A small-angle neutron scattering study of cholic acid-based organogel systems

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.

Protonation and deprotonation induced organo/hydrogelation: Bile acid derived gelators containing a basic side chain

Two bile acid derived molecules containing basic amino groups are reported to be efficient and unusual gelators of organic and aqueous solvents.

Gelation or molecular recognition; is the bis-(α,β-dihydroxy ester)s motif an omnigelator?

Understanding the gelation of liquids by low molecular weight solutes at low concentrations gives an insight into many molecular recognition phenomena and also offers a simple route to modifying the physical properties of the liquid. Bis-(α,β-dihydroxy ester)s are shown here to gel thermoreversibly a wide range of solvents, raising interesting questions as to the mechanism of gelation. At gelator concentrations of 5-50 mg ml⁻¹, gels were successfully formed in acetone, ethanol/water mixtures, toluene, cyclohexane and chloroform (the latter, albeit at a higher gelator concentration). A range of neutron techniques - in particular small-angle neutron scattering (SANS) - have been employed to probe the structure of a selection of these gels. The universality of gelation in a range of solvent types suggests the gelation mechanism is a feature of the bis-(α,β-dihydroxy ester) motif, with SANS demonstrating the presence of regular structures in the 30-40 Å range. A correlation between the apparent rodlike character of the structures formed and the polarity of the solvent is evident. Preliminary spin-echo neutron scattering studies (SESANS) indicated the absence of any larger scale structures. Inelastic neutron spectroscopy (INS) studies demonstrated that the solvent is largely unaffected by gelation, but does reveal insights into the thermal history of the samples. Further neutron studies of this kind (particularly SESANS and INS) are warranted, and it is hoped that this work will stimulate others to pursue this line of research.

Self-assembling chiral gelators for fluorinated media

Formulations involving partially and fully fluorinated media represent a technological challenge given the lipophobic and hydrophobic nature of such liquids. The identification of self-associating materials with which to control the viscosity and solubilizing characteristics of fluorinated solvents is a particularly interesting area of research. It is shown here that the presence of the stereogenic centers inherent in a family of bis-(alpha,beta-dihydroxy ester)s is an essential requirement for the thermoreversible gelation of mixtures of partially fluorinated liquids 2H,3H-perfluoropentane (HPFP) and 1H,1H-heptafluorobutanol (HFB). Gelation is driven by hydrogen bonding, which induces a nonpreferred conformation around the bis-(alpha,beta-dihydroxy ester) structural motif. An analysis of the melting temperature yields an enthalpy of melting that is consistent with three to four hydrogen bonds, commensurate with the end-group structure of the gelator. Small-angle neutron scattering demonstrated the existence of the common fibrillar structures whose dimensions showed no obvious correlation with the molecular structure of the gelator.

Pentapeptide based organogels: the role of adjacently located phenylalanine residues in gel formation

A terminally protected self-assembling pentapeptide Boc-Leu(1)-Val(2)-Phe(3)-Phe(4)-Ala(5)-OMe 1 bearing sequence similarity with Aβ17-21 (the fragment 17-21 of the amyloid β-peptide Aβ42) forms thermoreversible transparent gels in various organic solvents including benzene, toluene, m-xylene and 1,2-dichlorobenzene. A series of its variants have been synthesized in order to study the role of adjacently located phenylalanine residues and the protecting groups for gelation in different organic solvents. Replacement of any of the Phe residues of the Phe-Phe segment with any other hydrophobic α-amino acid residue drastically changes the gel forming properties indicating that both Phe residues have an important role in gel formation. These gels are characterised using field emission scaning electron microscopy (FE-SEM), transmission electron microscopy (TEM), FT-IR and wide angle X-ray scattering (WAXS) studies. WAXS studies of the peptide 1-benzene gel indicate that π-π interaction is responsible for gel formation and it reveals the necessity of the Phe residues in gel formation. Transmission electron microscopy (TEM) of the gels reveals a nanofibrillar morphology, which is obtained from the self-assembled gelators in the gel phase. These gels bind with a physiological dye, Congo red, and show a green birefringence under cross polarizers, which is a characteristic feature of amyloid fibrils.

Simple esters of cholic acid as potent organogelators: direct imaging of the collapse of SAFINs

Unusual gelation of a number of organic solvents by allyl cholate and other simple esters of cholic acid is reported. These gels were characterized by SEM, AFM, polarizing optical microscopy (POM), X-ray powder diffraction and other techniques. Electron micrographs of the xerogels revealed the presence of highly entangled 3D fibrillar networks. Direct imaging of the collapse of SAFINs and their thermal stability were performed using variable temperature polarizing optical microscopy (POM).

Small-Molecule Dynamics and Mechanisms Underlying the Macroscopic Mechanical Properties of Coordinatively Cross-Linked Polymer Networks

Specific metal-ligand coordination between bis-Pd(II) and Pt(II) organometallic cross-linkers and poly(4-vinylpyridine) in DMSO defines a three-dimensional associative polymer network. Frequency-dependent dynamic mechanical moduli of a series of four different bulk materials, measured across several decades of oscillatory strain rates, are found to be quantitatively related through the pyridine exchange rates measured on model Pd(II) and Pt(II) complexes. Importantly, the mechanism of ligand exchange in the networks is found to be the same solvent-assisted pathway observed in the model complexes, and so the bulk mechanical properties are determined by relaxations that occur when the cross-links are dissociated from the polymer backbone. It is how often the cross-links dissociate, independently of how long they remain dissociated, that determines the bulk mechanical properties. The quantitative relationship between bulk materials properties and the kinetics and mechanisms observed in model compounds holds promise for the rational, molecular design of materials with tailored mechanical properties.