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.
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.
This work reports a Dynamic Light Scattering study on aqueous micelles formed by tetradecyl dialkylammonium propanesulfonate surfactants (sulfobetaines; with alkyl = methyl, ethyl, n‑propyl and ...n‑butyl) within a range of surfactant concentrations (0.01–0.40 M) both in pure water and in the presence of various concentrations of NaBr, NaOH and NaClO4 (0.02–0.50 M NaBr; 0.10–1.00 M NaOH; 0.005–0.50 M NaClO4). From values of diffusion coefficients, D, we obtained micellar hydrodynamic radii, Rh, by application of the Stokes-Einstein relation. Plots of D vs. sulfobetaine concentrations can be qualitatively explained with a model based on a linear interaction theory, which allowed to separate thermodynamic and hydrodynamic perturbations to D. Results show that: i) formally neutral sulfobetaine micelles become negatively charged by preferential interaction with strongly interacting, “soft” anions; ii) the surface negative charge increases with the hydrophobicity of the anions; iii) bulkier alkyl substituents on the sulfobetaine head groups lead to less charged, less hydrated aggregates, which result in opposite perturbations to D; (iv) highly hydrated, high charge density hydroxide ions lead to an increase of micellar sizes through a disc-like growth pattern.
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•Sulfobetaine micelles were investigated by dynamic light scattering.•Micelles stay spherical in water and NaBr and NaClO4 solutions.•Micellar surfaces were differentially charged by interaction with salt anions.•Perchlorate ions are depleted from water by massive uptake from micelles.•Perrin's relation was used to estimate an oblate growth in the presence of NaOH.
Mixed cationic liposomes composed by different ratios of dimyristoyl-sn-glycero-phosphatidylcoline (DMPC) and a cationic gemini surfactant have been studied by various physicochemical tools as ...vehicles for m-tetrahydroxyphenylchlorin (m-THPC), a photosensitizer used in photodynamic therapy. Entrapment and location of m-THPC within the lipid double layer have been evaluated by different techniques and the new formulations have been tested on a stabilized cell line from a human colon tumor, COLO206. A correlation between the physicochemical features of formulations and their efficiency as photosensitizers vector was found.
The interaction of surfactants with proteins in aqueous solutions has been the subject of many investigations to understand the interactions between membrane proteins and lipids, structurally similar ...to synthetic surfactants. The effect of surfactant on enzyme structure and activity is the result of chemically selective interactions that may be influenced both by the enzyme structure and by the chemistry of the surfactant. For many years, surfactants have been considered as non-specific denaturants of proteins, even if in the literature several of them are reported to enhance activity and/or stability of some enzymes: the detergent can interact with the enzyme and cause a conformational change to a more active form and/or stabilize its native folded structure. Although the surfactant head group seems to have a determining role, other structural features of the detergent are also important in influencing the catalytic properties of an enzyme, i.e. head group size and its hydrophobic/hydrophilic balance. Up to now it is very difficult to predict the molecular features of the surfactant and an extensive investigation on the relationship between the surfactant chemical structure and the catalytic properties of enzyme is still required.
The SN2 reaction of Br− with methylnaphthalene-2-sulfonate (MeONs) in water is accelerated by micelles of tetradecyldialkyl amine oxide (alkyl = methyl, n-propyl) and rates increase sharply in HBr ...due to increased binding of Br− to the protonated amine oxide. Second-order rate constants at the micellar surface are similar to those at surfaces of trialkylammonium and sulfobetaine micelles. The reaction of OH− with MeONs is weakly inhibited by amine oxide micelles, showing that dispersive, as well as coulombic and charge-dipole, forces play a major role in the association of ions with surfaces of micellar aggregates.
The activity and stability of beef liver catalase have been investigated in the presence of different ionic and zwitterionic surfactants. All cationic and zwitterionic surfactants used in this work ...have no effect on the initial activity of catalase, but several of them allow the enzyme to retain a high residual activity for longer periods of time than those observed in the absence of any additives. However, the interactions between surfactants and catalase appear to be very peculiar, and certain zwitterionic surfactants have been found to remarkably slow down enzyme degradation, with the enzyme completely preserving its activity after several weeks at temperatures of up to 30 °C. This effect is probably due to an interaction between the surfactant and the intersubunit region of the protein; this interaction could stabilize the quaternary structure of the enzyme.
