The Hayden-O'Connell method is a method which may be used to calculate the vapor fugacity coefficients when using the Liquid Activity Methods to predict equilibrium K-values. The method is designed to handle systems in which the components dimerize in the vapor phase, such as carboxylic acids. These systems have large negative deviations from vapor ideality, even at low pressures.
To account for dimerization, Hayden and O'Connell developed an expression for fugacity coefficient based on the chemical theory of vapor imperfection. This theory assumes there are dimerization equilibria for a binary mixture of components A and B:
KA2
A1 + A1 <=> A2
KB2
B1 + B1 <=> B2
KAB
A1 + B1 <=> AB
Hayden and O'Connell related second virial coefficients to the dimerization equilibrium constants, KA2, KB2, and KAB and developed generalized second virial coefficients for simple and complex mixtures.
See "Hayden-O'Connell" under "Vapor Fugacities" in Chapter 2, Volume 1, of the PRO/II Reference Manual for more information.