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Quantitative conceptual models for Langmuir monolayers
Conceptual models for Langmuir monolayers are developed and quantitatively investigated. The first model utilizes an anisotropic potential and qualitatively reproduces the temperature dependent features of untilted phases in the $\pi$-T phase diagram of fatty acid monolayers. A cross-section potential is derived by modifying a Lennard-Jones potential and is used in a Monte Carlo simulation of an isobaric-isothermal ensemble that incorporates changes in both the size and shape of the periodic boundaries. A bound-rotator to free-rotator transition between well-defined rectangular and hexagonal symmetries is found in qualitative agreement with experiment. Atom-atom potential calculations are used to relate this model to real systems. A second model that considers straight strings of Lennard-Jones centers is used to investigate the effect of pressure on the tilting behavior of Langmuir monolayers. All strings, or molecules, exhibit the same tilt direction and magnitude. Both the length of the strings and the distance between centers affect the magnitude and direction of the tilt. Calculations are found to be significantly simplified by considering strings of infinite length. The model qualitatively reproduces the varied tilt behavior observed in real fatty acid films. At high temperatures a transition to a precessing phase is found. ^
David Robert Swanson,
"Quantitative conceptual models for Langmuir monolayers"
(January 1, 1995).
ETD collection for University of Nebraska - Lincoln.