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Measurements of Evaporation Kinetics of Pure Water and Salt Solutions

  • Author(s): Drisdell, Walter
  • Advisor(s): Cohen, Ronald C
  • et al.

The kinetics of vapor-liquid exchange in water are poorly understood, yet may be critically important in predicting changes in Earth's climate and understanding the water isotope record preserved in ice cores . In this thesis we present measurements of the kinetics of water evaporation. In Chapter 1 we review recent work on the subject, including our own liquid microjet technique which has higher precision than other methods.

In Chapter 2 we extend our earlier measurements of the evaporation kinetics of H2O by studying pure D2O. We find that the evaporation coefficient, which can be thought of as the percentage of evaporation "attempts" which succeed, is identical for the two isotopomers. We interpret this result using a previously developed transition state theory (TST) model of evaporation, which predicts the respective evaporation coefficients to be equal due to competing energetic and entropic effects.

In Chapter 3, we examine the evaporation kinetics of H2O evaporating from 3M ammonium sulfate solution. Ammonium sulfate was selected as it is the largest inorganic component of anthropogenic aerosol in the atmosphere. Again we find that the evaporation coefficient is unchanged relative to pure water. This is consistent with theoretical and experimental studies suggesting that both the ammonium ion and sulfate ion are repelled from the air-water interface, implying that these ions will not directly interact with evaporating water molecules. This result also suggests that inorganic components of atmospheric aerosol are unlikely to significantly affect evaporation kinetics.

In Chapter 4 we examine the evaporation kinetics of H2O from 4M sodium perchlorate solution. Perchlorate was selected as it is expected to be strongly enhanced in concentration at the air-water interface, and therefore more likely to directly influence the evaporation process. We find that the evaporation coefficient for this system is ~25% smaller than that for pure H2O, indicating that the perchlorate ions do indeed impede evaporation. Given experimental evidence for the perchlorate ion slowing the rotational motions of H2O molecules in its first solvation shell, and our TST predictions indicating that the evaporation kinetics of water are highly sensitive to the hindered rotational motions of surface water molecules, we suggest that perchlorate ions at the interface are inhibiting the evaporation of H2O molecules with which they are in direct contact. This result suggests that other surface-enhanced ions may also affect the evaporation kinetics through direct interactions with evaporating molecules and opens several interesting new avenues of study, which are discussed in Chapter 5.

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