In the preceding paper, we investigated a mixed assembly composed of a nonionic surfactant, n-octyl-β-D-glucopyranoside (OG), and an amphoteric lipid, 1,2-dioleolyl-sn-glycero-3-phosphocholine (DOPC), formed on hydrophilized solid substrates immersed in aqueous solutions containing OG and DOPC. The experimental data could be interpreted in terms of the phase equilibrium; thus, the partition equilibrium profile of OG between the bulk solution phase and the supported assembly phase was obtained, as well as that between the bulk solution and the dispersed assembly. The partition equilibrium profiles suggested that micellar-bilayer state transitions occur both in the supported assembly and in the dispersed one in a roughly synchronized manner, even though there are significant discrepancies between them. In this paper, we propose a simple thermodynamic model for the micellar-bilayer transition of the dispersed and supported assembly of OG and DOPC, assuming that the micellar and bilayer states are also pseudo-phases distinct from each other. Using this model, we analyzed these partition equilibrium profiles and concluded that the transition in the supported assembly should mainly be attributed to the transition in the dispersed assembly, which is partly modified by the interaction energy between the supported assembly and the substrate.
Keywords: micellar-bilayer state transition; n-octyl-β-D-glucopyranoside; 1,2-dioleolyl-sn-glycero-3-phosphocholine; phase separation model; regular solution approximation.