Spectroscopic and Computational Characterization of the Adsorption of Quaternary Ammonium Ions on Ceria-Supported...

The adsorption of molecular ions from aqueous solutions onto colloidal silica is utilized across a wide range of technologies. Measurement of such interactions using attenuated total reflectance/Fourier transform infrared (ATR/FT-IR) spectroscopy presents challenges due to the instability of colloidal silica particle films on an ATR's diamond crystal internal reflection element (IRE). Here, the instability of silica is addressed by first depositing a thin film of colloidal ceria particles on the IRE to improve silica's adhesion. This enables the application of ATR/FT-IR spectroscopy to characterize the adsorption of quaternary ammonium ions, including the poly(2-(trimethylamino)ethyl methacrylate) (pTMAEM) and compounds modeling pTMAEM's side chain functional groups, tetramethylammonium (TMA) and acetylcholine (AChl) ions, on low- and high-silanol density silica films at varying pH values. Density functional theory (DFT) calculations are performed to model the TMA-silica and pTMAEM-silica complexes and predict their infrared (IR) spectra. The adsorption of TMA, AChl, and pTMAEM to silica is demonstrated by IR bands in the range of 950-1250 cm^-1, which were dominated by transverse optical and longitudinal optical phonon modes. Langmuir adsorption constants based on optical phonon modes indicate that TMA is more strongly adsorbed to the low-silanol density films, whereas pTMAEM shows a stronger adsorption to silica overall, regardless of the silanol density. This suggested that both electrostatic and nonelectrostatic interactions contribute to TMA's adsorption, while the polymeric nature of pTMAEM enabled adsorption due to strong electrostatic interactions via multiple adsorption events. AChl's relatively weak adsorption to silica prevented the calculation of a Langmuir adsorption constant; however, the IR spectra suggested any adsorption was primarily due to the quaternary ammonium ion. The application of ATR/FT-IR spectroscopy in combination with DFT allowed a complete characterization of the adsorption of the quaternary ammonium ion onto ceria-supported colloidal silica, which may also be applied to other molecular ions, further characterizing and optimizing technologies employing similar materials.