external surface

https://doi.org/10.1351/goldbook.E02291
When a porous solid consists of discrete particles it is inconvenient to describe the outer boundary of the particles as the external surface.
Sources:
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 [Terms] [Paper]
PAC, 1976, 46, 71. (Manual of Symbols and Terminology for Physicochemical Quantities and Units - Appendix II. Definitions, Terminology and Symbols in Colloid and Surface Chemistry. Part II: Heterogeneous Catalysis) on page 79 [Terms] [Paper]