Oil-water mixtures are present in many products that range from foods to drug delivery systems. The stability of these items is intimately related to the interfacial properties of the oil-water interface that is formed, which can be tuned with judicious choice of amphiphilic molecules. This is commonly accomplished with a <i>catanionic </i>(a mixture of a cationic and anionic) surfactant. Adjusting the mixing ratio tunes the strength of the electrostatic interactions between surfactants and influences their adsorptive behavior at the interface. Vibrational sum frequency (VSF) and VSF scattering spectroscopies are employed to examine the molecular structure and conformation of a catanionic surfactant (DTAB/SDS) at the planar and curved nanoemulsion oil-water interfaces, respectively, as a function of mixing ratio. Nanoemulsion droplets, unlike the planar interface, are kinetically stabilized by repulsive electrostatic interactions. To understand why DTAB/SDS mixtures form stable, low charge nanoemulsions, we first examined the catanionic surfactant adsorbed at the thermodynamically stable planar interface. Insights garnered from the planar interface are extended to the nanoemulsion. The results of these studies aid in deriving a fundamental understanding of how catanionic surfactants influence the interfacial properties of nanoemulsions, which can lead to greater employment of catanionic surfactant stabilized systems.

Why do mixed cationic/anionic surfactant mixtures form stable, low charge nanoemulsions?,

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