The Environmental Chemical Sciences (ECS) program of the Division of Chemistry will support the research program of Prof. Marcelo Carignano of Northwestern University. Prof. Carignano and his students develop a polarizable water model to be used in full atomistic simulations to study water/vapor, ice/vapor and ice/water interfaces and their interaction with ions, trace atmospheric components and pollutant molecules in a temperature range relevant to atmospheric chemistry, which includes the water supercooled regime and ice. The project will increase our molecular level understanding of the role of water/vapor and ice/vapor interfaces in chemical reactions leading to cloud electrification and of the functionality and reactivity of the ice/water interface, which is a natural habitat for a diverse range of microbial organisms. The project will provide excellent educational opportunities for students, including some from under represented groups, desiring to work at the forefront of environmental science.
We have studied the properties of water in different forms. In particular, we pais special attention to the properties of ice and and ice/water interfaces. Also, we dedicated considerable time to understand anomalous behavior of liquid water below its equilibrium freezing temperature. The project as a whole resulted in the publication of twelve journal articles, and supported a postdoctoral scholar for three years. Water can be cooled below the temperature at which it freezes, zero degrees centigrades. This is done in the laboratory down to -10 C with relative easy. Lower temperature require a more controlled environment, but still water can be supercooled to approximately -40 C. The modeling of this phenomena is difficult, and challenges the quiality of water models and simulation techniques. During the course of our research we evaluated in a comparative manner the properties of different water models and how they can describe this so called 'supercooled regime'. In order for the water to form ice in the supercooled regime, it is necessary to form a nucleus of a minimum size. The formation of this nucles must happens by chance as the water molecules in liquid state move an form temporary ice like networks. Most of these networks are small, and they dissasemble (they liquify). There is a critical size for the nucleus that will result in the growth of ice affecting all the available water molecules. We have determined the size of this critical nucleus size for ice formation, anf found that for ten degrees below frezing it is necessary to have a nuceous of approximately 1000 molecules in order to freeze the whole system. The random organization of 1000 moeclues into an ice like pattern has a very low probability, and therefore this explains why is relatively easy to maintain qater in liquid state even though the temperature is below zero degrees centigrades. Most of our work related to this particular project centers around these two issues, and how different water models reproduce the experimental facts using computer simulation techniques.
