title: Self-Assembled Monolayers for the Control of Intermolecular Interactions Towards New Sustainable Water Cleaning Agents
Creators: name: Eigen, Andreas
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Self-assembled monolayers (SAMs) offer the possibility to modify the surface of materials in an astonishing range of options. The contribution of surfaces, in particular, becomes a dominating factor for the properties of nanomaterials, such as nanoparticles (NPs), and are in constant interaction with their environment. The wide range of possible chemical compositions of SAMs can control these interactions. This work focuses in detail on investigating such non-covalent interactions of SAMs. The aim is to understand the adsorption mechanisms between SAMs and organic substances to use the NPs developed here as efficient adsorbents for water purification. First, NPs are functionalized with different SAM-forming ligands (SFLs). The properties of the SAMs, such as their density on the surface, their effect on the whole hybrid SAM-NP system, and their dispersion properties, are investigated. The SAM-NPs are characterized in detail using thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), electrophoretic light scattering (ELS), and optical contact angle (OCA). For this purpose, phosphonic acid derivatives are used to attach the SAMs to the metal oxide surface of aluminum oxide () or iron oxide () NPs. The density of the SAM shows a pronounced dependency on the choice of SAM structure, particularly of the head group. The choice of SAM can change the wetting behavior of the dried SAM-NPs from superhydrophobic to hydrophilic. Most importantly, the chosen SAM can alter the zeta potential and thus the dispersion properties such as agglomeration. Although the zeta potential strongly depends on the adjusted pH value and all particles have similar maximum and minimum values, they show different isoelectric points (IEP). The IEP can be in both the acidic and basic range due to the choice of SAM, whereby partly fluorinated SFLs shift the IEP into the acidic range and positively charged SFLs shift the IEP into the basic range. Mixed SAMs consisting of two SFLs can adjust the mentioned properties even more precisely. Secondly, the adsorption behavior of different organic molecules on the SAM-NPs is investigated. During the work, different model molecules, also called water contaminants (CONs), are selected to study specific non-covalent interactions. The adsorption rates are determined by Steady-state absorption spectroscopy (UV-VIS), Steady-state fluorescence spectroscopy (FL), or High-performance liquid chromatography (HPLC), depending on the CON being analyzed. Polyaromatic hydrocarbons (PAHs) with increasing numbers of conjugated 𝜋 -systems show increased adsorption rates with increasing molecule size. The observation can be attributed to the increasing hydrophobicity of the PAHs. Likewise, an increased adsorption of PAHs is shown with increased hydrophobicity of the SAM-NPs used. Additionally, an increased adsorption rate onto phenyl-terminated SAMs can be observed, which indicates further interactions between the 𝜋 -systems of the SAM and the PAH. A second group of investigated CONs were charged molecules. Charged CONs show a pronounced preference depending on the NP surface charge, thus dependent on the pH value of the media. The number and type of charges have the most significant influence. In this way, it is possible to selectively remove one of the two substances from water samples containing a mixture of two different charged contaminants, depending on the SAM. However, if the surface charge is reduced by adjusting the pH value, hydrophobic interactions dominate the adsorption. The last chapter of the thesis investigates the interaction of mixed SAMs containing different ratios of alkyl SAMs and partially fluorinated SAMs with two amphiphiles (AM) containing either fluor or alkyl chains. These AMs combine hydrophobic and electrostatic interactions in separate parts of the molecule. The simultaneous addition of the two AMs creates an adsorption equilibrium of the two substances on the particle surface based on the forces at work. The acting forces thus influence the stoichiometric ratio of the AMs that adsorb. The selective accumulation of the respective chemically similar structures can be observed through chemical recognition. In the stoichiometric ratio of the two AMs, the proportion of the AM with fluorine chain is higher when a SAM-NP with fluorine SFL is used for adsorption. Conversely, the proportion of AM with an alkylated chain in the stoichiometric ratio is higher when an alkyl SFL is used for adsorption. This adsorption tendency can be controlled with the mixing ratio of the two SFLs on the NPs.