Anomalous Spectral Modulation of 4-Aminophthalimide inside Acetonitrile/AOT/n-Heptane Microemulsion: New Insights on Reverse Micelle to Bicontinuous Microemulsion Transition

Abstract

The behavior of acetonitrile/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/ n-heptane microemulsion, whether it remains as reverse micelle (RM) or bicontinuous microemulsion (BMC), has been controversial and even termed as a "problem system". Herein, we investigate the microemulsion using spectral and dynamical responses of a hydrophilic solvatochromic fluorophore 4-aminophthalimide (4-AP) at different w values (=[acetonitrile]/[AOT]). Interestingly, we found that emission parameters of 4-AP within the microemulsion vary differently at low and high w regimes. The quantum yield (ϕ) and lifetime (τ) of 4-AP first increase up to w = ∼1 and, thereafter, decrease upon a further increase in the w values. The emission maximum of 4-AP significantly shifts to a higher wavelength from 445 nm at w = 0 to 475 nm at w = 8. Interestingly, unlike aqueous RMs, the emission maximum at w = 1 matches with the emission maximum in neat acetonitrile and the emission maximum shifts to even longer wavelength at a higher w. Steady-state anisotropy also shows a break around w = 1; anisotropy decreases very sharply from w = 0 to 1 and, thereafter, remains nearly constant. Solvation dynamics becomes progressively faster with an increase in the acetonitrile content only in the low w regimes but remains almost independent of w after w > 2. All of the results collectively indicate that the morphology of the microemulsion may change at an intermediate w (∼1); below this, the system behaves like reverse micelles, and above this, the system may remain as BMC. The conjecture was further supported by dynamic light scattering measurements, where we observed a gradual increment of the average size at the low acetonitrile content (up to w = 1) but, thereafter, the size distribution becomes multimodal and sizes cannot be estimated correctly.

Crystal Morphology Prediction Models and Regulating Methods

“…Basically, microemulsion is an isotropic mixture of oil, water, and surfactants, in which crystals will exhibit different properties compared with those in bulk solution. The existence of droplet interface and the controllable structure of microemulsion droplets could help to enrich the crystallization environment, and the interfacial mass transfer caused by droplet collision will have a great influence on the crystal morphology [102]. Liu et al [103] used glycine as a model compound to crystallize in a microemulsion environment, and analyzed the structure and morphology of the crystals obtained by microemulsion crystallization, as shown in Figure 9.…”

Small Molecule Sorting: A Fluorescence Study of Microemulsions

“…Throughout this compositional tuning, microemulsions maintain transparency due to the sub-100 nm length scale of these domains . Microemulsions can be made with ionic or nonionic surfactants, with the former more commonly studied. ,− The use of nonionic surfactants, however, ensures that electrolytes added to the aqueous phase have minimal to no effects on the behavior of the surfactant . This advantage of nonionic surfactants over ionic ones is an important consideration for applications in electrochemical systems.…”

“…The use of fluorescence to study structural transitions in microemulsions dates back to 1988 when Chen et al employed steady-state fluorescence anisotropy to study the curvature and geometric constraints in a microemulsion composed of didodecyldimethylammonium bromide surfactant, alkanes, and water . Since then, a variety of fluorescence techniques have been used to study many different aspects of microemulsions, including morphology/structural changes, , small molecule localization, , surfactant hydration, , domain sizes and distributions, , and rotational behavior of small molecules. − Herein, we employ both steady-state and time-resolved fluorescence spectroscopies to examine localization and dynamics. First, as the water content of the microemulsion was observed to significantly influence the electron transfer kinetics, we directly monitor the surfactant hydration using the generalized polarization (GP) of the amphiphilic probe, laurdan .…”

Manipulating of Crystal Morphology and Polymorph by Crystallization in Microemulsions

“…Microemulsion shows great application potential in the field of crystallization because of its special properties. For example, various microemulsion media with a nanosized droplet (<0.1 μm) can dissolve many kinds of substances. , The existence of droplet interface and the controllable structure of the microemulsion droplet can enrich crystallization environments, which are different from traditional crystallization. Due to these reasons, some researches about microemulsion crystallization have been carried out and published.…”