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

摘要

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