Gap-Controlled Infrared Absorption Spectroscopy: A Unique Interface-Sensitive Spectroscopy Based on the Combination of Linear Spectroscopy and Multivariate Curve Resolution

We present an interface-sensitive spectroscopy method that integrates attenuated total reflection infrared absorption (ATR-IR) spectroscopy, a distance control system, and multivariate curve resolution (MCR). In this approach, we adjust the distance between the sample and the ATR prism while collecting a series of spectra that reflect various contributions from both bulk and interfacial regions. Subsequently, we utilize MCR to extract the spectral components specific to the interfacial region. Here, we validate this method through the analysis of interfacial water adjacent to self-assembled monolayers (SAMs), quartz, polymers, and polymer brush films. Our findings are compared with results from other interface-sensitive spectroscopic techniques, confirming the interface sensitivity of our approach. This method does not necessitate surface enhancement or nonlinear optical effects and imposes virtually no restrictions on the types of samples suitable for analysis. Furthermore, it allows us to assess the thickness of the interfacial region using a spectral component distinct from the bulk region, revealing insights into the relationship between the interfacial behavior of molecules and related phenomena. Additionally, this method can be seamlessly integrated into standard ATR-IR spectroscopes, offering a straightforward solution for interface-sensitive spectroscopy.