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Sphere-to-rod transitions of cetyltrimethylammonium (CTA⁺) micelles with dichlorobenzoate counterions are remarkably substituent dependent. Simultaneous estimates of the interfacial molarities of H2O, MeOH, and Cl^- and 2,6- and 3,5-dichlorobenzoate (2,6OBz and 3,5OBz) counterions were obtained by the chemical trapping method in mixed micelles of CTACl/CTA3,5OBz and CTACl/CTA2,6OBz without added salt. Increasing the CTA3,5OBz mole fraction produces a marked concurrent increase in interfacial 3,5OBz^- and a decrease in interfacial H2O concentrations through the sphere-to-rod transition. No abrupt concentration changes are observed with increasing CTA2,6OBz mole fraction. Counterion-specific changes in the interfacial water concentration may be a major contributor to the delicate balance of forces governing micellar morphology.

A molecular level description of the effect of urea on the stability of proteins or supramolecular aggregates such as micelles requires information about its interfacial concentration. Estimating interfacial urea concentrations has proved refractory because urea's association with interfaces is difficult to observe spectrometrically. Here we used a chemical trapping method based on product yields from spontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion to estimate the interfacial concentration of urea in micelles of cationic cetyltrimethylammonium halide (CTAX, X = Cl, Br), anionic sodium dodecyl sulfate, SDS, and sodium dodecyl sulfonate, SDSu, and zwitterionic N-hexadecyl-N,N-dimethylammonium propanesulfonate, HPS, surfactants. Up to 6−8 M added urea, the interfacial urea concentration is about 70% (HPS), 80% (CTABr), and 90% (CTACl, SDS, and SDSu) of its stoichiometric concentration in solution, that is, not much less than its concentration in bulk solution. These results are the first direct estimate of interfacial urea concentrations. We also obtained estimates of interfacial water and counterion concentrations in CTAX micelles and interfacial water and headgroup concentrations in SDSu micelles. Taken together, the results provide substantial evidence that urea destabilization of interfacial headgroup·counterion ion pairs is a major contributor to urea's effects on micellar properties such as increasing the critical micelle concentration and the degree of ionization and decreasing aggregation number.

Nitronaphthalimides are a particularly interesting group of nitroaromatics used for staining cells in hypoxia and in photodynamic therapy. The kinetics of the aromatic nucleophilic substitution of the nitro-group of 4-nitro-N-n-butyl-1,8-naphthalimide (4-NBN) by n-heptanethiol (RSH), thiophenol (ArSH) and 2-hydroxy-1-ethanethiol (HESH) in aqueous solutions containing micelles and in water–methanol mixtures were analyzed. The substitution of the nitro group by aromatic or aliphatic thiols led to fluorescent substitution products. Hexadecyltrimethylammonium chloride micelles (CTAC) increased the rate of thiolysis of 4-NBN by 1 × 10⁵ (RSH) and 4 × 10⁵ (ArSH) fold. For the reaction of ArSH with 4-NBN the calculated second-order rate constant in the micellar phase, k2m, is 1.4 times higher than that in the aqueous phase, k2w. For the reaction of ArSH with 4-NBN, k2m/k2w = 0.05. Both k2w and k2m decreased with increasing pKa of the thiol, suggesting that thiolate attack is not rate limiting in either the aqueous phase or micellar pseudophase. The high micellar rate acceleration is therefore largely due to concentration of the reagents in the micellar phase and effects on the thiol acid–base equilibrium. Copyright © 2003 John Wiley & Sons, Ltd.

The local concentrations of chloride, Clb, and bromide, Brb, in the interface of vesicles prepared with dioctadecyldimethylammonium chloride, DODAC, or bromide, DODAB, dipalmitoylphosphatidylcholine, DPPC, dimyristoylphosphatidylcholine, DMPC, and mixtures of DMPC, DPPC, and DODAC were determined by chemical trapping by analyzing product yields from spontaneous dediazoniation of vesicle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion. The values of Clb and Brb in DODAC and DODAB vesicles increase with vesicle size, in agreement with previous data showing that counterion dissociation decreases with vesicle size. Addition of tetramethylammonium chloride displaces bromide from the DODAB vesicular interface. The value for the selectivity constant for Br/Cl exchange at the DODAB vesicular interface obtained by chemical trapping was ~2.0, well within values obtained for comparable amphiphiles. In vesicles of DPPC the values of Clb were very sensitive to the nature of the cation and decreased in the order Ca²⁺ > Mg²⁺ > Li⁺ > Na⁺ > K⁺ = Cs⁺ = Rb⁺ ≥ TMA⁺. The effect of the cation becomes more important as temperature increases above the phase transition temperature, Tm, of the lipid. The values of Clb increased sigmoidally with the mol % of DODAC in vesicles prepared with DODAC/lipid mixtures. In sonicated vesicles prepared with DODAC and DMPC (or DPPC), the values of Clb reach local concentrations measured for the pure amphiphile at 80 mol % DODAC. These results represent the first extensive study of local concentration of ions determined directly by chemical trapping in vesicles prepared with lipids, synthetic amphiphiles, and their mixtures.

Phosphate esters are key compounds in important biological reactions. One family of enzymes, PTPases, catalyze the dephosphorylation of tyrosine residues from other proteins by a cystein side-chain nucleophilic attack at tyrosin phosphate. Very little is known about the intrinsic reactivity of thiol nucleophiles with phosphorus centers. To explore this important reaction, we have performed ab initio calculations on the trimethyl phosphate ester (TMP) thiolysis by (CH3S)^-. Results in the gas phase indicate that attack at TMP carbon is essentially predominant over phosphorus. Mechanisms are AnDn and exoergic for reaction at carbon and An + Dn with large activation barriers and endoergic reaction for attack on phosphorus. A trigonal-bipyramid intermediate was formed upon (CH3S)^- reaction at phosphorus and two different and competitive pathways were found for the elimination of methoxide from this intermediate. One of the elimination pathways is positioned in-line to the thiol group, as proposed in the enzymatic mechanism. If PTPases work by the same mechanism as the gas-phase reaction, these enzymes should drastically lower the activation barriers for attack at phosphorus.

Analysis of a variety of properties of supramolecular aggregates in aqueous urea supports an explanation for the urea effect that differs from traditional explanations based on the direct mechanism of urea−water solvation or the indirect mechanism via rupture of the three-dimensional (3-D) structure of water. The urea-induced effects investigated are increases in amphiphile critical micelle concentrations, ionization degrees (α), and aggregation numbers; decreases in percolation thresholds of reversed micelles; expansion of minimum areas of monolayers; increases in the radii of gyration of polyelectrolytes; changes in morphologies of sodium bis-2-ethylhexylsulfosuccinate thin films on glass substrates; and direct evidence for urea-induced reduction in ion pairing. All of these effects are attributed to an urea-induced enhancement of the hydrophilic properties of water that results in more strongly solvated polar groups and ions and a reduction in ion pair formation. The implications of this analysis for urea effects on protein 3-D structure are briefly highlighted.

Chemical trapping of bromide ions in reverse micelles prepared with hexadecyltrimethylammonium bromide, CTAB, in n-dodecane/CHCl3 and isooctane/n-hexanol has been obtained for 2,4,6-trimethylbenzenediazonium (1-ArN2⁺) and 2,4-dimethyl-4-hexadecylbenzenediazonium (16-ArN2⁺) tetrafluoroborates. Quantitative analysis of the reaction products of 1-ArN2⁺ and 16-ArN2⁺ with water and bromide ion, the corresponding phenol and bromo derivatives, and comparison with appropriate standard curves yielded the local concentrations of Br^- in the water pool, [Br]f, and micellar interface, [Br]b, in reverse CTAB micelles prepared in n-dodecane/CHCl3 and isooctane/n-hexanol. The determination of [Br]b in reverse micelles by chemical trapping with 16-ArN2⁺ can be obtained after correction for probe distribution between the reverse micelle and the organic solvent, especially in the case of n-dodecane/CHCl3. This correction was possible after demonstrating that 16-ArN2⁺, upon dediazoniation in wet n-dodecane/CHCl3, yields exclusively the corresponding bromo derivative. A Poisson−Boltzmann (PB) equation above a water/detergent molar ratio, W/S, of 14 appropriately describes the values of [Br]f. Comparison of the experimental values of [Br]b with those predicted by PB with changing W/S suggest that 16-ArN2⁺ extends from the interface 0.6−1.2 nm with increasing W/S. Both PB calculations and experimental data indicate that the degree of counterion dissociation from CTAB reverse micelles in n-dodecane/CHCl3 reaches a value of ca. 0.2 above W/S 15.

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 to the relative position of Nboc with respect to the interface.

Product yields measured by high-performance liquid chromatography from chemical trapping of Cl^-, Br^-, and H2O by an aggregate-bound arenediazonium ion in cetyltrialkylammonium halide {(CTRA)X, R = Me, Et, n-Pr, and n-Bu; X = Cl, Br}, micelles are used to estimate, simultaneously, interfacial counterion, Xm, and water, H2Om, concentrations as a function of [(CTRA)X] and tetramethylammonium halide concentrations, [(TMA)X]. The results are interpreted by using a two-site pseudophase model. Values of Xm and H2Om are estimated by assuming that when the product yields from reaction of a long-tail aggregate-bound arenediazonium ion in micelles are the same as the product yields from reaction of its short chain analogue in an aqueous quaternary ammonium ion salt solution, then Xm = [Xw] and H2Om = [H2Ow] in those solutions. The results show that Xm and H2Om are functions of headgroup size, surfactant concentration, and aqueous counterion concentration and type. Plots of Xm against [(CTRA)X] at a series of salt concentrations fall on separate curves. Xm increases gradually with added (CTRA)X and almost incrementally with added (TMA)X. However, plots of Xm and H2Om are essentially continuous functions of the aqueous counterion concentration, [Xw], at constant degree of micelle ionization, α. Three factors affect the shapes of these profiles. (a) An initial rapid increase in Xm is attributed to a salt-induced contraction of the micellar interfacial volume. (b) Above ca. 0.1 M [Xw], an incremental (slope of 1) increase in Xm with added counterion for (CTEA)Br, (CTPA)Br, and (CTBA)Br micelles is attributed to free movement counterions and co-ions between the interfacial region of the micelles (up to the micellar core) and the aqueous pseudophase. (c) Xm increases markedly for (CTMA)Br and (CTMA)Cl at their respective sphere-to-rod transitions ca. 0.1 M [Brw] and ca. 1.2 M [Clw]. The increases in Xm are accompanied by concomitant decreases in H2Om. Specific salt-induced rod formation is attributed to dehydration (partial) and tight ion pair formation between surfactant headgroups and counterions. Phase separation of (CTBA)Br micelles in 0.5 M (TMA)Br occurs when the interfacial water concentration is too low to maintain micelle stability. The dependence of Xm on [Xw] contradicts assumptions in the original pseudophase ion exchange model for aggregate effects on chemical reactivity that can be corrected, in part, by setting Xm equal to the sum the aqueous and interfacial counterion concentrations.

Products from spontaneous reaction of a short-chain, water soluble arenediazonium salt, 2,4,6-trimethylbenzenediazonium tetrafluoroborate (1-ArN2BF4), in aqueous micellar solutions of cetyltrimethylammonium halides ((CTA)X (X = Cl, Br)) are used to estimate the degree of ionization, α, and the ion exchange constant, KBr/Cl. The arenediazonium ion (1-ArN2⁺) reacts by rate-determining loss of N2 to give an aryl cation that traps available nucleophiles, i.e. H2O, Cl^-, and Br^-, to give stable phenol (1-ArOH) and halobenzene products (1-ArCl and 1-ArBr), respectively. Product yields are determined by HPLC from calibration curves obtained from independently prepared standards. Reproducible yields are obtained at very low halide ion concentrations, on the order of millimolar, well within the range needed to detect the “free counterions” in the aqueous intermicellar pseudophase. The basic assumption of the method is that 1-ArN2⁺ remains in the aqueous pseudophase at all (CTA)X and NaX concentrations. Trends in the Stern−Volmer constant for fluorescence quenching of Ru(bpy)3²⁺ by 1-ArN2⁺ in (CTA)Cl/NaCl solutions strongly support this assumption. The results obtained by this method are in good agreement with literature values:  α = 0.25 and 0.29 for (CTA)Br and (CTA)Cl, respectively, and KBr/Cl = 2.65 ± 0.4. Potential applications of the method are briefly discussed.

Direct and indirect mechanisms have been used to explain the effect of urea on surfactant aggregates. By studying formation properties, anion selectivities, and stabilities of cationic and zwitterionic micelles and of cationic vesicles, we confirm previous evidence in favor of the direct mechanism (Langmuir 1995, 11, 1715). Differential fluorescence suppression of 2,3-N-butylnaphthalimide by Br^- and Cl^- is used to investigate in a short time scale (nanoseconds) effects of urea on ion binding and exchange in micelles and vesicles. Enhanced degree of dissociation and decreased anion selectivity of the surfactant aggregates are observed. Micellar catalysis of the unimolecular decarboxylation of 3-carboxy-6-nitrobenzoxazol studied to investigate the effect of the additive on a long range (minutes) also shows a reduced anion selectivity. Formation properties of the thermodynamically stable (cmc) and nonstable aggregates (hydrodynamic radii and molecular weight) are measured to obtain parameters for analysis. Addition of urea caused increase in the cmc and led to increase in vesicles molecular weight and hydrodynamic radii. These results support preferential headgroup solvation in the aggregates (direct mechanism) by urea resulting in the loss, or size reduction, of the interfacial region for counterion specificity (size, polarizability, and other non-Coulombic terms) to manifest.

Products from the spontaneous reaction of a long-chain arenediazonium salt, 2,6-dimethyl-4-hexadecylbenzenediazonium tetrafluoroborate (16-ArN2BF4), in aqueous micellar solutions of sodium dodecyl sulfate (SDS), are used to estimate the local concentration of chloride and bromide ions at the micellar surface. The arenediazonium ion, 16-ArN2⁺, which is totally bound to the SDS micelle, reacts by rate-determining loss of N2 to give an aryl cation that traps available nucleophiles, i.e., H2O, Cl^-, and Br^-, to give stable phenol, 16-ArOH, and halobenzene products, 16-ArCl and 16-ArBr, respectively. Product yields, determined by HPLC, are related to local concentrations using calibration curves obtained from independent standards. The local concentrations determined by this method are consistent with co-ion concentrations calculated, using a cell model, by numerical integration of the Poisson−Boltzmann equation (PBE) taking into account salt-induced micellar growth. The salt dependence of the interfacial concentrations of Cl^- and Br^- are identical, indicating no specific interactions in the interfacial co-ion compartment. PBE calculations predict that, in micellar SDS, increasing the concentration of a particular halide salt (NaX) at constant concentration of another halide (NaY) should result in an increase in the local concentrations of both co-ions. Using this chemical-trapping method, this prediction was demonstrated experimentally.