| In this paper, variations of physicochemical properties(CMC, Krafft temperature, etc.) of a series of Gemini surfactant CmOhpNCn were investigated under different conditions, and a close relationship among molecular structure, physicochemical properties and aggregation morphologies was obtained. Meanwhile, to better understand the lyotropic liquid crystal(LLC) behaviors affected by surfactant molecules, the packing characteristics of CmOhpNCn in ionic liquid(IL) at different temperature were also studied. Relevant results may be helpful for constructing new Gemini surfactants and their applications. The detailed contents were described as follows:1. The physicochemical properties of a series of heterogemini surfactants N,N-dimethyl-N-[3-(alkyloxy)-2-hydroxypropyl]-alkylammonium bromide, abbreviated as CmOhpNCn(m, n=10, 8; 10, 14; 12, 8; 12, 10; 12, 12; 12, 14; 14, 8 and 14, 10), have been investigated in aqueous solution. The results of electric conductivity measurements indicated that Krafft temperatures were mainly related to the total number of carbon atoms, while the CMC values are affected by the combined factors of alkyl chain length, degree of dissymmetry and hydroxyl headgroup. And some unusual values may come from the poor water solubility induced by the relatively long alkyl chain n.The results of surface tension revealed that the molecular packing of CmOhpNCn affected by intermolecular hydrogen bonding and weak electrostatic repulsion was relatively loose even though the hydrophobicity of CmOhpNCn was stronger than that of conventional surfactants.2. The aggregation behaviors of CmOhpNCn(m, n=10, 8; 12, 8 and 14, 8) have been investigated in aqueous solution. Steady state, frequency sweep rheological measurements and dynamic light scattering(DLS) were adopted to study the micellar morphology affected by molecular structure, concentration and temperature, and a close relationship between them was established. Dynamic rheological results and cryo-TEM images revealed various micellar behaviors of CmOhpNC8 affected by different combined factors. When m value was increased, the formation ofquite long rodlike or wormlike micelles came from the enhanced hydrophobility and hydrogen bonding. And the effect of concentration on aggregation behavior was similar to that of m value except for the formation of micellar joints. With increasing the temperature, the decreased viscosity of C14OhpNC8 was due to the formation of micellar joints and weakening of hydrogen bonding which could be confirmed by DLS. The C14OhpNC8 aggregates formed below the Krafft temperature were more likely to be affected by temperature at high concentration. Furthermore, the loose network-like aggregates of CmOhpNC8 formed at a certain critical concentration can be attributed to the combined influences of hydrogen bonding and weak electrostastic repulsion.3. The self-assembly structures and lyotropic liquid crystal behaviors of CmOhpNCn(m+n=24, m=16,14,12) in ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate([OMIm]PF6) were investigated. Rheological measurements indicate that no long rodlike or wormlike micelles exist in Gemini surfactant / ionic liquid system. POM and SAXS results show that hexagonal, lamellar and bicontinuous cubic can be found in IL solution, and the stability of CmOhpNCn/[OMIm]PF6 LLC depends on ordered molecular packing and strong intermolecular force. Increasing the molecular dissymmetry of CmOhpNCn tends to destroy the dense molecular packing, resulting in the poor birefringence. With increasing temperature, the system phase structures varied from single phase(hexagonal or lamellar) to mixed phases and then to isotropic phase since the hydrogen bonding that drives the formation of LLC can be weakened at high temperature.The conculsions above would be expected as theoretical basis, including construction of new type Gemini surfactants, design of Gemini surfactants with special properties and applications of Gemini surfactants in non-aqueous system, etc. |
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