Many surfactants and lipids form interdigitated membranes with interlocking chains of the inner and outer leaflets of the membrane. However, the systematic mechanisms governing the formation and stabilization of interdigitated structures are not fully understood. This study investigates the effect of various small organic molecules on the bilayer-interdigitated membrane phase transition of a cationic surfactant, dimethyldioctadecylammonium chloride (DODAC), and the zwitterionic phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). The interdigitated DODAC membrane, which spontaneously formed in water, undergoes a phase transition to a noninterdigitated normal bilayer upon the addition of tetradecane (TD). The molar fraction of TD required for this transition, ϕIB, varied depending on the added small organic molecules and their concentration. A strong correlation was observed between the alkane-solvent interfacial tension (measured between liquid TD and the aqueous solutions) and ϕIB. As the interfacial tension decreased, ϕIB increased, which indicates that the stabilization of the interdigitated membrane is caused by a decrease in the interfacial tension at the alkyl chain ends exposed to the aqueous solution. A similar mechanism was confirmed for the phospholipid DPPC system. DPPC exhibited a phase transition from a bilayer to an interdigitated membrane when the interfacial tension between the alkane and aqueous solution decreased to a certain value, regardless of the molecular species of the small organic molecules. This study proposes a unified and systematic explanation for the stabilization of interdigitated membranes of both cationic surfactants and phospholipids, demonstrating that the interfacial tension at the alkyl chain ends is the dominant factor.
Sawada et al. (Tue,) studied this question.