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 ws values (=[acetonitrile]/[AOT]). Interestingly, we found that emission parameters of 4-AP within the microemulsion vary differently at low and high ws regimes. The quantum yield (ϕf) and lifetime (τf) of 4-AP first increase up to ws = ∼1 and, thereafter, decrease upon a further increase in the ws values. The emission maximum of 4-AP significantly shifts to a higher wavelength from 445 nm at ws = 0 to 475 nm at ws = 8. Interestingly, unlike aqueous RMs, the emission maximum at ws = 1 matches with the emission maximum in neat acetonitrile and the emission maximum shifts to even longer wavelength at a higher ws. Steady-state anisotropy also shows a break around ws = 1; anisotropy decreases very sharply from ws = 0 to 1 and, thereafter, remains nearly constant. Solvation dynamics becomes progressively faster with an increase in the acetonitrile content only in the low ws regimes but remains almost independent of ws after ws > 2. All of the results collectively indicate that the morphology of the microemulsion may change at an intermediate ws (∼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 ws = 1) but, thereafter, the size distribution becomes multimodal and sizes cannot be estimated correctly.