Small-angle X-ray scattering from the lamellar phase formed in a nonionic surfactant (C₁₆E₇)-water system. Analysis of peak position and line shape

The structure of the lamellar phase formed in the heptaethylene glycol n-hexadecyl ether (C₁₆E₇)-water system has been studied using small-angle X-ray scattering (SAXS). The concentration dependence of the repeat distance d follows the power law d ∝ φ _hc^-s where φ_hc is the volume fraction of hydrophobic layer. The exponent s obtained is unity as expected at 75 °C where the "L_α" phase extends to the lower concentration range. As the temperature decreases below about 70 °C, however, s decreases rapidly. Below about 55 °C, s becomes constant again and takes values of about 2/3. From the line shape analysis of SAXS in the "L_α" phase, half-thickness of the hydrophobic layer δ_hc and the thickness of the hydrophilic layer have been obtained. It has been shown that the anomalous behaviors of the exponent s cannot be explained by the variation of δ_hc with concentration and temperature. In other words, the structure of the "L_α" phase varies with concentration and temperature. Assuming that the bilayer sheets have water-filled defects, we have made a "map" showing how the fraction of the defects f_w depends on concentration and temperature. Strong correlation has been found among the f_w value, the appearance of the broad component superimposed on the first-order reflection, and the quadrupolar splitting of ²H₂O. Relations between the phase behaviors and the structures of the L₁, "L_α", and V₁ phases have been discussed based on these results.