Atmospheric Equilibrium of an Aqueous Solution Drop

In the previous sections we developed expressions for the water vapor pressure over a pure water droplet and a flat solution. Atmospheric droplets virtually always contain dissolved solutes, so we need to combine these two previous results to treat this general case. 

FIGURE 15.4 Variation of water vapor pressure ratio (p /p°) as a function of the salt molality (mol of salt per kg of water) of NaCI and (NH4)2804 at 25 °C. 

Let us consider a droplet of diameter Dp containing moles of water and n moles of solute (e.g., a nonvolatile salt). If the solution were flat the water vapor pressure over it would satisfy (15.14). Substituting this expression into (15.9) one obtains 

Let and Uy be the partial molar volumes of the two components in the solution and the total drop volume will satisfy 

Note that since we have assumed that the solute is nonvolatile, is the same regardless of the value of Dp. Using (15.19) and (15.15), we find that 

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Atmospheric Equilibrium of an Aqueous Solution Drop Containing an Insoluble Substance... 

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