Small-Angle X-ray Scattering from the Lamellar Phase Formed in a Nonionic Surfactant (C16E7)−Water System. Analysis of Peak Position and Line Shape

Abstract

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 ∝ φhc-s where φhc is the volume fraction of hydrophobic layer. The exponent s obtained is unity as expected at 75 °C where the “Lα” 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 “Lα” phase, half-thickness of the hydrophobic layer δhc 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 δhc with concentration and temperature. In other words, the structure of the “Lα” 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 fw depends on concentration and temperature. Strong correlation has been found among the fw 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, “Lα”, and V1 phases have been discussed based on these results.