両親媒子三元系におけるミクロ構造と相転移
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概要
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Amphiphiles, which are often called surfactants, have a hydrophilic polar head and hydrophobic nonpolar tails. It is natural to think that a mixture of water and oil is immiscible; however, it can be mixed in presence of a little amount of suitable amphiphiles. As a consequence of their amphibious structure almost all amphiphiles remain on the water-oil interface in an aqueous-oil-amphiphile ternary system and organize various topologically distinct structures depending upon the temperature and the concentration of the components. In a binary system with water (or oil), amphiphiles form bilayer and remain between two water (or oil) domains. The structures of a binary or ternary amphiphilic system can be classified into isotropic disordered phases such as micelles, vesicles, droplet or bicontinuous microemulsions and lyotropic ordered phases such as hexagonal arrays of cylinders and lamellar structures. Enormous efforts have been dedicated to examine the self-assembly, microstructures and dynamics of amphiphilic systems using several experimental methods such as small angle scattering, freeze fracture electron microscopy, NMR self-diffusion, static light scattering, electrical conductivity, etc. However, many interesting aspects are still unknown or little known. In the present thesis, we have investigated a ternary amphiphilic system consisting of water, n-octane and C_<l2>E_5 by means of small angle X-ray scattering (SAXS), small angle neutron scattering (SANS) and electrical conductivity. C_<l2>E_5 is a non ionic amphiphile and the phase diagram of this system shows a variety of structures as a function of both temperature and amphiphile concentration. For instance, in a dilute concentration of amphiphile if we increase temperature, the system shows a microemulsion phase at lower temperature, a lamellar phase at middle temperature and another microemulsion phase at higher temperature. These phases are called low temperature microemulsion (LTM) phase, middle temperature lamellar (MTL) phase and high temperature microemulsion (HTM) phase, respectively. Previous studies in this system have revealed many aspects such as self-assembly, microstructure and direct images of local structures. The aim of this thesis is to elucidate the mechanism of the phase transition and to clarify the structural differences in different phases of water/n-octane/C_<12>E_5 system. We have investigated the phase transition mechanism, microscopic structure parameters and interaction parameters by means of SAXS and SANS. To understand the structural evolution we have also investigated the electrical conductivity as a function of temperature in different phases. SAXS was measured at a mirror-monochromator point focusing camera using CuK_a characteristic line from a rotating anode X-ray generator (Rigaku, RU-200). SANS experiment was performed at SANS-U of JRR-3M at Japan Atomic Energy Research Institute. SAXS profiles were analyzed by means of phenomenological model, which is well established in amphiphilic systems to represent the scattering profiles. In each one-phase region (LTM, MTL or HTM phase) the scattering data shows a single scattering peak. However, we observed that two scattering peaks exist in a region between LTM and MTL or MTL and HTM phases. It means that two different structures coexist in the transition region between microemulsion and lamellar phase. We argue that in the transition region one peak represent microemulsion structure while the other one represent lamellar structure. From now, this region, where two peaks co-exist, will be named as coexistence region. The two peaks in coexistence region show a very systematic gradual change with respect to the peak positions and the intensity at peak positions. These behavior is in good agreement with the lever rule in phase diagram. In order to explain the experimental results in transition region we have supplemented the bending energy model developed by Andelman et. al
- 広島大学の論文
- 1997-12-28