Properties of Hydrogels Derived from Cationic Analogues of Bile Acid:  Remarkably Distinct Flowing Characteristics

Abstract

A new class of cationic bile acid derivatives was synthesized to gel water and aqueous electrolyte solutions. Investigations on these hydrogels are carried out at different length scales by a combination of physical techniques. Each of these hydrogels exhibits unique characteristics, thus providing a spectrum of thermal, optical, and mechanical properties. X-ray crystallographic investigation of the single crystals of two of the gelators shows significant differences in the solid-state packing. X-ray scattering experiments indicate that the gel state consists of a different morph than that in the solid. Electron microscopic investigations of the xerogels reveal the fibrous nature of the gel structure. These fibers are associated mainly through bundling processes. A detailed rheological study reveals significant differences in the mechanical properties of the three hydrogels. The storage modulus varies in the range (0.2−2) × 105 Pa at C = 2 wt % for these systems. The exponents of the scaling of the rheological parameters with the concentration for two of the systems agree well with those expected for cellular solids or strongly interacting colloidal gels. A third system exhibits a singular behavior with the energy of interaction between the colloidal flocs increasing with the concentration.

Bile acid alkylamide derivatives as low molecular weight organogelators: Systematic gelation studies and qualitative structural analysis of the systems

“…The presence of fibers from molecular scale to micrometer scale and their entanglement into a 3D network have been reported for physical gels of bile acid derivatives, and is generally accounted for their gelation ability [22][23][24][26][27][28]31,[34][35][36][37][38]. Our observations strengthen this well-established fact.…”

“…While there have been reports about lithocholic acid (LCA) derivative-based gels in the literature [23,24,92], a larger portion of research has been directed towards derivatives of deoxycholic acid (DCA) and cholic acid (CA) [21,22,[25][26][27]31,[34][35][36][37][38]. A common opinion about LCA derivatives (with only a single OH group attached to the steroidal skeleton) as gelators is that their ability to form the necessary hydrogen-bonding network to induce gelation is poor compared to DCA, CDCA (chenodeoxycholic acid), or CA derivatives, which contain two or three OH-groups in their framework [24,27].…”

“…Although further research on the subject was slow to catch on [10], by now, the potential of bile acid derivatives to act as low molecular weight gelators has been well established [19,20]. Several bile acid/salt-derived organo-and hydrogelators have been reported in the literature, having structural features such as alkylamide-phenylurea [21], amino acid alkyl ester [22,23], amidoalcohol [24,25], ester [26], alkyl amide [27][28][29], urea [27], aromatic rings [30], and cationic salts [25,31].…”

“…Examples of reported structures in solution-, sol-, gel-, or solid-state include various types of micelles [32,33], ribbon-or rod-like fibers [34][35][36][37][38], twisted ribbons [39], vesicles or spherical aggregates [29,40], lamellae [41], and tubules or nanotubes [42][43][44]. In general, physical gels of bile acids consist of one dimensional (1D) structures, such as various kinds of fibers of twisted, flat, or cylindrical form [22][23][24][26][27][28]31,[34][35][36][37][38]. The entanglement or junctioning of the 1D nano-or micro-structures provides the network that eventually immobilizes the solvent [10,11a].…”

Anion-tuning of supramolecular gel properties

“…173,180,182,183 The anion tuning of the gel characteristics was accomplished by changing the identity of the anion by changing the acid used to adjust the pH of the water solutions. Gel formation was changed to crystallisation by the use of chloride or nitrate resulting in the ability to precipitate a gel by the addition of salts containing these anions.…”