The effect of water thickness on the bending rigidity of inverted bilayers

The effect of water thickness on the mean curvature modulus kappa of an inverted bilayer has been investigated. Our system is a lamellar phase made up of a series of water films each surrounded by two ionic surfactant monolayers and separated with hydrophobic solvent. The elastic constant kappa has been estimated using the excess-area method and it is shown to decrease when the thickness of the water layer increases. This result cannot be interpreted in terms of electrostatic effects-an increase of kappa is expected instead-but may arise from a coupling between short-range surfactant interactions and curvature strains.