Dissolved helium atoms evaporate from liquids in super-Maxwellian speed distributions because their interactions are too weak to enforce full thermal equilibration at the surface as they are "squeezed" out of solution. The excess speeds of these He atoms reflect their final interactions with solvent and solute molecules at the surfaces of water and other liquids. We extend this observation by monitoring He atom evaporation from salty water solutions coated with surfactants. These surface-active molecules span neutral, anionic, and cationic amphiphiles: butanol, 3-methyl-1-butanol, pentanol, pentanoic acid, pentanoate, tetrabutylammonium, benzyltrimethylammonium, hexyltrimethylammonium, and dodecyltrimethylammonium, each characterized by surface tension measurements. The helium energy distributions, recorded in vacuum using a salty water microjet, reveal a sharp distinction between neutral and ionic surfactant films. Helium atoms evaporate through neutral surfactant monolayers in speed distributions that are similar to a pure hydrocarbon, reflecting the common alkyl chains of both. In contrast, He atoms appear to evaporate through ionic surfactant layers in distributions that are closer to pure salty water. We speculate that the ionic surfactants distribute themselves more loosely and deeply through the top layers of the aqueous solution than do neutral surfactants, with gaps between the surfactants that may be filled with salty water. This difference is supported by prior molecular dynamics simulations and ion scattering measurements of surfactant solutions.

中文翻译：

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通过氦原子蒸发探测水性表面活性剂的界面结构

溶解的氦原子以超麦克斯韦速度分布从液体中蒸发，因为它们的相互作用太弱，无法在表面被“挤出”溶液时实现完全热平衡。这些 He 原子的超速反映了它们与水和其他液体表面的溶剂和溶质分子的最终相互作用。我们通过监测涂有表面活性剂的咸水溶液中的氦原子蒸发来扩展这一观察。这些表面活性分子涵盖中性、阴离子和阳离子两亲物：丁醇、3-甲基-1-丁醇、戊醇、戊酸、戊酸酯、四丁基铵、苄基三甲基铵、己基三甲基铵和十二烷基三甲基铵，每种分子都通过表面张力测量来表征。使用盐水微射流在真空中记录的氦能量分布揭示了中性和离子表面活性剂薄膜之间的明显区别。氦原子通过中性表面活性剂单层蒸发，其速度分布与纯烃类似，反映了两者共同的烷基链。相比之下，He原子似乎通过离子表面活性剂层蒸发，其分布更接近纯盐水。我们推测，与中性表面活性剂相比，离子表面活性剂在水溶液的顶层分布得更松散、更深入，表面活性剂之间的间隙可能充满盐水。这种差异得到了先前的分子动力学模拟和表面活性剂溶液的离子散射测量的支持。