In this International Collaboration in Chemistry (ICC) project supported by a grant from the Chemical Structure, Dynamics and Mechanisms Program and the Office of International Science and Engineering, Professor Ranko Richert and his research group at Arizona State University will explore the role of the hydrogen bonds on the dynamics and structure of hydrogen-bonded liquids using dielectric relaxation and luminescent probe techniques. The collaborating group of Professor Christiane Alba-Simionesco of the Laboratoire Léon Brillouin in Saclay and Laboratoire de Chimie-Physique in Orsay, France will utilize x-ray and neutron scattering techniques to determine the structure of the same hydrogen-bonded liquids. The Saclay/Orsay group is supported by the Agence Nationale de la Recherce (ANR). Compared to other liquids, hydrogen-bonded systems engender molecular interactions and structures over extended distances, which complicate the interpretation of dielectric, calorimetric, and scattering data. The goal of this research is to provide a unified picture of hydrogen-bonded liquids which can account for dynamical behavior over a wide range of time scales. In order to study the role of hydrogen bonds in creating supramolecular structures in such liquids, the research program also includes the investigation of structure and dynamics of hydrogen-bonded liquids under nanoscopic confinement. The experimental work of both the US and French groups will be complemented by molecular dynamics simulations.
Hydrogen-bonded liquids such as water and alcohols are literally vital components of all biological systems, and essential in pharmaceuticals, food products, cryo-protection agents, microwave chemistry, and many other technologically important materials and processes. The research activity will provide a rich environment for the education and training of its student participants, as it encompasses experimental physical chemistry, computational chemistry, and materials science. The US students will also benefit from the international experience as they spend time in the laboratories of the French partner group. The research will also spawn outreach activities at the Arizona State Campus directed at K-12 audiences, including within the "Science is Fun" project established by the ASU Center for Solid State Science.
Hydrogen bonds form a particular case of attractive interactions among molecules, notably in cases where the hydrogen atom is bound to oxygen. This force is responsible for holding together the molecules in numerous liquids (opposed to these molecules forming a gas). The most prominent example for such a liquid is water, and practically all properties of water relevant to us are in some way connected to the hydrogen bond that facilitates the attractive force among two or more water (H2O) molecules. Hydrogen bonds are similarly important in many other systems, such as biological materials, food stuff, pharmaceuticals, cosmetic ingredients, products containing alcohols, etc. One typical experimental approach to the structure and dynamics that define the properties of these materials is by dielectric spectroscopy, a technique that rests upon the interaction of polar molecules with electric fields (as water in a microwave oven). The results of such experiments on alcohols have created a puzzle to scientists for many decades, as the most prominent signal detected by that technique had no satisfactory explanation regarding its microscopic origin. It was known that alcohols form compact ring-like or open chain-like structures of alcohol molecules interconnected by hydrogen bonds, but the effect of these features on the liquid properties was not clear. In this project, a measurement technique has been devised that provides electric fields high enough to promote a change toward less rings and more chains in liquid alcohols. In the figure, the measured red curve reflects the enhanced population of chains at low frequencies, where the structures are given enough time to form more chains. At high frequencies, the experiment does not provide sufficient time for rings to convert to chains, and this dependence on frequency of the alternating field tells us the time needed for the hydrogen bonded structures to change. Note that the insets in the figure are simplified schematic representations of the hydrogen bonded structures in an alcohol. Experiments of this kind allowed us to determine the time it takes for a ring to convert to a chain, and this conversion provides a satisfactory explanation for the long-standing puzzle concerning the relation between structure and dynamics in alcohols. The support associated with this project has helped to design novel experimental techniques to better study the relation between structure and dynamics in this important class of materials. The work has improved our knowledge of how these materials interact with electric fields, such as those created by cell phones or microwave ovens. It is anticipated that the results obtained for alcohols will enhance our understanding of many other materials in which hydrogen bonding plays a significant role. The project provided an undergraduate student, several graduate students, and a postdoctoral associate with training in a field that involves both the physics and chemistry of an important class of materials. These young scientists have generated scientific results that led to 18 publications in scientific journals of international circulation, two of which are review style publications aimed at making the subject accessible to the non-specialist.
