New Interfacial Rheology Characteristics Measured Using a Spinning Drop Rheometer at the Optimum Formulation. Part 2. Surfactant–Oil–Water Systems with a High Volume of Middle‐Phase Microemulsion

Abstract This article is a continuation of our first study on dilational interfacial rheology properties at optimum formulation for surfactant‐oil–water systems at low surfactant concentration just above the cμc. Here, we have investigated a high content of middle‐phase microemulsion with an optimum WIII phase behavior for a system containing sodium dodecyl sulfate, n‐pentanol, and kerosene. A new oscillating spinning drop interfacial rheometer was used to measure the interfacial properties. The very low dilational elasticity moduli and phase angle found at or near hydrophilic–lipophilic deviation (HLD) = 0 are related to the presence of the bicontinuous phase microemulsion and to the fast surfactant exchanges between the bulk and the interface, regardless of the phases involved in the measurement using the spinning drop apparatus, i.e., the two‐phase excess oil and excess water (O‐W) or the bicontinuous microemulsion and excess water (M‐W). We show that at or near optimum formulation, the interfacial tension and the dilational modulus for the M‐W case almost instantly reach equilibrium, because of the high surfactant content in the microemulsion and the fast exchanges between the bulk and the interface. In contrast, when both excess phases (O‐W) are measured, the changes in these properties are slower, due to the scarce presence of surfactants in both phases. The possibility of having almost all the surfactants trapped in the middle‐phase bicontinuous microemulsion could explain the emulsion instability in all the WIII range. This is behaving as if there were no surfactant available in the oil and water phases to stabilize the oil or water droplets thus formed.