Osmotic Method for Calculating Surface Pressure of Monolayers in Molecular Dynamics Simulations

Surface
pressure is a fundamental thermodynamic property related
to the activity of molecules at interfaces. In molecular simulations,
it is typically calculated from its definition: the difference between
the surface tension of the air–water and air-surfactant interfaces.
In this Letter, we show how to connect the surface pressure with a
two-dimensional osmotic pressure and how to take advantage of this
analogy to obtain a practical method of calculating surface pressure–area
isotherms in molecular simulation. As a proof-of-concept, compression
curves of zwitterionic and ionic surfactant monolayers were obtained
using the osmotic approach and the curves were compared with the ones
from the traditional pressure tensor-based scheme. The results shown
an excellent agreement between both alternatives. Advantageously,
the osmotic approach is simple to use and allows to obtain the surface
pressure–area isotherm on the fly with a single simulation
using equilibration stages.