Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, USA.
J Phys Chem A. 2010 Mar 4;114(8):2837-44. doi: 10.1021/jp911133j.
Probe molecule spectroscopy characterizes the surface environment of mixed NaCl/MgSO(4) (0.01-50 wt % MgSO(4)) aerosol particles as a model for marine aerosol. Two complementary measurements, the probe's excited state spectroscopy and photoionization efficiency, measure the electronic properties of the particle surface and monitor phase changes that are driven by changes in relative humidity (RH). The results illustrate that over a wide range of composition, these particles have a layered structure with NaCl in the core and primarily hydrated MgSO(4) at the surface. Modeling the spectroscopic data reveals that the surface layer is not a uniform shell and that the coumarin 314 probe molecules partition selectively to the MgSO(4) domains. The surface layer has a pi* value of 1.7, indicative of a very high interfacial polarity. In cases where MgSO(4) is a minor component (< or = 10 wt %), the NaCl component crystallizes at 44% RH, consistent with the single salt NaCl result. Deliquescence-mode experiments with these particles show that the MgSO(4) component forms a solution at 42% RH, prior to the full deliquescence of the particle. For mixed particles with 50 wt % MgSO(4), the crystallization of NaCl occurs at 35% RH, and the predeliquescence of MgSO(4) occurs at 38% RH owing to the contribution of MgCl(2) in the surface layer. A model surfactant, SDS, slightly lowers the RH of the NaCl formation to approximately 42% and leads to the formation of a thin soap film that persists to low values of RH.
探针分子光谱学表征了混合 NaCl/MgSO4（0.01-50wt%MgSO4）气溶胶颗粒的表面环境，作为海洋气溶胶的模型。两种互补的测量方法，探针的激发态光谱和光致电离效率，测量了颗粒表面的电子性质，并监测了由相对湿度（RH）变化驱动的相变化。结果表明，在广泛的组成范围内，这些颗粒具有分层结构，NaCl 位于核心，主要是水合的 MgSO4 位于表面。对光谱数据的建模表明，表面层不是一个均匀的壳，香豆素 314 探针分子选择性地分配到 MgSO4 区域。表面层的π*值为 1.7，表明界面极性非常高。在 MgSO4 是次要成分（≤10wt%）的情况下，NaCl 成分在 44%RH 下结晶，与单一盐 NaCl 的结果一致。这些颗粒的溶解模式实验表明，MgSO4 成分在 42%RH 下形成溶液，然后颗粒完全溶解。对于含有 50wt%MgSO4 的混合颗粒，NaCl 的结晶发生在 35%RH，而 MgSO4 的预溶解发生在 38%RH，这是由于表面层中 MgCl2 的贡献。一种模型表面活性剂 SDS 将 NaCl 形成的 RH 略微降低至约 42%，并导致形成薄的肥皂膜，该膜在低 RH 值下持续存在。
