For more than a century, density gradient theory (DGT) has been developed and applied for interfacial property calculations of pure and mixed fluid systems. However, due to the local density approximation, DGT has not been applicable to amphiphilic molecules. By developing a modified DGT model with the chain contribution to the free energy, this paper extends the application of the DGT model to heteronuclear chain molecules, such as surfactants. The chain contribution term is derived based on the work from the iSAFT model. With the help of the Stabihzed Density Gradient Theory (SDGT) algorithm developed in our previous work and the PC-SAFT equation of state (EoS), the modified DGT model is tested in water/oil/surfactant mixture systems, the results of which have been qualitatively verified with other theories and experimental data.