Phase-selective sorbent xerogels as reclamation agents for oil spills

Solvent selective gelation of cetyltrimethylammonium bromide: structure, phase evolution and thermal characteristics

We report herein the gelation behaviour of cetyltrimethylammonium bromide (CTAB), a cationic surfactant, in a variety of solvent compositions. A turbid gel of CTAB in a binary solvent mixture at a critical composition was observed to be 1 : 3 v/v toluene : water. The molecular structure of the as-formed gel was investigated by X-ray diffraction and microscopic techniques, namely, optical and polarizing microscopy, scanning electron microscopy and small-angle X-ray scattering (SAXS). The phase evolution has been studied using UV-visible transmittance measurements and the thermal characteristics of the gel by differential scanning calorimetry measurements. SAXS studies, in conjunction with molecular modelling, revealed the gel to assemble as lamellae with high interdigitation of bilayer assembly of CTAB molecules with predominant non-covalent interactions, where the gel lamellae were inferred from the interplanar spacings. Rheological studies revealed the viscoelastic nature of the CTAB gels. The ability to form a gel has been evaluated in several polar solvents, such as methanol and chloroform, and non-polar solvents, such as toluene and carbon tetrachloride.

Sodium Alginate Aerogel as a Carrier of Organogelators for Effective Oil Spill Solidification and Recovery

Marine oil spills pose a serious threat to the marine ecological environment. Phase-selective organogelators (PSOGs) are ideal candidates for oil spill gelation when used in combination with a mechanical recovery method. However, the toxicity of an organic solvent carrier has become a key problem when it is applied in the remediation of marine oil pollution. In this study, through an inexpensive and nontoxic ionic cross-linking and freeze-drying method, we successfully developed composite oil gelling agents that used a biomass sodium alginate aerogel as the carrier of 12-hydroxystearic acid (12-HSA). Simultaneously, carboxylated cellulose nanofibers (CNF-C) with large specific surface area and graphene oxide (GO) with excellent mechanical properties as reinforcing fillers were combined with an alginate matrix. 12-HSA, as a green and inexpensive organic gelator, was uniformly loaded on the aerogels by vacuum impregnation. The sodium alginate aerogel was capable of absorbing and storing oil due to its three-dimensional network skeleton and high porosity. Rheological studies have demonstrated that the organic gelator 12-HSA can be released from the aerogel substrate and self-assemble to form an oleogel with the absorbed oil quickly. The synergistic effect between absorption and congelation endows the composite oil gelling agent with efficient oil spill recovery capability. Based on eco-friendly, biodegradable, and simple synthesis methods, this composite oil gelling agent shows great potential for application in marine oil spill recovery.

Aluminum soap nanoparticles-lignin powder form phase-selective gelator as an efficient sorbent for oils/water separation

Rapid and efficient recovery of oil spill is the key link for oil spill remediation, and also a great challenge. Here, the organogelator-polymerized porous matrix composed of adsorbents and organogelators can provide a new strategy for solving this problem. The gelling mechanism of aluminum 12-hydroxystearate (Al HSA) to form spherical nano micelles in solvents was investigated via UV-vis, FT-IR, and XRD. A creative method for aluminum soap-lignin gelator (OTS-AL/Al HSA) syntheses was put forward through the saponification of 12-hydroxystearic acid (HSA) and lignin via epichlorohydrin (ECH) crosslinking. By adjusting the ECH content, the growth of Al HSA nanoparticles (15-40 nm) on lignin can be realized, and the accordingly increased roughness endowed gelator with better hydrophobicity (WCA of 134.6°) before octadecyltrichlorosilane (OTS) modification. Thanks to the porous structures, the gelator powder exhibited a high sorption capacity in the range of 3.5-5.2 g g-1 for oils and organic solvents. Rheological studies demonstrated high mechanical strength of gels (>1.6 × 105 pa) and the gelator still retained 70% sorption capacity after 6 gelation-distillation cycles. The gelation characteristics of OTS-AL/Al HSA were attributed to the rapid sorption of oils by lignin and the self-assembly of Al HSA nano micelles on lignin to form an aggregated network structure trapping oils, thus realizing the synergistic effect of oil sorption-gelation.

Influence of solute association on the phase behavior of 12-hydroxystearic acid/n-alkane solutions

The phase behavior of 12-hydroxystearic acid (12-HSA) in even-numbered alkanes ranging from octane (C₈) to hexatriacontane (C₃₆) was measured by visual observation of liquid + solid to liquid and liquid-liquid to liquid cloud points and liquid + solid to liquid + liquid transitions. In general solid phases were stabilized to low concentration and higher temperature with increasing alkane length. Liquid-liquid immiscibility was observed in larger alkanes starting with octadecane. The liquidus lines of shorter alkanes (octane to hexadecane) showing only liquid to liquid + solid transitions were fit with an attenuated associated solution model based on the Flory-Huggins lattice model assuming that 12-HSA forms a carboxylic acid dimer over all concentrations investigated. The fit results show that 12-HSA forms associated structures with degrees of association ranging from 3.7-4.5 dimers in the neat 12-HSA. At low concentrations, the 12-HSA is dissociated into dimers, however the free energy cost of dissociation stabilizes the solid phase giving a sharp knee at low concentrations. The role of 12-HSA association in its phase behavior and gelation behavior are discussed. More broadly, the importance of solute association in small molecule organogelators and its potential as a molecular design parameter similar to other component thermodynamic parameters, such as melting temperature and heat of fusion, is discussed.

Carboxylate Group-Based Phase-Selective Organogelators with a pH-Triggered Recyclable Property

Phase-selective organogelators (PSOGs) have recently attracted more attention because of their advantages in handling oil spills and leaked organic solvents. However, it is difficult to separate and recover the organic phase and PSOGs from organic gels due to the strong interaction between them. Aiming to enhance the separation and recovery performance of the organic phase and PSOGs, we synthesized a series of pH-responsive PSOGs by using itaconic anhydride and fatty amines with carbon chain lengths of C₁₂-C₁₈. Here, PSOGs have an excellent gelation ability in that amounts of organic solvents and fuel oil can be solidified at a low concentration (<3 wt %). It is worth noting that these gels are stronger, which is more convenient for removal by a salvage operation. More importantly, compared with traditional organogelators, pH-responsive PSOGs can easily recover the organic phase and fuel oil with an adjustment of the pH without extraction or distillation. Because of the transformation between the hydrophilicity and hydrophobicity of PSOGs by pH stimulation, 83.15% PSOGs are recovered in three-cycle experiments. In addition, the recycled PSOGs can be used to realize the removal of the organic phase again. Herein, we find that pH-responsive PSOGs could be used as promising and sustainable materials for separating and recovering organic solvents/oils and PSOGs.

Preparation of hydrophobic macroinimer-based novel hybrid sorbents for efficient removal of organic liquids from wastewater

Herein, the synthesis of hydrophobic macroinimer-based hybrid sorbents and their use in the removal of organic solvents from wastewater is explored. Polydimethylsiloxane (PDMS), 4,-4'-azobis-4-cyanopentanoyl chloride (ACPC), and methacryloyl chloride were reacted via bulk condensation polymerization to synthesize the macroinimer. The organogel systems were then prepared with macroinimer using different acrylic monomers of methyl acrylate, ethyl acrylate, and butyl acrylate without any additional crosslinker and initiator. The structural properties of the obtained final products were characterized by FT-IR, ¹H-NMR, and TGA. The effect of alkyl chain length and macroinimer moieties in the organogel networks, as well as the swelling capacities of the prepared gels, was evaluated for different organic solvents and oils. The maximum solvent absorbencies of macroinimer-based organogels were determined as 85.3%, 100.9%, 1422.1%, 1660.0%, 3809.3%, and 5032.2% for diesel oil, gasoline, acetone, benzene, tetrahydrofuran (THF), and dichloromethane (DCM), respectively. Furthermore, adsorption-desorption kinetics, selective absorption from oil/water mixtures, temperature effect on the absorption capacity, and reusability tests were investigated. Obtained results showed that the prepared organogels possessed high swelling, efficient absorption capacity, and good oil separation performance in the removal of organic solvents from wastewater. The temperature-dependent absorption study shows no significant change in absorption capacity. Thus, the prepared macroinimer-based organogels in the present study demonstrate potential as prospective sorbents for organic pollutant cleanup from wastewater.

Demonstrating Aqueous-Phase Low-Molecular-Weight-Gel Wicking of Oil for the Remediation of Oil Spilled into Surface Water

Oils spilled into surface water require effective and timely treatment. In this paper, we report on a low-molecular-weight gelator that can form gels in organic and aqueous phases. The aqueous gel was observed to absorb oils, which is proposed as a new class of materials for remediating oil spilled into surface water. The gels and the low-molecular-weight gelator have both fundamental and applied significance. Fundamentally, identifying the mechanisms that govern the formation of these gels and their resultant mechanical properties is of interest. Subsequently, these fundamental insights aid in the optimization of these gels for addressing spilled oil. First, we briefly compare the organic and aqueous gels qualitatively before focusing on the aqueous gel. Second, we demonstrate the ability of the aqueous gel to wick oils through experiments in a Hele-Shaw cell and compare our results to the Washburn equation for porous media. The Washburn equation is not entirely adequate in describing our results due to the change in volume of the porous media during the wicking process. Finally, we investigate mechanisms proposed to govern the formation of low-molecular-weight gels in the literature through rheological shear measurements during gel formation. Our experiments suggest that the proposed mechanisms are applicable to our aqueous gels, growing as anisotropic crystal networks with fractal dimensions between one and two dimensions from temporally sporadic nucleation sites.

Monopeptide-Based Powder Gelators for Instant Phase-Selective Gelation of Aprotic Aromatics and for Toxic Dye Removal

Through a combinatorial screening of 35 possible phase-selective monopeptide-based organogelators readily made at low cost, we identified five of them with high gelling ability toward aprotic aromatic solvents in the powder form. The best of them (Fmoc-V-6) is able to instantly and phase-selectively gel benzene, toluene, and xylenes in the presence of water at room temperature at a gelator loading of 6% w/v. This enables the gelled aromatics to be separated by filtration and both aromatics and the gelling material to be recycled by distillation. We also identified Fmoc-I-16 as the best gelator for benzyl alcohol, and the corresponding organogel efficiently removes toxic dye molecules by 82-99% from their highly concentrated aqueous solutions. These efficient removals of toxic organic solvents and dyes from water suggest their promising applications in remediating contaminated water resources.

Highly Efficient Recovery of Oils in Water via Serine-Based Organogelators

We report here the gelation of a series of petroleum products by serine derivatives. Among Fmoc (9-fluorenylmethoxycarbonyl) amino acids modified by long-chain amines with different substituents, serine-based compounds exhibit excellent performance in gel formation. Further studies on the variation of serine-based gelators demonstrate a considerable structure-property relationship between oil gelation performance and the molecular structure. Oils could be separated and collected by acid and distillation. Gelators exhibit the potential to be applied in an effective treatment of oil-containing water produced from frequent marine oil spills.

Polar Solvent-Induced Unprecedented Supergelation of (Un)Weathered Crude Oils at Room Temperature

Use of carrier solvents to assist dissolution of phase-selective organogelators (PSOGs) before application in oil gelation is a common approach for solution-based gelators. Because of the competition in H-bonds by the polar carrier solvent, decreased gelling ability of PSOGs was often observed. That is, while data are available, the previously documented biphasic minimum gelling concentrations (BMGCs) are much larger than the MGCs determined using heating-cooling cycle for the same PSOG against the same oil. In this study, we show that, by minimizing amount of polar carrier solvent used, the gelling ability of PSOGs actually can be enhanced very substantially, rather than being weakened. More specifically, we demonstrate that use of a minute amount of polar carrier solvents of different types (e.g., ethyl acetate, acetone, acetonitrile, and tetrahydrofuran) significantly enhances the gelling ability of seven structurally different organogelators in hydrophobic oil. In particular, with the use of 5 vol % essentially nontoxic ethyl acetate, application of this previously unexplored strategy onto four monopeptide-based PSOGs produces up to 11-fold improvement in biphasic gelling ability toward seven (un)weathered crude oils of widely ranging viscosities. While collectively overcoming many problematic issues (slow gelling action, low gelling ability, or a need to use hot or toxic solvent for dissolution of gelator) associated with PSOGs, this surprisingly simple yet powerful and reliable method produces unprecedented rapid supergelation of crude oil at room temperature, with BMGCs of as low as 0.38 w/v % (e.g., 3.8 g per liter of crude oil) and an averaged reduction in material cost of gelators by 85-97%.

Applying low-molecular weight supramolecular gelators in an environmental setting - self-assembled gels as smart materials for pollutant removal

This review explores supramolecular gels as materials for environmental remediation. These soft materials are formed by self-assembling low-molecular-weight building blocks, which can be programmed with molecular-scale information by simple organic synthesis. The resulting gels often have nanoscale 'solid-like' networks which are sample-spanning within a 'liquid-like' solvent phase. There is intimate contact between the solvent and the gel nanostructure, which has a very high effective surface area as a result of its dimensions. As such, these materials have the ability to bring a solid-like phase into contact with liquids in an environmental setting. Such materials can therefore remediate unwanted pollutants from the environment including: immobilisation of oil spills, removal of dyes, extraction of heavy metals or toxic anions, and the detection or removal of chemical weapons. Controlling the interactions between the gel nanofibres and pollutants can lead to selective uptake and extraction. Furthermore, if suitably designed, such materials can be recyclable and environmentally benign, while the responsive and tunable nature of the self-assembled network offers significant advantages over other materials solutions to problems caused by pollution in an environmental setting.

Preparation and investigation of temperature-responsive calix[4]arene-based molecular gels

Higher temperature enhances the strength and the toughness of the gel comprised of kerosene and a tetracholesteryl derivative based on calix[4]arene.

Toluene diisocyanate based phase-selective supramolecular oil gelator for effective removal of oil spills from polluted water

Due to tremendous resource wastes and great harm to ecological environment caused by the accidental oil spills, an alkyl bicarbamate supramolecular oil gelator was synthesized and applied to selectively gelate oils from oil/water mixtures. Interestingly, the oil gelator could be self-assembled in a series of organic solvents, i.e., edible oils and fuel oils to form 3D networks and then turned into thermally reversible organogels, allowing easy separation and removal of oil spills from oil/water mixtures. The possible self-assembly mode for the formation of organogels was proposed. What's more, the optimal conditions for using the oil gelator to recover oils were experimentally determined. Inspiringly, taking gasoline as the co-congealed solvent, a complete gelation of oil phase was achieved within 15 min with high oil removal rate and oil retention rate after convenient salvage and cleanup of oil gels from oil/water mixtures. The oil gelator had some advantages in solidifying oil spills on water surface, exhibiting fast oil gelation, convenient and thorough oil removal and easy recovery. This work illustrates the significant role of oil gelators in the potential cleanup of spilled oils for water purification.

Cross-linked poly(tetrahydrofuran) as promising sorbent for organic solvent/oil spill

In this study, a series of different molecular weights of poly(tetrahydrofuran) (PTHF), which is one of the most important commercial polymers around the world, was condensed with tris[3-(trimethoxysilyl)propyl]isocyanurate (ICS) to generate a cross-linked 3-dimensional network in order to obtain organic solvent/oil sorbents having high swelling capacity. The prepared sorbents show high and fast swelling capacity in oils such as dichloromethane (DCM), tetrahydrofuran (THF), acetone, t-butyl methyl ether (MTBE), gasoline, euro diesel, and crude oil. The recovery of the absorbed oils from contaminated surfaces, especially from water, and the regeneration of the sorbents after several applications are effective. The characterization and thermal properties of the sorbents are identified by Fourier transform infrared spectroscopy (FTIR), solid-state (13)C and (29)Si cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and thermal gravimetric analyses (TGA), respectively. The new usage area of PTHF is emerged by the preparation of PTHF-based network structure with high oil absorption capacity and having excellent reusability as an oil absorbent for the removal of organic liquids from the spill site.

Biocompatible hydrogelators based on bile acid ethyl amides

Four novel bile acid ethyl amides were synthetized using a well-known method. All the four compounds were characterized by IR, SEM, and X-ray crystal analyses. In addition, the cytotoxicity of the compounds was tested. Two of the prepared compounds formed organogels. Lithocholic acid derivative 1 formed hydrogels as 1% and 2% (w/v) in four different aqueous solutions. This is very intriguing regarding possible uses in biomedicine.

Self-assembled fibrillar networks comprised of a naturally-occurring cyclic peptide—LOB3

LOB3, a naturally-occurring orbitide, is capable of self-assembling into 1D nano-fibers and ultimately 3D molecular gel networks in acetonitrile.

Synthesis of multi-functional materials through self-assembly of N-alkyl phenothiazine linked poly(aryl ether) dendrons

Multi-functional materials developed from low molecular weight dendron gelators for phase selective gelation, a hydrophobic surface and invisible ink-gel formation.

Low molecular weight gels: potential in remediation of crude oil spillage and recovery

A molecular gelator which has strong gelation ability for different crude oils (light to heavy crudes), and a wide range of refinery products is reported for the first time for its potential application in oil spillage/recovery.