What effect does water or air velocities have on the uptake of chemicals by SPMDs?
Medium flow rates should have only small effects on the sampling rates of organic compounds under membrane control (i.e., log Kow < 4.4), because transport across the membrane is the rate-limiting step. In this case, the membrane is like a barrier or valve that allows only some fraction of the molecules contacting the membrane surface to permeate into the lipid. Increased flow or turbulence does not cause a significant rise in chemical concentration at the membrane surface but merely ensures that chemical concentration in the aqueous diffusion layer reflects that of the bulk water. As long as the chemical concentration at the membrane surface is not depleted by sampling (i.e., encounter volume or water volume exchanged at the membrane surface is sufficient), then chemical uptake is independent of flow regime. For compounds with log Kow values 4.4, the external boundary layer will likely control uptake rates for all but extremely turbulent conditions and very large molecules. Because fl
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