Thermodynamic Properties of Model Surfactant Forming Langmuir Monolayers

The study of Langmuir monolayer has attracted 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 showed the coexistence of two liquid phases, denominated as liquid expanded state (LES) and liquid condensed state (LCS). 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 phases were considered using Monte Carlo computer simulations in the Standard Virtual Gibbs Ensemble. The Caillette-Mathies phase diagrams were constructed and the critical parameters of the system were obtained using the Ising model and the rectilinear approximation. These critical parameters were identified varying the interaction between the surfactant molecules and the aqueous phase. Also the thermodynamic properties for the equilibria were obtained using the van Hoff equation.