Role of surface area-to-volume ratio in protein adsorption at the air–water interface

Adsorption isotherms of eight globular proteins at the air–water interface were analyzed using a modified adsorption model. The experimental adsorption data of eight globular proteins at the air–water interface obeyed the model. The Γsat predicted from the model agreed quite well with the experimentally determined values. The relative affinities of globular proteins decreased with increasing molecular weight of proteins and followed an M−2/3-relationship. Further analysis showed that this phenomena was related to the solvent accessible molecular surface area-to-volume ratio (As/V) of globular proteins. The results indicated that adsorption of globular proteins follows a Traube-rule-like behavior with surface area-to-volume ratio as the scaling parameter. Theoretical analysis in combination with experimental data indicated that for every doubling of molecular weight of globular proteins, the free energy change for adsorption becomes unfavourable by about 272 cal/mol.