A molecular dynamics study on the dependence of phase behaviors and structural properties of two-dimensional interfacial monolayer on surface area

The packing state and structure of monolayer at the air/water interface have important effect on its wide application. Using molecular dynamics (MD) simulation, phase transitions of sodium dodecyl sulfate (SDS) monolayer, dependent on the surface area per molecule, have been studied in the view of configuration entropy. With the reduction of surface area per molecule, three two-dimensional (2D) phases are defined, including gaseous, liquid expanded and liquid condensed phases. At small surface area per molecule, the 2D monolayer will collapse and 3D aggregates are formed in the bulk solution. We give a detailed description of the collapse mode from interfacial monolayer at the air/water interface to micelle in the solution at the molecular level. Using the quasi-harmonic (QH) approximation, the entropic change of SDS for 2D phase transition are first introduced, which is around −29.7 J mol−1 K−1 for the transition from 2D gaseous film to 2D liquid expanded film and −42.0 J mol−1 K−1 for the transition from 2D liquid expanded film to 2D liquid condensed film. The effect of different surface coverages on the dynamical and structural properties of SDS monolayer at the air/water interface is evaluated. Based on simulated results, a schematic diagram of 2D monolayer in different phases is proposed.