Chemically Reversible Organogels: Aliphatic Amines as “Latent” Gelators with Carbon Dioxide

Chemically Reversible Organogels:  Aliphatic Amines as “Latent” Gelators with Carbon Dioxid

Low Molecular-Mass Gelators with Diyne Functional Groups and Their Unpolymerized and Polymerized Gel Assemblies

A series of low molecular-mass organogelators (LMOGs) with conjugated diyne units, R−C⋮CC⋮C−R‘, has been synthesized from 10,12-pentacosadiynoic acid. R is a long alkyl chain and R‘ is a short or long alkyl chain containing an amide or ester group. The gelation efficiencies of these LMOGs and the parent acid (as assessed by the variety of liquids gelled, the amount of gelator needed for gelation, and the temporal and thermal stabilities of the gels) differ widely according to the nature of the substituents. An LMOG with an amide substituent is much more efficient than the corresponding molecule with an ester group, and LMOGs with longer R‘ chains are more efficient than those with shorter ones. When irradiated, some gel networks polymerize. In most cases, the polymerized aggregates phase-separate microscopically, but maintain the gel structure macroscopically. These gels are irreversibly photo- and thermo-chromic, and the thermal stabilities of some of the colored polymerized organogel networks are similar to those of the monomeric assemblies. The molecular packing of the LMOGs as neat powders and in gels before and after polymerization has been examined by X-ray diffraction techniques. This and analyses of IR, UV, and CD (in the case of a chiral diyne LMOG) data allow the nature of the aggregate assemblies before and after irradiation to be assessed. These monomeric organogels and their treatment with light and heat afford an approach to the synthesis of microheterogeneous polymerized networks from relatively simple molecules

Chemically Reversible Organogels via “Latent” Gelators. Aliphatic Amines with Carbon Dioxide and Their Ammonium Carbamates^†

Rapid and isothermal (at room temperature) uptake of CO2 by solutions or, in some cases, organogels comprised of a primary or secondary aliphatic amine (1) and an organic liquid leads to in situ chemical transformation to the corresponding alkylammonium alkylcarbamate (2) based gels. Chemical reversibility is demonstrated by removal of CO2 from 2-based gels upon gentle heating in the presence of nitrogen. This is a general strategy for reversible self-assembly or disassembly of molecular aggregates relying on the initiation or termination of ionic interactions. The dependence of the amine structure and the nature of the liquid component on the formation and stability of the 1 and 2 organogels are examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. In most cases, the 2 gelators are more effective (based on the minimum gelator concentration required at room temperature, the gelation temperature, and the duration of time a gel persists without bulk phase separation) and more diverse (based on the classes of liquids gelled) than their corresponding amines. The differences are attributed to the presence of ionic interactions between molecular segments of the alkylammonium alkylcarbamates that are stronger than the hydrogen-bonding interactions available between molecules of amines. The initial stages of aggregation in the gel assemblies (i.e., changes in the degree of aggregation of sols of some 2 gelators) have been examined as a function of concentration and temperature by NMR techniques

Primary Alkyl Amines as Latent Gelators and Their Organogel Adducts with Neutral Triatomic Molecules^†

A series of organogelator salts has been prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of a neutral triatomic molecule, CO2, NO2, SO2, or CS2. The organogels have been examined by differential scanning calorimetry, optical microscopy, and X-ray diffraction methods. The efficiency of each gelator has been assessed on the bases of the diversity of liquids it gelled, the minimum amount of it required for gelation, and the temporal and thermal stabilities of its gels. Thus, alkylammonium alkylcarbamates, amine−CO2 adducts, are the most effective gelators and the amine−NO2 adducts are the least efficient. Salts from longer n-alkylamines are better gelators than those from shorter homologues. Some of the salts are reconverted to their amine and triatomic constituents by heating, while others are transformed into new compounds. In the case of the CS2 adducts, H2S is expelled and the new species formed, N,N‘-dialkylthioureas, are also gelators

Detection of Pre-Sol Aggregation and Carbon Dioxide Scrambling in Alkylammonium Alkylcarbamate Gelators by Nuclear Magnetic Resonance^†

The initial stages of aggregation of a series of organogelator salts, prepared from n-alkylamines by the rapid in situ and isothermal (at room temperature) uptake of the neutral triatomic molecule, CO2, have been probed by NMR spectroscopy in the nongelled liquid, chloroform-d. Evidence for specific interactions of the ionic headgroups in the aggregates is presented. The influences of concentration and temperature on the processes leading to pre-sol aggregates of decylammonium decylcarbamate (2b) have been investigated in detail. NMR spectra of selectively deuterated (at the α-methylene group) and selectively 13C-enriched (at the carbonyl carbon) 2b demonstrate that CO2 is scrambled rapidly between the ammonium and carbamate parts of the molecule in chloroform solution. No scrambling of CS2 was detected in alkylammonium alkyldithiocarbamates under the same experimental conditions

Organogels with Complexes of Ions and Phosphorus-Containing Amphiphiles as Gelators. Spontaneous Gelation by in Situ Complexation

The properties of thermally reversible organogels that are formed spontaneously upon mixing a phosphonic acid monoester, monophosphonic acid, or bisphosphonate ester, each containing a long alkyl chain substituent, with one of several compounds of aluminum(III) and boron(III) in an organic liquid were studied by IR, NMR, optical microscopy, X-ray diffraction, and rheological techniques. Attempts to form gels with zirconium(IV) were unsuccessful. Gelation occurred at room temperature upon complexation, leading to the formation of entangled networks of elongated objects similar to giant, worm-like micelles. On the basis of the diversity of the liquids gelated, the minimum concentration of gelator required to make a gel at room temperature (typically <5 wt %), and the temporal and thermal stabilities of the gels, Al complexes of phosphonic acid monoesters were found to be better gelators than bisphosphonate complexes. Several of the gels formed from the monophosphonate−Al complexes were stable for very long periods when they were kept in sealed tubes at room temperature. When heated, they reverted to sols over wide temperature ranges. The nature of the gels and the complexes from which they were formed were correlated, especially for those with the phosphonic acid monoester. The results describe an interesting class of two-component gelators that can be made from freely flowing solutions by mixing the components at room temperature, without the need for a catalyst, radiation, or sonication. The properties of the gels can be modulated by careful choice of the structural variables in the phosphorus-containing latent gelators

Photochemical Phase Transition in Hydrogen-Bonded Liquid Crystals

Photochemical Phase Transition in Hydrogen-Bonded Liquid Crystal

Organogels with Fe(III) Complexes of Phosphorus-Containing Amphiphiles as Two-Component Isothermal Gelators

The properties of thermally reversible organogels in which the gelators consist of a phosphonic acid monoester, phosphonic acid, or phosphoric acid monoester and a ferric salt are probed by IR and NMR spectroscopies, optical microscopy, X-ray diffraction, rheology, and light and small-angle neutron scattering (SANS) techniques. This is one of a small number of two-component molecular gelator systems in which gelation can be induced isothermally. The data indicate that complexation between the phosphonate moieties and Fe(III) is accompanied by their in situ polymerization to form self-assembled fibrillar networks that encapsulate and immobilize macroscopically the organic liquid component. From SANS measurements, the cross-sectional radii of the cyclindrical fibers are ca. 15 Å. The efficiencies of the gelators (based on the diversity of the liquids gelated, the minimum concentration of gelator required to make a gel at room temperature, and the temporal and thermal stabilities of the gels) have been determined. With a common ferric salt and liquid component, phosphonic acid monoesters are generally more efficient than phosphinic acids or phosphoric acid esters. Of the phosphonic acid monoesters, monophosphonates are better gelator components than bisphosphonates, and introduction of an ω-hydroxy group on the alkyl chain directly attached to phosphorus reduces significantly gelation ability. Several of the gels are stable for very long periods at room temperature. When heated, they revert to sols over wide temperature ranges. The structures of the gelator complexes and the mechanism of their formation and transformation to gels in selected liquids are examined as well

Robust Organogels from Nitrogen-Containing Derivatives of (<i>R</i>)-12-Hydroxystearic Acid as Gelators: Comparisons with Gels from Stearic Acid Derivatives^†

Thirteen members of a new class of low molecular-mass organogelators (LMOGs), amides, and amines based on (R)-12-hydroxystearic acid (HSA; i.e., (R)-12-hydroxyoctadecanoic acid) and the properties of their gels have been investigated by a variety of structural and thermal techniques. The abilities of these LMOGs, molecules with primary and secondary amide and amine groups and the ammonium carbamate salt of 1-aminooctadecan-12-ol, to gelate a wide range of organic liquids have been ascertained. Their gelating efficiencies are compared with those of HSA and the corresponding nitrogen-containing molecules derived from stearic acid (i.e., HSA that lacks a 12-hydroxyl group). Several of the HSA-derived molecules are exceedingly efficient LMOGs, with much less than 1 wt % being necessary to gelate several organic liquids at room temperature. Generally, the self-assembled fibrillar networks of the gels consist of spherulitic objects whose dimensions depend on the protocol employed to cool the precursor sol phases. X-ray studies indicate that the LMOG molecules are packed in lamellae within the fibers that constitute the spherulites. In addition, some of the organogels exhibit unusual thixotropic properties: they recover a large part of their viscoelasticity within seconds of being destroyed by excessive strain shearing. This recovery is at least an order of magnitude faster than for any other organogel with a crystalline fibrillar network reported to date. Correlations of these LMOG structures (as well as with those that lack a hydroxyl group along the n-alkyl chain, a headgroup at its end, or both) with the properties of their gels, coupled with the unusual rheological properties of these systems, point to new directions for designing LMOGs and organogels

Reversible, Room-Temperature Ionic Liquids. Amidinium Carbamates Derived from Amidines and Aliphatic Primary Amines with Carbon Dioxide

Reversible, Room-Temperature Ionic Liquids. Amidinium Carbamates Derived from Amidines and Aliphatic Primary Amines with Carbon Dioxid