Phase transitions in Langmuir films of fatty acids: L2-L2'-L2" triple point and order of the transitions.

Abstract

Using Brewster angle microscopy we have studied the low-temperature region of the phase diagram of Langmuir monolayers of long-chain fatty acids. We have shown that the transitions between the three tilted phases, ${\mathit{L}}_{2}$, ${\mathit{L}}_{2}^{\ensuremath{'}}$, and ${\mathit{L}}_{2}^{\mathrm{\ensuremath{''}}}$ are first order, and have located the triple point precisely. We have observed a radical change of the tilt-azimuth texture at the ${\mathit{L}}_{2}$-${\mathit{L}}_{2}^{\mathrm{\ensuremath{''}}}$ phase transition, even though the tilt direction with respect to the lattice is the same in the two phases. The transition between the tilted phase ${\mathit{L}}_{2}^{\mathrm{\ensuremath{''}}}$ and the crystalline untilted phase CS is also studied. Its character possibly changes from second order below the ${\mathit{L}}_{2}$-${\mathit{L}}_{2}^{\ensuremath{'}}$-${\mathit{L}}_{2}^{\mathrm{\ensuremath{''}}}$ triple point to first order above. \textcopyright{} 1996 The American Physical Society.