Integrated Simulation System for Soft Materials.

We used DPD to calculate the interface tension between water and octane. Octane is expressed with 3 particles of DPD, and water is expressed with 1 particle of DPD. Because the volume of each particle was set to the volume of 3 water molecules, the standard volume Vbead is 54.3 [cm^3/mol], system number density is (ρ=3), and unit length Rc is 6.46 [Å].

The repulsion parameter a applied to each bead is derived from the following equations, which require the solubility parameter δ from the full atomistic model MD calculation (Table 1). The resulting value for the water/octane particle parameter is a_ij = 120.1. Furthermore, the particle parameter for particles of the same type is a_ii = 25.

Figure 1 shows the water/octane interface model using DPD particles. After setting the DPD particles to 3000 water molecules and 1000 octane molecules and arranging the formation of the interface, we used COGNAC to perform the calculations. The interface is maintained after relaxation, and the following formula was used to compute the interface tension from the stress values of various directions. Also, the unit length is used to give the interface tension a dimension. The above calculation resulted in a water/octane interface tension of 50.1 [dyn/cm]. These results are favorable to experimental data values of 51.7 [dyn/cm]^[1].

Figure 3 shows the interface tension a_ij dependency of this model. At a_ij >30 it is divided into two phases, and a_ij increases together with increases in interface tension.