The objectives of this study were to develop organogelators suitable for industrial use and to probe the viscosity-increasing mechanisms of such organic compounds. Butane 1,2,3,4-tetracar-boxamides, new organogelators that each has four chemical side chains, were synthesized. Two oleyl groups, each with another two side chains that varied from C4 to C18, were introduced to control the solubility and crystallinity of the compounds, and their solubility and rheological properties in isododecane were evaluated. The rheological properties of different amide compounds, N,N’,N’’,N’’’-1,2,4,5-tetra alkyl/alkenyl pyromellitamides with the same four chemical side chains, were also obtained to consider the skeleton’s effect on self-assembled structures due to hydrogen bonding among amide groups. The viscosity-increasing mechanism of the compounds was discussed through linkage of the molecular design, rheological evaluation, and morphological observations in this paper.
Viscosity control for hydrophobic fluids (oils) is necessary in a wide range of industrial applications and commercial goods, such as inks, paints, cosmetics, pharmaceutical products, and petroleum products.
Viscosity control agents for organic solvents (organogelators) have been developed widely over the past decade within the field of hydrophobic-supramolecular materials [
In this paper, we continue our research by using nonaromatic compounds, which are thought to be safer for commercial products. New organogelators, butane 1,2,3,4-tetracarboxamides, each with four chemical side chains, were synthesized by introducing two oleyl groups with another two side chains, which varied from C4 to C18. Since isododecane is widely used as a hydrophobic solvent for cosmetic and sanitary products, we selected isododecane as a test fluid in this study. The rheological property of PMDA was also evaluated to consider the influence of the skeleton. The viscosity-increasing properties of the compounds were evaluated not only by measuring their rheological properties but also by observing their morphologies using a transmission electron microscope (TEM), which can probe the self-assembled structures of compound molecules.
The chemical structure of the newly synthesized organogelators is that of butane 1,2,3,4-tetracarboxamide, as shown in
Oleyl amine (11.3 g, 42 mmol) was added dropwise to a stirred slurry of butane 1,2,3,4-tetracarboxylic dianhydride (4.2 g, 21 mmol) and pyridine (20 ml) under nitrogen gas at 50˚C. After the mixture was stirred for 3 h to give a homogeneous solution, diisopropylcarbodiimide (5.8 g, 48 mmol) and 2-ethylhexyl amine (5.4 g, 42 mmol) were added and the solution was stirred for 8 h. The solvent was removed in vacuo to give the crude product as a yellow gel-like material. This material was washed with methanol, and the resultant product was filtered to give compound 5.
The hydrophobic solvent used in this study was isododecane (Maruzen Petrochemical Co., Tokyo, Japan) without further purification. The synthesized compounds were carefully dissolved in the solvent under agitation for 12 h at 100˚C - 120˚C.
Compound No. | R’ |
---|---|
1 | n-C18H37 |
2 | n-C14H29 |
3 | n-C12H25 |
4 | n-C10H21 |
5 | 2-Ethylhexyl |
6 | n-C6H13 |
7 | n-C4H9 |
The solubility of the synthesized compounds to isododecane was assessed by both visual observation and quantitative evaluation of absorbance at an absorption wavelength in the vicinity of 660 nm, measured by an ultraviolet-visible spectrophotometer (UVmini1240, Shimadzu Corporation, Kyoto, Japan).
Rheological properties were measured using a cone and plate rheometer (NRM-2000R, Elquest Corporation, Chiba, Japan). The diameter and angle of the cone were 17.1 mm and 3˚, respectively. The equilibrium flow property was measured for the shear rate range from 0.01 - 500 s−1. The dynamic viscoelastic property was evaluated by the method of forced oscillation [
The measurements were conducted at a strain of 0.1 with varying angular velocity ω of sine wave oscillation from 0.06 - 63 rad/s. At the same time, the strain dependence of the sample’s viscoelasticity was also measured at a frequency of 1.0 Hz with strain ranging from 0.01 to 20. A Peltier plate was used to maintain a temperature of 25˚C during rheological measurements.
The self-assembled structures of the compounds were observed by a TEM (JEM-140, JEOL Corporation, Tokyo, Japan). An organogelator was dissolved in isododecane, and the sample was attached to a support film grid and dried for 2 h. The sample was then stained by osmium tetraoxide 2% aqueous solution for 4 h. The accelerating voltage of the TEM was 100 kV.
The equilibrium flow curves (shear viscosity η versus shear rate
Compound No. | |||||||
---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 6 | 7 | |
Absorbance (%) | 5.7 | 35.8 | 69.4 | 24.1 | 96.1 |
may have been caused by the interaction of compound molecules. Viscosities at
The dynamic viscoelastic properties of compounds 1 - 5 in isododecane are displayed in
We reported [
The morphology of compound 5 was studied by TEM.
which fibrous molecules with diameters of ca. 5 - 10 nm can be observed. The presence of such minute self-as- sembled structures might have contributed to the increasing viscosity in isododecane. The morphology of PMDA with the same alkyl side chains is shown in
The strength of the hydrogen bonding interaction was estimated by calculating the binding energy between a molecule of compound 5 and one of PMDA. The calculation was conducted using SCIGRESS Ver. 2.3 (Fujitsu Co., Tokyo, Japan) with PM5.
Butane 1,2,3,4-tetracarboxamides were newly synthesized by varying their chemical side chains. The solubili-
ties and rheological properties of compounds 1 - 7 were evaluated to obtain information valuable for the synthesis of effective synthesis of organogelators in isododecane. Self-assembled structures of the compounds’ molecules might be related to the rheological properties of compounds in isododecane. The hydrogen bonding of the amide group may become the core of the self-assembled structure, while the alkyl side chains might contribute the affinity of the core molecule to the solvent, which can be controlled by the solubility and crystallinity of compounds. The degree of the expanse might be related to the entanglement of each self-assembled structure. Also, the rheological properties of the compounds are related not only to the degree of entanglement but also to the rigidity of the structures, which can in some cases produce viscoelastic properties.