UPTAKE OF GAS-PHASE ALCOHOL AND ORGANIC-ACID MOLECULES BY WATER SURFACES

Mass accommodation coefficients (alpha) have been measured as a function of temperature in the range 260-291 K for methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol, ethylene glycol, chloroethanol, bromoethanol, iodethanol, formic acid, and acetic acid. The experimental method employs a monodispersed train of droplets (less-than-or-equal-to 230-mu-m in diameter) in a low-pressure flow reactor. Droplet-trace gas interaction times are in the range (2-10) x 10(-3) s. All mass accommodation coefficients show a negative temperature dependence and can be well expressed in terms of an observed Gibbs free energy as alpha/(1 - alpha) = exp(-DELTA-G double-ended-dagger obs/RT). Of the species studied, iodoethanol has the largest mass accommodation coefficient; for this molecule alpha-ranges from about 0.04 at 290 K to 0.2 at 260 K. 1-Propanol has the smallest alpha; it ranges from about 0.01 at 290 K to 0.07 at 260 K. The results show systematic trends. Most notably, when DELTA-G double-ended-dagger obs is expressed as DELTA-G double-ended-dagger obs = DELTA-H(obs) - T-DELTA-S(obs), it is observed that the magnitudes of both DELTA-H(obs) and DELTA-S(obs) correlate and increase in the sequence diols < acids < haloethanols < alkyl alcohols. The results have led to the development of a model for the uptake of gas-phase molecules by liquids.