Pseudophase Ion-exchange Model Research Articles

Efficiency of δ-Tocopherol in Inhibiting Lipid Oxidation in Emulsions: Effects of Surfactant Charge and of Surfactant Concentration.

Charged interfaces may play an important role in the fate of chemical reactions. Alterations in, for instance, the interfacial acidity of emulsions induced by the charge of the surfactant head group and associated counterions may change the ionization status of antioxidants, modifying their effective concentrations. The chemical reactivity between interfacial reactants and charged species of opposite charge (protons, metallic ions, etc.) is usually interpreted in terms of pseudophase ion-exchange models, treating the distribution of charged species in terms of partitioning and ion exchange. Here, we focus on analyzing the effects of charged interfaces on the oxidative stability of soybean oil-in-water (o/w) emulsions prepared with anionic (sodium dodecyl sulfate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and neutral (Tween 20) surfactants, and some of their mixtures, in the presence and absence of δ-tocopherol (δ-TOC). We have also determined the effective concentrations of δ-TOC in the oil, interfacial and aqueous regions of the intact emulsions. In the absence of δ-TOC, the relative oxidative stability order was CTAB < TW20 ~ TW20/CTAB < SDS. Surprisingly, upon the addition of δ-TOC, the relative order was SDS ≈ TW20 << TW20/CTAB < CTAB. These apparently surprising results can be rationalized in terms of the nice correlation that exists between the relative oxidative stability and the effective interfacial concentrations of δ-TOC in the various emulsions. The results emphasize the importance of considering the effective interfacial concentrations of antioxidants in interpreting their relative efficiency in emulsions.

Quantitative analysis of micellar effect on the reaction rate of cationic triphenylmethine dyes with water according to Berezin’s model

Several approaches quantitatively describe the effect of surfactant micellar solution on the reaction rate. The most used among them are Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. The last-named was developed by Bunton and Romsted. Piszkiewicz’s model is based on representations of the micellization according to the mass action law with the formation of a catalytic micelle, which consists of some surfactant molecules and a substrate. In our previously paper, this model was used to explain the kinetic micellar effect on the reaction of cationic triphenylmethine dyes with water once again showed the main disadvantages of this approach. Berezin’s model is based on another model of micelle formation viz. the pseudophase model, and the binding of reagents by micelles is considered as the distribution of a substance between two phases. In this work, we aim to consider the applicability of Berezin’s approach for the interaction of malachite green and brilliant green cations with water molecule as a nucleophile in aqueous systems of nonionic, anionic, cationic, and zwitterionic surfactants. On the whole, Berezin's model performed well when applied to the description of the micellar effect on the reaction of similar dye with the hydroxide ion. However, it was revealed that this model does not take into account the change in the local concentration of the HO– ions due to a compression of the double electric layer upon addition of reacting ions to the system, as well as the constant of association of the HO– ions with cationic head groups of surfactant. In this case, when water is used as a nucleophile, the question of the degree of nucleophile binding can be solved differently. The PIE model is also based on a pseudophase model of micellization, but a substrate binding by micelles is considered as an association in a stoichiometric ratio of 1:1, and a nucleophile concentration is expressed in a local concentration based on the neutralization degree of micelles. Given the latter, its approach cannot be applied to the kinetic micellar influence on the reaction of cationic triphenylmethine dyes with water.

Influence of surfactants and surfactant-coated IONs on the rate of alkaline hydrolysis of procaine in the presence of PEG

The influence of surfactant coated iron oxide nanocomposites (Surf-IONs) and surfactants on the rate of alkaline hydrolysis of procaine hydrochloride (2-diethylaminoethyl-4-aminobenzoic acid) in the presence of poly(ethylene glycol) (PEG) have been investigated under the varying reaction conditions. The nanoparticles were synthesised using the co-precipitation method and were characterised using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Vibrating sample magnetometer (VSM) and Thermogravimetric analysis (TGA). The concentration of NaOH was taken in overabundance over [procaine] for maintaining the reactions under pseudo-first-order conditions. The kinetic effect of poly(ethylene glycol); PEG of molecular weights 1500, 4000, 6000, and 8000 with the cetyltrimethylammonium bromide; CTABr and sodium dodecyl sulphate; SDS (either free or coated on iron oxide nanoparticles) were studied. A fall in the values of the rate constant on increasing the [surfactant] or [Surf-IONs] at fixed [PEG] was observed. The kψ-[PEG-surfactant] profile was treated using the pseudophase model, and pseudophase ion exchange model and the kinetic parameters were determined. The binding constant of procaine with PEG-surfactant aggregates was found to be lower than the respective micelles in the absence of PEG. The increase in the molecular weights of PEG decreased the values of binding constants and equilibrium constants.

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.

Micellar rate effects on the kinetics of nitrophenol violet anion reaction with HO– ion: Comparing Piszkiewicz's, Berezin's, and Pseudophase Ion-Exchange models

Triphenylmethine dyes are often used as the probes for studying the properties of micellar media and other organized solutions and the influence of the latter on the kinetics of the reactions. The reports by different authors are mainly devoted to the study of the accelerated alkaline fading of cationic triphenylmethine dyes, Ct+, in micelles of cationic surfactants, whereas the presence of anionic surfactants strongly decelerates the reaction Ct+ + HO– → carbinol. At the same time, the anionic dyes interact with hydroxide ion much more slowly even in cationic surfactant micelles. For instance, as shown by Grunwald and Duynstee as early as 1959, for a sulfonephthalein dye bromophenol blue anion, BPB2−, the reaction occurs so slowly that it is impossible to determine the rate constant. Hence, all the regularities of the nucleophilic attack of the hydroxide ion in cationic micelles were until now studied using cationic dyes. Therefore, it seemed to be significant to find an anionic triphenylmethine dye electrophilic enough to examine the rate of its reaction with alkali in the presence of cationic surfactants. As such a dye, we used another sulfonephthalein dye, nitrophenol violet, NPV (3,3′-dinitrophenolsulfonephtalein), which meet the above condition. Indeed, it was possible not only to (quantitatively) describe the decelerating effect of cationic micelles on the reaction NPV2− + HO−, but also utilize it for comparing the most popular Piszkiewicz's, Berezin's, and Pseudophase Ion-Exchange models of micellar kinetics in CTAB and CTAOH solutions. Zwitter-ionic, non-ionic, and anionic surfactants were used as well. The obtained results allow generalizing the concept of micellar effects on the kinetics of nucleophilic attack and validating the above-mentioned theories. Also, the results demonstrate the usefulness of the dye NPV for studying of the nucleophilic attack in systems with positively charged colloidal particles. For instance, in addition to the electrostatic deceleration of the reaction between two anions on lowering the polarity of the medium it was shown that the cationic head groups in the Stern layer display a “diverting” influence on the HO– ion.

Influence of surfactants/polyethylene glycols mixture on the kinetics of alkaline hydrolysis of tetracaine

The influence of the surfactants namely sodium dodecyl sulphate; SDS, cetyltrimethyl ammonium bromide; CTABr, and Triton-X-100; TX-100 and polymeric surfactants (viz. polyethylene glycols; PEG) on the rate of hydrolysis of tetracaine in the alkaline medium has been studied spectrophotometrically. The pseudo-first-order conditions were maintained by keeping [OH−] ≫ [tetracaine]. The rate of hydrolysis of tetracaine was found to be linearly dependent upon the concentration of NaOH in the concentration range from 2.0 × 10−2 mol dm−3 to 1.7 × 10−1 mol dm−3 at a fixed concentration of tetracaine at 4.0 × 10−5 mol dm−3. The values of rate constant were found to be independent on the concentration of tetracaine in the concentration range from 1.0 × 10−5 mol dm−3 to 6.0 × 10−5 mol dm−3 at a fixed concentration of NaOH at 5.0 × 10−2 mol dm−3. The rate of hydrolysis of tetracaine decreased with the increase in the [surfactants] and [PEG-surfactant]. Low molecular weight gave higher values of binding constant for tetracaine in comparison to higher analogues. This indicates that the lower molecular weight PEGs give more compact structure with CTABr. The increase in PEGs concentrations from 1% to 3% (w/v) increased the rate constant values slightly due to break in the water structure, interfacial adsorption of surfactants and lowering in viscosity of media. The observed results were treated in the light of pseudophase and pseudophase ion exchange models and the various kinetic parameters e.g., the binding constants for tetracaine with the micelles (Ks) and PEG-surfactant (Kp) and the values of rate constant in the micellar (km) and PEG-surfactant (kp) were determined.

Synergistic influence of inhibition of PEG-surfactant on the rate of alkaline hydrolysis of procaine

The effect of surfactants (sodium dodecyl sulphate; SDS, cetyltrimethyl ammonium bromide; CTABr, and Triton X-100; TX100) and polymeric surfactants (viz. polyethylene glycol; PEG) mixture on the rate of base catalyzed hydrolysis of procaine hydrochloride (2-diethylaminoethyl-4-aminobenzoic acid) has been studied under the pseudo-first-order conditions ([OH¯]≫[Procaine]). The rate of hydrolysis of procaine was found to be linearly dependent upon the [NaOH] in the range from 2.0×10−2moldm−3 to 8.0×10−1moldm−3 at fixed [procaine] (=5.0×10−5moldm−3) at 37°C. The values of rate constant were found to be independent on [procaine] in the concentration range from 1.0×10−5moldm−3 to 1.0×10−4moldm−3 at a fixed concentration of NaOH at 5.0×10−2moldm−3. The values of rate constant decreased with the increase in the concentrations of surfactants (SDS, CTABr and TX100). The rate of hydrolysis further decreased with the addition of PEGs of different molecular weights (1500, 4000, 6000 and 8000) in the reaction media containing surfactant. The increase in the concentration of PEGs from 1% to 5% decreased the rate of hydrolysis, thereby, increasing its stability. The increase in molecular weight of PEG also decreased the rate of hydrolysis. The kinetic parameters and binding constants (e.g. km, Kp for the different reaction media) were calculated for the reactions occurring in the aqueous and surfactant-PEG pseudophases by applying pseudophase ion exchange model. Maximum inhibition in the rate of hydrolysis was observed for PEG-TX100 aggregates.

Quantitative analysis of micellar effect on the reaction rate of alkaline fading of phenolphthalein

Quantitative treatment of the kinetic data of the reaction between phenolphthalein dianion and hydroxide ion in aqueous solutions containing variable concentration of various surfactants is presented. Following surfactants are used: Brij-35 (nonionic), sodium n-dodecyl sulfate (anionic), cetyltrimethylammonium bromide (cationic) and 3-(dimethyl-n-dodecylammonio)-propansulfonate (zwitterionic). The quantitative treatment is carried out basing of Piszkiewicz’s, Berezin’s, and Pseudophase Ion-Exchange (PIE) models. It is revealed that the Berezin’s model is a more applicable one for describing the effect of nonionic, anionic, and zwitterionic micellar systems. The values of the corresponding kinetic parameters are discussed. The effect of cetyltrimethylammonium hydroxide on the reaction is also examined and quantitatively described by the PIE model. The research of systems based on a cationic surfactant shows previously unknown effect called by us as “diverting influence”.

Interaction of hexadecylpyridinium bromide with glycine in aqueous medium using the Krafft temperature from conductivity measurement

Conductivity measurements were employed to investigate Kraft temperature (TK) of aqueous solution of hexadecylpyridinium bromide (HPyBr) and HPyBr+glycine with added counter-ions. The pseudophase ion exchange model was adopted for HPyBr in pure water, glycine (Gly) and potassium bromide (KBr). The TK values for HPyBr increase with the increase in concentration of Gly and KBr. However, the experimental results show that the degree of counter-ion dissociation or ionization (α) remains nearly constant as the total surfactant concentration or counter-ion concentrations of salt are varied at the slower rate of micellar growth. Although the (α) is dependent on micellar shapes and changes as the micellar shape changes. The TK measurements for ionic micelles can be used to estimate counter-ion dissociation (α). The TK increase with increase in the counter-ion concentration because the solubility of surfactant is reduced at faster rate than critical micellar concentration (CMC). Kraft temperature measurements were employed to find the interaction between HPyBr and Glycine.

Electron transfer reactions of ruthenium(II) complexes with polyphenolic acids in micelles

The electron transfer in a microhetrogeneous system is a perfect mimic of biological electron transfer. The electron transfer between biologically important phenolic acids and ruthenium (II) complexes is systematically studied in the presence of anionic and cationic micelles. The photophysical properties of these ruthenium (II) complexes with anionic and cationic micelles and their binding abilities with these two type of micelles are also studies using absorption, emission and excited state lifetime spectral techniques. Pseudophase Ion Exchange (PIE) Model is applied to derive mechanism of electron transfer in two types of micelles.

Micellar effects upon the rate of alkaline hydrolysis of triflusal

The rate of hydrolysis for triflusal was measured at varying concentrations of NaOH at four different temperatures (i.e. 25, 35, 45 and 55°C). The micelles of cetyltrimethylammonium bromide (CTABr), cetyltrimethylammonium chloride (CTACl), cetyltrimethylammonium hydroxide (CTAOH) and dodecyltrimethyl-ammonium bromide (DTABr) had catalytic effect on the rate of hydrolysis. CTABr, CTACl and DTABr gave maxima like curve for the rate–[surfactant] plot while CTAOH gave plateau like curve. The anionic sodium dodecyl sulfate (SDS) did not influence the rate of alkaline hydrolysis of triflusal. The non-ionic Brij-35 inhibited the rate of the hydrolytic reaction. The catalytic effect by cationic micelles was treated by applying the pseudophase ion exchange model while the inhibitive effect by non-ionic micelles has been described by using the Poisson–Boltzmann pseudophase model. The variation in kψ with the change in [surfactant] was used to determined various kinetic parameters e.g., binding constant (Ks), and micellar rate constant (km). The addition of electrolytes decreased the reaction rate in CTABr and CTAOH micelles.

Acid Hydrolysis of Bromazepam Catalyzed by Micelles, Reverse Micelles, and Microemulsions

Kinetics of the acid hydrolysis of bromazepam (Bz) has been investigated in micelles, reverse micelles, and microemulcions of cetyltrimethylammonium bromide (CTAB) by spectrophotometric method. The rate of the acid hydrolysis of Bz was found to be enhanced both below and above the critical micelle concentration (CMC) of CTAB in aqueous solution. The pseudo-first-order rate constant (k′) shows an initial decrease for both low and high H+concentrations. With further increase in [CTAB], at low [H+], thek′attains an almost constant value, while, at high [H+], thek′passes through a maximum and then decreases. The kinetic data for catalysis by micelles of CTAB was interpreted with the pseudophase ion exchange (PIE) model. In CTAB/cyclohexane/1-butanol/water microemulsions, as the water to surfactant ratio (wo) increases, the physicochemical properties and droplet sizes of microemulsions significantly change and distinct changes in reaction environment can be marked. The rate of the hydrolysis reaction exhibits excellent correlation with the physicochemical properties and droplet sizes of the microemulsions and reverse micelles of CTAB. At [H+] = 0.001 M, in reverse micelles and microemulsions of CTAB, thek′of the acid hydrolysis of Bz decreases sharply followed by a slight increase with increasingwo.

Photoinduced electron transfer reactions of ruthenium(II) phenanthroline complexes with dimethylaniline in aqueous and micellar media.

Four [Ru(NN)(3)](2+) complexes (NN = polypyridine) with ligands of varying hydrophobicity with different charges +2, 0 and -4 were synthesized. The photophysics and photoinduced electron transfer reactions of these Ru(II)-complexes with dimethylaniline (DMA) as the quencher have been studied in aqueous medium and ionic and non-ionic micellar medium. The extent of binding of the complexes with the surfactant interface is evident from the calculated binding constant values (K). Dimethylaniline (DMA) being a neutral quencher, the hydrophobic and electrostatic interactions competing with one another and their combined effect with the surfactants were reported by observing the quenching rate constant (k(q)) values. The formation of anilinium cation radical in transient absorption spectrum confirms the excited state electron transfer reactions of ruthenium(II) complexes with dimethylaniline. The calculated rate constant values (k(q)) are in good agreement with the experimental k(q) values giving quantitative evidence for the bimolecular reductive quenching rate constant for the complexes with DMA. Pseudophase ion exchange model is successfully applied to analyse the quenching data.

Micellar Effects upon the Alkaline Hydrolysis of Acetaminophen Prodrugs: Carboxylic and Carbonic Acid Esters

The rates of alkaline hydrolysis for four carboxylate esters (4-acetaminophenyl acetate, 4-acetaminophenyl butyrate, 4-acetaminophenyl heptanoate and 4-acetaminophenyl benzoate) and three carbonate ester (4-acetaminophenyl methylcarbonate, 4-acetaminophenyl ethylcarbonates and 4-acetaminophenyl propylcarbonates) prodrugs of acetaminophen derivatives were determined in aqueous and micellar media. The cationic micelles of cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium sulfate [(CTA)2SO4] catalyzed the rate of the reaction. The anionic sodium dodecyl sulphate (SDS) and non-ionic Brij-35 surfactants inhibited the rate of the reaction. Added salt did not influence the rate of the reaction in the aqueous medium but decreased the values of observed rate constant in the micellar media. The rate enhancement in cationic micelles was treated by applying the pseudophase ion exchange model while the inhibition effect on the rate of hydrolysis was described using the Poisson–Boltzman pseudophase model. The binding constant (K s ) for the acetaminophen derivatives increased with increasing hydrocarbon chain length while the values of micellar rate constant (k m ) decreased with the increase in chain length. The increase in values of K s and decrease in k m are ascribed to the hydrophobic interaction between the acetaminophen derivative and surfactant molecule and molecular orientation of the substrate in the micelles.

Effect of Urea on the Kinetics of the Alkaline Hydrolysis of Crystal Violet Catalyzed by Aqueous Micellar Solutions of Cetyltrimethylammonium Bromide

Kinetics of alkaline hydrolysis of crystal violet (CV) in micellar solutions of a cationic surfactant, cetyltrimethylammonium bromide (CTAB), both in absence and presence of urea was investigated. The reaction was catalyzed by micelles of CTAB and addition of urea caused a gradual decrease in the rate of the reaction. Kinetic results could be correlated with change in micellization behavior of CTAB by water structure breaking effect of urea. The kinetic profiles have been fitted with the simulated profiles on the basis of pseudophase ion-exchange model and best fits with reasonable parameters could be obtained.

Kinetics of the Micellar Effects on the Alkaline Hydrolysis of 2-Chloro-quinoxaline in Acetonitrile-Water Mixtures at 35°C

Pseudo-first-order reaction rate for alkaline hydrolysis of 2-chloroquinoxaline (2-CQX) is carried out in acetonitrile (AN)-water (H2O) mixtures at 35°C. Cationic surfactants as dodecyltrimethylammonium bromide (DOTAB) and an anionic surfactant as sodium dodecylsulphate (SDS) are used above their critical micelle concentration (cmc) to study the effect of micelles on reaction rate. When increasing the percentage of volume of AN, the rate profiles with DOTAB are shown to slightly increase with increasing surfactant concentration, while that with SDS are found to smoothly decrease. The micellar effect is explained in terms of a modified pseudo-phase ion exchange model. The binding constant (KS) between 2-CQX and DOTAB as micelle showed a decrease by increasing percentage of volume of AN, while that with SDS increased. The counterion micellar coverage degrees (β) are found to be 0.55 and 0.85 with DOTAB and SDS systems, respectively, at all range of volume percentage of AN. Finally, the calculated ratio between rate constants in water to that in the micelle region kw/kM at different volume percentage of AN indicated that DOTAB enhances the reaction rate while SDS inhibits it.

Micellar effect upon the rate of alkaline hydrolysis of carboxylic and carbonate esters

The alkaline hydrolysis of carboxylate (1-naphthylbutyrate) and carbonate esters (2-(methylsulfonyl)-ethyl-4-nitrophenylcarbonate) in the presence of different surfactants has been studied. The rate of hydrolysis of these esters was determined under pseudo first order condition in which the concentration of NaOH was kept in large excess over the [ester]. The cationic micelles of cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium sulfate ((CTA)2SO4) enhanced the rate of hydrolysis of esters to a maximum value and thereafter, the increasing concentration of surfactant decreased the reaction rate. The anionic micelles of sodium dodecyl sulfate (SDS) inhibited the rate of the hydrolysis. The reaction proceeds through the attack of OH− ions on the carbonyl carbon forming tetrahedral intermediate. The tetrahedral intermediate is unstable and collapses immediately to yield respective acid and alcohol. The micelles influence the stability of tetrahedral intermediate, in turn, altering the rate of hydrolysis. The variation in the rate of hydrolysis by micelles was treated by considering the pseudophase ion-exchange model and Menger–Portnoy model. The added salts viz. NaBr, NaCl, and LiCl inhibited the rate of the reaction in the presence of cationic and anionic micelles. The kinetic parameters i.e. km and Ks were determined from the rate–[surfactant] profile.

S-Nitrosocaptopril formation in ionic self-aggregation systems: effects of counterions

The reaction of the formation of S-nitrosocaptopril (NOcap) was studied in aqueous alkaline medium of surfactants resulting in the formation of micelles. The NOcap is formed in the nitroso group (NO) transfer from tert-butyl nitrite (tBN) to captopril (cap). Kinetics of first order in both tBN and cap concentration were observed and the reaction rate is independent of [OH− ]. These kinetic features indicate that the rate-limiting step is the nucleophilic attack of the thiolate ion of cap to the nitroso group of tBN. Anionic micelles of sodium dodecyl sulphate inhibit the reaction throughout all the concentration range, due to the reagents separation: tBN associates with micelles by hydrophobic effect, whereas the cap–thiolate anions are repelled from the micellar surface by electrostatic effect. Cationic micelles catalyse the reaction at surfactant concentrations just above the critical micelle concentration, but a further increase in the concentration of the surfactant inhibits the reaction. In this case, both reagents bind to the cationic micelles, but the reaction rate in the micellar pseudo phase is slower than that in the aqueous pseudo phase. In spite of the cap molecule existing in alkaline medium as a doubly charged anion, the results cannot be explained on the basis of the pseudophase ion exchange model, in part due to the high polarisability of the cap anion.

Kinetics of hydrolysis of procaine in aqueous and micellar media

AbstractThe kinetics of alkaline hydrolysis of procaine under the pseudo–first‐order condition ([OH−] ≫ [procaine]) has been carried out. N,N‐Diethylaminoethanol and p‐aminobenzoate anion were obtained as the hydrolysis product. The rate of hydrolysis was found to be linearly dependent upon [NaOH]. The addition of cationic cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DDTAB) and tetradecyltrimethylammonium bromide, and anionic sodium dodecyl sulfate (SDS) micelles inhibited the rate of hydrolysis. The maximum inhibitive effect on the reaction rate was observed for SDS micelles, whereas among the cationic surfactants, CTAB inhibited most. The variation in the rate of hydrolysis of procaine in the micellar media is attributed to the orientation of a reactive molecule to the surfactant and the binding constant of procaine with micelles. The rate of hydrolysis of procaine is negligible in DDTAB micelles. The observed results in the presence of cationic micelles were treated on the basis of the pseudophase ion exchange model. The results obtained in the presence of anionic micelles were treated by the pseudophase model, and the various kinetic parameters were determined. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 1–9, 2013

Aggregation of sodium dodecylsulfate in aqueous nitric acid medium

Nitric acid medium is invariably used for nitration of organic molecules. Although surfactants are known to influence reaction rates, little is known about the aggregation behavior of surfactants in nitric acid medium. Micellization characteristics of sodium dodecylsulfate (SDS) in aqueous nitric acid are investigated in this work by using the conductance method. The critical micelle concentration (cmc) and the aggregation number were also determined by the surface tension and the steady-state fluorescence methods, respectively. This study reveals that in acidic medium SDS exhibits both normal and unusual conductivity behaviors. Equations developed on the basis of the mixed electrolyte model, Debye–Hückel–Onsager approach, and the pseudophase ion-exchange model successfully simulate the conductivity data. The exchange of sodium and hydrogen counterions at the micellar surface has no significant effect on the cmc of SDS. Acid concentration, surfactant concentration, and cmc control the competitive binding of sodium and hydrogen counterions. Analysis of conductivity data revealed hydrolysis of about 12% SDS when [HNO3]⩾0.02moldm−3. Hydrolysis of SDS has been confirmed by nitrating some of the substituted phenols. It has been predicted that SDS+aqueous HNO3 medium with [HNO3]⩾0.02moldm−3 may be used as a green medium for nitration.