New ways to explore fundamentals of the formation of self-assembled molecular networks at the liquid-solid interface

The spontaneous organisation of molecules into ordered patterns or complex systems, i.e. self-assembly, emerges in nature and plays a crucial role in life. It enables the formation of intricate structures in biological membranes, guides DNA assembly, and directs protein folding. In supramolecular chemistry, rapid progress has sparked numerous possibilities for leveraging intermolecular interactions across various fields. The development of the Scanning Tunnelling Microscope (STM) has enabled the observation of molecular structures with atomic resolution on solid surfaces, paving the way for using self-assembly as a bottom-up approach to build nanomaterials. Understanding the complexity of molecular mechanisms controlling the self-assembly process at the interface between a liquid and a crystalline solid, i.e. the liquid/solid interface, is essential. This understanding is the basis for rational design and obtaining on-demand properties of materials needed to achieve long-standing technological goals. 

A good starting point for detailed fundamental research involves directing attention to the physical chemistry of the process at the molecular level. This thesis examines parameters influencing the supramolecular assembly by leveraging the synergy between computational modelling and STM experiments. Ultimately, to gain a comprehensive quantitative understanding of the forces that drive the formation of ordered networks at the solution/solid interface.  

In Chapter 3, using a model system, we investigate the effect of solution concentration on the formation of observed supramolecular structure. We show the process to be highly cooperative and susceptible to the change in concentration value. We also demonstrate that a change in structural features in the molecular structure, such as an increase in the alkyl chain length, linearly decreases the Gibbs free energy of monolayer formation. Focusing on the cooperativity aspect, in Chapter 4, we explore its origin. We find that cooperativity is present even in an assembly that is stabilised only by the formation of relatively weak van der Waals interactions. We conclude that cooperativity must be a local effect and develop a statistical thermodynamics (Ising) model that explains the observed behaviour in terms of molecule-molecule and molecule-substrate interactions. 

In Chapter 5, we focus on studying the temperature effect in the formation of self-assembled molecular networks at the liquid/solid interface. This allows us to determine the enthalpic and entropic contributions to the Gibbs free energy of monolayer formation and shows that the process is enthalpically driven in the case of the studied molecule. We further develop the Ising code by introducing temperature dependence. We show that even a relatively simple model can offer an excellent understanding of enthalpic and entropic contributions underlying the molecular self-assembly at the solid/liquid interface. 

Chapter 6 compares experimental STM setups that allow temperature modulation and test their potential limits. Based on STM measurements, Raman microscopy and X-ray diffraction experiments, we conclude that for long-term studies of the temperature effect, the choice of combination of assembling molecule and solvent is critical.  Chapter 7 provides other examples of complementarity between experimental measurements and computational modelling. We offer this as the most promising approach for in-depth studies of molecular behaviour at the liquid/solid interface. Finally, a summary of the conclusions and prospects is given in Chapter 8.