Richard G. Weiss

Organogels and Low Molecular Mass Organic Gelators

Nanostructured materials have many forms. One that has been somewhat neglected until recently is organogels, especially those whose three-dimensional structural networks are based on the self-assembly of low molecular-mass organic gelators (LMOGs). These thermoreversible materials consist of a small amount of LMOG and an organic liquid. Because of the wide diversity of structures of molecules known to function as LMOGs and the dearth of direct information concerning how they pack in their gel assemblies, it has been difficult to decipher the salient features that constitute an LMOG. The stepwise simplification of LMOG structures and the development of methods to determine their packing in organogels at the micrometer-to-angstrom distance regimes are discussed for the simplest known LMOGs to the more complex, such as CEP (see Figure), which is known to form molecular wires when gelling chloroform. Work from the laboratory of the authors is emphasized. In addition, an overview of current and potential applications for these materials is presented.

New Frontiers in Materials Science for Art Conservation: Responsive Gels and Beyond

The works of art and artifacts that constitute our cultural heritage are subject to deterioration, both from internal and from external factors. Surfaces that interact with the environment are the most prone to aging and decay; accordingly, soiling is a prime factor in the degradation of surfaces and the attendant disfigurement of a piece. Coatings that were originally intended to protect or contribute aesthetically to an artwork should be removed if they begin to have a destructive impact on its appearance or surface chemistry. Since the mid-19th century, organic solvents have been the method of choice for cleaning painted surfaces and removing degraded coatings. Care must be taken to choose a solvent mixture that minimizes swelling of or leaching from the original paint films, which would damage and compromise the physical integrity of all the layers of paint. The use of gels and poultices, first advocated in the 1980s, helps by localizing the solvent and, in some cases, by reducing solvent permeation into underlying paint layers. Unfortunately, it is not always easy to remove gels and their residues from a paint surface. In this Account, we address the removal problem by examining the properties of three classes of innovative gels for use on artwork--rheoreversible gels, magnetic gels, and peelable gels. Their rheological properties and efficacies for treating the surfaces of works have been studied, demonstrating uniquely useful characteristics in each class: (1) Rheoreversible gels become free-flowing on application of a chemical or thermal switch. For art conservation, a chemical trigger is preferred. Stable gels formed by bubbling CO(2) through solutions of polyallylamine or polyethylenimines (thereby producing ammonium carbamates, which act as chain cross-links) can be prepared with a wide range of solvent mixtures. After solubilization of varnish and dirt, addition of a weak acid (mineral or organic) displaces the CO(2), and the resulting free-flowing liquid can be removed gently. (2) Incorporation of magnetic, coated-ferrite nanoparticles into polyacrylamide gels adds functionality to a versatile system comprising oil-in-water microemulsions, aqueous micellar solutions, or xerogels that act as sponges. The ferrite particles allow the use of magnets both to place the gels precisely on a surface and to lift them from it after cleaning. (3) Novel formulations of poly(vinyl alcohol)-borate gels, which accept a range of organic cosolvents, show promise for swelling and dissolving organic coatings. This family of gels can be quite stiff but can be spread. They are non-sticky and have sufficient strength to be removed by peeling or lifting them from a sensitive surface. These three classes of gels are potentially very important soft materials to augment and improve the range of options available for conserving cultural heritage, and their interesting chemical-physical properties open a rich area for future scientific investigation.

Influence of Positional Isomers on the Macroscale and Nanoscale Architectures of Aggregates of Racemic Hydroxyoctadecanoic Acids in Their Molecular Gel, Dispersion, and Solid States

Inter/intramolecular hydrogen bonding of a series of hydroxystearic acids (HSAs) are investigated. Self-assembly of molecular gels obtained from these fatty acids with isomeric hydroxyl groups is influenced by the position of the secondary hydroxyl group. 2-Hydroxystearic acid (2HSA) does not form a molecular dimer, as indicated by FT-IR, and growth along the secondary axis is inhibited because the secondary hydroxyl group is unable to form intermolecular H-bonds. As well, the XRD long spacing is shorter than the dimer length of hydroxystearic acid. 3-Hydroxystearic acid (3HSA) forms an acyclic dimer, and the hydroxyl groups are unable to hydrogen bond, preventing the crystal structure from growing along the secondary axis. Finally, isomers 6HSA, 8HSA, 10HSA, 12HSA, and 14HSA have similar XRD and FT-IR patterns, suggesting that these molecules all self-assemble in a similar fashion. The monomers form a carboxylic cyclic dimer, and the secondary hydroxyl group promotes growth along the secondary axis.

Syntheses of cyclic carbonates with amidinium halide catalysts in reusable, reversible, room-temperature ionic liquids or acetonitrile

In situ-prepared, reversible, room-temperature ionic liquids (RTILs), amidinium carbamates, have been used as media for the syntheses of cyclic carbonates by cycloaddition of CO2 to epoxides. The amidinium carbamates were prepared by exposing equimolar mixtures of an easily synthesized amidine and a primary amine to CO2 gas. For comparison purposes, amidinium dithiocarbamates were also employed as the RTILs in some experiments. Reaction between CO2 and four epoxides in the RTILs, occurs in good yields (> 90% in many cases) at room temperature or 50 °C in the presence of an amidinium halide catalyst. Product and any unreacted epoxide were extracted easily upon addition of an immiscible, lower density liquid to an RTIL reaction mixture. This process was repeated three times with the same RTIL without any obvious decrease in catalytic activity; presumably, additional transformations could have been conducted. The influences of the type of catalyst, CO2 pressure, reaction time, and temperature on the reaction yields have been investigated. The relatively mild reaction conditions and ease of separation of products, as well as the ability of the amidinium carbamates to be reused in the presence or absence of water make this an attractive alternative to other procedures for the efficient syntheses of cyclic carbonates.

D-Sorbitol, a structurally simple, low molecular-mass gelator

A comprehensive examination of the ethanol gels produced with D-sorbitol, one of the simplest and lowest mass organogelator molecules known, is reported. Data from several spectroscopic and structural experiments reveal that the nature of the self-assembled fibrillar assemblies, spherulites for the most part, depends acutely on the manner in which the sol phase is treated. The critical concentration to form a gel at room temperature is ca. 1.5 wt% and a plateau value for melting the gels, ca. 50 °C, is reached near 3.5 wt%. Ostwald ripening, whose rate, again, depends on the history of the preparation, was also observed.

Cosolvent gel-like materials from partially hydrolyzed poly(vinyl acetate)s and borax.

A gel-like, high-viscosity polymeric dispersion (HVPD) based on cross-linked borate, partially hydrolyzed poly(vinyl acetate) (xPVAc, where x is the percent hydrolysis) is described. Unlike hydro-HVPDs prepared from poly(vinyl alcohol) (PVA) and borate, the liquid portion of these materials can be composed of up to 75% of an organic cosolvent because of the influence of residual acetate groups on the polymer backbone. The effects of the degree of hydrolysis, molecular weight, polymer and cross-linker concentrations, and type and amount of organic cosolvent on the rheological and structural properties of the materials are investigated. The stability of the systems is explored through rheological and melting-range studies. (11)B NMR and small-angle neutron scattering (SANS) are used to probe the structure of the dispersions. The addition of an organic liquid to the xPVAc-borate HVPDs results in a drastic increase in the number of cross-linked borate species as well as the agglomeration of the polymer into bundles. These effects result in an increase in the relaxation time and thermal stability of the networks. The ability to make xPVAc-borate HVPDs with very large amounts of and rather different organic liquids, with very different rheological properties that can be controlled easily, opens new possibilities for applications of PVAc-based dispersions.

Tetraalkylphosphonium trihalides. Room temperature ionic liquids as halogenation reagents.

Six room temperature ionic liquids (RTILs) comprised of a tetraalkylphosphonium cation (tridecylmethyphosphonium or trihexyltetradecylphosphonium) and a trihalide anion (Br(3)(-), BrCl(2)(-), or ClBr(2)(-)) have been prepared and characterized. Their ability to effect halogenation reactions with a variety of substrates has been explored. In general, halogenation reactions of alkenes proceed with high yields and stereo- and regioselectivities, whether performed in the absence or presence of a solvent (chloroform). Reactions of an alkyne and electrophilic substitution on an aromatic ring have been investigated as well. The facile preparation of the salts, their ease of handling, and the simplicity of product isolation should make these RTILs useful additions to the repertoire of halogenation reagents and the reagents of choice for specific transformations.

Stereoselective bromination reactions using tridecylmethylphosphonium tribromide in a "stacked" reactor.

A new reagent, tridecylmethylphosphonium tribromide, and new procedures for bromination reactions of unsaturated substrates (including one that allows several substrates to be reacted in sequence) are described. The procedures exploit the diffusion of components and the densities and immiscibilities of layers, including a fluorous spacer layer, within a reaction vessel. The stereoselectivities achieved in the reactions are superior in some cases to those found with other brominating reagents.

Anthraquinone–steroid based gelators of alcohols and alkanes

Gelation depends on key structural features of the gelator but not on specific gelator–liquid interactions other than those related to solubility.

Time-resolved C2 swan emission from short-pulse UV fragmentation of CO: evidence for two C2 formation mechanisms

Abstract We have studied C 2 Swan (d 3 Π → a 3 Π u ) emission resulting from multiphoton UV excitation of CO. Population of d 3 Π proceeds through distinct early and late processes, the former giving rise only to normal Swan emission. The late process is responsible for v = 6 enhancement (high-pressure bands), and it dominates time-averaged emission in an bands for ⪖ 10 Torr of CO.