Surfactant-free microemulsions of n-butanol, ethanol, and water

Surfactant-free microemulsions (SFMEs) refer to the microemulsions formed with two immiscible fluids (generally called the oil and water phases) in the presence of an amphi-solvent rather than the traditional surfactants. The understanding of SFMEs is still limited now. In the current work, the phase behavior of the ternary mixture of n-butanol (oil phase), ethanol (amphi-solvent), and water was investigated. The ternary phase diagram shows a single-phase homogeneous region and a multiphase region. The mesoscale structuring was observed within the macroscopically homogeneous single-phase zone near the phase boundary using dynamic light scattering (DLS) and small-wide angle X-ray scattering (SWAXS) techniques. That is, microemulsions are formed in the ternary mixture without the traditional surfactants. In addition, the electrical conductivity and micropolarity of the single-phase mixtures were determined, suggesting the existence of three types of microstructures, i.e., oil-in-water (O/W), bicontinuous (BC), and water-in-oil (W/O), for the SFMEs. The subregions corresponding to the three microstructures were roughly identified in the ternary phase diagram. n-Butanol is one of the extremely weak hydrophobic substances, and the detection of microemulsions in the mixture indicates that the formation of SFMEs is a universal phenomenon. This study provides a better understanding of the phase behavior of ternary liquid mixtures.