© Copyright 2000 American Chemical Society
NOVEMBER 14, 2000 VOLUME 16, NUMBER 23
Symposia Introductory Remarks The special issue of Langmuir “Colloid Science Matured” is devoted to four scientists who reached age 60 at the Millennium. In age sequence they are Evan Wyn-Jones, Raymond A. Mackay, Mats Almgren, and Josef F. Holzwarth. All contributors to this issue were “invited” to submit a paper, and I apologize for not having been able to ask all eminent members of the “colloid community”. Those working in colloid science have certainly come across these four names in the literature, because there are well over 500 publications available containing their contributions to the progress of science. More than 5000 citations in publications and a number of citation classics bearing one of these four names can also be found in the literature. Here I shall not attempt to give a full “Curriculum Vitae” of the four honored persons, but I will just highlight some features, which they have in common. All started their scientific careers after studying for the PhD by working in well-known laboratories abroad, and all were involved in the early days of physical characterization of micelles formed by surfactants. I still remember the engaged discussions of the 1970s concerning the structure of sodium dodecyl sulfate (SDS) micelles. Names such as Aniansson, Wall, Menger, Kahlweit, Robinson, Thomas, Zana, Lang, and Hoffmann, just to mention a few, come into my mind. The experimental developments of Mats Almgren in fluorescence lifetime studies, Ray Mackay in the application of electrochemical methods, and Evan Wyn-Jones in the use of ultrasonic techniques as well as the iodine-laser temperature-jump apparatus and the continuous-flow technique with integrating observation developed in my laboratory helped to create the structural and dynamic picture of SDS micelles which we accept today. In the 1980s, structural resolution of highly dynamic colloidal dispersions via dynamic light scattering or cryo-electron microscopy became available, and again these “four” were among the pioneers in the application of these techniques to colloid science. The 1980s and 1990s provided another tool to investigate the detailed structure of colloids, namely, SANS (small-angle neutron scattering). Despite problems with the limited number of facilities in the world, and the high costs involved, one is seeing the names of these “four” appearing again. SANS has also made available structural and composition
information of intermediate structures in aqueous solutions, which would otherwise not be accessible. In addition to their scientific work, all “four” have also been involved in the administrative duties serving the scientific community either in the Senate of their institutions and universities or at the National Science Foundation (R. Mackay). They served as Founders or Chairmen during Gordon Conferences and started new meetings for the European Science Foundation or NATO. In conclusion they not only are good examples of progressive and inventive scientists but also have demonstrated their willingness to contribute to the organizational side of science, which creates the necessary environment for the essential fruitful communication and exchange of ideas indispensable for the development of science. This leads me to the cover picture of this Special Issue. Colloid scientists all over the world know about “phase” diagrams. The cover picture is a new type of triangular diagram, which demonstrates how I see the fundamental questions in the area of colloid science. The key feature of all colloidal structures is their dynamic behavior, and by solving the dynamic behavior over the time range from picoseconds to years, we can hope to gain a perfect description of any colloidal system. Unfortunately this is not possible so far, as even the most advanced molecular dynamics simulations cover the time range from femtoseconds to only nanoseconds. Therefore we have to combine indirect experimental information from dynamic, structural, and thermodynamic techniques to obtain as true as possible a picture of colloidal systems. Most progress in the last 20 years was made by instrumental developments for achieving time-resolved structural information. Nevertheless we should keep in mind that a structure is only a time average over many dynamic events. Thermodynamic information provides only basic energy information and cannot, by definition, help with time-resolved structural or space information. In the past far too many investigations neglected this fundamental fact, and “the four” were among the first who shaped their research in such a way that the dynamic nature of colloidal systems came into the center of research to gain time-, space-, and energy-dependent information for producing as complete a picture of colloidal systems as possible. The cover also
10.1021/la000952i CCC: $19.00 © 2000 American Chemical Society Published on Web 11/07/2000
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Langmuir, Vol. 16, No. 23, 2000
shows that the sum of all dynamic information D between time zero and infinity contains all time-, energy-, and space-dependent information, which is not valid for thermodynamic or for structural information alone. As human beings we like to see pictures, but we often forget that a structure is only a time average of space information, typical for the technique applied, not an absolute picture. Nature uses colloidal systems because of their richness in time-dependent structures and their high structural mobilitysthis key feature is often forgotten, if we see a structure produced by X-ray diffraction. I remember a scientist from Calgary showing two slides during a conference. The first one showed an eagle gliding majestically in the air with his wings wide opensthe real system. The second was a box of well-arranged frozen chickenss close to representing its X-ray structure. Those two pictures demonstrated in a perfect way that a real living system in action is often very different to its X-ray structure and only develops its full features through a
permanent change of structures over the time frame from femtoseconds to years. Just to finish my description of the cover, we see two water dipoles representing the most important solvent in nature, a simplified vesicle, and some surfactant molecules. By mixing these ingredients in a clever way, we can synthesize the richness of highly mobile colloidal structures, many of which still await discovery by our younger colleagues. I do hope this short introduction into the following papers, which cover the “state of the art” in colloidal research, will provide a useful basis for more innovative investigations in colloid science. I wish to thank all contributors to this issue as well as my wife Ana at the Langmuir office in Berlin. The skilled support of Luis Netter on the computer to produce the artwork was outstanding. Josef F. Holzwarth LA000952I
