title: Thermodynamics of the liquid states of Langmuir monolayers.
abstract: The liquid states and the liquid-liquid equilibrium of surfactant molecules forming an interphase between air and water have been considered using Monte Carlo computer simulations. Specifically, the expanded and compressed liquid phases observed for surfactant molecules were characterized as a function of pressure and temperature. Simple modified beadlike potentials were implemented in order to describe the interparticle forces between the hydrophobic and hydrophilic portions of surfactant molecules at the air/water interface. A simulation box was defined such that the monolayer was exposed to an externally applied lateral pressure in a modified isothermal-isobaric ensemble, whereas the water bath was modeled in a canonical ensemble. The simulation resembles the experimental setup used to measure lateral pressure (Pi) versus area isotherms obtained with Langmuir troughs. The applied lateral pressure-surface area phase diagram clearly showed the coexistence of the expanded and compressed liquid phases within certain temperature and pressure ranges. Distribution functions of distances and enthalpies for the monolayer were computed to clearly identify each liquid phase and the coexistence region.
authors.affiliation: Department of Chemistry, University of Puerto Rico at Mayagüez, P.O. Box 9019, Mayagüez 00681-9019, Puerto Rico.
authors.forename: Leslie
authors.initials: L
authors.lastname: Villalobos
authors.forename: Yania M
authors.initials: YM
authors.lastname: López-Alvarez
authors.forename: Belinda
authors.initials: B
authors.lastname: Pastrana-Ríos
authors.forename: Gustavo E
authors.initials: GE
authors.lastname: López
mesh_terms: D003198:Computer Simulation; D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D044367:Phase Transition; D012996:Solutions; D013501:Surface-Active Agents / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D008051:Lipid Bilayers; D012996:Solutions; D013501:Surface-Active Agents