Uptake of water by organic films: the dependence on the film oxidation state

We report quartz crystal microbalance (QCM) measurements of the room temperature uptake of water by thin (<1 μm) organic films. The mass of water taken up by films of dodecane, 1-octanol, octanoic acid, 1,5-pentanediol, 1,8-octanediol and malonic acid was measured as a function of the ambient relative humidity (RH). All compounds tested here displayed some sorption of water. Uptake by dodecane is probably due to surface adsorption; in the other films, water dissolves into the film material. Malonic acid and 1,8 octanediol show deliquescence-like behaviour at relative humitidies near 72% and 95%, respectively. In general, the higher the oxidation state of the film compound, the greater is the amount of water sorbed. An absorptive partitioning model, using UNIFAC calculations of activity coefficients, yields semiquantitative agreement with the experimental results at low relative humidities for all compounds except dodecane. Model estimates of water uptake were generally higher than measurements at low RH and lower than measurements at high RH. Model-measurement deviations displayed a similar nonlinear behaviour with changes in RH for all compounds. The differences between the modelled and measured uptakes yield insight into the limitations of currently available model parameters.