Scattering study of the lyotropic lamellar phase in aqueous solutions of nonionic surfactants

Small angle x-ray scattering (SAXS) has been measured on the lamellar phase and concentrated micellar solutions for heptaethylene glycol-n-hexadecyl ether (C ₁₆ E ₇ )-water system. The repeat distance d follows the swelling law d ∝ φ _hc ^.s where φ _hc is the volume fraction of hydrophobic layers. In the higher temperature range, the exponent s is unity as expected. However, reduction of temperature induces gradual transition of s from unity to 2/3 which corresponds to the upper limit of theoretical values for the mesh phase. Based on these results and line shape analyses of SAXS, it has been shown that the deviation from the classical lamellar structure is enhanced as the temperature and concentration are reduced and the most pronounced near the boundary with bicontinuous cubic and micellar phases. Relations with phase behaviors and the structure of the micellar phase are also discussed.