Quantitative electronic structure and work-function changes of liquid water induced by solute

Recent advancement in quantitative liquid-jet photoelectron spectroscopy enables the accurate determination of the absolute-scale electronic energetics of liquids and species in solution. The major objective of the present work is the determination of the absolute lowest-ionization energy of liquid water, corresponding to the 1b(1) orbital electron liberation, which is found to vary upon solute addition, and depends on the solute concentration. We discuss two prototypical aqueous salt solutions, NaI(aq) and tetrabutylammonium iodide, TBAI((aq)), with the latter being a strong surfactant. Our results reveal considerably different behavior of the liquid water 1b(1) binding energy in each case. In the NaI(aq) solutions, the 1b(1) energy increases by about 0.3 eV upon increasing the salt concentration from very dilute to near-saturation concentrations, whereas for TBAI the energy decreases by about 0.7 eV upon formation of a TBAI surface layer. The photoelectron spectra also allow us to quantify the solute-induced effects on the solute binding energies, as inferred from concentration-dependent energy shifts of the I- 5p binding energy. For NaI(aq), an almost identical I- 5p shift is found as for the water 1b(1) binding energy, with a larger shift occurring in the opposite direction for the TBAI((aq)) solution. We show that the evolution of the water 1b(1) energy in the NaI(aq) solutions can be primarily assigned to a change of water's electronic structure in the solution bulk. In contrast, apparent changes of the 1b(1) energy for TBAI((aq)) solutions can be related to changes of the solution work function which could arise from surface molecular dipoles. Furthermore, for both of the solutions studied here, the measured water 1b(1) binding energies can be correlated with the extensive solution molecular structure changes occurring at high salt concentrations, where in the case of NaI(aq), too few water molecules exist to hydrate individual ions and the solution adopts a crystalline-like phase. We also comment on the concentration-dependent shape of the second, 3a(1) orbital liquid water ionization feature which is a sensitive signature of water-water hydrogen bond interactions.

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

In the framework of this thesis self-assembled supramolecular architectures of several molecular species on crystal metal surfaces are characterized. These investigations combine results obtained by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), X-Ray Photoelectron Spectroscopy (XPS), and Near-Edge X-Ray Absorption Fine Structure Spectroscopy (NEXAFS). The interest in controlling the formation of specific arrangements of supramolecular ensembles at surfaces is motivated to a great extent by the prospect to design molecular devices and nanomaterials with customized properties. The goal is a "spontaneous" creation of targeted, well-ordered molecular architectures, which shall be achieved by taking advantage of the molecule's propensity to self-assemble. Control means, in this context, predictability. This term address the accuracy of the prediction of the structures resulting from molecular self-assembly for systems exhibiting different characteristics. Here, system stands for a particular combination of a single-crystal surface and adsorbed molecular species. In a first part, the focus is given to the impact of a Pd(111) surface exhibiting high chemical activity. Due to surface induced deprotonation two resulting chemical states are found for the adsorbed terephthalic acid species. Deprotonation of the carboxyl moieties is observed to affect the topology of the molecular assemblies even though hydrogen bonds (H-bonds) are responsible for the stabilization of assemblies of both, intact and deprotonated molecular species. Therefore, the selective fabrication of a single specific molecular structure becomes a challenge when the surface is prone to induce chemical reactions. Secondly, a molecular species exhibiting conformational flexibility is studied on a largely inert Au(111) surface. It is observed that the characteristic conformational flexibility is also active on the surface, and the molecule is found to adopt different conformations depending on the locally most favorable bonding motif. In particular, phenyl groups may undergo a rotation such that one-dimensional chains can be stabilized by means of intermolecular H-bonds. This phenomenon is very interesting from an adsorbate-substrate interaction point of view, but it limits the predictability of supramolecular structures formed upon self-assembly. In a third step, a comparative study of the self-assembly of three closely related molecular species highlights the impact of concurrent competing intermolecular interactions. Molecules exhibiting an increasing number of possible interaction channels are found to assemble into an increasing number of distinct supramolecular architectures. The most flexible molecular species allowing for different hydrogen-bonding patterns as well as dipolar coupling self-assembles into various structurally complex architectures. Structural predictability, however, is low since the formation of one or the other assembly appears to depend on small variations in sample preparation conditions and on poorly controllable local surface properties. Finally, a system where the resulting structures fully agree with predictions evidences the possibility to control the formation of targeted specific supramolecular architectures. The formation of well ordered bimolecular ribbons and wires over extended length scales is accomplished by the rational choice of both, complementary building blocks for the selective formation of three-fold H-bonds and a nanostructured surface to guide the molecular self-assembly. Furthermore, the impact of H-bond formation on the electronic structure of self-assembled molecular building blocks is probed by STS measurements. The experimental data reveal shifts in the electronic levels depending on the H-bond configuration and on the local characteristics of the underlying surface. In order to improve the precision of such a study, single molecules, homo- and heteromolecular dimers as well as trimers are successfully isolated and assembled by molecular manipulation with the STM tip. Further STS studies on such artificially assembled structures are suggested.

EPFL2008

Molecular insights in aqueous systems

Yixing Chen

The unique structure and dynamics of the water hydrogen (H)-bond network enable a multitude of structures and chemical reactions in both bulk solutions and at interfaces. The underlying molecular interactions between water and dissolved electrolytes, organic molecules, and nanoscale interfaces are difficult to study and hence not fully understood, especially when it involves interactions of length scales larger than one nanometer. In this thesis, we perform second order nonlinear light scattering experiments to investigate extended hydration shells (over one nanometer) and interfacial structures of buried aqueous interfaces that surround nanodroplets. We investigate the spatial range of ion-water interactions by probing the molecular structure of the water network in dilute electrolyte solutions. We find that electrolytes induce long-range, non-ion-specific orientational order in the H-bond network of water (over distances up to 70 hydration shells or 20 nm). These modifications in the water network start at electrolyte concentrations as low as 10 micromolar. The amount of induced orientational order and the concentration at which it occurs is different between light and heavy water. We link the observed ion induced changes in the bulk H-bond network of water to ion induced surface tension anomalies that occur in the same concentration range. We also study the same interactions in solutions with higher electrolyte concentrations focusing on cations. We observe specific ion effects in both the local charge distribution and water ordering in extended hydration shells of ions. Cations with larger charge densities induce larger changes in these two observables that follow a direct Hofmeister trend. The ion-induced structural changes in the water network are strongly correlated with the viscosity B-coefficient and hydration free energy of cations. These correlations facilitate constructing a better molecular model for specific ion effects in aqueous solutions. Next, we study the nanoscale hydrophobe/water interface and related interactions by probing the molecular structure of the interface formed with amphiphilic alkanols. We find that alkanols form a fluid film at the oil nanodroplet surface. Long chain alkanols show similar molecular structures and number densities at the interface. With increasing alkanol density, interfacial water loses its initial orientational alignment. The structure of interfacial oil molecules is more distorted for shorter alkanols. This interfacial structure differs significantly from those of macroscopic planar alkanol/water and alkanol/air interfaces and charged surfactant/oil/water interface, which can be explained by the balance between dispersive and H-bonding interactions. Finally, we develop a new membrane model system: 3D-lipid monolayers with tunable molecular structure on oil nanodroplets in water. This in-situ prepared system mimics the structure of lipid droplets in cells. The interfacial structure of lipids can be tuned from a tightly packed monolayer (liquid condensed phase) to a dilute one (liquid condensed/liquid expanded coexistence phase) by varying the lipid/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system ideal for studies of molecular interactions of lipids in membranes.

EPFL2017

Molecular insights in aqueous systems

Yixing Chen

EPFL2017

Growth and characterization of specific self-assembled supramolecular architectures at crystal surfaces

Marta Canas Ventura

EPFL2008