Phase Behavior of Bipolar Fatty Acid Monolayers

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Abstract
The phase behaviors of the monohydroxy- and dihydroxy-substituted fatty acids at an air–water interface were investigated in the presence of different substrates. All of the hydroxylated fatty acids studied, except for 2-hydroxyoctadecanoic acid (2-HOA) and 9,10-dihydroxyoctadecanoic acid (9,10-DHOA), showed an intermediate region (“plateau”) for a transition from a liquid-expanded state (LE) to a liquid-condensed state (LC) in the π–A isotherms, regardless of the substrate composition. In contrast, 9,10-DHOA exhibited a pressure plateau region on only 3 M NaCl. In order to assess the effect of adding an extra polar group (i.e., a hydroxy group) for a 9,10-DHOA monolayer compared to a 9-hydroxyoctadecanoic acid (9-HOA) monolayer, the monolayer-substrate interactions were considered based on the thermodynamic parameters for the LE/LC phase transition, using a two-dimensional Clausius–Clapeyron equation. This can be explained by assuming that a greater energy was generated for breaking of hydrogen bonds between the two hydroxy groups of 9,10-DHOA and water. In this regard, the dynamic properties of the monolayer for these acids were examined using the surface pressure relaxation method as well as an equilibrium spreading pressure (ESP) measurement.