Experimental investigation of the structure of nonionic microemulsions and their relation to the bending elasticity of the amphiphilic film

Abstract

In this investigation we studied ternary microemulsions of the system nonionic surfactant (CiEj)/hydrocarbon/water. In this system droplet microemulsions were investigated by means of SANS experiments and measurements of the interfacial tension. The obtained structural quantities of the droplet microemulsion such as radius, polydispersity index, and macroscopic interfacial tension can be directly related to the bending constants (bending modulus κ and Gaussian modulus $$\bar \kappa$$ ) of the system. The sum of the elastic constants $$2\kappa + \bar \kappa$$ can be deduced independently from interfacial tension and polydispersity index (from SANS data in the shell contrast) and both are in good agreement for all systems. This sum increases with increasing thickness of the amphiphilic film, i.e. with increasing length of the surfactant chain. In contrast, variation of the chain length of the hydrocarbon does not influence the elastic properties of the surfactant film. A dilution series also allows for a determination of $$2\kappa + \bar \kappa$$ from the concentration dependence of the deduced radii and polydispersity indices. Again, good agreement is observed with the values obtained before. Experiments both on oil-in-water (O/W) and water-in-oil (W/O) droplets indicate that $$2\kappa + \bar \kappa$$ decreases with increasing temperature (W/O at higher temperature). In general, one can state that the properties of these microemulsions are well described by the elastic theory for the amphiphilic film. Structural parameters like radius and polydispersity are interrelated with the interfacial tension and if two of these quantities are known the third can be predicted reliably.