Role of hydration in phosphatidylcholine reverse micelle structure and gelation in cyclohexane: a molecular dynamics study

Abstract

In this work, we employ all-atom molecular dynamics simulations to examine the hydration response of phospholipid reverse micelles in cyclohexane. This ternary phospholipid-water-cyclohexane system is an important organogel forming system and the focus of this study is on gaining insight on the factors governing the gelation transition. We map the contributions rising from specific lipid-lipid and lipid-water interactions, and their response to increasing aggregate size and changes in water-to-lipid ratio. We find that, opposed to phospholipid-heptane organogels, in cyclohexane, lipid bridging and hydrogen bond driven stabilization of the lipid head group packing is at minor role in dictating the reverse micelle structural transitions corresponding to the organosol-organogel phase transition in this system. Instead, increasing the lipid head hydration changes the lipid packing factor directly which leads to gelation through the formation of long, wormlike micelles. Furthermore, the confined environment in the reverse micellar cores slows down the water dynamics significantly in comparison to fully hydrated phospholipid bilayers and at low water-to-lipid ratios this slow-down is even more significant. The findings map the role of hydration at microscopic level in these systems and could enable tailoring reverse micellar systems for applications relying on the structure and dynamics of the reverse micelles. Examples include such as drug transport, nanotemplating, or confined chemistry in the reverse micelle core water space, e.g., in catalysis.

Aggregation response of triglyceride hydrolysis products in cyclohexane and triolein

“…An overwhelming majority of computational studies of reverse micellar systems have focused on the anionic Aerosol OT surfactant (sodium bis-(2-ethylhexyl)sulfosuccinate) [37][38][39][40][41][42][43][44][45][46][47] . In re-cent years, however, there has been a surge of studies in which other surfactants and reverse micellar systems have been considered 21,[48][49][50][51][52][53][54][55][56][57][58][59][60] . Due to time-scale challenges, many of these studies have focused on preassembled aggregates, their structure, and the structure of confined water, while the self-assembly process and the resulting aggregate size distribution have been characterized only in a very few publications, see Refs.…”

Effect of temperature, water content and free fatty acid on reverse micelle formation of phospholipids in vegetable oil

“…While reverse micelles of phospholipids have been rather extensively studied in various organic solvents, like cyclohexane, benzene or toluene (31)(32)(33), very few investigations have used oil as the nonpolar media (13,14,26,27). Nevertheless, both experimental results and simulations have shown that the nature of the media has a strong influence on the aggregation behavior of phospholipids (34)(35)(36)(37). Notably, vegetable oils can solubilize considerably more water than cyclohexane or the other solvents that have been more frequently studied in the literature (13,38,39).…”

“…They also studied the interactions governing lecithin aggregate morphology in cyclohexane in comparison to those in, e.g. heptane (37). Abel et al have examined phospholipid reverse micelle structure and dynamics in benzene via similar methodology (41).…”

Physisorption of bio oil nitrogen compounds onto montmorillonite

“…We employ MD simulations with the umbrella sampling method to determine the adsorption conformations and energies of individual molecules on the Na-montmorillonite surface. As prior studies 22,[37][38][39][40][41] have raised concern on the performance accuracy of the aqueous environment force-field parametrizations used typically in atomistic detail MD simulations when taken to apolar solvent environments, we assess the modelling accuracy via density functional theory (DFT) electronic structure calculations with an implicit solvent model. The results enable an objective performance evaluation of the Charmm force field, in particular Charmm GenFF 42 parameters, for examining simple organic molecules in low dielectric, apolar solvent environments.…”