Membranes and water:An interesting relationship

The relationship between membranes and interfacial water is discussed. Various interfacial properties for a number of simulated membrane/water systems are compared. Although the simulated membranes consist of widely varying hydrophobic and hydrophilic components, some general properties emerge. A geometrical ordering principle is observed for all surfaces, resulting in a negative surface potential for all systems. The induced water order decays without any previously observed long-range oscillations. More specific properties relate to the dipolar charge distributions of the hydrophilic surfaces, which are akin to biomembranes. In contrast to the hydrophobic surfaces they have a broad interface with a high density. Compensation of local charge density is the main ordering principle for water in these systems. The interplay between membrane and water properties is further discussed in relation to the hydration force. It is concluded that the non-exponential decay of the surface molecules will lead to a more complicated decay of the hydration force than usually assumed, whether arising from water ordering or from surface protrusions.