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 (C 16 E 7 )−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 2 H 2 O. Relations between the phase behaviors and the structures of the L 1, “L α ”, and V 1 phases have been discussed based on these results.