Pseudophase Models Research Articles

Critical Micelle Concentrations in Surfactant Mixtures and Blends by Simulation.

We explore the use of coarse-grained dissipative particle dynamics simulations to predict critical micelle concentrations (CMCs) in polydisperse surfactant mixtures and blends. By fitting pseudo-phase separation models (PSMs) to aqueous solutions of binary surfactant mixtures at selected compositions above the CMC, we avoid the need for expensive simulations of more complex multicomponent mixtures performed as a function of dilution. The approach is demonstrated for sodium laureth sulfate (SLES) surfactants with polydispersity in the ethoxylate spacer. For this system, we find a modest degree of cooperativity in micelle formation, which we attribute to the reduced repulsion between charged headgroups for surfactants with dissimilar ethoxylate spacer lengths. However, this is insufficient to explain the lowered CMC often observed in commercial SLES samples, which we attribute to the presence of small amounts of unsulfated alkyl ethoxylates and/or traces of salt.

Influence of polyethylene glycol and surfactant mixed system on the rate of alkaline hydrolysis of benzocaine

The rate of alkaline hydrolysis of benzocaine has been studied spectrophotometrically in the presence of polyethylene glycols (PEGs), surfactants (cetyltrimethylammonium bromide; CTABr and sodium dodecyl sulphate; SDS) and in the mixed media of surfactants-polysurfactants (PEGs-CTABr and PEGs-SDS). The observed values of rate constant increased linearly with the increase in [NaOH] at fixed [benzocaine] obeying the equation: kobs = k2[NaOH]. The rate constant – [CTABr] plot showed peaked behaviour whereas in case of SDS, the rate constant decreased with increase in [SDS]. The increase in the molecular weight of PEGs increased the binding tendency of PEG-CTABr system for benzocaine, in turn, an increase in inhibitory effect on the rate of hydrolysis was observed. The obtained values of the binding constant decreased with the increase in the molecular weight of PEGs in PEG-SDS mixed system. The binding constant and the values of rate constant in the surfactant, PEG-surfactant media were determined by using Pseudophase Ion Exchange (PIE) and pseudophase models.

ChemInform Abstract: Effects of Surfactants on the Rate of Chemical Reactions

AbstractReview: 84 refs.

Catalytic Effect of Cationic Gemini Micelles on the Rate of Condensation between Glycyl-dl-Aspartic Acid and Ninhydrin in the Absence and Presence of Organic Solvents

The reaction between glycyl-dl-aspartic acid (Gly-dl-Asp) and ninhydrin has been investigated in cationic gemini surfactants [alkanediyl-α,ω-bis (dimethyltetradecylammonium bromide)] (14-m-14). The study was carried out as functions of [Gly-dl-Asp], [ninhydrin], solvent (v/v %) and [surfactant] at pH = 5.0 and 70 °C. The reaction followed first-order kinetics in [Gly-dl-Asp] and fractional-order kinetics in [ninhydrin]. The reaction is catalyzed by TTABr/14-m-14. Addition of an organic solvent at fixed [surfactant] increases the absorbance as well as the rate of formation of Ruhemann’s purple. The results obtained in micellar media are treated quantitatively in terms of pseudophase and Piszkiewicz kinetic models. The Eyring equation is valid for the reaction over the range of temperatures used and different activation parameters have been evaluated. The kinetic data have been used to calculate the micellar binding constants KS for Gly-dl-Asp and KN for ninhydrin.

Dynamics of cetyltrimethylammonium bromide mediated reaction of phenylsulfinylacetic acid with Cr(VI): Treatment of pseudo-phase models

micellar effect % Hammett correlation % Piszkiewicz cooperative model KR nema The influence of cetyltrimethylammonium bromide, CTon the oxidative decarboxylation of phenylsulfinylacetic acid, PSAA and several meta- and para-substituted PSAAs by Cr(VI) was investigated in 95 % H2O-5 % CH3CN medium. The rate profile displayed a peculiar trend with an initial rate increase at low [CTAB] followed by sharp rate inhibition at higher [CTAB]. The initial rate acceleration can be explained by strong binding of SO42- on the positively charged micellar surface. The specific partitioning of PSAA in micellar phase by hydrophobic interaction and the oxidizing species, HCrO3+ in aqueous phase by electrostatic repulsion accounted the rate retardation at higher [CTAB]. The Hammett plot with different substituted PSAAs showed an excellent correlation affording negative ? value which supports the proposed mechanism involving a sulfonium cation intermediate formation. The obtained ? value in CTmedium is found to be slightly lower than that in aqueous medium. The quantitative analysis of rate data for the inhibition shown by CTwas performed using Menger-Portnoy and Piszkiewicz pseudo-phase models. The binding constant for PSAA with micelle was evaluated from Piszkiewicz cooperative model.

Effects of Surfactants on the Rate of Chemical Reactions

Surfactants are self-assembled compounds that depend on their structure and electric charge can interact as monomer or micelle with other compounds (substrates). These interactions which may catalyze or inhibit the reaction rates are studied with pseudophase, cooperativity, and stoichiometric (classical) models. In this review, we discuss applying these models to study surfactant-substrate interactions and their effects on Diels-Alder, redox, photochemical, decomposition, enzymatic, isomerization, ligand exchange, radical, and nucleophilic reactions.

Micelle catalyzed oxidative degradation of norfloxacin by chloramine-T

Present work reports influence of micellar media (CTAB) upon the oxidative degradation of fluoroquinolone family drug norfloxacin (NOR). The reaction has been studied at constant temperature (308K). The stoichiometry of the reaction was found to be 1:4 and oxidation products were identified by LC–MS technique and other spectral studies. The reaction exhibited first-order-dependence on chloramine-T [CAT] and fractional-order dependence on [NOR]. The rate constant was independent of cationic micelles of cetyltrimethylammonium bromide [CTAB] at lower concentrations followed by a rapid increase in rate constant to a maximum and further gradual decrease in rate constant at higher CTAB was observed. In the presence of CTAB, both the oxidant and substrate are distributed between the aqueous and the micellar pseudo phases and then react to give the product. Compensation between water structure destruction and substrate–micelle interaction plays an important role in the presence of CTAB. The rate constants were obtained and the applicability of Piszkiewicz's, Raghvan–Srinivasan's and pseudo phase models allowed us to obtain the reactivity constants in the aqueous pseudo phase and in the micellar pseudo phase (k2w and k2m) and the binding constants of the reactants to the micellar phase (K1, K2). The effects of inorganic salts i.e. [SO42−], [HSO4−], [Cl−] and [Br−] on reaction rate have also been studied.

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.

PHYSICAL CHEMICAL STUDIES OF THE SURFACTANT DILAURYLDIMETHYLAMMONIUM BROMIDE (DLDMAB): AGGREGATION AND CATALYTIC PROPERTIES

The micellization of dilauryldimethylammonium bromide (DLDMAB) in water was studied by using surface tension measurements. The critical micellar concentration (CMC) was determined at 25°C, 32°C and 40°C and thermodynamic parameters such as the free energy of micellization (∆G°mic), enthalpy (∆H°mic), and entropy (∆S°mic) of micellization were measured. The CMC at 25°C was 4.93 x 10-5 M and the corresponding values of the thermodynamic parameters were: ∆G°mic = -5.87 kcal/mol; ∆H°mic = -1.12 kcal/mol and ∆S°mic = +16.00 e.u. Micelles of the surfactant DLDMAB act as catalysts for the alkaline hydrolysis of p-nitrophenyl diphenyl phosphate (NPDPP) with a maximum catalytic factor of approximately 120 compared to 80 for CTAB. Typical activation parameters measured for 1.00 x 10-3 M surfactant and 0.005 M NaOH were: Ea = 9.7 kcal/mo/; ∆H°≠ = 9.1 kcal/mol; ∆G°≠ = 19.6 kcal/mol and ∆S°≠ = -33.9 e.u. The kinetic results were also analyzed in terms of the pseudo-phase ion-exchange models (PPIE) and showed that the model is applicable to describe the experimental results.

Thermodynamics of mixed anionic/nonionic surfactant adsorption on alumina

The adsorption of sodium dodecyl sulfate and a polyethoxylated nonylphenol, and well defined mixtures thereof, was measured on γ-alumina. A pseudo-phase separation model to describe mixed anionic/nonionic admicelle (adsorbed surfactant aggregate) formation was developed, analogous to the pseudo-phase separation model frequently used to describe mixed micelle formation. In this model, regular solution theory was used to describe the anionic/nonionic surfactant interactions in the mixed admicelle and a patch-wise adsorption model was used to describe surfactant adsorption on a heterogeneous solid surface. The formation of mixed anionic/nonionic admicelles in the absence of micelles was accurately described by regular solution theory; mixed admicelle formation exhibited stronger negative deviations from ideality than mixed micelle formation. An adequate description of mixed anionic/nonionic admicelle formation in the presence of mixed micelles was obtained through a simultaneous solution of the pseudo-phase separation models for mixed admicelle and mixed micelle formation, and the appropriate mass balance equations. Anionic/nonionic mixed adsorption in the presence of mixed micelles was shown to correspond to an admicelle composition of approximately a 1:1 anionic/nonionic mole ratio throughout Regions II and III of the adsorption isotherm.

Diazeniumdiolate reactivity in model membrane systems

The effect of small unilamellar phospholipid vesicles on the acid-catalyzed dissociation of nitric oxide from diazeniumdiolate ions, R 1R 2N[N(O)NO] −, [ 1: R 1 = H 2N(CH 2) 3-, R 2 = H 2N(CH 2) 3NH(CH 2) 4-; 2: R 1 = R 2 = H 2N(CH 2) 3-; 3: R 1 = n-butyl-, R 2 = n-butyl- NH 2 + (CH 2) 6-; 4: R 1 = R 2 = nPr-] has been examined at pH 7.4 and 37 °C. NO release was catalyzed by anionic liposomes (DPPG, DOPG, DMPS, POPS and DOPA) and by mixed phosphatidylglycerol/phosphatidylcholine (DPPG/DPPC and DOPG/DPPC) covesicles, while cationic liposomes derived from 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic liposome DMPC did not significantly affect the dissociation rates of the substrates examined. Enhancement of the dissociation rate constant in DPPG liposome media (0.010 M phosphate buffer, pH 7.4, 37 °C) at 10 mM phosphoglycerol levels, ranged from 37 for 1 to 1.2 for the anionic diazeniumdiolate 4, while DOPA effected the greatest rate enhancement, achieving 49-fold rate increases with 1 under similar conditions. The observed catalysis decreases with increase in the bulk concentration of electrolytes in the reaction media. Quantitative analysis of catalytic effects has been obtained through the application of pseudo-phase kinetic models and equilibrium binding constants at different liposome interfaces are compared. The stoichiometry of nitric oxide release from 1 and 2 in DPPG/DPPC liposome media has been obtained through oxyhemoglobin assay. DPPG = 1,2-dipalmitoyl- sn-glycero-3-[phospho- rac-(1-glycerol)], DOPG = 1,2-dioleoyl- sn-glycero-3-[phospho- rac-(1-glycerol)], DMPS = 1,2-dimyristoyl- sn-glycero-3-[phospho- l-serine], POPS = 1-palmitoyl-2-oleoyl- sn-glycero-3-[phospho- l-serine], DOPA = 1,2-dioleoyl- sn-glycero-3-phosphate; DPPC = 1,2-dipalmitoyl- sn-glycero-3-phosphocholine, DMPC = 1,2-dimyristoyl- sn-glycero-3-phosphocholine, DOTAP = 1,2-dioleoyl-3-trimethylammonium-propane.

Micellar and salt effects on the interaction of [Cu(II)‐Gly‐Gly]+ with ninhydrin

AbstractThe effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the kinetics of interaction of copper dipeptide complex [Cu(II)‐Gly‐Gly]+ with ninhydrin has been studied spectrophotometrically at 70°C and pH 5.0. The reaction follows first‐ and fractional‐order kinetics, respectively, in complex and ninhydrin. The reaction is catalyzed by CTAB micelles, and the maximum rate enhancement is about twofold. The results obtained in the micellar medium are treated quantitatively in terms of the kinetic pseudophase and Piszkiewicz models. The rate constants (kobs or kΨ), micellar‐binding constants (kS for [Cu(II)‐Gly‐Gly]+, kN for ninhydrin), and index of cooperativity (n) have been evaluated. A mechanism is proposed in accordance with the experimental results. The influence of different inorganic (NaCl, NaBr, Na2SO4) and organic (NaBenz, NaSal) salts on the reaction rate has also been seen, and it is found that tightly bound/incorporated counterions are the most effective. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 556–564, 2007

The study of the influence of surfactant charge on alkaline hydrolysis reactions of acetylsalicylic acid (ASA) and triflusal (TFL) using spectrophotometric methods

In this research, the effects of micellar systems on alkaline hydrolysis reactions of acetylsalicylic acid (ASA) and triflusal (TFL) were found to be dependant upon the surfactant charge within the micelle. In cationic micelles, there is a catalytic effect at low concentrations of surfactant. However, this reaction is inhibited at higher surfactant concentrations. In anionic micelles, a catalytic effect occurs, while in zwitterionic and non-ionic micelles there is an inhibitory effect. Such reactions are attributable to changes in reactants on the micellar surface, or to the fact that both reactants are found in different microenvironments. The pseudophase (PS) and ion-exchange (PPIE) models were found to be consistent with the experimental result. Furthermore, the association constants for both drugs could be determined together with micellar rate constants in heterogeneous media.

Solvolysis of Tris- p-nitrophenyl-phosphate in aqueous and reverse micelles

The widespread use of toxic phosphates and phosphonates as insecticides, and their use as chemical weapons, has led to investigation of fast detoxification and decontamination methods. Micelles, microemulsions, cyclodextrines and liposomes have been used to accelerate phosphate ester decomposition by nucleophiles. Here, hydrolysis, methanolysis and hexanolysis of Tris- p-nitrophenyl phosphate (TNPP), a model for reactive phosphate esters, were studied in homogeneous phase, aqueous and reverse micelles. Kinetic micellar effects were quantitatively analyzed using pseudo-phase models. TNPP hydrolysis was catalyzed by cetyltrimethylammonium chloride (CTAC), cetyltrimethylammonium bromide (CTAB), and hexadecylammonium propanesulfonate (HPS), micelles by factors of five, CTAC, and three, CTAB, HPS, respectively. The calculated rate constants for spontaneous and acetate-catalyzed hydrolysis in the micellar phase were significantly higher than those in the aqueous phase. While in water and in methanol the effect of the acetate cation was negligible, the catalytic efficiency of acetate for hexanolysis depended on the nature of the cation with the K + salt being ca. 20 times more efficient than the tetraethylammonium salt in non-polar solvents. Sodium dodecylsulfate, SDS, micelles inhibited TNPP hydrolysis by a factor of eigth. Reverse micelles of CTAB in n-hexanol/isooctane (10:90, v/v) did not catalyze TNPP hydrolysis, but changed the bis- p-nitrophenyl phosphate/hexyl-bis- p-nitrophenylphosphate product ratio depending of CTAB concentration and water/detergent ratio.

On the Equivalence of the Pseudophase Related Models and the Brönsted Approach in the Interpretation of Reactivity under Restricted Geometry Conditions

The equivalence of the Pseudophase and related models, frequently used in the interpretation of kinetic data under restricted geometry conditions, and the well known Brönsted's equation, used in the interpretation of the reactivity in homogeneous media, has been proven in several cases. The generalisation in relation to other non-kinetic properties such as reaction free energy and redox potentials has also been considered.

Catalytic Effect of Polyoxyethylene t -octylphenol (Triton X-100) Surfactant on the Oxidation of EDTA by Chromic Acid

The effect of nonionic Triton X-100 (TX-100) micelles on the pseudo-first order rate constants ( k ƒ ) for the oxidation of EDTA by chromic acid has been studied at different perchloric acid concentrations. The micellar-mediated reaction was independent of chromic acid concentration whereas first- and complex-order dependence on [EDTA] and [HClO 4 ], respectively, were observed. Increase in the total concentration of TX-100 from 0.0 to 60.0 2 10 m 3 mol dm m 3 caused the k ƒ to increase by nearly 4-fold at constant temperature and [HClO 4 ]. The observed catalytic effect has been explained on the basis of pseudo-phase kinetic and Piszkiewicz models. The higher rate of EDTA oxidation in the micellar pseudo-phase is attributed to association of the reactants with the nonionic micellar head groups through hydrogen bonding. A mechanism is proposed in accordance with the experimental results obtained.

Effect of Vesicles of Dimethyldioctadecylammonium Chloride and Phospholipids on the Rate of Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate

Sonicated mixtures of dimethyldioctadecylammonium chloride (DODAC), egg phosphatidylcholine (PC), dimyristoyl phosphatidylcholine (DMPC), and dipalmitoyl phosphatidylcholine (DPPC) were used to analyze vesicle effects on the rate of decarboxylation of 6-nitrobenzisoxazol-3-carboxylic acid (Nboc). Electron microscopic images of the vesicles were obtained with trehalose, a know cryoprotector. Phase diagrams and phase transitions temperatures of the vesicle bilayers were determined. Nboc decarboxylation rates increased in the presence of vesicles prepared with both phospholipids and DODAC/phospholipid mixtures. Quantitative analysis of vesicular effects was done using pseudophase models. Phospholipids catalyzed up to 140-fold while the maximum catalysis by DODAC/lipid vesicles reached 800-fold. Acceleration depends on alkyl chain length, fatty acid insaturation of the lipids, and the DODAC/phospholipid molar ratio. Catalysis is not related to the liquid crystalline-gel state of the bilayer and may be related...

PHYSICAL CHEMICAL STUDIES OF THE AGGREGATION AND CATALYTIC PROPERTIES OF THE SURFACTANT CETYLDIMETHYLETHYLAMMONIUM BROMIDE (CDEAB)

The micellization of cetyldimethylethylammonium bromide (CDEAB) in water was studied by means of surface tensiometry. The critical micellar concentration (CMC) was determined at 25°, 32°, and 40°C, and thermodynamic properties such as the free energy of micellization (∆G°mic), enthalpy (∆H°mic) and entropy (∆S°mic) of micellization were measured. The CMC at 25°C was 9.05 x 10-4 M and the corresponding values of the thermodynamic parameters were: ∆G°mic = -4.15 kcal/mol; ∆H°mic = -1.37 kcal/mol and ∆S°mic = + 9.33 e.u. Micelles of the surfactant CDEAB were good catalysts for the alkaline hydrolysis of p-nitrophenyl diphenyl phosphate (NPDPP) with a maximum catalytic factor of approximately 80, comparable to that of CTAB. Typical activation parameters measured for 0.005 M NaOH were: Ea = 9.0 kcal/mol; ∆H°≠ = 8.4 kcal/mol; ∆G°≠ = 19.2 kcal/mol and ∆S°≠ = -36.3 e.u. The kinetic results were also analyzed in terms of the pseudo-phase ion-exchange models (PPIE) and showed that the model is applicable and gave reasonable fits

Micellar catalysis, a useful misnomer

Aggregates of amphiphiles often accelerate or ‘catalyze’ thermal reactions, but they also inhibit reactions and current pseudophase models treat both phenomena. Recent advances refine current models, offer new ones, treat solute and ion binding with more sophistication, enhance reactivity and selectivity with special catalysts, and apply the pseuodophase approach to more complex mixtures and novel applications.

Reactivity in aqueous association colloids. Descriptive utility of the pseudophase model

The ability of aqueous micelles and similar association colloids to control reaction rates is treated in terms of pseudophase models in which micelles and water are regarded as distinct reaction media, with consideration of reactant concentrations and rate constants in the two regions. An alternative model, based on transition state theory, considers the transfer free energy of the activated complex between water and micelles. Both models fit data for reactions in solutions of cetyltrimethylammonium hydroxide, for example. Their values in relating rate effects to reaction mechanism and structures of reactants and surfactants are discussed.