Aggregation of 12-Hydroxystearic Acid and Its Lithium Salt in Hexane: Molecular Dynamics Simulations.

ABSTRACT

12-Hydroxystearic acid (12HSA) is a well-known organogelator, and its metal salts and derivatives find roles in many important applications. The structures of aggregates of 12-hydroxysteric acid and its salts depend sensitively on cation type, but a fundamental understanding of this phenomenon is lacking. In this study, molecular dynamics simulations were conducted on the microsecond long time scales for (1) 12HSA and (2) its lithium salt, each at 12.5 wt % in explicit hexane solvent. Self-assembly was accelerated by using a modified potential to prohibit alkane chain dihedral gauche states (all-trans-12HSA) and then verified by continuation using standard force-field parameters. In three independent simulation, acceleration using "gauche-less" potentials resulted in self-assembled pseudocrystalline aggregates through formation of polarized five- and six-membered rings between inter-12-hydroxyl groups and head-to-head carboxylic acid dimerization. When subjected to the unmodified dihedral potential, two of the three structures remained stable after 1 µs of MD. Stable structures exhibited a "ring-of-rings" motif, composed of two six-membered acetic acid-dimerized ring bundles with six satellite rings, while the unstable structure did not. In strong contrast, the lithium salt produced a network of fibrils that spanned the volume of the sample. When lithium ions were substituted for carboxylic acid protons in the stable acid structures, they remained intact but lost their chiral nature. Both the acid and lithium structures displayed scattering peaks that agreed with experiment. Taken together, our results suggest that this ring-of-rings structure could be a primary feature of the self-assembly of 12HSA in organic solvents.