Self-assembled Fluorinated Organogelators for Surface Modification

A new class of alkyl-and perfluoroalkyl-containing urea and amide derivatives was synthesized from amino acid derivatives. Most of these compounds showed excellent gelation behavior in organic solvents at low concentrations. A few organogelators selected from the initial screening were used for surface modification of fibrous substrates to create hydrophobic and oleophobic composites. The hydrophobic and oleophobic behaviors of these composites were ascribed to a combination of increased surface roughness and the alkyl/fluorinated functionalities present in the gelator backbone.


Introduction
Low molecular weight organogelators (LMWOGs) are gaining increased attention due to their potential applications in a variety of industrial sectors related to materials, cosmetics, health care, food and oil technology [1][2][3][4][5][6][7][8][9].The design and elucidation of structure-property relationships of gelators is also a valuable area of research, in particular, establishing the relationship between the chemical structure of a gelator and its gelation properties in a given solvent [10][11][12][13][14][15][16][17][18].Thermally reversible physical gels are generally formed by self-assembly of the gelator molecules in solvent, leading to the OPEN ACCESS formation of a fibrous supramolecular network with the encapsulation of solvent.This complex process of self-assembly into supramolecular structures varies based on the functionalities present in the organogelator and external factors, including solvent, temperature, and pH.Various non-covalent interactions, such as hydrogen bonding, π-π stacking, hydrophobic interactions, and metal coordination, influence the formation and stabilization of the organogelator self-assemblies [1,2,[10][11][12][13][14][19][20][21].However, the non-covalent forces influencing gelation are relatively weak compared to covalent bonds, and a combination of many of these interactions is required to form a strong intermolecular network.Each molecule of an organogelator can establish several types of physical interactions with a neighboring molecule.For example, intermolecular hydrogen bonding usually facilitates the growth of linear, elongated aggregates with solvent entrapment.Evaporation of solvent from a gel results in the formation of a dried gel (xerogel) with a fibrous or web-like morphology [21].It is possible to alter the surface morphology of the substrate, and thus surface properties, if the gelation and subsequent evaporation of solvent are performed on a substrate.Hence, gel impregnation methodology could provide the low surface energies desired for fabricating superhydrophobic surfaces [19,[21][22][23][24]. Surfaces on which the contact angle of water exceeds 150°, and shows low contact angle hysteresis, are described as superhydrophobic or ultrahydrophobic [25][26][27].These surfaces repel water like a lotus leaf and are also considered 'self-cleaning', since any surface contamination is removed by water as the droplets roll across the surface [28][29][30].The self-cleaning ability of these surfaces is largely due to the increased surface area created by a two-tier micro and nano roughness morphology.It is possible to incorporate nano-or microscale morphologies on any substrate via organogelator impregnation, which could be further controlled by suitable gelling solvents, as well as concentration and temperature effects.In addition, incorporation of low surface energy functionalities like fluorinated groups in the gelator framework could further lower the surface energy and provide oleophobic properties to the modified surface [31].
Urea and amide derivatives are exceptionally well-suited candidates for the design of low molecular weight gelators owing to their rigidity, strength and ability to form highly directional hydrogen bonds [32].We recently demonstrated this concept by fabricating low surface energy surfaces via the surface modification of nonwoven substrates using partially fluorinated urea-amide gelators [33].The general structure and examples of these urea-amide gelators reported in that study are shown Figure 1.The compounds depicted in Figure 1 were prepared via amidation of the N-Boc protected amino acid with fluorinated amines, followed by deprotection and reaction with an alkyl isocynate.They showed excellent gelation behavior in organic solvents.Gelation in the presence of a nonwoven substrate led to a gel-impregnated surface, which upon drying produced a composite with porous microstructure morphology.The resulting composite also showed high water and hexadecane contact angles, indicative of excellent hydrophobic and oleophobic properties.Herein, we discuss the design and synthesis of two novel classes of partially fluorinated amide and urea-amide derivatives, where the positions of the hydrocarbon and fluorocarbon tail are reversed in the molecular structure in comparison to the gelator described in Figure 1.General structures of these partially fluorinated amide and urea-amide derivatives are shown in Figure 2.This report also describes the gelation behavior as a function of structural/functional variations of these derivatives and surface modification of nonwoven substrates using them.

General
All solvents and reagents, unless otherwise indicated, were purchased from commercial sources and used directly as supplied.Tyvek ® polyethylene nonwoven fabric was obtained from E. I. du Pont de Nemours and Company, Wilmington, DE, and Kolon ® gsm70 spun-bonded polyester fabric was obtained from Korea Vilene, Inc. 1 H and 19 F NMR spectra were recorded on a Bruker DRX 400 or 500 spectrometer.Chemical shifts are reported in ppm relative to an internal reference (CDCl 3 , CFCl 3 or TMS).All melting points reported are uncorrected.Contact angle (CA) measurements for both water and hexadecane on a surface were performed according to procedures in the manufacturer's manual using a Ramé-Hart Standard Automated Goniometer Model 200 employing DROP image standard software and equipped with an automated dispensing system with a 250 µL syringe and an illuminated specimen stage assembly.2H,2H,3H,3H-perfluorononoyl chloride, 2H,2H,3H,3Hperfluoroheptanoyl chloride, 1H,1H,2H,2H-perfluorooctyl isocyanate and 1H,1H,2H,2H-perfluorohexyl isocyanate were synthesized from the corresponding perfluoroalkylethyl iodides [34].

Synthesis of 2H,2H,3H,3H-Perfluorononoyl Chloride and 1H,1H,2H,2H-Perfluorooctyl Isocyanate
A four-neck reactor fitted with a stirrer, thermocouple, nitrogen purge, addition funnel and jacketed condenser was purged with nitrogen and charged with magnesium turnings (4.86 g, 202.5 mmol) and anhydrous ether (300 mL).To this suspension was slowly added a solution of 1H,1H,2H,2Hperfluorooctyl iodide (94.8 g, 200 mmol) in ether (100 mL) over a 2 h period.After the addition of about 10 mL of iodide, the mixture was heated to 35 °C, and when the reaction was initiated the rest of the iodide was added, keeping the temperature below 35 °C.The mixture was stirred for 2 h at room temperature (RT) and cooled to 5-10 °C.Excess dry ice (~15 g) was added in small portions with constant stirring.After 15 min, the mixture was quenched with H 2 SO 4 (25%) and the ether phase was separated.The aqueous phase was extracted with ether (2 × 100 mL).Combined ether phases were washed with NaOH solution (25%, 3 × 75 mL).The combined alkaline phases were heated at 100 °C for 10 min and neutralized using H 2 SO 4 (50%) after cooling to ice temperature.The precipitate formed was filtered and washed with water to obtain 2H,2H,3H,3H-perfluorononoic acid as a white crystalline solid (56.5 g, 14.1 mmol, 72%).mp.61-63 °C.
A 3-necked flask fitted with a stirrer, an addition funnel and a condenser with N 2 purge was charged with freshly distilled 2H,2H,3H,3H-perfluorononoyl chloride (16.26 g, 39.6 mmol).Trimethylsilyl azide (TMSN 3 , 4.57 g, 39.61 g) was added dropwise via the addition funnel, keeping the temperature about 5-10 °C.The mixture was then stirred at 10 °C for 30 min and heated at 75 °C for 1.5 h.After the reaction was complete (GC analysis), trimethylsilyl chloride was distilled off at atmospheric pressure and then the product was fractionally distilled under vacuum to produce 1H,1H,2H,2Hperfluorooctyl isocyanate as a colorless liquid (11.9 g, 30.6 mmol, 77%) bp.100-103 °C @ 30 mm Hg.

Gelation Experiments in Organic Solvents
Typically, 1-4 mg/mL of a gelator in an organic solvent in a closed vial was heated in a reactor block equipped to heat multiple vials.The vials were heated to 5-10 °C below the boiling point of the solvent until a clear solution was obtained, and then were allowed to cool to RT either by switching off the heat or by transferring the vials to a constant temperature water bath kept at 25 °C.The state of the solution was evaluated after 2-12 h.Stability of the gel was tested by inverting the vial, and gel samples for which the whole mass remains a single phase (no phase separation between solid and liquid phase) are considered stable.

Gel-Impregnation of Nonwoven Supports in Organic Solvents
Nonwoven fabrics (Tyvek ® polyethylene nonwoven fabric and Kolon ® spun-bonded polyester fabric (about 3.0 cm × 3.0 cm) were immersed in a suspension of amide or urea-amide gelator (1-3 mg/mL) in organic solvent kept in closed reaction flask equipped with a stir bar and temperature controller.The mixtures were heated to 5 °C below the boiling point of the solvent for 1-2 h until clear solutions formed.The flasks were then either rapidly cooled to RT by removing the oil bath or slowly cooled to RT by switching off the heat.Gel formation was usually observed in about 0.5-6 h, and the gels were allowed to age for an additional 6 h.The gelator-impregnated samples were removed and dried in a vacuum oven at RT overnight.The dried samples were weighed and used for contact angle measurements.

Organogelators: Design and Synthesis
Several new partially fluorinated organogelators were designed via careful manipulation of hydrogen bonding functionalities (urea and amide) and hydrophobic functionalities (fluorocarbon and hydrocarbon) in the molecule to promote gelation.Boc-protected amino acids, N-Boc-β-alanine and N-Boc-L-aspartic acid were chosen as the building blocks.First N-Boc-β-alanine and N-Boc-L-aspartic acid were transformed to the corresponding N-Boc-octylamides (1 and 3) by reacting with octylamine (Scheme 1).The resulting amides were deprotected to obtain the corresponding free amide-amine derivatives (2 and 4), the primary building blocks for partially fluorinated amide or urea-amide derivatives.Scheme 1. Synthesis of 3-amino-N-octylpropanamide (2) L-asparticdioctylamide (4) building blocks from N-Boc-β-alanine and N-Boc-L-aspartic acid.

Partially Fluorinated-bis-and Tris-Amides
The amide-amine building blocks (2 and 4) were used for the preparation of partially fluorinated amide derivatives as shown in Scheme 2. Reaction of 2 with 2H,2H,3H,3H-perfluorononoyl chloride or 2H,2H,3H,3H-perfluoroheptanoylchloride provided the bis-amide derivatives 5a or 5b, respectively, in very good yield.Similar reaction of 2 with pentafluorobenzoyl chloride provided the bis-amide 5c.Similarly, the reaction of 4 with corresponding fluorinated acid chlorides provided the tris-amide derivatives 6a-c in good yield.Scheme 2. Synthesis of amide-amine building blocks from N-Boc-β-alanine and N-Boc-L-aspartic acid.

Gelation in Organic Solvents
The majority of these newly synthesized amide and urea-amide derivatives showed gelation properties in organic solvents during their work-up or isolation.A systematic study was then performed to examine the gelation behavior of these partially fluorinated amide or urea-amide derivatives in various organic solvents.A methodology was developed where gelation was attempted with varying amounts of organogelator with a matching a solvent system.A suitable solvent system selection is based on initial solubility experiments.In general, a gelator that is too soluble in an organic solvent will stay dissolved without forming a gel, even at high concentrations.If the gelator is not soluble in an organic solvent, and partially or fully dissolves at high temperature, it may precipitate again as the temperature is lowered.Ideally, the organogelators should dissolve in a solvent at a temperature close to its boiling point and assemble into a network upon cooling, leading to a single gel phase that does not move upon shaking or inverting the vial.
In a typical screening experiment, 0.5-4 mg/mL slurries of the organogelators in various organic solvents were prepared and heated with stirring at a temperature 5 °C below the boiling point of the solvent to induce full dissolution.Gelation occurred upon cooling, as was evident by significant increase in viscosity and/or the formation of a translucent-to-opaque appearance, without the formation of precipitate.The majority of the partially fluorinated amide and urea-amide derivatives showed gelation properties in a variety of organic solvents at remarkably low concentrations, ranging from 0.5 to 4 mg/mL.The partially fluorinated amide derivative 5a showed gelation properties in acetone (2-3 mg/mL) and n-butanol (3-4 mg/mL) leading to partial gels.The corresponding perfluorobutyl derivative (5b) showed gelation in acetone at 3-4 mg/mL and was a clear solution in n-butanol at various concentrations.Both 5a and 5b existed either as clear solutions or slurries in THF, dichloromethane, methanol and toluene at various concentrations (0.5-4 mg/mL), and did not show any signs of gelation.Compound 5c did not gel under any conditions tested, indicating that a long-chain perfluoroalkyl group is probably needed to induce gelation of these bis-amide systems.We then examined the tris-amide systems 6a-c for their gelation abilities at various concentrations in a variety of organic solvents.Compound 6a gelled in acetone (2 mg/mL) and in n-butanol (4 mg/mL) to form opaque gels which were stable for several weeks at room temperature.Similarly, compound 6b formed stable hazy gels in acetone at (3 mg/mL) and in dichloromethane (2 mg/mL).Compound 6c formed gels in acetone at (1-2 mg/mL) and in methanol (2 mg/mL), indicating gelation could be induced on this tris-amide system even though it comprises a pentafluorophenyl group.
Gelation of urea-amide derivatives (7a-c and 8a-c) was then attempted in a variety of organic solvents under varying concentrations.Compound 7a gelled in THF (2 mg/mL) and acetone (2 mg/mL) forming stable gels, and partially gelled in n-butanol (3 mg/mL).Compound 7b gelled in acetone and THF (3 mg/mL), leading to stable gels.The pentafluorophenyl urea-amide derivative 7c remained as slurry or solution in many organic solvents, although partial gel formation was observed in dichloromethane (2 mg/mL).The urea-bis-amide derivatives 8a-c showed excellent gelation properties in a variety of organic solvents.Compound 8a gelled in both acetone and dichloromethane at 2-3 mg/mL concentrations.Compound 8a also showed partial gelation in THF and n-butanol at 2-4 mg/mL concentrations.Compound 8b gelled in dichloromethane (2 mg/mL) and acetone (3 mg/mL), leading to stable gels.Compound 8c showed excellent gelation behavior with a variety of organic solvents (toluene, THF, acetone, dichloromethane, n-butanol and methanol) at concentrations ranging from 0.5-3 mg/mL, and formed stable gels in several instances.Table 1 summarizes some of the best conditions for gel formation, and the minimum concentration of gelator where stable gels were obtained.In general, we conclude that the partially fluorinated urea-amide derivatives (7a-c and 8a-c) are better gelators than the corresponding amide systems (5a-c and 6a-c), possibly due to the ability of the urea moieties to form directional hydrogen bonds.In order to further understand the influence of amide groups in the gelation ability of these urea-amide systems, we synthesized the corresponding urea-ester derivatives, which in general did not show any gelation properties under the same gelation conditions as the urea-amide systems.For example, urea-ester derivatives 9 and 10 (comparable to urea-amides 7a and 8b) were synthesized via the esterification of N-Boc-β-alanine and N-Boc-L-aspartic acid with n-octanol, followed by deprotection and reaction of the amine with 1H,1H,2H,2H-perfluorooctyl isocyanate (Figure 3).Both 9 and 10 were oils and failed to gel in common organic solvents, indicating amide functionalities are desired in addition to the urea group in the backbone of these partially fluorinated compounds to obtain good gelation properties.

Surface Modification of Nonwoven Supports in Organic Solvents
The concept of fabricating low surface energy surfaces via surface modification of nonwoven substrates using self-assembly of partially fluorinated urea or amide gelators has been described in our earlier reports [29,31].The gelators self-assemble into fiber-like morphologies in solution, which grow sufficiently long and become entangled with one another, forming a gel with their solvent medium.In the presence of a solid or fibrous substrate, the gelation occurs on the surface of the substrate, creating a composite with an altered surface topology.Removal of the solvent from the gel composite leaves behind a network of assembled gelator fibers on the substrate, thus producing a composite surface with high surface area and roughness, which are desirable for superior hydrophobic/oleophobic properties.
A few of the partially fluorinated amide and urea-amide gelators that showed good gelation properties were gel-impregnated on nonwoven supports (Tyvek ® polyethylene nonwoven fabric and Kolon ® polyester fabric) in a suitable organic solvent.The nonwoven supports were then dried in a vacuum oven at room temperature to produce a composite of xerogel coated on the substrate.These surface-modified composites were then evaluated for hydrophobic and oleophobic properties.The tris-amide gelators (6a and 6b) were impregnated on the Tyvek ® and Kolon ® fabrics in acetone and dichloromethane, respectively, leading to the corresponding gel-impregnated composites.Similarly, the partially fluorinated urea-amide gelators (7a and 8c) that showed excellent gelation behavior were gel impregnated on both Tyvek ® and Kolon ® fabrics in acetone, leading to the corresponding surfacemodified composites.Examination of these composites by SEM showed a nanofibrous network of gelators impregnated on the fabric surface, vastly different from the micro structured surfaces of Tyvek ® and Kolon ® fabrics [31].Figure 4 shows SEMs at 2,000× and 10,000× magnification of the Kolon ® fabric surface-treated with gelator 7a with an average fiber diameter of 142 nm.variety of organic solvents, and in general the urea-amide derivatives were better gelators than the corresponding bis-or tris-amide derivatives.Analogous urea-ester derivatives failed to gel in any organic solvents, indicating a combination of urea and amide functionalities, or multiple amide groups, are desirable to induce gelation.Good organogelators derived from the initial screening were evaluated for surface modification of nonwoven substrates, and surface analysis indicated a gel-impregnated surface with porous microstructure morphology.Composites obtained via impregnation by gelators provided surfaces with excellent hydrophobic properties.The nonwoven surfaces impregnated with perfluoroalkyl-containing organogelators showed excellent oleophobic behavior.Superior hydrophobic and oleophobic behavior exhibited by these composites were attributed to a combination of micro-structured surface morphology created by the xerogel and the presence of fluorocarbon/hydrocarbon functionalities in the gelator backbone.

Figure 2 .
Figure 2. General structure of partially fluorinated amide and urea-amide derivatives described in this study.
a Minimum gel concentration for stable gel formation; b Hazy = partially transparent, solid gel.Transparent = completely transparent, solid gel.Opaque = solid gel, not transparent.Partial gel = solid gel with some movement.Precipitate = phase-separated liquid, with solid particles.