Do ionic and hydrophobic probes sense similar microenvironment in Triton X-100 nonionic reverse micelles?

Abstract

Rotational diffusion of two structurally similar ionic probes, rhodamine 110 and fluorescein, has been examined in nonionic reverse micellar system of Triton X-100/benzene-n-hexane/water as a function of mole ratio of the water to surfactant, W. This study has been undertaken to find out whether ionic and hydrophobic probes experience similar microenvironment in these reverse micelles. Experimental results indicate that, from W=0 to 3, the average reorientation time, which is a measure of the microviscosity experienced by the probe molecule, increases by 90% and 40% for rhodamine 110 and fluorescein, respectively, and from W=3 to 8, it decreases by 20% for both the probes. The increase in the average reorientation time with W has been rationalized on the basis of the flexible oxyethylene chains of the TX-100 surfactant being hydrogen bonded by the water molecules, which makes the core region less fluid. However, once the hydration of the oxyethylene chains is complete, further addition of water results in formation of water droplet; which renders the micelle-water interface in the core region less compact leading to a marginal decrease in the average reorientation time of the probe molecules. These explanations are consistent with the location of the probes and the structure of the Triton X-100/benzene-hexane/water reverse micelles. To compare how the microenvironment experienced by these ionic probes is different from the hydrophobic ones, results from our earlier work [J. Phys. Chem. B 108, 7944 (2004)] have been considered. Such a comparison revealed that both ionic and hydrophobic probes experience similar microenvironment in these reverse micelles until the hydration of the oxyethylene chains is complete. In case of hydrophobic probes, however, the onset of water droplet formation does not alter their microenvironment, which is due to their location in the reverse micellar cores.

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“…This situation can be compared to that reported by a series of interesting studies by Dutt exploiting both ionic and hydrophobic probes in non-ionic reverse micelles and also with that reported by Singh et al in a recent article. 8,[25][26][27] Interestingly, with increasing water content, only slightly perturbed steady state anisotropy prole was observed (vide Fig. 1d) which is in accordance to our initial results.…”

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“…In addition, we have also investigated the motion of probe molecules during this microstructural variation to monitor the dynamical properties of this SAILs-AMP containing aggregates. The dynamical properties of probe molecules in various self-assemblies depend upon its location and interaction with the surrounding microenvironments. − In organized assemblies, the solubilization sites and the mode of interaction of differently charged probe molecules depend upon the nature and charge of the aggregates. In the SAILs-AMP aggregates, the microstuctural nature will be different compared to that of the normal SAIL forming micelles.…”

What Governs Molecular Rotation at the Interfaces of Reverse Micelles?

“…Since the parameter microviscosity cannot be measured by direct experimental methods, it is difficult to predict the rotation of a probe molecule solubilized in an organized assembly. Among the organized assemblies, probe rotation in reverse micelles has been extensively investigated [2][3][4][5][6][7][8][9] and the observed reorientation times have been rationalized in terms of micellar packing, albeit in a qualitative manner. In other words, no quantitative relationship has been reported in literature.…”

“…However, two time constants were needed to fit the anisotropy decays of rhodamine 110 in the reverse micellar system and the functional form is given by Equation (1) rðtÞwhere t r1 and t r2 are the time constants associated with the decay of the anisotropy, b is the normalized amplitude corresponding to t r1 and r 0 is anisotropy at t = 0, whose value depends on the angle between absorption and emission transition dipoles of the probe molecule. The average reorientation time < t r > was calculated using Equation (2).It has been established from our earlier work [7][8][9] that rhodamine 110 is located in the interfacial region of the reverse micelles, and the two time constants arise as a consequence of the probe molecule undergoing slow lateral diffusion on the curved surface of the reverse micelle and a fast wobbling motion in an imaginary cone, which are coupled to the overall rotation of the micelle. However, the time constant for the overall rotation of the micelle in the present scenario has been found to be significantly longer than the two time constants obtained from the analysis and hence does not contribute to the decay of the anisotropy.…”

“…It has been established from our earlier work [7][8][9] that rhodamine 110 is located in the interfacial region of the reverse micelles, and the two time constants arise as a consequence of the probe molecule undergoing slow lateral diffusion on the curved surface of the reverse micelle and a fast wobbling motion in an imaginary cone, which are coupled to the overall rotation of the micelle. However, the time constant for the overall rotation of the micelle in the present scenario has been found to be significantly longer than the two time constants obtained from the analysis and hence does not contribute to the decay of the anisotropy.…”