Properties of chiral self-assembled monolayers at the solid-liquid interface investigated by FT-IR ATR spectroscopy

The main distinctive feature of modern surface science is that it deals with single crystal surfaces, which are well-defined from the viewpoint of their structure and composition. However, for the preparation of crystal surfaces and their analysis, the requirement of UHV equipment greatly limits applications. While atomically clean surfaces analyzed under artificial measurement conditions still play an important role for fundamental understanding, it should also be the goal of modern surface science to explore the role of surfaces under realistic conditions. Indeed, in recent years, surface science is moving to more complicated materials and to studies of the solid–liquid interface. The latter has great relevance in biological processes and technological applications, e.g. catalysis, lubrication and corrosion prevention. Also, the modification of surfaces by self-assembled monolayers (SAMs) is an important process taking place at the solid–liquid interface. SAMs are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface. Based on the constituent surfactant, a variety of SAMs with different properties and functionalities can easily be prepared with potential applications in various branches of surface technology. Furthermore, SAMs are excellent model systems amenable to investigating competing surfactant–substrate and surfactant–surfactant intermolecular interactions. The main goal of this thesis is to provide insight into the formation of SAMs at the solid–liquid interface by using in situ experimental techniques. Fourier-transform infrared spectroscopy (FT-IR) applied in the attenuated total internal reflection (ATR) mode was used as the prime technique to probe the self-assembly of chiral peptides or derivatives thereof from the liquid phase to gold surfaces. It is demonstrated that ATR is an excellent in situ analysis method that provides rich molecular-level information, such as kinetics of adsorption, structural changes and orientation of molecules within the SAM. The insight was further broadened by using ATR in combination with modulation excitation spectroscopy (MES). The latter benefits from highlighting spectral changes due to the periodic stimulation of the sample by an external parameter. MES was used to further explore the structure of SAMs and it was shown that the latter may undergo significant reversible structural changes by periodically stimulating the molecules within the adsorbate layer. One of the most interesting and valuable properties a chiral SAM or surface can have is its ability to discriminate between enantiomers of a chiral compound, which is crucial for heterogeneous enantioselective catalysis and chiral recognition. It is therefore desirable to develop techniques that can not only quantify enantiodiscrimination but also shed light on its origin. Such techniques should ideally provide molecular-level information and combine (surface-) sensitivity with selectivity for the chiral information, a combination of criteria that is difficult to meet simultaneously. It is demonstrated that ATR in combination with MES is a powerful technique to meet these requirements. For the first time, enantiodiscrimination between chiral SAMs and an analyte molecule was probed and unique molecular-level insight on the origin of chiral recognition was obtained. Additional experimental techniques used for this work were quartz crystal microbalance (QCM) and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). QCM is extremely sensitive to mass uptake on the sensor surface and the technique was used to investigate adsorption kinetics and self-assembly of molecules from the liquid phase in situ. Furthermore, computational methods at the density functional theory (DFT) level were routinely used for vibrational analysis of surfactant molecules and calculations helped to interpret experimental spectra.

Thèse de doctorat : Université de Neuchâtel, 2007 ; 1926