Study On The Organized Aggregates Based On Amphiphiles Containing Saturated Nitrogen Heterocycles

The amphiphiles can be assembled into various aggregates in different solvents, including micelle, liquid crystal, vesicle, gel, and also the organized aggregates in solid state. In recent years, it has been shown that the nitrogen containing heterocyclic amphiphiles, such as imidazolium and pyrrolidinium salts, can exhibit excellent performance properties. As building blocks, their uses in fabricating ordered molecular assemblies have attracted much attention with the aim to develop novel aggregate systems and provide potential applications. However, researches on amphiphiles constructed by long alkyl chain acid-soap complex or with morpholinium head group were less reported. As we know, the properties of amphiphile depend largely on its structure. Various headgroups can endow amphiphiles different self-assembling behaviors, which may influence their applications. Therefore, studies on self-assembling behaviors of such two kinds amphiphiles would be helpful for better understanding on the aggregation mechanisms for these nitrogen heterocyclic compounds.In addition, the amphiphiles have also been widely employed as capping agents to exert exquisite control over the nucleation and growth of nanocrystals or participate in self-organization of nanoparticles to generate multi-dimensional nanocrystals. Then, how to prepare organic/inorganic nanostractures with regular morphology mediated by self-assemblies of amphiphiles becomes an interesting exploration project.For these motivations, in this dissertation, various modern analytical instrumental methods including TEM、SEM、POM、SAXS、XRD、FTIR, UV-vis、 Raman and rheology measurements were used combining theoretical calculation to study the aggregation behaviours on two kinds nitrogen heterocyclic amphiphiles composed by acid-soap complex or containing morpholinium headgroup. The outline and main contents of this dissertation are listed as follows:In the first section, the research background for this work was introduced including the definition of amphiphiles and their aggregation in solution. The history and recent progress on formation of organized molecular assemblies by nitrogen heterocyclic amphiphiles were specially reviewed. On this basis, the research ideas, contents, and significance of this dissertation are depicted at the end of this part.In the second section, the hydrogen bond induced acid-soap complex has been prepared from a nitrogen-containing heterocyclic compound, pyrrolidine, and the myristic acid (MA). The complex composition and the intermolecular actions between these two component molecules have been characterized with techniques of nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and Fourier transformed infra-red (FTIR) spectroscopy. The molar ratio between the soap (pyrrolidium myristate, PM) and the acid (MA) is identified as1:1, which is different from the alkaline systems (with different stoichiometric ratios). The bilayer spacing of the complex is4.62nm. As an important driving force of assembly, the existence of hydrogen bond is further confirmed by a molecular mechanics calculation. DSC data show that the complex has a slightly lower melting temperature than that of myristic acid, whereas the alkaline acid soaps have significantly higher melting temperatures than the corresponding fatty acids. Introduction of the large size pyrrolidinium counter ion and the synergetic interactions between fatty acid and pyrrolidine soap in complex provide more hydrogen-bonding positions. Thus, such an acid-soap complex (PM-MA) exhibits amphiphilicity and its lyotropic liquid crystal phase in water has been observed over a wide concentration range. A lamellar phase structure is established using the polarized optical microscopy (POM) and smallangle X-ray scattering (SAXS) measurements. The change of lattice spacing indicates a typical one dimensional swelling scheme for the uptake of water by this lamellar phase. The frequency-independent storage and loss moduli could be found for these lamellar samples by rheological measurements, reflecting a highly viscoelastic nature. The obtained results should add new insights to better understand the structure and aqueous behavior of long chain acid-soaps with a large size nitrogen-heterocyclic counter ion. In the third section, the self-assemblies formed by morpholinium amphiphiles havebeen investigated. The work content can be divided into three parts:Investigation on the micellization behavior of N-Alkyl-N-Methylmorpholinine Bromide (CnMMB) in water; Investigation on the formation of wormlike micelle of N-methyl-N-hexadecylmorpholinium bromide (C16MMB) induced by organic salts in water; Investigation on the LLC phases formed by fatty acid morpholininium salt (CnAM) in water. The obtained results are described as follows:(1) The micellar behaviors of CnMMB (n=1214,16) were studied through the surface tension (y) and electrical conductivity measurements. Compared with the critical micelle concentration (cmc) values of N-Alkyl-N-Methylpyrrolidine Bromide (CnMPB) and N-Alkyl-N-Methylpiperidine Bromide (CnPDB), the surface activity of CnMMB is slightly less than those of CnMPB and CnPDB. The oxygen atom existence instead of methylene unit in saturated N-heterocyclic ring might increase the effective positive charge of the hydrophilic group, which slightly increases hydrophilicity of the polar headgroup and causes larger cmc values of CnMMB. The slightly increased hydrophilicity by oxygen atom will also decrease the efficiency of y reduction. The intermediate size of morpholinium headgroup results in the saturated adsorption amount (Γmax) and minimum surfactant molecule adsorption area (Amin) values just between those of CnPDB and CnMPB. Furthermore, based on the calculated thermodynamic parameters (△Gm0,△Hm0and△Sm0) for CnMMB micelle formation from the electric conductivity measurements, the micellar formation of CnMMB is found to be entropy-driven at low temperature and enthalpy-driven at high temperature. We expect this work can help us to better understand the effect of hydrophilic headgroup on the micellar behaviors of saturated nitrogen containing heterocyclic cationic surfactants.(2) The wormlike micelles formation by amphiphile N-alkyl-N-methyl-morpholinium bromide mixed with sodium salicylate have been investigated in aqueous solution. The typical rheological parameters and the viscoelastic properties of wormlike micelles were characterized by rheological measurements and cryogenic-transmission electron microscope (Cryo-TEM). Rheological results showed that the viscoelastic properties of wormlike micelles could be fitted well by the Maxwell model. The formation of wormlike micelles in this system could be further confirmed by the Cryo-TEM image. We believe that this work not only advances our understanding on the formation of wormlike micelles, but also enriches the application fields of nitrogen-containing heterocyclic amphiphiles.(3) A series of morpholinium fatty acid salt were prepared and characterized by several techniques such as1H NMR and FTIR. Their thermotropic and self-assembling behaviors in water were further investigated by DSC, TGA, POM,2H NMR and SAXS measurements. The lyotropic liquid crystal behaviors were analyzed in detail. With increased amphiphile concentration, the hexagonal and lamellar LLCs could be formed in C12AM/H2O and C14AM/H2O systems, but only the lamellar phase could be observed in C16AM/H2O system.In the fourth section, researches were focused on crystallization of the N-methyl-N-hexadecylmorpholinium bromide (C16MMB) surfactant in Au nanoparticle (Au NPs) sol. A large-scale production of uniform rectangular Au@C16MMB crystal sheets with uniform surface enhanced Raman scattering (SERS) activity could be induced. The scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopic measurements have been adopted to characterize the obtained crystal sheet morphology and structure. The island-like Au NPs aggregates were observed to distribute uniformly over the surface of C16MMB crystal, which provide sufficient SERS-active sites. Results from Zeta potential measurement and molecular dynamic simulation indicated that the C16MMB molecules were electrostatically adsorbed on Au NPs to reverse the nanoparticle surface charge to be positive. The concentration and crystallization capability of C16MMB in solution play a key role to induce the formation of the self-organized Au@C16MMB crystal sheets. A surfactant crystallization induced assembling mechanism has been proposed. Raman tests of Au@C16MMB as an ideal SERS substrate exhibit the high enhancement factor and good uniformity. It is believed that this approach could provide a unique way to fabricate uniform noble metal nanoparticle-based SERS substrates. Thanks for the financial supports from the National Natural Science Foundation of China (No.20973104,21033005and21373127).