Pseudophase Ion Exchange Research Articles

A General Formulation Encompassing the Effects of Salts and Micelles (Direct and Reverse) on Ionic Reactions

Given the common behavior of ionic reactions in micellar and salt solutions and in microemulsions, a general approach has been developed for the interpretation of kinetic results in these media. This approach takes as a starting point the Bronsted equation. It has been checked by employing kinetic results for cation/cation \(([\mathrm{Ru}(\mathrm{NH}_{3})_{5}\mathrm{py}^{2+}] + [\mathrm{Co}(\mathrm{NH}_{3})_{4}\mathrm{pzCO}_{2}^{2+}])\), anion/anion \((\mathrm{I}^{-}+ \mathrm{IrCl}_{6}^{2-})\) and cation/anion \(([\mathrm{Ru}(\mathrm{NH}_{3})_{5}\mathrm{py}^{2+}] + \mathrm{S}_{2}\mathrm{O}_{8}^{2-})\) reactions. The approach can be easily generalized to cases in which more than two pseudophases (or more than one receptor) are present in the reactive system, as well as cases in which the reaction can follow more than two reaction paths. The approach is consistent with (but more general than) the Pseudophase and related models, such as the Pseudophase Ion Exchange Model.

Micellar Effects on Alkaline Hydrolysis of 3,5-Dinitro-2-chloro Benzotriflouride and 2,4-Dinitrochloro Benzene in Aqueous-Acetonitrile Mixtures

Reaction rate for alkaline hydrolysis of the substrates 3,5-dinitro-2-chloro benzotriflouride (DNCBTF) (1) at 30°C and 2,4-dinitrochloro benzene (DNCB) (2) at 50°C separetely with NaOH as nucleophile is carried out spectrophotometrically in mixed aqueous-acetonitrile solvents. In each system, cationic surfactant as dodecyltrimethyl ammonium bromide (DoTAB), or anionic one as sodium dodecyle sulfate (SDS) is used in wide range of concentrations to study the effect of micelle on the reaction rate. The micellar effect is explained in term of modified pseudo phase ion exchange model. Pseudo first order rate constant, kobs is obtained for each of the nucleophile and for both substrates 1 and 2 at all range of XAN · kobs at given [OH−] and in presence of any substrate is found to increase with the increase of DoTAB,while decrease with the increase of SDS as micellar phases. Critical micelle concentrations (CMCs) in similar trend are observed to increase in DoTAB while decrease in SDS systems by increasing acetonitrile (AN) content. Micellar binding constant (KS) between any type of given substrate and the formed micelle, is found to decrease in presence of DoTAB and increase in SDS micellar phases by increasing AN content. Finally, the ratios between pseudo first order rate constants for hydrolysis in micellar phase kM to that in the bulk phase kw for DoTAB and SDS systems are found to be greater than and smaller than unity respectively at any given XAN where the data indicated for (1) is always higher than those for (2). The results concluded that micelle DoTAB is working as a catalyst for the reaction rate, while that for SDS is considered as an inhibitor.

Drug Surfactant and Polymer Mediated Acid Catalyzed Hydrolysis of Acetohydroxamic Acid

The effects of drug-surfactant and polymer on the acid hydrolysis of acetohydroxamic acid (AHA) have been studied at room temperature in 2% (v/v) dioxane medium. Kinetic data corresponding to the reaction with drug micelles are analyzed by using the pseudo-phase ion exchange model. The binding constant and first-order rate constant in micellar media in presence of AMYTP+-Drug, AMYTP+-β-Cyclodextrine, AMYTP+-SDS, AMYTP+-PEG, AMYTP+-TX-100, AMYTP+-CTAB, have been studied in 0.5 M HCl media.

ChemInform Abstract: Effects of Monoalkyl Phosphate Surfactants Upon the Acid Hydrolysis of Dioxolanes.

The pseudophase ion-exchange (PPIE) model was tested in the presence and absence of buffer for the acid-catalyzed hydrolysis of 2-(p-methoxyphenyl)-1,3-dioxolane (p-MPD) and 2-(2,4-dimethoxyphenyl)-2-ethyl-1,3-dioxoloane (2,4-DPED) in the presence of sodium monoalkyl phosphate surfactants. The maximum increase in the observed rate constant was about 10-fold for both substrates

Effects of Cationic Micelles on the Rate of Alkaline Hydrolysis of N-(2-methoxyphenyl)phthalimide

Pseudo-first-order rate constants (kobs), obtained for the reaction of N-(2-methoxyphenyl)phthalimide (1) with HO− at a constant [NaOH] and varying concentrations of cetyltrimethylammonium bromide ([CTABr]T = 0.0–0.20 M), have been analyzed in terms of both pseudophase ion-exchange (PIE) model and Berezin's pseudophase (BPP) model. Although both models give the best observed data fit with least-squares values not significantly different from each other, the calculated values of KS from BPP model appear to be more reliable compared to those from PIE model. The nonlinear decrease of kobs with the increase of [NaBr] at the constant [NaOH] and [CTABr]T is explained in terms of pseudophase micellar model coupled with an empirical equation which could indirectly give the measure of the ion-exchange Br−/HO−.

Kinetics Study of Malachite Green Fading in the Presence of TX-100, DTAB and SDS

The rate constants of alkaline fading of malachite green (<TEX>$MG^+$</TEX>) was measured in the presence of nonionic (TX-100), cationic (DTAB) and anionic (SDS) surfactants. This reaction was studied under pseudo-first-order conditions at 283∼303 K. The rate of fading reaction showed noticeable dependence on the electrical charge of the used surfactants. It was observed that the reaction rate constants were increased in the presence of TX-100 and DTAB and decreased in the presence of SDS. According to Hughs-Ingold rules for nucleophilic substitution reactions, the electric charge of MG/surfactant compound along with decrease in dielectric constant of <TEX>$MG^+$</TEX> micro-environment in this compound varies the rate of fading reaction. Binding constants of surfactant molecules to <TEX>$MG^+$</TEX> were calculated using cooperativity, pseudo-phase ion exchange and classical models and the related thermodynamic parameters were obtained by classical model. The results show that the binding of <TEX>$MG^+$</TEX> to TX-100 is exothermic and binding of <TEX>$MG^+$</TEX> to DTAB and SDS in some concentration ranges of the used surfactants is endothermic and in the other ones is exothermic.

Hydrophobic N‐Diazeniumdiolates and the Aqueous Interface of Sodium Dodecyl Sulfate (SDS) Micelles

Zwitterionic diazeniumdiolates of the form RN[N(O)NO(-)](CH(2))(2)NH(2) (+)R, where R=CH(3) (1), (CH(2))(3)CH(3) (2), (CH(2))(5)CH(3) (3), and (CH(2))(7)CH(3) (4) were synthesized by reaction of the corresponding diamines with nitric oxide. Spectrophotometrically determined pK(a)(O) values, attributed to protonation at the terminal oxygen of the diazeniumdiolate group, show shifts to higher values in dependence of the chain lengths of R. The pH dependence of the decomposition of NO donors 1-3 was studied in buffered solution between pH 5 and 8 at 22 degrees C, from which pK(a)(N) values for protonation at the amino nitrogen, leading to release of NO, were estimated. It is shown that the decomposition of these diazeniumdiolates is markedly catalyzed by anionic SDS micelles. First-order rate constants for the decay of 1-4 were determined in phosphate buffer pH 7.4 at 22 degrees C as a function of SDS concentration. Micellar binding constants, K(SM), for the association of diazeniumdiolates 1-3 with the SDS micelles were also determined, again showing a significant increase with increasing length of the alkyl side chains. The decomposition of 1-3 in micellar solution is quantitatively described by using the pseudo-phase ion-exchange (PIE) model, in which the degree of micellar catalysis is taken into account through the ratio of the second-order rate constants (k(2m)/k(2w)) for decay in the micelles and in the bulk aqueous phase. The decay kinetics of 1-3 were further studied in the presence of cosolvents and nonionic surfactants, but no effect on the rate of NO release was observed. The kinetic data are discussed in terms of association to the micelle-aqueous phase interface of the negatively charged micelles. The apparent interfacial pH value of SDS micelles was evaluated from comparison of the pH dependence of the first-order decay rate constants of 2 and 3 in neat buffer and the rate data obtained for the surfactant-mediated decay. For a bulk phase of pH 7.4, an interfacial pH of 5.7-5.8 was determined, consistent with the distribution of H(+) in the vicinity of the negatively charged micelles. The data demonstrate the utility of 2 and 3 as probes for the determination of the apparent pH value in the Stern region of anionic micelles.

A Comparative Analysis of Pseudophase Ion-Exchange (PIE) Model and Berezin Pseudophase (BPP) Model: Analysis of Kinetic Data for Ionic Micellar-mediated Semi-ionic Bimolecular Reaction

Pseudo-first-order rate constants (kobs) for the reaction of N-benzylphthalimide (NBPT) with HO − have been determined at 2.0 × 10 −4 M NBPT, 1.0 × 10 −3 and 2.0 × 10 −3 M NaOH as well as varying concentrations of cetyltrimethylammonium bromide ([CTABr]T = 0.0-1.7 × 10 −1 M). The effects of [CTABr]T – CMC (with CMC representing the critical micelle concentration of CTABr) on kobs have been analyzed in terms of Berezin’s pseudophase (BPP) model and pseudophase ion-exchange (PIE) model. Although both models give the best observed data fit with least-squares values not significantly different from each other, the calculated values of KS from BPP model appear to be more reliable compared to those from PIE model because the values of KS from BPP model are similar to the corresponding KS values determined spectrophotometrically.

Kinetics of Reaction of Oximate α‐Nucleophiles with p‐nitrophenyl Acetate in Alkyltriphenyl Phosphonium Bromide Micelles

Rate constants for the nucleophilic hydrolysis of p‐nitrophenyl acetate by the butane 2,3‐dione monoximate ion at pH 7.9 and 27°C have been measured kinetically in alkyltriphenylphosphonium (C12–, C14–, C16–) bromide micellar media. Addition of surfactants led to a significant increase in the pseudo‐first order rate constants. The rate‐surfactant profiles are fitted quantitatively in terms of pseudo phase ion exchange model. There is a decrease in the observed rate with decreasing hydrophobic chain length for the same surfactant head group. Cetyltriphenylphosphonium bromide is more reactive than cetyltrimethylammonium bromide.

Effect of CTAB and SDS micelles on the excited state equilibria of some indole probes

The absorption and fluorescence spectral characteristics of some biologically active indoles have been studied as a function of acidity and basicity (H_/pH/H o) in cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium dodecylsulphate, SDS) and aqueous phases at a given surfactant concentration. The prototropic equilibrium reactions of these probes have been studied in aqueous and micellar phases and apparent excited state acidity constant ( p K a * ) values are calculated. The probes show formation of different species on changing pH. Various species present in water, CTAB and SDS have been identified and the equilibrium constants have been determined by Fluorimetric Titration method. The fluorescence spectral data suggest the formation of oxonium ion through the excited state proton transfer reaction in highly acidic media and formation of photoproducts due to the base catalyzed auto-oxidative reaction in basic aqueous solutions. Variations in the apparent p K a * value have been observed in different media. The change in the apparent p K a values depends upon the solubilising power of the micelles, as well as on the location of the protonating site in the molecule. The observation about increase in p K a * values in SDS and decrease in CTAB compared to pure water for various equilibria is consistent with the pseudophase ion-exchange (PIE) model.

Kinetics of the alkaline hydrolysis of isoproturon in CTAB and NaLS micelles

AbstractKinetics of the alkaline hydrolysis of isoproturon has been studied in the absence and presence of cetyltrimethylammonium bromide (CTAB) and sodium lauryl sulfate (NaLS) micelles. CTAB micelles were found to enhance the rate of reaction, while NaLS micelles inhibited the reaction rate. The reaction obeyed first‐order kinetics in [isoproturon] and was linearly dependent on [NaOH] at lower concentration. The rate of reaction became independent at higher [NaOH]. At lower [NaOH] the reaction proceeded via formation of hydroxide ion addition complex, while at higher [NaOH] the reaction occurred via deprotonation of NH, leading to the formation of isocyanate. The values of kw, km, and Ks were determined by considering the pseudophase ion exchange model. The activation parameters have also been reported. The effect of added salts (NaCl and KNO3) on the reaction rate has also been studied. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 39–45, 2007

Degradation of the Pesticide Fenitrothion as Mediated by Cationic Surfactants and α-Nucleophilic Reagents

The reaction of fenitrothion with a series of alpha-nucleophile oximates having pK(a) values in the range of 7.7-11.8 was studied both in the absence and presence of cetyltrimethylammonium (CTA(+)) surfactants. Reaction with CTA-oximates was found to proceed through two pathways: S(N)2(P) and S(N)2(C); an S(N)Ar pathway was not observed. Accordingly, the observed rate constants were dissected into the two corresponding S(N)2(P) and S(N)2(C) pathways. Use of the pseudophase ion exchange (PPIE) model for micellar catalysis in the CTA(+) system allowed evaluation of micellar second-order rate constant (k(2m)) parameters and binding constants, (K(S)). K(S) values for CTA-oximates were found to vary with the counterion, and the rate enhancement depended on a combination of K(S) and k(2m) values. k(2m)/k(2w) values ranged from 0.0025 to 0.64, suggesting that a concentration effect is mainly responsible for the rate enhancement. In the absence of surfactant, an alpha-effect (i.e., k(alpha)/k(normal)) varying from 8 to 450 was observed for the oximate reaction, decreasing with increasing pK(a). It is proposed that differential solvation (transition-state imbalance) is a cause of the alpha-effect in this system.

Micellar catalysis on the electron transfer reactions of iron(iii)-polypyridyl complexes with organic sulfides—importance of hydrophobic interactions

The oxidation of organic sulfides with iron(III)-polypyridyl complexes [Fe(NN)3]3+ proceeds through an electron transfer mechanism and an increase in the methanol content in the methanol-water mixture favors the reaction. The reaction is catalyzed by both the anionic surfactant, sodium dodecyl sulfate (SDS) and the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The micellar catalysis in the presence of SDS is accounted for in terms of strong binding of the cationic oxidant with the anionic surfactant and the development of positive charge on sulfur center of substrate in the transition state. The micellar catalysis observed on the reaction involving a trication, [Fe(NN)3]3+, in the presence of CTAB indicates the importance of hydrophobic interaction between the micelle and hydrophobic ligand of [Fe(NN)3]3+. The micellar catalysis is explained in terms of a pseudophase ion exchange model.

Rate-retarding effects of mixed anionic/non-ionic micelles on the alkaline hydrolysis of the chloropentamminocobalt(III) complex: Role of the anionic surfactant chain nature

Rate data for the alkaline hydrolysis of the chloropentaamminecobalt(III) cation in the presence of mixed micelles composed of (i) anionic sodium decylsulphate (SDeS) and non-ionic dodecylpenta(oxyethylene glycol) monoether (C 12E 5) surfactants and (ii) anionic sodium perfluorooctanoate (SPFO) and non-ionic C 12E 5 surfactants has been obtained at T 298 K and constant electrolyte concentration 0.08 mol dm −3 ([NaOH] = 0.01 mol dm −3, [NaClO 4] = 0.07 mol dm −3) over a wide range of total surfactant concentration ( C t) and anionic mole fraction ( χ). The critical micelle concentrations (c.m.c.s) of the mixed micelles have been determined over the entire χ range by means of surface tension measurements. The experimental data have been analyzed by the regular solution model for non-ideal mixing. The estimated negative values of the interaction parameter β indicate that attractive interactions between the two types of surfactants in the mixed anionic/non-ionic micelles take place. The kinetic data have shown that, at all mole fraction examined, at surfactant concentrations below the critical micelle concentration the rate constant values are not significantly influenced by the presence of surfactants while beyond the c.m.c. value, in all cases examined, the rate constant is conspicuously inhibited. The reaction rates decrease with increasing C t (at constant χ) and χ (at constant C t) to an extent which depends on the nature of the anionic surfactant used, i.e., the hydrogenated anionic surfactant is much more effective in inhibiting the hydrolysis process than the fluorinated one. The kinetic data, interpreted quantitatively by applying the pseudo-phase ion exchange model, indicate that two reaction paths contribute to the overall hydrolysis rate and that the binding constant of the complex ( K M) decreases markedly as χ decreases. These findings have been attributed to the reduction of the charge density at the mixed micellar surface. Moreover, the K M values indicate that the capacity of the hydrogenated anionic micelles to bind the cobalt complex is higher than that of the fluorinated anionic micelle-forming surfactant. The present findings have been compared with those previously obtained concerning the effects of other aggregated systems upon the rate of the same hydrolysis reaction.

Nucleophilic Dephosphorylation of p-Nitrophenyl Diphenyl Phosphate in Cationic Micellar Media

The kinetics of nucleophilic dephosphorylation of p-nitrophenyl diphenyl phosphate by hydroxamate ions (R'(C=O)N(RO-)) have been investigated in aqueous cationic micellar media at pH 9.12 and 27 degrees C. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase. N-Substituted hydroxamate ion shows higher reactivity over the unsubstituted hydroxamate ions in cationic micellar media. The kinetic data are discussed in terms of the pseudophase ion exchange model.

Micellar effects of a triazole-based cationic gemini surfactant on the rate of a nucleophilic aromatic substitution reaction

The reaction between 2,4-dinitrochlorobenzene (DNCB) and hydroxide ion was studied spectrophotometrically at 25 °C in micelles of a triazole-based cationic gemini surfactant 18-triazole-18 or micelles of the conventional cationic surfactant CTAB. Both CTAB and 18-triazole-18 accelerated this nucleophilic aromatic substitution reaction. The binding constant of the substrate to the micelle, KS, for 18-triazole-18 (KS=335 M−1) was found to be much larger than that for CTAB (85 M−1) by fitting the kinetic results with pseudophase ion-exchange (PIE) model, which suggests that DNCB binds with gemini micelles more easily than it does with CTAB micelles. It was also found that 18-triazole-18 catalytic system was in accordance with PIE model at surfactant concentrations below ca. 0.5 mM, above which the increase of viscosity and the change of micelle size with increased surfactant concentration may remarkably influence the reaction. This was quite different from the reaction catalyzed by micelles of the conventional surfactant CTAB.

Micellar Catalyzed Degradation of Fenitrothion, an Organophosphorus Pesticide, in Solution and Soils

We report on a study of the decomposition of fenitrothion (an organophosphorus pesticide that is a persistent contaminant in soils and groundwater) as catalyzed by cetyltrimethylammonium (CTA+) micelles. The CTA micelles were associated with two types of counterions: (1) inert counterions (e.g. CTABr) and (2) reactive counterions (e.g. CTAOH). The reactive counterion surfactants used were hydroxide anion (HO-) as a normal nucleophile and hydroperoxide anion (HOO-) and the anion of pyruvaldehyde oxime (MINA-) as two alpha-nucleophiles. The reactivity order followed: CTABr < CTAOH < CTAMINA << CTAOOH. Treatment of the rate data using the Pseudo-Phase Ion Exchange (PPIE) model of micellar catalysis showed the ratio k2M/k2w to be less than unity for all the surfactants employed. Rather than arising from a "true catalysis", we attributed the observed rate enhancements to a "concentration effect", where both pesticide and nucleophile were incorporated into the small micellar phase volume. Furthermore, the CTAOOH/CTAOH pair gave an alpha-effect of 57, showing that the alpha-effect can play an important role in micellar systems. We further investigated the effectiveness of reactive counterion surfactants in decontaminating selected environmental solids that were spiked with 27 ppb fenitrothion. The solids were as follows: the clay mineral montmorillonite and SO-1 and S0-2 soils (obtained from the Canadian Certified Reference Materials Project). The reactive counterion surfactant solutions significantly enhanced the rate of fenitrothion degradation in the spiked solids over that obtained when the spiked solid was placed in contact with either 0.02 M KOH or water. The rate enhancements followed the order CTAOOH >> CTAMINA approximately CTAOH > KOH >> water. We conclude that reactive counterion surfactants, especially with alpha-nucleophiles, hold great potential in terms of remediating soils contaminated by toxic organophosphorus esters.

Study of Kinetics of Bromophenol Blue Fading in the Presence of SDS, DTAB and Triton X-100 by Classical Model

In this paper, kinetics of reaction between Bromophenol blue (BPB) and OH − , called fading, has been studied through a spectrophotometric method in the presence of nonionic Triton X-100 (TX-100), anionic sodium dodecyl sulfate (SDS) and cationic dodecyl trimethyla mmonium bromide (DTAB) surfactants. The influence of changes in the surfactant concentration on the observed rate constant was investigated. The results are treated quantitatively by pseudophase ion-exchange (PPIE) model and a new simple model called “classical model”. The binding constants of BPB molecules to the micelles and free molecules of surfactants, their stoichiometric ratios and thermodynamic parameters of binding have been evaluated. It was found that SDS has nearly no effect on the fading rate up to 10 mM, whereas TX-100 and DTAB interact with BPB which reduce the reaction rate. By the use of fading reaction of BPB, the binding constants of SDS molecules to TX-100 micelles and their Langmuir and Freundlich adsorption isotherms were obtained and when mixtures of DTAB and TX-100 were used, no interaction was observed between these two surfactants.

Counterion exchange selectivity coefficients at water-in-oil microemulsion interface

The counterion binding at a water/Aerosol-OT (AOT)/heptane microemulsion interface was treated in the context of the pseudo-phase ion exchange formalism. Two approaches were used to determine the selectivity coefficient for copper/sodium counterion binding at the AOT microemulsion interface: measurements of the Cu(II) concentration taken up by the reverse micelle in a Winsor II microemulsion system and steady-state emission quenching measurements of an anionic water-solubilized probe, the tris-(4,4′-dicarboxy-2,2′-bipyridine) ruthenium (II) ion. In addition, the selectivity coefficient for methyl viologen/sodium at the microemulsion surface was determined by the same photophysical technique. The value for copper (II)/sodium exchange ( K Cu/Na) is 1.1±0.3 and that for methyl viologen/sodium ( K MV/Na) is 0.9±0.3. The results show that the pseudo-phase ion exchange model can be used to obtain the selectivity coefficient in a microemulsion system.

Ethyl Cyclohexanone-2-carboxylate in Aqueous Micellar Solutions. 1. Ester Hydrolysis in Cationic and Nonionic Micelles

Hydrolysis rates for the ester of ethyl cyclohexanone-2-carboxylate (ECHC) were measured in aqueous dioxane mixtures and in aqueous micellar solutions of cationic, nonionic, and mixed cationic−nonionic detergents. The following cationic surfactants were used: dodecyltrimethylammonium bromide (DTABr), tetradecyltrimethylammonium bromide (TTABr), tetradecyltrimethylammonium chloride (TTACl), tetradecyltrimethylammonium acetate (TTAAc), and cetyltrimethylammonium bromide (CTABr). Poly(ethylene oxide)-9-dodecyl ether (C12E9) and poly(ethylene oxide)-20-cetyl ether (C16E20) were used as the nonionic surfactants. The kinetic profiles of the reactions were investigated as a function of surfactant concentration. In aqueous strong mineral acid (HCl or HBr) solutions of the cationic surfactants DTABr, TTABr, TTACl, and CTABr, the reaction was greatly inhibited at surfactant concentrations above the critical micelle concentration; the same behavior was observed for the influence of the nonionic surfactants and for the mixed surfactants DTABr + C12E9 analyzed in an equimolar mixture of both surfactants. Micellar effects were analyzed by means of the pseudophase model. When the hydrolysis reaction was studied in aqueous buffered solutions of acetic acid−acetate, the shape of the observed rate constant versus surfactant concentration (of TTACl and TTABr) profiles depended on surfactant concentration. At low surfactant concentration, first-order rate constants pass through maxima with increasing surfactant concentration followed by a gradual but steady decrease in the rate as the surfactant concentration increases further. By contrast, the first-order rate constant increases throughout the whole concentration range of TTAAc used. Micellar effects observed for the reaction in aqueous buffered solutions were analyzed qualitatively or quantitatively by means of the pseudophase ion exchange (PPIE) model. In every experimental situation, the reaction at the micellar interface is not negligible; however, the second-order rate constant in this region is lower than that measured in water.