Dilution-driven gel-sol-gel-sol transitions present intriguing behaviors of surfactant mixtures, particularly at varying concentrations. New research highlights how mixing erucyl dimethyl amidopropyl betaine (EAPB) with sodium salicylate (NaSal) triggers remarkable changes at the molecular level. The findings demonstrate the sequential transformation of long wormlike micelles (WLMs) to complex micellar structures, characterized by hexagonal liquid phases and varying densities. The research helps pave the way for applications where switchable viscosities are highly advantageous.
Researchers aim to leverage the unique properties and transitions of surfactant mixtures for industrial use, particularly within fields such as oil recovery and enhanced fluid delivery systems. Surfactants are known for their ability to self-assemble based on concentration and external environmental factors, such as temperature and chemical additives.
This study, published on March 8, 2025, conveys how concentrated EAPB solutions undergo transformations by incorporating aromatically structured hydrotropes like NaSal. Gradually introducing NaSal changes the surfactants’ properties, leading to their morphological evolution. Such transitions begin with the creation of hexagonal liquid crystal phases before reaching the highly concentrated spherical micelles (HCSMs), then shifting to the hexagonal close-packed micellar phase (HCP).
The core sequence of events shows these mixed surfactant systems can drastically decrease viscosity, particularly at specific molar ratios of NaSal and EAPB during dilution. Within the study, mixed solutions were seen to change from solid-like to liquid-like states with very low viscosities, making them easier to inject and manipulate compared to conventional polymeric alternatives.
At low ratios, or R values below 0.75, the aqueous mixtures exhibited gel-like signatures, with high zero-shear viscosity readings ranging from 106 to 107 mPa·s. By equimolar mixing of NaSal and EAPB (R = 1), researchers observed viscosity reductions, dipping drastically down to less than 100 mPa·s as R ranged from 1.25 to 1.75.
Remarkably, the viscosity of the NaSal and EAPB mixture sharply decreased by approximately 7 orders of magnitude, landing at the minimal viscosity of about 2.9 mPa·s when EAPB concentration reached roughly 300 mmol/kg. Higher concentrations, as noted near the range of 625 mmol/kg, reverted the solution to isotropic gels with increasing viscosity.
Notably, as the EAPB concentration lowered to below 5 mmol/kg, the transformed aqueous solutions created Newtonian fluids with significantly low viscosities of 1 mPa·s at concentrated EAPB levels of 1 mmol/kg. This adaptability of viscosity opens new avenues for custom fluid formulations.
Interestingly, upon dilution of the initial concentrated solution, equilibrium viscosity values could reach as high as 2800 mPa·s after about 7 days, showcasing the applicability of this transition within practical scenarios. The study did observe various intermediate states, including HCSMs, WLMs, and microstructural behaviors through rheological responses.
The underlying mechanisms behind these transitions primarily stem from co-assembly processes at the micellar level, accentuating competitive interactions aided by environmental triggering.
Factors affecting this transformation range from dilution levels to environmental stimuli. Researchers utilized careful rheological measurements to capture the fine details of changes occurring during these transitions across specified EAPB concentrations.
Through rigorous studies of NMR spectroscopy, the binding dynamics between sodium salicylate and EAPB paint vivid pictures of molecular interactions as they form micellar structures capable of adjusting viscosity dramatically based on concentration and dilution strategies.
These findings contribute significantly to the surfactant science field, enhancing our comprehension of scenarios where tunable viscosity can be optimally utilized. The systematic layering of micellar formations driven by dilution strategies can render new products beneficial for varied uses: from cleaning products to enhanced delivery systems for pharmaceuticals.
Overall, the exploration of dilution-induced morphological transitions not only elucidates the complex interplay among surfactants but also signifies progress toward practical applications of wormlike micelle systems within industrial and technological usages.