MOLECULAR PHYSICS, 2002, VOL. 100, NO. 2, 265 ± 272 Development of a force ®eld for molecular simulation of the phase equilibria of per¯uoromethylpropyl ether H.-C. LI 1;2 , C. MCCABE 1;2 , S. T. CUI 1;2 , P. T. CUMMINGS 2;3 , and H. D. COCHRAN 1;2 * 1 Department of Chemical Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA 2 Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6224, USA 3 Departments of Chemical Engineering, Chemistry, and Computer Science, University of Tennessee, Knoxville, TN 37996-2200, USA (Received 3 May 2001; revised version accepted 24 July 2001) A ®rst step towards the development of a general, realistic potential model for per¯uoroether compounds has been to parameterize a united atom model for a short chain per¯uoroether per¯uoromethylpropyl ether (CF 3 CF 2 CF 2 OCF 3 ). The potential model takes the usual form in which separate bond bending and torsional terms describe the intramolecular interactions with the addition of van der Waals and electrostatic terms to describe the non-bonded interactions. Ab initio quantum calculations have been carried out to obtain the partial charges and intra- molecular torsional and bending potentials. Phase equilibrium data were then used to optimize the van der Waals interaction parameters through Gibbs ensemble Monte Carlo simulations. The resulting model reproduces vapour±liquid equilibrium densities, the critical temperature and the critical density of per¯uoromethylpropyl ether, in good agreement with those from experiment. 1. Introduction In the past decade, per¯uoroethers have attracted interest from a wide range of communities because of their importance in a number of applications and research areas. Per¯uoropolyethers, and per¯uorocar- bons in general, have a unique combination of physical and chemical properties which makes them suitable for a wide range of potential applications in the medical, bio- technology, electronic, and oil and gas industries [1, 2]. Per¯uoropolyethers are extremely unreactive, non-toxic, non-¯ammable ¯uids, making them excellent candidates as high performance lubricants [3±5]. For example, Fomblin 1 and Krytox 1 are widely used as lubricants for magnetic recording media because of their low vapour pressure, oxidative stability, and relatively stable structures [6±8]. In the pharmaceutical and bio- medical industries, per¯uoropolyethers have attracted a lot of attention because of their chemical and biochem- ical inertness. This inertness, when combined with their high solubility of oxygen and other respiratory gases, has led to a number of applications, such as, blood plasma substitutes, eye ¯uid, infusion ¯uid in the treat- ment of burns, and in cosmetics [1]. Additionally, sev- eral simple per¯uoroethers can be used as potential CFC substitutes [9]. A particularly topical example is the possible use of functionalized per¯uoropolyethers as surfactants that can form microemulsions which dis- perse water in supercritical carbon dioxide [10±12], helping to make CO 2 a more versatile replacement sol- vent in the chemical and related industries. Thus, it would be of great value to be able to model per¯uoropolyethers with molecular simulation tech- niques. While the thermophysica l properties (including phase equilibrium and rheology) of hydrocarbons have been examined extensively through molecular simula- tion, and numerous force ®elds have been proposed in the literature [13±17], simulation studies and force ®eld development for per¯uorinated compounds are much less advanced [18, 19]. Essentially, previous work for per¯uoro compounds has been for per¯uoroalkanes, and has focused on modelling the solid phase diagram of PTFE using explicit atom models to describe the helical nature of the carbon backbone (see, e.g. [13, 20±23]). In the early 1990s, in a study of the properties of liquid supported monolayers of long chain amphi- philic molecules, Rice and coworkers [24±26] developed a united atom model for CF 3 and CF 2 groups in per- Molecular Physics ISSN 0026±8976 print/ISSN 1362±3028 online # 2002 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/0026897011008633 6 * Author for correspondence. e-mail: hdc@ornl.gov