  Hypothesis: As a fluid approaches three phase coexistence, adsorption may
take place by the successive formation of two intervening wetting films. The
equilibrium thickness of these wetting layers is the result of a delicate
balance of intermolecular forces, as dictated by an underlying interface
potential. The van der Waals forces for the two variable adsorption layers may
be formulated exactly from Dzyaloshinskii-Lifshitz-Pitaevskii theory, and
analytical approximations may be derived that extent well beyond the validity
of conventional Hamaker theory.
  Calculations: We consider the adsorption equilibrium of water vapor on Silver
Iodide where both ice and a water layers can form simultaneously and compete
for the vapor as the triple point is approached. We perform numerical
calculations of Lifshitz theory for this complex system and work out analytical
approximations which provide quantitative agreement with the numerical results.
  Findings: At the three phase contact line between AgI/water/air, surface
forces promote growth of ice both on the AgI/air and the water/vapor
interfaces, lending support to a contact nucleation mode of AgI in the
atmosphere. Our approach provides a framework for the description of adsorption
at three phase coexistence, and allows for the study of ice nucleation
efficiency on atmospheric aerosols.
