Structure of microemulsions of AOT and water in dodecane

The dielectric properties of the system water/AOT/dodecane are studied as a function of volume fraction of the dispersed phase and molar ration (water/surfactant). Data shows that the spherical model is valid only at lown values or low values. At high concentrations of dispersed phase, one has to consider micellar aggregation or deformation.     

Tris(pyrazol-1-yl)-s-triazine (TPT): X-ray crystallographic,computational, and gas-phase electron diffraction studies

The conformation of the TPT molecule has been analyzed using experimental and computational techniques. The solid-state molecular structure shows similar conformational features to those in the 2-pyrimidine and phenyl derivatives although a different pattern of bond angles in the triazine ring was observed. The AM1 calculations predicted two conformations of comparable stability (E=1.8 kcal/mol) differing in the orientation of one pyrazole ring. While the minimum energy conformation corresponds to a model displayingC _3h symmetry ( ₁= ₂= ₃=0°), the other minimum ( ₁= ₂=0°, ₃=180°) is close to that observed in the solid state. The electron diffraction results are consistent with a planar or nearly planar conformation in agreement with the preceding studies.On leave from the Depto. Química. Universidade Federal Rural do Rio de Janeiro. Itaguai (RJ) 23851 Brazil.     

Small-Angle Neutron Scattering Study of Nonionic Surfactant Micelles and Phase Transitions in w/o Microemulsion

The structure of micelles formed by a four component water-in-oil nonionic microemulsion surfactant polyoxyethene (20) sorbitan monoleate (Tween 80), sorbitan monolaurate (Span 20) at ethyl oleate and deuterated water interface have been probed by small-angle neutron scattering (SANS). The total surfactant concentration in each of the samples studied (Tween 80: Span 20) is fixed at 3:2. The deuterated water content is variable at 5–60% w/w. The experimental SANS data from all the seven samples are fit well by spherical micelles interacting with hard sphere potential. Increased deuterated water leads to spherical to lamellar and rod-like micelle geometry featured in the SANS scattering data. The observed change in micelle geometry supports the characterization of phase transition between the self-assembled micelles of the nonionic microemulsion.      

Lipase-catalyzed enantioselective esterifications using different microemulsion-based gels

Chiral esters with high optical purity have been synthesized at 298.2 K from racemic 2-octanol and alkanoic acids using the commerical lipases fromChromobacterium viscosum (CV) orCandida sp. (SP 525) immobilized in microemulsion-based gelatin gels. The microemulsions consisted of water and alkanes stabilized by the anionic surfactant sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the naturally occurring zwitterionic surfactant soybean lecithin, respectively. The enzymes were solubilized both in water-in-oil (W/O) microemulsions and in microemulsions with a bicontinuous structure. Different microstructures of the gels were chosen since the enzyme may undergo conformational changes in different environments resulting in different catalytic efficiencies toward competing substrates. Therefore, it is of great fundamental interest to know the phase behaviour and the microstructures of the used microemulsion systems. Phase diagrams were determined at 298.2 K for the systems water-hexane-AOT and ethanol/water (11)-hexadecane-soybean lecithin. The former system exhibited a large one-phase W/O microemulsion region, while in the latter a small one-phase region with bicontinuous structure was present. The kinetic enantiomeric ratios (E-values), as determined from enantiomeric excess (e.e.) values at a conversion below 0.5, were higher both in the W/O microemulsion as well as in the bicontinuous microemulsion using the SP 525 lipase, than using the CV lipase. On the other hand, the conversions were higher using gels based on W/O microemulsions (AOT stabilized) than using gels based on microemulsions with a bicontinuous structure (lecithin stabilized).     

Etude par viscosité et autodiffusion de solutions aqueuses de divers n-alkyl-benzènesulfonates

Resumé Les propriétés physico-chimiques de solutions aqueuses de différents alkyl-p-benzènesulfonates de pureté contrôlée ont été déterminées par diverses méthodes: solubilité, viscosité, autodiffusions. Les micelles formées par len-octyl-p-benzènesulfonate de sodium (C₈ SO₃ Na) et len-dodécyl-p-benzènesul-fornate de sodium (C₁₂ Na) sont globulaires dans tout le domaine de concetration exploité; les micelles (C₈ SO₃ Na + C₁₂ SO₃ Na) sont de taille proche de celles de C₈ SO₃ Na.La détermination du nombre de contre-ions liés aux micelles de C₈ SO₃ Na confirme cette structure.

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Summary Various methods, i. e. solubility, viscosity and self-diffusion have been employed to determine the physico chemical parameters of aqueous solutions of puren-alkyl-p-benzene-sulfonates. Micelles of sodiumn-octyl-p-benzene sulfonate (C₈ SO₃ Na) are of globular shape; mixed micelles formed by these compounds (C₈ SO₃ Na + C₁₂ SO₃ Na) show a very slight swelling compared with C₈ S03 Na micelles.The degree of counter-ion association of C₈ SO₃ Na micelles is in good accordance with a globular shape of the micelles.

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Zusammenfassung Löslichkeit, Viskosität und Selbst-Diffusion wurden benutzt, um physikalisch-chemische Parameter von wäßrigen Lösungen reinern-Alkyl-p-benzenesulfonate zu bestimmen. Mizellen von Natriumn-Oktylpbenzenesulfonar (C₈ SO₃ Na) haben eine globulare Form; Mischmizelle von C₈ SO₃ Na und C₁₂ SO₃ Na zeigen eine sehr geringe Quellung im Vergleich mit Mizellen von C₈ SO₃ Na.Der Grad von Gegenionenassoziation zu C₈ SO₃-Mizellen ist in guter Übereinstimmung mit einer globularen Form der Mizellen.

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Avec 6 figures et 7 tableaux

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The Lund Institute of Technology, Lund (Suède).

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Groupe de Dynamique des phases condensées, USTL.     

Enthalpy of dilution and heat capacity of aqueous solutions of tetrabutylammonium butyrate as a function of temperature

Enthalpies of dilution of tetrabutylammonium butyrate are reported as a function of temperature between 10° and 50°C. Heat capacities of aqueous solutions of tetrabutylammonium butyrate were measured in order to obtain values of _cp at 15°, 25°, and 35°C. These data were combined with the enthalpy of dilution data to obtain _cp as a function of molality and temperature. The apparent molal heat content _L decreased with increasing temperature in concentrated solutions but increased with increasing temperatures in dilute solutions (below0.7 m). Over the temperature range studied _cp shows a maximum as a function of molality at approximately 0.5m. The decrease in _cp with increasing concentration of hydrophobic solute is consistent with the view that the hydrophobic hydration cages, formed under the influence of the tetrabutylammonium and butyrate ions, are saturated at about 0.5m, and that at higher molalities increased overlap of the hydration cages occurs.     

Thermodynamic properties of alkali halides. II. Enthalpies of dilution and heat capacities in water at 25°C

The enthalpies of dilution and volumetric specific heats of most alkali halides were measured in water at 25°C with flow microcalorimeters in the concentration range 0.01 to 1m. Apparent molal relative enthalpies _L, derived from the enthalpies of dilution, can be represented by a parametric equation in molality. Combining _L with osmotic data, excess entropies can be calculated. Excess free energies, enthalpies, and entropies are compared at 0.5m, and the observed trends are consistent with a model of structural interactions in aqueous alkali halide solutions. The apparent molal heat capacities _C were fitted with the equation _C= _C ^° +A_C(d₀m)^1/2+B _C m. The _C ^° are, in general, additive to better than 1 J-K^–1-mole^–1 and reflect mostly the structural part of ion-solvent interactions. Taking _C ^° (H⁺)=0, conventional ionic _C ^° are obtained. The parameterB _C for different pairs of ions follows approximately the same trends as the corresponding parameterB _V for apparent molal volumes and seems to reflect structural interactions between the ions.     

Apparent molal volumes and heat capacities of some 1:1 electrolytes in anhydrous methanol at 25°C

A flow calorimeter and flow densimeter have been used to measure volume specific heats and densities of solutions of LiCl, LiBr, NaCl, NaBr, KF, KBr, Kl, CsF, and Bu₄NBr in anhydrous methanol at 25°C. The concentrations ranged from approximately 0.01m to close to saturation in some cases. Apparent molal heat capacities _cp and volumes _v have been evaluated and extrapolated to infinite dilution to obtain _cp ^o and _v ^o . Nearly all the heat capacities in methanol are negative. However, with the exception of the lithium halides and Bu₄NBr they are more positive than heat capacities of the corresponding salts in water. The dependence of the heat capacities on ionic radii is generally opposite in methanol solutions from that observed for aqueous solutions. In agreement with others, the _v ^o data indicate that electrostriction in methanol solutions is greater than in aqueous solutions.     

Phase behaviour and physicochemical properties of microemulsions with a non-ionic surfactant (IGEPAL)

The non-ionic surfactant pentaethylenglycol-4-octylphenyl ether (igepal CA-520) represents a good industrial alternative to the long-tail members of the C_iE_j family. In this paper, the phase behaviour of the microemulsion system igepal CA-520/n-decane/brine is studied in detail. An isotropic phase was found, as well as liquid crystalline and cream-like structures, depending on composition and temperature. Such structures can either form single-phase homogeneous mixtures, or coexist with other structures when phase separation takes place. Below surfactant concentration of about 20%, more complicated phase equilibria develop as temperature changes. The presence of different additives shifts the temperature ranges where the different phases exist, while keeping the general shape of the phase diagram, which agrees with the general rules for non-ionic surfactants. Complementary rheology experiments reveal a change from non-Newtonian to Newtonian behaviour during the phase transition from a lamellar phase to the isotropic microemulsion. A structure of water droplets associated in clusters can be proposed from SANS and electrical conductivity.     

Isentropic compressibilities of alcohol-water mixtures at 25°C

The sound velocities of aqueous solutions of methanol, ethanol, 2-propanol, tertbutanol and 2-butoxyethanol (BE) were measured over the whole mole fraction range at 25°C. The isentropic apparent and partial molar compressibilities, _K,S and _S,A were derived from these data. In the case ofBE, the isothermal partial molar compressibilities were also calculated. _K,S and _S,A for all alcohols except BE initially decrease slightly with the mole fraction and then increase sharply, especially with the higher members, to the value of the pure liquid. In the case of BE, _K,S and _S,A do not go through an initial minimum and the latter goes through a sharp maximum. The compressibilities of water in these mixtures are significantly lower than those of pure water itself. These data can be correlated with other properties and are consistent with the existence of microphase transitions in these aqueous organic mixtures.     

3D Monte Carlo Simulations of Diffusion Limited Cluster Aggregation up to the Sol-Gel Transition: Structure and Kinetics

Three-dimensional (3D) Monte Carlo simulations of diffusion limited cluster aggregation at different concentrations () show a crossover from a flocculation regime at short times to a percolation regime close to the gel time (t_g). Contrary to suggestions in the literature t_g is independent of the system size (L) for large L. The structural and temporal crossovers between flocculation and percolation take place at characteristic values of the cluster mass (m_c) and the time (t_c) which depend on . After normalisation by these characteristic values the crossovers are independent of except for very small clusters and at short times. The concentration dependence of m_c and t_c indicates that the crossover takes place at a given cumulated volume fraction of the clusters independent of . At low concentrations the -dependence of t_g is determined by the cluster growth in the flocculation regime.     

Diffusion–Migration Currents on Microelectrodes: Comparing Two Approximate Approaches

Two electroneutrality and constant-field solutions of the diffusion–migration problem in steady-state conditions on microelectrodes, where both approximations admit analytical solutions, are compared. Analytical equations for the potential drop across the diffusion layer ₀ in terms of RT/F and the migration factor Y are obtained for three- and four-component systems containing two and three types of electroactive ions, respectively, and one type of ions that takes no part in the reaction. Both methods yield virtually identical ₀. The migration coefficients at large absolute values of ₀ noticeably differ. The Y vs. ₀ dependences in the two approaches different. The Y values yielded by these methods are close only at |₀| 1. For real electrochemical reactions considered, the electroneutrality condition at the limiting current is fulfilled at electrode radii >1 m and electrolyte concentrations >0.1 mM.     

Karplus-type relationships between scalar coupling constants: 3JHH molecular versus 4hJHH supramolecular coupling constants

The ³J_HH coupling constants in six H–X–Y–H systems (ethane, methylamine, methanol, hydrazine, hydroxylamine and hydrogen peroxide) and ^4hJ_HH coupling constants in four H–...XH...Y–H, namely [H₃NHNH₃]⁺ (two arrangements), HOHNH₃ and HOHOH₂ have been calculated theoretically as a function of the torsion angle . For covalent situations, the corresponding Karplus equations have been fitted to calculated ³J_HH=acos2 +bcos +c. The a, b and c terms have been analyzed as a function of the electronegativities of X and Y. In the case of ammonium/ammonia complexes (proton shared and not), water/ammonia, and water dimer the values are low (maximum 0.5 Hz) but follow closely a Karplus relationship. Supplementary material is available in the online version of this article at Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Brillouin light scattering from a biopolymer gel: hypersonic sound waves in gelatin

Rayleigh-Brillouin light scattering experiments were performed on gelatin gels in order to obtain the (hypersonic) sound velocity and sound attenuation as a function of gelatin concentration. The results show that in the high frequency regime there is a strong coupling in the gel between the dynamics of the network and that of the fluid. The network sound velocity varies with ^1/2. Sound attenuation increases with increasing gelatin concentration.     

Polymerisation of liquid crystalline phases in binary surfactant/water systems

The binary phase behaviour of two potentially polymerisable quaternary ammonium surfactants in water has been investigated. Allyldodecyldimethylammonium bromide (ADAB) a single-chain surfactant displays a conventional phase progression upon increasing concentration. Whereas the doublechain analogue allyldidodecylmethylammonium bromide (ADDAB) forms two lamellar liquid crystalline phases built from surfactant bilayers, which transform via a first order phase transition. The formation of two distinct lamellar phases and their coexistence has been evidenced by optical microscopy, small-angle x-ray scattering and D₂O deuterium quadrupolar nuclear magnetic resonance spectroscopy. The lamellar phase formed at higher surfactant compositions is a normal lamellar phase (typeL ) consisting of bilayers which are on average parallel and flat. The lower compositional lamellar phase (typeL ) in contrast may not be comprised of planar bilayers but rather aggregates having a high degree of curvature in comparison to those of theL phase. The presence of the allyl polymerisable moiety in the head group position of these surfactants has the effect of reducing the rigidity of the surfactant and increasing its solubility in comparison to nonpolymerisable analogues. Polymerisation of the surfactants was attempted by using thermal and photochemical initiation in isotropic and self-assembled systems. Polymerisation occurred to approximately 30% for DADB but did not occur for ADDAB. Where polymerisation did occur the polymer was incorporated into the monomer matrix by interweaving between the surfactant aggregates. The polymers had a molecular wieght not greater than 8000 Daltons, independent of the monomer concentration of the original solution and type of polymerisation.     

Application of the Bjerrum Association Model to Electrolyte Solutions. II. Enthalpies of Dilution in Water and Nonaqueous Solvents

The association theory based on the Bjerrum model, which had been developed for the treatment of apparent and partial molar volumes of electrolyte solutions, is extended to apparent molar relative enthalpies L _2,. Experimental enthalpies of dilution for tetrabutylammonium bromide in acetonitrile, propylene carbonate, and -butyrolactone and for lithium perchlorate and sodium thiocyanate in acetonitrile were obtained and analyzed with this model. Literature data for various electrolytes in water, acetonitrile, and n-propanol were also reanalyzed. Through the Bjerrum equations, enthalpies of dilution can be extrapolated to infinite dilution and reliable L _2, obtained for associated electrolytes. The model can be used to estimate the association constant K _A of the electrolyte and these K _A are compared with literature values (generally obtained from conductivity). Considering the difference in the concentration ranges investigated in L _2, and measurements, K _A extracted from L _2, generally fall within an expected range of deviation from the ones obtained by conductivity, provided that no specific interactions are present in solution.     

Mathematical structure of chromatographic optimization based on information theory: I. Effect of mobile phase composition on precision

Summary The influence of mobile phase composition X on the precision of liquid-chromatographic analysis is interpreted in terms of the derivate of the mutual information for peak j with respect to X, d_j/dX. The sign and magnitude of d_j/dX depend on the operational conditions of X or on the details of chromatograms (e.g., resolutions R_s), but d_j/dX always indicates the direction of X toward more information _j. The sensitivity function s_j(=1/(k_j+1)) is examined on the basis of information theory. Optimization is formulated as a nonlinear programming problem.     

Apparent molar heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Apparent molar volumes V and heat capacities C_p, of NaCl, KCl, KNO₃, AgNO₃, KI, NaBPh₄ and Ph₄PCl have been measured in acetonitrile (AN)-water mixtures up to x_AN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to x _AN =0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption _tX(PH₄P⁺) = _tX(BPh₄-) where X=V or C _p and _tX is the change in X for a species on transfer from H₂O to AN-H₂O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, _tX for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H₂O solutions. _tV and _tC_p, for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag⁺ and AN.     

The dependence of build-up233U,232U,233Pa and fission products from ThO2 irradiated in HFETR on integral thermal neutron fluxes and neutron spectra

In this paper the dependence of build-up²³³U,²³²U,²³³Pa and fission products from ThO₂ irradiated in HFETR on integral thermal neutron fluxes and neutron spectra have been investigated. The yields of all above nuclides in ThO₂ increase with the increase of integral thermal neutron fluxes at different neutron spectra. The values of²³³U/²³²Th increase with the increases of _th and decreases with the increase of fast/thermal neutron ratios (_f/_th). The values of²³²U/²³³U increase with the increase of both _th and _f/_th ratio. The amount of fission products relative to original irradiated thorium decreases with the increase of _f/_th ratios. These results could be used to evaluate the behaviour of thorium-based nuclear fuel in reactor.     

Apparent molal heat capacities of transfer from H2O to D2O of tetraalkylammonium bromides

The volumetric specific heats (in J-K^–1-cm^–3) of tetraalkylammonium bromides (R ₄ NBr) have been measured at 25°C in the concentration range 0.02 to 0.4 aquamolal in H ₂ O and D ₂ O with a differential flow microcalorimeter. The apparent molal heat capacities _c, calculated from the specific heats and known densities, were fitted with the equation _c= _c ^o +A_cm^1/2+B_cm whereA _c is the Debye-Hückel limiting slope andB _c is an adjustable parameter.The standard heat capacity of transfer C _ptr ^o = _c ^o (D ₂ O- _c ^o (H ₂ O) of R ₄ NBr is positive forR equal ton-propyl andn-butyl and negative for methyl and ethyl. Except for Me ₄ NBr in H ₂ O, allB _c are negative and become more so as the size of the cation increases;B _c is usually more negative in D ₂ O. These results can be interpreted with a two-state model for water and show that a positive C _ptr ^o is evidence that the solute is an overall structure maker, while a negative value indicates a net structure breaker. The negativeB _c is consistent with the existence of strong solute-solute structural (mostly hydrophobic-hydrophobic and hydrophobic-hydrophilic) interactions in the solution.