Molecular Dynamics: Investigating the Self-Association of Stearic Acid and Heteroassociation of Stearic Acid–Water in Cyclohexane

[Image: see text] The self-association of molecular additives determines the chemical potential in the bulk and, in turn, the adsorbed amount onto a surface for a number of important commercial applications such as wind turbines. Molecular dynamics simulations have been utilized as a technique to study the self-association of model additive, stearic acid, and heteroassociation of stearic acid–water, in cyclohexane as a function of temperature. Reasonable values of the enthalpies and equilibrium constants were determined for stearic acid in cyclohexane. The role of water, nearly always present in commercial systems, in solution association was also studied to determine the thermodynamics parameters of hydration (i.e., acid–water heteroassociation). There are very few other studies reporting on these important heteroassociation parameters. The association constants and enthalpies of association obtained from molecular dynamics are in good agreement with experimental data in the literature. A combination of Fourier transform infrared (FTIR) data and molecular dynamics simulation results allows the fraction of open dimers (single hydrogen-bonded dimers) to be estimated in cyclohexane (which is not possible from the experimental FTIR data alone). The fraction of open dimers of stearic acid in cyclohexane at room temperature is ∼1.5% at 25 °C and ∼4% at 70 °C.