The Synthesis, Gel Properties And Gelation Mechanism Of Amino Acid-Type Low Molecular-Mass Organogelators

A large number of low molecular-mass organogelators were reported, since of the discovery of organogel. The gel properties of low molecular-mass organogelators, mechanism of action in different solvents (including the balance of the weak interaction between molecules), state of aggregation in organic solvents and micro forces have been extensive researched, by comprehensively evaluating the thermodynamics, kinetics and other properties of low molecular-mass organogelators. The study showed that low molecular-mass organogel has broad applying prospects.So far, the research of low molecular-mass organogel focused on preparing new gelators. It is a direct and effective way to obtain new low molecular-mass organogelators by structurally modifying the group of known gelators, which have special functions as structural building blocks.The amino acid molecules with the characters of rich sources, varied structures, easy modification and biocompatibility were selected as the main structure parts, synthesized a series of gelators, by modifying the hydroxy, amino or carboxyl and introducing the benzyloxycarbonyl or long chain alkyl structure fragments to the amino acid structures. Gel performance tests in more than 20 kinds of organic solvents, revealed that these amino acid derivatives were excellent gelators though their different gel properties and gelling in a variety of organic solvents at concentrations less than 3.0 wt%. The new amino acid-type low molecular-mass organogelators were designed and achieved through step-by-step modifying the chain segments and the grafting ratio, and adding a new structure to the amino acid molecule fragments.In this paper, the gel-sol phase transition temperature (TGS) of the amino acid-type low molecular-mass organogels was determined by the falling ball method, which also is the basis of evaluating organic gel stability. The gelator structure, concentration and organic solvent polarity, which impacted on the stability of low molecular-mass organogels, were studied. The research showed that the complexity of gelator structure resulted in increasing of the forms of binding force or intensity, not only between the gelators, but also between the gelators and the solvents, and implied gels with higher TGS were more stable. The interaction between the gelator molecules was enhanced with increasing the concentration of gelators, and the stability of the gel was enhanced with increasing of TGS.Because variety of non-covalent interactions, such as hydrogen bonding, occurred between the highly polar solvent molecules and gelators, more stable gels were formed and showed high TGS values. Kamlet-Taft model also proved the influence of the solvents on formation process of low molecular-mass organogels by the three aspects:ability of giving the hydrogen bonding a, ability of accepting the hydrogen bonding ? and the polarity parameter ?*. The gel-sol phase transition enthalpy of low molecular-mass organogels has been achieved from the van't Hoff equation.Studies of the organic gel micro-structures by scanning electron microscopy (SEM) showed that amino acid-type low molecular-mass organogelators in different organic solvents self-assemble to form fibrous, rod-like or flake three-dimension networks. The supramolecular aggregation morphology depended on the gelators and solvents, that is, gathering way is different, not only with different gelators in the same solvent but also with the same kind of gelator self-assembling in different solvents. Thus the formation of organogel morphology was different, and various in the space size of the aggregates. The size of the aggregates was not only determined by the nature of the gelators, but also affected by the concentrations, the greater the concentration of gelators, the larger the size of aggregate space.XRD patterns of the amino acid-type low molecular-mass organogelators changed after forming organic gels, which showed that a new molecular arrangement was employed after gelling, that is, gelators self-assembling were happened in the process of forming gels. Studies of Fourier transform infrared spectroscopy (FT-IR) showed that the hydrogen bonds formed between the organogelator molecules, and was the main driving force to form three-dimension networks, in which hydrophobic interaction between long-chain alkyl groups played an assistant role in the self-assembly process. By the molecular structure simulation software, the self-assembled molecular model of amino acid-type low molecular-mass organogelators in the solvent was deduced:the repeated bilayers formed gel aggregates. The spatial arrangements of the bilayers were linked by means of head-to-head. Long chain alkyl paralleled or interspersed with each other, to form different forms of nano-scale supramolecular structures.The possible mechanism of amino acid-type low molecular-mass organogelators gelling in organic solvents is:organogelator molecules in organic solvents through hydrogen bonding and hydrophobic interactions self-assembly to form a one-dimension structure, which further wind cross-linking to form three-dimension supramolecular network fibers, and then form gels through bounding solvent molecules by the hydrogen bond strength, ?-? stacking interaction, van der Waals force, capillary action and surface tension.