The position of the
Gibbs surface is often defined experimentally as that surface which encloses the volume of space
from which the solid excludes helium gas (the so-called helium dead-space), and is
associated with the assumptions that the volume of the solid is unaffected by the
adsorption of component
i, and that helium is not adsorbed by the solid. This requires that the measurement
of the helium dead-space be made at a sufficiently high temperature.
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 595