2019 Volume 66 Issue 3 Pages 103-112
We have developed a novel low-molecular-mass oil-gelling agent that is electrically neutral, has no nitrogen atoms and consists only of cyclic sugar alcohols and saturated linear fatty acids. The cyclic sugar alcohols were 1,5-anhydro-D-glucitol (1,5-AG) and 1,5-anhydro-D-mannitol (1,5-AM) derived from starch via 1,5-anhydro-D-fructose. Various saturated linear fatty acids with 10 to 18 and 22 carbon atoms were introduced into all the hydroxy groups of 1,5-AG. Various saturated linear fatty acids with 13 to 18 and 22 carbon atoms were introduced into all the hydroxy groups of 1,5-AM. Initially, the gelling ability increased as the carbon number increased, but the gelling ability decreased as the carbon number increased beyond 17 carbons. This trend was similar for both 1,5-AG and 1,5-AM. A comparison of 1,5-AG and 1,5-AM derivatives revealed that 1,5-AG derivatives had greater gelling abilities for different kinds of oils at the same fatty acid length. Further, it was confirmed by SEM observations that a three-dimensional fibrous structure was formed, and this network structure formed the gel and held the oil. Here, we report the synthesis and characteristics of a novel low-molecular-weight gelling agent and its gelation mechanism.
1,5-AG, 1,5-anhydro-D-glucitol; 1,5-AM, 1,5-anhydro-D-mannitol; DS, degree of substitution; DMF, N,N-dimethylformamide; SEM, scanning electron microscopy.
Gelling agents are added to a liquid such as water or oil and to give it new physical properties such as the thickening or solidification of those liquids. There are two types of gels: hydrogels and organogels. Hydrogels are gelled water-based fluids,1)2)3)4)5)6)7)8) and organogels are gelled oil-based fluids.9)10)11) These are widely used in various fields, such as cosmetics, foods, paints, and lubricants, and distinct properties are needed for each application. Many organogelators use sugar as a backbone. For example, 1,3:2,4-di-O-benzylidene-D-glucitol,12) methyl 4,6-O-benzylidene-α-D-galactopyranoside,13) 4-dodecanamidophenyl β-D-glucopyranoside,14) and D-glucitol derivatives.15) However, compounds with benzylidene smell of benzaldehyde, and their use in odor sensitive products is avoided. In addition, complex compounds that require multiple steps for synthesis have the disadvantage of being expensive. There is need for development of organogelators that improve these drawbacks. Recently, 1,5-anhydro-D-glucitol (1,5-AG) and 1,5-anhydro-D-mannitol (1,5-AM) were successfully synthesized from starch via 1,5-anhydro-D-fructose.16)17)18) In 1,5-AG and 1,5-AM, the anomeric hydroxy groups of glucose and mannose, respectively, are deoxygenated. Since the anomeric hydroxy group is the most reactive hydroxy group among the hydroxy groups on glucose and mannose, 1,5-AG and 1,5-AM, which are deoxygenated here, are more stable than native glucose and mannose, respectively, under acidic, alkaline, and high-temperature conditions. Therefore, they are potentially applicable in industry. In particular, since it has the effect of suppressing coloration in organic reaction, it can be applied to applications that dislike the coloration of sugar, such as cosmetic applications. We report herein the development of novel organogelators consisting of 1,5-AG and 1,5-AM as backbones for cosmetic use.
General methods. 1H-1H COSY, HMBC, HSQC, 1H NMR, and 13C NMR spectra were obtained in CDCl3 on Bruker BioSpin spectrometers (AV 400, Bruker Corporation, Madison, MA, USA). Chemical shifts are expressed in parts per million (ppm) relative to the signal of either residual CHCl3 or Me4Si in CDCl3 (7.24 or 0.00 ppm, respectively). To characterize the signals, the following abbreviations are used: s = singlet, d = doublet, t = triplet, m = multiplet, dd = doublet of doublets. MALDI-TOF MS spectra were recorded in positive ion mode on an Applied Biosystems instrument (4700 proteomics analyzer, Applied Biosystems, Waltham, MA, USA). 2,5-Dihydroxybenzoic acid was used as the matrix. Optical rotations were determined in chloroform solutions on a Jasco instrument (P-1020-GT, Jasco corporation, Tokyo, Japan) at ambient temperature. SEM (scanning electron microscopy) observations was made using a JEOL instrument (JSM-7600F, JEOL Ltd., Tokyo, Japan). The deoiled sample was coated with osmium with an osmium coater from Filgen (OPC60A, Filgen Inc., Nagoya, Japan). Liquid paraffins #350 and #70 were products of Yamakei Sangyo (Osaka, Japan). Olive squalene, olive oil, jojoba oil, and canola oil were products of Tree of Life Co., Ltd. (Tokyo, Japan). Hydrogenated polyisobutene (kinetic viscosity 20.1 mm2/s) and (kinetic viscosity 300 mm2/s) were products of NOF Corporation (Tokyo, Japan). Pentaerythrityl tetraisostearate, triethylhexanoin, diisostearyl malate, and polyglyceryl-2 triisostearate were products of The Nisshin Oillio Group, Ltd. (Tokyo, Japan). Isopropyl myristate was a product of Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). Ethylhexyl palmitate, and caprylic/capric triglyceride were products of Nikko Chemicals Co., Ltd. (Tokyo, Japan). Soybean oil, castor oil, and ethanol (99.5) were products of FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). 2-Octyl-1-dodecanol was a product of Sigma-Aldrich Co. LLC (St. Louis, MO, US). Dimethicone (viscosity 10 mm2/s), cyclopentasiloxane, and diphenylsiloxy phenyl trimethicone were products of Shin-Etsu Chemical Co., Ltd. (Tokyo, Japan). The gelation test was performed using a 2-mL screw-capped glass bottle. Each sample was then accurately weighed into this bottle, and each oil was additionally added to a total of 1.00 g. All reagents and solvents were reagent grade.
Preparation of per-O-saturated linear fatty acid-protected 1,5-AG.
1,5-Anhydro-2,3,4,6-tetra-O-decanoyl-D-glucitol (1). 1,5-AG (152 mg, 0.926 mmol) was dissolved in N,N-dimethylformamide (DMF, 4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Decanoyl chloride (915 μL, 4.51 mmol) was added to the mixture, and the reaction was carried out at 90 °C for 6 h. After the addition of methanol (4 mL), the reaction mixture was washed with 2 M aq HCl and extracted with chloroform. After concentration under reduced pressure, the residue was purified by silica gel column chromatography (toluene) to obtain compound 1 as a white solid (606 mg, 84 %).
1,5-Anhydro-2,3,4,6-tetra-O-undecanoyl-D-glucitol (2). 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Undecanoyl chloride (993 μL, 4.51 mmol) was added, and the reaction was carried out at 90 °C for 6 h. The work-up and purification were carried out in the same manner as in the case of compound 1, and compound 2 was obtained as a white solid (632 mg, 82 %).
1,5-Anhydro-2,3,4,6-tetra-O-lauroyl-D-glucitol (3). 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Lauroyl chloride (1.07 mL, 4.51 mmol) was added, and the reaction was carried out at 90 °C for 5 h. Methanol (4 mL) was added to the reaction solution, a solid appeared when the reaction mixture was cooled to room temperature. The solid was separated by filtration through filter paper, washed with methanol and dried under reduced pressure to afford compound 319) as a white solid (730 mg, 88 %).
1,5-Anhydro-2,3,4,6-tetra-O-tridecanoyl-D-glucitol (4). To a stirred solution of tridecanoic acid (3.0 g, 14.0 mmol) and DMF (1.08 mL, 14.0 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (5.1 mL, 70.0 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give tridecanoyl chloride. This was used without further purification. 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Tridecanoyl chloride (1.15 mL, 4.51 mmol) was added as described above, and the reaction was carried out at 90 °C for 4 h. After adding methanol (4 mL) to the reaction solution, a solid appeared when the reaction mixture was cooled to room temperature. The solid was separated by filtration through filter paper, washed with methanol and dried under reduced pressure. DMF was added to the obtained solid, and the solution was heated to 90 °C to completely dissolve the solid. The solution was then cooled to room temperature to obtain the recrystallized solid. The solid was separated by filtration through filter paper, washed with methanol and dried under reduced pressure to afford compound 4 as a white solid (778 mg, 89 %).
1,5-Anhydro-2,3,4,6-tetra-O-myristoyl-D-glucitol (5). 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Myristoyl chloride (1.22 mL, 4.51 mmol) was added, and the reaction was carried out at 90 °C for 6 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4, and compound 5 was obtained as a white solid (723 mg, 78 %).
1,5-Anhydro-2,3,4,6-tetra-O-pentadecanoyl-D-glucitol (6). To a stirred solution of pentadecanoic acid (3.0 g, 12.4 mmol) and DMF (953 μL, 12.4 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (4.5 mL, 61.9 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give pentadecanoyl chloride. This was used without further purification. 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Pentadecanoyl chloride (1.29 mL, 4.51 mmol) as described above was added, and the reaction was carried out at 90 °C for 4 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4, and compound 6 was obtained as a white solid (867 mg, 88 %).
1,5-Anhydro-2,3,4,6-tetra-O-palmitoyl-D-glucitol (7). 1,5-AG (304 mg, 1.85 mmol) was dissolved in DMF (8 mL) and pyridine (1.20 mL, 14.9 mmol) under a nitrogen atmosphere at 50 °C. Palmitoyl chloride (2.73 mL, 9.02 mmol) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 7 as a white solid (1.87 g, 90 %).
1,5-Anhydro-2,3,4,6-tetra-O-heptadecanoyl-D-glucitol (8). To a stirred solution of heptadecanoic acid (3.0 g, 11.1 mmol) and DMF (854 μL, 11.1 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (4.0 mL, 55.5 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give heptadecanoyl chloride. This was used without further purification. 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Heptadecanoyl chloride (1.43 mL, 4.51 mmol) as described above was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 8 as a white solid (959 mg, 88 %).
1,5-Anhydro-2,3,4,6-tetra-O-stearoyl-D-glucitol (9). 1,5-AG (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Stearoyl chloride (1.52 mL, 4.51 mmol) was added, and the reaction was carried out at 90 °C for 6 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4, and compound 9 20) was obtained as a white solid (981 mg, 86 %).
1,5-Anhydro-2,3,4,6-tetra-O-behenoyl-D-glucitol (10). To a stirred solution of behenic acid (4.43 g, 13.0 mmol) and DMF (1.0 mL, 13.0 mmol) in 1,2-dichloroethane (37 mL) was added thionyl chloride (4.7 mL, 65.0 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give behenoyl chloride. This was used without further purification. 1,5-AG (114 mg, 0.694 mmol) was dissolved in DMF (3 mL) and pyridine (450 μL, 5.58 mmol) under a nitrogen atmosphere at 50 °C. A solution of behenoyl chloride (1.27 g, 3.36 mmol) as described above in toluene (2 mL) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and crystallization were carried out in the same manner as in the case of compound 3, and compound 10 was obtained as a white solid (970 mg, 96 %).
Preparation of per-O-saturated linear fatty acid-protected 1,5-AM.
1,5-Anhydro-2,3,4,6-tetra-O-tridecanoyl-D-mannitol (11). To a stirred solution of tridecanoic acid (3.0 g, 14.0 mmol) and DMF (1.08 mL, 14.0 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (5.1 mL, 70.0 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give tridecanoyl chloride. This was used without further purification. 1,5-AM (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Tridecanoyl chloride (1.15 mL, 4.51 mmol) as described above was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 11 as a white solid (718 mg, 82 %).
1,5-Anhydro-2,3,4,6-tetra-O-myristoyl-D-mannitol (12). 1,5-AM (114 mg, 0.694 mmol) was dissolved in DMF (3 mL) and pyridine (450 μL, 5.58 mmol) under a nitrogen atmosphere at 50 °C. Myristoyl chloride (911 μL, 3.36 mmol) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and crystallization were carried out in the same manner as in the case of compound 3 to afford compound 12 as a white solid (615 mg, 88 %).
1,5-Anhydro-2,3,4,6-tetra-O-pentadecanoyl-D-mannitol (13). To a stirred solution of pentadecanoic acid (3.0 g, 12.4 mmol) and DMF (953 μL, 12.4 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (4.5 mL, 61.9 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give pentadecanoyl chloride. This was used without further purification. 1,5-AM (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Pentadecanoyl chloride (1.29 mL, 4.51 mmol) as described above was added, and the reaction was carried out at 90 °C for 4 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 13 as a white solid (653 mg, 66 %).
1,5-Anhydro-2,3,4,6-tetra-O-palmitoyl-D-mannitol (14). 1,5-AM (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Palmitoyl chloride (1.37 mL, 4.51 mmol) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 14 as a white solid (822 mg, 79 %).
1,5-Anhydro-2,3,4,6-tetra-O-heptadecanoyl-D-mannitol (15). To a stirred solution of heptadecanoic acid (3.0 g, 11.1 mmol) and DMF (854 μL, 11.1 mmol) in 1,2-dichloroethane (30 mL) was added thionyl chloride (4.0 mL, 55.5 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h The reaction mixture was concentrated in vacuo to give heptadecanoyl chloride. This was used without further purification. 1,5-AM (152 mg, 0.926 mmol) was dissolved in DMF (4 mL) and pyridine (600 μL, 7.43 mmol) under a nitrogen atmosphere at 50 °C. Heptadecanoyl chloride (1.43 mL, 4.51 mmol) as described above was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 15 as a white solid (711 mg, 65 %).
1,5-Anhydro-2,3,4,6-tetra-O-stearoyl-D-mannitol (16). 1,5-AM (114 mg, 0.694 mmol) was dissolved in DMF (3 mL) and pyridine (450 μL, 5.58 mmol) under a nitrogen atmosphere at 50 °C. Stearoyl chloride (911 μL, 3.36 mmol) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and recrystallization were carried out in the same manner as in the case of compound 4 to afford compound 16 as a white solid (734 mg, 86 %).
1,5-Anhydro-2,3,4,6-tetra-O-behenoyl-D-mannitol (17). To a stirred solution of behenic acid (4.43 g, 13.0 mmol) and DMF (1.0 mL, 13.0 mmol) in 1,2-dichloroethane (37 mL) was added thionyl chloride (4.7 mL, 65.0 mmol) at room temperature, and the mixture was stirred at room temperature for 3 h. The reaction mixture was concentrated in vacuo to give behenoyl chloride. This was used without further purification. 1,5-AM (114 mg, 0.694 mmol) was dissolved in DMF (3 mL) and pyridine (450 μL, 5.58 mmol) under a nitrogen atmosphere at 50 °C. A solution of behenoyl chloride (1.27 g, 3.36 mmol) as described above in toluene (2 mL) was added, and the reaction was carried out at 90 °C for 5 h. The work-up and crystallization were carried out in the same manner as in the case of compound 3 to afford compound 17 as a white solid (961 mg, 95 %).
Gelation test of 1,5-AG derivatives and 1,5-AM derivatives differing in fatty acid length. 1,5-AG derivatives or 1,5-AM derivatives were accurately weighed and placed in a screw-capped glass bottle, and liquid paraffin #350 or olive squalane was added to afford concentrations of 1, 2, 5, 10, and 20 wt%. After clear dissolution at 100 °C, the solutions were left to stand for 12 h at room temperature. The system was judged to be gelled if the fluidity of the solution was lost and it did not drop when the screw bottle was inverted.
Gelation test of each 1 wt% palmitate ester compound with different backbones in liquid paraffin #350. Each D-glucose or sugar alcohol (myo-inositol, D-glucitol, and xylitol) coupled with palmitic acid was synthesized in the manner described for compound 7 (data not shown). The degree of substitution (DS) of each palmitate compound (D-glucose, myo-inositol, D-glucitol, and xylitol) was 5, 6/5 = 75:25, 5, and 4, respectively. These compounds were accurately weighed and placed in a screw-capped glass bottle, and liquid paraffin #350 was added to a concentration of 1 wt%. After clear dissolution at 100 °C, the solutions were left to stand for 12 h at room temperature. The system was judged to be gelled if the fluidity of the solution was lost and it did not drop when the screw bottle was inverted.
Gelation test on various oils using compound 7. Compound 7 was accurately weighed and placed in a screw-capped glass bottle, and various oils were added to concentrations of 1, 5, and 10 wt%. After clear dissolution at 100 °C, the solutions were left to stand for 12 h at room temperature. In the cases of dimethicone and cyclopentasiloxane, 120 °C was required for clear dissolution. The system was judged to be gelled if the fluidity of the solution was lost and it did not drop when the screw bottle was inverted.
SEM observations of compound 7 forming an organogel in liquid paraffin #350. Compound 7 was accurately weighed and placed in a screw-capped glass bottle, and liquid paraffin #350 was added to a concentration of 1 wt%. After clear dissolution at 100 °C, the solution was left to stand over 12 h at room temperature. An aliquot of the resulting gel was placed in an acetone solution and allowed to stand at room temperature for 7 days to remove the oil. The obtained deoiled sample was removed to aluminum foil, washed carefully with acetone and naturally dried. The sample together with aluminum foil was placed on a sample table and osmium-coated with an osmium coater. The obtained coated sample was observed by SEM.
We synthesized 1,5-AG and 1,5-AM linked with saturated linear fatty acids of various lengths (Figs. 1 and 2). Furthermore, they were found to be gelling agents for oil, and the differences in the gelling abilities of the two derivatives were examined. After suspending 1,5-AG or 1,5-AM in a mixed solution of DMF and 8 equivalents of pyridine, the temperature was raised to 50 °C for dissolution. Then, the chloride of each fatty acid was added, and the reaction was performed at 90 °C for 4 to 6 h. After completion of the reaction, the products were purified by crystallization, except compounds 1 and 2, which possess shorter fatty acid chains than other and were thus purified by column chromatography. When a peak corresponding to an impurity derived from a fatty acid was detected by NMR, the obtained crystals were dissolved in DMF and further purified by recrystallization to obtain a highly pure product. The yields were determined and are shown in Figs. 1 and 2. Each reaction proceeded smoothly and had a relatively high yield. The characteristic signals associated with H-2, 3, 4, and 6, 6´ of the 1,5-AG or 1,5-AM residues in the 1H NMR spectra of all the target compounds shifted to a lower magnetic field. This indicated that all hydroxy groups were successfully protected, and the target compounds were smoothly obtained.
The yield of each compound was as follows: 1, 84 %; 2, 82 %; 3, 88 %; 4, 89 %; 5, 78 %; 6, 88 %; 7, 90 %; 8, 88 %; 9, 86 %; and 10, 96 %.
The yield of each compound was as follows: 11, 82 %; 12, 88 %; 13, 66 %; 14, 79 %; 15, 65 %; 16, 86 %; and 17, 95 %.
| Carbon number of fatty acid | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 22 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1,5-AG | |||||||||||
| Compound | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
| 1 | Gelling concentration in liquid paraffin #350 (wt%) | N.G. | 20 | 2 | 1 | 1 | 1 | 1 | 1 | 2 | 10 |
| 2 | Gelling concentration in olive squalane (wt%) | 10 | - | 5 | 5 | 1 | - | 1 | - | 5 | - |
| 1,5-AM | |||||||||||
| Compound | - | - | - | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
| 3 | Gelling concentration in liquid paraffin #350 (wt%) | - | - | - | 10 | 1 | 1 | 1 | 1 | 2 | 10 |
N.G., No gelation; -, No data.
| Entry | Backbone | Form |
|---|---|---|
| 1 | 1,5-AG (7) | Gel |
| 2 | 1,5-AM (14) | Gel |
| 3 | D-Glucosea | dropped |
| 4 | myo-Inositolb | dropped |
| 5 | D-Glucitolc | dropped |
| 6 | Xylitold | dropped |
aDS = 5, α:β = 61:39. bDS6:DS5 = 75:25. cDS = 5. dDS = 4.
We first synthesized compound 7, in which palmitic acid was linked to 1,5-AG, and found that the 1 wt% liquid paraffin solution lost fluidity and gelled. Therefore, we investigated the change in the gelation ability as function of the length of the fatty acid. As described above, fatty acids of various lengths were introduced into 1,5-AG, and their gelation abilities for liquid paraffin were investigated (Table 1, entry 1). Then, compound 1, with the shortest fatty acid (10 carbon atoms), did not gel even when 20 wt% was added. However, the gelation concentrations were 20, 2, and 1 wt% when the number of carbons in the fatty acid was increased to 11, 12, and 13, respectively, and gelation occurs with only 1 wt% when the number of carbons in the fatty acid is 13 to 17. However, it was also found that the gelation concentration increased as the number of carbons in the fatty acid was increased above 18. That is, it was found that there was an optimal fatty acid chain length for gelation, and approximately 15 carbons in the fatty acid showed the best gelling ability. Next, the influence of the fatty acid chain length on the gelation of olive squalane was examined (Table 1, entry 2). Similarly, it was found that approximately 16 carbons in the fatty acids was ideal, and deviations from this value decreased the gelation ability. That is, it was found that regardless of the oil, 1,5-AG showed higher gelling abilities when the number of carbons in the fatty acid was 15 or 16. In the future, ease of procurement of raw materials will be important in industrial production. Palmitoyl chloride having 16 carbon atoms is industrially produced and available at low cost. In consideration of availability, C16 palmitic acid was selected as the best gelator for 1,5-AG. Moreover, the gelation ability as a function of the length of fatty acid was also examined with 1,5-AM, which is slightly structurally different from 1,5-AG (Table 1, entry 3). Even in the case of liquid paraffin, which gelated over a wide range of conditions in the case of 1,5-AG, the gelation range at 1 wt% in 1,5-AM was narrow, 14 to 17 carbons in the fatty acids. That is, the gelation ability of 1,5-AG was higher than that of 1,5-AM.
Unmark, 1 wt%; *, 5 wt%; **, 10 wt%.
This time, the length of the fatty acid was fixed at 16, the sugar-like backbone was derivatized, and the changes in gelation ability were examined (Table 2 and Fig. 3). D-Glucose, myo-inositol, D-glucitol, and xylitol were used as the backbone, and palmitic acid was introduced in the manner described for compound 7. The material was dissolved in liquid paraffin at 1 wt%, but all compounds were fluid and none of them gelled. That is, 1,5-AG and 1,5-AM are excellent backbones for gelling agents.
| Entry | Oil | Gelling concentration (wt%) |
|---|---|---|
| 1 | Paraffin liquid #350 | 1 |
| 2 | Paraffin liquid #70 | 1 |
| 3 | Olive squalane | 1 |
| 4 | Hydrogenated polyisobutene (kinetic viscosity 20.1 mm2/s) | 5 |
| 5 | Hydrogenated polyisobutene (kinetic viscosity 300 mm2/s) | 5 |
| 6 | Pentaerythrityl tetraisostearate | 1 |
| 7 | Isopropyl myristate | 5 |
| 8 | Ethylhexyl palmitate | 5 |
| 9 | Caprylic/capric triglyceride | 1 |
| 10 | Triethylhexanoin | 5 |
| 11 | Olive oil | 5 |
| 12 | Jojoba oil | 1 |
| 13 | Canola oil | 5 |
| 14 | Soybean oil | 5 |
| 15 | Castor oil | 1 |
| 16 | Diisostearyl malate | 1 |
| 17 | Polyglyceryl-2 triisostearate | 1 |
| 18 | 2-Octyl-1-dodecanol | 1 |
| 19 | Ethanol (99.5) | 5 |
| 20 | Dimethicone (viscosity 10 mm2/s) | 10 |
| 21 | Cyclopentasiloxane | 5 |
| 22 | Diphenylsiloxy phenyl trimethicone | 5 |
Furthermore, the gelation ability of compound 7, which is a palmitic acid form of 1,5-AG, was examined with various oils (Table 3 and Fig. 4). Compound 7 gelated at 1 wt% with the following oils: hydrocarbon oils such as liquid paraffin and olive squalane; ester oils such as pentaerythrityl tetraisostearate, caprylic/capric triglyceride, jojoba oil, castor oil, diisostearyl malate, and polyglyceryl-2 triisostearate; and alcohol oils such as 2-octyl-1-dodecanol. Additionally, it is interesting to note that ethanol (99.5) was gelled at 5 wt%. Unfortunately, the ethanol evaporated when the lid was opened. In addition, it gelled at 5 wt% with silicone oils (both cyclopentasiloxane and diphenylsiloxy phenyl trimethicone). Thus, various oils can be gelled, although the amount required for gelation varies depending on the type of oil. Compound 7 has been developed for cosmetic use. Thus, it has been found that most oils commonly used in cosmetics can be gelled. Cosmetic consumers tend to prefer naturally derived ingredients. 1,5-AG is a component that is also present in the human body, and furthermore, this gelling agent to which natural fatty acids are bound will be a raw material meeting consumer need.
The structures in the oils were observed using SEM to elucidate the mechanism of oil fluidity loss when dissolving compound 7. After gelling liquid paraffin at 1 wt%, a small amount of the gel was transferred to acetone and deoiled for 7 days. After coating the deoiled sample with osmium, SEM was performed, and a fibrous and highly entangled material with fibers 100 to 300 nm in diameter was observed (Fig. 5).
Compound 7, with a molecular weight of 1117.79, has been found to aggregate to form large fibers and gel to entrap oil in the network. As a result of investigating the temperature stability of this compound 7, the gel of the concentration of 1 and 5 % in liquid paraffin dissolved at 50 °C. This property, which dissolves at relatively low temperature, is a remarkable feature not found in other low molecular weight gelling agents. This is considered to be attributable to the chemical structure of the present compound which produces large fibers only by van der Waals force. A report disclosing how these small molecules are oriented and form fibrous materials will be published in due course.
The authors declare no conflicts of interest.
This research was conducted using NMR and MS equipment owned by the Advanced Analysis Center, National Agriculture and Food Research Organization (NARO), JP. We thank H. Ono (Advanced Analysis Center, NARO, JP) and his staff for the MALDI-TOF-MS measurements. And SEM observations was conducted using SEM equipment owned by National Institute of Animal Health, NARO, JP. We thank N. Tanimura (National Institute of Animal Health, NARO, JP) and his staff for expert assistance in SEM observations.
