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 Apr 17;34(15):4688-4694. doi: 10.1021/acs.langmuir.8b00579. Epub 2018 Apr 2.
Langmuir monolayer self-assembled at the air-water interface represents an excellent model for studying phase transition and lipid polymorphism in two dimensions. Compared with numerous studies of phospholipid phase transitions induced by isothermal compression, there are very scarce reports on two-dimensional phase transitions induced by isobaric heating. This is mainly due to technical difficulties of continuously regulating temperature variations while maintaining a constant surface pressure in a classical Langmuir-type film balance. Here, with technological advances in constrained drop surfactometry and closed-loop axisymmetric drop shape analysis, we studied the isobaric heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer. It is found that temperature and surface pressure are two equally important intensive properties that jointly determine the phase behavior of the phospholipid monolayer. We have determined a critical point of the DPPC monolayer at a temperature of 44 °C and a surface pressure of 57 mN/m. Beyond this critical point, no phase transition can exist in the DPPC monolayer, either by isothermal compression or by isobaric heating. The melting process of the DPPC monolayer studied here provides novel insights into the understanding of a wide range of physicochemical and biophysical phenomena, such as surface thermodynamics, critical phenomena, and biophysical study of pulmonary surfactants.
在气-水界面上自组装的朗缪尔单分子层是研究二维相转变和脂质多晶型的理想模型。与众多研究等温压缩诱导的磷脂相变的文献相比，关于等压加热诱导的二维相变的报道非常少。这主要是由于在经典的朗缪尔型膜天平上，连续调节温度变化而同时保持恒定的表面压力存在技术困难。在这里，通过受限滴表面张力法和闭环轴对称液滴形状分析技术的进步，我们研究了二棕榈酰磷脂酰胆碱（DPPC）单分子层的等压加热过程。结果发现，温度和表面压力是共同决定磷脂单分子层相行为的两个同等重要的强度性质。我们在温度为 44°C 和表面压力为 57 mN/m 处确定了 DPPC 单分子层的临界点。超过这个临界点，无论是等温压缩还是等压加热，DPPC 单分子层都不可能存在相变。这里研究的 DPPC 单分子层的熔融过程为理解广泛的物理化学和生物物理现象提供了新的见解，例如表面热力学、临界现象和肺表面活性剂的生物物理研究。
