Microviscosity molecules

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. 

Vesicle size, bilayer fluidity, membrane permeability, microviscosity, ability to bind small molecules, suseeptibility to pore formation, flip-flop rates, extent of water penetration, lateral amphiphile diffusion, vesiele fusion, and kinetic medium effeets (some of which will be discussed briefly below) all depend on the paeking of... 

Although usually the exact values of the effective radius, reg, and of the microviscosity, /, are not known, rotational correlation times for similar systems with different molecular weight can be well compared using this equation, supposing that the microviscosity and the density of the molecules are the same, thus the ratio of the r3eff values can be expressed by the ratio of the molecular weights. 

In this approach the molecule is treated as a sphere of radius a in a uniform cavity characterized by a microviscosity ij and an apparent molar volume Vm. Over the years many studies correlating T, with solution viscosity have been carried out. The record of linear correlations is patchy, partly because the microviscosity is not an experimentally available number. However, in large, relatively flat... 

We suggested that addition of a saturated fatty-acid tail (buoy) that incorporates in the antioxidant molecule membrane may make the membrane more rigid and thus contribute to the therapeutic effect of ichfan. As is seen from Table 1, after introduction of the compoimd to mice, the microviscosity of the membrane near-surface sites studied by the method of EPR spin probes either changes insignificantly or increases the latter is a desirable effeet. It should be emphasized particularly for membranes isolated from a coarse fraction of synaptosomes because AD is associated mostly with damages of nerve fibers. 

Steady-state depolarization measurements of the fluorescence of probe molecules dispersed in the hydrocarbon interior of a lipH bilayer have been used extensively as a probe of the lipid microviscosity. This applkation has been reviewed Two inherent assumptions in the method, however, cast some doubt on the results obtained. Firstly the probe molecule is assumed to be a rigid spherical body rotating isotropically, a situation unlikely considering the heterogeneous nature of the system. Secondly, it is amimed that the hydrocarbon interior can be modelled using homogeneous solvents. These problems can be lai ely overcome with the use of time-resolved fluorescence depolarization measurements. 

However, it was the microviscosity, rather than the bulk viscosity which was probed by using picosecond time-resolved fluorescence methods, since the value of C was constant within a homologous series of solvents but was different for monofunctional and difunctional alcohols. The probe molecule has a natural (radiative) lifetime, Xp of 4.6ns, so the reduction in lifetime results from the competition between fluorescence ( p 2xl0 s ) and the high rate of internal conversion (A ic lO" s ). This was attributed to rotation of the N-tolyl group of the dye, which then has to sweep through the solvent and hence probes the local environment. 

Since photoisomerization involves motion of one-half of the probe molecule against the other, it remains unaffected by the overall motions of the organized assemblies, such as the motion of different segments of DNA, or of the peptide chains of a protein or the surfactants in a micelle. In contrast, in optical anisotropy studies, the bending and twisting motion of the macromolecular chains of DNA, proteins, or micelles become superimposed on the orientational motion of the probe. As a result, photoisomerization seems to be a better probe for the microviscosity of complex biological assemblies. 

Trimeric and oligomeric surfactants have also been prepared (Zana, 1995 Sumida, 1998 In, 2000 Onitsuka, 2001). Their CMC values are even smaller than those of the analogous geminis. As the number of hydrophobic groups per molecule increases for gemini quaternary C12 ammonium compounds with polymethylene -(CH2) j spacers, their surface layers become more dense, their micellar microviscosity increases, and their micellar shape changes from spherical to wormlike, to... 

An important feature of the mosaic or subunit model of the molecular organization of lipid and lipid-protein films and membranes (4, 5,12) is the cohesion between subunits and between molecules within each subunit (2, 13). The latter can be measured as microviscosity by polarization of fluorescence (14) of the surface film the former is measured as surface viscosity by various techniques (15,16). The relevance of this parameter to the exploration of biological surfaces cannot be overemphasized. Surface viscosity, in fact, has also been invoked as a force stabilizing pulmonary alveoli (6, 7,13, 16). 

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