort out the rate effects due to equilibrium anion-transfer mechanism for transfer of anions from the aqueous to the organic phase. The reactivity of the reaction by PTC is controlled by the rate of reaction in the organic phase, the rate of reaction in the aqueous phase, and the mass transfer steps between the organic and aqueous phases [27-29]. In general, one assumes that the resistances of mass transfer and of chemical reaction in the aqueous phase can be neglected for a slow reaction in the organic phase by LLPTC. 

Role of reactions in the aqueous phase and water solubility of... 

Asua et al. [49] have developed a model for estimating k, taking into consideration the possible reactions in the aqueous phase as well as the competition between termination and desorption when a monomer radical enters a particle already containing a radical. For low values of n, the authors arrived at the expression ... 

First, the equilibrium constant for the reaction in the aqueous phase was determined, and it was found that the equilibrium of the reaction was shifted almost entirely toward the left = 2 X 10 ). The yield of the ester obtained for reaction in the aqueous phase was as low as 0.01%. Next, four different organic solvents (chloroform, benzene, carbon tetrachloride, and diethyl ether) were selected the partition coefficient was determined for each of the four solvent-water binaries. Then the biphasic equilibrium constant was found from Equation 18.7 and plotted as a function of a (Figure 18.3). 

The first term on the right represents the homogeneous reaction rate in the aqueous phase and is not affected by the microphase. Clearly, the reaction rate on the microphase (or any additional reaction in the aqueous phase due to the microphase) can be obtained by subtracting the observed aqueous phase rate without a microphase from the observed (total) rate with a microphase. 

Competition between HA and HB leading to the following key equilibrium reaction in the aqueous phase (see Figure 25.2) ... 

Rate constant for reaction in the aqueous phase, appropriate units. 

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