Understanding of interfacial tension and interface thickness of liquid/liquid interface at a finite concentration of alkyl phosphate by molecular dynamics simulation

Abstract

The Mulliken charge embedded newly developed OPLS force field has been applied for computation of various structural and dynamical properties of water–dodecane biphasic system. The interface thickness has been investigated in the presence of finite concentration of third component, a lipophilic ligand, tri-isoamyl phosphate (TiAP) and acid employing molecular dynamics simulation. The physical properties at the interface like interface structure, interfacial tension, interface thickness have been studied for a wide range of mole fraction of TiAP and with various aqueous phase acidity. The interfacial tension is found to be decreased with increasing mole fraction of TiAP whereas increased with the acid concentration. The inverse relation between interface thickness and interfacial tension has been established. The surface tension computed from pressure tensor was found to be in excellent agreement with the experimental results, which is then used to determine the interface thickness employing capillary wave theory (CWT) using a simple scheme of finding the bulk correlation length by adopting conductor like screening model for real solvents. Furthermore, a simple equation has been obtained by fitting the MD results of total interface thickness for a wide range of composition of third component. The value of interface thickness calculated from the model equation was found to be in good agreement with the available experimental and MD results for a wide variety of oil-water biphasic systems. Additionally, the stability of the dimer of TiAP with different orientation was established by computing the dimerization energy using DFT which was also captured in the MD simulation. Furthermore, the present MD data shows that the water extraction is enhanced at higher mole fraction of TiAP because of wider interface thickness at higher composition of TiAP which induced the water transport to the organic phase. Again, the calculated values reveal that the water extractability is decreased with increasing aqueous phase acidity as the interface thickness is reduced at higher acidity. The present results might be useful in the frontier area of interfacial science and engineering as well as in partitioning of drug molecules at the biological interfaces.