Initial Aggregation Process of Thickener in Lithium Soap Greases: All-Atom Molecular Dynamics Simulation

Grease is an organogel consisting of low molecular mass gelators that exhibit self-assembly. We performed all-atom molecular dynamics simulations of two types of lithium soaps, lithium stearate (LiS) and lithium 12-hydroxystearate (Li12HS), in hexane as a solvent to investigate the aggregation process. We found that the hydroxy group, present in Li12HS but absent in LiS, significantly affects the aggregation process. LiS forms plate-like reverse micelles with a cluster of the head groups inside and the tail chains oriented outward, a structure that is qualitatively consistent with previous experimental results. At room temperature, Li12HS also forms small reverse micelles, but these micelles are interconnected by their tails, whose hydroxy side chains form bonds with the head group clusters, leading to the formation of a (meta)stable network structure. Analyzing the types of bonds involving the hydroxy groups based on their bonding partners, we found that most types hinder the transition from the network structure to an experimentally observed crystal-like structure, while bonds between hydroxy groups contribute to this crystal-like structure. At high temperatures, the number of these bonds decreases significantly, which allows the aggregates to undergo structural fluctuations and to adopt various conformations without being trapped in metastable states. This temperature effect, as demonstrated by our simulation involving initial heating followed by cooling to room temperature, promotes the transition from a network structure to a crystal-like structure. Our findings provide molecular-level insights into the initial aggregation dynamics of lithium soaps and into the mechanisms through which thermal cycling alters aggregation pathways and final structures.