Oil-water interfaces are present in many products that range from detergents to drug delivery systems. The functionality of these products is intimately related to their interfacial properties which can be tuned with judicious choice of surface-active agents. This is usually accomplished with a <i>catanionic</i> surfactant (a mixture of a cationic and anionic). Adjusting the mixing ratio and concentration tunes the strength of the electrostatic interactions between surfactants and influences their adsorptive behavior, compared to pure cationic and anionic surfactants. 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. Unlike the planar interface, nanoemulsion droplets are kinetically stabilized by 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 surface. 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.

Characterization of low charge nanoemulsions stabilized by cationic/anionic surfactant mixtures

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