Effects of a Cosurfactant on the Stretching and Bending Elasticities of a Surfactant Monolayer

A thermodynamic theory is developed of the elasticity of two-component monolayers consisting of an insoluble surfactant and a soluble cosurfactant. Deriving general expressions for the effective stretching and bending elastic moduli of the monolayer, we find the exchange of cosurfactant with the environment to reduce both moduli. A simple model of the monolayer is introduced specifying the dependence of the bending rigidity and the bending moment on the numbers of surfactant and cosurfactant molecules. Numerical estimates of the effect of a cosurfactant show that the stretching elastic modulus remains positive but may be reduced by an order of magnitude, while the bending rigidity can become even negative in certain conditions.