Colloids and Surfaces最新文献

The formation of interfaces and emulsions in systems comprising perfluoromethyldecalin (PFMD) and non-fluorinated oils with different polarities was investigated. The measured interfacial tension between the pure liquids was quantitatively interpreted in the framework of the Fowkes' model. The interfacial activity of several kinds of non-fluorinated, perfluorinated, ionic and non-ionic surfactants was explored. The perfluorinated non-ionic compound perfluoroalkyi-polyoxyethylene (PFPE) proved to be most effective as a surfactant in these systems. The critical micelle concentration and the areas per molecule at interfaces with non-fluorinated oils were determined for this substance. In some cases where fluorinated alcohols were used as co-surfactants, ultra-low interfacial tension and microemulsion formation were observed. The stability of the emulsion using PFPE as a surfactant was also investigated. It was shown that the higher polarity of the non-fluorinated oils led to higher emulsion stability. The addition of fluorinated alcohols decreased the stability and reversed the macroemulsion type. The observed effects are discussed from the viewpoint of the chemical structure of the compounds.

The photo-oxidation of surfactants catalyzed by a TiO₂ semiconductor, which is one of the methods suggested to solve the problem of water pollution, is reviewed on the basis of our earlier reported experimental results. The photodegradation rates of different kinds of surfactants and of various moieties (aromatic, oxyethylene and alkyl groups) in the surfactant structure are discussed. A possible photo-oxidation mechanism at the TiO₂/water interface is proposed.

The phase behaviour of different non-ionic surfactant/non-ionic polymer/water systems has been studied. The surfactants investigated are tetraethylene glycol mono(n-dodecyl) ether (C₁₂E₄) and octaethylene glycol mono(n-dodecyl) ether (C₁₂E₈), and the polymers chosen are a linear random copolymer of ethylene oxide and propylene oxide (UCON) and ethyl hydroxyethyl cellulose (EHEC); all components display a lower consolute curve. The phase behaviour is strongly dependent on the distances from the phase separation limits of the two components, except that the phase behaviour of systems with a given polymer is similar. However, on changing the polymer, a marked change in phase behaviour is displayed. For systems with UCON, the miscibility between the surfactant and the polymer is of crucial importance in determining the phase behaviour. The polymer and the surfactants simply act as a preferred solvent for each other compared to water. In this case, the cloud points of the ternary mixtures depend essentially on the component with the lowest solubility, i.e. UCON in the C₁₂E₈/UCON/water mixture and C₁₂E₄ in the C₁₂E₄/UCON/water mixture. When the component with the higher solubility (C₁₂E₈ in C₁₂E₈/UCON/water and UCON in C₁₂E₄/UCON/water) is substituted for water, the lower consolute boundary shifts upwards. In systems with EHEC, however, the lower consolute boundaries pass through a minimum on adding surfactant to a binary EHEC—water solution, which could be attributed to an attractive interaction and the formation of a strong complex between the polymer and the surfactant.

Model polypeptides with amino acid sequences occurring in the components of clupeine and salmine were solubilised in reverse micelles of the system sodium bis(ethylhexyl) sulfosuccinate (AOT)—H₂O—iso-octane at [H₂O]/[AOT] ratios, w₀, of 4 and 10 for studying their conformation by circular dichroism (CD). The CD spectra were run between 200 and 300 nm. In the case of (Arg—Ala)_n the CD spectra obtained are typical of those for the α helix whereas those of (Arg—Gly)_n with n ≈ 15 are like random-coil spectra. The CD spectra of the sequential polypeptide (Arg₃—Gly)_n are typical of the α helix, in contrast to (Arg₃—Pro)_n. In the series (Arg_m—Ser)_n with m = 1, 2, 3, 4 and n = 15, 8, 5, 3, only the polymer with m = 2 appears to exist in the α-helical conformation.

It is concluded that the protamines studied attain an α-helical conformation only in the C-terminal region with an extended conformation in the central and N-terminal regions.

Bimetallic particles of platinum and palladium have been prepared by reduction, with hydrazine at arabient temperature, of H₂PtCl₆ and PdCl₂ dissolved in microemulsions consisting of water, hexadecane and pentacthylene glycol dodecyl ether. The particles were deposited onto Al₂O₃. Examination by scanning transmission electron microscopy and extended X-ray adsorption fine structure measurements of particles with atomic ratio Pt:Pd = 35:65 showed that the particles are true Pt/Pd alloys. Indirect evidence was also obtained that particles containing only 4% Pd are alloys. The diameter of the particles ranges from 10 to 100 nm; there is evidence that they are formed by aggregation of particles in the diameter range 2–5 nm.

The catalytic properties of the Pt/Pd/Al₂O₃ catalysts prepared from microemulsions were similar to those of catalysts prepared by conventional impregnation. Catalysts with low Pd content had the same selectivity in the isomerization of 2-methylpentane and hydrogenolysis of methylcyclopentane as a highly dispersed Pt catalyst (or a pure Pd catalyst). The catalytic cracking of 2-methylpentane indicates that the particle surfaces behave as Pt surfaces up to a Pd concentration of 60% in the particles. Isomerization of hexanes is dominated by the cyclic mechanism even at Pd concentrations as low as 4%; for these very low Pd concentrations hydrogenolysis of methylcyclopentane is the same as observed for very small Pt particles. It is concluded that Pd is enriched in the particle surfaces in such a way that they consist of Pd and some highly dispersed Pt. even at bulk Pd concentrations as low is 4%.

Molecular aggregation states of sodium and zinc 10-undecenoates in aqueous systems with different water contents were investigated in relation to their polymerizabilities. The aggregation states were examined by thermal analysis, and γ-ray-initiated post-polymerization in various states was monitored by IR spectral changes. In the case of sodium 10-undecenoate, coagel and gel states with lamellar structures and a liquid crystal state consisting of a regular hexagonal array of cylindrical aggregates of the molecules can appear, depending on the temperature in the range of water contents of 30–60%. However, zinc 10-undecenoate exhibits coagel, gel-I, gel-II and lamellar liquid crystal states in a wide range of water contents. The polymerizability of the sodium salt increased in the order coagel, gel and liquid crystal with a hexagonal structure, whereas the zinc salt polymerized most effectively in the gel-I phase with a regular lamellar structure. It is concluded that the regular arrangement of the monomer molecules together with conformational freedom is an important factor for the polymerization.

The charge distribution in the surface region and the isoelectric point of guinea-pig polymorphonuclear leucocytes were investigated on the basis of their electrophoretic mobility data at different pH values and ionic strengths of the medium. As a result, it was shown that the distribution of the negative charges arising from the acidic groups is fairly uniform while the positive charges arising from the basic groups increase with increasing inward distance from the boundary of the surface region and the medium, and that the isoelectric point is dependent on the ionic strength of the medium. The dependence of the isoelectric point on the ionic strength was found to decrease after the cells were treated with neuraminidase.

Solution properties or mixed fluorocarbon—hydrocarbon surfactant systems have been investigated with dynamic and static light scattering and with fluorescence-probe measurements.

The systems are mixtures of hexadecylpolyoxyethylene ether (C₁₆POE₂₀) with ammonium perfluorooctanoate (APFO) and/or with ammonium perfluorodecanoate (APFDe). When the concentration of fluorocarbon surfactant increases, the mutual diffusion coefficient and the second virial coefficient of mixed micelles increase, but the micelle molecular weight and the micropolarity decrease. The mutual diffusion coefficient and the second virial coefficient of mixed micelles in C₁₆POE₂₀/APFDe are larger than those in the C₁₆POE₂₀/APFO system, but the micelle molecular weight of C₁₆POE₂₀/APFDe is smaller than that of C₁₆POE₂₀/APFO.

It is found that C₁₆POE₂₀, a hydrocarbon non-ionic surfactant, mixes well with APFO and/or APFDe, a fluorocarbon anionic surfactant, and the mixed micelle is formed more easily with the surfactant containing a longer fluoroalkyl chain (APFDe) than with that having a shorter fluoroalkyl chain (APFO).

Mixed solutions containing sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) have been investigated by tracer self-diffusion (Na⁺ and Cl⁻ ion diffusion) and potentiometry with Br⁻- and DTA⁺-specific electrodes, with particular emphasis on the binding of counter-ions to the mixed micelles in the SDS-rich and DTAB-rich isotropic systems. At DTAB mole fractions (X) between 0 and 0.38 it has been shown that no Br⁻ ion binding occurs, whereas all added DTA⁺ ions are incorporated in the mixed micelles. In the DTAB-rich systems, all added DS⁻ ions are incorporated into mixed micelles, and the degree of Br⁻ ion binding decreases upon increasing added SDS. However, it remains well above zero at the upper precipitation boundary (X = 0.88), in contrast to the degree of Na⁺ ion binding which tends toward zero at the lower precipitation boundary (X = 0.38). This difference in counter-ion binding in the two composition ranges correlates well with the reported changes of micelle size in these ranges. Finally, in the range 0.32 < X < 0.38, which corresponds to turbid but stable systems, the self-diffusion data suggest the existence of vesicles and/or more-complex surfactant assemblies. Direct examination of the solutions by phase-contrast light microscopy and cryo-transmission electron microscopy (cryo-TEM) has confirmed this conclusion, and has shown the existence or highly polydisperse, sometimes open, vesicles which are spontaneously formed in solutions of mixtures of SDS and DTAB at X = 0.34. Cryo-TEM also shows that some thread-like micelles coexist with the vesicles at X = 0.34. At X = 0.30 the micelles are mostly spheroidal or slightly elongated. The micrographs also show some open vesicles.