Department of Mechanical Engineering , University of Hawaii at Manoa , Honolulu , Hawaii 96822 , United States.
Department of Pediatrics, John A. Burns School of Medicine , University of Hawaii , Honolulu , Hawaii 96826 , United States.
Langmuir. 2018 Jul 24;34(29):8694-8700. doi: 10.1021/acs.langmuir.8b01544. Epub 2018 Jul 12.
The polymorphism of phospholipid monolayers has been extensively studied because of its importance in surface thermodynamics, soft matter physics, and biomembranes. To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance. However, because of experimental artifacts caused by film leakage, the Langmuir balance fails to study the reversibility of two-dimensional surface phase transitions. We have developed a novel experimental methodology called the constrained drop surfactometry capable of providing a leakage-proof environment, thus allowing reversibility studies of two-dimensional surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC) as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structures of the DPPC monolayer also show reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.
磷脂单层的多态性已被广泛研究，因为它在表面热力学、软物质物理和生物膜中具有重要意义。迄今为止，磷脂单层的相行为几乎仅通过经典的 Langmuir 型膜天平进行研究。然而，由于膜泄漏引起的实验假象，Langmuir 天平无法研究二维表面相转变的可逆性。我们开发了一种称为约束滴表面张力法的新实验方法，该方法能够提供防漏环境，从而允许对二维表面相转变的可逆性进行研究。使用二棕榈酰磷脂酰胆碱 (DPPC) 作为模型系统，我们研究了单层中恒温恒压相变的可逆性。结果发现，不仅等温压缩和膨胀等温线，而且等压加热和冷却等压线，完全相互叠加，没有滞后。DPPC 单层的微观横向结构也显示出不仅在等温压缩和膨胀过程中而且在等压加热和冷却过程中的可逆性。可以得出结论，磷脂单层中的二维表面相转变是可逆的，这与纯物质体相中相变的可逆性一致。我们的结果提供了对表面热力学、相变材料以及膜和肺表面活性剂的生物物理研究的更好理解。
