It may happen that a component is virtually insoluble in both of two adjoining phases, but is present as a monolayer between them. Such a layer can be produced by spreading.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 591 (https://doi.org/10.1351/pac197231040577)