Ultrahigh Vacuum Studies of the Fundamental Interactions of Chemical Warfare Agents and Their Simulants with Amorphous Silica Amanda Rose Wilmsmeyer Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Chemistry John R. Morris, Chair Louis A. Madsen James M. Tanko Brian M. Tissue Edward F. Valeev August 3, 2012 Blacksburg, VA Keywords: surface chemistry, chemical warfare agent simulants, hydrogen bonding, ultrahigh vacuum, temperature programmed desorption, infrared spectroscopy Ultrahigh Vacuum Studies of the Fundamental Interactions of Chemical Warfare Agents and Their Simulants with Amorphous Silica Amanda Rose Wilmsmeyer Abstract Developing a fundamental understanding of the interactions of chemical warfare agents (CWAs) with surfaces is essential for the rational design of new sorbents, sensors, and decontamination strategies. The interactions of chemical warfare agent simulants, molecules which retain many of the same chemical or physical properties of the agent without the toxic effects, with amorphous silica were conducted to investigate how small changes in chemical structure affect the overall chemistry. Experiments investigating the surface chemistry of two classes of CWAs, nerve and blister agents, were performed in ultrahigh vacuum to provide a well-characterized system in the absence of background gases. Transmission infrared spectroscopy and temperature-programmed desorption techniques were used to learn about the adsorption mechanism and to measure the activation energy for desorption for each of the simulant studied. In the organophosphate series, the simulants diisopropyl methylphosphonate (DIMP), dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), dimethyl chlorophosphate (DMCP), and methyl dichlorophosphate (MDCP) were all observed to interact with the silica surface through the formation of a hydrogen bond between the phosphoryl oxygen of the simulant and an isolated hydroxyl group on the surface. In the limit of zero coverage, and after defect effects were excluded, the activation energies for desorption were measured to be 57.9 ± 1, 54.5 ± 0.3, 52.4 ± 0.6, 48.4 ± 1, and 43.0 ± 0.8 kJ/mol for DIMP. DMMP, TMP, DMCP, and MDCP respectively. The adsorption strength was linearly correlated to the magnitude of the frequency shift of the (SiO-H) mode upon simulant adsorption. The interaction strength was also linearly correlated to the calculated negative charge on the phosphoryl oxygen, which is affected by the combined inductive effects of the simulants’ different substituents. From the structure-function relationship provided by the simulant studies, the CWA, Sarin is predicted to adsorb to isolated hydroxyl groups of the silica surface via the phosphoryl oxygen with a strength of 53 kJ/mol. The interactions of two common mustard simulants, 2-chloroethyl ethyl sulfide (2-CEES) and methyl salicylate (MeS), with amorphous silica were also studied. 2-CEES was observed to adsorb to form two different types of hydrogen bonds with isolated hydroxyl groups, one via the S moiety and another via the Cl moiety. The desorption energy depends strongly on the simulant coverage, suggesting that each 2-CEES adsorbate forms two hydrogen bonds. MeS interacts with the surface via a single hydrogen bond through either its hydroxyl or carbonyl functionality. While the simulant work has allowed us to make predictions agent-surface interactions, actual experiments with the live agents need to be conducted to fully understand this chemistry. To this end, a new surface science instrument specifically designed for agent-surface experiments has been developed, constructed, and tested. The instrument, located at Edgewood Chemical Biological Center, now makes it possible to make direct comparisons between simulants and agents that will aid in choosing which simulants best model live agent chemistry for a given system. These fundamental studies will also contribute to the development of new agent detection and decontamination strategies. ii Acknowledgements My completion of graduate school would not have been possible without the support of family and friends every step of the way. First, I would like to thank my husband Kyle, for his daily encouragement, patience, and love. I always knew you were behind me in my pursuits and I am forever grateful for that. I would also like to thank my parents for always encouraging me to do my best and providing me with the confidence to do just that. My brother, Ryan, kept me laughing and always put a smile on my face. I am also thankful for the support of my friend Meghan Rodriguez. Even though we were states apart, we still went though this together, and being able to share the experience made it much more manageable. The members of the Morris Group and other friends around the department made daily life of graduate school enjoyable. Josh Uzarksi, Dimitar Panayotov, and Wes Gordon taught me more than they probably realize and my success in graduate school began with their mentorship. Erin Davis, Jessica Lu, Will Alexander, Steve Burrows, Alec Wagner, Yafen Zhang, Tommy Rockhold, Josh Abelard, and Guanyu Wang all provided friendship and support in lab and I would not be the scientist I am today without all of their help. I would also like to thank Dave Simmons and Darrell Link from the engineering machine shop for always allowing me to bounce ideas off of them and their expertise allowed me to do better science. Diego Troya has helped me tremendously in developing a more complete understanding of my research and collaborating with him has been a true pleasure. All of my committee members have been incredibly supportive throughout this journey and were always willing to offer additional support. Finally, I would especially like to thank my advisor, John Morris. I could not have asked for a better advisor. He challenged me to work harder than I thought I could, but also encouraged me when I felt defeated. He has taught me how to become an excellent researcher and teacher and I thank him for always pushing me forward. iii Table of Contents List of Figures............................................................................................................................... vii List of Tables ............................................................................................................................... xiii Index of Acronyms ........................................................................................................................xv Chapter 1. Introduction and Motivation ....................................................................................1 Thesis Statement ..............................................................................................................................1 1.1 Background................................................................................................................................1 1.1.1 Nerve Agents ..............................................................................................................4 1.1.1.1 History..........................................................................................................5 1.1.1.2 Physical Properties.......................................................................................6 1.1.1.3 Nerve Agent Simulants................................................................................7 1.1.2 Vesicant Agents..........................................................................................................9 1.1.2.1 History..........................................................................................................9 1.1.2.2 Physical Properties.....................................................................................10 1.1.2.3 Vesicant Agent Simulants..........................................................................10 1.2 CWA Reactions on Surfaces....................................................................................................11 1.2.1 Metal Oxides.............................................................................................................12 1.2.2 Heterogeneous Catalysts...........................................................................................16 1.3 Silica ........................................................................................................................................17 1.3.1 Silica Structure..........................................................................................................17 1.3.2 Adsorption to Silica ..................................................................................................19 1.4 Summary and Overview of Thesis...........................................................................................22 Chapter 2. Experimental Approach for Simulant-Surface Studies .......................................24 2.1 Vacuum Considerations...........................................................................................................24 2.2 Main Chamber .........................................................................................................................26 2.3 Sample Preparation ..................................................................................................................27 2.4 Simulant Dosing ......................................................................................................................30