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.

中文翻译：

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磷脂单层中的可逆相变

由于磷脂单分子层的多态性在表面热力学，软物质物理学和生物膜中的重要性，因此已进行了广泛的研究。迄今为止，磷脂单分子层的相态几乎是用经典的朗缪尔型薄膜天平进行研究的。但是，由于薄膜泄漏引起的实验伪像，Langmuir天平无法研究二维表面相变的可逆性。我们开发了一种新颖的实验方法，称为约束液滴表面测量法，能够提供防泄漏的环境，从而可以对二维表面相变进行可逆性研究。使用二棕榈酰磷脂酰胆碱（DPPC）作为模型系统，我们研究了单层中等温和等压相变的可逆性。发现不仅压缩和膨胀等温线而且加热和冷却等压线彼此完全重叠而没有滞后。DPPC单层的微观横向结构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也显示出可逆性。结论是，磷脂单层中的二维表面相变是可逆的，这与本体纯物质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。彼此完全重叠而没有滞后。DPPC单层的微观横向结构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也显示出可逆性。结论是，磷脂单层中的二维表面相变是可逆的，这与本体纯物质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。彼此完全重叠而没有滞后。DPPC单层的微观横向结构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也显示出可逆性。结论是，磷脂单层中的二维表面相变是可逆的，这与本体纯物质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。结论是，磷脂单层中的二维表面相变是可逆的，这与本体纯物质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。结论是磷脂单层中的二维表面相变是可逆的，这与本体纯物质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。