Classical density functional theory for associating fluids in orienting external fields

We develop a classical density functional theory (DFT) for two site associating fluids in spatially uniform external fields which exhibit orientational inhomogeneities. The Helmholtz free energy functional is obtain using Wertheim’s thermodynamic perturbation theory and the orientational distribution function is obtained using DFT in the canonical ensemble. It is shown that an orienting field significantly enhances association by ordering the molecules thereby reducing the entropic penalty of association. It is also shown that association enhances orientational order for fixed field strength.