Case Studies

Evaluation of Interfacial Tension Using DPD

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 aij = 120.1. Furthermore, the particle parameter for particles of the same type is aii = 25.


Table 1 Mol Volume/Solubility Parameters of Water and Octane
Material
V [cm^3/mol]
δMD [(J/cm^3)^(1/2)]
δexp[1] [(J/cm^3)^(1/2)]
water
18.1
52.6
47.9
octane
162.6
16.0
15.6

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 1 Interface Model (Upper area is water, lower area is octane)
Figure 2 Changes in Interface Tension Over Time

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


Figure 3 Relationship Between Interface Tension and aij


Reference
[1] A. Maiti and S. McGrother, J. Chem. Phys., 120, 3, 15 (2004)





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