Book Reviews March 2006

• Crystallography of the Polymethylene Chain: An Enquiry into the structure of Waxes by Douglas L. Dorset reviewed by Andrew Bond

Title:  Crystallography of the Polymethylene Chain: An Enquiry into the structure of Waxes
Author:   Douglas L. Dorset
Publisher:  Oxford University Press 2004 IUCr Monographs of Crystallograpy 17
Price:  Price: £85.00 (hardback)
ISBN   0-19-852-9082 232 pages

This seventeenth title in the series of IUCr Monographs on Crystallography concerns the study of waxes, with particular emphasis on the structures of pure n-alkyl compounds and their binary mixtures as models for multicomponent systems containing linear polymethylene chains. The author�s own research in this field spans nearly 35 years, and his significant contributions to the development of electron crystallography in particular have provided many new insights into earlier powder X-ray diffraction and spectroscopic studies of these systems. Electron diffraction images, mostly taken from the author�s own archives, are prominent throughout the book, reflecting both the author�s expertise and the importance of electron crystallography in this area. The back-cover synopsis of the book specifies that it aims to elucidate three main features of waxes: �firstly, which modifications of molecular components are allowed for maintaining stable solid solutions; secondly, what happens when stability conditions are traversed and fractionation begins and thirdly, the structure of fractionated arrays�.

The opening chapter provides a more detailed statement of the book�s scope, citing a multitude of everyday products and industries that rely on a thorough understanding of various waxes. One principal scientific objective is noted to be a critical evaluation of this understanding, specifically of the widely-accepted �Le Roux model� for wax structure. In short, this model suggests that a polydisperse wax � that is, a linear polymethylene chain assembly that contains a distribution of chain lengths, chain unsaturation, functionalisation and branching � can be considered to comprise four zones: the first, a mostly crystalline region in which polymethylene chains are arranged in a manner that resembles the pure materials; the second, a rigid amorphous zone that incorporates chain branches; the third, an amorphous region between the ends of the chains that may contain solvent molecules, and the fourth, a more fluid region that contains volatile lower molecular weight components. The book follows a course intended to assess this model by examining the structures of various pure wax components and their binary solid solutions in the most crystalline forms available.

Following the preliminary remarks, Chapter 1 comprises a concise and helpful primer concerning aspects of polymorphism, phase diagrams and solid solutions. X-ray diffraction, electron diffraction and differential scanning calorimetry are featured techniques. Chapter 2 treats the general packing features of linear polymethylene chains in the manner of Kitaigorodskii, including characteristic electron diffraction patterns that assist with the later discussion. Chapter 3 describes in some detail the known crystal structures of the n-alkanes (paraffins), establishing the structural basis for their extensive polymorphism. Inclusion in the discussion of recent experimental data is a useful expansion on earlier accounts. This chapter also touches upon branched and aromatically substituted paraffins, but highlights that experimental knowledge in this area remains somewhat sparse. Thermodynamic and structural aspects of disorder and phase transitions in the pure paraffins is expanded upon in Chapter 4, which again incorporates a good blend of crystallographic, spectroscopic and calorimetric data.

Chapters 5 and 6 provide a substantial discussion on binary paraffin systems. In this section, Kitaigorodskii�s well-cited rules for stabilisation of solid solutions are evaluated against newly available single-crystal electron diffraction data. While volumetric rules concerning size and shape matching of the molecular components are found to be largely robust, previous suggestions regarding the symmetries of the pure components do not stand. Some fractionated binary structures are discussed, in which phase separation is shown to involve both lateral and longitudinal chain segregation � i.e. the structures do not simply comprise layers of the pure components segregated at the methyl-group interfaces. The influence of unsaturation and heteroatom substitution on general chain packing and on the formation of binary phases is touched upon in Chapter 6, but experimental information is currently less abundant in this area.

The following four chapters are devoted to functionalised derivatives of the n-alkanes, considering respectively alcohols, carboxylic acids, fatty acid esters and cholesteryl esters. Each survey encompasses the structural features of the pure materials and also their binary solid solutions. These chapters are valuable summaries of a large and very diverse literature, and each is an excellent starting point for researchers wishing to consider the solid-state behaviour of these classes of compounds.

In Chapter 11 the author brings together all of the information from the preceding chapters, first providing an excellent summary, then discussing how the knowledge can be applied to polydisperse waxes incorporating much broader chain distributions. Specific examples are given in which structural knowledge provides insight into the physical properties of polydisperse waxes. Ultimately, Dorset arrives at the conclusion that growing experimental evidence requires re-evaluation of the Le Roux model, then goes on to suggest numerous areas for further progress.

As a whole, the book is impressive. The author brings together a huge amount of literature to provide a coherent and comprehensive account of the research area as it stands today. The pioneering research is faithfully documented, but it is critically re-evaluated where more recent results suggest this to be necessary. The range of analytical techniques discussed imparts an appreciation of their complementarity for examining these systems, while the focus on electron crystallography is justified and clearly written by an expert. The overall presentation, including plentiful diagrams and photographs, is excellent. The only possible negative comment regarding this book is that it seems likely to find a rather limited audience. Of the series of IUCr Monographs to date, the topic seems to be the most esoteric, and despite the fact that the author makes an admirable effort to stress its relevance in the opening remarks, it is perhaps unlikely that the book will find its way into the general reading of many crystallographers. Nonetheless, those who do venture into the book will be richly rewarded � it is a worthy addition to the series.

Andrew Bond