(667f) New Theory for Associating Fluids: Effects of Steric Hindrance, Ring Formation and Double Bonding

Equations of state based on Wertheim’s first order perturbation theory (TPT1) have proven to be a powerful tool in the modeling of associating fluids. In TPT1 the effect of bond angle is neglected such that the equation of state for associating molecules with a bond angle of 60 degrees is identical to that for associating fluids with bond angles of 180 degrees; that is, the effect of steric hindrance between association sites is neglected. Also neglected in TPT1 is the ability for molecules to associate into rings and double bond. For large bond angles these assumptions are valid; however, we show through molecular simulation that as bond angles are decreased steric hindrance between sites, association into rings and the possibility of double bonding must all be accounted for.

To model these small bond angle systems we develop a new molecular theory which captures each one of these small bond angle effects (steric hindrance, rings, double bonds). We explicitly include the effect of bond angle in each contribution to the free energy to develop an equation of state which is accurate over the full range of bond angles. We show by comparison to molecular simulation that the theory accurately predicts the pressure, internal energy and distributions of molecules in the various types of associated clusters (chains, double bonded, 3-mer rings, 4-mer rings, etc…). Further, we use the theory to demonstrate the effect of bond angle on the phase diagram of patchy colloid fluids.