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.

Gas–liquid phase equilibrium of a model Langmuir monolayer captured by a multiscale approach

“…2,9,20 Theoretical frameworks such as Landau theory of phase transitions have been used with some success in predicting condensed phases and coexistence boundaries of Langmuir monolayers 21 qualitatively. Molecular dynamics (MD) and Monte Carlo (MC) simulations 1,2,8,10,[22][23][24] of the full 3D system including the liquid phase do not only provide more quantitative information but have also broadened our insight into the structure and phase behavior of Langmuir monolayers. Current state-of-the-art atomistic simulations 25 were so far successful in predicting the structure of gas and solid phases of phospholipid Langmuir monolayers (76 nm 2 in size, with 76 to 144 surfactants at each interface).…”

Phase equilibria in model surfactants forming Langmuir monolayers

“…Initial choices for the volume and number of particles were obtained from our previous study 16 on model Langmuir monolayer and were set so each box will have the proper density of the phase being modeled, i.e., either a LES-V or LCS-LES. A total of 1600 surfactant molecules were used in each simulation box.…”