(588b) Vapor Pressure of Low Volatile Chemicals from a Knudsen Effusion Technique

Vapor pressure data are vital to understanding impacts that substances, specifically pesticides and other agrochemicals, may exert on the environment. They enter into atmospheric deposition models for such chemicals which determine the fate and transport of these species in the environment. At normal application temperatures (i.e. room temperature) the vapor pressures of many of these chemicals are too low to be determined by conventional means. An isothermal Kundsen effusion technique was designed and developed in our laboratory for such measurements. The effusion mass as a function of time is measured in our technique using a thickness shear mode (TSM) acoustic wave sensor, which allows for extremely high (few nanograms) sensitivity. This sensitivity allows for much more rapid determination of low vapor pressures (10^-1 to 10^-5 Pa) than is possible by other Knudsen effusion techniques. Basing the effusion mass measurement on the TSM sensor as in our apparatus eliminates the typically seen dependence on vibration in conventional microbalance-based effusion techniques. Full design details of our apparatus and specifically the Knudsen cell, based on original equations derived by Knudsen, and many corrections that have been noted in the literature for cell and effusion-hole dimensions, will be presented. The limitation placed by utilizing the TSM sensor, viz., that the effusing mass must coat the sensor uniformly, will be discussed with suggestions for turning this limitation to an advantage. The accuracy of our method was validated by a comparison of known vapor pressure data acquired via an alternative TGA method¹ to vapor pressure data acquired in our laboratory with measurements on anthracene (320-360 K), catechol (290-310 K), hydroquinone (320-340 K), caffeic acid (410-430 K), ferulic acid (360-390 K), gentisic acid (360-380 K), and myoinositol (440-460 K). Our method was then utilized in the determination of vapor pressure data for pyriproxifen (290-310 K) and cypermethrin (290-310 K), active ingredients in pesticides of which specific vapor pressure data have not previously been available.

References

1. Chen, X., V. Oja, W. G. Chan, and M. R. Hajaligol (2006). Vapor Pressure Characterization of Several Phenolics and Polyhydric Compunds by Knudsen Effusion Method. J. Chem. Eng. Data 2006, Vol. 51, 386-391.