Open Access Article
Yuki Kitamura
a,
Yusuke Yasuda
b,
Junya Metokic,
Shogo Tsujinoc,
Kazuma Yoshimurad,
Takahide Watanabec and
Kazushi Fujimoto
*abe
aGraduate School of Science and Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan. E-mail: k-fuji@kansai-u.ac.jp
bOrganization for Research and Development of Innovative Science and Technology, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan
cTechnical Division, The Nisshin OilliO Group, Ltd, 1 Shinmori-cho, Isogo-ku, Yokohama 235-8558, Japan
dHousehold-Use & Wellness Foods Business Strategy, The Nisshin OilliO Group, Ltd, 1 Shinmori-cho, Isogo-ku, Yokohama 235-8558, Japan
eDepartment of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan
First published on 20th November 2025
Triglycerides (TGs) exhibit diverse flow behaviours that are essential for applications ranging from food and pharmaceuticals to renewable fuels and advanced lubricants. Despite sharing nearly identical atomic compositions and similar bulk thermodynamic properties, different TGs display markedly different viscosities, and the microscopic origins of these distinct behaviours remain unclear. Herein, we performed all-atom molecular dynamics simulations to investigate three representative TGs with different chain lengths and degrees of unsaturation: trioctanoin (8
:
0), triolein (18
:
1), and trilinolenin (18
:
3). Our results qualitatively reproduced the experimental viscosity ordering, with 18
:
1 being the most viscous and 8
:
0 showing a slightly higher viscosity than 18
:
3. Neither the cohesive energy density, melting point correlation, nor molecular size could account for these differences. Likewise, single-molecule conformational statistics and mesoscale cluster morphologies revealed no decisive distinctions that explain the observed viscosity variations. Instead, structural analyses revealed that the local parallel alignment of C-chain segments produced junctions that formed extended network structures. These networks were abundant in 18
:
1, intermediate in 8
:
0, and absent in 18
:
3, directly correlating with the observed viscosities. Overall, fine-scale packing appears to govern the dynamics of TGs.
TGs are commonly referred to as edible oils and have been extensively used in human diets for centuries. According to the U.S. Department of Agriculture Economic Research Service, global vegetable oil production and consumption are projected to exceed 220 million tons in the 2025/26 fiscal year.13 In addition to conventional edible oils, functional products with health benefits—such as medium-chain TG oil—are now commercially available and widely marketed. The flow behaviour of these oils is determined by their viscosity, which directly affects food processing in homes, industries, and restaurants,14,15 and may influence aspects of nutritional utilisation.
As a general feature of oils, several thermodynamic properties, such as density and cohesive energy, are markedly similar among TGs because they share the same atomic composition. In contrast, their dynamic properties significantly vary. Empirical trends observed across fatty acid and edible oil viscosity data suggest that viscosity increases with chain length and decreases with the number of double bonds.14,16 These observations are partially consistent with the behaviours of the TGs examined in the present study. For instance, trilinolenin (18
:
3) exhibits a viscosity of around 24.64 mPa s, which is lower than that of triolein (18
:
1, 84.18 mPa s), in line with the expected fluidising effect of multiple double bonds. However, despite its substantially longer carbon chains, trilinolenin also shows a lower viscosity than the medium-chain trioctanoin (8
:
0, 26.65 mPa s), as confirmed by our rheological measurements (Table 1). Such a trend cannot be rationalised by conventional empirical correlations. This discrepancy indicates that viscosity cannot be explained solely by chain length or unsaturation; rather, a molecular-level framework based on collective organisation and intermolecular energetics is required to capture the distinct dynamics of TGs.
8 : 0 |
18 : 1 |
18 : 3 |
|
|---|---|---|---|
| D∞ (10−12 m2 s−1) | 5.2 | 0.68 | 3.6 |
| η (mPa s) | 100 (26.65) | 345 (84.18) | 92 (24.64) |
| Rg (nm) | 0.52 | 0.88 | 0.84 |
| Rh (nm) | 0.42 | 3.2 | 0.61 |
| Edc (J cm−3) | −315 | −312 | −317 |
| ρ (g cm−3) | 0.955 | 0.917 | 0.940 |
Molecular dynamics (MD) simulations have been employed to elucidate various TG phenomena, such as the crystallisation of tripalmitin and triolein at physiological temperatures,17 coarse-grained modelling and crystallisation of systems of tridecanoin and mixed stearin,18,19 and interfacial assembly of tri-cis-6-hexadecenoin at air–water interfaces,20 among others.2,3,7,9,21–27 These studies have clarified the packing, phase behaviour, and interfacial ordering in TGs. However, a direct connection between molecular-scale structure and viscosity has not been established, and thus the microscopic origin of their distinct dynamic behaviour remains elusive.
In this work, we employ all-atom MD simulations to investigate three homologous TG molecules differing in the chain length and degree of unsaturation, namely trioctanoin (8
:
0), triolein (18
:
1), and trilinolenin (18
:
3) (Fig. 1). By systematically comparing their diffusion and viscosity, we demonstrate that bulk thermodynamic properties are insufficient to explain the observed trends. By examining the alkyl chain conformational distributions, local alignment, and collective network organisation, we observe how fine-scale structural features govern the viscosity of TGs. This molecular-level framework provides insight for the design of oils and lubricants with tailored flow properties.
![]() | (1) |
The shear viscosity, η, was computed using the Green–Kubo relation:28
![]() | (2) |
| Ec = Esystem − Eisolated, total = Einter, | (3) |
![]() | (4) |
θ), which gives the probability of finding two C-chain units separated by a distance r between their central carbon atoms, with a relative orientation cos
θ. Here, a “C-chain unit” refers to a contiguous segment of seven carbons in 8
:
0 or eight carbons in 18
:
1 and 18
:
3 (see Fig. 1). Accordingly, each fatty acid chain of 8
:
0 and 18
:
3 contains one C-chain unit, whereas each fatty acid chain of 18
:
1 contains two C-chain units. A pronounced feature at r ≈ 0.5 nm and cos
θ ≈ 1 corresponds to parallel alignment of the C-chain segments.
| B(t) = 〈b(t)b(0)〉, | (5) |
• The distance between the centre atoms of the two C-chain units was less than 0.6 nm (contact condition).
• The end-to-end length of the C-chain was larger than 0.8 nm for 8
:
0 or 1.0 nm for 18
:
1 and 18
:
3 (straight condition).
• The relative orientation satisfied cos
(θ) > 0.95 (orientation condition).
The angle brackets denote an ensemble average.
Data were analysed using RheoCompass software (Anton Paar GmbH, Graz, Austria). After confirming that the samples exhibited Newtonian behaviour, the viscosity was determined as the average of the values obtained from 6 to 50 s during the measurement. Temperature control was achieved using a Peltier system with a circulating fluid bath, and all measurements were conducted at 20 °C.
:
0, 18
:
1, and 18
:
3 is shown in Fig. 2. The density of 18
:
1 continued to gradually increase until approximately 200 ns and then equilibrated, whereas the other systems stabilised earlier. Accordingly, only data collected after 300 ns were used in the subsequent analyses; only the first 300 ns of post-equilibration data are shown in the following figures, except for Fig. 3 and 9. Profiles of the potential energy, temperature, and pressure as functions of time are shown in the SI and confirm that all three systems reached equilibrium under the present conditions. The stabilised values are listed in Table 1, along with the other calculated data.
:
0, 18
:
1, and 18
:
3 were approximately 100, 345, and 92 mPa s, respectively (Table 1; experimentally determined values are also shown in parentheses). The calculated viscosities are roughly four times larger than the experimental values, but the relative ordering among the three TGs is consistent with the experiment. This discrepancy is comparable to that reported in [ref. 27] investigating twelve TGs, where the calculated viscosities were several times higher than the experimental values. Among the TGs, 18
:
1 exhibited the highest viscosity, and the value of 8
:
0 was slightly larger than that of 18
:
3, suggesting that 18
:
1 molecules have the greatest resistance to motion.
Fig. 4 shows the MSD and the corresponding logarithmic plot for 8
:
0, 18
:
1, and 18
:
3. The MSDs were fitted in their linear regimes to obtain the diffusion coefficients, D∞ (Table 1). As with the viscosities, the diffusion coefficients of 8
:
0 and 18
:
3 were nearly identical, whereas that of 18
:
1 was the lowest, further indicating that 18
:
1 molecules are the most hindered in their motion. Hydrodynamic radii Rh were calculated using the Stokes–Einstein relation and compared with the radii of gyration Rg, whose distribution is shown in Fig. 5. The geometric Rg of 8
:
0 was the smallest among the three TGs, whereas the Rg of 18
:
1 was nearly identical to that of 18
:
3, despite the presence of three double bonds in 18
:
3. Notably, 18
:
1 had the largest Rh, but 8
:
0 and 18
:
3 exhibited nearly the same value (differing by only 0.19 nm). This mismatch between Rg and Rh signifies that molecular size alone cannot account for the observed diffusion behaviour, necessitating energy and structural analyses.
![]() | ||
Fig. 5 Probability distributions of the radius of gyration Rg for 8 : 0 (red), 18 : 1 (blue), and 18 : 3 (green). | ||
:
0, 18
:
1, and 18
:
3 differ by less than 2%. Similarly, the equilibrium densities are close in value. Using the Hansen solubility parameters listed in the Database of Safer Solvents44 for 8
:
0, 18
:
1, and 18
:
3, the solubility parameters (SP) of each triglyceride TG were estimated to be 17.1, 16.8, and 17.1, respectively. Applying these SP values to the Hildebrand equation45 to calculate the CED, the resulting values for 8
:
0, 18
:
1, and 18
:
3 were −292, −282, and −292 J cm−3, respectively. These values show good agreement with MD simulations, indicating that the three TGs have nearly identical CED values. Thus, bulk cohesion and packing density cannot explain the observed differences in diffusion.
Another widely used empirical descriptor is the melting temperature (Tm), which correlates with viscosity via Vogel–Fulcher–Tammann-type relations. A similar temperature dependence, scaled by Tm, is found in Andrade's model, an empirical relation widely used to describe viscosity as a function of temperature and to rationalise structure–property trends. In both cases, the viscosity is governed by how far the system is from the melting point. Here, we considered the melting points of the three TGs: 8
:
0 (Tm ≈ 9–10 °C),46 18
:
1 (Tm ≈ −5.5 °C),47 and 18
:
3 (Tm ≈ −23–24 °C).48 Based on these melting points, one would expect the viscosity to decrease in the order 8
:
0 > 18
:
1 > 18
:
3 because lower melting points generally correspond to more fluid systems. However, our experiments and simulations reveal that 18
:
1 exhibits a markedly higher viscosity than both 8
:
0 and 18
:
3, which have similar values, contradicting this empirical expectation. This discrepancy indicates that neither CED nor melting point arguments can account for the distinct dynamic behaviour.
The three TGs studied here display similar overall molecular conformations (Fig. 6). Thus, the pronounced differences in viscosity and hydrodynamic radii among 8
:
0, 18
:
1, and 18
:
3 cannot be attributed to molecular shape effects.
:
0 system, a single large cluster was observed, whereas multiple smaller clusters (i.e., discrete aggregates) formed in the 18
:
1 and 18
:
3 systems (Fig. 7(b and c)). The size distributions of the aggregates exhibited an approximately exponential decay. Furthermore, the 18
:
3 system formed a greater number of smaller aggregates than the 18
:
1 system, likely because the kinks introduced by the three double bonds hinder tight packing of the hydrocarbon chains. These structural differences suggest that chain length and unsaturation influence mesoscale morphology.
However, the cluster morphology of 18
:
1 resembled that of 18
:
3, despite their viscosities differing by a factor of four. Conversely, although the cluster morphology of 8
:
0 was markedly different from that of 18
:
3, their viscosities were comparable. Therefore, cluster structures alone cannot account for the viscosity differences among the TGs, and more local analyses are required.
:
0 and 18
:
1, pronounced peaks appear at r = 0.5 nm and cos
θ = 1, corresponding to two C-chain units aligned in parallel. In contrast, only a weak peak was observed for 18
:
3. These locally aligned C-chain units act as junction points, which, when connected through the glycerol backbones, generate extended network structures in 8
:
0 and 18
:
1. Considering the number of C-chain segments, 8
:
0 contains three C-chain units per molecule, whereas 18
:
1 contains six. Note that 18
:
3 also has three C-chain units, but the large kinks introduced by the double bonds adjacent to these segments hinder their parallel packing, thereby preventing effective junction formation.
![]() | ||
Fig. 8 Distributions of the C-chain alignment for 8 : 0 (a), 18 : 1 (b), and 18 : 3 (c). The right panels show representative snapshots at r = 0.5 nm and cos θ = 1, corresponding to different contact pairs. | ||
Our findings suggest that network formation is responsible for the markedly reduced diffusion—higher viscosity—observed in 18
:
1. Although the compact geometry of 8
:
0 would typically imply a much higher diffusion coefficient, its mobility is comparable to that of 18
:
3 because of network formation. Quantitatively, the peak intensities in Fig. 8 indicate that the density of aligned junctions is highest in 18
:
1, moderate in 8
:
0, and negligible in 18
:
3, correlating with the observed order of viscosity.
Fig. 9 presents the bond correlation function B(t), which characterises the lifetime of the junction structures. For 8
:
0 and 18
:
3, B(t) reached zero at roughly 100 ns, indicating that the local alignments between C-chain units are relatively short-lived compared with those in 18
:
1. Conversely, for 18
:
1, a significant fraction of correlations persisted at 100 ns, demonstrating that C-chain–C-chain alignments are more long-lived. These results support the view that the anomalously high viscosity of 18
:
1 arises not only from the formation of network junctions but also from their enhanced temporal stability.
Furthermore, we calculated the relaxation times of B(t) for 8
:
0, 18
:
1, and 18
:
3 by fitting the data to a double exponential function:
, where T1 and T2 represent the relaxation times associated with fast vibrational bond fluctuations and long-lived C-chain alignment, respectively. T1 was found to be 2.5 ps for all three TGs, whereas the T2 values for 8
:
0, 18
:
1, and 18
:
3 were determined to be 3, 14, and 5 ns, respectively. From the perspective of the star polymer relaxation analogy,49 each TG molecule contains three fatty acid chains, and for the entire molecule to relax and diffuse, all three chains must relax first (Fig. 10). Thus, the presence of aligned C-chains delays molecular relaxation and leads to increased viscosity.
![]() | ||
| Fig. 10 Schematic illustration of TG diffusion mechanics based on the star polymer relaxation analogy. | ||
In summary, the viscosities of TGs can be rationalised by the emergence—or absence—of network structures mediated by the parallel alignment of C-chain units. The ease of forming such networks is determined by the balance between the number of available C-chain units and the disruption due to kinks introduced by double bonds.
:
0), triolein (18
:
1), and trilinolenin (18
:
3). The simulations quantitatively reproduced the ordering; 18
:
1 was the most viscous, whereas 8
:
0 and 18
:
3 exhibited nearly identical viscosities.
Bulk thermodynamic descriptors, including CED, density, and melting point correlations of fatty acids, could not explain the viscosity trends among the three TGs. Similarly, single-molecule conformations were essentially comparable across the three systems and thus cannot account for the observed differences. Mesoscale cluster morphologies also failed to clarify the trend; although 18
:
1 and 18
:
3 exhibited similar aggregation patterns, their viscosities differed by a factor of four, and 8
:
0 displayed a distinct morphology but a comparable viscosity to 18
:
3. The decisive factor was the presence or absence of locally aligned C-chain units that acted as network junctions. Such junctions were abundant in 18
:
1, intermediate in 8
:
0, and negligible in 18
:
3. Consequently, the viscosity of 18
:
1 was markedly larger, and that of 8
:
0—which was expected to be much lower owing to the small molecular size—was elevated by network formation, yielding a value comparable to or slightly higher than that of 18
:
3.
These findings demonstrate that fine-scale structural motifs, rather than bulk thermodynamics or molecular size alone, govern the dynamic properties of TGs. The molecular-level framework developed herein provides new guidelines for tailoring the flow behaviour of oils and lubricants through the design of chain length, unsaturation, and alignment propensity.
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