2019/11/27(Wed) 10:30 -11:30 P101會議室 1F, P101 Meeting Room
Title
Molecular dynamics simulation of NaCl and NaI aqueous solutions/air interface from supercooled to ambient temperatures
Speaker
Dr. Hock-Seng Nguan (Institute of Atomic and Molecular Sciences, Academia Sinica)Abstract
The preference of halides ions to the water-air interface was first predicted through molecular dynamics (MD) simulation [1] and later confirmed by series of experiments [2-4]. Interestingly, the degree of the preferences of the ions to the water surface are found to match with the Hofmeister’sseries[5], which classify the order of ability for ions to salt out or salt in a protein.
It is still debated that whether the driving force behind the ions surface propensity on the water-air interface is similar to that of the water-proteins interface. In this work, we study the role of temperatures on the ions surface propensity, which is rarely been studied. Here, we perform MD simulations of aqueous NaCl and NaI solutions in slab geometry and interface with vacuum at a series of temperatures, ranging from supercooled water temperature of 240 K up to ambient temperature around 342 K. Our results show that the ions propensities to the water-air interface are greatly enhanced in the supercooled water, such that at temperature of 240 K the ions densities near the surface are about 2 times of its bulk densities. We observe that the ions accumulation near the water surface is more significant at temperature below the water temperature of maximum density (TMD) than at temperature higher than the TMD. This observation suggests that the water structures different between the two temperature regions give rise to the two different temperature dependent behaviors. Based on the tetrahedral order parameters analysis, we found that below the TMD, the tetrahedral ordered water structures are preferred over the presence of the ions, which could disturb the tetrahedral order, hence the ions prefer to stay at the water surface where tetrahedral order is lower. Furthermore, the free energy profiles of the ions near the surface are also analyzed and their thermodynamics properties changes across different temperatures are shown. Our results could imply the high concentration of ions at the surface of sea-spray-aerosol with at low temperatures, which could influence the surface chemistry and the cloud formations.
Language
演講語言 (Language): in English
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