Surface excess

The interfacial excess of a species relative to bulk reference phases extrapolated up to a chosen reference surface. An excess number density per area is denoted $\Gamma$; its molar counterpart is $\Gamma /N_A$. When an interface is dominated by a molecular layer, an effective area per adsorbed molecule is $\Sigma =1/\Gamma$. The reference-surface convention matters in multicomponent systems.