The origin of the phase transition between interdigitated to bilayer of ionic surfactant
イオン性界面活性剤膜の指組膜－二重膜相転移の起源解明
両親媒性分子の自己集合構造は分子間の様々な相互作用のバランスによって決まるが、電気的な相互作用やvan der Waals相互作用、疎水性相互作用、立体的な反発力などが複雑に関係するために、構造形成メカニズムとこれらの相互作用の関係性を知ることは存外困難である。我々は、とくに界面活性剤が作る二分子膜を題材に、分子間相互作用と膜構造の関係性を研究してきた。ここでは、親水基に電荷をもつ界面活性剤を用い、直鎖状のアルカンを添加したときの膜構造の変化を観察した。親水基同士の電気的な反発と疎水鎖同士のvan der Waals力や疎水性相互作用のバランスが議論できると期待される。カチオン性界面活性剤としてDODAC（Dimethyldioctadecylammonium Chloride）を、アルカンとしてn-テトラデカンを用い、とくに膜内で疎水鎖が秩序立った相における疎水鎖の秩序配列構造と膜の厚みがアルカン濃度に応じてどう変化するのかを調べた。
DODAC膜の間には強い静電反発が働くため、膜間はできるだけ広がろうとし、その間隔は系内における膜の体積分率で決まる。この関係を利用し、X線小角回折から求めた膜間距離から膜厚を算出した。すると、テトラデカン無添加時には30 Å程度であったものが、25 mol%程度以上では55 Å程度になることが分かった。これはテトラデカン添加によって指組膜構造から二重膜構造へと転移したことを意味している。アルカン濃度が低い時には疎水鎖を水にさらすエネルギーロスよりも親水基同士の距離を離すエネルギーゲインのほうが大きく指組膜となるが、濃度が高くなると水にさらされる疎水鎖が増えてエネルギーロスが大きくなるために二重膜へと転移すると考えられる。二重膜へと転移すると親水基間の距離が短くなるために、できるだけ分子間距離を離そうとして充填度が疎になると考えられる。実際に、膜内での疎水鎖の充填度をX線広角回折で調べると、高濃度側でより疎であることが確認された。ほかの疎水性分子（末端官能基を変えたテトラデカン誘導体）を添加した場合にも同様の相転移が観測された。
上記のエネルギーバランスを理論的に記述し、添加物による自由エネルギー変化を観測したところ、理論的にも指組膜から二重膜への相転移を再現できた。さらに膜内での分子パッキングも再現されたため、上記のモデルのようなメカニズムでこの相転移が起こることが裏付けられた。
Mafumi Hishida, Naofumi Shimokawa, Yuki Okubo, Shun Taguchi, Yasuhisa Yamamura, and Kazuya Saito, Langmuir, 36, 14699-14709 (2020).
The self-assembled structure of an amphiphilic molecule is determined by the balance of various intermolecular interactions, but because of the complex relationships among electrical interactions, van der Waals interactions, hydrophobic interactions, and steric repulsive forces. The relationship between the structure formation mechanism and these interactions is surprisingly difficult to understand. We have been studying the relationship between intermolecular interactions and membrane structure, especially in the case of bilayers made by surfactants. Here, we used surfactants with charged hydrophilic groups and observed the changes in the membrane structure when linear alkanes were added. It is expected that the balance between the electrical repulsion between hydrophilic groups and the van der Waals force or hydrophobic interaction between hydrophobic chains can be discussed. We used DODAC (Dimethyldioctadecylammonium Chloride) as a cationic surfactant and n-tetradecane as an alkane to investigate how the ordered structure of the hydrophobic chains and the thickness of the membrane change with alkane concentration, especially in the phase where the hydrophobic chains are ordered in the membrane. In particular, we investigated how the ordered structure of the hydrophobic chains and the thickness of the membrane changes with alkane concentration in the phase where the hydrophobic chains are ordered in the membrane.
Because of the strong electrostatic repulsion between DODAC membranes, the distance between the membranes becomes as wide as possible, which is determined by the volume fraction of the membrane in the system. Using this relationship, the thickness of the membrane was calculated from the distance between the membranes obtained from small-angle X-ray diffraction. It was found that the thickness of the membranes increased from about 30 Å when no tetradecane was added to about 55 Å when more than 25 mol% was added. This indicates that the addition of tetradecane caused a transition from a interdigitated membrane structure to a bilayer structure. When the alkane concentration is low, the energy gain that separates the hydrophilic groups from each other is greater than the energy loss that exposes the hydrophobic chains to water, resulting in a interdigitated membrane structure. However, as the concentration increases, the number of hydrophobic chains exposed to water increases and the energy loss increases, resulting in the transition to bilayer. As the distance between hydrophilic groups becomes shorter during the transition to bilayer, the packing in-plane becomes sparse in order to keep the intermolecular distance as large as possible. In fact, when the packing of the hydrophobic chains in the membrane was examined by X-ray wide-angle diffraction, it was confirmed that the packing was more sparse at higher concentrations. The same phase transition was observed when other hydrophobic molecules (tetradecane derivatives with different terminal functional groups) were added.
By theoretically describing the above energy balance and calculating the free energy change due to the additives, we were able to reproduce the phase transition from interdigitated membrane to bilayer membrane theoretically. Furthermore, the molecular packing in the membrane was also reproduced, supporting the mechanism of this phase transition as described in the model above.
Mafumi Hishida, Naofumi Shimokawa, Yuki Okubo, Shun Taguchi, Yasuhisa Yamamura, and Kazuya Saito, Langmuir, 36, 14699-14709 (2020).