Gordon Brittany P, Moore Frederick G, Scatena Lawrence F, Valley Nicholas A, Wren Sumi N, Richmond Geraldine L
Department of Chemistry , University of Oregon , 1253 University of Oregon , Eugene , Oregon 97403 , United States.
Department of Physics , Whitman College , Walla Walla , Washington 99362 , United States.
J Phys Chem A. 2018 Apr 19;122(15):3837-3849. doi: 10.1021/acs.jpca.8b01193. Epub 2018 Apr 9.
Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.
已知大气中的小分子醛和酮在二次有机气溶胶（SOA）的形成中起着重要作用。然而，它们中的许多物质很难通过实验进行分离，因为它们容易形成水合和低聚物。羟基丙酮（HA）在这类物质中较为特殊，它在主要以未水合单体形式存在的情况下对SOA有贡献。这使得HA成为类似二次有机羰基化合物的一个有价值的模型体系。在本文中，结合计算分子动力学模拟和密度泛函理论计算，使用振动和频（VSF）光谱以及威尔海姆板表面张力测定法研究了HA在气 - 水界面的表面行为。实验结果表明，HA具有高度的表面活性，并且在界面处呈有序排列。此外，即使在高HA浓度下，在界面处也观察到了取向水。光谱特征还揭示了界面处存在顺式和反式HA构象体，且取向不同。分子动力学结果表明，在次表面（约5 Å深）和表面之间，HA的取向会因构象而异。这些结果共同描绘了一个由多个HA构象体和溶剂化水组成的高度动态但统计有序的界面。这些结果对HA在水性颗粒中的行为具有启示意义，这可能会影响其在大气和SOA形成中的作用。
