Water activity in lamellar stacks of lipid bilayers: "Hydration forces" revisited

TL;DR

This study investigates water activity in lipid bilayer stacks using osmotic pressure and x-ray scattering, offering a thermodynamic model that challenges traditional hydration force concepts.

Contribution

It introduces a thermodynamic framework to interpret water-lipid interactions without relying on hydration forces, based on experimental osmotic and structural data.

Findings

Osmotic pressure profiles show classical exponential decay with hydration.

The type of interaction (bound or unbound) depends on lipid mixture ratios.

A thermodynamic model successfully explains the observed behaviors.

Abstract

Water activity and its relationship with interactions stabilising lamellar stacks of mixed lipid bilayers in their fluid state are investigated by means of osmotic pressure measurements coupled with small-angle x-ray scattering. The (electrically-neutral) bilayers are composed of a mixture in various proportions of lecithin, a zwitterionic phospholipid, and Simulsol, a non-ionic cosurfactant with an ethoxylated polar head. For highly dehydrated samples the osmotic pressure profile always exhibits the "classical" exponential decay as hydration increases but, depending on Simulsol to lecithin ratio, it becomes either of the "bound" or "unbound" types for more water-swollen systems. A simple thermodynamic model is used for interpreting the results without resorting to the celebrated but elusive "hydration forces"