Gel Formation: Phase Diagrams Using Tabletop Rheology and Calorimetry

In previous chapters, we have attempted to define the term “gel” in a consistent and meaningful way, while recognizing that the gel state is often easier to recognize than to define (if it looks like “Jello”, feels like “Jello”, and responds like “Jello”, it must be a gel [1]). This leads us to the theme of the present chapter, the elucidation of phase diagrams and composition maps for molecular gels. Typically, this is the first step in a given study. Other chapters are devoted to detailed analyses of gels and their phase transitions (see Chapters 1–4). It is worthwhile to recall that molecular gel samples are usually prepared by dissolving a small-molecule gelator in a liquid, usually by warming [1]. At this point, the gelator is unaggregated or in small aggregates and this system is referred to as a sol. The sol is then cooled below a transition temperature whereupon it transforms into a gel. Typically, the higher the gelator concentration c, the higher is the sol-gel transition temperature Tgel and a plot of Tgel against c constitutes a phase boundary. A phase diagram is a map of T vs. c showing the sol-gel boundaries as well as the boundaries of any multi-phase or lyotropic liquid crystal regions that are also present. Most articles on molecular gels usually have an accompanying phase diagram. While studying gelators, especially those with novel structures, the focus is on determining the entire phase diagram rapidly and using as little gelator as is practically feasible. Thus, there is a need for simple, reliable, and