Nonuniform fluids in the grand canonical ensemble

Nonuniform simple classical fluids are considered quite generally. The grand canonical ensemble is particularly suitable, conceptually, in the leading approximation of local thermodynamics, which figuratively divides the system into approximately uniform spatial subsystems. We review the procedure by which this approach is systematically corrected for slowly varying density profiles, and suggest a model that carries the correction into the domain of local fluctuations. The latter is assessed for substrate bounded fluids, as well as for two‐phase interfaces. The peculiarities of the grand ensemble in a two‐phase region stem from the inherent very large number fluctuations. A primitive model shows how these are quenched in the canonical ensemble. This is taken advantage of by applying the Kac‐Siegert representation of the van der Waals decomposition, with petit canonical corrections, to the two‐phase regime.