With respect to porous solids, the surface associated with pores communicating with the outside space may be called the internal surface. Because the accessibility of ores may depend on the size of the fluid molecules, the extent of the internal surface may depend on the size of the molecules comprising the fluid, and may be different for the various components of a fluid mixture. This effect is known as the molecular sieve effect.
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 585 (https://doi.org/10.1351/pac197231040577)