Structural changes and interaction parameters in amphiphilic system C12E5/water/n-octane

A ternary amphiphilic system consisting of water, n-octane and nonionic amphiphile C12E5 has been studied by means of small-angle X-ray scattering in order to understand the differences of structures and interaction parameters among the low-temperature microemulsion (LTM), middle-temperature lamellar (MTL) and high-temperature microemulsion (HTM) phases. Scattering spectra for microemulsion phases are analyzed by the phenomenological scattering formula of Teubner and Strey (TS), whereas the Ornstein-Zernike scattering formula was combined with the TS formula for the lamellar phase. From the fitting parameters we have determined structural parameters such as the repeat distance D, correlation length ξ and 2πξ/D for both the lamellar and the two microemulsion phases. Observed results suggest that the LTM phase is more ordered than the HTM phase. Thermal fluctuations in the HTM phase may make the structure less ordered than that in the LTM phase. Again a finite value of kξ suggests the existence of an imperfectly ordered MTL phase. Microscopic interaction parameters were determined in terms of Gompper and Schick's (GS) microscopic spin-lattice theory for a suitable lattice parameter d=15 A. Observed interaction parameters have reasonable agreement with GS's prediction. The inequality condition ¦L¦>J/4, suggests strong amphiphilic interaction for all three phases.