Molecular simulations of model Langmuir monolayers.

摘要

The study of Langmuir monolayers has generated the attention of researchers because of their unique properties and their not well understood phase equilibrium. These monolayers exhibit interesting phase diagrams where the unusual liquid-liquid equilibrium can be observed for a single component monolayer submitted to an external applied pressure. In this study, the thermodynamic properties of a model Langmuir monolayer were determined using two types of computer simulations. First, Monte Carlo simulations in the Isothermal-Isobaric ensemble were used to obtain adsorption isotherms of Langmuir monolayers. The results clearly show coexistence of two liquid phases, denominated as liquid expanded state (LES) and liquid condensed state (LCS). Radial distribution function and distribution functions of enthalpies for the monolayer were also computed to clearly identify each liquid phase and the coexistence region. A second model was used to obtain the critical properties for this model system. The phase equilibrium between the liquid phases and the LES-Vapor (V) phases has been considered using Monte Carlo computer simulations in the Standard Virtual Gibbs Ensemble. The Caillete-Mathias phase diagrams were constructed and two models were implemented in order to determine the critical parameters of the system. Specifically, the Ising Model and the rectilinear approximation were used to identify the critical temperature (Tc*) and the critical density (rhoc*), respectively. These critical parameters were identified by varying the interaction between the surfactant molecules and the aqueous phase. Finally, we have identified the coexistence between the LES and LCS states, in agreement with experimental and theoretical evidence in the literature. Furthermore, we have successfully identified the critical parameters Tc* and rhoc* for the LES-LCS and the LES-V equilibrium of the monolayer.