This paper relates to the upgrading of model biogas mixtures, typically 60/40 CH4/CO2, by clathrate (gas) hydrates, which have recently been considered as a safe alternative to high-pressure or ...liquefied gas storage, and as an economic, chemical-free process for the separation of gas mixtures. Several factors affecting the driving force to hydrate formation are considered, such as the degree of overpressurization and the presence of chemical promoters. Promoters used were several anionic and zwitterionic surfactants which are demonstrated to affect the hydrate-forming ability of water. Some lignin derivatives were also tested. Promoted hydrates were also compared to hydrate-based separation starting from nonpromoted water. Separation experiments were conducted under pressures of 4 and 2.5 MPa at 274 K, under either pressure-dropping or constant pressure conditions. Results show that the separation ability of clathrate hydrates as determined by the separation factor S is highest when no promoters are added to the water phase; the well-known promoter sodium dodecyl sulfate (SDS) shows a value of S which is approximately half the value of that in pure water while higher separations were obtained with some lignin derivatives and a non-surface-active naphthalenesulfonate derivative. We also show that the contribution of CO2 solubility in water to S is a main player in the overall process. Finally, the separation ability of hydrates seems to be inversely proportional to the amount of gas mixture enclathrated, i.e., the occupancy.
Critical micelle concentrations (CMCs) of several surfactants in water have been determined by conductivity measurements under hydrate-forming conditions (2
°C; 40
bar methane) and under the same ...conditions with nitrogen. Control experiments were also performed at room temperature and pressure. Surfactants investigated were the anionics sodium dodecylsulfate (SDS), sodium laurate (SL), sodium oleate (SO), 4-dodecylbenzenesulfonic acid (DBSA), and the cationics dodecylamine hydrochloride (DAHCl) and dodecyltrimethylammonium chloride (DTACl). For SO, DBSA and DTACl, CMC values were found to vary slightly under hydrate-forming conditions with respect to normal P–T, whereas SDS, SL and DAHCl solutions underwent precipitation before reaching the CMC under hydrate-forming conditions. Interestingly, no micellar formation was observed for any surfactants in the concentration range where strong hydrate promotion was previously reported.
A push-pull N-methylpyridinium fluorescent dye with a pyrenyl group as the electron-donor portion was investigated within the nano-heterogeneous media provided by some micellar systems. The molecule ...was studied by stationary and time-resolved spectroscopic techniques in spherical micellar solutions and viscoelastic hydrogels, in order to throw light on the role played by twisting in its excited state deactivation. As proven by femtosecond fluorescence up-conversion and transient absorption experiments, the excited state dynamics of the molecule is ruled by charge transfer and twisting processes, which, from the locally excited (LE) state initially populated upon excitation, progressively lead to twisted (TICT) and planar (PICT) intramolecular charge transfer states. The inclusion within micellar aggregates was found to slow down and/or limit the rotation of the molecule with respect to what had previously been observed in water, while its confinement within the hydrophobic domains of the gel matrixes prevents any molecular torsion. The increasing viscosity of the medium, when passing from water to micellar systems, implies that the detected steady-state fluorescence comes from an excited state which is not fully relaxed, as is the case with the TICT state in micelles or the LE state in hydrogels, where the detected emission changes its usual orange colour to yellow.
Oxytetracycline (OX) and doxycycline (DX) are antibiotics belonging to the family of tetracyclines. We present a UV-Visible steady state and time-resolved experimental study of OX and DX and their ...biologically active Mg(2+) complexes loaded within a hydrogel matrix. Hydrogels are a three dimensional network of worm-like micelles, mutually intertwined, forming a pattern of hydrophobic domains and water pools. We resorted to a hydrogel, made of a zwitterionic N-oxide surfactant (p-dodecyloxybenzyldimethylamine N-oxide, pDoAO), which showed promising features as a drug vehicle. The spectral and photophysical properties of the drugs are significantly altered by the inclusion in the hydrophobic domains of the gel and these variations are indicators of the permeation ratio of the drug in between the micelles forming the gel network. We thus get a clear picture of the distribution of the drug molecules and metal chelates into the two different kinds of environment, where the hydrophobic domains are also able to cause a gel-induced deprotonation of these two drugs. Furthermore, the amphoteric nature of the surfactant is responsible for its peculiar acid-base behaviour: under acidic pH conditions, the surfactant gets protonated and the stability of the gel network is damaged. This feature can be thus exploited for the pH controlled release of the tetracycline drugs.
We have measured the diffusion coefficients, D, of aqueous micelles formed by cetyltriethyl-, cetyltripropyl- and cetyltributyl-ammonium bromides (CTEABr, CTPABr and CTBABr, respectively) and ...cetyltriethyl- and cetyltripropyl-ammonium hydroxides (CTEAOH and CTPAOH, respectively) by dynamic light scattering (DLS) at several temperatures from 15 to 55 °C and a range of surfactant (0.01–0.05 M) and salt (0.02–0.06 M NaBr; 0.05–0.3 M NaOH) concentrations. From values of D, we derived the respective fractional ionization values of micellar surfaces. For surfactants with bromide counterion we obtained fits of the diffusivity data using the linear interaction/DLVO approach, thus yielding estimates of the micellar hydrodynamic radius, Rh, and the micellar fractional ionization, α, which ranged from 0.26 to 0.35. For CTEAOH and CTPAOH, the fits appeared to be poorly sensitive to changes in the London-Van der Waals interactions, as expressed by the Hamaker constant, and only a large fractional ionization could account for the observed diffusivities.
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•Cetyltrialkylammonium bromide and hydroxide micelles were investigated by dynamic light scattering.•Linear inteaction-DLVO theory was adopted as a simple approach to evaluate interaction behavior of micelles.•Micelles remained essentially spherical throughout the temperature and ionic strength ranges.•Fractional ionization, α, for Br- micelles ranged from 0.26 to 0.35, with Hamaker parameters increasing with the head group bulk.•Fractional ionization for OH- micelles is much higher (0.58 to 0.65); fits are rather insensitive to the Hamaker parameter.
pDoAO oxide forms viscoelastic gel-like wormlike micellar solutions in water without additive; the system reverses to fluid when acid is added (pH
<
2).
Formation and properties of viscoelastic ...wormlike aqueous micellar solutions of the zwitterionic surfactant
p-dodecyloxybenzyldimethylamine oxide (
pDoAO) were studied. Semi-dilute aqueous solutions of
pDoAO show a sharp increase in viscosity, which exceeds 160 cST for concentrations >50
mM, leading to viscoelastic solutions. Viscoelasticity relates to the surfactant charge type. In fact this viscoelastic system reverses to fluid when acid is added (pH
<
2), which changes the system to cationic. Under acidic conditions the system resembles solutions of the similar cationic surfactant
p-dodecyloxybenzyltrimethylammonium bromide, (
pDoTABr) in terms of viscosity. Properties of aqueous solutions of
pDoAO were investigated by dynamic light scattering (DLS), rheology and small angle neutron scattering (SANS). Data support the idea that small micelles grow in length (wormlike or threadlike micelles) as surfactant concentration increases and viscoelastic solutions form as micelles become entangled. The micellar diameter as calculated by different techniques is about 5
nm.
•We run an experiment to test the traveler's dilemma using individual and group data.•The size of the penalty/reward did not affect subjects’ choices in the first-period.•The size of the ...penalty/reward affects subjects’ behaviour in the repeated game.•Groups behave more rationally: they were always closer to the Nash equilibrium.•The reference point did not encourage coordination around the Pareto optimal choice.
We provide an experimental test of the traveler's dilemma using individual and group data. Our investigation aims to assess whether individual decisions differ significantly from group decisions. Experimental findings reported in this paper show that: (1) groups are always more rational – i.e. their claims are closer to the Nash equilibrium; (2) the size of the penalty/reward influences convergence to the equilibrium both when decisions are taken individually or in groups; and (3) groups are more sensitive to the size of the penalty/reward.
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•Kinetic micellar effects on SN2 reactions were investigated in the range 25–45°C.•The kinetic data were quantitatively analysed by the pseudophase model.•The activation parameters ...were evaluated from the Eyring equation.•The nature of halide ion influences the kinetic temperature effect in micellar media.•The surfactant head group size influences the temperature effect in micellar media.
Reactivity of methyl naphthalene-2-sulphonate, MeONs, with H2O, Br− and Cl− in water and in cationic micelles of cetyltrialkylammonium surfactants, n-C16H33N+R3X−, R=Me (CTA+), n-Pr (CTPA+), X=Br, Cl, CH3SO3, has been investigated in the temperature range 25–45°C. Micellar rate effects were analysed by using the pseudophase treatment, and the second-order rate constants in the micellar pseudophase were evaluated at the various temperatures. Values of these rate constants increase with temperature, and the effect is less important in micelles than in water and more important for chloride than for bromide ions. Micelles lead to an ion behaviour discrimination, whose extent depends on surfactant type and on temperature, with maximum effect in CTPA+ at 25°C and with bromide being always more reactive than chloride.
Quantitative analysis of the temperature effect by the Eyring equation showed that micelles speed up reaction of MeONs with halide ions by decreasing the activation enthalpies, which is partially offset by decreases in the activation entropies. The rate acceleration by increase in surfactant head group size has only enthalpic origin for bromide and only entropic origin for chloride: this different behaviour was rationalised taking into account both solvation of anions and the hydrophobic effect.
Ionic liquids based on 1,3-dialkylimidazolinium methanesulfonate have been used as effective reusable reaction media in the esterification of several carboxylate sodium salts with different alkyl ...halides. Products are easily isolated by extraction with ether, and the protocol is mild and green, compared to the existing methods based on toxic solvents. Proper ‘design’ of the ionic liquid allows us to obtain esters always in quantitative yields.
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Ionic liquids based on 1,3-dialkylimidazolinium methanesulfonate have been used as efficient reusable reaction media in the esterification of several carboxylic acids with alkyl halides catalyzed by ...fluoride ions. The method has wide applicability, and it is mild and green; it is useful for the protection of acids, via ester formation, for alkali labile molecules.
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