The gas–liquid interface of water is environmentally relevant due to the abundance of aqueous aerosol particles in the atmosphere. Aqueous aerosols often contain a significant fraction of organics. As aerosol particles are small, surface effects are substantial but not yet well understood. One starting point for studying the surface of aerosols is to investigate the surface of aqueous solutions. We review here studies of the surface composition of aqueous solutions using liquid-jet photoelectron spectroscopy in combination with theoretical simulations. Our focus is on model systems containing two functional groups, the carboxylic group and the amine group, which are both common in atmospheric organics. For alkanoic carboxylic acids and alkyl amines, we find that the surface propensity of such amphiphiles can be considered to be a balance between the hydrophilic interactions of the functional group and the hydrophobic interactions of the alkyl chain. For the same chain length, the neutral alkyl amine has a lower surface propensity than the neutral alkanoic carboxylic acid, whereas the surface propensity of the corresponding alkyl ammonium ion is higher than that of the alkanoic carboxylate ion. This different propensity leads to a pH-dependent surface composition which differs from the bulk, with the neutral forms having a much higher surface propensity than the charged ones. In aerosols, alkanoic carboxylic acids and alkyl amines are often found together. For such mixed systems, we find that the oppositely charged molecular ions form ion pairs at the surface. This cooperative behavior leads to a more organic-rich and hydrophobic surface than would be expected in a wide, environmentally relevant pH range. Amino acids contain a carboxylic and an amine group, and amino acids of biological origin are found in aerosols. Depending on the side group, we observe surface propensity ranging from surface-depleted to enriched by a factor of 10. Cysteine contains one more titratable group, which makes it exhibit more complex behavior, with some protonation states found only at the surface and not in the bulk. Moreover, the presence of molecular ions at the surface is seen to affect the distribution of inorganic ions. As the charge of the molecular ions changes with protonation, the effects on the inorganic ions also exhibit a pH dependence. Our results show that for these systems the surface composition differs from the bulk and changes with pH and that the results obtained for single-component solutions may be modified by ion–ion interactions in the case of mixed solutions.

中文翻译：

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两个官能团的表面故事：水性羧酸、烷基胺和氨基酸的表面倾向

由于大气中含有丰富的水性气溶胶颗粒，水的气液界面与环境相关。水性气溶胶通常含有很大一部分有机物。由于气溶胶颗粒很小，表面效应很大但尚未得到很好的理解。研究气溶胶表面的一个起点是研究水溶液的表面。我们在这里回顾了使用液体喷射光电子能谱结合理论模拟对水溶液表面组成的研究。我们的重点是包含两个官能团的模型系统，羧基和胺基，这两个官能团在大气有机物中都很常见。对于链烷羧酸和烷基胺，我们发现这种两亲物的表面倾向可以被认为是官能团的亲水相互作用和烷基链的疏水相互作用之间的平衡。对于相同的链长，中性烷基胺具有比中性链烷羧酸更低的表面倾向，而相应的烷基铵离子的表面倾向高于链烷羧酸根离子。这种不同的倾向导致与本体不同的 pH 依赖性表面组成，其中中性形式具有比带电形式高得多的表面倾向。在气溶胶中，链烷羧酸和烷基胺经常一起出现。对于这样的混合系统，我们发现带相反电荷的分子离子在表面形成离子对。这种合作行为导致比在广泛的环境相关 pH 范围内预期的更富含有机物和疏水性的表面。氨基酸含有一个羧基和一个胺基，生物来源的氨基酸存在于气溶胶中。根据侧基的不同，我们观察到表面倾向从表面耗尽到富集 10 倍。半胱氨酸包含一个更多的可滴定基团，这使其表现出更复杂的行为，一些质子化状态仅在表面而不是在大部分。此外，表面分子离子的存在被认为会影响无机离子的分布。由于分子离子的电荷随质子化而变化，因此对无机离子的影响也表现出 pH 依赖性。