Kinetic coupled with UV spectral evidence for near-irreversible nonionic micellar binding of N-benzylphthalimide under the typical reaction conditions: an observation against a major assumption of the pseudophase micellar model

Pseudo-first-order rate constants (k(obs)) for alkaline hydrolysis of N-benzylphthalimide (1) show a nonlinear decrease with the increase in [C(m)E(n)]T (total concentration of Brij 58, m = 16, n = 20 and Brij 56, m = 16, n = 10) at constant [CH(3)CN] and [NaOH]. These nonionic micellar effects, within the certain typical reaction conditions, have been explained in terms of the pseudophase micellar (PM) model. The values of micellar binding constants (KS) of 1 are 1.04 x 10(3) M(-1) (at 1.0 x 10(-3) M NaOH) and 1.08 x 10(3) M(-1) (at 2.0 x 10(-3) M NaOH) for C(16)E(20) as well as 600 M(-1) (at 7.6 x 10(-4) M NaOH) and 670 M(-1) (at 1.0 x 10(-3) M NaOH) for C(16)E(10) micelles. The pseudo-first-order rate constants (kM) for hydrolysis of 1 in C(16)E(20) micellar pseudophase are approximately 90-fold smaller than those (kW) in water phase. The values of kM for hydrolysis of 1 in C(16)E(10) micelles are almost zero. Kinetic coupled with UV spectral data reveals significant irreversible nonionic micellar binding of 1 molecules in the micellar environment of nearly zero hydroxide ion concentration at >or=0.14 M C(16)E(20) and 1.0 x 10(-3) M NaOH while such observations could not be detected at or=3 x 10(-3) M C(16)E(10) and 7.6 x 10(-4) M NaOH, while the rate of hydrolysis of 1 is completely ceased at >or=0.05 M C(16)E(10) and 7.6 x 10(-4) M NaOH. The rate of hydrolysis of 1 at 5.0 x 10(-2) and 8.8 x 10(-2) M C(16)E(10) and 1.0 x 10(-3) M NaOH reveals the formation of presumably phthalic anhydride, whereas such observation was not observed in the C(16)E(20) micellar system under similar experimental conditions.     

Kinetic and rheological measurements of the effects of inert 2-, 3- and 4-bromobenzoate ions on the cationic micellar-mediated rate of piperidinolysis of ionized phenyl salicylate

The effects of the concentration of inert organic salts, [MX], (MX=2-, 3- and 4-BrBzNa with BrBzNa=BrC(6)H(4)CO(2)Na) on the rate of piperidinolysis of ionized phenyl salicylate (PS(-)) have been rationalized in terms of pseudophase micellar (PM) coupled with an empirical equation. The appearance of induction concentration in the plots of k(obs) versus [MX] (where k(obs) is pseudo-first-order rate constants for the reaction of piperidine (Pip) with PS(-)) is attributed to the occurrence of two or more than two independent ion exchange processes between different counterions at the cationic micellar surface. The derived kinetic equation, in terms of PM model coupled with an empirical equation, gives empirical parameters F(X/S) and K(X/S) whose magnitudes lead to the calculation of usual ion exchange constant K(X)(Br) (=K(X)/K(Br) with K(X) and K(Br) representing cationic micellar binding constants of counterions X(-) and Br(-), respectively). The value of F(X/S) measures the fraction of S(-) (=PS(-)) ions transferred from the cationic micellar pseudophase to the aqueous phase by the optimum value of [MX] due to ion exchange X(-)/S(-). Similarly, the value of K(X/S) measures the ability of X(-) ions to expel S(-) ions from cationic micellar pseudophase to aqueous phase through ion exchange X(-)/S(-). This rather new technique gives the respective values of K(X)(Br) as 8.8±0.3, 71±6 and 62±5 for X(-)=2-, 3- and 4-BrBz(-). Rheological measurements reveal the shear thinning behavior of all the surfactant solutions at 15mM CTABr (cetyltrimethylammonium bromide) indicating indirectly the presence of rodlike micelles. The plots of shear viscosity (η) at a constant shear rate (γ), i.e. η(γ), versus [MX] at 15 mM CTABr exhibit maxima for MX=3-BrBzNa and 4-BrBzNa while for MX=2-BrBzNa, the viscosity maximum appears to be missing. Such viscosity maxima are generally formed in surfactant solutions containing long stiff and flexible rodlike micelles with entangled and branched/multiconnected networks. Thus, 15 mM CTABr solutions at different [MX] contain long stiff and flexible rodlike micelles for MX=3- and 4-BrBzNa and short rodlike micelles for MX=2-BrBzNa.     

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in aqueous cationic micelles: kinetic evidence of microinterface property changes

We studied decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, 1, as a kinetic probe to investigate microinterface properties of aqueous micelles formed by cationic surfactants of increasing head group bulk, i.e., cetyltrialkylammonium bromide, with alkyl=Me (CTABr), Et (CTEABr), n-Pr (CTPABr), n-Bu (CTBABr) and p-octyloxybenzyltrialkylammonium bromide surfactants with alkyl=Me (pOOTABr), n-Pr (pOOTPABr), and n-Bu (pOOTBABr), and the longer p-dodecyloxybenzyltrimethylammonium bromide (pDoTABr) at concentrations higher than 0.05 M. The pseudophase kinetic treatment fails to fit the data that show anomalies with abrupt increases in k(obs) for CTPABr and CTBABr (but not for CTEABr) and with smooth and continuos increase of k(obs) for all p-alkyloxybenzyltrilakylammonium bromides. Abrupt and successive modifications of the micellar interface properties, undergoing only when the polar head or the alkyl chain have some covalent structure, account for the observed kinetic behavior.     

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.     

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.     

Indomethacin solubilization induced shape transition in CnE7 (n=14,16) nonionic micelles

Shape transitions were examined with regard to the solubilization of the poorly water-soluble drug indomethacin (IMC) in the nonionic surfactants heptaethylene oxide tetradecyl (C14E7) and hexadecyl (C16E7) ethers by means of a dynamic light scattering technique. The cloud points of the pure C14E7 and C16E7 micelles ranged from 58 to 62 degrees C and from 52.1 to 55.6 degrees C, respectively, at surfactant concentrations of 1 to 10 mM. The cloud points of IMC-solubilized micelles increased by approximately 1 to 5 degrees . The sizes of the pure C14E7 micelles were 4 to 14 nm at 20 to 40 degrees C at a concentration of 2 to 20 mM. The apparent hydrodynamic radius (R happ) of pure C16E7 micelles varied with temperature and concentration. C16E7 surfactant formed small spherical micelles at 20 and 25 degrees C at concentrations below 5 mM; the size of the micelles was approximately 5 nm. On the other hand, from 30 to 40 degrees C and at a higher concentration, C16E7 formed elongated cylindrical micelles, and these elongated micelles entangled or overlapped each other. The R happ of the IMC-solubilized C14E7 micelles at 20 to 40 degrees C and of C16E7 micelles at 20 degrees C increased compared to that of pure micelles. On the other hand, the cylindrical micelles of C16E7 decreased in size and turned into spherical ones because of the hydrophobicity between the micelles caused by solubilization of IMC. This phenomenon was confirmed by transmission electron microscope (TEM) images.     

Effects of

Effects of cetyltrimethylammonium bromide (CTABr) micelles on second-order rate constants (k(n)(obs)) for nucleophilic reactions of amines (piperidine and n-butylamine) with ionized phenyl salicylate (PS(-)) reveal a nonlinear decrease with the increase in [D(n)] (where [D(n)] = [CTABr](T) - cmc) at a constant [NaBr] and 35 degrees C. The observed data, at a constant [NaBr], fit reasonably well to a pseudophase model of micelles, and such a data fit gives kinetic parameters such as CTABr micellar binding canstant (K(S)) of PS(-). The effect of [NaBr] upon K(S) is explained with the empirical relationship K(S) = K(S)(0)/(1 + psi[NaBr]), where psi is an empirical parameter.     

Reverse micellar aggregates: effect on ketone reduction. 1. Substrate role

The reduction of three aromatic ketones, acetophenone (AF), 4-methoxyacetophenone (MAF), and 3-chloroacetophenone (CAF), by NaBH(4) was followed by UV-vis spectroscopy in reverse micellar systems of water/AOT/isooctane at 25.0 degrees C (AOT is sodium 1,4-bis-2-ethylhexylsulfosuccinate). The first-order rate constants, k(obs), increase with the concentration of surfactant due to the substrate incorporation at the reverse micelle interface, where the reaction occurs. For all the ketones the reactivity is lower at the micellar interface than in water, probably reflecting the low affinity of the anionic interface for BH(4)(-). Kinetic profiles upon water addition show maxima in k(obs) at W(0) approximately 5 probably reflecting a strong interaction between water and the ionic headgroup of AOT; at W(0) < 5 by increasing W(0) BH(4)(-) is repelled from the anionic interface once the water pool forms. The order of reactivity was CAF > AF > MAF. Application of a kinetic model based on the pseudophase formalism, which considers distribution of the ketones between the continuous medium and the interface, and assumes that reaction take place only at the interface, gives values of the rate constants at the interface of the reverse micellar system. At W(0) = 5, we conclude that NaBH(4) is wholly at the interface, and at W(0) = 10 and 15, where there are free water molecules, the partitioning between the interface and the water pool has to be considered. The results were used to estimate the ketone and borohydride distribution constants between the different pseudophases as well as the second-order reaction rate constant at the micellar interface.     

Persistence length of wormlike micelles composed of ionic surfactants: self-consistent-field predictions

The persistence length of a wormlike micelle composed of ionic surfactants C(n)E(m)X(k) in an aqueous solvent is predicted by means of the self-consistent-field theory where C(n)E(m) is the conventional nonionic surfactant and X(k) is an additional sequence of k weakly charged (pH-dependent) segments. By considering a toroidal micelle at infinitesimal curvature, we evaluate the bending modulus of the wormlike micelle that corresponds to the total persistence length, consisting of an elastic/intrinsic and an electrostatic contribution. The total persistence length increases with pH and decreases with increasing background salt concentration. We estimate that the electrostatic persistence length l(p,e)(0) scales with respect to the Debye length kappa(-1) as l(p,e)(0) approximately kappa(-p) where p approximately 1.98 for wormlike micelles consisting of C(20)E(10)X(1) surfactants and p approximately 1.54 for wormlike micelles consisting of C(20)E(10)X(2) surfactants. The total persistence length l(p,t)(0) is a weak function of the head group length m but scales with the tail length n as l(p,t)(0) approximately n(x) where x approximately 2-2.6, depending on the corresponding head group length. Interestingly, l(p,t)(0) varies nonmonotonically with the number of charged groups k due to the opposing trends in the electrostatic and elastic bending rigidities upon variation of k.     

Effects of nonionic micelles on dephosphorylation and aromatic nucleophilic substitution

Terminal hydroxyl groups of micellized dodecyl (10) and (23) polyoxyethylene glycol (C12E10 and C12E23, respectively) are deprotonated by OH-, and the alkoxide ions react with 2,4-dinitrochlorobenzene (DNCB) giving ethers, but do not react with p-nitrophenyl dimethyl and diethyl phosphate (pNPDMP and pNPDEP, respectively), which react wholly with OH-. These substrates have similar hydrophobicities and should locate similarly in the micellar interfacial regions. These differences in reactivities are due to differences in relative nucleophilicities of OH- and alkoxide ions in aromatic nucleophilic substitution and dephosphorylation in micelles. Reactivities of DNCB and the related ethers indicate that local concentrations of OH- are similar in the aqueous and micellar interfacial regions but those of the alkoxide ions are higher in C12E10 than in C12E23 micelles due to differences in the molar volumes of the interfacial regions. With time the initially formed ethers react at similar rates with OH- generating 2,4-dinitrophenoxide ion.     

Platinum chalcogenido MCM-41 analogues. High hexagonal order in mesostructured semiconductors based on Pt(2+) and [Ge(4)Q(10)](4-) (Q = S, Se) and [Sn(4)Se(10)](4-) adamantane clusters

Highly periodic hexagonal honeycombs of platinum-germanium chalcogenide and platinum-tin selenide frameworks were prepared by linking corresponding [Ge(4)Q(10)](4)(-) (Q = S, Se) and [Sn(4)Se(10)](4)(-) clusters with Pt(2+) ions. The non-oxidic honeycombs designated as C(n)PyPtGeQ and C(n)PyPtSnSe were templated by the lyotropic liquid-crystalline phase of alkylpyridinium surfactant [C(n)H(2)(n)(+1)NC(5)H(5)]Br (C(n)PyBr) with n= 12, 14, 16, 18, 20, and 22. Although the materials are amorphous at the microscale, they have crystalline mesoporosity with well-ordered and aligned surfactant-filled cylindrical pores. In addition to high mesoscopic order, the pore-pore separation is adjustable with the surfactant chain length (i.e., value of n). The quality of these materials, as judged by the degree of hexagonal order, rivals or exceeds that reported for the highest quality MCM-41 silicates. The materials have the lowest band gap reported so far for mesostructured chalcogenides solids, in the range 1.5 < E(g)< 2.3 eV. The C(n)PyPtGeS analogues show intense photoluminescence at 77 K when excited with light above the band gap.     

Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO(4)). The observed second-order rate constant (k(obs)) of sulfoxidation of thioaniosoles by [(N4Py)Fe(IV)(O)](2+) increases linearly with increasing concentration of HClO(4) (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+), the observed second-order rate constant (k(et)) of electron transfer from one-electron reductants such as [Fe(II)(Me(2)bpy)(3)](2+) (Me(2)bpy = 4,4-dimehtyl-2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+) increases with increasing concentration of HClO(4), exhibiting second-order dependence on HClO(4) concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe(II)(Me(2)bpy)(3)](2+) to [(N4Py)Fe(IV)(O)](2+) to yield [Fe(III)(Me(2)bpy)(3)](3+) and [(N4Py)Fe(III)(OH(2))](3+). The one-electron reduction potential (E(red)) of [(N4Py)Fe(IV)(O)](2+) in the presence of 10 mM HClO(4) (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E(red) vs log[HClO(4)] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe(IV)(O)](2+). The PCET driving force dependence of log k(et) is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k(obs) values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+) with the k(et) values of PCET from one-electron reductants to [(N4Py)Fe(IV)(O)](2+) at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe(IV)(O)](2+) to thioanisoles rather than outer-sphere PCET.     

Does the solid-liquid crystal phase transition provoke the spin-state change in spin-crossover metallomesogens?

Three types of interplay/synergy between spin-crossover (SCO) and liquid crystalline (LC) phase transitions can be predicted: (i) systems with coupled phase transitions, where the structural changes associated to the Cr<-->LC phase transition drives the spin-state transition, (ii) systems where both transitions coexist in the same temperature region but are not coupled, and (iii) systems with uncoupled phase transitions. Here we present a new family of Fe(II) metallomesogens based on the ligand tris[3-aza-4-((5-C(n))(6-R)(2-pyridyl))but-3-enyl]amine, with C(n) = hexyloxy, dodecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, R = hydrogen or methyl (C(n)-trenH or C(n)-trenMe), which affords examples of systems of types i, ii, and iii. Self-assembly of the ligands C(n)-trenH and C(n)-trenMe with Fe(A)2 x xH2O salts have afforded a family of complexes with general formula [Fe(C(n)-trenR)](A)2 x sH2O (s > or = 0), with A = ClO4(-), F-, Cl-, Br- and I-. Single-crystal X-ray diffraction measurements have been performed on two derivatives of this family, named as [Fe(C6-trenH)](ClO4)2 (C(6)-1) and [Fe(C6-trenMe)](ClO4)2 (C(6)-2), at 150 K for C(6)-1 and at 90 and 298 K for C(6)-2. At 150 K, C(6)-1 displays the triclinic space group P, whereas at 90 and at 298 K C(6)-2 adopts the monoclinic P2(1)/c space group. In both compounds the iron atoms adopt a pseudo-octahedral symmetry and are surrounded by six nitrogen atoms belonging to imino groups and pyridines of the ligands C(n)-trenH and C(n)-trenMe. The average Fe(II)-N bonds (1.963(2) A) at 150 K denote that C(6)-1 is in the low-spin (LS) state. For C(6)-2 the average Fe(II)-N bonds (2.007(1) A) at 90 K are characteristic of the LS state, while at 298 K they are typical for the high-spin (HS) state (2.234(3) A). Compound C(6)-1 and [Fe(C18-trenH)](ClO4)2 (C(18)-1) adopts the LS state in the temperature region between 10 and 400 K, while compound C(6)-2 and [Fe(Cn-trenMe)](ClO4)2 (n = 12 (C(12)-2), 18 (C(18)-2)) exhibit spin crossover behavior at T(1/2) centered around 140 K. The thermal spin transition is accompanied by a pronounced change of color from dark red (LS) to orange (HS). The light-induced excited spin state trapping (LIESST) effect has been investigated in compounds C(6)-2, C(12)-2 and C(18)-2. The T(1/2)LIESST is 56 K (C(6)-2), 48 K (C(16)-2), and 56 K (C(18)-2). On the basis of differential scanning calorimetry, optical polarizing microscopy, and X-ray diffraction findings for C(18)-1, C(12)-2, and C(18)-2 at high temperature a smectic mesophase SX has been identified with layered structures similar to C(6)-1 and C(6)-2. The compounds [Fe(C(n)-trenH)](Cl)2 x sH2O (n = 16 (C(16)-3, s = 3.5, C(16)-4, s = 0.5, C(16)-5, s = 0), 18 (C(18)-3, s = 3.5, C(18)-4, s = 0.5, C(18)-5, s = 0), 20 (C(20)-3, s = 3.5, C(20)-4, s = 0.5, C(20)-5, s = 0)) and [Fe(C18-tren)](F)2 x sH2O (C(18)-6, s = 3.5, C(18)-7, s = 0) show a very particular spin-state change, while [Fe(C18-tren)](Br)2 x 3H2O (C(18)-8) together with [Fe(C18-tren)](I)2 (C(18)-9) are in the LS state (10-400 K) and present mesomorphic behavior like that observed for the complexes C(18)-1, C(12)-2, and C(18)-2. In compounds C(n)-3 50% of the Fe(II) ions undergo spin-state change at T(1/2) = 375 K induced by releasing water, and in partially dehydrated compounds (s = 0.5) the Cr-->SA phase transition occurs at 287 K (C(16)-4), 301 K (C(18)-4) and 330 K (C(20)-4). For the fully dehydrated materials C(n)-5 50% of the Fe(II) ions are in the HS state and show paramagnetic behavior between 10 and 400 K. In the partially dehydrated C(n)-4 the spin transition is induced by the change of the aggregate state of matter (solid<-->liquid crystal). For compound C(18)-6 the full dehydration to C(18)-7 provokes the spin-state change of nearly 50% of the Fe(II) ions. The compounds C(n)-3 and C(18)-6 are dark purple in the LS state and become light purple-brown when 50% of the Fe(II) atoms are in the HS state.     

Mixed micelle formation among anionic gemini surfactant (212) and its monomer (SDMA) with conventional surfactants (C12E5 and C12E8) in brine solution at pH 11

The micellization of anionic gemini surfactant, N,N'-ethylene(bis(sodium N-dodecanoyl-beta-alaninate)) (212), and its monomer, N-dodecanoyl-N-methyl alaninate (SDMA), and polyethoxylated nonionic surfactants, C(12)E(5) and C(12)E(8), has been studied tensiometrically in pure and mixed states in an aqueous solution of 0.1 M NaCl at pH 11 to determine physicochemical properties such as critical micellar concentration (cmc), surface tension at the cmc (gamma(cmc)), maximum surface excess (Gamma(max)) and minimum area per surfactant molecule at the air/water interface (A(min)). The theories of Rosen, Rubingh, Motomura, Maeda, and Nagarajan have been applied to investigate the interaction between those surfactants at the interface and in the micellar solution, the composition of the aggregates formed, the theoretical cmc in pure and mixed states, and the structural parameters as proposed by Tanford and Israelachvili. Various thermodynamic parameters (free energy of micellization and interfacial adsorption) have been calculated with the help of regular solution theory and the pseudophase model for micellization.     

胶束电动毛细管色谱分析间接紫外吸收检测人体血小板活化因子

研究了人体血小板活化因子中1-O-十六烷基乙酰甘油磷酰胆碱与1-O-八烷基乙酰甘油磷酰胆碱的胶束电动毛细管色谱分离条件与间接紫外吸收检测条件。首次采用高效毛细管电泳法成功地分析了人体血小板活化因子中两种主要磷酰胆碱的含量。     

Billion-fold acceleration of the methanolysis of paraoxon promoted by La(OTf)3 in methanol

The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = (-)OS(O)(2)CF(3)) as a function of at 25 degrees C. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La(3+)-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La(3+)(2)((-)OCH3)n, where n assumes values of 1-5, gives the equilibrium distribution of each as a function of. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2).k(2)(2:)n for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La(3+)(2)((-)OCH3)1, La(3+)(2)((-)OCH3)2, and La(3+)(2)((-)OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La(3+)](total), at neutral, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximately 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La(3+)(2)((-)OCH3)2 species is proposed.     

Changes in the relative contribution of specific and general base catalysis in cationic micelles. The cyclization of substituted ethyl hydantoates

The rate-surfactant profiles for the HO(-)- and AcO(-)-catalyzed ring closure of two ethyl hydantoates, E2 and E3, to hydantoins with three cetyltrimethylammonium salts (CTAX, X = Br(-), Cl(-), or AcO(-)) are measured in 0.02 and 0.2 M acetate buffers 50% base with starting pH 4.65. Marked accelerations associated with large pH increases are found in 0.02 M buffered CTAOAc. Smaller accelerations and smaller pH changes are observed in 0.2 M buffered CTAOAc and CTACl. From these profiles, the micellar rate constants for the specific base- and general base-catalyzed reactions, and, respectively, of E2 and E3 are obtained separately. The resulting values of k(2,m)/k(w), E2/E3 rate constant ratios, and kinetic solvent isotope effects, KSIEs, are consistent with a strong predominance of the HO(-) reaction in the dilute buffer, while in the more concentrated buffer, specific and general catalysis compete for the two substrates. This result is in sharp contrast with that observed in water in which the reaction of E2 is almost exclusively specifically catalyzed. The increase in the general base-catalyzed pathway for E2 is attributed not to an increase in the rate constant for this pathway in micelles but to a smaller decrease than that for the specific catalysis (k(2,m)/k(w) = 0.2 and 0.4 for the specific and general catalysis, respectively). The different responses of the rate constants to the micellar media are interpreted as a larger effect of the interfacial polarity on the specific than on the general catalysis. The apparent contradiction between the rate constant decreases and the marked accelerations in micellar media is discussed in terms of pH changes, i.e., [HO(-)] changes, and of acetate inclusion via ion exchanges at micellar interfaces.     

Influence of the addition of alcohol on the reaction methyl-4-nitrobenzenesulfonate + Br(-) in tetradecyltrimethylammonium bromide aqueous micellar solutions

The reaction methyl-4-nitrobenzenesulfonate + Br(-) was studied in tetradecyltrimethylammonium bromide (TTAB) aqueous micellar solutions in the absence and in the presence of various amounts of n-hexanol, n-pentanol, and n-butanol. Kinetic micellar effects provoked by the addition of the linear alcohols can be rationalized by using simple pseudophase kinetic models. The equilibrium binding constants of the methyl-4-nitrobenzenesulfonate molecules to the cationic micelles decreases when [alcohol] increases. The (k(2)(m)/V(m)) values found are practically the same for the different TTAB-alcohol micellar solutions studied, independent of the nature and concentration of the alcohol present in the reaction medium. This has been explained by considering the balance of two factors operating on reactivity in opposite ways: (1). an increase in the volume of the micellar interfacial region upon increasing alcohol concentration, and (2). a decrease in the polarity of the interfacial region as the amount of alcohol present in the micellar solutions increases.     

Polyoxyethylene alkyl ether nonionic surfactants: physicochemical properties and use for cholesterol determination in food

Polyoxyethylene alkyl ethers, C(n)E(m), are nonionic surfactants made of an alkyl chain with n methylene groups and a hydrophilic part with m oxyethylene units. C(n)E(m) nonionic surfactants are very useful in chemical analysis. The commercially available products are often a mixture of several C(n)E(m) molecules with different m values. Pure C(n)E(m) surfactants are now available. The physicochemical parameters: critical micelle concentration (c.m.c.), molar volume, density, cloud-point temperature and hydrophile-lipophile balance value for pure C(n)E(m) surfactants were collected from the literature. Regression analyses were carried out on the data. They showed that strong correlations existed between the structure of the molecule (n and m values) and its physicochemical properties. General equations linking the c.m.c., molar volume, density and cloud-point temperature of the C(n)E(m) surfactants and their structure (n and m values) are proposed and discussed. The use of these surfactants in chemical analysis is illustrated by the determination of cholesterol in egg yolk. Cholesterol was separated from the bulk yolk by cloud-point extraction using the C(12)E(10) surfactant. It was quantitated using micellar liquid chromatography. The C(12)E(23) surfactant was used to prepare the micellar mobile phase that allowed the separation of cholesterol and the use of an enzymatic detector.     

Kinetics and mechanism of platinum-thallium bond formation: the binuclear [(CN)5Pt-Tl(CN)](-) and the trinuclear [(CN)5Pt-Tl-Pt(CN)5](3-) complex

Formation kinetics of the metal-metal bonded binuclear [(CN)(5)Pt-Tl(CN)](-) (1) and the trinuclear [(CN)(5)Pt-Tl-Pt(CN)(5)](3-) (2) complexes is studied, using the standard mix-and-measure spectrophotometric method. The overall reactions are Pt(CN)(4)(2-) + Tl(CN)(2)(+) <==> 1 and Pt(CN)(4)(2-) + [(CN)(5)Pt-Tl(CN)](-) <==> 2. The corresponding expressions for the pseudo-first-order rate constants are k(obs) = (k(1)[Tl(CN)(2)(+)] + k(-1))[Tl(CN)(2)(+)] (at Tl(CN)(2)(+) excess) and k(obs) = (k(2b)[Pt(CN)(4)(2-)] + k(-2b))[HCN] (at Pt(CN)(4)(2-) excess), and the computed parameters are k(1) = 1.04 +/- 0.02 M(-2) s(-1), k(-1) = k(1)/K(1) = 7 x 10(-5) M(-1) s(-1) and k(2b) = 0.45 +/- 0.04 M(-2) s(-1), K(2b) = 26 +/- 6 M(-1), k(-2b) = k(2b)/K(2b) = 0.017 M(-1) s(-1), respectively. Detailed kinetic models are proposed to rationalize the rate laws. Two important steps need to occur during the complex formation in both cases: (i) metal-metal bond formation and (ii) the coordination of the fifth cyanide to the platinum site in a nucleophilic addition. The main difference in the formation kinetics of the complexes is the nature of the cyanide donor in step ii. In the formation of [(CN)(5)Pt-Tl(CN)](-), Tl(CN)(2)(+) is the source of the cyanide ligand, while HCN is the cyanide donating agent in the formation of the trinuclear species. The combination of the results with previous data predict the following reactivity order for the nucleophilic agents: CN(-) > Tl(CN)(2)(+) > HCN.