Micelles relaxation times

The adsorption kinetics in micellar systems was studied first by Lucassen (1975). He defined a micelle relaxation time... 

Chemical-shift measurement has proven to be useful in determining cmc, surfactant chain conformation, and the extent of water penetration into micelle. Menger [39] found that as many as three methylene groups of an alkyl chain near the head group are hydrated in a micelle. Relaxation times are related to translational and rotational molecular motions. These also can give us informa-... 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Similar conclusions were obtained from lH and 31P NMR and also from IR studies of egg phospholecithin reversed micelles in benzene by Boicelli et al. 58 61). According to the results of these experiments the water structure within the reversed phospholecithin micelles alters considerably compared with water in bulk. This becomes evident from the shortening of the relaxation time T, of the water protons split into two relaxation times T1A and T1B, indicating that there are at least two... 

The addition of salts modifies the composition of the layer of charges at the micellar interface of ionic surfactants, reducing the static dielectric constant of the system [129,130]. Moreover, addition of an electrolyte (NaCl or CaCli) to water-containing AOT-reversed micelles leads to a marked decrease in the maximal solubihty of water, in the viscosity, and in the electrical birefringence relaxation time [131],... 

In addition, water motion has been investigated in reverse micelles formed with the nonionic surfactants Triton X-100 and Brij-30 by Pant and Levinger [41]. As in the AOT reverse micelles, the water motion is substantially reduced in the nonionic reverse micelles as compared to bulk water dynamics with three solvation components observed. These three relaxation times are attributed to bulklike water, bound water, and strongly bound water motion. Interestingly, the overall solvation dynamics of water inside Triton X-100 reverse micelles is slower than the dynamics inside the Brij-30 or AOT reverse micelles, while the water motion inside the Brij-30 reverse micelles is relatively faster than AOT reverse micelles. This work also investigated the solvation dynamics of liquid tri(ethylene glycol) monoethyl ether (TGE) with different concentrations of water. Three relaxation time scales were also observed with subpicosecond, picosecond, and subnanosecond time constants. These time components were attributed to the damped solvent motion, seg-... 

This approach was followed by Yushmanov for the localization of papaverine in ionic micelles.42 Another interesting application was reported by Chien43 who measured 19F NMR relaxation times of trifluor-omethyl labelled atrazine induced by paramagnetic probes gadolinium ethylenediamine tetraacetic acid and 2,2,6,6-tertramethyl-piperidine-N-oxyl. The results showed that atrazine solubilized by humic micelles occupied a hydrophobic domain accessible only to neutral hydrophobic molecules. 

The relaxation times vary with time itself only when the backbone becomes full-stretched (A=q), and then in such a way as to maintain this maximum stretch until the flow no longer tends to stretch the molecules further. The history of relaxation time Zj, needs to be taken into account in the integral part of the dynamic equations, just as for wormlike micelles [72]. The stress itself is a function of both molecular variables ... 

The internal rotational relaxation times of 1-pyrene carboxaldehyde in sulfonate systems may offer some indication of the extent of probe binding to the inverted micelle. In the absence of any background fluorescence interference to the time-dependent anisotropy decay profile, the internal rotational relaxation time should correlate with the strength of binding with the polar material in the polar core. However, spectral interference from the aromatic moieties of sulfonates is substantial, so that the values of internal rotational relaxation time can only be used for qualitative comparison. 

SDS micelles [188-190]. These results may be a consequence of a lack of template-induced orientation or of the orientational forces being too weak to overcome the orientational preferences between an excited and a ground state molecule. It is certainly the case in all of the micellar examples cited that the solvent relaxation times should allow molecules to reorient themselves at the interface (should they so choose) on timescales which are comparable to those necessary for an excited molecule to form its photoproducts. 

Nuclear magnetic resonance (NMR) has been used to study segmental motions in block copolymer solutions. The mobility of protons in polymer chains in dilute solutions has been probed using high-resolution H NMR. Association of chains into micelles leads to a reduction in mobility in the core, which leads to a broadening of the respective NMR lines that has been studied for a number of systems, as described by Tuzar and Kratochvil (1993). The sol-gel transition in concentrated solutions has been located via ]H transverse relaxation time experiments, as outlined in Chapter 4. 

The second possible geometry will arise if the micellization proceeds by aggregation of copolymer chains that already have a glassy head. Although little is known about the structure of individual glassy chains in solution, the characteristic relaxation time ofthe chain is likely to depend on the amount of solvent actually present in the collapsed globule. In the extreme case of zero solvent... 

Several spectroscopic techniques, namely, Ultraviolet-Visible Spectroscopy (UV-Vis), Infrared (IR), Nuclear Magnetic Resonance (NMR), etc., have been used for understanding the mechanism of solvent-extraction processes and identification of extracted species. Berthon et al. reviewed the use of NMR techniques in solvent-extraction studies for monoamides, malonamides, picolinamides, and TBP (116, 117). NMR spectroscopy was used as a tool to identify the structural parameters that control selectivity and efficiency of extraction of metal ions. 13C NMR relaxation-time data were used to determine the distances between the carbon atoms of the monoamide ligands and the actinides centers. The II, 2H, and 13C NMR spectra analysis of the solvent organic phases indicated malonamide dimer formation at low concentrations. However, at higher ligand concentrations, micelle formation was observed. NMR studies were also used to understand nitric acid extraction mechanisms. Before obtaining conformational information from 13C relaxation times, the stoichiometries of the... 

The phase separation model is particularly useful for describing the amount of micellized amphiphile and how molecular properties vary with amphiphile concentration. The average of a quantity Q (which can be a diffusion coefficient, a NMR chemical shift, a NMR relaxation time etc.) is determined by the fractions micellized,... 

When the system has reached its quasi-equilibrium state a slower process, involving the relaxation to the true equilibrium, becomes measureable. This process involves a change in the number of micelles. The formation or dissolution of a micelle involves according to scheme (5.1) the appearence of aggregates of size at the minimum of the size distribution curve, and since these aggregates occur with low probability the process can be a very slow one. Aniansson and Wall showed that this process is also characterized by an exponential decay with a relaxation time r2,... 

The type of molecular information that can be obtained from measurements of the fast relaxation time is illustrated in Table 5.1 where data for a series of sodium alkylsulfates are presented. One can see that for micelles of short-chain surfactants,... 

In principle, the proportion of K-casein on the external surface can be studied by using cross-linked or immobilized enzymes, antibodies, lectins, or other specific functionalities. This approach is not without its problems, however. The rate of exchange of K-casein between the micelle and milk serum has not been established but may have a relaxation time of several hours, similar to that for p-casein (Creamer et al., 1977). If this rate is of the order of the time of the experiment, or faster, the proportion apparendy in the surface will be overestimated. Likewise, solubilization of micellar K-casein, e.g., by dissociation during the experiment, would lead to a similar error. 

In other cases, several discrete relaxation times or distributions of relaxation times can be found [39]. This is typically the case if the stress relaxation is dominated by reptation processes [42 ]. The stress relaxation model can explain why surfactant solutions with wormlike micelles never show a yield stress Even the smallest applied stress can relax either by reptation or by breakage of micelles. For higher shear rates those solutions typically show shear thinning behaviour and this can be understood by the disentanglement and the orientation of the rod-like micelles in the shear field. 

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