Thixotropic Supramolecular Gel Based on l-Lysine Derivatives

The dimer l-lysine derivatives, in which two Nα,Nε-diacyl-l-lysines were crosslinked by calcium ion, were synthesized through a simply synthetic procedure and their gelation properties were examined. These compounds functioned as an organogelator; especially, the gelators possessing both a linear and a branched alkyl chains had the better organogelation ability and formed the thermally stable and rigid organogel. In addition, some organogels had a thixotropic property, which were responsive to a mechanical stimulus and reversibly underwent the gel–sol transition at room temperature. The thixotropic behavior was confirmed by visual contact and rheological experiments. Furthermore, it was assumed the mechanism of the thixotropic behavior.

. Rheological data for kerosene gels based on gelators A-C.

Gel (G′ > G′′) Sol (G′ < G′′)
Kerosene gels: For 1-10, 50, 100, 200, 300, and 301 cycles, the measurement was carried out on the rheometer. For the gels in the screw capped test tube, the operation of the shearing by a shaker (10 s) and then standing at 30 min was repeatedly conducted.  The AFM samples were prepared as follows: the gelator A was dissolved in kerosene (10 mg/mL) in a screw capped test tube by heating, and then the test tube was allowed to stand at 25 °C for 2 h. The gel was broken by shaking (change into the sol), and then the sample was allowed to stand at 25 °C for 30 min. The gel re-formed. The gel was first sample for the AFM, and the AFM sample was prepared on mica by spin coating. The re-formed gel sample in the test tube was broken again by shaking, and the sol was imidiately spin-coated on mica. This is the sol sample. The sol in the test tube was allowed to stand at 25 °C for 30 min, the gel formed again. The second gel sample was prepared from it.

S9
Compared with the images before shearing and after standing, the diameter and amount of the nanofibrous assemblies in the image after shearing became bold and small, respectively. The nanofibers separated from the networks are difficult to fix on mica, leading to the low density of the nanofibers (sample after shearing). Furthermore, the nanofibers may aggregate to the thicker nanofibers during the preparation of AFM sample. In contrast, for the samples prepared from the gels, the network structures can inhibit further aggregation to make the thicker nanofibers and be easy to fix the gel network on mica. The fact is one of the evidences for the degradation of network structures by shearing. Figure S9. CD spectra of kerosene gels of gelators A (10 mg/mL) and gelators B (15 mg/mL) and their 1-propanol solutions. The first gel means the kerosene gel formed after the heating dissolution of the gelators.
The circular dichroism spectra were measured by using a JASCO Circular Dichroism J-600 spectrometer. In 1-propanol, no CD spectra were obtained because these gelators did not form the gel and have any self-assemblies. For gelator A, the first gel showed the positive CD peak at 202 nm. After shearing, the peak became a little narrow band very slightly shifted to 201 nm. The CD peak of the gel re-formed after standing little changed. For gelator B, the first gel showed the positive CD peak at 201 nm. After shearing, the peak a little broadened and very slightly shifted to 202 nm. The CD peak of the gel re-formed after standing little changed. The FE-SEM samples were prepared in vacuum by freeze-dried samples for cyclohexane gel and the room temperature-dried samples for chloroform.
Scheme S1. Tentative illustration of intermolecular interactions in self-assembled nanofibers.
Gelator A Gelator B Gelator C

Cyclohexane gel
Chloroform gel