Micelles local polarities

Most of the characteristics invoked to explain rate accelerations and rate retardations by micelles are valid for vesicles as well. For example, the alkaline hydrolysis of A-methyl-A-nitroso-p-toluenesulfonamide is accelerated by cationic vesicles (dioctade-cyldimethylammonium chloride). This rate acceleration is the result of a higher local OH concentration which more than compensates for the decreased polarity of the vesicular pseudophase (compared to both water and micelles) resulting in a lower local second-order rate constant. Similar to effects found for micelles, the partial dehydration of OH and the lower local polarity are considered to contribute significantly to the catalysis of the Kemp elimination " by DODAB vesicles. Even the different... 

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-... 

Since the dynamics of the twisted intramolecular charge transfer (TICT) process is very sensitive to the polarity of the medium, the local polarity of an organized medium may also be determined from the rate of the HCT process. For TNS, which is nearly nonfluorescent in water ((j)f = 10 and Xf = 60 ps), the emission quantum yield and lifetime increases nearly 50 times on binding to cyclodextrins and more than 500 times on binding to a neutral micelle, TX -100 [86]. Such a dramatic increase in the emission intensity and lifetime arises because of the marked reduction of the nonradiative HCT process inside the less polar microenvironment of the cyclodextrins and the micelle. Determination of the micropolarities of various organized assemblies using TICT probes has been surveyed quite extensively in several recent reviews [5b-d,f,86]. Therefore, in this chapter we will focus only on some selected works not covered in the earlier reviews. 

The local polarity of microheterogeneous solutions can be examined by studying the emission of intramolecular heteroexcimers with little charge separation (sometimes called mixed excimers ). These show measurable emission in polar solvents, in contrast to heteroexcimers of high dipole moment (i.e. more than 14 D). Zachariasse et al. (167) investigated a series of micelles, microemulsions, and phosphatidylcholine bilayers using the probe l-(4-biphenylyl)-3-(penta-methyOpropane ... 

Compared to other methods, the advantage of PCS is the ability to detect a very low critical micelle concentration [158, 160-163] (CMC) and a very low critical aggregation concentration [158, 160, 161] (CAC) as they often appear in block copolymer solutions. This could for example be demonstrated by Colombani et al. who could access the CMC of a diblock copolymers by PCS, but only obtained an upper estimate analyzing the absorption band of pyrene which is very sensitive to local polarity of its surrounding [162]. 

The shape of the fine structure in the absorption and fluorescence spectra of pyrene is very sensitive to local polarity (15a). The so-called Py--scale" of solvent polarities (15b) is a manifejstation of the Hamm effect (15c), whereby locally anisotropic electric fields relax the forbiddenness of the (0,0) band in Si -> So transitions of symmetrical aromatic chromophores. Thomas has used pyrene fluorescence to probe local polarity in aqucious micelles and in aqueous solutions of polyelec trolytes (15d). 

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

Sodium cholate is insoluble in chloroform and in nonpolar solvents in general, but it is very soluble in alcohol and in water. Lecithin, on the contrary, is soluble in chloroform and only swells in water without dissolving in it. These differences in solubility are evidently related to the molecular structure and to the position of the hydrophilic groups in each of these molecules. The lecithin molecule has two important paraffinic chains and a group of hydrophilic functions (choline phosphate) localized at one end. In the presence of water, the lecithin molecules are oriented with their hydrophilic groups toward the water, and they hide their paraffinic chains inside a structure formed of two superposed layers of molecules. Conversely, in a nonpolar solvent the paraffinic chains are turned toward the solvent, while the polar groups are hidden inside the micelle. 

In other media like micelles, cyclodextrin, binary solvent mixtures, and proteins (47-55), lifetime distributions are routinely used to model the decay kinetics. In all of these cases the distribution is a result of the (intrinsic or extrinsic) fluorescent probe distributing simultaneously in an ensemble of different local environments. For example, in the case of the cyclodextrin work from our laboratory (53-55), the observed lifetime distribution is a result of an ensemble of 1 1 inclusion complexes forming and coexisting. These complexes are such that the fluorescent probe is located simultaneously in an array of environments (polarities, etc.) in, near, and within the cyclodextrin cavity, which manifest themselves in a distribution of excited-state lifetimes (53-55). In the present study our experimental results argue for a unimodal lifetime distribution for PRODAN in pure CF3H. The question then becomes, how can a lifetime distribution be manifest in a pure solvent ... 

The rate and yields of dimerization for isophorone (51)48) has been found to be enhanced in micellar microemulsions relative to homogeneous solutions (Scheme XVIII). In homogeneous solutions, the ratio of head-to-head/head-to-tail (HH/HT) dimers increases with increasing polarity, but in micelles polarity effects alone cannot fully explain the observed ratios. The change in the regioselectivities and high yields is attributed to the localization of the substrate in the vicinity of the Stem layer where polarity and orientational effects are quite strong. 

Methoxy-l-nitronaphthalene (73a) and 1-nitronaphthalene (73b) undergo photochemical aromatic substitution reactions with cyanide (Scheme XXVIII). A two-fold increase in the quantum yield for the reaction is observed for (73a) when the reaction occurs in HDTC1 compared to aqueous solution 73). However, a 6800-fold catalytic increase in quantum yield is observed for (73b). SDS micelles decrease the quantum yield compared to aqueous solutions. The higher local concentration of cyanide near the HDTC1 micelles can explain a least partially the increase in quantum yield. However, the 6800-fold increase for (73b) is also due to a polarity effect on the reaction. This was demonstrated by an increase in the quantum yield of the reaction with decreasing polarity. 

Fluorescent organic compounds have been widely used as molecular-microscopic probes in biophysical studies of the local environment in micelle-forming surfactant solutions, in phospholipid dispersions, and in membranes. It is assumed that the nature of the probe environment is reflected in its emission characteristics [i.e. position and intensity of emission maxima, vibrational fine structure, quantum yields, excited-state lifetime, polarization of the fluorescence) cf [112, 115, 360] for reviews. 

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