Low molecular weight organogelators derived from threefold symmetric tricarbamates

A group of new low molecular weight organogelators based on threefold symmetric tricarbamate were synthesized and characterized. The tricarbamates with long alkyl chains were able to gelate a wide range of polar and nonpolar organic solvents such as acetonitrile and cyclohexane, generally at concentrations lower than 20 g/L. The best organogel formation was obtained using a threefold symmetric tricarbamate in n-dodecane, in which a sufficiently transparent gel was formed at the critical gelation concentration 1 g/L. Intermolecular hydrogen bonding by the tricarbamate in a nonpolar solvent benzene-d₆ was indicated by ¹H NMR spectra. Its maximum UV absorption was 11 nm higher in chloroform than in n-dodecane, and this red shift indicated increased conjugation between the benzene core and the carbamate substituents, which confirmed a change in its conformation from nonpolar to polar solvent. The self-assembling behavior of the tricarbamate in dilute solutions was investigated by TEM. Fiber-like networks were observed in a large range of solution concentrations.     

Low molecular weight analytes in water by spectral interferometry using a competitive immunoassay

The optical detection principle of reflectometric interference spectroscopy (RIFS) is applied for the first time to the immunological detection of low molecular weight analytes. Dinitrophenol/Anti-Dinitrophenol is used as a model system for pesticide detection. The spectrometric principle allows the sensitive determination of small changes in the thickness of a thin film caused by the reaction of an antigen and its antibody. Changes in the optical thickness correlate with the concentration of the analyte. Time resolved measurements allow the dynamic monitoring of the antigen-antibody-interaction. Detection limits currently achieved are in the ppb-range.     

Low molecular weight fluorescent organogel for fluoride ion detection

The design, synthesis, and the photophysical properties of a Low Molecular Weight Gel (LMWG) based on AB(3) and AB(2) type poly(aryl ether) dendrons with an anthracene chromophore attached through an acylhydrazone linkage are described. The gel is utilized for an efficient 'naked eye' detection of fluoride ions (as low as 0.1 equiv with respect to the gelator concentration), through a reversible gel-sol transition, which is associated with a color change from deep yellow to bright red.     

Low molecular weight organogels based on long-chain carbamates

Thermoreversible organogels were prepared from carbamates with alkyl side chains of different lengths. Gelation was possible only up to an alkyl side chain length of 12 carbons, beyond which crystallization occurs, due to the dominant van der Waals interaction between the alkyl chains. This is in contrast to other alkane-based organogels, in which gelating efficiency increased with the length of the alkane chain (see Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237). The critical concentration for gelation decreases drastically with an increase in the side chain length. Xerogels of these show birefringent fibers with uniform cross section and unlimited growth in one direction. The extent of this unlimited growth is affected by the length of the alkyl side chain in the carbamate, which finally ceases the gel formation ability of the carbamate. Cryogenic scanning electron microscopy images of the gels are similar to those of xerogels. From X-ray diffraction of the fibers, we propose that the growth direction is along the plane of hydrogen bonds between the carbamate molecules. The thickness of the fibers depends on the length of the alkyl side chain. Morphological differences are seen between gels prepared by slow cooling and quenching of the solution. Thus, the morphology of the fibrous xerogels of the carbamates can be tailored for specific applications, by the choice of the alkyl side chain length and the rate of cooling the solution.     

Glucofuranose derivatives as a library for designing and investigating low molecular mass organogelators

We designed and synthesized a class of saccharide-based gelators having three free OH groups in the glucofuranose fragment. The gelating abilities of fourteen compounds were examined to systematically study the influence of the hydrophobic fragment connected to the C2′ carbon. Also the correlation between the saccharide crystal structure and its gelating properties was examined, showing limited usefulness in this particular case. SEM observations were carried out in order to investigate the hierarchical structure of xerogels and changes depending on different gel concentration.     

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

A series of amino- and hydroxyalkyl amides of bile acids have been synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), (1)H and (13)C nuclear magnetic resonance spectroscopy (NMR), as well as electrospray ionization mass spectrometry (ESI-MS) measurements. The ability of the synthesized molecules to promote gel formation was systematically investigated. Out of 396 combinations formed by 11 compounds and 36 different solvents, 22 gel-containing systems were obtained with 1% (w/v) gelator concentration. Apart from one exception, the gelator compounds were lithocholic acid derivatives. This challenges the general trend of bile acid-based physical gelators, according to which the gelation ability of lithocholic acid derivatives is poor. A correlation between the values of Kamlet-Taft parameters and solvent preferences for gelators was observed. The morphologies of the solid and gel structures studied with scanning electron microscopy (SEM) showed variability from fibers to spherical microscale aggregates, the latter of which are unique among bile acid-based organogels. The gels exhibited more complex behavior than was previously established with bile acid derivatives, judging by the microscale diversity present in gelating and non-gelating systems and the tendency for polymorphism. This study underlines the importance of both the molecular and colloidal scale aspects of the gelation phenomenon.     

Low‐molecular‐weight organogelators as functional materials for oil spill remediation

Oil spills from tankers are one of the major types of man‐made disasters that impact the marine environment, and they have been shown to have long‐lasting effects. On prevention of the spread of oil through rapid cleanup of spills, low‐molecular‐weight organogelators have received much attention because of their ability to tune their properties through rational design. In this mini‐review, I present a brief summary of studies focused on the remediation of oil spills via a chemical method, which involves the use of low‐molecular‐weight organogelators that form organogels with fuel oils or organic solvents. Moreover, recent attempts to create new improved molecular organogels composed of commercially available simple organogelators via a mixing induced enhancement method for solidifying oil are also discussed. In addition, polymer organogelators for oil spills are discussed in relation to low‐molecular weight gelators. Copyright © 2015 John Wiley & Sons, Ltd.     

Low molecular weight alkane-fed solid oxide fuel cells for power and chemicals cogeneration

This paper presents a review of low molecular weight alkane-fed solid oxide fuel cells(SOFCs), which,unlikely the conventional use of SOFCs for only power production, are utilized to cogenerate produce useful chemicals at the same time. The cogeneration processes in SOFC have been classified according to the different types of fuel. C₂ and C₃ alkenes and synthesis gas are the main cogenerated chemicals together with electricity. The chemicals and energy cogeneration in a fu...     

Dipeptide-based low-molecular-weight efficient organogelators and their application in water purification

The development of new low-molecular-weight gelators for organic solvents is motivated by several potential applications of gels as advanced functional materials. In the present study, we developed simple dipeptide-based organogelators with a minimum gelation concentration (MGC) of 6-0.15 %, w/v in aromatic solvents. The organogelators were synthesized using different L-amino acids with nonpolar aliphatic/aromatic residues and by varying alkyl-chain length (C-12 to C-16). The self-aggregation behavior of these thermoreversible organogels was investigated through several spectroscopic and microscopic techniques. A balanced participation of the hydrogen bonding and van der Waals interactions is crucial for efficient organogelation, which can be largely modulated by the structural modification at the hydrogen-bonding unit as well as by varying the alkyl-chain length in both sides of the hydrophilic residue. Interestingly, these organogelators could selectively gelate aromatic solvents from their mixtures with water. Furthermore, the xerogels prepared from the organogels showed a striking property of adsorbing dyes such as crystal violet, rhodamine 6G from water. This dye-adsorption ability of gelators can be utilized in water purification by removing toxic dyes from wastewater.     

Thixotropic twin-dendritic organogelators

Twin-dendritic organogelators have been prepared through selective functionalization of N-(3-aminopropyl)-1,3-propanediamine (APPDA) with self-assembling dendrons by using 1,1'-carbonyldiimidazole (CDI). Subsequent modification of the APPDA linker provided an additional degree of structural diversity by which to tailor the gelator self-assembly in bulk or in the gel state. These compounds are able to gel cyclohexane, toluene, n-butyl acetate, ethyl acetate, dichloromethane, and tetrahydrofuran. 3,4-Disubstituted apical branching units provided the most efficient organogelators and show a propensity to form thixotropic gels, wherein the gel recovers its elasticity after being subjected to shear. Structural and retrostructural analysis of the twin-dendritic organogelators reveals the bulk structural characteristics to be indicative of the subsequent gel properties. Diverse self-organized arrays were identified in bulk and all are able to form gels, thus indicating the role of quasiequivalence in mediating self-assembly in the gel state. Furthermore, we have found that porous columnar mesophases provide a strategy by which to prepare thixotropic gels. We demonstrate the importance of weak lateral hydrogen bonding within a column stratum versus hydrogen bonding along the length of the column for forming porous columnar mesophases and, by extension, thixotropic gels.     

Low molecular weight by-products in the Ziegler polymerization of higher terminal olefins

For the polymerization of n-octadecene-1 with catalysts derived from titanium tetrachloride and triethylaluminum, it has been shown that, in addition to polymerization of the olefin, the formation of isomerized olefin occurs. The latter is predominantly trans-n-octadecene-2 and its formation is favored by increase in Al:Ti mole ratio, in catalyst concentration, and in reaction temperature. It has also been shown that 1,1-disubstituted ethylene is present in the nonpolymeric reaction products. The influence of added trans-n-octadecene-2 or trans-n-octadecene-7 on the polymerization of n-octadecene-1 has been studied, and it is shown that the 2-isomer has the more pronounced effect on polymer yield and intrinsic viscosity. It has also been shown that no significant copolymerization of terminal with nonterminal octenes or octadecenes occurs under these conditions. Results indicate that, in polymerizations of this kind, the interaction of catalyst with isomerized monomer is probably an important factor in determining polymer yield and molecular weight. The isomerization reaction is also of interest as a general preparative method for trans-2-olefins.     

Phosphorescent platinum acetylide organogelators

The series of platinum acetylide oligomers (PAOs) with the general structure trans,trans-[(RO)3Ph-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-(Ar)-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-Ph(OR)3], where Ar = 1,4-phenylene, 2,5-thienylene, or bis-2,5-(S-2-methylbutoxy)-1,4-phenylene and R = n-C12H25 gel hydrocarbon solvents at concentrations above 1 mM. Gelation is thermally reversible (T(gel-sol) approximately 40-50 degrees C), and it occurs due to aggregation of the PAOs resulting in the formation of a fibrous network that is observed for dried gels imaged by TEM. The influence of aggregation/gelation on the photophysical properties of the PAOs is explored in detail. Aggregation induces a significant blue shift in the oligomers' absorption spectra, and the shift is attributed to exciton interactions arising from H-aggregation of the chromophores. Strong circular dichroism (CD) is observed for gelled solutions of a PAO substituted with homochiral S-2-methylbutoxy side chains on the central phenylene unit. The CD is attributed to formation of a chiral supramolecular aggregate structure. The PAOs are phosphorescent at ambient temperature in solution and in the aggregate/gel state. The phosphorescence band is blue-shifted ca. 20 nm in the aggregate/gel, and the shift is assigned to emission from an unrelaxed conformation of the triplet excited state. Phosphorescence spectroscopy of mixed aggregate/gels consisting of a triplet donor/host oligomer (Ar = 1,4-phenylene) doped with low concentrations of an acceptor/trap oligomer (Ar = 2,5-thienylene) indicates that energy transfer occurs efficiently in the aggregates. Triplet energy transfer involves exciton diffusion among the host chromophores followed by Dexter exchange energy transfer to the trap chromophore.     

Low molecular weight heparin enhances prostacyclin production by cultured human endothelial cells.

Confluent cultures of vascular endothelial cells derived from the human umbilical vein were incubated in a serum-free medium in the presence of low molecular weight heparin (LMWH) with molecular weights of 4000-6000 dalton (Da), or of unfractionated heparin (UFH) with average molecular weight 12,000 Da, and prostacyclin production was determined by radioimmunoassay for 6-keto-prostaglandin F1 alpha, the stable metabolite of prostacyclin. LMWH at 1 U/ml as anti-factor Xa activity significantly increased prostacyclin production after 6h or longer; however, UFH at 1 USP U/ml did not induce such a significant change. The LMWH-induced increase in prostacyclin production occurred at 0.1 U/ml and above after 6 h of treatment. Since prostacyclin is both a potent inhibitor of platelet aggregation and a vasodilator, it was suggested that the increased endothelial cell prostacyclin production induced by LMWH may be a component of the anticoagulant activity of the drug.     

Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution

A detailed, mechanistic model for high-density polyethylene pyrolysis was created based on the modeling framework developed in our previous work and was used to study the time evolution of low molecular weight products formed. Specifically, the role that unzipping, backbiting, and random scission reaction pathways play in the evolution of low molecular weight species was probed. The model tracked 151 species and included over 11,000 reactions. Rate parameters were adapted from our previous work, literature values, and regression against experimental data. The model results were found to be in excellent agreement with experimental data for the evolution of condensable low molecular weight products. The time evolution curves of specific low molecular weight products indicated that the random scission pathway was important for all species, while the backbiting pathway played a complementary role. Net rate analysis was used to further elucidate the competition between the pathways. Net rate analysis of end-chain radicals showed that the unzipping pathway was not competitive with the other pathways, as expected based on experimental yields of ethylene. The random scission pathway was found to be controlling, with the backbiting pathway playing a more minor role for product formation. By comparing the net rates for formation of specific mid-chain radicals via intramolecular hydrogen shift reactions, the contribution of the backbiting pathway was shown to vary, with radicals formed via the most facile x,x + 4-intramolecular hydrogen transfer reactions being favored.     

Determination of molecular weight dependences and characterization of molecular weight distributions

Molecular weight dependences of the square of the radius of gyration as well as of the coefficients of diffusion and friction of monodisperse polystyrenes in toluene at 20°C are determined experimentally. For this purpose, the z-averages of these quantities and $\text{M}$_w as determined by light scattering were subjected to the polydispersity correction procedures as described previously. In addition, a method is suggested for the checking of known data with respect to molecular weight distribution, using measured values in conjunction with the polydispersity correction procedures.     

Novel deoxycholic acid alkylamide-phenylurea-derived organogelators

Three monomeric and three dimeric deoxycholic acid (DCA) alkylamido-phenylurea derivatives are designed based on known gelators and are synthesized and characterized by ¹H and ¹³C NMR spectroscopy, ESI-TOF mass spectrometry, and elemental analyses. Among them, a monomeric derivative forms supramolecular gels in CHCl₃ and chlorobenzene, whereas a dimeric derivative gels THF and higher 1-alkanols containing 7-10 carbons. The morphologies of their xerogels are studied by scanning electron microscopy (SEM). No signature of macroscopic chirality of the gels is visible.     

Superheated water chromatography of low molecular weight polyethylene glycols with ultraviolet detection.

Superheated water chromatography (SWC) with ultraviolet detection was applied to the separation of low molecular weight polyethylene glycols (PEGs). PEG oligomers could be detected sensitively when the detection wavelength was set at 190 nm. The effect of column temperature on the separation of PEG oligomers was investigated. The elution time of all PEG oligomers decreased with increase in the column temperature; linear relationships were obtained between ln k and 1/T. A temperature-programmed SWC separation enabled the baseline separation of a PEG 200 sample within 50 min.