Study on water evaporation through 1-alkanol monolayers by the thermogravimetry method.

The influence of 1-alkanol monolayers on the rate of water evaporation has been studied by measuring water loss per unit time using thermogravimetry. The evaporation rate of water from the surface covered by an insoluble monolayer for each of four saturated 1-alkanols (C(13)OH, C(15)OH, C(17)OH, and C(19)OH) was measured as a function of temperature and alkyl chain length, where the monolayer was under equilibrium spreading pressure. The evaporation rate decreased with increasing alkyl chain length or increasing molecular interaction among 1-alkanol molecules in the insoluble monolayer. Using the Arrhenius equation, the activation energy for the water evaporation was calculated from the temperature dependence of the evaporation rate, which showed that the activation energy decreased with increasing temperature. On the other hand, the activation energy increased with increasing alkyl chain length, which indicates that the activation energy includes the energy to cross the insoluble monolayer at the air/water interface. This energy increased almost linearly with alkyl chain length, when the length is longer than a dodecyl group. This means that water molecules need more energy to escape from the liquid to the gaseous phase across a membrane of longer 1-alkanols, which becomes more evident at lower temperatures. The temperature dependence of the activation energy was slightly larger for longer 1-alkanols than for shorter ones.