BET Surface Area & VSSA Nanomaterial Classifier

Particle Bed

Particle diameter D 40 nm

Material

Surface roughness / porosity f 1.0×

N₂ Adsorption (BET)

Relative pressure p/p₀ 0.05

Regulatory Classification

—VSSA vs. 60 m²/cm³ threshold

Geometric SSA

—

Measured BET SSA

—

VSSA

—

Monolayer coverage θ

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How it works

The EU's nanomaterial definition (2011/696/EU, used in REACH nanoform dossiers and referenced by ISO/TC229) has two alternative criteria: 50%+ of particles by number between 1–100 nm, or a volume-specific surface area (VSSA) above 60 m²/cm³. VSSA is measured directly by gas adsorption, not imaging.

BET equation (multilayer adsorption):
  p / [V(p0-p)] = 1/(Vm·C) + (C-1)/(Vm·C) · (p/p0)
  θ(p/p0) = V/Vm = C·x / [(1-x)(1-x+Cx)],  x = p/p0

Geometric surface area (ideal sphere, D in nm):
  SSA_geo [m²/g] = 6000 / (ρ[g/cm³] · D[nm])
  VSSA [m²/cm³]  = f · SSA_geo · ρ = f · 6000 / D[nm]

Particle diameter — smaller particles pack far more surface per gram; VSSA of an ideal smooth sphere scales as 1/D and is independent of density.
Material / density — sets the mass-specific SSA (m²/g), the figure quoted on a REACH technical dossier.
Roughness / porosity f — real powders are never perfect spheres; necking, internal pores and surface fractality inflate the BET-measured area above the geometric value, exactly like a real instrument reading.
p/p₀ slider & Sweep — scrubs the N₂ relative pressure through the BET linear range (0.05–0.35); the dot cloud on each particle visualizes adsorbed-gas coverage θ climbing toward a full monolayer.

Note the consistency check baked into the geometry: for a perfectly smooth sphere (f = 1), VSSA = 60 m²/cm³ falls almost exactly at D = 100 nm — the same number the EU's size-based and surface-area-based criteria converge on independently.

Nanoparticle Adsorbed N₂ (monolayer)

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