In the past few decades great strides have been made in chemistry at the nanoscale, where the atomic granularity of matter and the exact positions of individual atoms are key determinants of structure and dynamics. Less attention, however, has been paid to the mesoscale—it is at this scale, in the range extending from large molecules (10 nm) through viruses to eukaryotic cells (10 microns), where interesting ensemble effects and the functionality that is critical to macroscopic phenomena begins to manifest itself and cannot be described by laws on the scale of atoms and molecules alone. Thus, mesoscale systems bridge the molecular and the macroscopic. The progress made in nanoscience can serve us well as we explore the mesoscale, as noted in a recent report for the Basic Energy Sciences Advisory Committee (BESAC) at the Department of Energy (DOE) (BESAC Subcommittee on Mesoscale Science 2012): “With our recently acquired knowledge of the rules of nature that govern the atomic and nanoscales, we are now well positioned to unravel and control the complexity that determines functionality at the mesoscale.”
Understanding phenomena at the mesoscale presents opportunities for developing new functionality of materials and understanding of biological and interfacial systems, as well as challenges for analysis and description. As Whitesides et al. noted, “the distinctive properties of mesoscale systems arise when the characteristic length of a process of interest, such as ballistic movement of an electron, excitation of a collective resonance by light, diffusion of a redox active molecule close to an electrode, or attachment and spreading of a eukaryotic cell is similar to a dimension of the structure in (or on) which it occurs. These processes involve interactions with many atoms or molecules rather than interactions with small, localized ensembles of atoms or molecules” (Kumar et al. 1995).
The complexity of mesoscale systems is due to their collective and often nonlinear behavior, and their study requires new approaches to synthesis, fabrication, and analysis beyond those used to study the molecular and macroscopic realms. Some mesoscale structures, such as micelles and liquid crystals, are transient yet thermodynamically stable, while others, particularly those from the biological world such as folded proteins, the capsids of viruses, and the calcium carbonate in mollusk shells, are longer lived.
Jennifer Curtis, Distinguished Professor of Chemical Engineering and Associate Dean for Research in the College of Engineering at the University of Florida, co-chair of the Chemical Sciences Roundtable (CSR), and a member of the workshop organizing committee, explained that the DOE BESAC report and other ad hoc workshops on the mesoscale have had more of a materials science focus rather than a chemistry focus. To explore how knowledge about mesoscale phenomena can affect chemical research and development activities and vice versa, the CSR decided to organize and convene a day-and-a-half workshop on mesoscale chemistry with the following statement of task:
1 The planning committee’s role was limited to planning the workshop, and the workshop summary has been prepared by the workshop rapporteur as a factual summary of what occurred at the workshop. Statements, recommendations, and opinions expressed are those of individual presenters and participants and are not necessarily endorsed or verified by the National Research Council, and they should not be construed as reflecting any group consensus.
An ad hoc committee will plan and conduct a public workshop in November 2014 in Washington, DC. Advances in theoretical, computational, synthetic, and analytical techniques have supported the extension of research into the study and development of mesoscale structures and processes. In the nanoscale to microscale size range, interesting ensemble effects exist that present opportunities for developing new functionality of materials, and understanding of biological systems and interfacial systems, as well as challenges for analysis and description.
This 1.5-day symposium will focus on the research on chemical phenomena at the mesoscale, and participants will be invited to actively participate in discussions responding to panels of speakers to identify opportunities and challenges in chemical and chemical engineering research at the mesoscale with a particular focus on collective and emergent behaviors at this scale. Questions considered during the workshop will include the following:
- What is the current state of the art of research at the mesoscale in chemistry and chemical engineering?
- What is the particular value to chemists and chemical engineers of studying and exploiting individual and collective behaviors at this scale?
- What opportunities and challenges exist for research in this area?
The workshop held November 6-7, 2014, in Washington, DC, focused on exploring the collective and emergent behaviors at the mesoscale. Participants examined the opportunities that utilizing those behaviors can have for developing new catalysts, adding new functionality to materials, and increasing our understanding of biological and interfacial systems. The workshop also highlighted some of the challenges for analysis and description of mesoscale structures. It is the hope of the CSR that, as the interest in mesoscale chemistry continues to grow, this workshop will provide a snapshot of the current research on chemical phenomena at the mesoscale and will provide inspiration for ideas for research and research programs. Over the course of the workshop, a few important points were raised repeatedly by various participants. These points are as follows:
- There exists a lack of broadly applicable theoretical and conceptual frameworks for understanding and predicting phenomena at the mesoscale.
- Studying molecular-level interactions using both theoretical and empirical tools is useful to understand and improve our ability to adequately describe mesoscale phenomena.
- A distinguishing aspect of mesoscale chemistry research is the need to understand and investigate a system as a whole as opposed to breaking down the system into discrete parts.
- Metastable and nonequilibrium phenomena have a larger effect at the mesoscale than at other length scales.
- Gaps remain in the understanding of the emergence of structure and/or function through collective interactions and motion.
- Entropy plays an increasingly important role in describing behavior at the mesoscale.
- Having fine control over these states in the laboratory is a continuing challenge.
- Modeling and measurement challenges are presented by the stable or metastable states that exist at the mesoscale.
This publication summarizes the presentations and discussions that occurred throughout the workshop (see Appendix A for the agenda), highlighting the key lessons presented and the resulting discussions among the workshop participants (see Appendix D for a list of attendees). Chapter 2 presents an overview of mesoscale phenomena and how they bridge the behavior of atomic, molecular, and nanoscale structures and those of the macroscopic world. Chapter 3 discusses the role that mesoscale phenomena play in catalysis and how those phenomena can inform strategies for synthesizing and stabilizing catalysts. Chapter 4 examines membrane behavior as a function of mesoscale interactions and how the ability to manipulate ensemble-scale phenomena may lead to the development of novel microchemical systems. Chapter 5 recounts the presentations and discussions
on the mesoscale aspects of biomineralization and geochemical systems, as well as the development of microanalytical tools to probe geochemical heterogeneity and interpret the geochemical record on, within, and beyond Earth. Chapter 6 describes some of the computational approaches that are being used to better understand mesoscale phenomena and design novel materials and protein complexes, improve solar energy conversion, and represent thermodynamic properties of molecular ensembles.
In accordance with the policies of the National Research Council, the workshop did not attempt to establish any conclusions or recommendations about needs and future directions and focused instead on issues identified by the speakers and workshop participants. The beginning of each chapter contains a list of key points. These key points reflect statements made by speakers and participants at the event and do not reflect a consensus view of the Roundtable or the National Research Council. The organizing committee’s role was limited to planning the workshop. The workshop summary has been prepared by workshop rapporteurs Kathryn Hughes, Camly Tran, and Joe Alper as a factual summary of what occurred at the workshop.