Theory and simulation of chain molecules with multiple bonding sites in an associating solvent

Abstract

Although polyethylene oxide (PEO) is soluble in water, polymethylene oxide (PMO) is not, even though PMO has more association sites. Some suggest this is due to orientation effects in the water hydrogen-bond network. A simulation and theory study of the effect of bonding site density on thermodynamic properties and extent of bonding of a linear flexible chain in a hydrogen-bonding solvent is performed. Predictions from Wertheim's theory are compared against simulation results. Thermodynamic properties and extent of bonding were obtained. The solvent molecules are modeled as hard spheres with four association sites in a tetrahedral arrangement. The chains are flexible and consist of six tangent segments of hard spheres with bonding sites that interact with the solvent molecules. A solvent molecule can also form a bond with a second solvent molecule. The association interaction is modeled with an orientation-dependent square-well. The total number of bonding sites on each chain is varied and the effects studied. This is another test of the theory for the case of mixtures of associating molecules of different sizes. The Metropolis Monte Carlo technique was chosen to perform simulations in the canonical and isothermal–isobaric ensembles. Good agreement was found between theory and simulation.

Keywords:

• associating molecules
• cross and self bonding
• thermodynamic perturbation theory
• Monte Carlo simulation
• SAFT

Acknowledgements

We gratefully acknowledge financial support from the Robert A. Welch Foundation (grant C-1241), the Petroleum Research Fund, and the National Science Foundation (grant CBET-0756166). A.J.G.C. acknowledges support from Tecnológico de Monterrey (grant CAT-125).