The behavior of surfactant monolayers, such as lung surfactant films, at air-water interfaces, is significantly challenging to study both experimentally and computationally. Lung surfactant (LS), a mixture of phospholipids, cholesterol, and surfactant proteins, is essential for breathing, as it regulates surface tension of the alveolar air-liquid interface during the expansion/contraction cycles. Monolayers of dipalmitoyl phosphatidyl choline (DPPC), a major LS component, exhibit temperature-dependent 2D phase transitions between liquid condensed (LC), liquid expanded (LE) and gaseous phases, upon film expansion and contraction, which are characterized by the surface pressure-area (P-A) isotherm. Understanding the 2D phase behavior of lipid monolayers is crucial to elucidate the mechanism of LS functioning, which has not been completely explained. While the experimental measurements can be influenced by conditions of the experimental setup, molecular simulations are limited mainly by the lack of proper forcefields. Here, we develop a quantitively consistent, dissipative particle dynamics (DPD) model to simulate phase behavior of DPPC monolayers at different temperatures. By systematic parametrization and utilizing the DPD gas model (Wang et al. Langmuir 2020, 36, 14686-14698) developed previously by our group, our model reproduces air-water surface tension at a wide range of temperatures. We calculate P-A isotherms at different temperatures demonstrating the LE-LC transition characterized by a phase coexistence plateau in the PA-isotherm (Figure 1). Our DPD model with its characteristic simplicity and feasibility, can be extended to a variety of phenomena that include effects of cholesterol, salt effects, and nanoparticle interactions with LS as well as the behavior of other surfactant monolayers.

<i>Figure </i><i>1</i><i>. (a) Pressure-area isotherms of DPPC monolayers at T=293K, from experiments and DPD simulations at no salt condition. (b) Top view of a monolayer in DPD simulation exhibiting LC (towards red)-LE (towards blue) coexistence. S is the order parameter indicating tail orientation along the normal.</i>

Figure 1. (a) Pressure-area isotherms of DPPC monolayers at T=293K, from experiments and DPD simulations at no salt condition. (b) Top view of a monolayer in DPD simulation exhibiting LC (towards red)-LE (towards blue) coexistence. S is the order parameter indicating tail orientation along the normal.

Alexander Neimark

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