Spectroscopic properties of polycyclic aromatic compounds Part 6. The nitromethane selective quenching rule visited in aqueous micellar zwitterionic surfactant solvent media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 58 representative PAH solutes dissolved in micellar N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate and in micellar N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate solvent media. Results of measurements show that zwitterionic surfactants can be considered, for the most part, as providing a polar solubilizing media as far as the nitromethane selective quenching rule is concerned. Nonalternant PAHs that contain electron donating methoxy- and hydroxy-functional groups (and methyl-groups to a much lesser extent) are noted exceptions.     

On the localization of water-soluble porphyrins in micellar systems evaluated by static and time-resolved frequency-domain fluorescence techniques

Fluorescence quenching of meso-tetrakis-4-sulfonatophenyl (TPPS(4)) and meso-tetrakis-4-N-methylpyridil (TMPyP) porphyrins is studied in aqueous solution and upon addition of micelles of sodium dodecylsulfate (SDS), cetyltrimethylammonium chloride (CTAC), N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and t-octylphenoxypolyethoxyethanol (Triton X-100). Potassium iodide (KI) was used as quencher. Steady-state Stern-Volmer plots were best fitted by a quadratic equation, including dynamic (K(D)) and static (K(S)) quenching. K(S) was significantly smaller than K(D). Frequency-domain fluorescence lifetimes allowed estimating bimolecular quenching constants, k(q). At 25 degrees C, in aqueous solution, TMPyP shows k(q) values a factor of 2-3 higher than the diffusional limit. TPPS(4) shows collisional quenching with pH dependent k(q) values. For TMPyP quenching results are consistent with reported binding constants: a significant reduction of quenching takes place for SDS, a moderate reduction is observed for HPS and almost no change is seen for Triton X-100. Similar data were obtained at 50 degrees C. For CTAC-TPPS(4) system an enhancement of quenching was observed as compared to pure buffer. This is probably associated to accumulation of iodide at the cationic micellar interface. The attraction between CTAC headgroups and I(-), and repulsion between SDS and I(-), enhances and reduces the fluorescence quenching, respectively, of porphyrins located at the micellar interface. The small quenching of TPPS(4) in Triton X-100 is consistent with strong binding as reported in the literature.     

Micellar solutions of sulfobetaine surfactants in water-ethylene glycol mixtures: surface tension, fluorescence, spectroscopic, conductometric, and kinetic studies

Micellization in water-ethylene glycol (EG) N-dodecyl, N-tetradecyl, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-12, SB3-14, and SB3-16, respectively) micellar solutions, with the weight percent of EG changing within the range 0-40, was studied by means of surface tension measurements. Information about the influence of the added EG on the aggregation number of the sulfobetaine micelles and on the polarity of the interfacial region of micelles was obtained through fluorescence and spectroscopic measurements. Surface tension measurements also provide information about the dependence of the surface excess concentration, the minimum area per surfactant molecule, the surface pressure at the cmc, and the standard Gibbs energy of adsorption on the added weight percent of the organic solvent. The Gordon parameter of the water-EG mixtures was also estimated by means of surface tension measurements. The thermodynamic and structural changes originated by the presence of EG control the micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br(-) occurring in the water-EG sulfobetaine micellar solutions. Information about the distribution of bromide ions between the bulk and micellar pseudophases was obtained through conductivity measurements. The kinetic micellar effects were quantitatively explained by using the pseudophase kinetic model.     

Interactions between zwitterionic and conventional anionic and cationic surfactants

Critical micelle concentration (cmc) values have been determined for the mixed zwitterionic/anionic surfactant systems of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-12)/sodium dodecyl sulfate (SDS), N-dodecyl-N,N-(dimethylammonio)butyrate (DDMAB)/SDS, N-octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (ZW3-08)/sodium octyl sulfate (SOS), and the zwitterionic/cationic systems of ZW3-12/dodecyltrimethylammonium bromide (DTAB), DDMAB/DTAB. Conductivity studies and nuclear magnetic resonance (NMR) spectroscopy were the methods employed for cmc determinations. The degree of nonideality of the interaction in the micelle (beta(m)), for each system, was determined according to Rubingh's nonideal solution theory. Evidence was found for the existence of strong interactions between zwitterionic and anionic surfactants in each of the zwitterionic/anionic systems. The ZW3-08/SOS and DDMAB/SDS systems behaved synergistically at all mole fractions studied while the ZW3-12/SDS system exhibited synergistic behavior above mole fractions of 0.30. Greater negative deviations from ideal behavior were demonstrated in the DDMAB/SDS system than in the other two zwitterionic/anionic systems. The zwitterionic/cationic systems of ZW3-12/DTAB and ZW3-08/OTAB displayed only slight deviations from ideal behavior, therefore indicating near ideal mixing.     

Interaction of pyrimethamine and sulfadiazine with ionic and neutral micelles: Electronic absorption and fluorescence studies

The binding of two antitoxoplasmosis drugs, pyrimethamine (PYR) and sulfadiazine (SDZ) to cationic cetyltrimethylammonium chloride (CTAC), anionic sodium dodecylsulfate (SDS), zwiterionic N-hexadecyl-N,N-dimethyl-2-ammonium-1-propanesulfonate (HPS) and neutral polyoxyethylene-dodecyl-ether (Brij-35^®) micelles was studied using absorption and fluorescence spectroscopic methods. The pK_a of PYR changed in the presence of charged anionic, cationic and zwiterionic micelles, indicating that interaction is influenced by the micellar charge. For SDZ, pK_a changes were lower than 1 for all micelles, suggesting the occurrence of low binding constants in addition to a reasonable influence of the micellar charge. The values of binding constants K_b, obtained from fluorescence measurements, for PYR to CTAC micelles were very low at pH 4.0, where the drug is in a complete protonated state, increasing at pH 9.0 to long-chained CTAC and HPS micelles since this factor also favors accomodation of the neutral drug in the hydrophobic compartments. For SDZ the binding constants were determined from optical absorption measurements. Low binding constants were observed to charged surfactant micelles, with influence of micellar charge. It must be stated however that those values can be underestimated due to the relatively low sensitivity of the method based on absorption measurements.     

阳离子表面活性剂混合胶团对2,4-二硝基氯苯水解反应的 催化作用

研究了阳离子表面活性剂混合胶团对2,4-二硝基氯苯(DNCB)碱性水解反应的催化作用。结果表明：(1)在十六烷基三甲基溴化铵(CTAB)和十六烷基溴化吡啶(CPB)混合溶液中DNCB水解一级速率常数k1与混合胶团中CTAB或CPB的摩尔分数有直线关系,表面活性剂形成理想的混合胶团。(2)辛基三甲基省化铵(OTAB)与CTAB,CPB的cmc值相差很大,在它们的混合胶团中OTAB含量极少,DNCB水解k1与CPB/OTAB混合胶团中CPB摩尔分数的关系与直线呈负偏差。(3)在CTAB(或CPB)与OTAB混合体系中OTAB起溴盐作用,使催化活性降低。用假相离子交换(PIE)模型对所得结果给出了定量的处理和解释。     

手性胶束中的不对称Micheal反主尖

本文研究了手性表面活性剂N,N-二甲基-N-十二烷基麻黄素溴化铵及N,N-二甲基-N-十六烷基麻黄素溴化铵所形成的手性胶束体系中,以六氢吡啶作为碱催化剂,硝基甲烷或硫酚对查耳酮类化合物的Michael加成反应.     

Interfacial concentrations of chloride and bromide in zwitterionic micelles with opposite dipoles: experimental determination by chemical trapping and a theoretical description

Interfacial concentrations of chloride and bromide ions, with Li(+), Na(+), K(+), Rb(+), Cs(+), trimethylammonium (TMA(+)), Ca(2+), and Mg(2+) as counterions, were determined by chemical trapping in micelles formed by two zwitterionic surfactants, namely N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and hexadecylphosphorylcholine (HDPC) micelles. Appropriate standard curves for the chemical trapping method were obtained by measuring the product yields of chloride and bromide salts with 2,4,6-trimethyl-benzenediazonium (BF(4)) in the presence of low molecular analogs (N,N,N-trimethyl-propane sulfonate and methyl-phosphorylcholine) of the employed surfactants. The experimentally determined values for the local Br(-) (Cl(-)) concentrations were modeled by fully integrated non-linear Poisson Boltzmann equations. The best fits to all experimental data were obtained by considering that ions at the interface are not fixed at an adsorption site but are free to move in the interfacial plane. In addition, the calculation of ion distribution allowed the estimation of the degree of ion coverage by using standard chemical potential differences accounting for ion specificity.     

The Use of Thermodynamics for the Indirect Determination of Neutral Surfactants Activities in Mixtures of Potassium Polyvinyl Sulfate (PVSK), Dodecyltrimethylammonium Bromide (DTABr), and N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12)

Mixtures of zwitterionic and cationic surfactants were studied in the presence of an anionic polyelectrolyte. We used N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), dodecyltrimethyl ammonium bromide (DTA+Br-), and polyvinyl sulfate (potassium salt) (PVS-K+). The composition of the system was determined by measuring the activity of DTABr with an ion-selective electrode, and deducing the activity of SB12 from thermodynamic equations. This method provides information about the distribution of all species (activities of both surfactants and composition of the micelles), by experimental determination of only one quantity. SB12 affected the adsorption of DTABr onto PVSK. Copyright 1999 Academic Press.     

Small-angle X-ray scattering and electron paramagnetic resonance study of the interaction of bovine serum albumin with ionic surfactants

Small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) techniques have been used to monitor the interaction of bovine serum albumin (BSA) with ionic surfactants such as anionic sodium dodecyl sulfate (SDS), zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonium-1-propane sulfonate (HPS), and cationic cethyltrimethylammonium chloride (CTAC) at pH 7.0. The SAXS results have shown that in the presence of 5 mM SDS and HPS the radius of gyration (Rg) almost does not change as compared to the BSA free-surfactant solution; its value is ca. 30 Angstroms. In the presence of 5 mM CTAC the SAXS data indicate the presence of a particle with a Rg of at least 63 Angstroms, suggesting that in this case, a kind of protein aggregation takes place. In the presence of SDS and HPS surfactants at concentrations above 10 mM, a characteristic broad peak in the region of 0.12-0.18 Angstroms(-1) indicates the presence of micelle-like aggregates in solution. The SAXS curves are consistent with the "pearl necklace" model, where micelle-like aggregates are randomly distributed around the polypeptide chain. EPR results using 5-DSA and 16-DSA spin labels show that in the presence of BSA the EPR spectra are composed of two label populations, one contacting the protein and a second one due to label localization in the micelles. Evidence is also obtained for a competition of the surfactants with the spin labels for the high-affinity binding sites of the stearic acid spin labels as monitored by changes in the fractions of the two label populations as the surfactant concentration is increased. The effect of SDS seems to be stronger in the sense that increased SDS concentration leads to a complete transfer of spin labels from close protein contact sites to micelles, while for HPS, a significant immobilization of probe apparently remains even at higher surfactant concentrations. These two techniques are quite useful since SAXS monitors the overall properties of the scattering particle, while EPR gives information on the dynamics inside this particle and associated with label localization and motion.     

Properties of polyethylene glycol (23) lauryl ether with cetyltrimethylammonium bromide in mixed aqueous solutions studied by self-diffusion coefficient NMR

NMR self-diffusion coefficient measurements have been used to study the properties of polyethylene glycol (23) lauryl ether (Brij-35) with cetyltrimethylammonium bromide (CTAB) in the mixed aqueous solutions with different mole fractions of CTAB. By fitting the self-diffusion coefficients to the two-state exchange model, the critical micelle concentrations of the two solutes in the mixed solutions (cmc*1 and cmc*2) were obtained. The critical mixed micelle concentrations (cmc*) were then evaluated by the sum of cmc*1 and cmc*2, which are in good agreement with the results measured by the surface tension method. The cmc* values are lower than those of the ideal case of mixing, which indicates that the behavior of the CTAB/Brij-35 system is nonideal. Moderate interactions between CTAB and Brij-35 in their mixtures can be deduced from the interaction parameters (betaM) based on the cmc* obtained by the NMR self-diffusion method. The compositions (x1) of the mixed micelles at different total surfactant concentrations were also evaluated. By using these results, a possible mechanism of mixed micellar formation and a picture of the formation of nonsimultaneous CTAB/Brij-35 binary mixed micelle were proposed. In contrast to the case of CTAB/TX-100 system, Brij-35 molecules have a tendency to form micelles first at any mole fraction of CTAB. The mixed micellar self-diffusion coefficients (Dm) increase slightly at lower CTAB molar ratios, and then speed up with increasing CTAB mole fraction.     

On the interaction of bovine serum albumin with ionic surfactants: temperature induced EPR changes of a maleimide nitroxide reflect local protein dynamics and probe solvent accessibility

The interaction of bovine serum albumin (BSA) with the ionic surfactants sodium dodecylsulfate (SDS, anionic), cetyltrimethylammonium chloride (CTAC, cationic) and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS, zwitterionic) was studied by electron paramagnetic resonance (EPR) spectroscopy of spin label covalently bound to the single free thiol group of the protein. EPR spectra simulation allows to monitor the protein dynamics at the labeling site and to estimate the changes in standard Gibbs free energy, enthalpy and entropy for transferring the nitroxide side chain from the more motionally restricted to the less restricted component. Whereas SDS and CTAC showed similar increases in the dynamics of the protein backbone for all measured concentrations, HPS presented a smaller effect at concentrations above 1.5mM. At 10mM of surfactants and 0.15 mM BSA, the standard Gibbs free energy change was consistent with protein backbone conformations more expanded and exposed to the solvent as compared to the native protein, but with a less pronounced effect for HPS. In the presence of the surfactants, the enthalpy change, related to the energy required to dissociate the nitroxide side chain from the protein, was greater, suggesting a lower water activity. The nitroxide side chain also detected a higher viscosity environment in the vicinity of the paramagnetic probe induced by the addition of the surfactants. The results suggest that the surfactant-BSA interaction, at higher surfactant concentration, is affected by the affinities of the surfactant to its own micelles and micelle-like aggregates. Complementary DLS data suggests that the temperature induced changes monitored by the nitroxide probe reflects local changes in the vicinity of the single thiol group of Cys-34 BSA residue.     

Solvolysis of Tris-p-nitrophenyl-phosphate in aqueous and reverse micelles

The widespread use of toxic phosphates and phosphonates as insecticides, and their use as chemical weapons, has led to investigation of fast detoxification and decontamination methods. Micelles, microemulsions, cyclodextrines and liposomes have been used to accelerate phosphate ester decomposition by nucleophiles. Here, hydrolysis, methanolysis and hexanolysis of Tris-p-nitrophenyl phosphate (TNPP), a model for reactive phosphate esters, were studied in homogeneous phase, aqueous and reverse micelles. Kinetic micellar effects were quantitatively analyzed using pseudo-phase models. TNPP hydrolysis was catalyzed by cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB), and hexadecylammonium propanesulfonate (HPS), micelles by factors of five, CTAC, and three, CTAB, HPS, respectively. The calculated rate constants for spontaneous and acetate-catalyzed hydrolysis in the micellar phase were significantly higher than those in the aqueous phase. While in water and in methanol the effect of the acetate cation was negligible, the catalytic efficiency of acetate for hexanolysis depended on the nature of the cation with the K⁺ salt being ca. 20 times more efficient than the tetraethylammonium salt in non-polar solvents. Sodium dodecylsulfate, SDS, micelles inhibited TNPP hydrolysis by a factor of eigth. Reverse micelles of CTAB in n-hexanol/isooctane (10:90, v/v) did not catalyze TNPP hydrolysis, but changed the bis-p-nitrophenyl phosphate/hexyl-bis-p-nitrophenylphosphate product ratio depending of CTAB concentration and water/detergent ratio.     

Cleavage of phosphate diesters mediated by Zn(II) complex in Gemini surfactant micelles

The cleavage of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) catalyzed by the Zn(II)-biap (biap: N,N-bis(2-ethyl-5-methylimidazole-4-ylmethyl)aminopropane) complex has been investigated spectrophotometrically in a micellar solution of cationic Gemini surfactant 16-2-16 [bis(hexadecyldimethylammonium)ethane bromide] and CTAB (hexadecyltrimethylammonium bromide) at 25+/-0.1 degrees C. The experimental results reveal that a higher rate of acceleration (about 2016-fold) of HPNP cleavage promoted by the Zn(II)-biap complex has been observed in the 16-2-16 micellar solution in comparison with the background rate (k(0)) of HPNP spontaneous cleavage at 25 degrees C. Reaction rates of HPNP cleavage in CTAB micellar solutions are only about 40% of that in Gemini 16-2-16 micelles under comparable conditions. In addition, the cleavage rates of HPNP in Gemini micelles and in CTAB micelles are respectively 29.5 times and 12 times faster than that in aqueous buffer. Especially, a "sandwich absorptive mode" has been proposed to explain the acceleration of HPNP cleavage in a cationic micellar solution.     

Study of the Reaction 2-(p-Nitrophenyl)Ethyl Bromide + OH(-) in Sulfobetaine Aqueous Micellar Solutions in the Presence and Absence of Added Salts

The reaction of dehydrobromination of 2-(p-nitrophenyl)ethyl bromide with hydroxide ions has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14. The kinetic effects of added salts (NaF, NaCl, NaBr, and NaNO(3)) on the reaction rate in SB3-14 aqueous micellar solutions have also been studied. They were rationalized by considering the binding of the anions, which come from the salt, to the sulfobetaine micelles and their competition with the reactive hydroxide ions for the micellar surface. The equilibrium binding constant of the 2-(p-nitrophenyl)ethyl bromide to the sulfobetaine micelles was estimated by recording the changes in the spectra of the organic substrate when the SB3-14 concentration in the micellar medium changed. This value was in agreement with that obtained from fitting of kinetic data. The second-order rate constant in the micellar pseudophase revealed that the reaction is faster in SB3-14 micelles than in water. This acceleration seems independent of the presence of added salts and can be explained by considering that SB3-14 micelles favor reactions in which charge is delocalized in the transition state. Copyright 2001 Academic Press.     

Trifluoperazine effects on anionic and zwitterionic micelles: a study by small angle X-ray scattering

In this work small angle X-ray scattering (SAXS) studies on the interaction of the phenothiazine trifluoperazine (TFP, 2-10 mM), a cationic drug, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) and the anionic surfactant sodium dodecyl sulfate (SDS, 40 mM) at pH 4.0, 7.0, and 9.0 are reported. The data were analyzed through the modeling of the micellar form factor and interference function, as well as by means of the distance distribution function p(r). For anionic micelles (SDS), the results evidence a micellar shape transformation from prolate ellipsoid to cylinder accompanied by micellar growth and surface charge screening as the molar ratio TFP:SDS increases in the complex for all values of pH. Small ellipsoids with axial ratio nu=1.5+/-0.1 (long dimension of 60 A) grow and reassemble into cylinder-like aggregates upon 5 mM drug incorporation (1 TFP:8 SDS monomers) with a decrease of the micelle surface charge. At 10 mM TFP:40 mM SDS cylindrical micelles are totally screened with an axial ratio nu approximately 4 (long dimension approximately 140 A at pH 7.0 and 9.0). However, at pH 4.0, where the drug is partially diprotonated, 10 mM TFP incorporation gives rise to a huge increase in micellar size, resulting in micelles at least 400 A long, without altering the intramicellar core. For zwitterionic micelles (HPS), the results have shown that the aggregates also resemble small prolate ellipsoids with averaged axial ratio approximately nu=1.6+/-0.1. Under TFP addition, both the paraffinic radius and the micellar size show a slight decrease, giving evidence that the micellar hydrophobic core may be affected by phenothiazine incorporation rather than that observed for the SDS/TFP comicelle. Therefore, our results demonstrate that the axial ratio and shape evolution of the surfactant:TFP complex are both dependent on surfactant surface-charge and drug:surfactant molar ratio. The results are compared with those recently obtained for another phenothiazine drug, chlorpromazine (CPZ), in SDS and HPS micelles (Caetano, Gelamo, Tabak, and Itri, J. Colloid Interface Science 248 (2002) 149).     

Porphyrin effects on zwitterionic HPS micelles as investigated by small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

In this work, small-angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) studies on the interaction of three anionic mesotetrakis (4-sulfonatophenyl) porphyrins, TPPS4, FeTPPS4, and ZnTPPS4, at concentrations in the 2-10 mM range, with micelles of the zwitterionic surfactant 3-(N-hexadecyl-N,N-dimethylammonium) propane sulfonate (HPS, 30 mM) at pH 4.0 and 9.0 are reported. The SAXS results demonstrate that, upon addition of all species of porphyrins, the HPS micelle of prolate shape reduces its axial ratio from 1.8 +/- 0.2 (in the absence of porphyrin) to 1.5 +/- 0.1. Such an effect is accompanied by a shrinking of the paraffinic shortest semiaxis from 22.5 +/- 0.5 A to 18.0 +/- 0.2 A. This shows that the micellar hydrophobic core is affected by porphyrin incorporation, independent of the type of porphyrin and pH. Concurrently, EPR results demonstrate an increase in the micellar packing as noticed from the increase in motional restriction for both nitroxides. Furthermore, increase of the porphyrin concentration induces the appearance of a repulsive interference function over the SAXS curve of zwitterionic micelles, which is typical of an interaction between surface-charged micelles. Such a finding gives strong evidence that the negatively charged porphyrin molecule must accommodate in the HPS micelle dipole layer close to the inner positive charges (near the hydrophobic core), inducing a surface charge (probably a negative one associated with the HPS sulfonate external groups) in the original zwitterionic (overall neutral) micelle. Such a porphyrin location is favored by both electrostatic and hydrophobic contributions, giving rise to binding constant values that are quite large compared to the binding of cationic drugs to HPS micelles (Caetano, W.; Barbosa, L. R. S.; Itri, R.; Tabak, M. J. Coll. Int. Sci. 2003, 260, 414).     

Investigations on mixed micelles of binary mixtures of zwitterionic surfactants and triblock polymers: a cyclic voltammetric study

Cyclic voltammetry has been employed to investigate the mixed micellar behavior of the binary mixtures of different zwitterionic surfactants such as 3-(N,N-dimethylhexadecylammonio)propane sulfonate (HPS), 3-(N,N-dimethyltetradecylammonio)propane sulfonate (TPS) and 3-(N,N-dimethyldodecylammonio)propane sulfonate (DPS) with three triblock polymers (L64, F127 and P65) by using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as an electroactive probe at 25 °C. Critical micellar concentration (cmc) has been determined from the plots of variation in peak current (i_p) versus the total concentration of surfactant/triblock polymer. Diffusion coefficient of the electroactive species has also been reported. The regular solution theory approximation has been used to determine various micellar parameters of ideal systems. The variation in micellar mole fraction (X₁) of the zwitterionic surfactant supports the formation of mixed micelles, which are rich in triblock polymer component in the surfactant rich region of the mixture and vice versa. The regular solution interaction parameter (β) suggests the formation of mixed micelles due to the synergistic interactions in case of HPS/TPS/DPS + F127/P65 systems and gets affected by EO/PO ratio of triblock polymers.     

手性胶束中的有机反应(Ⅴ)——手性胶束中查耳酮的不对称环氧化

本文报道查耳酮用H₂O₂在手性表面活性剂N,N-二甲基-N-十二烷基麻黄素澳化铵(1a)及N,N-二甲基-N-十六烷基麻黄素溴化铵(1b)形成的手性胶柬水溶液中的不对称环氧化反应,得到相应的手性α,β-环氧酮,对映体过量为5～8%.     

Electric-field-driven surface aggregation of a model zwitterionic surfactant

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to describe the electric-field-controlled surface aggregation of N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDAPS), a model zwitterionic surfactant, at a Au(111) electrode surface. At concentrations below the critical micelle concentration (CMC), the monomer adsorbs and aggregates at the surface. The charge on the metal (sigmaM) controls the orientation of adsorbed molecules and consequently the film structure. At high negative (sigmaM < -5 microC cm-2) charge densities, a spongy, disordered film is formed in which the polar heads are turned toward the solution. At high positive (sigmaM > +5 microC cm-2) charge densities, a planar film with "blisters" is observed with the polar heads of DDAPS turned to the metal. Hemicylindrical aggregates are observed in the intermediate charge density range (-5 < sigmaM < +5 microC cm-2). At bulk concentrations higher than the CMC, micelles adsorb and the structure of these films is controlled by the fusion of the adsorbed micelles. STM and AFM images provided direct visualization of this field-driven surface aggregation of the zwitterionic surfactant.