Microviscosity

Sun Y-P and Saltiel J 1989 Application of the Kramers equation to stiibene photoisomerization in / -alkanes using translational diffusion coefficients to define microviscosity J. Phys. Chem. 93 8310-16... 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

The strong interactions between the water molecules also become obvious from NMR measurements by Tsujii et al..57) 13C-NMR experiments were used for determining the microviscosity of water in reversed micelles of dodecylammonium-propionate with 13C glycine cosolubilized. It was found that the apparent viscosity of the water-pool corresponds to the viscosity of a 78 % aqueous glycerol solution, obviously as a consequence of the extended network formation by strong hydrogen bonding. 

Thus, the size and the reactivity of the initiator-derived radicals and the medium viscosity (or microviscosity) are important factors in determining the initiator efficiency. Thus, the extent of the cage reaction is likely to increase with... 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

With further understanding how molecular rotors interact with their environment and with application-specific chemical modifications, a more widespread use of molecular rotors in biological and chemical studies can be expected. Ratiometric dyes and lifetime imaging will enable accurate viscosity measurements in cells where concentration gradients exist. The examination of polymerization dynamics benefits from the use of molecular rotors because of their real-time response rates. Presently, the reaction may force the reporters into specific areas of the polymer matrix, for example, into water pockets, but targeted molecular rotors that integrate with the matrix could prevent this behavior. With their relationship to free volume, the field of fluid dynamics can benefit from molecular rotors, because the applicability of viscosity models (DSE, Gierer-Wirtz, free volume, and WLF models) can be elucidated. Lastly, an important field of development is the surface-immobilization of molecular rotors, which promises new solid-state sensors for microviscosity [145]. 

Kung CE, Reed JK (1986) Microviscosity measurements of phospholipid bilayers using fluorescent dyes that undergo torsional relaxation. Biochemistry 25 6114—6121... 

Lukac S (1984) Thermally induced variations in polarity and microviscosity of phospholipid and surfactant vesicles monitored with a probe forming an intramolecular charge-transfer complex. J Am Chem Soc 106 4386 -392... 

Vogel M, Rettig W (1987) Excited state dynamics of triphenylmethane-dyes used for investigation of microviscosity effects. Ber Bunsenges Phys Chem 91 1241-1247... 

A second approach with respect to anisotropic flavin (photo-)chemistry has been described by Trissl 18°) and Frehland and Trissl61). These authors anchored flavins in artificial lipid bilayers by means of C18-hydrocarbon chains at various positions of the chromophore. From fluorescence polarization analysis and model calculations they conclude, that the rotational relaxation time of the chromophore within the membrane is small compared to the fluorescence lifetime (about 2 ns74)). They further obtain the surprising result that the chromophore is localized within the water/lipid interface, with a tilt angle of about 30° (long axis of the chromophore against the normal of the membrane), irrespective of the position where the hydrocarbon chain is bound to the flavin nucleus. They estimate an upper limit of the microviscosity of the membrane of 1 Poise. 

In accordance with the above, it is clear that the organized media may play an important role in the development of CL reactions. This role may be shown in the improvement of the sensitivity, precision, and selectivity of many CL reactions, due principally to the change of the microenvironment of the CL system. Organized media can alter the microviscosity, local pH, polarity, reaction pathway or rate, etc. This situation allows application of these organized media to determination of organic and inorganic analytes in different kinds of matrices using CL reactions. A summary of these applications is shown in Table 5. 

Bagchi M, Hassoun EA, Stohs SJ. 1992a. Endrin-induced increases in hepatic lipid peroxidation, membrane microviscosity, and DNA damage in rats. Arch Environ Contain Toxicol 23 1-5. 

Polymers dynamics of polymer chains microviscosity free volume orientation of chains in stretched samples miscibility phase separation diffusion of species through polymer networks end-to-end macrocyclization dynamics monitoring of polymerization degradation... 

Vesicles characterization of the bilayer microviscosity, order parameters phase transition effect of additives internal pH permeability... 

The diffusion constant D with the underlying microviscosity , and the two order parameters , <(P4> reflecting the degree of orientational constraint have been successfully determined from the fluorescence anisotropy decay in... 

Micellar systems Internal microviscosity of micelles Fluidity and order parameters (e.g. bilayers of vesicles)... 

Microviscosity, fluidity, molecular mobility. Estimation by means of fluorescent probes... 

We should first emphasize that viscosity is a macroscopic parameter which loses its physical meaning on a molecular scale. Therefore, the term microviscosity should be used with caution, and the term fluidity can be alternatively used to characterize, in a very general way, the effects of viscous drag and cohesion of the probed microenvironment (polymers, micelles, gels, lipid bilayers of vesicles or biological membranes, etc.). 

The term microviscosity is often used, but again no absolute values can be given, and the best we can do is to speak of an equivalent viscosity, i.e. the viscosity of a homogeneous medium in which the response of the probe is the same. But a difficulty arises as to the choice of the reference solvent because the rotational relaxation rate of a probe in various solvents of the same macroscopic viscosity depends on the nature of the solvent (chemical structure and possible internal order). 

These preliminary considerations should be borne in mind during the following discussion on the various methods of characterization of microviscosity . 

In conclusion, the method of intramolecular excimer formation is rapid and convenient, but the above discussion has shown that great care is needed for a reliable interpretation of the experimental results. In some cases it has been demonstrated that the results in terms of equivalent microviscosity are consistent with those obtained by the fluorescence polarization method (described in Section 8.5), but this is not a general rule. Nevertheless, the relative changes in fluidity and local dynamics upon an external perturbation are less dependent on the probe, and useful applications to the study of temperature or pressure effects have been reported. 

The changes in correlation time upon an external perturbation (e.g. temperature, pressure, additive, etc.) reflects well the changes in fluidity of a medium. It should again be emphasized that any microviscosity value that could be calculated from the Stokes-Einstein relation would be questionable and thus useless. 

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