Use of attenuated total reflection infrared spectroscopy for analysis of partitioning of solutes between thin films and solution

The paper examines attenuated total reflection (ATR) spectroscopy as a tool for quantifying the partitioning of small molecular species between a solution and a thin film, while the film is directly exposed to the solution for equilibration. For the case of a thin film having a thickness substantially smaller than the decay length of the evanescent wave, we developed suitable linear relationships that relate the measured absorption of the characteristic band to the concentration of the species under study in the film and in solution. In the application of ATR-Fourier transform infrared spectroscopy, the method is particularly suitable for films a few tens to hundreds of nanometers thick and for solutes that preferentially partition into the film. As an example, the partitioning isotherm of 1-pentanol between water and a thin polyamide film separated from a reverse osmosis membrane was determined experimentally, and the limitations of the method are discussed.