Micelles solvent relaxation

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

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. 

Telgmann and Kaatze studied the stmcture and dynamics of micelles using ultrasonic absorption in the 100-KHz to 2-GHz frequency range [100]. They detected several relaxation times in the long (ps), intermediate (10 ns), and fast (0.1-0.3 ns) time scale. The longest relaxation time has been attributed to the exchange of monomer between bulk and the micelles, and the fastest to the rotation of the alkyl chains of the surfactants in the core of the micelle. The intermediate relaxation time has not been assigned to any particular motion. We will discuss later that the intermediate relaxation times in the 10-ns time scale may well be due to solvent relaxation in the Stem layer. 

Keywords Block copolymer micelles Fluorescence anisotropy Fluorescence correlation spectroscopy Molecular dynamics simulations Monte Carlo simulations Solvent relaxation method Time-resolved fluorescence... 

The second example concerns the multidisciplinary study of the micelliz-ing block copolymer polystyrene-( -poly(2-vinylpyridine)-ft-poly(ethylene oxide) (PS-PVP-PEO), which shows a high tendency to aggregation and the formation of micellar clusters [88,89]. It shows the application of SRM for studying the mobility and structural details of different domains in micelle-like polymeric nanoparticles. The fluorescence technique reveals interesting features of studied systems that are hardly accessible by other techniques. Section 3.3 is devoted to the development of the methodology of the solvent relaxation technique for studying nanostructured self-assembling systems. 

First, we studied the solvent relaxation in solutions of diblock copolymer micelles. A commercially available polarity-sensitive probe, patman (Fig. 10, structure I), frequently used in phospolipid bilayer studies [123], was added to aqueous solutions of PS-PEO micelles. The probe binds strongly to micelles because its hydrophobic aliphatic chain has a strong affinity to the nonpolar PS core. The positively charged fluorescent headgroup is supposed to be located in the PEO shell close to the core-shell interface. The assumed localization has been supported by time-resolved anisotropy measurements. 

As we intended to study the pH-dependent hydration of PEO in triblock copolymer micelles, we measured the solvent relaxation for patman embedded in PS-PEO micelles both in acidic (0.01 M HCl) and alkaUne (0.01 M NaOH) solutions for comparison. Because we found only marginal differences in the relaxation behavior, we can conclude that the dye itself does not exhibit any pH-dependent changes after binding to micelles and that the solvation of short PEO does not change much with pH (it is very important to emphasis that the PEO blocks are significantly shorter than those in the studied PS-PVP-PEO copolymer). 

Consequently, the solvent relaxation was studied in PS-PVP micelles in 0.01 M HCl solution. The micelles are stable in acidic solution, where they are positively charged. Nevertheless, our earlier studies suggest that the PVP layer partially collapses around the core because PVP is only slightly protonized close to the nonpolar PS core [132], A fairly high value of the residual anisotropy (around 0.2) measured at the maximum emission intensity (467 nm) suggests that patman is embedded in considerably rigid and little protonized domains close to the PS-PVP interface, which means that its location in PS-PVP micelles is similar to that in PS-PEO micelles. 

Han and coworkers [38] determined the phase behavior of the ternary system consisting of [bmim][PFJ,TX-100, and water at 25 °C. By cyclic voltammetry method using potassium ferrocyanide, K Fe(CN)g, as the electroactive probe, the water-in-[bmim][PFJ, bicontinuous, and [bmim][PFJ-in-water microregions of the microemulsions were identified (Fig. 16.7). The hydrodynamic diameter of the [bmim] [PFJ-in-water microemulsions is nearly independent of the water content bnt increases with increasing [bmim] [PF ] content due to the swelling of the micelles by the IL. Sarkar and coworkers [39-41] reported the solvent and rotational relaxation studies in [bmim][PFJ-in-water microemulsions and water-in-[bmim][PFJ microemulsions using different types of probes, coumarin 153 (C-153), coumarin 151 (C-151), and coumarin 490 (C-490). The solvent relaxation time is retarded in the IL-in-water microemulsion compared to that of a neat solvent. The retardation of solvation time of water in the core of the water-in-IL microemulsion is several thousand times compared to pnre water. Nozaki and coworkers [42] reported a broadband dielectric spectroscopy study on a microemnlsion composed of water. 

Humpolickova J, Stepanek M, Prochazka K, Hof M (2005) Solvent relaxation study of pH-dependent hydration of poly(oxyethylene) shells in polystyrene-block-poly (2-vinylpyridine)-block-poly(oxyethylene) micelles in aqueous solutions. J Phys Chem A 109(48) 10803-10812. doi 10.102l/jp053348v... 

Abstract We discuss applications of selected fluorescence spectroscopy techniques for the studies of block copolymer micelles in aqueous solution, focusing on solvent relaxation measurements using polarity-sensitive fluorescent probes, oti fluorescence quenching studies, and on using fluorescent pH indicators for studies... 

While there are many papers in the literature on the solvent relaxation in solutions of proteins, surfactant micelles, and phospholipid vesicles studied by time-resolved emission spectroscopy [25-27], similar studies focused on block copolymer micelles are scarce. In most cases, amphiphilic fluorescent probes localized in the inner part of the shells of amphiphilic block copolymer micelles in aqueous solutions were used for the studies. The studies reveal the heterogeneity of the binding sites of the probe that manifest itself by multiple-exponential fluorescence decays. In the case of block copolymer micelles, interpretation of the relaxation behavior can be complicated by redistribution of the probe molecules in the micelles during its excited-state lifetime of the probe [28]. The redistribution occurs as a result of the increased polarity of the excited probe as compared with its ground electronic state. 

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-... 

In the early 1960s it became evident that the reaction environment had an important role in dictating the course of photochemical conversions acting on the course of the relaxation processes and stabilizing photoproducts.17 A constrained medium such as that of a porous matrix or a micelle provides the restricted environment to stop any bimolecular processes that could lead to degradation of products. These effects, however, are subtle. For instance, confinement of a molecule within a host instead of leading to inhibition of reactions of the trapped substrate often results in enhanced reactivity and selectivity because confinement does not mean steric inhibition of all motions of the entrapped host molecule which may eventually enjoy less restriction of some motions than in common solvents. 

The photochemistry and photophysics of trans-stilbene derivatives (24) have been utilized by Whitten and co-workers to understand the relaxation characteristics of media such as micelles, monolayers, and LB films [144,145]. For example, the d>,rans to Cis for stilbene derivatives show the following trend solvent system methylcyclohexane > SDS micelle multilayer assemblies (with arachidic acid). In fact, no isomerization is observed in multilayer assemblies. This is the trend expected on the basis of how readily the media can respond to stilbene shape changes during isomerization process. 

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... 

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