Polar solvent-AOT interactions control non-aqueous/AOT/n-heptane reverse micelle polar core sizes.

The microenvironment of the polar core generated in different ionic liquid reverse micelle (IL RM) systems were investigated using the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) as an absorption probe and dynamic light scattering (DLS) technique. The novel RM systems consist of two different ILs--1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf2N)--sequestrated by two different surfactants--Triton X-100 (TX-100) and benzyl-n-hexadecyldimethylammonium chloride (BHDC)--in order to make IL/surfactant/benzene RMs. The effect of the variation of Ws (Ws=[IL]/[surfactant]) on the QB spectroscopy was used to characterize these nonaqueous RMs. DLS results confirm the formation of these IL RM systems because increasing Ws increases the droplet sizes. Moreover it is demonstrated that the structure of the sequestrated ILs depends strongly on the type of surfactant use to create the RMs.

Nonaqueous microemulsions of AOT in n-hexane and n-heptane by using six polar solvents as water substitutes such as glycerol (GY), ethylene glycol (EG), propylene glycol (PG), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA) were studied and compared with the corresponding aqueous reverse micelles. The microenvironment generated by these systems was sensed by following the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB). By varying Ws (Ws ) [polar solvent]/[AOT]), the existence of an interaction between the hydrogen bond donor solvents EG, PG, and FA and QB was detected by the changes in the absorption spectra. These changes were interpreted as caused by the partition of the probe between the micelle interface and the polar solvent core by hydrogen bond interactions. The hydrogen bond association constants were calculated, being the largest for FA. In the case of GY, as well as for water, QB seems to be anchored at the interface and no partition was detected. For solvents with no hydrogen bond donor ability such as DMF and DMA the polarity sensed by QB increases with Ws being always larger than the polarity of the neat solvent. The value of the critical micellar concentration (cmc) was also investigated by using acridine orange base as absorption and fluorescence probe. The values of the cmc found for these systems are around (7 ( 2) × 10 -3 M which are higher than those for AOT in comparable aqueous systems.

We have investigated, for the first time, the effect of the composition of the nonpolar organic media on the benzyl-n-hexadecyl-dimethylammonium chloride (BHDC) reversed micelles (RMs) properties at fixed temperature. To achieve this goal we have used the solvatochromic behavior of 1-methyl-8-oxyquinolinium betaine (QB) as absorption probe and dynamic light scattering (DLS), to monitor droplet sizes, interfacial micropolarity, and sequestrated water structure of water/BHDC/n-heptane:benzene RMs. DLS results confirm the formation of the water/BHDC/n-heptane:benzene RMs at every n-heptane mole fraction (X(Hp)) investigated, that is, X(Hp) = 0.00, 0.13, 0.21, 0.30, and 0.38. Also, DLS was used to measure the RMs diffusion coefficient and to calculate the apparent droplet hydrodynamic diameter (d(App)) at different compositions of the nonpolar organic medium. The data suggest that as the n-heptane content increases, the interdroplet attractive interactions also increase with the consequent increment in the droplet size. Moreover, the interdroplet attractive interactions can be "switched on (increased)" or "switched off (decreased)" by formulation of appropriate n-heptane:benzene mixtures. Additionally, QB spectroscopy was used to obtain the "operational" critical micellar concentration (cmc) and to investigate both the RMs interfacial micropolarity and the sequestrated water structure in every RMs studied. The results show that BHDC RMs are formed at lower surfactant concentration when n-heptane or water content increases. When the interdroplet interaction "switches on", the RMs droplet sizes growth expelling benzene molecules from the RMs interface, favoring the water-BHDC interaction at the interface with the consequent increases in the interfacial micropolarity. Therefore, changing the solvent blend is possible to affect dramatically the interfacial micropolarity, the droplet sizes and the structure of the entrapped water.

The properties of the base acridine orange (AOB) in n-heptane/AOT/water reverse micelles were investigated by using absorption, fluorescence, and single photon counting techniques. For comparison, studies in homogeneous media (water and n-heptane) were also performed. The absorption spectra of AOB in water at pH < 10 in the range of [AOB] ) 10 -6 -10 -4 M show two bands at 467 and 492 nm which were attributed to the dimer ((AOBH)2 2+ ) and monomer (AOBH + ) species, respectively. At pH > 10 in the same [AOB] range, only the basic form, AOB, was detected with a band in the visible at λmax ) 435 nm, which obeys Lambert-Beer's law. This species was also the only one detected in n-heptane at 417 nm. These results show that only the protonated base dimerizes as confirmed by fluorescence techniques. The absorption spectra of AOB in the micellar media at various W0 (W0 ) [H2O]/[AOT]) and pH ) 4, working above the operational critical micellar concentration and below [AOT] ) 3 × 10 -3 M, show the disappearance of the band originally present in n-heptane and the appearance of the bands attributable to the protonated base mostly as the dimer, (AOBH)2 2+ . Above this concentration, the absorption band corresponding to AOBH + is the predominant one. There are two isosbestic points, at 426 and 475 nm. Three processes can account for the behavior of AOB in the reverse micelles: (1) distribution of the dye between the organic phase and the micellar interface followed by AOB protonation to give AOBH + , (2) dimerization of AOBH + at the interface at low [AOT], and (3) the conversion of (AOBH)2 2+ to AOBH + by the micelle at [AOT] > 3 × 10 -3 M. The spectral changes allow us to estimate the equilibrium constants for the processes at different W0; the dimerization process is favorable at low water content. The fluorescence spectra of these species show at 650 nm the band for (AOBH)2 2+ and at 550 nm the one for AOBH + . By varying the experimental conditions, the fluorescence decay times for AOB (in n-heptane and water) as well as for AOBH + and (AOBH)2 2+ in water and in AOT reverse micelles were determined. The results were used to explain the micellar influence on AOB's protonation and aggregation processes. The fluorescence decay times at low W0 are higher than that obtained in bulk water. On increasing W0, the fluorescence decay times tend to the value obtained in water at the same pH.