Pulling simulation predicts mixing free energy for binary mixtures. Issue 41 (12th October 2022)

• Record Type: Journal Article
• Authors: Mkandawire, Wezi D.; Milner, Scott T.
• Is Part Of: Soft matter. Volume 18:Issue 41(2022)
• Page Start: 7998
• Page End: 8007
• Publication Date: 2022-10-12
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/d2sm01065h
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : By applying attractive or repulsive harmonic potentials to every molecule in a mixture, and observing the resulting concentration profile, the chemical potential versus mole fraction can be determined. Abstract : Predicting the mixing free energy of mixing for binary mixtures using simulations is challenging. We present a novel molecular dynamics (MD) simulation method to extract the chemical potential μ ( X ) for mixtures of species A and B. Each molecule of species A and B is placed in equal and opposite harmonic potentials ±(1/2) U ex ( x ) centered at the middle of the simulation box, resulting in a nonuniform mole fraction profile X ( z ) in which A is concentrated at the center, and B at the periphery. Combining these, we obtain U ex ( X ), the exchange chemical potential required to induce a given deviation of the mole fraction from its average. Simulation results for U ex ( X ) can be fitted to simple free energy models to extract the interaction parameter χ for binary mixtures. To illustrate our method, we investigate benzene–pyridine mixtures, which provide a good example of regular solution behavior, using both TraPPE united-atom and OPLS all-atom potentials, both of which have been validated for pure fluid properties. χ values obtained with the new method are consistent with values from other recent simulation methods. However, the TraPPE-UA results differ substantially from the χ obtained from VLE experimental data, while the OPLS-AA results are in reasonable agreement with experiment, highlighting the importance of accurate potentials in correctly representing mixture behavior.