Supramolecular hydrogels based on 1,3,5‐benzene tricarboxamides
In this thesis the pH-sensitive aggregation behavior of low molecular weight (lmw) compounds based on the 1,3,5-benzene tricarboxamide (BTA) self-assembly motif is investigated in aqueous systems to elucidate their potential to form supramolecular hydrogels. The thesis covers four main subjects: i) the self-assembly behavior of pH-sensitive BTAs in aqueous systems is investigated with regard to their ability to form nano-fiber networks and supramolecular hydrogels; ii) the formation of supramolecular chromophores during gelation is studied using spectroscopic methods; iii) the possible application of the hydrogel as an adsorption material for small dye molecules and the controlled delivery is elucidated; and iv) the formation of controlled hydrogel films on conductive substrates via electrogelation is investigated. Five different BTA derivates were synthesized to show that a sensitive balance between lipophilic and hydrophilic interactions must be fulfilled to enable pH-sensitive aggregation in water. Besides the hydrophobic shielding of the BTA core, the introduction of three carboxylic acid groups as lateral headgroups proved to be mandatory to achieve stable gel formation upon change of the pH value. Different gelation methods were investigated for the carboxylic acid derivative and the properties of the resulting hydrogels were compared. Upon addition of aqueous acid solutions only inhomogeneous gels can be obtained. The diffusion of hydrochloric acid gas leads to macroscopically homogenous hydrogels with a gradient in mechanical stability. The addition of glucono-delta-lactone (GdL), a slow proton donating agent, enables the formation of stable homogenous gels with a homogeneous mechanical stability. Gels prepared with hydrochloric acid gas or GdL have a very low critical gelation concentration (cgc) of 2 g L 1. Even at this low gelator concentrations the gels are thermally stable up to the boiling point of water. They are also stable in various organic solvents and by a solvent exchange the hydrogels can be transferred to stable organogels. The gel formation process is proposed to occur step-wise and to proceed from completely charged and dissolved molecules via partially charged anisotropic aggregates to a completely uncharged three-dimensional network. The aggregation process could also be observed using spectroscopic methods. The dissolved gelator molecules do not show any photoluminescence independent of the used solvent, while in bulk and upon gelation a blue luminescence occurs. This gelation- or aggregation-induced emission enhancement (AIEE) gives interesting insight in the formation of the chromophore and thus the molecular arrangement at an early stage of gelation. These findings support the proposed structural model. Furthermore, the possible application of the hydrogel as an adsorption material in waste water treatment or as a matrix material for the controlled delivery of small molecules was investigated. The hydrogels showed a high adsorption potential even at very low concentrations of rhodamine B, a red dye that acted as model guest compound. The adsorption data were analyzed using different isothermal and kinetic models. The results indicate a favorable and strong but slow adsorption. Furthermore, the hydrogel system fulfills basic requirements for the application as matrix material in the controlled delivery of drugs. To investigate the release behavior, hydrogels were prepared in the presence of the model compound rhodamine B. This encapsulation technique is a fast and efficient loading method compared to loading a gel via adsorption. The subsequent release of dye in water showed that only low amounts of dye were released. The release is very slow and follows the generalized Fickian diffusion model. The dissolution and release behavior of gels loaded with rhodamine B was investigated in different biological relevant media, such as phosphate buffered saline (PBS), simulated body fluid (SBF), fasted state simulated gastric fluid (SGF), and fasted state simulated intestinal fluid (SIF). Most interestingly were the studies using SGF and SIF as these allow for conclusions regarding the oral administration of the gel system. The stability of the gels in SGF in combination with their release behavior in SIF renders this hydrogelator a promising matrix material. Furthermore, the “electrogelation” method was used to prepare defined hydrogel films on conductive substrates. This gelation technique utilizes the decrease of the pH value in the vicinity of an anode due to the electrolysis of water. For the systematic screening of the electrogelation parameters an experimental set up was developed. The influence of the gelation parameters was investigated in respect to the gel film properties in dried state. The findings of this study show that by tuning the electrode distance, the applied potential, the gelator concentration, the concentration of the background electrolyte, and the gelation time defined hydrogel films can be prepared by electrogelation.
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