The interaction of hexafluoroacetic anhydride with methane sulphonic acid and with sulphuric acid

The reaction between methanesulphonic acid and hexafluoroacetic anhydride yields methanesulphonyltrifluoroacetic anhydride.The reaction of an excess of sulphuric acid with hexafluoroacetic anhydride in a 1:1 mixture of nitromethane and dichloromethane results in the formation of trifluoroacetyl hydrogen sulphate. However, at higher concentrations of hexafluoroacetic anhydride, bis-trifluoroacetyl sulphate is formed. The rates of the reactions are dependent on the concentration of hexafluoroacetic anhydride and independent of the concentration of the acids when these are in excess. At low concentration of sulphuric acid a more complex relationship is observed.     

The melting behaviour of concentrated sulphuric acid investigated by DSC

The DSC-investigation of different concentrated sulphuric acid solutions shows that the results of this dynamic measurement method are in good agreement with those of steady state methods [1]. The diverse DSC-melting peaks can be interpreted by using the known behaviour of the system SO₃/H₂O [1] as the melting of different congruent melting phases or eutectics, respectively. The observed melting enthalpies are in the correct order.

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Zusammenfassung DSC-Untersuchungen an Schwefelsäurelösungen verschiedener Konzentration zeigten, daß die Ergebnisse dieses dynamischen Meßverfahrens mit den Ergebnissen von Gleichgewichtsverfahren in guter Übereinstimmung stehen. Die verschiedenen DSC-Schmelzpeaks können unter Zuhilfenahme des bekannten Verhaltens des Systemes SO₃/H₂O, nämlich mit dem Schmelzen unterschiedlicher kongruenter Schmelzphasen bzw. Eutektika erklärt werden. Die ermittelten Schmelzenthalpien liegen in der richtigen Größenordnung.

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, , . , SO₃/H₂O, . .

     

Structure formation of surfactants in concentrated sulphuric acid: a light scattering study

A common cationic surfactant,n-hexadecylammonium hydrogensulphate, dissolved in concentrated sulphuric acid, has been studied by static and dynamic light scattering. Micelle formation has been observed even in this unusual solvent. An apparent molar mass of 45 500±4.5% was found for the aggregates. A translational diffusion coefficientD ₀=5.5×10^–9 cm²/s was measured which gave a hydrodynamically effective radius ofR _h=17.7 nm. The geometric radius of gyration wasR _g=76.2 nm. The ratioR _g/R _h=4.33 is indicative for rodlike structures. Assuming a polydispersity ofL _w/L _n=2 this corresponds to a cylinder ofL _w=152 nm. An axial ratiop _w=(L _w/d)=60.4 nm was estimated which leads to a cylinder diameter of 2.53 nm. At surfactant concentrations higher than 5% (w/vol) the rod-like micelles aggregate to form more globular structures. The time correlation function, recorded by dynamic light scattering, exhibited a two-step decay which indicates a bimodal distribution of particle sizes. The fast motion coincides with that of the micelles at low concentrations while the other is slower than the fast one by three orders of magnitude and corresponds to the translational motion of large clusters.     

Micellization and interfacial behavior of binary and ternary mixtures of model cationic and nonionic surfactants in aqueous NaCl medium

Mixed micelle formation and interfacial properties of aqueous binary and ternary combinations of hexadecyltrimethylammonium bromide (C(16)Br), hexadecylbenzyldimethylammonium chloride (C(16)BzCl) and polyoxyethylene (20) cetyl ether (Brij58) at 25 degrees C in 30 mM aqueous NaCl have been studied in detail employing tensiometric and fluorimetric techniques. The micellar and adsorption characteristics like composition, activity coefficients, mutual interaction parameters and free energy of micellization have been estimated using the theoretical approaches of Clint, Rosen, Rubingh, Blankschtein et al., Rubingh-Holland and Maeda. A comprehensive account of the comparative performance of these models on the selected cationic-cationic-nonionic surfactant mixtures at constant ionic strength has been presented. The Blankschtein model predicted lower synergism than from Rubingh's method because it neglects the contribution due to steric interaction between surfactant head groups of different sizes and charges. Free energy of micellization calculated using Maeda's approach, which employs interaction parameter and micellar mole fraction from Rubingh's model as inputs, shows good correlation with that calculated from commonly used phase separation model. The present study also reveals that the modified Rubingh-Holland method along with the Rosen's model can be applied to analyze the interfacial characteristics of ternary surfactant mixtures with a fair degree of success thereby widening the domain of applicability of this model.     

Micellization and interfacial properties of alkyloxyethylene sulfate surfactants in the presence of multivalent counterions

Alkyloxyethylene sulfates are a special class of surfactants that are unusually stable in the presence of multivalent counterions and are not as prone to precipitation as anionic surfactants without intermediate ethoxy groups in the molecule. However, formation of micelles, their structure, and the properties of monolayers of these surfactants exhibit very interesting and sometimes unexpected properties depending on the nature of the ions dissolved in the solution. This paper presents a brief overview of our recent efforts to reveal the nature of these properties, including some new results. We show that the strong binding of multivalent (and particularly trivalent counterions) triggers a sphere-to-cylinder shape transition of the micelles and facilitates their further growth, even at very low ionic strength. The properties of surfactant monolayers are coupled to those of the micelles in the bulk and are governed also by multivalent counterion binding. The effect of multivalent counterions on the aggregation and structure formation in anionic surfactant solutions has both fundamental and practical importance.     

Polarographic determination of nitrate in concentrated sulphuric acid solutions

The possibility of the polarographic determination of nitrates in the presence of hydroquinone in 65–95% sulphuric acid solutions was investigated. It has been found that nitrates in 95% acid have one polarographic wave without inflection point the height of which is proportional to nitrate concentration. In 85% sulphuric acid and in other acids investigated with lower concentrations, nitrates are not polarographically active. If both nitrate and hydroquinone are present in H₂SO₄, two waves are observed on the polarogram. The more negative wave is completely defined. The height of this wave is proportional to nitrate concentration in the range 5×10^?5?5×10^?3M.     

Hydrolytic thermal degradation of polyamidobenzimidazole in concentrated sulphuric acid

Thermal analysis has been carried out on the process of degradation of polyamidobenzimidazole in solution in 97.8% sulphuric acid for 0.2 to 200 hr at 20 to 110°. Hydrodynamic and optical methods were used to establish that the degraded macromolecules exhibit the same equilibrium rigidity and optical anisotropy of unit length as the undegraded molecules of polyamidobenzimidazole of various molecular weights. Thermal degradation proceeds according to the law of probability. The activation energy of degradation, 134 kJ/mol, coincides with that for low molecular weight amides. Hence, the main degradation mechanism is the breaking of the macromolecules at the amide bonds. This phenomenon can be used for analytical purposes to achieve a controlled decrease in the length of the macromolecules. In 112.5% oleum, thermal degradation proceeds much faster and the activation energy is lower by a factor of two-three.

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Zusammenfassung Die Thermoanalyse der Zersetzung von Polyamidobenzimidazol wurde in Lösungen von 97.8% Schwefelsäure während 0.2 bis 200 h bei 20 bis 110° durchgeführt. Hydrodynamische und optische Methoden wurden eingesetzt um festzustellen, daß die abgebauten Makromoleküle dieselbe Gleichgewichtsrigidität und optische Anisotropie der Längeneinheit aufweisen wie die nicht abgebauten Moleküle von Polyamidobenzimidazol verschiedenen Molekulargewichts. Die thermische Zersetzung verläuft nach dem Gesetz der Wahrscheinlichkeit. Die Aktivierungsenergie der Zersetzung, 134 kJ/mol, stimmt mit den der Amide niedrigen Molekulargewichts überein. Folglich besteht der Hauptmechanismus der Zersetzung in der Spaltung der Makromoleküle in den Amidbindungen. Dieses Phänomen kann für analytische Zwecke verwendet werden um eine gesteuerte Abnahme der Länge der Makromoleküle zu erzielen. In 112.5%igem Oleum verläuft die Zersetzung viel schneller und die Aktivierungsenergie ist um einen Faktor von 2 bis 3 niedriger.

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Résumé On a effectué l'analyse thermique de la dégradation du polyamidobenzimidazole en solutions 97.8% d'acide sulfurique pendant 0.2 à 200 h, à 20–110°. On s'est servi de méthodes hydrodynamiques et optiques et établi que les macromolécules dégradées montrent la même rigidité à l'équilibre et la même anisotropie optique par unité de longueur que les molécules non dégradées du polyamidobenzimidazole de divers poids moléculaires. La dégradation thermique procède selon une loi de probabilité. L'énergie d'activation de la dégradation, 134 kJ/mole, coincide avec celle des amides à faibles poids moléculaires. Par conséquent, le mécanisme principal de la dégradation consiste en la rupture des macromolécules aux liaisons amides. Ce phénomène peut être appliqué en analyse, afin de réaliser une diminution contrôlée de la longueur des macromolécules. Dans de l'oléum à 112.5%, la dégradation s'effectue bien plus vite et l'énergie d'activation est plus faible d'un facteur 2 à 3.

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97.8% 0.2–200 20–110°. , , . . 134 / . , . . 112.5% , — .

     

Vesicle-micelle transition in aqueous mixtures of the cationic dioctadecyldimethylammonium and octadecyltrimethylammonium bromide surfactants

The vesicle-micelle transition in aqueous mixtures of dioctadecyldimethylammonium and octadecyltrimethylammonium bromide (DODAB and C(18)TAB) cationic surfactants, having respectively double and single chain, was investigated by differential scanning calorimetry (DSC), steady-state fluorescence, dynamic light scattering (DLS) and surface tension. The experiments performed at constant total surfactant concentration, up to 1.0 mM, reveal that these homologous surfactants mix together to form mixed vesicles and/or micelles, depending on the relative amount of the surfactants. The melting temperature T(m) of the mixed DODAB-C(18)TAB vesicles is larger than that for the neat DODAB in water owing to the incorporation of C(18)TAB in the vesicle bilayer. The surface tension decreases sigmoidally with C(18)TAB concentration and the inflection point lies around x(DODAB) approximately 0.4, indicating the onset of micelle formation owing to saturation of DODAB vesicles by C(18)TAB molecules. When x(DODAB)>0.5 C(18)TAB molecules are mainly solubilised by the vesicles, but when x(DODAB)<0.25 micelles are dominant. Fluorescence data of the Nile Red probe incorporated in the system at different surfactant molar fractions indicate the formation of micelle and vesicle structures. These structures have apparent hydrodynamic radius R(H) of about 180 and 500-800 nm, respectively, as obtained by DLS measurements.     

Micellization and surface properties of aqueous solutions of binary mixtures of tween-80 and bromide cetyltrimethylammonium

The physicochemical properties of solutions of mixtures of cationic (cetiltrimethylammonum bromide) and nonionic (polyoxyethylene (20) sorbitan monooleate) surfactant, i.e., Tween-80, have been studied. The critical concentration of micellization, adsorption at the interface of a mixture of surfactants solution/air, and the minimum area occupied by a surfactant molecule have been determined. It was shown that, in the whole region of ratios between surfactants, sinergic effects are observed for both micellization and for the process of adsorption at the interface of a mixture of surfactants solution/air. The results obtained have been analyzed according to the regular solution theory (RST).     

Vesicle-micelle transition in mixtures of dioctadecyldimethylammonium chloride and bromide with nonionic and zwitterionic surfactants

We have investigated the effect of mixing spontaneously formed dispersions of the cationic vesicle-forming dioctadecyldimethylammonium chloride and bromide (DODAX, with X being anions Cl- (C) or Br- (B)) with solutions of the micelle-forming nonionic ethylene oxide surfactants penta-, hepta-, and octaethyleneglycol mono-n-dodecyl ether, C12E(n) (n = 5, 7, and 8), and the zwitterionic 3-(N-hexadecyl-N,N-dimethylammonio)propane sulfonate (HPS). We used for this purpose differential scanning calorimetry (DSC), turbidity, and steady-state fluorescence spectroscopy to investigate the vesicle-micelle (V-M) transition yielded by adding C12E(n) and HPS to 1.0 mM vesicle dispersions of DODAC and DODAB. The addition of these surfactants lowers the gel-to-liquid crystalline phase transition temperature (T(m)) of DODAC and DODAB, and the transition becomes less cooperative, that is, the thermogram transition peak shifts to lower temperature and broadens to disappear when the V-M transition is complete, the vesicle bilayer becomes less organized, and the T(m) decreases, in agreement with measurements of the fluorescence quantum yield of trans-diphenylpolyene (t-DPO) fluorescence molecules incorporated in the vesicle bilayer. Turbidity data indicate that the V-M transition comes about in three stages: first surfactants are solubilized into the vesicle bilayer; after saturation, the vesicles are ruptured, and, finally, the vesicles are completely solubilized and only mixed micelles are formed. The critical points of bilayer saturation and vesicle solubilization were obtained from the turbidity and fluorescence curves, and are reported in this communication. The solubility of DODAX is stronger for C12E(n) than it is for HPS, meaning that C12E(n) solubilizes DODAX more efficiently than does HPS. The surfactant solubilization depends slightly on the counterion, and varies according to the sequence C12E5 > C12E7 > C12E8 > HPS.     

Chromotropic acid-formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid

The procedure for formaldehyde analysis recommended by the National Institute for Occupational Safety and Health (NIOSH) is the Chromotropic acid spectrophotometric method, which is the one that uses concentrated sulphuric acid. In the present study the oxidation step associated with the aforementioned method for formaldehyde determination was investigated. Experimental evidence has been obtained indicating that when concentrated H₂SO₄ (18 mol l⁻¹) is used (as in the NIOSH procedure) that acid is the oxidizing agent. On the other hand, oxidation through dissolved oxygen takes place when concentrated H₂SO₄ is replaced by concentrated hydrochloric (12 mol l⁻¹) and phosphoric (14.7 mol l⁻¹) acids as well as by diluted H₂SO₄ (9.4 mol l⁻¹). Based on investigations concerning the oxidation step, a modified procedure was devised, in which the use of the potentially hazardous and corrosive concentrated H₂SO₄ was eliminated and advantageously replaced by a less harmful mixture of HCl and H₂O₂.     

Selfcondensation of some γ-keto acids in sulphuric acid

A novel type of selfcondensation of some aliphatic and aromatic γ-keto acids (capable of forming γ-lactols) in concentrated sulphuric acid is described. A mechanism for the reaction is proposed, where the key intermediates are the carbonium ion formed by loss of the OH group in the γ-lactols and the α,β-unsaturated γ-lactone with an exocyclic double bond in γ-position formed by loss of one molecule of water from the γ-lactols. The proposal is supported by deuteration experiments. Structure determinations are based on IR, PMR and MS.     

The investigation of the solvent extraction of molybdenum from concentrated solution of sulphuric acid

The solvent extraction of molybdenum(VI) from sulphuric acid solutions with di-(2-ethylhexyl)-phosphoric acid (HDEHP) and monododecylphosphoric acid (HDDP) in n-heptane has been studied (a) as a function of the concentration of sulphuric acid, molybdenum and the extractant; (b) in the presence of copper and zinc in the aqueous phase and (c) in the presence of tri-n-butylphosphate (TBP) in the organic phase. The distribution of the sulphuric acid between aqueous and organic phase has also been studied.     

Optical anisotropy and conformation of polyamidebenzimidazole in an organic solvent and in concentrated sulphuric acid

Flow birefringence, intrinsic orientation and intrinsic viscosities of polyamidebenzimidazole (PABI) and poly-meta-phenylene-isophthalamide (PPIP) molecules have been studied for a wide range of molecular weights. The optical and conformational properties of PABI molecules in dimethyl acetamide (DMAA) and concentrated sulphuric acid are compared. The conformation of PABI (kinetically rigid coil) is shown to be practically the same in both solvents. This fact indicates the practical absence of a significant role of protonization of molecules during formation of conformation of aromatic polyamides macromolecules in sulphuric acid. There is comparison of the coefficients G = [χ/g]RT/M[η]η₀ for PABI and PPIP molecules in sulphuric acid and DMAA. Values of G of 0.65 ± 0.05 for PABI and 0.30 ± 0.03 for PPIP correspond to those for rigid and flexible Gaussian coils respectively.     

A reinvestigation of the reaction between titanium(iv) and salicylic acid in concentrated sulphuric acid

It is shown that the coloured species formed between TiO(2+) and salicylic acid in concentrated sulphuric acid may be a pi-complex rather than a chelate. Similar species are formed by other hydroxybenzoic acids where chelating sites for TiO(2+) are not available. The ultraviolet and NMR spectral characteristics of hydroxybenzoic acids in water, concentrated sulphuric acid and deuterochloroform are presented as evidence for the pi-complex formation. The reaction of VO(2+) with salicylic acid in concentrated sulphuric acid is shown to be due to increasing electron-donation in the pi-component of the V-O bond of VO(H(2)O)(5)(2+) as the concentration of sulphuric acid is increased, and not to the presence of salicylic acid.     

Effect of polyacrylic acid on the corrosion behaviour of aluminium in sulphuric acid solution

Electrochemical techniques using both ac and dc as well as surface analyses approach were used to investigate the corrosion inhibition characteristics of polyacrylic acid (PAA) for pure cast aluminium in 0.5 M H₂SO₄ at 30 ± 1 °C. The effect of iodide ion additives was also studied. The results obtained indicate that PAA inhibited the corrosion of pure cast aluminium in the acid medium by adsorption onto the metal surface following Frumkin adsorption isotherm model. Inhibition efficiency increases with an increase in PAA concentration and synergistically enhanced by the addition of iodide ions. A mixed inhibition mechanism is proposed for the inhibitive effects of PAA as revealed by potentiodynamic polarisation technique. Synergism parameter evaluated was found to be greater than unity, indicating that the enhanced inhibition efficiency of PAA on addition of iodide ions was synergistic in nature. Fourier transform infrared analyses revealed that the synergistic effect of iodide ions and PAA is due to co-adsorption of iodide ions and PAA molecules which is cooperative in nature.