Pulse radiolysis study of redox reactions of safranine T in sodium dodecylsulphate (SDS) micellar medium

Reaction of hydrated electrons with safranine T (SF⁺), a phenazine dye useful as sensitizer in photogalvanic cell and the transient semireduced species formed by this reaction have been studied in SDS micellar medium using the technique of pulse radiolysis. Thee _aq ⁻ reaction with SF⁺ in the micellar environment was only marginally slower (5.1 × 10⁹ dm³ mol⁻¹ s⁻¹) as compared to that in homogeneous aqueous medium (2.2 × 10¹⁰ dm³ mol⁻¹ s⁻¹) explicable on the basis of our finding that although a large fraction of the dye gets localized near the micelle Stern layer where the molecule experiences a dielectric.constant of ≈40, a small but significant concentration of the dye exists in the aqueous bulk as charge pair complex with the anionic surfactant monomer (association constant for the formation of the complex being 2.8 × 10⁴ dm³ mol⁻¹). The transient semireduced absorption band observed in the micellar medium showed a red shift of ≈ 50 nm and also the decay of the transient, which was very fast with 2k = 1 × 10⁹ dm³ mol⁻¹ s⁻¹ in aqueous medium, was stable in the SDS micellar medium over a few tens of milliseconds suggesting that the radical is incorporated deeper than the parent molecule in the SDS micelle. The effect of this stability on the photogalvanic conversion needs to be examined.     

Generation of 8‐oxo‐7,8‐dihydroguanine in G‐Quadruplexes Models of Human Telomere Sequences by One‐electron Oxidation

The mechanistic aspects of one‐electron oxidation of G‐quadruplexes in the basket (Na⁺ ions) and hybrid (K⁺ ions) conformations were investigated by transient absorption laser kinetic spectroscopy and HPLC detection of the 8‐oxo‐7,8‐dihydroguanine (8‐oxoG) oxidation product. The photo‐induced one‐electron abstraction from G‐quadruplexes was initiated by sulfate radical anions (SO₄˙^?) derived from the photolysis of persulfate ions by 308 nm excimer laser pulses. In neutral aqueous solutions (pH 7.0), the transient absorbance of neutral guanine radicals, G(‐H)˙, is observed following the complete decay of SO₄˙^? radicals (~10 μs after the actinic laser flash). In both basket and hybrid conformations, the G(‐H)˙ decay is biphasic with one component decaying with a lifetime of ~0.1 ms, and the other with a lifetime of 20–30 ms. The fast decay component (~0.1 ms) in G‐quadruplexes is correlated with the formation of 8‐oxoG lesions. We propose that in G‐quadruplexes, G(‐H)˙ radicals retain radical cation character by sharing the N1‐proton with the O⁶‐atom of G in the [G˙⁺: G] Hoogsteen base pair; this [G(‐H)˙: H⁺G G˙⁺: G] leads to the hydration of G˙⁺ radical cation within the millisecond time domain, and is followed by the formation of the 8‐oxoG lesions.     

Study of Partitioning and Solubilization of Some New Chalcones Between Aqueous and Micellar Phases

The partitioning behavior of four newly synthesized chalcones between aqueous and micellar phases of ionic surfactants (SDS and CTAB) was investigated using ultraviolet-visible spectroscopy. The simple absorption spectra were recorded to study the interaction between these drugs and surfactants (in the concentration range below critical micelle concentration to above critical micelle concentration). The absorption data is also used to determine the number of additive molecules incorporated per micelle of the surfactant. The partition coefficient (K_x) of additives between bulk water phase and the micellar phase was determined in the range of 5.52 × 10⁺⁴ to 5.06 × 10⁺⁵ at 298 K by differential spectroscopic method. The corresponding standard free energy of partition ΔG°_p obtained was in the range of ?27.05 kJmol^?1 to ?32.54 kJmol^?1. The relative solubility of additives between aqueous and micellar phases in different micellar concentrations was also estimated. The results showed that the chalcones are preferably soluble in cationic surfactant micelles.     

Spectral,kinetic, and redox properties of basic fuchsin in homogeneous aqueous and sodium dodecyl sulfate micellar media

Effect of anionic surfactant on the optical absorption spectra and redox reaction of basic fuchsin, a cationic dye, has been studied. Increase in the absorbance of the dye band at 546 nm with sodium dodecyl sulfate (SDS) is assigned to the incorporation of the dye in the surfactant micelles with critical micellar concentration (CMC) of 7.3 × 10^?3 mol dm^?3. At low surfactant concentration (<5 × 10^?3 mol dm^?3) decrease in the absorbance of the dye band at 546 nm is attributed to the formation of a dye–surfactant complex (1:1). The environment, in terms of dielectric constant, experienced by basic fuchsin inside the surfactant micelles has been estimated. The association constant (K_A) for the formation of dye–SDS complex and the binding constant (K_B) for the micellization of dye are determined. Stopped‐flow studies, in the premicellar region, indicated simultaneous depletion of dye absorption and formation of new band at 490 nm with a distinct isosbestic point at 520 nm and the rate constant for this region increased with increasing SDS concentration. The reaction of hydrated electron with the dye and the decay of the semireduced dye are observed to be slowed down in the presence of SDS. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 629–636, 2003     

Micellar effect on the electron transfer reaction of chromium(V) ion with organic sulfides

The rate of electron transfer from organic sulfides to [Cr^V(ehba)₂]⁻ (ehba-2-ethyl-2-hydroxy butyric acid) decreases with a decrease in the polarity of the medium. The anionic surfactant, SDS and the cationic surfactant, CTAB have different effects on the kinetics of this reaction. The micellar inhibition observed in the presence of SDS is probably due to the decrease in the polarity and the electrostatic repulsion faced by the anionic oxidant from the anionic micelle and the partition of the hydrophobic substrate between the aqueous and micellar phases. The micellar catalysis in the presence of CTAB is attributed to the increase in the concentration of both reactants in the micellar phase. This micellar catalysis is observed to offset the retarding effects of the less polar micellar medium and the unfavorable charge-charge interaction between the + charge developed on S center in the transition state and the cationic micelle. This catalysis is contrary to the enormous micellar inhibition observed with IO₄⁻, HSO₅⁻ and HCO₄⁻ oxidation of organic sulfides.     

Kinetics and mechanism of the formation of silver nanoparticles by reduction of silver (I) with maltose in the presence of some active surfactants in aqueous medium

The kinetics and mechanism of the formation of silver nanoparticles by reduction of Ag⁺ with maltose were studied spectrophotometrically by monitoring the absorbance change at 412 nm in aqueous and micellar media at a temperature range 45–60 °C. The reaction was carried out under pseudo-first-order conditions by taking the [maltose] (>tenfold) the [Ag⁺]. A mechanism of the reaction between silver ion and maltose is proposed, and the rate equation derived from the mechanism was consistent with the experimental rate law. The effect of surfactants, namely cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant), on the reaction rate has been studied. The enthalpy and the entropy of the activation were calculated using the transition state theory equation. The particle size of silver sols was characterized by transmission electron microscopy and some physiochemical and spectroscopic tools.     

Spectroscopic Investigations on the Interaction of Crystal Violet with Nonionic Micelles of Brij and Igepal Surfactants in Aqueous Media

The behavior of the triphenylmethane dye crystal violet in aqueous solutions containing polyoxyethylene nonionic surfactants was investigated using absorption and fluorescence spectroscopic techniques. The interactions of the dye were examined in micellar media in order to prevent dye aggregation and to ensure maximum dye and surfactant interaction. The relative fluorescence enhancements and the binding constants of the dye to the surfactant micelles were determined. The micropolarities of the micellar environment sensed by the pyrene probe were estimated from the I ₁/I ₃ intensity ratios of the fluorescence spectra of pyrene. The fluorescence quenching of pyrene by hexadecylpyridinium chloride was investigated in aqueous surfactant mixtures at a fixed concentration of surfactant in order to determine the aggregation numbers. Attempts were made to correlate the binding constants obtained in this investigation to various micellar parameters.     

Kinetics of the autoxidation of adrenaline and [copper(II)(adrenaline)]2+ in alkaline aqueous and micellar media

The kinetics of autoxidation of adrenaline and [Cu(adrenaline)]²⁺ complex by dissolved oxygen in alkaline aqueous and micellar media has been studied. The reaction is initiated by the removal of amino-H⁺ protons of adrenaline by hydroxide ion, followed by cyclization. The values of (1/k _obs) for the autoxidation of both species were found to be linearly dependent upon 1/[OH^?]. The reaction follows a consecutive pathway in which the intermediate adrenochrome remains stable for few minutes, and then undergoes further reactions to yield adrenolutin and other products. The [Cu(adrenochrome)]⁺ complex is stable for a few hours. Studies on the effects of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) on the reactivity of both species revealed different behaviors. The micelles of CTAB catalyzed the rates of autoxidation for both species, whereas SDS micelles inhibited the autoxidation of adrenaline but catalyzed the rate of autoxidation of [Cu(adrenaline)]²⁺. Addition of the reactive counterion surfactant, cetyltrimethylammonium hydroxide (CTAOH) initially increased the rate constant with the increasing [CTAOH], until it reached a plateau for k _ψ ?[CTAOH]. Salts such as NaCl, NaBr, tetramethyl ammonium bromide, and tetraethyl ammonium bromide increased the rate when added at lower concentrations, but had negligible effect at higher concentrations. The results obtained in micellar media were treated according to Berezin’s Pseudophase Model. The various kinetic parameters for the reactions occurring in aqueous and in micellar media are reported.     

Role of the Surfactant Structure in the Behavior of Hydrophobic Ionic Liquids within Aqueous Micellar Solutions

The behavior of an ionic liquid (IL) within aqueous micellar solutions is governed by its unique property to act as both an electrolyte and a cosolvent. The influence of the surfactant structure on the properties of aqueous micellar solutions of zwitterionic SB‐12, nonionic Brij‐35 and TX‐100, and anionic sodium dodecyl sulfate (SDS) in the presence of the “hydrophobic” IL 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim][PF₆]) is assessed along with the possibility of forming oil‐in‐water microemulsions in which the IL acts as the “oil” phase. The solubility of [bmim][PF₆] within aqueous micellar solutions increases with increasing surfactant concentration. In contrast to anionic SDS, the zwitterionic and nonionic surfactant solutions solubilize more [bmim][PF₆] at higher concentrations and the average aggregate size remains almost unchanged. The formation of IL‐in‐water microemulsions when the concentration of [bmim][PF₆] is above its aqueous solubility is suggested for nonionic Brij‐35 and TX‐100 aqueous surfactant solutions.     

Micellar catalysis on picolinic acid promoted hexavalent chromium oxidation of glycerol

Under pseudo-first-order conditions, monomeric Cr(VI) was found to be kinetically active in the absence of picolinic acid (PA), whereas in the PA-promoted path, the Cr(VI)–PA complex undergoes nucleophilic attack by the substrate to form a ternary complex which subsequently experiences redox decomposition, leading to glyceraldehydes and Cr(IV)–PA complex. The uncatalyzed path shows a second-order dependence on [H⁺], whereas the PA-catalyzed path shows zero-order dependence on [H⁺]. Both the uncatalyzed and PA-catalyzed path show a first-order dependence on [glycerol]_T and [Cr(VI)]_T. The PA-catalyzed path is first order in [PA]_T. All these observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant cetyl pyridinium chloride (CPC) and anionic surfactant sodium dodecyl sulfate (SDS) on the PA-catalyzed path have been studied. CPC inhibits, whereas SDS accelerates the reaction. Here, SDS is a catalyst for glyceraldehydes production and at the same time reduction of carcinogenic hexavalent chromium to nontoxic trivalent chromium. The reaction proceeds simultaneously in both aqueous and micellar phase. Micellar effects have been explained by considering the preferential partitioning of reactants between the micellar and aqueous phase. The Menger–Portnoy model, Piszkiewicz cooperative model, and pseudo-phase ion exchange model have been tested to explain the observed micellar effect.     

Micellar catalysis of chromium(VI) oxidation of ethane-1,2-diol in the presence and absence of 2,2′-bipyridine in aqueous acid media

The kinetics and mechanism of the Cr(VI) oxidation of ethane-1,2-diol in the presence and absence of 2,2′-bipyridine (bipy) in aqueous acid media were studied under the conditions [ethane-1,2-diol]_T ? [Cr(VI)]_T. Under the kinetic conditions, monomeric Cr(VI) was found to be kinetically active in the absence of bipy, whereas in the bipy-catalyzed path the Cr(VI)-bipy complex was the active oxidant. In this path, the Cr(VI)-bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes redox decomposition (through 2e transfer) leading to hydroxyethanol and the Cr(IV)-bipy complex. The Cr(IV)-bipy complex then participates further in oxidation of organic substrate, ultimately converted into inert Cr(III)-bipy complex. The uncatalyzed path shows a second-order dependence on [H⁺], while the bipy-catalyzed path shows a first-order dependence on [H⁺]. Both the uncatalyzed and bipy-catalyzed paths show first-order dependence on [ethane-1,2-diol]_T and on [Cr(VI)]_T. The bipy-catalyzed path is first-order in [bipy]_T. All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N-cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalyzed and bipy-catalyzed paths were studied. CPC inhibits both the uncatalyzed and bipy-catalyzed paths, whereas SDS catalyzes the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases.     

Role of manganese(II), micelles,and inorganic salts on the kinetics of the redox reaction of L‐sorbose and chromium(VI)

The kinetics of the reduction of chromium(VI) to chromium(III) by L ‐sorbose in HClO₄ was studied between 30 and 80°C at various concentrations of reactants and acidities in both aqueous and micellar sodium dodecyl sulfate (SDS)/TritonX‐100(TX‐100) solutions. Under pseudo‐first‐order conditions the reaction rate is fractional‐order in [L ‐sorbose] and [H⁺], and first‐order in [Cr^VI] both in the absence and in the presence of surfactant micelles. The reaction is accelerated by addition of manganese(II) and is routed through the same mechanism as shown by the kinetic studies in the absence and presence of surfactants. The rate enhancement in presence of SDS/TX‐100 micelles indicates that essentially all the reactive species are bound to micelles under the experimental conditions. The observed catalyses are explained with the modified Menger and Portnoy model. Inorganic salts (NaBr, LiBr, NH₄Br) inhibit the reaction in presence of SDS micelles, which confirms exclusion of the reactive species of chromium(VI) from the reaction site. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 543–554, 2003     

Nucleophilic debenzoylation of p‐nitrophenyl benzoate in cationic micellar media

Pseudo‐first‐order rate constants for the nucleophilic debenzoylation reaction of p‐nitrophenyl benzoate with various hydroxamate ions [RC = ONHO^?] were investigated in aqueous cationic micellar media at pH 7.8 and 27°C. The kinetic rate data of the reaction revealed that the nucleophilic reactivity sequence of these hydroxamate ions is generally benzohydroxamic acid > salicylhydroxamic acid > acetohydroxamic acid. The k_obs value increases upon addition of cationic surfactants to the reaction medium involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. The effect of surfactant head and tail group is discussed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 106–112, 2010     

Kinetics and mechanism of the redox reaction between malachite green and iron(III) in aqueous and micellar media

The kinetics of the oxidation of malachite green (MG⁺) by Fe(III) were investigated spectrophotometrically by monitoring the absorbance change at 618 nm in aqueous and micellar media at a temperature range 20–40 °C; I = 0.10 mol dm^?3 for [H⁺] range (2.50–15.00) × 10^?4 mol dm^?3. The rate of reaction increases with increasing [H⁺]. The reaction was carried out under pseudo-first-order conditions by taking the [Fe(III)] (>10-fold) the [MG⁺]. A mechanism of the reaction between malachite green and Fe(III) is proposed, and the rate equation derived from the mechanism was consistent with the experimental rate law as follows: Rate = (k ₄ + K ₁ k ₅[H⁺]) [MG⁺][Fe(III)]. The effect of surfactants, such as cetyltrimethylammonium bromide (CTAB, a cationic surfactant) and sodium dodecylsulfate (SDS, an anionic surfactant), on the reaction rate has been studied. CTAB has no effect on the rate of reaction while SDS inhibits it. Also, the effect of ligands on the reaction rate has been investigated. It is proposed that electron transfer proceeds through an outer-sphere mechanism. The enthalpy and the entropy of the activation were calculated using the transition state theory equation.     

Photolysis of 1,2-dihydroquinolines in micellar solutions of anionic and cationic surfactants

The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4–12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s^?1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species—carbocations in the aqueous phase and aminyl radicals in micelles—is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.     

Kinetics and mechanism of intramolecular general base-catalyzed alkanolysis of phenyl salicylate in the presence of anionic micelles

The alkanolysis of ionized phenyl salicylate, PS^?, has been studied in the presence and absence of micelles of sodium dodecyl sulphate, SDS, at 0.05 M NaOH, 30 or 32°C and within the alkanol, ROH, (ROH = HOCH₂CH₂OH and CH₃OH) contents of 15–74 or 92%, v/v. The alkanolysis of PS^? involves intramolecular general base catalysis. At a constant concentration of SDS, [SDS]_T, the observed pseudo first-order rate constants, k_obs, for the reactions of ROH with PS^? obtained at different concentration of ROH, [ROH]_T, obey the relationship: k_obs = k[ROH]_T/(1 + K_A[ROH]_T) where k is the apparent second-order rate constant and K_A is the association constant for dimerization of ROH molecules. Both k and K_A decrease with increase in [SDS]_T. At a constant [ROH]_T, the rate constants, k_obs, show a decrease of nearly 2-fold with increase in [SDS]_T from 0.0–0.3M. These results are explained in terms of pseudo-phase model of micelle. The rate constants for alkanolysis of PS^? in micellar pseudophase are insignificant compared with the corresponding rate constants in aqueous-alkanol pseudophase. This is attributed largely to considerably low value of [ROH] in the specific micellar environment where micellar bound PS^? molecules exist. The increase in [ROH]_T decrease the value of the binding constant of PS^? with SDS micelle. The effects of anionic micelles on the rates of alkanolysis of PS^? are explained in terms of the porous cluster micellar structure.     

Environmentally responsive micelles from polystyrene–poly[bis(potassium carboxylatophenoxy)phosphazene] block copolymers

Amphiphilic diblock copolymers that contained hydrophilic poly[bis(potassium carboxylatophenoxy)phosphazene] segments and hydrophobic polystyrene sections were synthesized via the controlled cationic polymerization of Cl₃P?NSiMe₃ with a polystyrenyl–phosphoranimine as a macromolecular terminator. These block copolymers self‐associated in aqueous media to form micellar structures which were investigated by fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The size and shape of the micelles were not affected by the introduction of different monovalent cations (Li⁺, K⁺, Na⁺, and Cs⁺) into the stable micellar solutions. However, exposure to divalent cations induced intermicellar crosslinking through carboxylate groups, which caused precipitation of the ionically crosslinked aggregates from solution. This micelle‐coupling behavior was reversible: the subsequent addition of monovalent cations caused the redispersion of the polystyrene‐block‐poly[bis(potassium carboxylatophenoxy)phosphazene] (PS–KPCPP) block copolymers into a stable micellar solution. Aqueous micellar solutions of PS–KPCPP copolymers also showed pH‐dependent behavior. These attributes make PS–KPCPP block copolymers suitable for studies of guest retention and release in response to ion charge and pH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2912–2920, 2005     

Activity of α‐Chymotrypsin in Cationic and Nonionic Micellar Media: Ultraviolet and Fluorescence Spectroscopic Approach

α‐Chymotrypsin (α‐CT) activity was measured in aqueous buffer with the following alkyltriphenylphosphonium bromide surfactants in the series cetyl, tetradecyl, and dodecyl as a tail length. For the sake of comparison with mixed micellar investigation on activity of α‐CT, cationic cetyltriphenylphosphonium bromide (CTPB) and nonionic surfactant Triton X‐100, Brij‐56, Brij‐35, Tween 20, and Igepal Co‐210 have been used. The p‐nitrophenyl acetate (PNPA) hydrolysis rate was determined at the surfactant concentration of both cationic and mixed micellar systems by a UV–vis spectrophotometer. The catalytic reaction follows the Michaelis–Menten mechanism, and the catalytic efficiency (k_cat/K_M) was evaluated for both homogeneous and mixed‐micellar media. The maximum catalytic efficiency was observed at 5 mM concentration of CTPB, but the highest catalytic efficiency, 572 M^?1 s^?1, was measured in the presence of mixed micellar (7.5 mM CTPB + 2.5 mM Tween‐20). The fluorescence (FL) spectra showed the differences of α‐CT conformations in the presence of cationic surfactants. The FL results suggest that the influence of cationic surfactant on proteolysis arises from the interaction with the α‐CT. The binding constant, k_sv, of α‐CT with cationic aggregates was determined in the buffer using the Stern–Volmer equation by the fluorescence spectroscopic approach.