Full characterization of liquid, semi-liquid, gel, or solid systems requires direct, supramolecular-level information (i.e., images, which show how molecules arrange to form clusters of various sizes and shapes). Cryogenictemperature transmission electron microscopy (cryo-TEM) is the method of choice for obtaining such direct imaging of liquid or semi-liquid specimens, thermally fixed into a vitreous or quasi-solid state. Cryo-TEM provides highresolution direct images of the assemblies in the system. Thus, it can elucidate the nature of the basic building blocks that make up the systems, covering a wide range of length scales from few nanometers to several microns. In addition, coexistence of many different assemblies present in the examined systems is quite easily observed in the micrographs. The interpretation of data cryo-TEM produces is usually quite straightforward, not model-dependent. In contrast, experimental interpretation data from "indirect methods", such as scattering techniques, is model dependent and is complicated when the system contains more than one type of aggregate or a broad size distribution. In the case of molecular gels, the unique rheological properties are the result of supramolecular aggregates. Those may be regular or irregular, homogenous or very diverse. Thus, cryo-TEM is most useful to image the range of the nanostructures present in those systems. In some cases, direct images provide the only way to prove a suggested or a theoretically predicted model. That was demonstrated for example in the case of the theoretically predicted branched micelles , the shape of the "end-caps" of thread-like micelles [2, 3], and the exact nanostructure and mechanism of formation of lithocholate nanotubes [4, 5]. While micrographs are most useful, at the same time one should keep in mind that cryo-TEM is not a strictly quantitative technique. It is the technique of choice to determine the structural building blocks of complex fluid systems, but the quantitative data should be usually provided by other techniques, such as small-angle X-ray scattering (SAXS) (see Chapter 11), small-angle neutron scattering (SANS) (see Chapter 10), or nuclear magnetic resonance (NMR). Another advantage of these scattering techniques is that they probe the bulk of the system, not just a small sample of it; they thus provide a real statistical average. However, in a very heterogeneous system, such an average may be difficult to interpret. In addition, these techniques are "model dependent" they are not "observer-dependent". In fact, the best experimental approach is to apply cryo-TEM to collect data on the nature of the nano-building blocks of the system, use that information to construct a physical model that is used to interpret data from the above mentioned "indirect techniques", and then check whether those latter results agree with cryo-TEM images to rule out possible artifacts. Below we describe the basic aspects of cryo-TEM. That is followed by a review of the applications of the technique to the study of gel and gel-like systems. The interested reader will find more details about the technique and its application to other systems in references [6, 7].
|Title of host publication||Molecular Gels|
|Subtitle of host publication||Materials with Self-Assembled Fibrillar Networks|
|Number of pages||22|
|ISBN (Print)||1402033524, 9781402033520|
|State||Published - 2006|