Interaction Geometry Causes Spectral Line-Shape Broadening at the Solid/Liquid Interface

Abstract

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.

Cooperative Multivalent Weak and Strong Interfacial Interactions Enhance the Adhesion of Mussel-Inspired Adhesives

“…In the case of sm-Tyr, the peaks were observed at cos θ ≈ −0.78 and −0.14, whereas sm-DOPA displayed peaks at cos θ ≈ −1.0 and −0.18. The observation of two peaks in the orientation distribution has been reported previously . The interplay between the dipolar, dispersive, and hydrogen bonding interactions at the interface often leads to two or more probable molecular orientations .…”

“…The overall interaction enthalpy of a molecule with surface hydroxyl groups (Δ H molecule ) is the sum of the individual weak and strong acid–base interactions. , The determination of the type of interactions ( i ), number of interactions ( n i ), and enthalpy of a single interaction (Δ H i ) enables the calculation of Δ H molecule as described below. , Δ H i can be obtained experimentally from SFG spectroscopy, and n i can be calculated from MD simulations. , …”

“…The observation of two peaks in the orientation distribution has been reported previously. 57 The interplay between the dipolar, dispersive, and hydrogen bonding interactions at the interface often leads to two or more probable molecular orientations. 56 For example, the peak at lower cos θ (−0.78 for sm-Tyr and −1 for sm-DOPA) configurations originates from the near perpendicular orientation of the molecule through which the hydrogen bonds can be formed at the interfaces.…”

“…60,62 ΔH i can be obtained experimentally from SFG spectroscopy, and n i can be calculated from MD simulations. 57,58…”

“…We have used surface-sensitive sum-frequency generation (SFG) spectroscopy to measure the strength of the acid–base interactions (hydrogen bonding is a subset of acid–base interactions). In previous SFG experiments, we have demonstrated that the sapphire surface −OH group peak (−OH sap ) shift is directly correlated to the strength of the acid–base interactions between the −OH sap and the interacting groups. , The shifts of the −OH sap peak can be directly related to the interfacial enthalpic interactions. ,− We have used this approach to measure the enthalpy of interaction between −OH sap and model compounds that represent the four functionalities of interestPhe, Ser, Tyr, and DOPA. To relate the enthalpic contribution to the work of adhesion, we need information on the interaction strength and the number of acid–base interactions with the surface OH sap .…”