Efficient and eco-friendly low-molecular-weight gelators based on l-phenylalanine as promising remediation tool for oil pollution

2.1 Materials

Z-Phe-OH, decyl amine and octadecyl amine were purchased from TCI, Japan. HBTU was purchased from Iris, Japan. N,N-Diisopropylethylamine, solvents and all others Chemicals were purchased from Sigma-Aldrich.

2.2 Analysis

All the ¹H and ¹³C NMR spectra were recorded on a Bruker AV500 spectrometer [500.13 MHz for ¹H, 125.77 MHz for ¹³C]. Elemental analyses were performed by using an EAI CE-440 CHN/O/S elemental analyzer [Exeter Analytical, Inc.]. Atmospheric pressure chemical ionization [APCI] mass spectrometry was obtained on a Burker MicrOTOF II-SDT1.

The structure changes between gels samples were studied by ATR-FTIR spectroscopy [Perkin Elmer]. All spectra were obtained by 32 scans and 4 cm⁻¹ resolutions in wavenumbers ranging from 4000 to 450 cm⁻¹. The morphology and topography of the prepared samples were analyzed by scanning electron microscopy [SEM, Quanta FEG 250, FEI]. Differential scanning calorimetry [SETRAM DSC evo 131, scan rate 5 °C/min] was used to study the thermal behavior. All calculations were applied on the second heating/cooling cycle. The critical gelation concentrations [CGC] were determined by virtual checking in various fluids with different nature by test tube inversion (Suzuki yet al., 2006).

2.3 Synthesis of 1 and 2

In a 250 mL flask, Z-Phe-OH [5 g, 16.70 mmol] was dissolved in chloroform [50 mL], followed by addition of DIPEA [6.0 mL, 2.0 eq, 33.41 mmol] and decyl amine [3.56 g, 1.1 eq, 18.37 mmol] in case of gelator 1 or octadecylamine [3.56 g, 1.1 eq, 16.62 mmol] in case of gelator 2. The reaction mixtures were added to DMF [20 mL] containing HBTU [7.0 g, 1.1 eq, 18.37 mmol] and DIPEA [6.0 mL, 2.0 eq, 33.41 mmol]. The mixtures were stirred for 24 h at room temperature until completion of the reaction [monitored by TLC]. The reaction mixtures were diluted with chloroform [50 mL], washed two times consequently with 5% HCl, 10% K₂CO₃, brine and water. The resultant organic phase was dried over anhydrous MgSO₄. The resultant solids were purified by dissolving in chloroform and reprecipitated by diffusion using petroleum ether and the precipitates were collected by filtration, dried in vacuo.

3.1 Organogelation properties

The screening for the gelation performance was tested in 15 selected solvents: aromatics, linear and cyclic alkanes, alcohols, chlorinated, esters, ketones and different oils. The screening revealed that the prepared LMWGs exhibited good gelation behavior especially in aromatic and alkanes solvents [Table 1]. This behavior revealed the importance of the π-π stacking interactions between the LMWGs and the used aromatic solvent. In case of alkanes that may have some considerations regarding the solubility especially with presence of long alkyl chains. This may be appeared in gelification of many solvents. Gelator 2 had the ability for gelation in more solvents. This may be due to presence of excess and longer alkyl chain which enhanced the solubility that needed as a primary step for gelation. We observed also transparent gel formed in case of gelification of the aromatic solvents and oils rather than opaque gels in case of alkanes solvents. This may be attributed to small nanoscale structures formed through gelification of aromatic solvents and oils, whereas the opaque gels resulted from the formation of larger nano/microscale structure (Edwards et al., 2011).

3.2 NMR spectroscopy

Fig. 1 showed visible and sharp signals for the gel of gelator 2 in d₈-toluene [1 wt%] in¹H NMR at 25 °C. That was consistent with what reported by Sakurai et al. regarding the wet gel possess a good thermal motion (Sakurai et al., 2003). Clear downfield shift for several protons were also observed; that referred to their participation in the hydrogen bonding and the deshielding of the protons in the gel state [i.e. H_c and H_d]. Moreover, deshielding shift was also observed for the aromatic protons [i.e. H_a and H_g]. This confirmed the primary importance of the hydrogen bonding formation for the gelation process with support of π-π stacking.

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Fig. 1

a) ¹H NMR spectra (in toluene‑d₈ at 25 °C) of gelator 2 in 0.05 wt% solution (bottom) and 1 wt% gel state (top). b) 2D-NOESY ¹H NMR spectra (in toluene‑d₈ at 25 °C) of gelator 2 (1 wt%). c) Schematic representation of the proposed 1D columnar assembly of gelators.

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3.3 Fourier-transform infrared investigation

To study the gelation process of the supramolecular network of gels that normally arranged via physical interactions, FT-IR spectra for gelators 1 and 2 were recorded in ethanol, toluene with various concentrations and solid [Figs. 6S–8S, see supporting information].

The measurements were performed in an ethanolic solution in which the gelators could not form gel; due to competition of the ethanol molecules and its participation in the hydrogen bonding rather than the gelator molecules. This provided the information of the non-hydrogen bonded amide groups; to assign the absorption bands corresponding to free amide groups. The characteristic bands of gelator 2 in an ethanolic solution were single bands at 3290, 1647 and 1531 cm⁻¹, respectively, in the regions of —NH stretch region, the carbonyl amide I and the amide II which complied with literature. Moreover, absorption band at 1696 cm⁻¹ was observed which assigned to C⚌O stretching band of the urethane unit.

More measurements were conducted in gel state using toluene at different concentrations 1, 3 and 5 wt%. The first observation showed the similarity between the gelation pattern of hydrogen bonding in solid and gel state. The results showed also that the absorption band 1647 cm⁻¹ corresponding to the bonded carbonyl amide I in the solid state remain in the gel state even at 1 wt%. Additionally, appearance absorption bands corresponding to free carbonyl at 1652, 1655, 1661 cm⁻¹, respectively, in case of 5, 3, 1 wt%. The same behavior was noticed the absorption band corresponding to the bonded C⚌O stretching and 1678 cm⁻¹ of the urethane unit; absorption band corresponding to free C⚌O group appeared at 1688 cm⁻¹.

As shown in Fig. 7S, the absorption bands assigned to symmetric and antisymmetric stretching vibrations of the long alkyl chains were observed at 2913 cm⁻¹ and 2846 cm⁻¹ in an ethanolic solution, but in the solid state and gel state these absorption bands shifted to 2924 cm⁻¹ and 2855 cm⁻¹, respectively, for symmetric and antisymmetric one. The observed shift could be assigned to the mobility decrease of the alkyl chains and its participate in strong aggregate formation by van der Waals interactions (Luo, Liu, and Liang, 2001; Suzuki et al., 2006).

The FT-IR study showed the intermolecular hydrogen bonding between the amide groups was highly important for the self-assembly process. In addition to, the clear participation of long alkyl chains in the gelation process.

3.4 Thermal stabilities study

The gel strength of the prepared organogels had evaluated using differential scanning calorimetry (DSC). The using of thermal stability as indication was due to its dependence on the nature, concentration of the gelators molecules and the nature of the gelified solvents. For better investigation we used different solvents (i.e. toluene and paraffin oil) and the evaluation was done at different concentrations of organogelators. The results [Fig 14S, see supporting information] showed that the values of T_gel for the organogels were increased by increasing the concentrations of the gelators, which may be due to increasing the density of 3D fibrillar network.

The values of gel-sol transition enthalpies (ΔH) indicated the strength of the intermolecular interactions. ΔH could be calculated from the following Schrader’s equation (Amanokura et al., 1998; Bielejewski et al., 2008; Seo and Chang, 2005): $$\text{Log}\left[\text{c}\right]=\frac{-\mathrm{\Delta }\text{H}}{2.303\text{R}}\times \frac{1}{{\text{T}}_g}+\text{constant}$$ where [c]: gelator concentration (mol.l⁻¹), ΔH: gel melting enthalpy (kJ·mol⁻¹), R: gas constant and T_gel: gel-sol temperature (Kelvin).

The ΔH values were calculated from the slopes of the linear curves in [Fig. 14S right]. The values were found 127, 123 and 40 for 2 in paraffin oil, 2 in toluene and 1 in paraffin oil respectively. The melting enthalpy of organogels prepared using gelator 2 in paraffin oil was higher than analogous in toluene and in case of gelator 1 in paraffin oil, which confirmed the important of presence of long alkyl chain (i.e. 18).

3.5 Morphological study of the xerogel

The three-dimensional network which acquired the gelator the ability to immobilize the fluid phases was formed by the entanglement of self-assembled nanofibers. SEM was used for evaluating the porous structure of the prepared xerogels, obtained from drying of 2 organogels 1 wt% in toluene. The images of SEM showed a high entangled fibrillar network for the prepared xerogels [Fig. 2b]. Fig. 2c showed effect of gelator- solvent interactions on the fibre size, with a diameter of several hundred of nanometers long 256–569 nm. Formation of larger nano/microscale structure may be due to the decreasing of the interactions between the gelator and the solvent molecules (Edwards et al., 2011).

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Fig. 2

a) Investigation of gelation ability and selective removal of various oils using 1 and 2. b, c) SEM images of xerogel of 2 prepared by drying of organogels 1 wt% in toluene.

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3.6 Investigation of the 1-D self-assembly process (stacking-up)

The primary step for formation of a three-dimensional fibrillar network that responsible for immobilizing fluid phases was the stacking-up of the gelator molecules. The stacking up had two possibilities head-to-head (Yabuuchi et al., 2007) or head-to-tail (Iqbal et al., 2008). 2D-NOESY ¹HNMR experiments provided information about which protons are spatially close and intermolecular correlated (Allix et al., 2010). The study of gel in d₈-toluene provided us an evidence for long alkyl chain protons spatially correlated CH₂ protons of Z unit [Fig. 1]. This referred to the stacking up of gelators 1 and 2 is head-to-tail one [Fig. 1].

3.7 Phase-selective gelation of different oils from mixtures with water

The gelation ability of the prepared gelators was investigated in various commercial oils, which used widely in daily life and in industrial sector. As shown in Fig. 2a, different oils (i.e. olive, soya bean, sunflower and paraffin oil) were tested by test tube inversion method. Fig. 2 demonstrated selective removal of waste mineral oil from its mixture with water by using an ethanolic solution of gelator 2 via formation of gel that can be easily removed by filtration.

The synthesis of these types of functional LMWGs is highly promising, to solve many problems in the river Nile [i.e. oil spill, oil leakage from different tankers, etc.] and in many industries which produce oily waste [i.e. cheese industry, cosmetics, oil refining, etc.]. The studies are running to modify the design of the gelators with more universal gel applications. Also, the scaling up for production of these LMWGs is under investigation.