Squaramide- and Urea-based Supramolecular Gels: Characterization and Environmental Applications

Supramolecular gels are promising materials that can be used as adsorbent for the removal of different classes of pollutants from water matrices. This thesis explores the synthesis and characterization of eighteen new low molecular weight gelators (LMWGs), eleven based on the squaramide scaffold and seven based on the urea scaffold, their ability to form supramolecular gels, as well as the characterization of the novel materials and their use as pollutants adsorbent from water matrices. Although urea-based gels have been reported to be efficient for water remediation application, no report about the use of squaramide-based gelators have been reported so far. Chapter 1 gives an overview on supramolecular chemistry and supramolecular gels, with a focus on squaramide-based and urea-based LMWGs. Chapter 2 presents the synthesis, characterization, and gelation behaviour of fifteen novel squaramide/urea-based LMWGs, L1–L15. Gelation tests were performed using three different stimuli: temperature variation, pH adjustment, and solvent switching. Not all compounds successfully formed stable gels. Specifically, L3, L4, L7 and L8 did not produce gels under any of the tested conditions. All successfully formed gels were comprehensively characterized using multiple techniques to investigate their properties across different length scales. Mechanical properties were assessed via rheological measurements, conducted in triplicate to ensure reproducibility. The internal morphology of the corresponding xerogels was examined using transmission electron microscopy (TEM), while small-angle X-ray scattering (SAXS) was performed on wet gel samples to avoid drying artifacts. To further investigate the potential crystalline nature of the materials, powder X-ray diffraction (P-XRD) was carried out on selected samples and gelators L5 and L14 yielded crystals suitable for single-crystal X-ray diffraction (SC-XRD) analysis. Lastly, fluorescence spectroscopy was employed to study changes occurring during gel formation in gelators containing fluorophorogenic fragments (L1, L2, L6, and L10). Chapter 3 explores the potential applications of supramolecular gels for water treatment. The most stable and mechanically stiff gels obtained from gelators L1, L2, L5, L6, L10, L12 and L13 were tested as potential pollutant adsorbent. Three experimental setups were employed to assess the gels pollutant removal capabilities: contact tests, flow tests, and dispersion tests, using a fixed gel-to-contaminated solution volume ratio of 1:2 (v/v). The pollutants investigated included dyes (Rose Bengal, Nile Blue A, Naphthol Yellow S), heavy metals, per- and polyfluoroalkyl substances (PFAS, specifically PFOA and PFOS), and non-steroidal anti-inflammatory drugs (NSAIDs, represented by sodium ibuprofen). For dye removal, quantitative analysis was performed using UV-Vis spectroscopy. Regarding heavy metal removal, quantitative analysis was carried out via inductively coupled plasma optical emission spectroscopy (ICP-OES). Quantitative analysis of these contaminants was performed using liquid chromatography–mass spectrometry (LC-MS). Chapter 4 describes the synthesis of new bis-urea gelators, L16, L17, and L18, and their ability to form gels. To investigate the possibility to form metallogels, gelation tests were conducted in the presence of various metals. Preliminary screening of heavy metal adsorption was conducted using ICP-OES and ICP-MS, highlighting the possible application of these materials in environmental cleanup.Appendix A reports a novel family of squaramide-based compounds (A1–A8), designed to evaluate the effect of an indolyl substituent on anion binding and transmembrane anion transport properties. Appendix B presents a small library of acyclic squaramides (B1–B5) investigated as ionophores for the potentiometric detection of non-steroidal anti-inflammatory drugs (NSAIDs).