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Ester 3-(octanoyloxy)-4-nitrobenzoic acid is a standard for the assay of the activity of phospholipase enzymes (the main toxins involved in tissue-damaging after snake bites). Because of its amphiphilic nature, it probably behaves as a surfactant but the instability of the ester bond prevents its characterization. Its ether analogue, 3-(octyloxy)-4-nitrobenzoic acid, is also an interesting compound as it is an inhibitor of the phospholipase activity and can be accepted as a model of the ester derivative. Its sodium salt, stable at basic pH, was studied by surface tension measurements, isothermal titration calorimetry, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The aggregation number, fraction of bound counterions, critical micelle concentration and thermodynamic parameters involved in demicellization were obtained. The positive value for the change in the heat capacity for demicellization indicates that a larger hydrophobic surface area of each monomer is exposed to a hydrophilic environment after dissociation. Semiempirical calculations are in agreement with DLS and TEM measurements. For several carboxylate surfactants, the plot of enthalpy vs. entropy is linear. Although the slope has been named the compensation temperature, Tc, it merely is an experimental temperature without any extra-thermodynamic meaning.

This work reports on the multi stimuli sensitivity of a cholic acid derivative in a buffer. The molecules self-organize in single walled tubules at room temperature and pH 8.0–9.5. By increasing the pH to 10.0–12.0, these tubules open up and form scrolls, which transform by aging into different tubular structures. A further evolution from scrolls or tubules into ribbons is induced by increasing the temperature. Moreover, a transition into rolled lamellae can be triggered by adding NaCl. All transitions are reversible and accompanied by drastic molecular rearrangements, manifested by spectroscopic and imaging techniques. Such rich multi stimuli responsiveness is usually accomplished by block copolymers or peptides and seldom by pure surfactants. The reported surfactant provides in addition an uncommon variety of structures. The sharpness of the spectroscopic response suggests that the system could be employed in physico-chemical sensors with a rather narrow dynamic range.

New double-chain surfactants can be designed in which nitrilotriacetic acid plays the role of glycerol in phospholipids. The synthetic methodology is applied to the synthesis of (2-(bis(2-dodecylamino)-2-oxoethyl)amino)acetic acid. This is a symmetric dimeric surfactant but the methodology allows the synthesis of asymmetric double-chain surfactants as well. The Krafft temperature is 16–18 °C. The critical aggregation concentration of the new surfactant (4.76 × 10−5 mol dm−3 in NaOH 0.01 M and 25 °C; surface tension measurements) is two hundred times lower than that for sodium 2-dodecanamidoacetate, which can be considered its monoalkyl surfactant analog, indicating the enhanced surfactant properties resulting from the presence of two hydrophobic alkyl chains. The shape of the aggregates was investigated by transmission electron microscopy. At 40 °C, the main structure present in solution corresponds to vesicles and, from the size distribution of their radii, values of (0.34 ± 0.06)kT and 56.7 ± 2.6 nm were obtained for the effective bending elasticity constant and the spontaneous radius of curvature of vesicles, respectively.

Tubules formed by self-assembly of organic molecules have vast potential for nanotechnology applications, and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, which are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is provided by means of cryogenic transmission electron microscopy and represents, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level.

The kinetics of the reaction between phenylurea (PhU) and nitrous acid in aqueous perchloric acid solution at 25 °C have been studied spectrophotometrically over the acidity range pH 2.2–4.0 (µ 0.20 mol dm–3) by the initial-rate method and over the range [H+] 0.50–3.00 mol dm–3(µ 2.0 mol dm–3, except for [H+] 3.0 mol dm–3) by the integration method. The proposed reaction mechanism for both weakly and strongly acidic media involves the rapid formation of an O-nitroso compound (I) followed by two separate reaction paths, the rate-controlling step in both cases being the loss of a proton by (I). In this step (I) is transformed to either of two conjugate bases, one of which subsequently undergoes rearrangement to secondary N-nitrosophenylurea (PhUNO) and the other to the unstable primary N-nitrosophenylurea, which a series of fast steps converts into benzenediazonium ion (PhN2+). These fast steps involve the nitrosating agent, which explains the stoicheiometry of the reaction ([PhU]:[HNO2] 1 : 2). The ratio between the formation constants of benzenediazonium ion and N-nitrosophenylurea is 0.3 in the absence of base catalysis. In keeping with the nature of the rate-controlling step, general base catalysis was found to be exerted in weakly acid media by both nitrite ion and the carboxylate anions of the buffers used, but not by halide ions.

The kinetics of N-nitrosation of thioproline have been studied under a variety of experimental conditions. The experimental rate equation obtained in the absence of nucleophiles suggests that under these conditions nitrosation initially takes place at the sulphur atom, the nitroso group subsequently being transferred to the nitrogen atom. The fact that a primary isotope effect and general base catalysis are observed, together with the experimental rate equation, implies that the rearrangement is intramolecular, and that the rate-controlling step is the loss of a proton from the nitrogen atom of the S-nitrosated intermediate. Cl–, Br–, SCN–, and thiourea all catalyse the rection via the formation of the corresponding nitrosyl compounds, which react directly with the unprotonated thioproline nitrogen atom in both the negatively charged and neutral forms of the amino acid. The observed rate constants for these reactions are discussed in terms of the nitrosating agent and the substrate. In the case of Br– and Cl– the plot of reaction rate against concentration of catalyst levels off at high values of the latter, showing that the protonated N-nitrosamine intermediate is in the steady state. The characteristics of the curve, which provide a measure of the susceptibility of the nitrosamine to nucleophiles, show that this is much less than that of aromatic amines of similar basicity.

The kinetics of the reactions between nitrous acid and the 2,4,6-trimethyl and 4-bromo derivatives of phenylurea have been studied at 25 °C in aqueous perchloric acid solution ([H+] 0.5–2.0 mol dm–3 and µ 2.0 mol dm–3). The mechanism suggested for the trimethyl derivative is completely analogous to that proposed for phenylurea itself: an initial fast equilibrium step in which the electrophile NO+ attacks the urea oxygen atom is followed by two parallel reaction paths, both of which begin with the rate-controlling loss of a proton by the protonated O-nitroso compound to produce both of its two possible conjugate bases. One of these bases undergoes internal rearrangement to the N-nitroso compound, and the other, after a series of fast steps, yields the corresponding benzenediazonium ion. This is the only final product, since the formation of the N-nitroso compound is reversible. In the case of 4-bromophenylurea the loss of proton by the protonated O-nitroso compound no longer determines the reaction rate of the second of the above reaction paths. The observed kinetics and the suggested reaction mechanism for various ureas allow a value of the order of 0.15 mol–1 dm3 to be calculated for the equilibrium constant between NO+ and substrate on the one hand and the corresponding protonated O-nitroso compound on the other. This value implies that for 2,4,6-trimethylphenylurea and phenylurea proton loss is not diffusion-controlled, which is in keeping with the general base catalysis found previously for phenylurea.

The kinetics of nitrosation of thiomorpholine have been studied under different experimental conditions. At low nitrite concentration and high acidity the reaction rate is first order with respect to nitrite and thiomorpholine and independent of acidity. The results are interpreted through a mechanism in which the S-nitroso intermediate with a deprotonated amino group must acquire a boat conformation to facilitate the migration of the NO group from the sulphur to the nitrogen atom. The mechanism is supported by the low isotope effect observed when the reaction is studied in D2O. In the pH range 2.3–3.6 and high nitrite concentration, the reaction rate is second order with respect to nitrite, the accepted mechanism being the nitrosation of deprotonated thiomorpholine by dinitrogen trioxide.