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| - A Simple Molecular Model for the Spontaneous Curvatureand the Bending Constants of Nonionic SurfactantMonolayers at the Oil/Water Interface
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| - A simple analytical model for the spontaneous curvature and the bending constants of nonionic surfactantmonolayers at an oil/water interface is developed with the goal of allowing correlation and prediction ofphase behavior. The surfactant molecules are treated as diblock copolymers grafted to the interface. Thechange in the free energy of the surfactant monolayer due to the bending of the interface is calculated asa sum of the contributions of the hydrophobic and the hydrophilic blocks. The equilibrium thickness ofa spherical and cylindrical surfactant monolayer is found by minimizing the sum of the free energy ofmixing and the stretching energy of the hydrophilic and the hydrophobic blocks. Comparison with theHelfrich model gives expressions for the spontaneous curvature H0, the bending elasticity κ, and the saddlesplay modulus κ, of the surfactant monolayer. An analytical expression is obtained for the phase inversiontemperature, which correlates well with experimental data for a wide series of nonionic surfactants CiEjand oils (n-alkanes). The model gives the temperature dependence of the spontaneous curvature and thebending parameters of nonionic surfactant monolayers. Phase boundaries in C12E5/octane/water microemulsions are calculated without use of any adjustable parameters and are in a good agreement withexperimental data.
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