Evaporation rates and vapor pressures of the even-nunibered C8-C18 monocarboxylic acids

Temperature-dependent vapor pressures of the even-numbered alkanoic monoacids from C-8-C-18 were measured using temperature-programmed desorption (TPD). In TPD, the evaporation rates from the samples are directly measured and the vapor pressures are subsequently determined from the Hertz-Knudsen equation. Our measurements indicate that the vapor pressures of the solid even-numbered alkanoic acids decrease monotonically with increasing carbon number by more than 6 orders of magnitude going from C-8 to C-18. The enthalpies of sublimation increase monotonically with carbon number, from similar to 110 to 205 M/mol. The liquid-phase vapor pressure was measured for oleic acid, a C-18 alkenoic acid. Comparison to the estimated liquidphase vapor pressure for the corresponding C-18 alkanoic acid indicates that the liquid-phase vapor pressures of these two compounds are identical. Our measured solid-phase vapor pressures for the C-14 and larger alkanoic acids are lower than in previous studies. We attribute these differences to the influence of residual solvent molecules on the previous measurements, Which cause the measured vapor pressures to be too large.