Counterion Effects on Aggregate Structure of 12-Hydroxystearate Salts in Hexane: A Quantum Mechanical and Molecular...

Salts of 12-hydroxystearate are important organogelators and grease thickeners, but a structural rationale for their rheological properties remains elusive. We use quantum mechanical calculations and molecular dynamics (MD) simulations to analyze aggregate structures for (1) ( R)-12-hydroxystearic acid (( R)-12HSA), (2) lithium ( R)-12-hydroxystearate (( R)-Li12HS), and (3) sodium ( R)-12-hydroxystearate (( R)-Na12HS). First, quantum mechanical calculations were used to establish the structure and complexation energies of dimers of acetic acid, lithium acetate, and sodium acetate. The expected acetic acid dimer is predicted, and both the lithium acetate and sodium acetate dimer formed a C_{2 h}-symmetric structure. All dimers were sufficiently stable to allow modeling them as pseudocovalent complexes in all-atom, explicit solvent MD. After microsecond-long MD, all systems produced strong ringlike ordered nuclei. The C_{2 h} lithium salt molecules produced aggregates that had the most efficient packing at the head group and a higher frequency of hydroxyl hydrogen bonding compared to the sodium salt. This ordering propensity explains the high melting temperature of ( R)-Li12HS. Also, the higher frequency of hydrogen bonding leads to fewer solvent-exposed hydrogen bond partners. This explains why lithium is a common counterion in high-temperature and water-resistant greases.