Theory and Simulation of Liquids, Solutions, and Nucleation Processes

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Crystal nucleation from melts or solution is an important process that occurs practically everywhere in our surrounding environment. The nucleation of ice crystals in liquid water, and of common salts such as NaCl (rock salt) from solution are important examples considered in this proposal. The normal freezing point of water is 0 oC, but pure water can be supercooled to near -35 oC before freezing occurs. This means that essentially all ice nucleation on earth or in the atmosphere is facilitated by contact with a foreign body. This process is called heterogeneous nucleation, and the foreign body is termed an ice nucleus. Heterogeneous ice nucleation is an important process in the atmosphere because ice nucleation by atmospheric aerosols results in ice clouds, which in turn influence the global climate. Although heterogeneous ice nucleation has long been a subject of investigation, little is known about the microscopic nature of a good ice nucleus. This absence of knowledge is mainly because the small sizes and fast times relevant to nucleation are not easily accessible by current experiments. The proposed research will employ computer simulations to examine these questions. Computer simulations can be viewed as "experiments'' on representative models of real systems, which allow us to follow physical processes at the molecular level. This is a very powerful approach which yields a microscopic understanding of physical phenomena that is not otherwise available. We will use molecular dynamics simulations to examine particularly important atmospheric aerosols, such clays and other mineral particles, as well as to obtain a deeper understanding of the general factors that influence heterogeneous ice nucleation. We will also employ molecular simulation to investigate the crystallization of salts and other molecular solids from solution. The purpose is again to probe the underlying mechanisms of crystal birth, and identify the crystal and solution properties which enhance or inhibit crystal nucleation and growth. The state and function of many physical and biological systems depends on a balance between moieties that interact favourably with water (hydrophilic) and those that do not (hydrophobic). Molecules that have both hydrophilic and hydrophobic parts are called amphiphilic. We will explore how competing hydrophilic and hydrophobic interactions influences the nanoscale structure of several model solutions containing amphiphilic solutes. This work promises new insight into why some small molecules function more effectively than others as protein denaturants. Denaturants are molecules that cause proteins to change their structure to a less compact (unfolded) state in aqueous solution.