Abstract

Triglycerides (TGs) have nearly identical atomic compositions and similar bulk properties, such as density and cohesive energy. Nevertheless, their viscosities vary widely. Simple empirical rules—that viscosity increases with chain length and decreases with the number of double bonds—cannot explain the order observed in our measurements: triolein (18:1) is by far the most viscous, whereas trioctanoin (8:0) and trilinolenin (18:3) show comparable viscosities despite the much longer chains of 18:3. This indicates that chain length and unsaturation alone do not determine the flow behavior of TGs.

We performed all-atom molecular dynamics (MD) simulations of three TGs—8:0, 18:1, and 18:3. The simulations yielded self-diffusion coefficients and shear viscosities, and the calculated viscosity trend was consistent with the experimental results. To identify the molecular origin of the differences, we systematically examined common descriptors, including cohesive energy density, molecular size represented by the radius of gyration and hydrodynamic radius, single-molecule conformations, and mesoscale aggregate shapes.

None of these descriptors accounted for the viscosity trend. The cohesive energy densities differed by less than 2%. Molecular size, single-molecule conformation, and aggregate morphology also failed to reproduce the observed trend. Instead, the decisive factor was the local parallel alignment of carbon-chain segments. These aligned segments act as junctions that connect through glycerol backbones to form extended network structures. Such networks were abundant in 18:1, less frequent in 8:0, and almost absent in 18:3, matching the measured viscosity order. The junctions were also longer-lived in 18:1. In a star-polymer-like picture, a TG molecule can diffuse only after all three chains have relaxed; therefore, long-lived aligned chains retard molecular motion and increase viscosity. These results demonstrate that fine-scale packing, particularly local ordering of carbon chains, governs TG dynamics more strongly than bulk thermodynamics or molecular size. This insight provides a molecular basis for designing oils and lubricants with tailored flow properties.

Building on this framework, we further examined the distribution and transport of water in these oils. Water molecules were found mainly near the ester carbonyl (C=O) groups of the TGs. Rather than diffusing independently, water motion was coupled to the diffusion of the surrounding TG framework. Thus, the local oil structure controls not only the flow of TG molecules themselves but also the transport of dissolved water.

References

[1] Y. Kitamura, Y. Yasuda, J. Metoki, S. Tsujino, K. Yoshimura, T. Watanabe, and K. Fujimoto, RSC Adv., 15, 45514 (2025).