Water-air equilibrium

Air-Water Equilibrium and Mass Transfer — Odors and Reaeration in Sewers... 

Freshwater mammals such as heaver may leave odors on the surface of their ponds and olfactorily sample the water or layer of air immediately above it. Lipids on water may form micelles, small blobs of molecules (from Latin mica, a grain, crumb, morsel) that enhance evaporation into the air layer by increased chemical potential. Some seahirds hunt hy odor (e.g. Hutchison and Wenzel, 1980 Nevitt, 1999). They may respond to prey volatiles (from krill, squid, or fish) that rise to the water surface and evaporate into the air. The air-water equilibrium for dilute solutions can be expressed by using partition coefficients, relative volatility, or Henry s law (Thibodeaux, 1979). 

As illustrated in Figure E8.1.1, a relative humidity of 100% is at air-water equilibrium for H2O. The relative humidity does not usually rise beyond 100% because water vapor condenses. At a relative humidity below 100%, water evaporates. At 100% relative humidity, the partial pressure of water vapor in the air is equal to the pressure at equihbrium, also called the vapor pressure. 

A relative humidity of 70% means that we are at 70% of air-water equilibrium or... 

Note that is not necessarily constant with varying p,. In fact, evaluation of the air-water equilibrium distribution ratio as a function ofp, is one of the methods that can be used to assess the concentration dependence of y-w of an organic compound, regardless whether the compound is a gas, liquid, or solid at the temperature considered (see below). 

Inspection of Eq. 6-14 reveals that we do not need to learn anything new to understand air-water equilibrium partitioning of neutral organic compounds. All we have to do is to recall how chemical structures (controlling intermolecular interactions) and environmental factors (e.g., temperature, presence of salts or organic cosolvents in the aqueous phase) affect the vapor pressure and the aqueous activity coefficient of a given compound. Hence, our discussion of air-water partitioning can be quite brief. 

When considering the air-water equilibrium partitioning of an organic acid or base, we may, in general, assume that the ionized species will not be present in the gas phase. The air-water distribution ratio of an organic acid, >,aw (note that we speak of a ratio and not of a partition constant since we are dealing with more than one species), is then given by ... 

P 8.2 Air-Water Equilibrium Distribution of Organic Acids and Bases in Fog... 

Schreitmuller, J., Ballschmiter, K. (1995) Air-water equilibrium of hexachlorocyclohexanes and chloromethoxy-benzenes in North and South Atlantic. Environ. Sci. Technol. 29, 207-215. 

HOPs from HWMs can find their way into surface water and ground water. The persistence of these chemicals in water can depend on how easily they can volatilize from solution. To derive the air-water equilibrium relationship we start with the fugacity relationship for the air phase ... 

Table 3.A shows the calculated mol fractions for the air-water equilibrium. 

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