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We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x-ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a ‘‘flat section’’ or a ‘‘kink;’’ we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a ‘‘tilted hexagonal’’ phase, even at temperatures where the untilted phase is hexagonal (rotator-II). [ABSTRACT FROM AUTHOR]

We have studied the effect of pressure and pH on Langmuir monolayers of heneicosanoic acid in the presence of calcium ions in the subphase. At very low pH the monolayer is unaffected by calcium ions; as the pH is increased, the head groups are pulled closer together and the chain tilt at zero pressure decreases. As a result, the phase transitions seen along the isotherms occur at lower pressures. When the isotherms become featureless, the chains are untilted at all pressures. At very high pH, a new untilted structure is observed. [ABSTRACT FROM AUTHOR]