Solvatochromic probes micelles

Before using a solvatochromic probe in the SCF state, it is important to perform calibrations in well-defined one-phase liquid systems. A variety of solvatochromic indicators have been used previously to probe AOT reverse micelles at atmospheric pressure. For example El Seoud et al. (2 investigated malachite green and thymol blue, which are hydrophobic. They are insoluble in heptane despite their hydrophobicity. Accordingly it was found that these probes are absorbed at the surfactant/oil interface, but do not partition into the water pool. The Xmax and UV absorbance for thymol blue change significantly for Wo below 6, but become constant for Wq from 8.3 to 22.2. UV-VIS and positron annihilation studies showed that even less hydrophobic probes such as nitrophenols are also solubilizied in the surfactant/oil interface. 

Figure 1 shows the aggregation behavior of AOT in liquid cyclohexane and supercritical fluid ethane. The systems are one-phase without added water. Surfactant aggregation is indicated by the solvatochromic probe pyridine A -oxide. Pyridine A -oxide was used because of its small size and large dipole moment (/x = 4.3 D), which allow it to partition to the center of reverse micelles instead of being trapped at the surfactant interface. This molecule is a blue shift indicator in that its U V absorption maximum shifts to lower... 

Spectroscopic studies were performed on water in supercritical CO2 microemulsions using an ammonium carboxylate PFPE surfactant (24). FTIR spec-toscopy was used to identify a bulk water phase within the microemulsion capable of solubilizing ionic species and supporting inorganic reactions. In addition, the UV-visible spectrum of the solvatochromic probe methyl orange indicated three microenvironments within the microemulsions (a) a polar microenvironment like that found in dry PFPE reverse micelles (b) bulk water microenvironment and (c) an acidic microenvironment due to CO2 dissolved in water. 

The system of SDS/DTAB has been also investigated by Wan et al. ° The system involves the evolution of vesicles on mixing micelles of SDS and DTAB at a weight ratio of 1.6 1. Included in the reaction mixture was a low concentration of the solvatochromic probe 2,6-diphenyl-4-(2,4,6-triphenyl-l-pyridino) phenoxide ( (30)), which has been used previously to probe the interfacial properties of micelles and vesicles. Wan et al.i suggest that a vesicle interfacial environment is established rather rapidly, on the time scale of... 

All three methods give similar values of interfacial potentials typical results for some of micelles and vesicles are listed in Table 3. Also listed are estimates of interfacial dielectric constants (e), determined by comparing the position of absorption bands of solvatochromic indicators in the surfactant assemblies with that of reference 1,4-dioxane water mixtures with known e values. More generally, luminescence probe analysis [49], thermal leasing [50] and absorption spectroscopy [47, 51] are techniques that have all been utilized to measure local polarities in micelles and vesicles. It is important to note that these methods presume knowledge of the loca-... 

Other related kinds of medium-dependent behavior have been observed, such as second-sphere coordination effects (e.g., with crown ethers, cyclodextrins ), and solvatochromic medes have been used as probes of their environment in polymers, micelles, " zeolites, inorganic glasses, and surfaces, etc. They may be used in this way either by virtue of their response to the electric field experienced in a particular environment, or because of specific interactions between the probe and the environment, e.g., hydrogen bonding. Solvatochromism is also a useful predictor of nonlinear optical behavior (see Chapter 9.14), because the same properties which give rise to strong solvatochromism are also necessary for large, second-order, nonlinear optical coefficients (/3). ... 

Most of the indicator solutes used in the literature are polar and they probe the polar region of a micelle. It has been found that the n -, a-, and p-values for the aqueous phase, as determined from the value of the parameter in the aqueous phase iP in Equation 7.12), are almost equal to the values in pure water. Thus, the addition of surfactant molecules at concentrations below cmc does not have any significant influence on the solvation properties of water. On the contrary, values of the solvatochromic parameters in the micellar phase differ significantly from those in the aqueous phase. It appears that a micellar phase is characterized by a high polarity. This is rationalizable in view of the fact that the probes reside in the polar part of the micelle and report the values accordingly. 

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