Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as kinetic probe for piperazinium-based cationic micelles

A new class of cationic surfactants containing the heterocyclic piperazinium ring in their covalent structure was prepared; cetyldialkylpiperazinium halides, CRPX, with alkyl=Me (CMPX), Et (CEPX), n-Pr (CPPX), and with halide=Br and Cl. They were characterized by measures of critical micellar concentration, cmc, and ionization degree, alpha, and also by use of the decarboxylation of 6-nitrobenzisoxazole-3-carboxylate as a kinetic probe to investigate the properties of the microinterface they provide in aqueous solutions. The pseudophase kinetic treatment fails to fit the data at high [surfactant], which show anomalies with abrupt increase in k(obs), especially for CEPX and CPPX. Data can be fitted up to [surfactant] ca. 0.1 M, and 0.2 M in some cases. Compared with cetyltrialkylammonium halides, values of k'(M) indicate a water-richer microenvironment and less important interface property changes with increasing head group bulk. The reaction could be studied both in the presence and in the absence of NaOH; comparison shows that NaOH affects only the shape of the kinetic profile at low [surfactant], without affecting the microenvironment provided by mature micelles.     

Effect of ethanol on micellization and on decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous cationic micelles

The effects of ethanol on the critical micellar concentration (cmc) and the rates of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) have been investigated in aqueous cationic surfactants of the cetyltrialkylammonium family with bromide [CT(R)ABr], chloride [CT(R)ACl], and nitrate [CT(R)ANO3] counterions, and methyl (CTAX), n-propyl (CTPAX), and n-butyl (CTBAX) as the head group alkyl moieties. Effects upon cmc and reactivity are similar, featuring a break at the ethanol mole fraction, x(EtOH), of ca. 0.055; these effects have been related to changes in solvent structure, with formation of a clathrate at x(EtOH) = 0.055. Rate data in CTBABr were further investigated and fitting of raw kinetic data to the pseudophase model is possible up to x(EtOH) = 0.1, showing an unexpected decrease of rate constant values in the micellar pseudophase, kM', as ethanol content increases: a significant variation of micellar ionization degree could account for this kinetic effect.     

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in mixed micelles of zwitterionic and positively charged surfactants

The rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, NBOC, was determined in micelles of N-hexadecyl-N,N,N-trimethylammonium bromide or chloride (CTAB or CTAC), N-hexadecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (HPS), N-dodecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (DPS), N-dodecyl-N,N,N-trimethylammonium bromide (DTAB), hexadecylphosphocholine (HPC), and their mixtures. Quantitative analysis of the effect on micelles on the velocity of NBOC decarboxylation allowed the estimation of the rate constants in the micellar pseudophase, k(m), for the pure surfactants and their mixtures. The extent of micellar catalysis for NBOC decarboxylation, expressed as the ratio k(m)/k(w), where k(w) is the rate constant in water, varied from 240 for HPS to 62 for HPC. With HPS or DPS, k(m) decreased linearly with CTAB(C) mole fraction, suggesting ideal mixing. With HPC, k(m) increased to a maximum at a CTAB(C) mole fraction of ca. 0.5 and then decreased at higher CTAB(C). Addition of CTAB(C) to HPC, where the negative charge of the surfactant is close to the hydrophobic core, produces tight ion pairs at the interface and, consequently, decreases interfacial water contents. Interfacial dehydration at the surface in equimolar HPC/CTAB(C) mixtures, and interfacial solubilization site of the substrate, can explain the observed catalytic synergy, since the rate of NBOC decarboxylation increases markedly with the decrease in hydrogen bonding to the carboxylate group.     

Temperature effects upon aqueous micellar-assisted decarboxylation of 6-nitrobenzisoxazole-3-carboxylate and its 5-methyl derivative

An investigation of temperature effects upon first-order rate constants in the micellar pseudophase for decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) and its 5-methyl derivative (6-NBIC-5-Me) was carried out. Surfactants used were cationic cetyltrialkylammonium bromide with alkyl = methyl (CTABr), ethyl (CTEABr), n-propyl (CTPABr), and n-butyl (CTBABr). The investigation shows that micelles speed up reactions by decreasing enthalpies of activation. Increase in head group bulk further speeds reactions still by a small decrease in the enthalpies, for both substrates. Values of DeltaH# and DeltaS# for 6-NBIC in the various surfactants give linear isokinetic plot, with CTBABr as outlier.     

Premicellar accelerated decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion and its 5-tetradecyloxy derivative

Didodecyldialkylammonium chloride and bromide (alkyl = Me, Et, n-Pr, n-Bu) accelerate the spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion, 1,H, and its 5-tetradecyloxy derivative, 1,OTD. With most of these surfactants, first-order rate constants, kobs, go through maxima in very dilute surfactant and then decrease and go through minima as association colloids form. These phenomena are not explicable in terms of substrate-induced micellization. However, kobs increases in the N-alkyl sequence Me < Et < n-Pr < n-Bu, as is typical of decarboxylations in association colloids of single-chain surfactants. Reaction in premicelles is accelerated by an initial increase in 1,H. The factors that control relative rates of spontaneous reactions in premicelles and in the association colloids, in particular, depletion of water at the reaction center and association of substrate and quaternary ammonium centers, are discussed with respect to the roles of substrate and surfactant hydrophobicities.     

Kinetic and spectroscopic investigations of aqueous micelles of cationic surfactants

The aqueous micellar solutions of monocationic surfactants N-hexadecyl-N,N,N-trimethylammonium bromide (CTABr), N-hexadecyl-N,N,N-trimethylammonium nitrate (CTANO₃), N,N,N-tributyl-N-hexadecylammonium bromide (CTBABr) and gemini surfactants 1,4-bis(N-hexadecyl-N,N-dimethylammonium)ethane dibromide (C-E-C2Br), 1,4-bis(N-hexadecyl-N,N-dimethylammonium)propane dibromide (C-P-C2Br), and 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (C-B-C2Br) were studied with a solvatochromic probe, 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, better known as Reichard’s ET-30 dye. The local polarity at the probe site (E_T) was calculated from the wavelength maximum of the lowest-energy intramolecular charge-transfer ϖ-ϖ* absorption band of ET-30. The results were compared with a kinetic investigation of the cyclization of 2-(3-bromopropyloxy)phenoxide (PhBr7) in micelles; this reaction is a model for S_N2 reactions and it depends on medium polarity.     

Local mobility and structure of cationic amphiphilic diblock copolymer micelles in aqueous solutions

The local mobility and organization of micelles formed by the cationic diblock copolymer PS-poly(N-ethyl-4-vinylpyridinium bromide) in dilute aqueous solutions is studied by spin-probe ESR spectroscopy. Micelles composed of a hydrophobic PS core and a lyophilizing polyelectrolyte corona are prepared by two methods: dialysis from a nonselective solvent and direct dispersion of the diblock copolymer in water under long-term heating. Velocity-sedimentation studies and static and dynamic light-scattering measurements show that the micelles obtained by dialysis have smaller mean hydrodynamic sizes and weight-average molecular masses and are less polydisperse than micelles prepared by direct dispersion. The ESR spectra of spin probes localized in micelles of both types are found to be identical. This finding suggests that their local structure is independent of the dispersion procedure and molecular-mass characteristics. Probes are localized in the outer layer of the PS core near the core/shell boundary, and their local mobility is a factor of ∼2 higher than the local mobility of probes in the phase of the solid PS. It is inferred that the structure of the outer layer of the PS core in micelles is looser than the structure of PS in the solid phase. The localization sites of spin probes are partially penetrated by water.     

Wormlike micelles formed by mixed cationic and anionic gemini surfactants in aqueous solution

The formation and the properties of wormlike micelles in aqueous solutions of mixed cationic and anionic gemini surfactants, 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3(OH)-12) and O,O'-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C(12)?C(12)), have been studied by steady-state and dynamic rheological measurements at 25°C. With the addition of a small amount of C(12)?C(12) into the solution of 12-3(OH)-12, the total surfactant concentration of which was always kept at 80 mmol L(-1), the solution viscosity was strongly enhanced and its maximum was much larger than that of the mixed system of propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3-12) and C(12)?C(12). The results of dynamic rheology measurements showed that 12-3(OH)-12/C(12)?C(12) formed longer wormlike micelles in comparison with 12-3-12/C(12)?C(12). This was attributed to the effect of hydrogen bonding occurring between 12-3(OH)-12 molecules, which was an effective driving force promoting micellar growth. As few C(12)?C(12) participated in the micelles, the electrostatic attraction between the oppositely charged head groups of 12-3(OH)-12 and C(12)?C(12) made the molecules in the aggregates pack more tightly. This reinforced the hydrogen-bonding interactions and greatly promoted the micellar growth.     

6-氯哒嗪-3-甲酸甲酯的合成

以乙酰丙酸乙酯为起始原料,经过环合、溴代、消除、重铬酸钾氧化、酯化、POCl3氯代6步反应合成6-氯哒嗪-3-甲酸甲酯,目标化合物总收率42%,本方法操作简单,成本低廉,分离纯化容易,收率高,为6-氯哒嗪-3-甲酸甲酯的规模化合成奠定了基础.     

Soluble complexes of sodium poly(isoprene-b-methacrylate) micelles with cationic surfactants in aqueous media

Water-soluble complexes between sodium poly(isoprene-b-methacrylate) (NaIMA) amphiphilic block copolymer micelles and two cationic surfactants with different hydrophobic tail lengths, namely, dodecyltrimethylammonium bromide (DTMAB) and octyltrimethylammonium bromide (OTMAB), were prepared by mixing individual aqueous solutions of block copolymers and surfactants. The complexes were characterized in terms of size, overall charge, and micropolarity by dynamic light scattering, zeta-potential measurements, and fluorescence spectroscopy. Properties of the systems were investigated as a function of surfactant concentration and surfactant type and state in the initial solutions, as well as temperature. Experiments reveal surfactant complexation at the coronal sodium poly(methacrylate) (NaMA) chains, followed by an increase in mass and a decrease in size of the micelles. Complexation of individual surfactant micelles was observed when the DTMAB concentration in the starting solutions was higher than the surfactant cmc. The complexes show a temperature dependence of their dimension due to the hydrophobic effect.     

Ultrafast dynamics of water in cationic micelles

The effect of confinement on the dynamical properties of liquid water is investigated for water enclosed in cationic reverse micelles. The authors performed mid-infrared ultrafast pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted HDO in D(2)O in cetyltrimethylammonium bromide (CTAB) reverse micelles of various sizes. The authors observe that the surfactant counterions are inhomogeneously distributed throughout the reverse micelle, and that regions of extreme salinity occur near the interfacial Stern layer. The authors find that the water molecules in the core of the micelles show similar orientational dynamics as bulk water, and that water molecules in the counterion-rich interfacial region are much less mobile. An explicit comparison is made with the dynamics of water confined in anionic sodium bis(2-ethythexyl) sulfosuccinate (AOT) reverse micelles. The authors find that interfacial water in cationic CTAB reverse micelles has a higher orientational mobility than water in anionic AOT reverse micelles.     

Rheology of wormlike micelles in aqueous systems of a mixed amino acid-based anionic surfactant and cationic surfactant

Amino acid-based anionic surfactant, N-dodecanoylglutamic acid, after neutralizing by 2, 2′, 2″-nitrilotriethanol forms micellar solution at 25 °C. Addition of cationic cosurfactants hexadecyltrimethylammonium chloride (CTAC), hexadecylpyridinium chloride (CPC), and hexadecylpyridinium bromide (CPB) to the semi-dilute solution of anionic surfactant micellar solutions favor the micellar growth and after a certain concentration, entangled rigid network of wormlike micelles are formed. Viscosity increases enormously ～4th order of magnitude compared with water. With further addition of the cosurfactants, viscosity declines and phase separation to liquid crystal occurs. The wormlike micelles showed a viscoelastic behavior and described by Maxwell model with a single stress-relaxation mode. The position of viscosity maximum in the zero-shear viscosity curve shifts towards lower concentration upon changing cosurfactant from CPB to CTAC via CPC; however, the maximum viscosity is highest in the CPB system showing the formation of highly rigid network structure of wormlike micelles. In all the systems, viscosity decays exponentially with temperature following Arrhenius type behavior.     

Comparison of the adsorption of cationic diblock copolymer micelles from aqueous solution onto mica and silica

The similarities and differences in the adsorption behavior of diblock poly(2-(dimethylamino)ethyl methacrylate)-b-poly(2-(diethylamino)ethyl methacrylate) (XqPDMA-PDEA, where X refers to a mean degree of quaternization of the PDMA of either 0, 10, 50, or 100 mol%) copolymers at the mica/ and silica/aqueous solution interfaces have been investigated. These diblock copolymers form core-shell micelles with the PDEA chains located in the cores and the more hydrophilic PDMA chains forming the cationic micelle coronas at pH 9. These micelles adsorb strongly onto both mica and silica due to electrostatic interactions. In situ atomic force microscopy (AFM) has demonstrated that the mean spacing and the dimension of the adsorbed micelles depend on both the substrate and the mean degree of quaternization of the PDMA blocks. In particular, the morphology of the adsorbed nonquaternized 0qPDMA-PDEA copolymer micelles is clearly influenced by the substrate type: these micelles form a disordered layer on silica, while much more close-packed, highly ordered layers are obtained on mica. The key reasons for this difference are suggested to be the ease of lateral rearrangement for the copolymer micelles attached to the solid substrates and the relative rates of relaxation of the coronal PDMA chains.     

Adsorption and kinetic studies of cationic and anionic dyes on pyrophyllite from aqueous solutions

The adsorption of cationic Methylene Blue (MB) and anionic Procion Crimson H-EXL (PC) dyes from aqueous medium on pyrophyllite was studied. Changes in the electrokinetics of pyrophyllite as a function of pH were investigated in the absence and presence of multivalent cations. The results show that pyrophyllite in water exhibits a negative surface charge within the range pH 2-12. Pyrophyllite is found to be a novel adsorbent for versatile removal of cationic and anionic dyes. The negative hydrophilic surface sites of pyrophyllite are responsible for the adsorption of cationic MB molecules. The adsorption of anionic PC dye is possible after a charge reversal by the addition of trivalent cation of Al. Nearly 2 min of contact time are found to be sufficient for the adsorption of both dyes to reach equilibrium. The experimental data follow a Langmuir isotherm with adsorption capacities of 70.42 and 71.43 mg dye per gram of pyrophyllite for MB and PC, respectively. For the adsorption of both MB and PC dyes, the pseudo-second-order chemical reaction kinetics provides the best correlation of the experimental data.     

Light-induced viscosity changes of aqueous solutions containing 9-substituted anthracenes solubilized in cetyltrimethylammonium micelles

Micellar solutions of various 9-substituted anthracenes show rheopectic, thixotropic and viscoelastic behaviour depending on composition. Large viscosity changes are induced by the small changes in solubilizate concentration caused by photodimerization of the anthracenes at wavelengths exceeding 300 nm. Photocleavage of the dimers at lower wavelengths allows viscosity modulation by suitable choice of irradiation conditions.     

Polyamides: Decarboxylation and desamination in nylon 6 equilibrium polymer

Rate and extent of both decarboxylation and desamination in the temperature range of 250–290°C were studied on equilibrium polymers obtained by the hydrolytic polymerization of ε-caprolactam. Mechanisms have been proposed that are characterized by the participation of the equilibrium monomer in the considered decomposition reactions. The mechanisms suggested accommodate both the experimental results of this study and findings of previous investigations as reported in the literature.     

A new semi-empirical kinetic method for the determination of ion exchange constants for the counterions of cationic micelles

A new method, based upon semi-empirical kinetic approach, for the determination of ion exchange constant for ion exchange processes occurring between counterions at the cationic micellar surface is described in this review article. Basically, the method involves a reaction kinetic probe which gives observed pseudo-first-order rate constants (k_obs) for a nucleophilic substitution reaction between the nonionic and anionic reactants (R and S) in the presence of a constant concentration of both reactants as well as cationic micelles and varying concentrations of an inert inorganic or organic salt (MX). The observed data (k_obs, versus [MX]) fit satisfactorily (in terms of residual errors) to an empirical equation which could be derived from an equation explaining the mechanism of the reaction of the kinetic probe in terms of pseudophase micellar (PM) model coupled with another empirical equation. This (another) empirical equation explains the effect of [MX] on cationic micellar binding constant (K_S) of the anionic reactant (say S) and gives an empirical constant, K_X/S. The magnitude of K_X/S is the measure of the ability of X⁻ to expel S⁻ from a cationic micellar pseudophase to the bulk aqueous phase through ion exchange X⁻/S⁻. The values of K_X/S and K_Y/S (where Y⁻ is another inert counterion) give the ion exchange constant, K_X^Y (= K_X/K_Y where K_X and K_Y represent cationic micellar binding constants of X⁻ and Y⁻, respectively). The suitability of this method is demonstrated by the use of three different reaction kinetic probes and various MX.     

Encapsulation of curcumin in cationic micelles suppresses alkaline hydrolysis

The alkaline hydrolysis of curcumin was studied in three types of micelles composed of the cationic surfactants cetyl trimethylammonium bromide (CTAB) and dodecyl trimethylammonium bromide (DTAB) and the anionic surfactant sodium dodecyl sulfate (SDS). At pH 13, curcumin undergoes rapid degradation by alkaline hydrolysis in the SDS micellar solution. In contrast, alkaline hydrolysis of curcumin is greatly suppressed in the presence of either CTAB or DTAB micelles, with a yield of suppression close to 90%. The results from fluorescence spectroscopic studies reveal that while curcumin remains encapsulated in CTAB and DTAB micelles at pH 13, curcumin is dissociated from the SDS micelles to the aqueous phase at this pH. The absence of encapsulation and stabilization in the SDS micellar solution results in rapid hydrolysis of curcumin.