Adsorption to ice of n-alcohols (ethanol to 1-hexanol), acetic acid, and hexanal

The gas-to-ice uptakes of a variety of small organic molecules (C2 to C6 n-alcohols, acetic acid and hexanal) have been measured in a coated-wall flow tube coupled to an electron-impact mass spectrometer. The ice films are prepared by freezing liquid water and the temperature range covered is between 213 and 245 K. With the exceptions of 1-hexanol and high partial pressures of 1-pentanol, the surface coverages at 228 K are adequately described by a standard Langmuir adsorption isotherm model consistent with a saturated surface coverage of 2 to 3 x 10(14) molecules/cm(2), where the area is the geometric surface area of the film. For similar partial pressures, I-hexanol exhibits very much larger uptakes than the other species, an indication that multilayer adsorption is occurring. The equilibrium constant for the partitioning between the gas-phase organic and its adsorbed form has been measured from the linear dependence of the surface coverage on pressure at low partial pressures. The temperature dependence of these equilibrium constants is used to derive the following heats of adsorption to ice at temperatures close to 228 K (in kcal/mole): -17.5 +/- 2.8 for acetic acid, -17.1 +/- 1.6 for 1-pentanol, -16.2 +/- 0.9 for 1-butanol, -14.8 +/- 0.4 for ethanol, and -15.5 +/- 1.4 for hexanal. The gas-to-surface equilibrium constants at 228 K correlate closely with the free energy change for gas-to-liquid condensation, indicating that polar organics adsorbing to ice participate in H-bonding interactions analogous to those in the pure liquid. The 228 K equilibrium constants do not correlate well with the aqueous Henry's Law constants, an indication that dissolution into a liquidlike layer of water on the surface has a minor impact on the uptakes.