Location of spectroscopic probes in self-aggregating assemblies. I. The case for 5-doxylstearic acid methyl ester serving as a benchmark spectroscopic probe to study micelles

A strategy to locate spectroscopic probes in micelles is presented which involves establishing a "benchmark" probe, i.e., one whose position is well-known and against which other probe positions may be established. Theoretically calculated values of the fraction of the micelle polar shell occupied by water, H(shell), are compared with experimental values measured with the spin probe 5-doxylstearic acid methyl ester (5DSE) for a series of sodium n-alkyl sulfate micelles as functions of both the aggregation numbers and the alkyl chain length. The theoretical values involve one adjustable parameter that may be taken to be the volume in the polar shell inaccessible to water, V(dry). Under the hypothesis that the thickness of the polar shell (5 Angstroms) remains constant as either the aggregation number or the chain length is varied, we find excellent agreement between the theoretical predictions and the experimental results, using the same value of V(dry) for chain lengths 8-12 and for aggregation numbers varying from approximately 38 to 130. We argue that these are compelling reasons that 5DSE follows the zero-order model (ZOM) of probe location. The ZOM applies to any probe that rapidly diffuses within the confines of the micelle polar shell and nowhere else. Thus, 5DSE can serve as a benchmark in the sodium alkyl sulfate micelles. As a further check, results are also presented for ammonium dodecyl sulfate micelles, where 5DSE is also found to follow the ZOM, i.e, no further adjustable parameters are needed to pass from the sodium alkyl sulfate micelles to ammonium dodecyl sulfate micelles. In contrast, results are also presented for a similar spin probe 16-doxylstearic acid methyl ester (16DSE) that is found not to adhere to the ZOM in any of the micelles. A simple first-order correction to the ZOM in which 16DSE is displaced slightly from the polar shell is shown to account for the results well. The necessary displacements, which range from about 0.7 Angstroms outside the polar shell to 1.3 Angstroms inside, are not correlated with either chain lengths or aggregation numbers; however, they correlate rather well with H(shell). Calibrations of 6-, 7-, 10-, and 12DSE spin probes are presented in the Appendix, making them available to measure microviscosities and effective water concentrations.     

A reinterpretation of the hydration of micelles of dodecyltrimethylammonium bromide and chloride in aqueous solution

The hydration of dodecyltrimethylammonium (DTAB) micelles is reinterpreted in light of the results of the companion paper (immediately preceding this paper) that showed that the location of the spin probe 16-doxylstearic acid methyl ester (16DSE) changes as a function of the aggregation number, N, of anionic micelles, i.e, that it does not conform to the zero-order model (ZOM). The ZOM requires that the NO(*) moiety diffuse throughout the Stern layer of the micelle and nowhere else. By using the ZOM as a working hypothesis, the previous interpretation (J. Phys. Chem. B 2002, 106, 1926) of 16DSE data proposed that an increasing number of alkyl chain methyl groups occupied the Stern layer as N increased. In this work, the spin probe 5-doxylstearic acid methyl ester that was found to fulfill the ZOM in anionic micelles was measured as a function of N in DTAB and was found to obey the ZOM in this cationic micelle as well. Thus, a simple model of the hydration of micelles that is successful in anionic micelles is also successful in DTAB. The previous results for 16DSE are reinterpreted here as being due to small displacements of the NO(*) moiety as a function of N.     

Investigation of SDS, DTAB and CTAB micelle microviscosities by electron spin resonance

Electron spin resonance spectroscopy (ESR) of the nitroxide labelled fatty acid probes (5-, 16-doxyl stearic acid) was used to monitor the micelle microviscosity of three surfactants at various concentrations in aqueous solution: sodium dodecyl sulphate (SDS), dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB). At low surfactant concentration, there is no micelle, the ESR probe is dissolved in water/surfactant homogeneous phase and gives his microviscosity. At higher surfactant concentration, an abrupt increase in microviscosity indicates the apparition of micelles and, the solubilization of the probes in micelles. The microviscosity of the three surfactants, in a large surfactant range, was obtained as well as the critical micelle concentration (CMC). The microviscosity increased slightly with the increase in surfactant concentration. Phosphate buffer lowered the CMC value and generally increased the microviscosity.     

Effect of the nature of the counterion on the properties of anionic surfactants. 5. Self-association behavior and micellar properties of ammonium dodecyl sulfate

Micelles formed in water from ammonium dodecyl sulfate (AmDS) are characterized using time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonance (EPR), conductivity, Krafft temperature, and density measurements. TRFQ was used to measure the aggregation number, N, and the quenching rate constant of pyrene by dodecylpyridinium chloride, k(Q). N depends only on the concentration (C(aq)) of ammonium ions in the aqueous phase whether these counterions are derived from the surfactant alone or from the surfactant plus added ammonium chloride as follows: N = N0(C(aq)/cmc0)(gamma), where N0 is the aggregation number at the critical micelle concentration in the absence of added salt, cmc0, and is equal to 77, 70, and 61 at 16, 25, and 35 degrees C, respectively. The exponent gamma = 0.22 is independent of temperature in the range 16 to 35 degrees C. The fact that N depends only on C(aq) permits the determination of the micelle ionization degree (alpha) by employing various experimental approaches to exploit a recent suggestion (J. Phys. Chem. B 2001, 105, 6798) that N depends only on C(aq). Utilizing various combinations of salt and surfactant, values of alpha were obtained by finding common curves as a function of C(aq) of the following experimental results: the Krafft temperature, N, k(Q), the microviscosity of the Stern layer determined from the rotational correlation time of a spin probe, 5-doxyl stearic acid methyl ester, and the spin-probe sensed hydration of the micelle surface. The values of alpha, determined from applying the aggregation number-based definition of alpha to all of these quantities, were within experimental uncertainty of the values alpha = 0.19, 0.20, and 0.21 derived from conductivity measurements at 16, 25, and 35 degrees C, respectively. The volume fraction of the Stern layer occupied by water decreases as N increases. For AmDS micelles, both the hydration and its decrease are predicted by a simple theory of micelle hydration by fixing the parameters of the theory for sodium dodecyl sulfate and employing no further adjustable parameters. For a given value of N, the hydration decreases as the temperature increases.     

Physicochemical characterization of phospholipid solubilized mixed micelles and a hydrodynamic model of interfacial fluorescence quenching

Mixed micelles of solubilized dimyristoyl phosphatidylcholine (DMPC) and the zwitterionic detergent dodecyldimethylammoniopropane sulfonate are characterized employing time-resolved fluorescence quenching (TRFQ), electron spin resonance (ESR), and surface tensiometry toward the goal of investigating interfacial reactions using these micelles as host reaction media. The properties measured are the micelle aggregation numbers, interfacial hydration index, microviscosity, and the critical micelle concentrations for various molar fractions, XDMPC, of DMPC, 0相似文献   

Fluorescence probes study on the mixed cationic-anionic surfactant solutions

Pyrene andp-N,N-dimethylaminostyrylphenylmalononitrile are used as the fluorescence probes to study the micro environment and observe the excimer formation of pyrene in sodium alkylcarboxylate and alkyltrimethyl ammonium bromide mixed solutions. The micro polarity, micro-dielectric constants and micro viscosity of the self-organized assemblies in the mixed cationi-anionic surfactant solution were compared before and after sonication, which may form different organized assemblies (micelle or vesicle). The micelle and vesicles have almost the same polarity for the probing molecules whereas the microviscosity differs. The variation of fluorescence quenching curves also shows the different effect of the change of assembly forms (micelles to vesicles). Thus some novel physicochemical properties about the micro environment of the cationic-anionic surfactant assemblies were found from this report.     

Studies of bio-mimetic medium of ionic and non-ionic micelles by a simple charge transfer fluorescence probe N,N-dimethylaminonapthyl-(acrylo)-nitrile

In this report we have studied micellization process of anionic, cationic and non-ionic surfactants using N,N-dimethylaminonapthyl-(acrylo)-nitrile (DMANAN) as an external fluorescence probe. Micropolarity, microviscosity, critical micellar concentration of these micelles based on steady state absorption and fluorescence and time resolved emission spectroscopy of the probe DMANAN show that the molecule resides in the micelle-water interface for ionic micelles and in the core for the non-ionic micelle. The effect of variation of pH of the micellar solution as well as fluorescence quenching measurements of DMANAN provide further support for the location of the probe in the micelles.     

Distribution of fluorocarbon quencher among micelles via pyrene fluorescence probe method

 Fluorescence-quenching of pyrene in micellar system has been investigated using 1,1,2,2-tetrahydroheptadecafluorodecylpyridinium chloride (HFDePC). The new fluorocarbon quencher has a similar quenching ability as hexadecyl-pyridinium chloride (CPC) towards pyrene in hydrocarbon micelles if only a quencher molecule is solubilized in a micelle. The fluorocarbon quencher randomly distributed among micelles if the average occupancy number of probes per a micelle was small enough. The fluorescence behavior of pyrene was examined for hexadecyl-trimethylammonium chloride (CTAC) and HFDePC mixtures. The variation of fluorescence intensity gave second cmc, reflecting the micellar immiscibility of fluorocarbon and hydrocarbon surfactants. The second cmc can be simulated by material balances of both surfactants supposing the coexistence of two kinds of mixed micelles. The fluorescence-quenching behavior suggested the enhanced micellar immiscibility probably due to nonrandom distribution of fluorocarbon quenchers among micelles. Received: 13 March 1997 Accepted: 24 May 1997     

Probing solubilization sites in block copolymer micelles using fluorescence quenching

The solubilization sites provided by micelles formed by a diblock copolymer with one neutral hydrophobic block, polystyrene, and one charged hydrophilic block, poly(acrylic acid) or poly(methacrylic acid), have been studied by fluorescence quenching of pyrene by polar and nonpolar quenchers. Pyrene solubilized into these micelles is distributed between the inner corona and the micelle core. The fraction of pyrene residing in the inner corona is almost unity for star micelles, where the corona-forming blocks are larger than the core-forming blocks, and around 0.5 for crew-cut micelles where the opposite situation prevails. The kinetics of the quenching process depends on the pyrene location, i.e. is static in the micelle core, and largely dynamic in the inner corona at low quencher concentration. The rate constants for fluorescence quenching by nitromethane are shown to increase with increasing pH.     

Hydrophobie effects on photophysical and photochemical processes: VI. Character of aggregates formed by hydrocarbon with long chain in poor solvents

Dodecyl β-naphthoate and 3-octadecyl pyrene were used as fluorescence probes for estimation of microscopic polarity in aggregates formed by long chain hydrocarbon Cn (n-10, 12, 14, 16, 18) in dimethyl sulfoxide-water mixed solvent. The polarity in the aggregates is similar to that in cyclohexane. The average aggregation number (N?) was determined by using time resolved fluorescence technique. N? is dependent on substrate chain length and varies from 15 to 30. The microviscosity was also measured by fluorescence depolarization method. Above phase transition temperature, the microviscosity in aggregates is slightly larger than that in micelle, but below this temperature, the microviscosity in aggregates is as large as that in solid phase.     

POLARIZATION STUDY OF 5(E)-STYRYL-1,3-DIMETHYL URACIL IN DIISOOCTYL SULFOSUCCINATE (AEROSOL-OT) REVERSED MICELLE SOLUTIONS

The degree of polarization around the water pool generated in aerosol- OT/hexane/water reversed micelle solutions was studied by monitoring the depolarization of solubilized probe fluorescence. The 5(E)-styryI-1, 3-dimethyl uracil (SDU) used in this study was moderately polar and proved to be solubilized between the interface and hydrocarbon phases in reversed micelle solution by a fluorescence quenching study with hydrophilic and hydrophobic quenchers. The degree of polarization monitored by SDU was parallel with that obtained from the fluorescence depolarization study of hydrophylic probes solubilized in the water pool and interface. A dramatic increase in microviscosity was observed when ω value became small.     

分子动力学模拟研究荧光分子芘在磺基甜菜碱胶束中的增溶

采用分子动力学模拟研究了荧光分子芘在磺基甜菜碱两性表面活性剂聚集体中的增溶现象．结果表明,芘分子自发地自溶液中增溶进入胶束疏水内核的栅栏层区域．当胶束溶液中芘分子的局部浓度增大时,两个芘分子可以同时增溶进胶束的栅栏层区域,此时两个芘分子形成π-π共轭堆积的激发态络合物．但是由于荧光分子之间的弱兀．兀相互作用,激发态络合物在胶束中是不稳定的,表现为两个芘分子的多次结合和分离．模拟表明,分子动力学方法可以在分子水平上研究荧光探针分子在表面活性剂胶束中的增溶位点,解释荧光分子在胶束中的动力学现象．     

Micropolarity and microviscosity of Pluronics L62 and L64 core–shell aggregates in water at various concentrations and additives examined by absorption and fluorescence probes

The microstructure of aggregates formed in aqueous solutions of the triblock copolymers poly(ethylene oxide) (PEO)–poly(propylene oxide)–PEO, Pluronics L62 and L64, and the effect of additives (n-butanol, n-hexanol, and o-xylene) on the local polarity and viscosity were investigated using several absorption and fluorescence probes with different hydrophilic/hydrophobic trade-offs. The absorption probe was 2,2,6,6-tetramethylpiperidine-1-oxyl and the fluorescence probes pyrene (P), 1-anilinonaphthalene-8-sulfonic acid (ANS), 1,10-bis(1-pyrene) decane (PD), and N-[5-(dimethylamino)naphthalene-1-sulfonyl]hexadecylamine (Dansyl). The specific absorption and fluorescence parameters, sensitive to changes in micropolarity and microviscosity, were related to the hydration calibration curves carried out in homogeneous tetraethylene glycol/water mixtures. Thus, the effective local hydration of the molecular probe solubilized in the guest aggregate was quantified, and at the same time, the probe location is established with respect of the corona and core aggregate. The ANS and PD probes evidence differences in microviscosity and track the effect of the block copolymer structure and additive concentration on microviscosity.     

The nature of the micellar stern region as studied by reaction kinetics. 2

The nature of rate-retarding effects of cationic micelles on the water-catalyzed hydrolyses of a series of para-substituted 1-benzoyl-1,2,4-triazoles (1a-f) and 1-benzoyl-3-phenyl-1,2,4-triazole (2) has been studied using kinetic methods. A comparison is drawn between medium effects in the micellar Stern region and in model solutions for the micellar Stern region. Simple model solutions involving concentrated aqueous solutions of a small ionic molecule resembling the surfactant headgroup, as reported before,(1) were improved. New model solutions for alkyltrimethylammonium bromide micelles contain both tetramethylammonium bromide (TMAB), mimicking micellar headgroups, and 1-propanol, mimicking hydrophobic tails. The rate-retarding effect of micelles on the hydrolysis of 1a-f and 2 is caused by the high concentration of headgroups as well as by hydrophobic tails in the Stern region where 1a-f and 2 bind to the micelle. Individual contributions of these interactions are quantified. Rate-retarding effects found for different probes, with different sensitivities for interactions as they occur when the probe binds to the micellar Stern region, as well as the micellar Stern region's micropolarity as reported by the E(T)(30) probe, are satisfactorily reproduced by new model solutions containing both TMAB and 1-propanol.     

Spectroscopic monitoring for the formation of mesoporous MCM-41 materials upon microwave irradiation

The mesoporous MCM-41 materials were prepared in very short crystallization time (∼40 min) upon microwave irradiation in comparison with conventional hydrothermal heating method. With both microwave irradiation and hydrothermal heating, the MCM-41 formation via supramolecular templating method has been monitored by fluorescence and electron spin resonance (ESR) spectroscopy. Pyrene as a fluorescence probe and 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethyl piperidinyloxy iodide (CAT16) as a spin probe were respectively dissolved into the micelle solutions to form the MCM-41 precursor gels. These probes allow the monitoring of the supramolecular interaction between the anionic silicate species and the cationic surfactant molecules during the MCM-41 formation. Analyses of fluorescence and ESR spectra indicate that the fast increase of hydrophobicity and microviscosity at the solubilzation sites of the probes results from the accelerated condensation of silicates onto the micelle surface upon microwave irradiation. The fluorescence change from the silicate L-center in the MCM-41 precursor gel also probes the fast silicate condensation upon microwave irradiation. It seems that the fast formation of MCM-41 upon microwave irradiation is ascribed to the microwave-susceptible head groups of surfactant molecules in addition to fast dissolution of the precursor gel.     

Comparison of microenvironments of aqueous sodium dodecyl sulfate micelles in the presence of inorganic and organic salts: a time-resolved fluorescence anisotropy approach

Microenvironments of aqueous sodium dodecyl sulfate (SDS) micelles was examined in the presence of additives such as sodium chloride and p-toluidine hydrochloride (PTHC) by monitoring the fluorescence anisotropy decays of two hydrophobic probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and coumarin 6 (C6). It has been well-established that SDS micelles undergo a sphere-to-rod transition and that their mean hydrodynamic radius increases from 19 to 100 A upon the addition of 0.0-0.7 M NaCl at 298 K. A similar size and shape transition is induced by PTHC at concentrations that are 20 times lower compared to that of NaCl. This study was undertaken to find out how the microviscosity of the micelles is influenced under these circumstances. It was noticed that the microviscosity of the SDS/NaCl system increased by approximately 45%, whereas there was a less than 10% variation in the microviscosity of the SDS/PTHC system. The large increase in the microviscosity of the former system with salt concentration has been rationalized on the basis of the high concentration of sodium ions in the headgroup region of the micelles and their ability to strongly coordinate with the water present in this region, which decreases the mobility of the probe molecules.     

EPR spin probe and spin label studies of some low molecular and polymer micelles

The rotational mobility of spin probes of different shape and size in low molecular and polymer micelles has been studied. Several probes having nitroxide fragment localized either in the vicinity of micelle interface or in the hydrocarbon core have been used. Upon increasing the number of carbon atoms in hydrocarbon chain of detergent from 7 to 13 (sodium alkyl sulfate micelles) or from 12 to 16 (alkyltrimethylammonium bromide micelles) the rotational mobility of spin probes is decreased by the factor 1.5–2.0. The spin probe rotational mobility in polymer micelles (the complexes of alkyltrimethylammonium bromides and polymethacrylic or polyacrylic acids) is less than mobility in free micelles of the same surfactants. The study of EPR-spectra of spin labeled polymethacrylic acid (PMA) indicated that formation of water soluble complexes of polymer and alkyltrimethylammonium bromides in alkaline solutions (pH 9) does not affect the polymer segmental mobility. On the other hand, the polymer complexes formation in slightly acidic water solution (pH 6) breaks down the compact PMA conformation, thus increasing the polymer segmental mobility. Possible structures of polymer micelles are discussed.