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