Time Dependence of Gel Formation in Lyotropic Nematic Liquid Crystals: From Hours to Weeks

The combination of lyotropic liquid crystals (LLCs) and low-molecular-weight gelators (LMWGs) for the formation of lyotropic liquid crystal gels (LLC gels) leads to a versatile and complex material combining properties of both parent systems. We gelled the calamitic nematic NC phases of a binary and ternary system using the LMWG 3,5-bis-(5-hexylcarbamoyl-pentoxy)-benzoic acid hexyl ester (BHPB-6). This binary system consists of the surfactant N,N-dimethyl-N-ethyl-1-hexadecylammonium bromide (CDEAB) and water, whereas the ternary system consists of the surfactant N,N,N-trimethyl-N-tetradecylammonium bromide (C14TAB), the cosurfactant n-decanol, and water. Though containing similar surfactants, the gelled NC phases of the binary and ternary systems show differences in their visual and gel properties. The gelled NC phase of the binary system remains clear for several days after preparation, whereas the gelled NC phase of the ternary system turns turbid within 24 h. We investigated the time evolution of the gel strength with oscillation rheology measurements (a) within the first 24 h and (b) up to two weeks after gel formation. The shape of the fibers was investigated over different time scales with freeze fracture electron microscopy (FFEM). We demonstrate that despite their similarities, the two LLC gels also have distinct differences.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Light-modulation of gel stiffness: a glyconucleoside based bolaamphiphile as a photo-cleavable low molecular weight gelator

Photo-cleavable glyconucleoside bolaamphiphiles containing a nitrophenyl unit feature gelation abilities in aqueous media. The stiffness of the resulting gels can be modulated upon light irradiation thanks to the photocleavage reaction of nitrophenyl moieties.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

A crown-ether-based moldable supramolecular gel with unusual mechanical properties and controllable electrical conductivity prepared by cation-mediated cross-linking

Supramolecular gels that possess high mechanical properties and unusual electrical conductivity were prepared by incorporating Cs⁺.

Simultaneous Measurement of the Dissolution Kinetics of Responsive DNA Hydrogels at Multiple Length Scales

Recent years have seen increasing study of stimulus-responsive hydrogels constructed from aptamer-connected DNA building blocks. Presumably due to a lack of simple, quantitative tools with which to measure gel responsiveness, however, the literature describing these materials is largely qualitative. In response, we demonstrate here simple, time-resolved, multiscale methods for measuring the response kinetics of these materials. Specifically, by employing trace amounts of fluorophore-quencher labeled cross-linkers and the rheology of entrapped fluorescent particles, we simultaneously measure dissolution at molecular, hundred-nanometer, and hundred-micron length-scales. For our test-bed system, an adenine-responsive hydrogel, we find biphasic response kinetics dependent on both effector concentration and depth within the gel and a dissolution pattern uniform at scales longer than a few times the monomer-monomer distance. Likewise, we find that, in agreement with theoretical predictions, dissolution kinetics over the hundred nanometer length scale exhibit a power-law-like dependence on the fraction of disrupted cross-links before a distinct crossover from solid-like to liquid-like behavior.

Hydrogels of Superlong Helices to Synthesize Hybrid Ag-Helical Nanomaterials

The gelation behavior of mixtures of sodium deoxycholate (NaDC) and glutathione (GSH) in water is investigated. The system exhibits a structural transition of self-assembled hydrogels from nanofibers to nanohelix structures, and then to helical ribbons with increasing GSH concentration. Superlong helical nanofibers with left- and right-handed orientations are produced by tuning the concentration of GSH at a fixed concentration of NaDC. Random coil and β-sheet structures are significant for the formation of the helical structures, and are indicated by circular dichroism (CD) and Fourier transform infrared (FT-IR) spectra. The mechanical strength of the "weak" hydrogels is enhanced by the introduction of appropriate suitable amount of AgNO₃. Furthermore, the controlled growth of Ag nanoparticles at spatially arranged locations along the nanohelices (hybrid Ag-helical nanomaterial) is readily achieved by UV reduction of Ag (I) ions on the supramolecular helical templates.

Aggregation Behavior of 6-Isocassine andN-Methyl-6-Isocassine: Insights into the Biological Mode of Action of Lipid Alkaloids

The aggregation behavior of 6-isocassine and N-methyl-6-isocassine, two piperidin-3-ol alkaloids isolated respectively from the barks of Prosopis nigra and P. affinis, was investigated using a combination of NOE experiments and diffusion measurements in solvents of varying polarity and hydrogen bonding capacity. While the NOE enhancements for N-methyl-6-isocassine are positive, regardless of the solvent, those for 6-isocassine shift from negative to positive when going from chloroform- d to methanol- d4solution. In addition, despite the self-diffusion coefficients of both compounds being virtually identical in methanol- d4, N-methyl-6-isocassine diffuses nearly twice as fast as the non-methylated alkaloid in chloroform- d. The changes in rotational and translational dynamics observed between solvents for 6-isocassine suggest that the molecule forms dimeric head-to-head aggregates in non-polar aprotic environments, a behavior that could help explain the biological mode of action that has been proposed for this type of alkaloids.

A low molecular weight hydrogel with unusual gel aging

We describe a dipeptide hydrogel with unusual aging characteristics. Over time, a transformation from a turbid gel to a transparent gel occurs which is initiated from the air-water interface. Here, we investigate this transition and discuss the implications of this aging on the bulk properties of the gel.

Small angle neutron scattering (SANS) studies on the structural evolution of pyromellitamide self-assembled gels

The kinetics of aggregation of two pyromellitamide gelators, tetrabutyl- (C4) and tetrahexyl-pyromellitamide (C6), in deuterated cyclohexane has been investigated by small angle neutron scattering (SANS) for up to 6 days. The purpose of this study was to improve our understanding of how self-assembled gels are formed. Short-term (< 3 h) time scales revealed multiple phases with the data for the tetrabutylpyromellitamide C4, indicating one-dimensional stacking and aggregation corresponding to a multifiber braided cluster arrangement that is about 35 Å in diameter. The corresponding tetrahexylpyromellitamide C6 data suggest that the C6 also forms one-dimensional stacks but that these aggregate to a thicker multifiber braided cluster that has a diameter of about 62 Å. Over a longer period of time, the radius, persistence length, and contour length all continue to increase in 6 days after cooling. These data suggest that structural changes in self-assembled gels occur over a period exceeding several days and that fairly subtle changes in the structure (e.g., tail-length) can influence the packing of molecules in self-assembled gels on the single-to-few fiber bundle stage.

Structural insight into the aggregation of L-prolyl dipeptides and its effect on organocatalytic performance

NMR and organocatalytic studies of four dipeptides derived from L-proline are described. Results indicate that important conformational changes around the catalytic L-proline moiety are observed for free dipeptides upon changing the adjacent amino acid. Also, an aggregation process is detected as the concentration increases. The self-association of the dipeptides has been fitted to a cooperative binding model. All the compounds have been assayed as catalysts for the conjugated addition of cyclohexanone to trans-β-nitrostyrene in toluene. In agreement with the structural studies, noticeable changes in the catalytic performance are detected upon changing the catalyst concentration, as the catalyst is activated by self-aggregation.

Self-assembled structures of amphiphiles regulated via implanting external stimuli

This review article has summarized recent achievements of manipulating amphiphilic molecules and their self-assembled structures via different external stimuli.

Salt-induced control of supramolecular order in biocatalytic hydrogelation

Biocatalytic action and specific ion effects are both known to have dramatic effects on molecular self-assembly and hydrogelation. In this paper, we demonstrate that these effects are highly cooperative. Biocatalytic hydrogelation of Fmoc peptides in the presence of salts combines kinetic (through enzymatic catalysis) and thermodynamic (specific ion and protein templating) contributions when applied in combination. Spectroscopic data (obtained by fluorescence spectroscopy and circular dichroism) revealed that hydrophobic interactions are greatly affected, giving rise to differential chiral organization and supramolecular structure formation. The kinetic effects of catalytic action could be removed from the system by applying a heat/cool cycle, giving insight into the thermodynamic influence of both protein and salt on these systems and showing that the effects of catalysis, templating, and salts are cooperative. The variable molecular interactions are expressed as variable material properties, such as thermal stability and mechanical strength of the final gel-phase material. To gain more insight into the role of the enzyme, beyond catalysis, in the underlying mechanism, static light scattering is performed, which indicates the different mode of aggregation of the enzyme molecules in the presence of different salts in aqueous solution that may play a role to direct the assembly via templating. Overall, the results show that the combination of specific salts and enzymatic hydrogelation can give rise to complex self-assembly behaviors that may be exploited to tune hydrogel properties.

Vibrational Circular Dichroism Shows Reversible Helical Handedness Switching in Peptidomimetic l-Valine Fibrils

We elucidate the supramolecular organization in the form of microsize fibrils of gels formed by a l-Valine peptidomimetic compound. Analysis was based on circular dichroism spectroscopies, vibrational (VCD) and electronic (CD), supported by microscopy (atomic force and scanning electron). We show how the VCD spectra give account of the micrometric structure of the fibrils formed by the helicoidal arrangement of simpler proto-fibrils, which are organized in a lower hierarchical level. This ability is used to monitorize a fully reversible change in the handedness of the helix by modulating different external stimuli as pH or ionic strength, thus providing the first observation by VCD of such a phenomenon in a short peptide.

Molecular hydrogels from bolaform amino acid derivatives: a structure-properties study based on the thermodynamics of gel solubilization

Insight is provided into the aggregation thermodynamics associated to hydrogel formation by molecular gelators derived from L-valine and L-isoleucine. Solubility data from NMR measurements are used to extract thermodynamic parameters for the aggregation in water. It is concluded that at room temperature and up to 55 °C, these systems form self-assembled fibrillar networks in water with quite low or zero enthalpic component, whereas the entropy of the aggregation is favorable. These results are explained by considering that the hydrophobic effect is dominant in the self-assembly. However, studies by NMR and IR spectroscopy reveal that intermolecular hydrogen bonding is also a key issue in the aggregation process of these molecules in water. The low enthalpy values measured for the self-assembly process are ascribed to the result of a compensation of the favorable intermolecular hydrogen-bond formation and the unfavorable enthalpy component of the hydrophobic effect. Additionally, it is shown that by using the hydrophobic character as a design parameter, enthalpy-controlled hydrogel formation, as opposed to entropy-controlled hydrogel formation, can be achieved in water if the gelator is polar enough. It is noteworthy that these two types of hydrogels, enthalpy-versus entropy-driven hydrogels, present quite different response to temperature changes in properties such as the minimum gelator concentration (mgc) or the rheological moduli. Finally, the presence of a polymorphic transition in a hydrogel upon heating above 70 °C is reported and ascribed to the weakening of the hydrophobic effect upon heating. The new soft polymorphic materials present dramatically different solubility and rheological properties. Altogether these results are aimed to contribute to the rational design of molecular hydrogelators, which could be used for the tailored preparation of this type of soft materials. The reported results could also provide ground for the rationale of different self-assembly processes in aqueous media.

Supramolecular catalysis with extended aggregates and gels: inversion of stereoselectivity caused by self-assembly

L-Proline-L-valine dipeptide derivatives, which self-assemble in toluene, have been studied as stereoselective catalysts in the conjugate addition of cyclohexanone to trans-beta-nitrostyrene. Remarkable effects on the stereoselectivity are observed associated to the aggregation of the catalyst. Outstanding differences were observed between the catalytic activity of compound 1, which forms supramolecular gels in toluene, and compound 2, which is not a gelator. In the former case, the enantioselectivity of the reaction was almost insensitive to changes in catalyst concentration and temperature, but in the case of compound 2, the catalytic activity was very much affected by those variables. Structural studies indicate that the results can be rationalized by taking into account significant conformational changes experienced by the catalytic L-proline derivatives associated with the aggregation process. The results highlight that catalyst self-assembly is a very important issue to consider in the stereoselective outcome of organocatalytic reactions. Especially relevant is the fact that the use of supramolecular gels as organocatalysts emerges as a technique that affords reliable and constant stereoselectivity in different conditions with the added value of easy catalyst recovery.

Switchable performance of an L-proline-derived basic catalyst controlled by supramolecular gelation

An L-proline-derived low molecular weight gelator forms gels in nitromethane and nitroethane and acts as a basic catalyst for the Henry nitroaldol reaction of these solvents with 4-nitrobenzaldehyde and 4-chlorobenzaldehyde. The reported catalyst is efficient only upon aggregation into self-assembled fibrillar networks. The formation of the gels is associated to a basicity boost of the L-proline residues. Gel dissociation blocks the catalytic efficiency for the nitroaldol reaction but enhances a reaction pathway leading to alkenes. Because of the reversible nature of supramolecular gels, subtle temperature changes allow for a reversible sol-gel transition associated to an activation of the catalyst. The catalytic gel from nitroethane is significantly more active than the one from nitromethane probably because of its different structure as revealed by X-ray diffraction and thermal stability studies. The results shown indicate that in solution the L-proline moiety catalyzes the reaction of nitroalkanes with aldehydes via iminium intermediates while efficient nitroaldol reactions are promoted in the gel phase through an ionic pair type mechanism. The fact that upon aggregation the amino acid-based molecule used as gelator plays both a structural (gel formation) and catalytic role is interesting for the point of view of life origin studies.

A supramolecular hydrogel as a reusable heterogeneous catalyst for the direct aldol reaction

An L-proline based supramolecular hydrogel is used as an efficient heterogeneous organocatalyst for the direct aldol reaction with high stereoselectivity (up to 90% ee) and recyclability (up to 3 runs). The reversible nature of this self-assembled supramolecular system allows for easy recovery and regeneration of the catalyst.

Pyromellitamide gelators: exponential rate of aggregation, hierarchical assembly, and their viscoelastic response to anions

The gelation and aggregation properties of a newly synthesized structurally simplified tetrahexyl pyromellitamide 2 have been studied and compared to the previously reported tetra(ethylhexanoate) pyromellitide 1, indicating that the ester groups in the latter significantly impede its aggregation. Morphology studies (AFM and TEM) on the aggregates formed by tetrahexyl pyromellitamide 2 in cyclohexane revealed highly uniform aggregates with different dimensions at different starting concentrations, suggesting that this molecule aggregates in a hierarchical fashion from a one-dimensional supramolecular polymer through hollow tubes or compressed helices to a network structure and then to a gel. This hypothesis is further supported by viscosity measurements that indicate a crossover point where individual supramolecular fibers get entangled at concentrations above ca. 3 mM in cyclohexane. Addition of 1 equiv of tetraalkylammonium salts of chloride or bromide, however, caused the viscosities of these pyromellitamide solutions to drop by a factor of 2-3 orders of magnitude, demonstrating the sensitivity of these aggregates to the presence of small anions. The sensitivity to anions does depend on the solubility of the salts used as small anion salts with little solubility in cyclohexane did not show this effect. Time-dependent viscosity studies showed that the aggregation of pyromellitamide 2 follows an exponential rate law, possibly related to the columnar rearrangements that are associated with the observed 6 angstroms contraction in d spacing in the XRD pattern of these gels. These results, particularly on the importance of kinetics of aggregation of self-assembled pyromellitamide gels, will be useful for future development of related materials for a number of applications, including tissue engineering and drug delivery.