Van der Waals Interactions between Nonpolar Alkyl Chains and Polar ZnO Surfaces in Gas Sensing Dynamics of Aliphatic Carboxylic Acids

Understanding the adsorption behavior of carboxylic acids on oxide surfaces is important in various chemical processes, including heterogeneous catalysis and chemical sensing. Traditionally, these behaviors have been attributed mainly to interactions between the polar carboxyl group and the polar oxide surface. Here, we demonstrate a vital yet previously underestimated effect of van der Waals interactions between the nonpolar alkyl chains and the polar ZnO surfaces on gas adsorption sensing of aliphatic carboxylic acids. Adsorption behaviors of carboxylic acids with different chain lengths (C3-C9) on single-crystalline ZnO nanowires were investigated by gas chromatography/mass spectrometry (GCMS), infrared spectroscopy (IR), and <i>p</i>-polarized multiple-angle incidence resolution spectroscopy (pMAIRS). GCMS analysis showed a distinct chain-length-dependent temperature shift in the adsorption maxima, with shorter acids reaching their maxima at lower surface temperatures, whereas longer acids required progressively higher temperatures over the range of room temperature to 150 °C. IR spectra revealed that the conversion of carboxyl groups into carboxylate species coordinated to surface Zn cation sites determines the surface retention of carboxylic acids. pMAIRS measurements further indicated that these transitions are restricted by alkyl-ZnO interactions, particularly at lower temperatures. These findings are consistent with density functional theory (DFT) calculations showing that the cumulative effect of individually weak van der Waals interactions between nonpolar C-H groups and polar ZnO surfaces becomes the major contributor to adsorption energy for longer-chain carboxylic acids. Moreover, quartz crystal microbalance (QCM) measurements successfully discriminated carboxylic acid homologues by their distinct adsorption dynamics.