Supramolecular Fibers in Gels Can Be at Thermodynamic Equilibrium: A Simple Packing Model Reveals Preferential Fibril Formation versus Crystallization

Abstract

Low Molecular Weight Gelators are able to form nanostructures, typically fibers, which entangle to form gel-phase materials. These materials have wide-ranging applications in biomedicine and nanotechnology. While it is known that supramolecular gels often represent metastable structures due to the restricted molecular dynamics in the gel state, the thermodynamic nature of the nanofibrous structure is not well understood. Clearly, 3D extended structures will be able to form more interactions than 1D structures. However, self-assembling molecules are typically amphiphilic, thus giving rise to a combination of solvophobic and solvophilic moieties where a level of solvent exposure at the nanostructure surface is favorable. In this study, we introduce a simple packing model, based on prisms with faces of different nature (solvophobic and solvophilic) and variable interaction parameters, to represent amphiphile self-assembly. This model demonstrates that by tuning shape and 'self' or 'solvent' interaction parameters either the 1D fiber or 3D crystal may represent the thermodynamic minimum. The model depends on parameters that relate to features of experimentally known systems: the number of faces exposed to the solvent or buried in the fiber; the overall shape of the prism; and the free energy penalties associated with the interactions can be adjusted to match their chemical nature. The model is applied to describe the pH dependent gelation/precipitation of well-known gelator Fmoc-FF. We conclude that, despite the fact that most experimentally produced gels probably represent metastable states, one-dimensional fibers can represent thermodynamic equilibrium. This conclusion has critical implications for the theoretical treatment of gels.A wide range of small molecules which are able to selfassemble into supramolecular gels have been investigated over the years as promising new materials with potential applications in nanotechnology and biomedicine.

Controlling supramolecular polymerization through multicomponent self‐assembly

“…295 Shifts of the apparent pKa value is a well-known phenomenon for polymeric weak acids, self-assembled fatty acids and amphiphilic block copolymers, [298][299][300][301][302][303][304][305] and has been used by many research labs in order to fine-tune the pH-triggered self-assembly of linear 306,307 and aromatic peptide amphiphiles. 85,[308][309][310][311] A similar effect has been reported by Matile and coworkers in the b-sheet directed self-assembly of poctiphenyl b-barrel pores embedded in lipid bilayers. 312 Weakening of the basic character of protonated lysine and histidine side chains (apparent pK a of 7 and 4.5, respectively), as well as a weakening of the acidic character of aspartic acid side chains (apparent pK a of 6) was observed.…”

Augmenting Photoinduced Charge Transport in a Single‐Component Gel System: Controlled In Situ Gel–Crystal Transformation at Room Temperature

“…Correlating the events occurring during the self‐assembly process with increase in TPAPAE concentration, we believe that somewhere between switching in between the transparency and opaqueness of the TPAPAE gel, the balance has been shifted more towards the molecular aggregation force than the solubilizing gelator–solvent interactions. An extended 3D structure of fibers can form more interactions and more ordered networks than a 1D structure . Our FE‐SEM results provide evidence regarding the regulated self‐assembly process, where the prevailing solvophobic forces induced order in the fibers with maximal CSO at higher TPAPAE concentrations.…”

“…The four approximations which conceptually explore the room‐temperature‐controlled in situ phase change from wire‐like fibrils to crystals is described in Scheme . Approximation 1 in Scheme states that if the entangled 1D wires in a 3D network exhibit a minimal contact surface overlap (CSO) between them, then the interaction in between the wires is negligible, in comparison to interaction of the components forming the 1D wire . This is of crucial importance for most stable organogels, in which a proper balance between the solvophilic and solvophobic forces renders the gel to attain a reversibly accessible local minimum, which avoids the further transformation of the system to attain a global minimum, i.e., the crystal state .…”

A Ferrocene–Tryptophan Conjugate: The Role of the Indolic Nitrogen in Supramolecular Assembly

“…Some fibers on average range between 40–80 nm in width . Fibril structures have been reported as the main morphological structure of relatively strong gels . Redox triggering in compounds is a novel and significant property.…”