Evolution toward the X Phase of Fatty Acid Langmuir Monolayers on a Divalent Cation Solution

The structure of docosanoic acid monolayers spread over chloride salt solutions of copper was investigated by means of isotherm measurements, grazing incidence X-ray diffraction, and Brewster angle microscopy, as a function of the ion concentration and at two subphase pHs (5.5 and 7.5). The X phase is evidenced immediately above a concentration threshold which depends on the pH. The sequence of phases leading to this rigid phase involves two different processes depending on the pH. The initial L(2h) phase evolves toward an X-like phase through a phase transition which is first order at pH 7.5 while it is second order at pH 5.5. The transition is then followed by a continuous evolution toward the X phase.