Quantification of energy of activation to supramolecular nanofibre formation reveals enthalpic and entropic effects and morphological consequence

The dimensions of supramolecular fibres formed from a system that starts far from equilibrium because of fast solvent – anti-solvent mixing is determined by the balance between enthalpy and entropy in different solvent mixtures.

[b] The critical stress value is defined as the cross-over point between storage and loss moduli.

Materials
All reagents and solvents used in this work were of analytical grade. MilliQ water was used for the sample preparation. Compound 1,3-bis[(3-octadecyl-1-imidazolio)methyl]benzene dibromide was synthetized as previously reported in literature. 1

Methods
Sample preparation. All mixtures were prepared by addition of water to an ethanolic solution of gelator, varying concentration of the latter from 1 to 12 mM, and water-ethanol proportion between 5:5 and 9:1. 1 mL samples were prepared in 3 mL closed vials, sealed after gentle mixing (with a micropipette) and letting them stand undisturbed at 292 K to allow the self-assembly. The vialinversion test was used to assess gel formation. A standard preparation protocol was adopted to ensure the maximum reproducibility over all the samples: Stock solutions of gelator in ethanol were made having concentration ten times higher than those analysed. For example, the sample 12 mM in water-ethanol 9:1 was prepared by addition of 0.9 mL of water to 0. with concentration 12 mM and water ethanol ratio 7:3 was obtained combining 0.1 mL of stock solution 120 mM with 0.2 mL of ethanol, then 0.7 mL of water were added.

Kinetic measurements
Extinction measurements were carried out with an Agilent Cary 5000 UV-Vis-NIR spectrophotometer equipped with the Peltier 1×1 cell holder system. 10 mm quartz cuvettes were used for all the measurements. The kinetics of fibre formation was monitored following the increase of optical density at 700 nm, where no contribution to the signal comes from absorbed light is possible (the molecule absorbs in the UV region) but only by scattering from the sample, as seen in the following dataset for the 9:1 water:ethanol sample at 306 K. At lower wavelengths the scattering signal saturates the absorption spectrometer.
Ethanolic solutions of 1·2Br and MilliQ water were thermally equilibrated at the desired temperature for 15 minutes before mixing. 2 mL samples were prepared using different water-ethanol proportions and a final 1·2Br concentration of 8 mM. The mixing was done very quickly with a micropipette and then the cuvette was returned to the sample holder (generally within 2-3 s) for data acquisition. The fitting of the data to the models given in the main text was carried out using the software Origin Pro performing a non-linear curve fit using Levenberg Marquardt method as iteration algorithm.

Characterization
Scanning Electron Microscopy (SEM) was performed on a JEOL 7100F FEG-SEM instrument. Samples were prepared as 1mL volume in 3 mL sealed vials. Small aliquots of gel were deposited on aluminium SM 13 stubs, dried at room temperature under vacuum (fast drying achieved within 5-10 seconds after casting to minimise possible drying effects) and coated with a 5-nm thick layer of Iridium. Image acquisition was carried out using a working distance between 5-10 mm and 5 kV accelerating voltage to minimize sample charging. The rheological analysis was performed with an Anton Paar MCR 302 rheometer equipped with temperature controller and parallel plate geometry setup (PP50 stainless steel, 50 mm diameter, 1 mm gap between plates). All the measurements were carried out at 298 K.
Samples were made as 6 mL volume in 7 cm diameter petri dishes and sealed to prevent solvent evaporation. Gels were analysed 2 days after preparation. Samples were carefully transferred on the rheometer plate without breakage and the extra material was removed to suit the working geometry. prepared in a 10 mL sealed vial for each sample analysed, which were dried under vacuum to obtain xerogels after 3 days from preparation. The powder was collected and placed on brass sample holder for data acquisition in 2-Theta scale between 2-30°, with step size of 0.013°. Diffractograms analysis for phase identification was carried out with the software CRYSFIRE suite, using Dicvol91 2 as automatic powder indexing program to obtain the unit cell. Software Xfit-Koalariet 3 was adopted for peak fitting using Pseudo-Voight functions.