Water vapor adsorption isotherms on self-assembled monolayers using a quartz crystal microbalance

The purposes of the paper are to discuss the uncertainties encountered in determining water vapor adsorption isotherms using a quartz crystal microbalance (QCM) and to present the results of measuring three isotherms on self-assembled monolayers (SAMs) formed with alkanethiols. The isotherms were measured using a vacuum QCM, which has a resolution of about 1 ng/cm², corresponding to a minimum surface coverage of about 3%. SAMs were formed at 295 K on gold-coated 6-MHz crystals from 2 to 3 mM solutions of octadecanethiol (ODT, HS(CH₂)₁₇CH₃), methylmercaptohexadecanoate (MMHD, HS(CH₂)₁₅COOCH ₃), or mercaptohexadecanoic acid (MHA, HS(CH₂)₁₅COOH), which provide surfaces consisting only of the organic functional groups of CH₃, COOCH₃ and COOH, respectively. Alkanethiols form a thiolate bond to the gold via the HS group on each molecule. After mounting the crystals on the QCM holders and evacuating at 293 K, the water isotherms were obtained at 293 K by admitting water incrementally in 2-torr steps from very high vacuum up to 16 torr and then decrementally in 2-torr steps until very high vacuum was again attained. Mass changes were monitored in each run with five different QCMs of the same type of SAM. The uptakes only range from 10 ng to 50 ng at 10 torr and from 30 ng to 160 ng at 16 torr; so, statistical approaches were necessary to reduce the uncertainty in the measurements, which are close to the detectability limits of the QCMs, especially for the ODT. To achieve the detectability limits, the components replaced and corrections made to the data are discussed. The isotherms on all three SAMs are Brunauer-Emmett-Teller (BET) Type 2 with c values of 2 to 10 and heats of adsorption of 11.3 to 11.7 kcal/mole, as determined from BET plots of the data. As expected, the water vapor uptakes at any pressure on the end-groups are in the order COOH>COOCH₃>>CH₃