Self-assembled thiol monolayers with carboxylic acid functionality: Measuring pH-dependent phase transitions with the quartz crystal microbalance

The resonant frequency of piezoelectric 5-MHz quartz crystal microbalances (QCMs), coated with self-assembled monolayers prepared from HS(CH²)₁₅COOH, exhibited unusually large sigmoidal increases (≈1200 Hz for AT-cut quartz) when the pH of aqueous solutions in contact with the monolayer was increased. The Δf-pH curves for the monolayers indicated pK_a values (K_a = the ionization constant) and transition region widths that were significantly greater than the values for carboxylic acids in solution. The frequency shift observed during titration was essentially independent of counterion (Mⁿ⁺ = Na⁺, K⁺, Cs⁺, Ca²⁺) and could not be explained on the basis of simple mass changes. Double-resonator experiments with AT- and BT-cut quartz resonators suggested that an increase in tensile stress with increasing pH contributed partially to the QCM response. The major contribution to the frequency response is attributed to changes in the viscoelastic properties of the hydrodynamic layer in contact with the QCM.