Presence Of CTAB Micelles Research Articles

Interface and doping in carbon dots influence charge transfer and transport

Nanoscale carbon-based materials have the potential for large-scale optoelectronic and biomedical applications due to their tunable photo-physics. Here, we explore the impact of doping and the role of intervening media on both electron transfer and transport from carbon dots (CD) to an acceptor menadione (MD). In a binary mixture compared to undoped-CD (UCD), the amine-based nitrogen-doped-CD (ACD) reflects significant changes in its photo-physics like excitation-independent fluorescence, enhanced lifetime, along with a distinct drop in electron transfer rate (kET) to MD. With micro-emulsion, kET from UCD to MD is enhanced while a substantial reduction is observed for ACDs, emphasizing the critical role of doping, surface states, encapsulation, and interfaces of CTAB micelles. The impact of doping and Coulombic interaction at interfaces on the charge transport phenomena with redox-active MD is further investigated by current-sensing atomic force microscopy. For similar bias-voltage, ACD reflects a higher conductance than UCD, and the conductance is enhanced in the presence of CTAB micelles. Moreover, at a low voltage regime, the current-voltage curve shows a linear growth, while at higher voltage, Fowler-Nordheim (F–N) tunneling is observed. The presence of MD, however, leads to an abrupt rise in conductivity resulting in a transition from F–N to direct tunneling.

Exploring the diethylaminoflavylium derivatives multistate system of chemical reactions in the presence of CTAB micelles: thermodynamic reversibility achieved through different kinetic pathways

The flavylium cation and trans-chalcone species are thermodynamically stable at pH 0.5 and 6.0, respectively, switching reversibly between these two pH values. However, the thermodynamic stability is achieved through different pathways depending on whether pH is increasing or decreasing.

Chemistry of 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium. A photochromic system switching from yellow to green

The network of chemical reactions of the bio-inspired compound 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium is identical to anthocyanins. A photochromic system switching from yellow to green is obtained. The presence of a cathecol susbtituent can extend the network to metal inputs. • The so-called styrylflavylium compounds are promising alternatives to anthocyanins and related compounds. • They follow the same network of reactions of anthocyanins including hydration, tautomerization and cis – trans isomerization. • The absorption spectra of styrylflavyliums are red shifted in comparison with the flavylium analogues. • A switching photochromic system from yellow to green is obtained in the presence of CTAB micelles. The bio-inspired compound 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium (styrylflavylium) was synthesized through the insertion of a double bond between rings B and C of the flavylium cation. The pH dependent network of chemical reactions of the new compound is similar to the one of anthocyanins and related compounds. The styrylflavylium is the dominant species at very low pH values. At moderately acidic pH values the respective quinoidal base is formed by deprotonation of the styrylflavylium in competition with the hydration reaction to give an analogous of the anthocyanins hemiketal. The cis -chalcone is formed through the hemiketal (ring C opening) and the trans -chalcone by the cis–trans isomerization of the former. All the rate and equilibrium constants of these reactions have been calculated, permitting to completely characterize the styrylflavylium network. While in water no photochromism could be observed in the presence of CTAB micelles, formation of the flavylium cation upon irradiation of the trans -chalcone takes place. Complexation of the quinoidal base with Al 3+ was clearly identified.

Photochemistry of 7-Hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium and Related Compounds

2-Styryl-1-benzopyrylium derivatives exhibit deeper hues and absorption spectra that are substantially red-shifted when compared with their 2-phenyl-1-benzopyrylium analogues. They follow the same pH and light-dependent network of chemical reactions previously described for 2-phenyl-1-benzopyrylium compounds. In this work, the photochromic properties of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium chloride are reported. This compound was fully characterized by UV-vis absorption, fluorescence emission, pH jumps, and flash photolysis, and its properties were compared with the analogue 7,4'-dihydroxyflavylium (7-hydroxy-2-(4-hydroxyphenyl)-1-benzopyrylium). The trans-chalcones of both compounds lacking the hydroxyl in position 2 were synthesized and used as model compounds since they exhibit cis-trans isomerization but cannot be involved in the other processes resulting from the ring closure. The transient absorption of two triplets attributed to the chalcones Ct/Ct(-), and a tautomer was detected by nanosecond flash photolysis, independent of the existence of the 2-hydroxyl substituent. The experimental results are compatible with the main formation of cis-chalcone from the singlet state. In the case of the styryl derivatives, the fraction of triplet formed from excitation of Ct is much higher, and the fraction of isomerization is much smaller. For this reason, the photochemistry of 7-hydroxy-2-(4-hydroxystyryl)-1-benzopyrylium in water is much less efficient than that of its parent 7,4'-dihydroxyflavylium; however, in the presence of CTAB micelles, intense red colors can be obtained upon irradiation, confirming the usefulness of this family of compounds as photochromic systems.

Decomposition of Monochloramines of Amino Acids from Water by Surfactant Micelles

Decomposition of monochloramines derived from amino acids were studied in buffer solution pH=7 in the presence of different concentrations of CTAB micelles. The acceleration of decomposition reaction of monochloramines of amino acids in the presence of CTAB micelles is ascribed to the increased reactivity in the micellar pseudophase as compared with that in aqueous phase as inferred from the obtained values of km and kw. From studying the removal efficiency of monochloramines, it was observed that the removal efficiency increases with increasing surfactant concentration, temperature, and the hydrophobicity of side chain of monochloramines of amino acids.

Kinetics and Mechanism of the Reaction between Ascorbic Acid Derivatives and an Arenediazonium Salt: Cationic Micellar Effects

The effects of tetradecyltrimethylammonium bromide, TTAB, and hexadecyl-trimethylammonium bromide, CTAB, micellar systems on the reaction of 3-methylbenzenediazonium, 3MBD, tetrafluoroborate with ascorbic acid, VC, and with the hydrophobic derivatives 6-O-dodecyl-L-ascorbic acid, VC12, and 6-O-palmitoyl-L-ascorbic acid, VC16, were investigated at different pH values by employing a combination of UV-vis spectroscopy and high-performance liquid chromatography, HPLC, techniques. Previous studies in the absence of surfactant showed that the reaction between 3MBD and VC derivatives takes place through a rate-limiting decomposition of a transient diazo ether, DE, formed from reaction between 3MBD and the monoanion form of ascorbic acid, VC-, in a rapid preequilibrium step. In the presence of a fixed [CTAB], the kinetics of the reaction of 3MBD with VC follows a saturation kinetics similar to that observed in its absence, but for the reaction with VC12 and VC16, only the first linear portions of the saturation profiles could be obtained because k(obs) values become too large. HPLC analyses of the reaction mixtures show that no unexpected products are detected, suggesting that cationic micelles do not modify the mechanism of the reaction. Analyses of the kinetic data allowed estimations of the rate constant for the decomposition of the diazo ether and of the equilibrium constant for the formation of DE in the presence of CTAB micelles, which is approximately 6 times higher than in its absence; this suggests that CTAB micelles promote diazo ether formation. At constant [antioxidant], the variations of k(obs) for the reactions with VC, VC12, or VC16 follow bell-shaped curves, with rate enhancements of up to 2-3-fold for VC with respect to the value in the absence of surfactant. The rate maximum for the reaction of 3MBD with VC is reached at [CTAB] = 0.02 M suggesting a CTAB-induced rate increase, i.e., micellar catalysis; meanwhile the rate maximum for the reaction with VC12 and VC16, which may behave as amphiphilic compounds, is reached at [CTAB] approximately 1 x 10(-4) M, a concentration about 10 times lower than its critical micelle concentration, cmc, in pure water, but only approximately 3 times lower than the cmc of VC16, suggesting the formation of reactive CTAB-VC12 and CTAB-VC16 premicellar aggregates. Kinetic and HPLC results are consistent with the predictions of the pseudophase model and are interpreted in terms of 3MBD ions sampling in the aqueous bulk phase and the micellar effects on the different equilibrium involved. The results should contribute to a better understanding of the role of compartmentalized systems on the efficiency with which hydrophilic and hydrophobic reductants such as ascorbic acid derivatives interact with potentially mutagenic and carcinogenic ArN2+ ions.

Esterolysis of long-chain p-nitrophenyl picolinates by hydrophobic Zn 2+ complexes in the presence of a cationic surfactant

The introduction of an increasing alkyl-chain length into a series of homologous esters (5-alkoxy-4′-nitrophenyl picolinates) leads to the enhancement of the overall esterolysis reaction rate catalyzed by lipophilic substituted Zn 2+-complexed 2-hydroxymethylbenzimidazoles in the presence of CTAB micelles.

Study of the Interaction of Propoxur and Its Metabolites with Surfactants

The interaction between propoxur and different surfactant molecules has been studied in order to determine the experimental conditions required to obtain sensitized spectrometric measurements. It has been found that a fluorescence enhancement of 1.4-fold for propoxur in the presence of CTAB micelles provides a limit of detection of 5 × 10−8M. On the other hand, the hydrolysis products of propoxur, 2-isopropoxyphenol and 2-isopropoxyphenolate interact strongly with CTAB micelles and provide fluorescence enhancement of 2-fold and 19.5-fold, respectively. Spectrophotometric and fluorometric measurements have confirmed that the interaction takes place in the excited state and without modification of the pK of 2-isopropoxyphenol.

Catalysis by surfactant aggregates in aqueous solutions

Abstract

Time-resolved micelle-stabilized room-temperature phosphorimetry for simultaneous determination of gallium and indium

Ga(III) and In(III) react with 7-iodo-hydroxyquinoline-5-sulphonic acid (Ferron) in the presence of CTAB micelles, forming complexes which exhibit analytically useful room-temperature phosphorescence (RTP). The RTP features of the two complexes are similar and the RTP spectra overlap. However, there is sufficient difference between the two phosphorescence decay rates for both metals to be determined by time-resolved room-temperature phosphorimetry.

Relevance of cell kinetics and hormonal receptor status on response to hormonal therapy of advanced breast cancer

The bio-inspired compound 7,8-dihydroxy-2-(4-dimethylaminostyryl)-1-benzopyrylium (styrylflavylium) was synthesized through the insertion of a double bond between rings B and C of the flavylium cation. The pH dependent network of chemical reactions of the new compound is similar to the one of anthocyanins and related compounds. The styrylflavylium is the dominant species at very low pH values. At moderately acidic pH values the respective quinoidal base is formed by deprotonation of the styrylflavylium in competition with the hydration reaction to give an analogous of the anthocyanins hemiketal. The cis-chalcone is formed through the hemiketal (ring C opening) and the trans-chalcone by the cis–trans isomerization of the former. All the rate and equilibrium constants of these reactions have been calculated, permitting to completely characterize the styrylflavylium network. While in water no photochromism could be observed in the presence of CTAB micelles, formation of the flavylium cation upon irradiation of the trans-chalcone takes place. Complexation of the quinoidal base with Al3+ was clearly identified.

Interactions between alkyl xanthates and cationic micelles

Interactions between methyl, ethyl, n-buthyl, n-amyl, and n-hexyl xanthates with monomers and micelles of cetyltrimethylammonium bromide CTAB were studied. Xanthates have a pronounced absorbance peak at 301 nm which is shifted to 306 nm when in presence of CTAB micelles. A maximum difference in absortivities between the micellar bound and free xanthates was found at 313 nm. This wavelength was used to measure the amount of xanthate associated to micelles. An insoluble ion pair formed between xanthates and CTA monomers which is gradually redissolved by increasing amounts of CTAB was detected. Association constants of xanthates to CTAB micelles as well as ionic exchange constants between xanthates and added Br − ions in presence of CTAB micelles were calculated and from their values the transfer free energies Δ μ x 0 of the methyl and methylene groups of the alkyl chain of xanthates were calculated from both the association and ionic exchange constants.

Micellar and Metal Ion Catalysis in the Pyridoxal-Promoted α,β-Elimination of S-Phenylcysteine

Abstract The effects of metal ions and detergents on the pyridoxal-catalyzed α,β-elimination reaction of S-phenylcysteine have been investigated in 0.01 M borate buffer at 50 °C (the copper ion catalysis at 40 °C in the presence of CTAB micelles). Both copper(II) and nickel(II) ions markedly accelerated the elimination reaction of S-phenylcysteine. Since the catalytic efficiency of nickel(II) seems to be obtained due to the poor coordination ability of the leaving group in the present reaction, the previous working hypothesis which was provided for the elucidation of catalytic roles of metal ions in the α,β-elimination of O-phosphothreonine has now been approved. Among cationic, anionic, and non-ionic detergents employed in this work as apoenzyme models, only CTAB of cationic nature showed a considerable rate acceleration even in the absence of metal ions. The micellar binding constant was evaluated from the saturation-type correlation between reaction rate and CTAB concentration. The surfactant micelles demonstrated some enzyme-like features of catalysis in the pyridoxal-promoted α,β-elimination reaction, where the reaction is controlled by the entropy term. The CTAB catalysis was greatly inhibited by organic and inorganic anions due to the electrostatic nature of the micellar surface. The catalytic mechanisms for the micellar reactions and their significance as an enzyme model system are discussed.