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.