Competition between gelation and crystallisation of a peculiar multicomponent liquid system based on ammonium salts

Abstract

An exemplar competition between gelation and crystallisation phenomena was examined with an unusual synergistic multicomponent (organo)gelator solution (MGS), which consists of a well-defined methanolic solution of (1R,2R)-1,2-diaminocyclohexane L-tartrate containing 2.4 equiv of concentrated hydrochloric acid. The optimal composition of the MGS was determined through meticulous solubility, gelation and structural studies, which support a transient gelation mechanism based on the kinetic self-assembly of the tartrate salt driven by hydrogen-bonding interactions, involving ammonium nitrogen donors and hydroxyl oxygen acceptors, and electrostatic interactions. The hydrochloric acid is involved in the solubilisation of the salt through an ionic dissociation-exchange process, which ends up with the formation-precipitation of (1R,2R)-1,2-diaminocyclohexane dihydrochloride. As a consequence, an irreversible destruction of the gel takes place, which indicates the metastable nature of this phase that cannot be accessed from the thermodynamically equilibrated state. Gelation of a variety of oxygenated and nitrogenated solvents with moderate polarity occurred efficiently using extremely low MGS concentrations at low temperatures, and the gel phase was confirmed by dynamic rheological measurements. Several features make the described MGS unique: (1) it is a multicomponent solution where each component and its stoichiometry plays a key role in the reproducible formation and stabilization of the gels; (2) it is formed by simple, small, and commercially available chiral building blocks (dissolved in a well-defined solvent system), which are easily amenable for further modifications; (3) the gelation phenomenon takes place efficiently at low temperature upon warming up the isotropic solution, conversely to the typical gel preparation protocol; (4) the formed organogels are not thermoreversible despite the non-covalent interactions that characterize the 3D-network. 45 non-covalent bonds, predominantly hydrogen-bonding, van der Waals, charge-transfer, dipole-dipole, π-π stacking, and coordination interactions, which usually lead to reversible gel-tosol phase transitions-. Furthermore, systems based on both types of connections are also known. 12,13 The solid-like appearance of 50 gel materials is the result of the entrapment of the liquid (major component) into the compartments of a solid 3D-matrix of a large surface area (minor component), typically through surface tension

Gels with sense: supramolecular materials that respond to heat, light and sound

“…39,94,[98][99][100] This change in phase may involve a salting-out process, 101,102 rearrangement of hydrogen bonds 94,98,103 or recombination of labile metal-ligand interactions. 104,105 The relative stabilities of products are often highly sensitive to the environment of the gelator, and in particular the solvent, which in some multicomponent systems might be temperature-dependent.…”

Non-invasive and continuous monitoring of the sol–gel phase transition of supramolecular gels using a fast (open-ended coaxial) microwave sensor

“…However, it should be noted that gelation of liquids at RT 10,11 or induced by ultrasound treatment 12 instead of heating-cooling has also been described. 15,16 Due to the brittleness of these materials, it is usually easier to monitor the gel-sol transition rather than the sol-gel for the construction of phase diagrams according to both the gel-sol transition temperature (T GS ) and the sol-gel transition temperature (T SG ). Bottom: a typical gelation mechanism of physical gels, where (a) is usually a heating step and (b) a cooling step.…”

“…The precursor isotropic solution is introduced in a pyrex vial and placed inside an open coaxial re-entrant (microwave) cavity sensor. 15 A number of gels with different solvents and compositions could be easily obtained from this library at well-defined concentrations. This reflected signal is measured continuously to determine the resonance frequency and quality factor of the sensor 23 during gelation to monitor the transition process.…”

“…Fig. 15 This collection of gelators offered a versatile scenario for the proof-of-concept of the detection of the sol-gel transition in physical gels by continuously monitoring the dielectric properties of the materials. We have reported elsewhere the fundamental details of the microwave system with a different sensor head.…”

“…In this context, oxalic acid has been proven to be the lowest molecular weight organic ligand able to form stable supramolecular metallogel networks in the presence of metal salts. 15 Gel-crystal transitions have been previously related to the Ostwald rule of stages. of concentrated HCl (37 wt% in aqueous solution) to a variety of organic solvents pre-cooled close to their freezing points.…”