Modeling surface and line tensions of nanoconfined water using a single atomistic simulation

In this work, the liquid-vapor (γlv), solid-liquid (γsl), and solid-vapor (γsv) surface tensions, as well as the line tension (τ) of water confined between planar rigid walls modeled as graphene sheets, are calculated from a single molecular dynamics simulation. While γlv and γsl are explicitly evaluated far from the contact lines between the liquid, vapor, and solid regions, γsv is deduced. Following a thermodynamic approach based on the description of the free energy, the line tension is determined from the three surface tensions, the pressures of the liquid and vapor phases, and the derivative of the free energy with respect to the length supporting the contact line. This analysis shows that the line tension cannot be reduced solely to the excess energy associated with line contact deformations. By relating the thermodynamic variables to the total stress along the x-direction, the mechanical and thermodynamic approaches are found to be consistent. We show that the line tension of confined water is negative and that neither temperature nor the degree of confinement affects its sign, which is consistent with other published results obtained using different methods. The main advantage of this approach lies in the ability to determine the three surface tensions and the line tension from a single atomistic simulation.