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Line shape broadness in vibrational spectra is usually associated with structural heterogeneity of the surrounding environment. At the solid/liquid interface, surface-sensitive sum frequency generation spectroscopy (SFG) has shown a variety of distributions of the vibrational frequency for sapphire surface hydroxyl groups in contact with several liquids. Even though the broadness in SFG spectra could be associated with the surface heterogeneity and diverse interfacial interactions, the origin remains elusive in experiments. To better understand the physical picture of interfacial interactions, and hence broadness, we perform SFG along with molecular dynamics (MD) simulations of liquid molecules in contact with sapphire. In the SFG spectra, line-shape broadness of the sapphire hydroxyl group vibrational frequency increases in the following order: chloroform, acetone, and dimethyl sulfoxide. The broadness in the interaction energy distributions calculated from the MD simulations for the molecules interacting with surface hydroxyl groups follows the same order. MD simulations show that liquid molecules are seen to interact locally with multiple sapphire hydroxyl groups. The number of interactions depends on the location of a molecule with respect to the surface hydroxyl groups, which relate to the packing of individual molecules on surface sites. Regardless of the number of hydroxyl groups that a molecule interacts with, the strength of the strongest interaction remains similar. However, neighboring hydroxyl groups interact with the same molecule with weaker energies, creating broadness in the interaction energy distribution for the strongly interacting (acetone and dimethyl sulfoxide) species. The energy distribution profiles correlate well with the experimental SFG spectra, highlighting the ability to interpret spectroscopic features with the physical insights gained from MD simulations.

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The competitive adsorption of molecules on a surface has both beneficial and detrimental effects for technological applications, such as chromatography (material separation) and protein adsorption on medical implants. A comprehensive understanding of adsorption can aid in the design of surfaces with desired functional properties. Molecular dynamics (MD) simulations serve as a direct approach to quantify interfacial behavior. In this study, we use MD simulations to gain insights into the adsorption of acetone−chloroform mixtures on a solid sapphire substrate. Acetone segregates preferentially to the sapphire surface because of hydrogen bonding between the oxygen atom of the acetone molecules and the sapphire surface hydroxyl groups. Both acetone and chloroform possess two probable orientations next to sapphire. Orientation analysis reveals that the presence of chloroform alters the way acetone interacts with the surface, and vice versa. Two analysis methods were developed and utilized to calculate the surface segregation: radial-cut and Z-cut. By comparing the results from the two MD simulation analysis methods, we gain insights into hydrogen-bonding-driven surface segregation and illustrate challenges in defining the surface phase. The surface segregation calculated using the radial-cut method matches with the Defay−Prigogine adsorption model with the differences in interfacial energies of individual components calculated using the Badger−Bauer and Dupre−Fowkes formalism. In addition, the surface segregation from the radial-cut method is found to correlate well with previously reported sum-frequency generation spectroscopy results. The current study paves the way for the overall understanding of adsorption, which can help in designing new surfaces for controlling adsorption.