Binding of Small Molecules to a Silica Surface: Comparing Experimental and Theoretical Results

A multiscale method for systematically generating predictive models for probe–surface interactions and its independent experimental verification is described. The interaction of three probe molecules (H2O, NH3, and NO) with silica was studied using experiment, theoretical quantum chemistry, and molecular dynamics calculations. Quantum chemical (QC) methods were used to compute binding enthalpies and vibrational (infrared, IR) spectra of molecule–surface pairs for three unique surface silanol sites. The probe–surface IR spectral shifts induced by the interaction of the probe molecules with the surface silanol sites were also computed and compared to experiment. The computed IR results are comparable to those of experiment and (a) verified that the surface that has been created using simulation is indeed similar to the experimental surface and (b) shed insight into the underlying physical process leading to the observed shifts. The theoretically determined enthalpies of adsorption (ΔH ads) compared well with experiment falling within the uncertainty of those measured using inverse gas chromatography. For water, ΔH ads,350K = −13.5 kcal/mol (calculated) versus −13.6 ± 2.8 kcal/mol (experimental, 330 K < T expt < 370 K). For ammonia, ΔH ads,353K = −15.2 kcal/mol (calculated) versus −12.7 ± 2.9 kcal/mol (experimental, 323 K < T expt < 383 K). Finally, for nitric oxide, ΔH ads,253K = −4.23 kcal/mol (calculated) versus −4.03 ± 0.35 kcal/mol (experimental, 243 K < T expt < 263 K).