https://doi.org/10.1351/goldbook.S05806
When the area of the @[email protected] between two phases is proportional to the mass of one of the phases (e.g. for a solid @[email protected], for an @[email protected] or for an @[email protected] ), the specific surface area (\(a\),\(s\) or preferably \(a_{s}\)) is defined as the surface area divided by the mass of the relevant phase.