Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USA.
J Am Chem Soc. 2013 Jun 12;135(23):8496-9. doi: 10.1021/ja403437c. Epub 2013 May 29.
Understanding the freezing of salt solutions near solid surfaces is important in many scientific fields. Here we use sum frequency generation (SFG) spectroscopy to study the freezing of a NaCl solution next to a sapphire substrate. During cooling we observe two transitions. The first corresponds to segregation of concentrated brine next to the sapphire surface as we cool the system down to the region where ice and brine phases coexist. At this transition, the intensity of the ice-like peak decreases, suggesting the disruption of hydrogen-bonding by sodium ions. The second transition corresponds to the formation of NaCl hydrates with abrupt changes in both the SFG intensity and the sharpness of spectral peaks. The similarity in the position of the SFG peaks with those observed using IR and Raman spectroscopy indicates the formation of NaCl·2H2O crystals next to the sapphire substrate. The melting temperatures of the hydrates are very similar to those reported for bulk NaCl·2H2O. This study enhances our understanding of nucleation and freezing of salt solutions on solid surfaces and the effects of salt ions on the structure of interfacial ice.
理解靠近固体表面的盐溶液冻结过程对于许多科学领域都很重要。在这里，我们使用和频发生（SFG）光谱学来研究氯化钠溶液在蓝宝石衬底附近的冻结过程。在冷却过程中，我们观察到两个转变。第一个转变对应于浓缩盐水在蓝宝石表面附近的分凝，当我们将系统冷却到冰和盐水相共存的区域时。在这个转变过程中，类似冰的峰的强度降低，表明钠离子破坏了氢键。第二个转变对应于 NaCl 水合物的形成，伴随着 SFG 强度和光谱峰尖锐度的突然变化。SFG 峰的位置与使用红外和拉曼光谱观察到的位置相似，这表明在蓝宝石衬底附近形成了 NaCl·2H2O 晶体。水合物的熔点与大块 NaCl·2H2O 的熔点非常相似。这项研究增强了我们对盐溶液在固体表面上的成核和冻结过程以及盐离子对界面冰结构的影响的理解。
