As noted earlier, in the case of advective transport, diffusion of a chemical is dependent on the structure of the molecule. The structure of the molecule determines the size and weight of the molecule. The later parameter affects the rate of diffusion, and the diffusion coefficient of a molecule is inversely related to MW.
DA (\|F(1,MW))1/2
in cm2/sec
This relationship stems from the fact that different chemicals in gas phase (under isothermal and isobaric conditions) possess the same mean kinetic energy since:
KE = \|F(1,2) m1v12 = \|F(1,2) m2v22
v \|R(1/m)
Diffusion v
Diffusion is also affected by several other molecular properties polarizability, dipole movement. These properties can affect the magnitude of intermolecular forces. Obviously, vapor pressure and boiling point are important properties.
Vapor pressure is a measure of the tenacity with which the molecules of the chemical are attached to each other. If a chemical has high intermolecular attraction (generally a result of dipole-dipole or charge-dipole interactions) then only a few of the molecules will escape into the gas phase; the rate of evaporation for such molecules is low.
Similar is the case with boiling point. Boiling point is also a guide to the strength of intermolecular attraction. The temperature at which the vapor pressure of a chemical equals the vapor pressure of the atmosphere is defined as the boiling point. If the boiling point is high, large amounts of energy, usually in the form of heat, must be applied to the chemicals.
Generally, chemicals with vapor pressure of less than 10-7 mmHg (at room temperature ~25š) will be present mostly in soil and will be detected in air or atmosphere in negligible amounts. As discussed earlier, the magnitude of van der Waals' forces and dipole-dipole interactions determine the solubility of chemicals in organic solvents and water. The solubility of a chemical will affect the diffusion and volatilization of a chemical from water to air.
The distribution of a chemical between water and air can be mathematically described in terms of equilibrium constant (Kwa):
Kwa = \|F(mg chemical / ml water,mg chemical / ml air)
The Kwa value can be used for approximating the mechanism of chemical movement in the soil system.
If Kwa is 1, the contaminant movement would be entirely through the diffusion in the soil. If the value is 100, then movement will still be primarily through diffusion in air. If Kwa is 10,000, then chemical movement will occur both through the diffusion and mass flow in the two phases. If Kwa is 106, then chemical movement will occur primarily through mass flow in the water phase.
An estimate of a chemical tendency to volatilize from a solution can be estimated by Henry's Law:
KH = \|F(Vp,C) or Vp = (KH)(C)
(c)Stagnant pockets within the soil particles
(d)Dp DA
DSA = \|F(DA (PSA)10/3,(PT)2)