Linking micellar structures to hydrogelation for salt-triggered dipeptide gelators

Abstract

Some functionalised dipeptides can form hydrogels when salts are added to solutions at high pH. We have used surface tension, conductivity, rheology, optical, confocal and scanning electron microscopy, (1)H NMR and UV-Vis spectroscopy measurements to characterise fully the phase behaviour of solutions of one specific gelator, 2NapFF, at 25 °C at pH 10.5. We show that this specific naphthalene-dipeptide undergoes structural transformations as the concentration is increased, initially forming spherical micelles, then worm-like micelles, followed by association of these worm-like micelles. On addition of a calcium salt, gels are generally formed as long as worm-like micelles are initially present in solution, although there are structural re-organisations that occur at lower concentrations, allowing gelation at lower than expected concentration. Using IR and SANS, we show the differences between the structures present in the solution and hydrogel phases.

Opening a Can of Worm(‐like Micelle)s: The Effect of Temperature of Solutions of Functionalized Dipeptides

“…Small‐angle neutron scattering (SANS) data was also collected (Supporting Information, Figure S6 and Table S2). In agreement with our previous work,9 the data before heating is best fit to a hollow cylinder with a radius of 36.9±0.1 Å and a core radius of 19.1±0.2 Å. After a heat/cool cycle, the radius decreases to 31.2±0.1 Å (in line with the SAXS data) and the core radius also decreases to 9.2±0.3 Å.…”

“…The concentration range coincides with our previous work on the phase diagram for 2NapFF, where we showed that worm‐like micelles are formed above around 1 mg mL −1 , with a significant increase in the viscosity above 5 mg mL −1 9. Hence, this heat‐induced behavior requires that specific aggregates exist at room temperature, rather than this being a heat‐induced structural transition.…”

“…After stirring overnight, a slightly viscous solution was formed at a pH of approximately 11. We have previously shown that these solutions contain worm‐like micelles 6b, 9. These solutions are susceptible to shear forces, exhibiting enhanced extensional viscosity after exposure to shear rates of between 10 3 and 10 6  s −1 10.…”

“…At high pH, these can be dispersed by adding base to deprotonate the free carboxylic acid. Typically, spherical or worm‐like micelles are formed,6b although this is concentration dependent 9. We have described this scenario in detail and have shown that solutions of the worm‐like micelles can be gelled at high pH by adding a divalent cation 6.…”

“…The solution that has been exposed to a heat/cool cycle can be pushed out as a single strand essentially. Solutions of 2NapFF at high pH can be gelled by adding a divalent cation 6a, 9. A homogeneous gel is formed after standing overnight (initially, local gelation occurs where the salt solution contacts the 2NapFF solution).…”