Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48105, United States.
J Phys Chem B. 2021 Mar 11;125(9):2389-2397. doi: 10.1021/acs.jpcb.0c07546. Epub 2021 Mar 1.
Cationic surfactant mixed with fatty alcohol as cosurfactant in excess water can form stable emulsions, known as "lamellar gel networks," that contain extended and interconnected networks of swollen bilayers, including ones with in-plane liquidlike disorder (L phase) and solidlike order (L phase). To study their structure and thermodynamics, molecular dynamics (MD) simulations with lateral pressure and temperature scans along reversible pathways were used to drive reversible phase changes, including formation at negative lateral pressure of the L phase with interdigitated tails of opposing leaflets. Thermodynamic integration, with extrapolations to infinitely slow scans, yielded a free energy difference between the interdigitated L and non-interdigitated L phases of 2.4 ± 0.5 kJ/mol, which is consistent with the spontaneous formation of the L phase under atmospheric pressure in simulation. Thermodynamic cycles involving temperature and lateral pressure for which the free energy difference is identically zero were constructed as negative controls to verify the method. Using lateral pressure, including negative lateral pressure, helps avoid kinetic bottlenecks that occur when temperature alone is used as the control variable. The method, using negative lateral pressure, should be widely applicable to other bilayers to identify molecular properties that control interdigitation and other bilayer properties.
阳离子表面活性剂与过量水混合的脂肪醇作为助表面活性剂可以形成稳定的乳液，称为“层状凝胶网络”，其中包含扩展和相互连接的溶胀双层网络，包括具有平面液态无序（L 相）和固态有序（L 相）的网络。为了研究它们的结构和热力学性质，使用分子动力学（MD）模拟，沿可逆转路径施加侧向压力和温度扫描，以驱动可逆相变，包括在负侧向压力下形成具有相反叶瓣交错尾部的 L 相。通过热力学积分，并进行无限缓慢扫描的外推，得出交错 L 相与非交错 L 相之间的自由能差为 2.4±0.5kJ/mol，这与模拟中在大气压下 L 相自发形成的情况一致。构建了涉及温度和侧向压力的热力学循环，其中自由能差为零，作为负对照来验证该方法。使用包括负侧向压力在内的侧向压力有助于避免仅使用温度作为控制变量时发生的动力学瓶颈。该方法，使用负侧向压力，应广泛适用于其他双层膜，以确定控制交错和其他双层膜性质的分子性质。
