Temperature effect on the fluorescence anisotropy decay dynamics of Coumarin-153 dye in Triton-X-100 and Brij-35 micellar solutions

The fluorescence anisotropy decay dynamics of the fluorescent probe Coumarin-153 (C153) have been investigated in two neutral micelles, Triton-X-100 (TX-100) and Brij-35 (BJ-35), at different temperatures and analyzed on the basis of the well-known two-step model. Because steady-state fluorescence spectra of the above probe do not show any noticeable changes with respect to temperature, for either of the studied micelles, suggests a similar polarity in the microenvironment around the probe at all the temperatures studied. The anisotropy results indicated that, for both the micelles, the fluidity inside the Palisade layer increases with temperature. However, the temperature effect on the anisotropy decay is relatively more pronounced in TX-100 than in BJ-35. It is inferred that the temperature effect on the anisotropy decay in the BJ-35 micelle is mainly due to the thermal effect on the microviscosity in the micellar phase. In the case of TX-100, the results indicate that, along with the above thermal effect, an additional effect is observed due to the increased size and hydration of the micelle with temperature, with the result being that the fluorescence anisotropy decay in TX-100 is more sensitive to temperature than in BJ-35. In the TX-100 micelle, our studies show that with an increase in temperature, even though the micellar size increases substantially, the distance of the probe from the micellar core does not increase that significantly. Thus, with increasing temperature, the probe undergoes a relative migration toward the micellar core to avoid the increased hydration in the micellar Palisade layer.     

Effect of lithium chloride on the palisade layer of the Triton-X-100 micelle: two sites for lithium ions as revealed by solvation and rotational dynamics studies

Dynamic Stokes' shift measurements using coumarin 153 as the fluorescence probe have been carried out to explore the effect of added electrolyte, lithium chloride (LiCl), on solvation dynamics in the Triton-X-100 (TX-100) micelle and thus to understand the changes in micellar Palisade layer, especially the entrapped water structures in the Palisade layer. At all concentrations of LiCl, the spectral shift correlation function shows biexponential decay. At lower LiCl concentrations, the longer solvation time is seen to decrease, although the shorter solvation time is not affected much. At higher LiCl concentrations, both longer and shorter solvation times increase with electrolyte concentration. The present observations have been rationalized assuming two possible modes of interaction of the Li+ ions in the micellar palisade layer. For LiCl concentrations below about 1.5 M, the Li+ ions appear to bind preferably to the ether groups of surfactant molecules, and the increased micellar hydration with the added salt effectively makes the solvation dynamics faster. At higher LiCl concentrations, available ether binding sites for the Li+ ions seem to get occupied effectively and the excess Li+ ions start remaining in the Palisade layer as strongly hydrated free ions. Because of strong hydration of the Li+ ions, the mobility of the entrapped water molecules in the micellar Palisade layer decreases significantly, causing the solvation dynamics to slow at higher LiCl concentrations. The fluorescence anisotropy results in the present systems are also in support of the above inferences drawn from solvation dynamics results. The present results with LiCl salt are found to be substantially different than those obtained in our earlier study (Kumbhakar et al. J. Phys. Chem. B 2005, 109, 14168) with salts such as NaCl, KCl, and CsCl. These differences are attributed mainly to the binding of the Li+ ions with the surfactant ether groups, which seems to be unlikely for the other alkali cations.     

Effect of micellar environment on Marcus correlation curves for photoinduced bimolecular electron transfer reactions

Photoinduced electron transfer (ET) between coumarin dyes and aromatic amine has been investigated in two cationic micelles, namely, cetyltrimethyl ammonium bromide (CTAB) and dodecyltrimethyl ammonium bromide (DTAB), and the results have been compared with those observed earlier in sodium dodecyl sulphate (SDS) and triton-X-100 (TX-100) micelles for similar donor-acceptor pairs. Due to a reasonably high effective concentration of the amines in the micellar Stern layer, the steady-state fluorescence results show significant static quenching. In the time-resolved (TR) measurements with subnanosecond time resolution, contribution from static quenching is avoided. Correlations of the dynamic quenching constants (k(q) (TR)), as estimated from the TR measurements, show the typical bell-shaped curves with the free-energy changes (DeltaG(0)) of the ET reactions, as predicted by the Marcus outersphere ET theory. Comparing present results with those obtained earlier for similar coumarin-amine systems in SDS and TX-100 micelles, it is seen that the inversion in the present micelles occurs at an exergonicity (-DeltaG(0)> approximately 1.2-1.3 eV) much higher than that observed in SDS and TX-100 micelles (-DeltaG(0)> approximately 0.7 eV), which has been rationalized based on the relative propensities of the ET and solvation rates in different micelles. In CTAB and DTAB micelles, the k(q) (TR) values are lower than the solvation rates, which result in the full contribution of the solvent reorganization energy (lambda(s)) towards the activation barrier for the ET reaction. Contrary to this, in SDS and TX-100 micelles, k(q) (TR) values are either higher or comparable with the solvation rates, causing only a partial contribution of lambda(s) in these cases. Thus, Marcus inversion in present cationic micelles is inferred to be the true inversion, whereas that in the anionic SDS and neutral TX-100 micelles are understood to be the apparent inversion, as envisaged from two-dimensional ET theory.     

Nature of the water molecules in the palisade layer of a triton X-100 micelle in the presence of added salts: A solvation dynamics study

The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.     

Dynamics of solvent and rotational relaxation of Coumarin 153 in a room temperature ionic liquid, 1-butyl-3-methylimidazolium octyl sulfate, forming micellar structure

The solvent and rotational relaxation of Coumarin 153 (C-153) was investigated by picosecond time-resolved fluorescence spectroscopy in a room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium octyl sulfate ([C4mim][C8SO4]). This is a typical RTIL, which form micellar structure above certain concentration of the RTIL (0.031 M). Dynamic light scattering (DLS) measurements show that the average hydrodynamic diameter ( Dh) of a [C4mim][C8SO4]-water micelle is 2.8 (+/-0.2) nm. Both the solvent and rotational relaxation of C-153 are retarded in this micelle compared to the solvation time of a similar type of dye in neat water. However, the solvent relaxation in this ionic liquid surfactant is different from that of a conventional ionic surfactant. The slow component of the solvation dynamics in C8H17SO4Na or TX-100 micelle is on the nanoseconds time scale, whereas in [C4mim][C8SO4] micelle the same component is on the subnanoseconds time scale. The different molecular motions with different time scale is the main reason behind this difference in the solvation time in micelles composed of RTIL with other conventional micelles.     

Spectral studies of N-nonyl acridine orange in anionic, cationic and neutral surfactants

The spectroscopic and photophysical properties of N-nonyl acridine orange - a metachromatic dye useful as a mitochondrial probe in living cells - are reported in water and microheterogeneous media: anionic sodium dodecylsulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB) and neutral octylophenylpolyoxyethylene ether (TX-100). The spectral changes of N-nonyl acridine orange were observed in the presence of varying amount of SDS, CTAB and TX-100 and indicated formation of a dye-surfactant complex. The spectral changes were also regarded to be caused by the incorporation of dye molecules to micelles. It was proved by calculated values K(b) and f in the following order: K(bTX-100)>K(bCTAB)>K(bSDS) and f(TX-100)>f(CTAB)>f(SDS). NAO binds to the micelle regardless the micellar charge. There are two types of interactions between NAO and micelles: hydrophobic and electrostatic. The hydrophobic interactions play a dominant role in binding of the dye to neutral TX-100. The unexpected fact of the binding NAO to cationic CTAB can be explained by a dominant role of hydrophobic interactions over electrostatic repulsion. Therefore, the affinity of NAO to CTAB is smaller than TX-100. Electrostatic interactions play an important role in binding of NAO to anionic micelles SDS. We observed a prolonged fluorescence lifetime after formation of the dye-surfactant complex tau(SDS)>tau(TX-100)>tau(CTAB)>tau(water), the dye being protected against water in this environment. TX-100 is found to stabilize the excited state of NAO which is more polar than the ground state. Spectroscopic and photophysical properties of NAO will be helpful for a better understanding of the nature of binding and distribution inside mammalian cells.     

Dynamics of solvent and rotational relaxation of coumarin 153 in room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate confined in Brij-35 micelles: a picosecond time-resolved fluorescence spectroscopic study

The dynamics of solvent and rotational relaxation of Coumarin 153 (C-153) in ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and in the ionic liquid confined in Brij-35 micellar aggregates have been investigated using steady-state and time-resolved fluorescence spectroscopy. We observed slower dynamics in the presence of micellar aggregates as compared to the pure IL. However, the slowing down in the solvation time on going from neat IL to IL-confined micelles is much smaller compared to that on going from water to water-confined micellar aggregates. The increase in solvation and rotational time in micelles is attributed to the increase in viscosity of the medium. The slow component is assumed to be dependent on the viscosity of the solution and involves large-scale rearrangement of the anions and cations while fast component is assumed to originate from the initial response of the anions during excitation. The slow component increases due to the increase in the viscosity of the medium and increase in fast component is probably due to the hydrogen bonding between the anions and polar headgroup of the surfactant. The dynamics of solvent relaxation was affected to a small extent due to the micelle formation.     

Microenvironment in the corona region of triblock copolymer micelles: temperature dependent solvation and rotational relaxation dynamics of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.     

Femtosecond solvation dynamics in a neat ionic liquid and ionic liquid microemulsion: excitation wavelength dependence

Solvation dynamics in a neat ionic liquid, 1-pentyl-3-methyl-imidazolium tetra-flouroborate ([pmim][BF4]) and its microemulsion in Triton X-100 (TX-100)/benzene is studied using femtosecond up-conversion. In both the neat ionic liquid and the microemulsion, the solvation dynamics is found to depend on excitation wavelength (lambda(ex)). The lambda(ex) dependence is attributed to structural heterogeneity in neat ionic liquid (IL) and in IL microemulsion. In neat IL, the heterogeneity arises from clustering of the pentyl groups which are surrounded by a network of cation and anions. Such a nanostructural organization is predicted in many recent simulations and observed recently in an X-ray diffraction study. In an IL microemulsion, the surfactant (TX-100) molecules aggregate in form of a nonpolar peripheral shell around the polar pool of IL. The micro-environment in such an assembly varies drastically over a short distance. The dynamic solvent shift (and average solvation time) in neat IL as well as in IL microemulsions decreases markedly as lambda(ex) increases from 375 to 435 nm. In a [pmim][BF4]/water/TX-100/benzene quaternary microemulsion, the solvation dynamics is slower than that in a microemulsion without water. This is ascribed to the smaller size of the water containing microemulsion. The anisotropy decay in an IL microemulsion is found to be faster than that in neat IL.     

Ultrafast photoinduced electron transfer from dimethylaniline to coumarin dyes in sodium dodecyl sulfate and triton X-100 micelles

The primary steps of photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to five coumarin dyes are studied in an anionic micelle [sodium dodecyl sulfate (SDS)] and a neutral micelle [triton X-100 (TX-100)] using femtosecond upconversion. The rate of PET in micelle is found to be highly nonexponential. In both the micelles, PET displays components much faster (approximately 10 ps) than the slow components (180-2900 ps) of solvation dynamics. The ultrafast components of electron transfer exhibit a bell-shaped dependence on the free energy change. This is similar to Marcus inversion. The rates of PET in TX-100 and SDS micelle are, in general, faster than those in cetyltrimethylammonium bromide (CTAB) micelle. In the SDS and TX-100 micelle, the Marcus inversion occurs at -DeltaG0 approximately 0.7 eV which is lower than that (approximately 1.2 eV) in CTAB micelle. Possible causes of variation of PET in different micelles are discussed.     

Effect of added ionic liquid on aqueous Triton X-100 micelles

Addition of ionic liquids to aqueous surfactant solutions can alter/modify physicochemical properties of such systems in favorable manner. Changes in the properties of aqueous solutions of a useful nonionic surfactant Triton X-100 (TX-100) are assessed upon addition of 2.1 wt% of a common and popular ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6). It is shown that the solubility of 'hydrophobic' bmimPF6 in aqueous TX-100 increases with TX-100 concentration. This observation combined with the conductivity data strongly indicates partitioning of bmimPF6 into TX-100 micellar phase. Behavior of a variety of molecular absorbance [methyl orange, phenol blue, and N,N-diethyl-4-nitroaniline] and fluorescence [phenyl on the TX-100, pyrene, pyrene-1-carboxaldehyde, 2-(p-toluidino)naphthalene-6-sulfonate, and 1,3-bis-(1-pyrenyl)propane] probes further confirm this observation. Statistically insignificant increase in critical micelle concentration (cmc) and decrease in aggregation number (N(agg)) of TX-100 micelles are observed upon addition of 2.1 wt% bmimPF6. Based on the overall data, it is inferred that ionic liquid bmimPF6 partitions into the TX-100 micellar phase; presence of bmimPF6 both close to the core as well as in the palisade layer of TX-100 micelles is suggested. Presence of favorable interactions (e.g., H-bonding, dipole-induced dipole, among others) between bmimPF6 and TX-100 is proposed to be the reason for these observations.     

Micellization of sodium dodecyl sulfonate and Triton X-100 in polyacrylamide water solution studied by 1H NMR relaxation and two-dimensional nuclear overhauser enhancement spectroscopy

Micellization of Triton X-100 (TX-100) and sodium dodecyl sulfonate (SDSN) in the presence of partially hydrolyzed polyacrylamide (PAAM) was studied by ¹H NMR spin–spin and spin–lattice relaxation. Relaxation experiment results show that TX-100 behaves differently from SDSN in micellization in the presence of PAAM. PAAM causes a decrease in the critical micellar concentration of SDSN, while it has no influence on the critical micellar concentration of TX-100. The lack of cross peaks between protons of PAAM and those of TX-100 and SDSN in the 2D nuclear Overhauser enhancement spectroscopy (NOESY) spectra confirms self-aggregation of TX-100 and SDSN in the presence of PAAM. The identity of each of the corresponding interproton distances of TX-100 with and without the addition of PAAM further confirms the formation of normal TX-100 micelles in the presence of PAAM. Besides, the distances between protons on the hydrophilic and hydrophobic chains in TX-100 micelles, calculated from the 2D NOESY spectra, are remarkably shorter than those for an extended hydrophilic poly(oxyethylene) chain. This implies that the hydrophilic chain is curled upon micellization. Received: 2 February 1999 Accepted in revised form: 2 June 1999     

Interaction of ionic liquid with water in ternary microemulsions (Triton X-100/water/1-butyl-3-methylimidazolium hexafluorophosphate) probed by solvent and rotational relaxation of coumarin 153 and coumarin 151

The interaction of ionic liquid with water in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])/Triton X-100 (TX-100)/H2O ternary microemulsions, i.e., "[bmim][PF6]-in-water" microregions of the microemulsions, has been studied by the dynamics of solvent and rotational relaxation of coumarin 153 (C-153) and coumarin 151 (C-151). The variation of the time constants of solvent relaxation of C-153 is very small with an increase in the [bmim][PF6]/TX-100 ratio (R). The rotational relaxation time of C-153 also remains unchanged in all micremulsions of different R values. The invariance of solvation and rotational relaxation times of C-153 indicates that the position of C-153 remains unaltered with an increase in R and probably the probe is located at the interfacial region of [bmim][PF6] and TX-100 in the microemulsions. On the other hand, in the case of C-151, with an increase in R the fast component of the solvation time gradually increases and the slow component gradually decreases, although the change in solvation time is small in comparison to that of microemulsions containing common polar solvents such as water, methanol, acetonitrile, etc. The rotational relaxation time of C-151 increases with an increase in R. This indicates that with an increase in the [bmim][PF6] content the number of C-151 molecules in the core of the microemulsions gradually increases. In general, the solvent relaxation time is retarded in this room temperature ionic liquid/water-containing microemulsion compared to that of a neat solvent, although retardation is very small compared to that of the solvent relaxation time of the conventional solvent in the core of the microemulsions.     

Dynamics of solvent and rotational relaxation of coumarin-153 in room-temperature ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate confined in poly(oxyethylene glycol) ethers containing micelles

We have investigated solvent and rotational relaxation of coumarin 153 (C-153) in room-temperature ionic liquid (RTILs) 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF(4)]) and the ionic liquid confined in alkyl poly(oxyethylene glycol) ethers containing micelles. We have used octaethylene glycol monotetradecyl ether (C(14)E(8)) and octaethylene glycol monododecyl ether (C(12)E(8)) as surfactants. In the [bmim][BF(4)]-C(14)E(8) micelle, we have observed only a 22% increase in solvation time compared to neat [bmim][BF(4)], whereas in the [bmim][BF(4)]-C(12)E(8) system, we have observed approximately 57% increase in average solvation time due to micelle formation. However, the slowing down in solvation time on going from neat RTIL to RTIL-confined micelles is much smaller compared to that on going from water to water confined micellar aggregates. The 22-57% increase in solvation time is attributed to the slowing down of collective motions of cations and anions in micelles. The rotational relaxation times become faster in both the micelles compare to neat [bmim][BF(4)].     

Effect of ionic surfactants on the hydration behavior of triblock copolymer micelles: a solvation dynamics study of coumarin 153

Dynamic fluorescence Stokes shift measurements of coumarin 153 (C153) have been carried out to study the influence of ionic surfactants (sodium dodecyl sulfate, SDS and hexadecyltrimethylammonium chloride, CTAC) on the hydration behavior of aqueous poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)20 (P123) block copolymer micelles. Increase in SDS or CTAC concentration at a fixed P123 concentration induces the steady-state emission spectra of C153 to shift gradually toward lower energy. This is attributed to an increase in polarity (due to enhanced hydration) experienced by the probe as a consequence of incorporation of ionic head groups in the Corona region. The observed dynamic fluorescence Stokes shift value decreases more in mixed micellar systems than in pure copolymer micelles and the trends are quite similar in the presence of SDS and CTAC. The spectral shift correlation functions were observed to be nonexponential in nature. Critical analysis of the spectral shift correlation function indicates a fast solvation component (<0.2 ns) in P123 micelles, which was absent in the presence of ionic surfactants. Due to increased hydration in the presence of ionic surfactants, the initial fast solvation event was elusive in mixed copolymer-surfactant systems, reflecting the absence of faster solvation component and reduced observed Stokes shift in mixed systems. It has been argued that in the low surfactant concentration region, increase in hydration with the incorporation of ionic head groups in the Corona region is mainly due to increase in mechanically trapped water content. However, at higher surfactant concentrations, bound water content dominates and leads to slower solvation dynamics. The present results also indicate that though CTAC alters the Corona hydration more efficiently than SDS, the overall influence of ionic surfactants on the Corona hydration is grossly similar irrespective of the cationic or anionic nature of the surfactants. Interaction of SDS and CTAC with poly(ethylene oxide)(100)-poly(propylene oxide)(70)-poly(ethylene oxide)(100) (F127) block copolymer micelles has also been studied to comprehend the effect of copolymer composition. The overall trends in dynamic fluorescence Stokes shift and solvation times are similar in both the copolymer micelles.     

Unique liquid crystal behavior in water of anionic fluorocarbon-hydrocarbon hybrid surfactants containing oxyethylene units

In this present study, we report on new methodology for determining the Critical Micelle Concentration (CMC) of a neutral surfactant Triton X-100 (TX-100) both in aqueous and non-aqueous media based on a non-invasive approach. The presence of the phenyl moiety of TX-100 was made use of as an intrinsic fluorophore and steady-state and time-resolved spectroscopy has been used to characterize the micellar systems. There are reports that external fluorophores may bring about some structural changes in the systems and the perturbations caused by these fluorophores in micellar systems may affect the shape and size of the micelles. We have also used three probes namely ANS, Rh6G and C-480 to determine the CMC of TX-100 both in aqueous and non-aqueous media and the values obtained agree very well with those estimated by the non-invasive techniques. Interestingly, for our system, we have conclusively proved that the external probes have almost no effect on the process of micellization. Although, both the invasive and non-invasive technologies report almost the same values of CMC, yet the latter methodology is free from any external perturbations and this makes the micellar/reverse micellar system, which may interact with other biological systems less prone to any physical distortions.     

Dynamics of solvation and rotational relaxation of Coumarin 153 in ionic liquid confined nanometer-sized microemulsions

The effects of confinement of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate on solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in Triton X-100/cyclohexane microemulsions have been explored using steady-state and picosecond time-resolved emission spectroscopy. The steady-state and rotational relaxation data indicate that C-153 molecules are incorporated in the core of the microemulsions. The average rotational relaxation time increases with increase in w ([bmim][BF(4)]/[TX-100]) values. The solvent relaxation in the core of the microemulsion occurs on two different time scales and is almost insensitive to the increase in w values. The solvent relaxation is retarded in the pool of the microemulsions compared to the neat solvent. Though, the retardation is very small compared to several-fold retardation of the solvation time of the conventional solvent inside the pool of the microemulsions.     

Influence of electrolytes on the microenvironment of F127 triblock copolymer micelles: a solvation and rotational dynamics study of coumarin dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements of coumarin 153 (C153) and coumarin 151 (C151) as fluorescence probes have been carried out to understand the influence of electrolytes (NaCl and LiCl) on the hydration behavior of aqueous (ethylene oxide)100-(propylene oxide)70-(ethylene oxide)100 (EO100-PO70-EO100, F127) block copolymer micelles. A small blue shift in the fluorescence spectra of C153 has been observed in presence of electrolytes due to the dehydration of the oxyethylene chains in the PEO-PPO region, although fluorescence spectra of C151 remain unaltered. The close vicinity of bulk water for C151 probably negates the effect of dehydration in the PEO region. Fluorescence anisotropy measurements indicate a gradual increase in microviscosity with electrolyte concentrations. The partial collapse of copolymer blocks in the presence of electrolytes has been suggested as a reason for the increase in microviscosity along with the strong hydration of ions in the corona region. The interplay between the ion hydration and the mechanically trapped water content, and specific interaction of ions, such as complexation of Li+ ions with the copolymer block, is found to control solvation dynamics in the corona region. In addition to that, it has been established that Na+ ions reside deep into the corona region whereas Li+ ions prefer to reside closer to the surface. Owing to its higher lyotropicity, LiCl influences the corona hydration to a greater extent than NaCl and sets in micelle-micelle interaction above the 2 M LiCl concentration, as reflected in the saturation of solvation time constants. The formation of larger clusters of F127 micelles above 2 M LiCl has been confirmed by dynamic light scattering measurements; however, such cluster formation is not evident with NaCl.     

Fluorescence Anisotropy of Probes Solubilized in Micelles of Tetradecyltrymethylammonium Bromide: Effect of Ethylene Glycol Added

This paper deals with the effect of ethylene glycol on the micelle formation of tetradecyltrimethylammonium bromide. The effect of ethylene glycol addition on the fluorescence anisotropy of several probe molecules residing in different regions of the micelle was investigated to address the solvent penetration in the micelle structure. Fluorescence depolarization measurements were carried out on micellar systems containing two different hydrophobic dyes, namely, perylene and diphenylbutadiene, and a hydrophilic one, fluorescein. The steady-state anisotropy values obtained in these experiments were used to estimate the microviscosity of the corresponding micellar regions. It is observed that the microviscosity in the hydrophobic regions of micelles were roughly constant with EG addition, indicating that the micellar interior does not undergo significant structural changes by the presence of cosolvent in the solution. However, the microviscosity at the micellar surface, as determined by using fluorescein as a probe, is found to increase with EG addition. This perturbation of the micellar surface is ascribed to the solvent penetration in this region of the micelle, where there is probably participation in the solvation layer of the micelle headgroups. Copyright 2000 Academic Press.     

维生素A在胶束溶液中衰变动力学及机理研究

利用紫外光谱方法测定了维生素A及其乙酸酯在不同胶束水溶液中的衰变速率常数和衰变活化参数. 数据显示, 维生素A及其乙酸酯在阴离子胶束溶液SDS中的衰变速度要远大于在阳离子胶束溶液CTAB和中性胶束溶液TX-100中的衰变速度. 机理分析表明, 维生素A及其乙酸酯在水溶液中的衰变是经过先质子化, 再脱去一分子水或乙酸生成碳正离子中间体, 碳正离子再脱去质子, 经重排后得到最终产物脱水维生素A.