Thermodynamic and Interfacial Analysis of Polypropylene Glycol: Dependence of the Gradient Energy Coefficient on Molecular Weight Using Simha–Somcynsky and Cahn–Hilliard Theories

This study presents a theoretical analysis of the thermophysical and interfacial properties of polypropylene glycol with different molecular weights using a combined Simha–Somcynsky equation of state and Cahn–Hilliard diffuse-interface framework. Properties including gradient energy coefficient, surface tension, density, specific volume, hole fraction, and occupied volume fraction were evaluated over 313–473 K and 0.1–150 MPa. Results show that increasing molecular weight reduces specific volume and density but increases surface tension and gradient energy coefficient, while increasing temperature produces opposite trends. Numerical accuracy was assessed through volume deviation measures. The approach reliably captures molecular-weight-dependent interfacial behavior relevant to PPG material design applications.