Using Rheology to Understand Transient and Dynamic Gels

Abstract

Supramolecular gels can be designed such that pre-determined changes in state occur. For example, systems that go from a solution (sol) state to a gel state and then back to a sol state can be prepared using chemical processes to control the onset and duration of each change of state. Based on this, more complex systems such as gel-to-sol-to-gel and gel-to-gel-to-gel systems can be designed. Here, we show that we can provide additional insights into such systems by using rheological measurements at varying values of frequency or strain during the evolution of the systems. Since the different states are affected to different degrees by the frequency and/or strain applied, this allows us to better understand and follow the changes in state in such systems.

Keywords: dissipative; dynamic; gel; rheology; transient.

Figure 1

(a) Chemical structure of the gelator 1ThNapFF used in the study. This compound undergoes deprotonation in presence of urea and urease due to production of ammonia. Base-catalysed hydrolysis of methyl formate reduces the pH and regenerates the structure of 1ThNapFF. (b) Cartoon representing the effect of variation of strain on the G′ profile of a dynamic system undergoing gel-to-sol-to-gel transition. The final value of G′ decreases as the applied strain increases.

Figure 2

(a) Photograph of the gel-to-sol-to-gel transition brought about by the sequential increase and decrease in pH. Note that bubbles appear during the phase transitions which can be seen in the sol and final gel state; (b) Frequency sweep for a stable gel formed at low pH; (c) Strain sweep for a stable gel formed at low pH. For both (b,c), no enzyme, urea or methyl formate were added and hence, the solvent-triggered gel remains stable. In all cases, initial concentration of 1ThNapFF = 2 mg/mL, urea = 0.01 M, urease = 0.2 mg/mL and volume of methyl formate is 100 μL. The black data represent G′ and the red data represent G″.

Figure 3

Variation of G′ (black), G″ (red), and pH (green) with time for 1ThNapFF in presence of urea–urease reaction and methyl formate at (a) 1 rad/s, (b) 10 rad/s and (c) 50 rad/s. Throughout all measurements, the strain value was fixed at 0.5%. Comparisons of (d) final G′ and (e) yield points of the three systems. For (d), the values of G′ at 0.5% strain from the strain sweeps are considered. In all cases, initial concentration of 1ThNapFF = 2 mg/mL, urea = 0.01 M, urease = 0.2 mg/mL and volume of methyl formate is 100 μL. Note that the pH and rheology measurements were performed with two different vials under identical conditions. The data were then compared to investigate the variation of rheological properties to that of the changes of pH with time.

Figure 4

Variation of G′ (black), G″ (red), and pH (green) with time for 1ThNapFF in the presence of urea–urease reaction and methyl formate at strain values of (a) 0.05%, (b) 5%, (c) 10%, (d) 20%, (e) 50%, (f) 100% and (g) 200%. Throughout all measurements, the frequency value was fixed at 10 rad/s. Comparisons of (h) final G′ and (i) yield point of the systems. For (h), the values of G′ at 0.5% strain from the strain sweeps are considered. In all cases, initial concentration of 1ThNapFF = 2 mg/mL, urea = 0.01 M, urease = 0.2 mg/mL and volume of methyl formate is 100 μL. Note that the pH and rheology measurements were performed with two different vials under identical conditions. The data were then compared to investigate the variation of rheological properties to that of the changes of pH with time.