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 oil 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.