The soft cluster composed of polyethyleneglycol mono-dodecyl ether (AEO_n) and bovine serum albumin (BSA) was explored by surface tension and steady-state fluorescence quenching technique.There occur two obvious break points in the γ-lg_c curves of the above cluster.[Γ_∞], the specific saturation capacity of clusterization in respect of AEO_n on BSA, keeps constant when the concentration of BSA increases.[N_P]₁, the critical aggregation number of bound micelle, is 54 for AEO₆ and 52 for AEO₇, which means that BSA associates with bound micelles of AEO_n rather than monomeric AEO_n between the two critical concentrations c₁～c₂.The thermodynamic model of clusterization between AEO_n and BSA was deduced from the mass action law.The standard Gibbs free energy of clusterization is negative, which indicates that clusterization is spontaneous and needs no outside driving force as well.ΔH°>0, ΔS°>0 and ΔG°<0, make known that clusterization is entropy driving process, and the supramolecular force is primarily hydrophobic interaction.(ΔG°)_clu and (ΔG°)_mic are both less than zero and c<1cmc, so the soft cluster of AEO_n and BSA forms first by supramolecular interaction when concentration of AEO_n approaches c₁.For clusterization having finished at c_2', micellization begins at c₂ accompanied by monomeric AEO_n reaching cmc.The above thermodynamic model of clusterization can provide reasonable explanation to the experiments that supramolecular clusterization appears at first whereas self-assembly micellization follows.

Key words: nonionic surfactant, polyethyleneglycol mono-dodecyl ether(AEO_n), bovine serum albumin(BSA), soft cluster, thermodynamic model