Journal
COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS
Volume 648, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.colsurfa.2022.129154
Keywords
HLD; Formulation; Turbulent apparent viscosity; Winsor III; Microemulsions; Cationic surfactants; Rheology; SAXS
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Funding
- Italian Ministry of Research MUR under the program PON Ricerca e Innovazione 2014-2020 -Dottorati innovativi con caratterizzazione industriale XXXV Ciclo project [DOT1302393]
- Center for Colloid and Surface Science (CSGI)
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A new method called HLD-titration has been developed recently to determine the composition at which a microemulsion is balanced. This method allows for a fast and low-cost evaluation of the surfactants characteristic parameters by observing the consistent maximum level reached by the three-phasic system under stirring. The study also found that the turbulent apparent viscosity reaches a minimum at the Winsor III phase equilibrium, and investigated the microstructural evolution of the microemulsion using techniques such as small-angle X-ray scattering, diffusion nuclear magnetic resonance, and rheo-small-angle light scattering.
Recently a new, time saving, approach to the determination of the composition at which a microemulsion is balanced has been developed using the so called HLD-titration. The key of the method is the observation that in correspondence of the balanced state where the microemulsion coexist with excess oil and water (Winsor III phase equilibrium), the level reached by the three-phasic system under stirring has a consistent maximum allowing a fast and low-cost readout of the balanced state that permits the evaluation of the surfactants characteristic parameters (SCPs). To better understand how the formation of a balanced microemulsion is related to rheology, here a titration experimental approach was adopted under turbulent flow meanwhile fluid friction. A mixture of equal volumes of brine and oil and didodecyldimethylammonium bromide (DDAB) is titrated with dodecyltrimethylammonium bromide (LTAB), in the Couette cell of a rheometer under continuous rotation, promoting the transition from a Winsor II (w/o microemulsion coexisting with brine) to a Winsor III phase equilibrium. The turbulent apparent viscosity was measured after each addition of the LTAB. We demonstrate that the turbulent apparent viscosity attains a minimum at the Winsor III phase equilibrium. Furthermore, we have investigated the microstructural evolution of the microemulsions found at the different DDAB/LTAB ratios that reproduces the HLD-titration by means of small-angle X-ray scattering (SAXS) and diffusion nuclear magnetic resonance (NMR) and rheo- small-angle light scattering (rheo-SALS). The anisotropy index, measured by rheo-SALS, increases upon increasing the shear rate suggesting the ability of oil and water domains, in the balanced state, to elongate along the streamlines.
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