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Why write this book? A number of reasons come to mind. Today, a vast number of publications on clathrate hydrates continue to appear in journals dealing with the many different, often non‐overlapping areas of hydrate research. Increasingly, these studies focus especially on engineering and geological aspects, as well as potential applications. Molecular science has provided the fundamental underpinnings for much of this work; however, the earlier work has become more difficult to find, and there have not been major comprehensive monographs or books focused on scientific aspects of these substances for some 40 years. It is to fill this gap that preparation of this book was undertaken. We also feel there are a number of misconceptions or questionable approaches that have propagated over the years, and this book provides an opportunity to revisit some of these points.

The 150 years that it took for the first observed phenomenon of gas hydrates to be properly explained make for a fascinating story of the intertwining of the then current hydrate science, advances in technology, and the evolution of chemical concepts. After an Introduction and summary of the “classical period,” the book presents 16 chapters outlining hydrate science in different areas of specialization (see below). Each chapter provides a short summary of the respective methodology and is written with an emphasis on the experience of the authors with significant feedback from the editors and the other chapter authors. The book also provides comprehensive tabulated information on the structural, compositional, spectroscopic, thermodynamic properties, and molecular simulations of clathrate hydrates. With this information gathered in one place, it will be a valuable resource for both experienced researchers, and researchers and graduate students of science and engineering just starting their studies of these fascinating substances. The authors have aimed at making each chapter as comprehensive as possible, but in a work of this scope, valuable work will inevitably not be discussed.

A summary of the chapter contents follows.

Chapter 1 reports the major highlights in the development of clathrate hydrate knowledge and then highlights contributions of the National Research Council of Canada group where the authors of this volume have worked or which they were in close collaboration.

Chapter 2 gives a more detailed historical outline of the study of clathrate hydrates from the classical period up to 1970 when the hydrate crystallographic structure became known and the statistical mechanical model of clathrate hydrates was developed. We surveyed some of the primary literature of this period to clarify some of the historical aspects of these substances discovered by the early researchers.

Chapters 3 and 4 introduce the different hydrate cages made of hydrogen‐bonded water molecules and discuss the classification of clathrate hydrates as part of the larger family of supramolecular compounds, and their techniques of synthesis, respectively. Hydrates are presented as solid solutions with their stability being a lattice property. Comprehensive tables are presented of the known guest molecules, and summaries of their structural and physical properties are given in this chapter. The different classes of clathrate hydrate phase equilibria are presented.

Chapters 5 and 6 discuss structural aspects of clathrate hydrates, semi‐clathrates, and salt hydrates. The importance of unconventional guest–host interactions like hydrogen and halogen bonding is introduced. Different ways of looking at hydrate structures based on layered structures and space filling cages using the Frank–Kasper approach are presented, and related non‐hydrate clathrates are introduced.

Chapter 7 introduces thermodynamics and statistical mechanics of clathrate hydrates with discussion of calorimetric methods and the van der Waals–Platteeuw theory and some of its extensions. Many recent engineering applications and extensions are not directly discussed as they are discussed in Chapter 16 or are beyond the scope considered for this book. Tables of hydrate composition and thermochemical information are presented.

Chapter 8 gives a summary of the application of molecular simulation methods to study clathrate hydrate properties. Methods of characterizing structural and dynamic properties of clathrate hydrates are discussed. Most of the emphasis is on classical molecular dynamics and Monte Carlo results, but quantum mechanical calculations of confinement effects for small molecules such as hydrogen and methane in the clathrate hydrate cages are also reviewed. A table is given for systems studied to date using molecular simulation methods.

Chapters 9, 10, 11, and 13 discuss X‐ray and neutron diffraction and scattering, general and specialized NMR methods and IR/Raman methods for studying clathrate hydrates, respectively. The techniques are briefly introduced, and the often complementary information they provide on clathrate hydrates are described. The use of these methods in unraveling the structure and dynamics of guest–lattice interactions is summarized.

Chapter 12 presents information, mainly from dielectrics and solid‐state NMR, on the molecular motion of guest and host molecules. The relationship between cage geometry and guest dynamics is introduced, as is the effect of guest–host hydrogen bonding on water molecule dynamics.

Chapter 14 presents the rate and mechanisms of hydrate formation and decomposition from both macroscopic (process) and microscopic (mechanism) points of view. Classical nucleation theory introduces a number of key parameters that are pertinent to both homo‐ and heterogeneous nucleation mechanisms of hydrate formation. Emphasis is placed on hydrate processes as phase changes occurring in the presence of mass and temperature gradients rather than chemical reactions occurring in isotropic and isothermal systems. The various factors that modify kinetics of hydrate formation are introduced and discussed from results gathered from both experimental and molecular simulations. The hydrate memory effect and possible mechanisms of kinetic hydrate inhibition (using both polymeric substances and antifreeze proteins) are discussed in this chapter.

Chapter 15 deals with mechanical properties of clathrate hydrates, including acoustic velocity, elastic constants, thermal expansion, and thermal conductivity. Experimental and theoretical backgrounds for the study of these properties are given. Some anomalous effects seen in the temperature dependence of the thermal conductivity of hydrate phases, but absent in ice, are discussed in detail in this chapter.

Chapter 16 presents with selected potential applications of clathrate hydrate compounds, including flow assurance, natural gas recovery, desalination, concentration of aqueous solutions, and the storage of natural gas, hydrogen, and gas separation. This chapter is meant to be an overview of some applications, which will be well studied in the near future.

The authors' work described in this book has contributions from many colleagues, staff, and students at the National Research Council of Canada who are named in Chapter 1. The contributions of these individuals are gratefully acknowledged. The work described would not have been possible without the material support of the National Research Council of Canada from ∼1960 to 2011. As in any field, the progress in clathrate hydrate research is the result of collaboration and contributions from researchers in many countries. We hope to have given proper representation of contributions from researchers from all parts of the world. We apologize for any omissions, which are the result of the limited scope of some of the discussions in this book.

We would like to acknowledge the contributions of Donald Davidson (1925–1986) of the National Research Council of Canada as one of the pioneers in modern hydrate research in Canada. Don (i) initiated a multi‐technique approach to studying gas hydrates; (ii) provided mentorship to generations of hydrate researchers at the NRC; and (iii) wrote a number of monographs and book chapters, which are exemplary for their clarity and are still useful today. At the time, the National Research Council was a fertile multidisciplinary environment where cutting‐edge dielectric, NMR, single‐crystal X‐ray crystallographic, and simulation techniques were being developed, and almost immediately being used in understanding these substances. After Don's passing, this tradition was maintained at the NRC.

The editors would like to thank the authors of the chapters in this book for their contributions and their patience with our numerous requests for editorial changes. We would particularly like to thank Christopher I. Ratcliffe, Amadeu K. Sum, Peter Englezos, and Dennis D. Klug who also commented extensively on chapters other than their own.

We would like to thank our wives Beth and Dorothy for support and dealing with the seemingly unending demands on our time while we worked on this book. We thankfully acknowledge their indirect, but important contributions in getting this book project completed.