Gel Formation: Phase Diagrams Using Tabletop Rheology and Calorimetry

4. Conclusions and Perspectives

In this chapter, we have described how the phase diagram for molecular gelators can be obtained using simple, straightforward techniques. The phase diagram is a plot of temperature vs. gelator concentration showing the location of sol-gel boundari(es) as well as any multi-phase or lyotropic regions. The first set of techniques involve application of rheological principles to detect gelation. These include: (a) tube inversion; (b) falling sphere; and (c) rising bubbles. Calorimetry studies are also useful in directly measuring the enthalpy of gelation (melting). The latter quantity can also be obtained by analyzing an Arrhenius plot of the gelator concentration as a function of the gelation temperature.

Among the rheology-based methods, tube inversion is by far the most popular and convenient, and arguably also the least ambiguous. As a starting point for studying gels, it is preferable to use tube inversion over falling ball or other alternatives. If falling ball must be used, it is important to use a heavy ball and a sufficiently large vessel in order to obtain a clean measurement. Finally, where possible, the simple “tabletop” rheological methods should be benchmarked using data from conventional rheometry.