From Nonfunctional Lamellae to Functional Nanotubes

Designing Heat-Set Gels for Crystallizing APIs at Different Temperatures: A Crystal Engineering Approach

An organic salt crystallizes through different kinds of charge-assisted hydrogen-bonded networks depending on carboxylic functionality number and the degree of amine. These H-bonded packing patterns are often robust and predictable, so one can design a supramolecular salt with a certain purpose. In some cases, two different crystalline packing patterns can be found in Primary Ammonium Dicarboxylate (PAD) salts at different temperatures. Two kinds of supramolecular bonding, namely, charge-assisted hydrogen bonding and weak van der Waals interactions stabilize the two states. A small increase in the carbon chain length in a primary amine enhances the additional van der Waals interactions with the packing so that the 2D hydrogen-bonded network (HBN) transforms into a 1D HBN at room temperature. Such van der Waals interactions can be controlled by external heat, so a temperature-dependent 1D to 2D phase change is feasible. When certain moieties, such as azo and bipyridine, are introduced into the carboxylic acid backbone, the acids become insoluble in most organic solvents, raising their melting point, and resulting in heat-set gels. In the presence of an API, temperature and solvent-dependent polymorphic crystals can be grown in the heat-set gel medium and by simply cooling down the mixture, the API crystals can be separated easily.

Supramolecular Gels Derived from Simple Organic Salts of Flufenamic Acid: Design, Synthesis, Structures, and Plausible Biomedical Application

Following supramolecular synthon rationale in the context of crystal engineering, a nonsteroidal-anti-inflammatory-drug (NSAID), namely flufenamic acid (FA) and its β-alanine monopeptide derivative (FM), were converted to a series of primary ammonium monocarboxylate (PAM) salts. Majority of the PAM salts (∼90%) showed gelation with various solvents including water and methyl salicylate (important solvents in topical gel formulation). Structure-property correlation studies based on single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) data provided intriguing insights into the structure of the gel network. Furthermore, one of the gelator salts (S7) displayed anticancer activity on a highly aggressive human breast cancer cell line (MDA-MB-231) ,as revealed by MTT, PEG₂, and cell migration assays.

Designing Supramolecular Gelators: Challenges, Frustrations, and Hopes

This article is a personal account of the author, who serendipitously entered the field of supramolecular gels nearly two decades ago. A supramolecular synthon approach in the context of crystal engineering was utilized to develop a working hypothesis to design supramolecular gelators derived from simple organic salts. The activity not only provided a way to occasionally predict gelation, but also afforded clear understanding of the structural landscape of such supramolecular materials. Without waiting for an ab initio approach for designing a gel, a large number of supramolecular gelators derived from organic salts were designed following the working hypothesis thus developed. Organic salts possess a number of advantages in terms of their ease of synthesis, purification, high yield and stability and, therefore, are suitable for developing materials for various applications. Organic salt-based gel materials for containing oil spills, synthesizing inorganic nanostructures and metal nanoparticles, sensing hazardous gas and dissolved glucose, adsorbing dyes, and facilitating drug delivery in self-delivery fashion have been developed. The journey through the soft world of gelators which was started merely by serendipity turned out to be rewarding, despite the challenges and frustrations in the field.

A correlation study between hydrogen-bonded network and gelation ability of three galactose derivatives

The hydrogen bonding network in the crystals of the three saccharides was correlated with their gelling ability or inability, and unexpectedly, a 2D hydrogen-bonded system was found to show efficient gelation, whereas a 1D hydrogen bonding system was a nongelator.

Chiral gels derived from secondary ammonium salts of (1R,3S)-(+)-camphoric acid

In order to have access to chiral gels, a series of salts derived from (1R,3S)-(+)-camphoric acid and various secondary amines were prepared based on supramolecular synthon rationale. Out of seven salts prepared, two showed moderate gelation abilities. The gels were characterized by differential scanning calorimetry, table top rheology, scanning electron microscopy, single crystal and powder X-ray diffraction. Structure property correlation based on X-ray diffraction techniques remain inconclusive indicating that some of the integrated part associated with the gelation phenomena requires a better understanding.

Combinatorial library of primaryalkylammonium dicarboxylate gelators: a supramolecular synthon approach

Following the supramolecular synthon approach, a combinatorial library comprising 35 organic salts derived from 7 dicarboxylic acids (malonic-, succinic-, adipic-, L-tartaric-, maleic-, phthalic-, and isophthalicacid) and 5 primaryalkyl amines Me-(CH2)n-NH2 (n = 11-15) was prepared and scanned for gelation. About 66% of the salts in the combinatorial library were found to show moderate to good gelling ability in various polar and nonpolar solvents including commercial fuels such as petrol. The majority of the salts having a rigid, unsaturated anionic backbone (maleate, phthalate, and isophthalate) did not show gelation; only the corresponding hexadecylammonium salts showed gelation. Some of the representative gels were characterized by rheology, small-angle neutron scattering (SANS), optical microscopy (OM), and scanning electron microscopy (SEM). Single-crystal structures of two gelator and two nongelator salts were also discussed in the context of supramolecular synthon and structure-property correlation.