Density and Dynamic Viscosity of Hydrotrope-Assisted Surfactant Free Microemulsions Formed with Hydrophobic Deep Eutectic Solvents

In surfactant-free microemulsions (SFMEs), a hydrotrope assists in microemulsion formation. Use of natural hydrophobic deep eutectic solvents (HDESs) as the oil phase imparts environmental benefits. Density and dynamic viscosity are reported for hydrotrope ethanolamine (ETA)-assisted HDES-based SFME solutions formed using n-decanoic acid (DA) as one constituent of the HDES and one of the five structurally different closed or open ring terpenoids [thymol, l(−)-menthol, linalool, β-citronellol, and geraniol] as the other constituent in equimolar ratio with water as the polar medium. The amount of water was varied at a fixed concentration of hydrotrope where the formation of the microemulsion was optimum. Density decreased linearly with an increase in temperature for all systems. At higher water fractions, the SFMEs exhibit higher density than that expected, ideally providing evidence for the presence of water pools. Because of the inherent complexity associated, the temperature dependence of the dynamic viscosity adheres better to the Vogel–Fulcher–Tammann (VFT) formulation instead of the Arrhenius equation. As water is added to a high-viscosity (HDES/ETA) system, viscosity initially decreases but increases at higher water contents. It is attributed to the increasing size of the microemulsions formed as they create a hindrance to the overall fluidity. The density and dynamic viscosities of the SFME systems reveal important structural information about these organized assemblies.