Colloids and Surfaces最新文献

The electrokinetic sonic amplitude (ESA) of aqueous suspensions of a poly(methyl methacrylate) latex and of the colloidal silica, Ludox®-TM, is reported as a function of particle volume fraction. The use of these readily obtainable materials as standards for the calibration of ESA apparatus is described. Such calibration procedures facilitate the conversion of ESA measurements to electrophoretic mobilities. We report results obtained, in parallel experiments, with two separate instrumentation systems in two of our respective laboratories. The precision of these techniques appears competitive with other classical electrophoretic methods.

The adsorption of ABA block copolymers, where A is poly(ethylene oxide) (PEO) and B is poly(propylene oxide) (PPO), on carbon black was investigated at room temperature (20 ± 2°C). Two series of block copolymers were studied: series A containing 56 PO units and 16–148 EO units per chain; series B containing 30 PO units and 30 or 77 EO units per chain. The results showed a decrease in the amount of adsorption Γ (μmol m⁻²) with increase in the ethylene oxide (EO) chain length. Adsorption increased as the number of propylene oxide (PO) units in the chain decreased. The area per molecule σ showed a linear increase with n^{$\text{1}\text{2}$}, where n is the number of EO units, indicating that adsorption is governed by the size of the PEO chain.

Concentrated dispersions of carbon black that were stabilised using the block copolymers were studied using steady state (shear stress—shear rate) and oscillatory measurements. The relative viscosity—volume fraction φ curves were compared with the theoretical curves for hard-sphere dispersions (calculated according to the Dougherty—Krieger equation) to obtain the adsorbed layer thickness as a function of φ. Storage modulus G′-φ curves showed a rapid increase above a critical volume fraction φ_cr for the carbon dispersions. With the ABA block copolymer containing the shortest EO chain (16 EO units per chain) φ_cr was significantly lower than the values obtained with the other block copolymers, indicating weak flocculation of the suspension. All other block copolymers stabilised the carbon dispersions, and φ_cr decreased with increase in the EO chain length, as expected.

The critical flocculation temperature (CFT) of carbon dispersions, stabilised using a block copolymer containing 56 PO units and 37 and 148 EO units per chain, was measured as a function of K₂SO₄ concentration using rheological measurements. The results showed that for both polymers the CFT is lower than the θ temperature of the PEO chain at the same electrolyte concentration. This indicated that the block copolymers were not as effective for stabilisation of carbon black dispersions as the previously studied ABC surfactants containing the nonylphenyl group in addition to the PPO chain, indicating that the nonylphenyl group plays a major role in anchoring the chain to the carbon surface.

The linear stability of a liquid film is examined, when its thickness is time dependent, for the case of a head-on collision of two bubbles. An analytical expression for surface perturbations is given for long waves. For symmetric disturbances, the amplification factor remains less than unity if the wavelength is shorter than a critical value. For antisymmetric disturbances, the film is unstable.

Surfactant adsorption studies carried out with surfactants suitable for mobility control by foams in gas-flooding enhanced oil recovery are described. The extent of adsorption of three surfactants (an anionic surfactant, a betaine, and a sulfobetaine) is measured on four types of rock representing reservoir materials (berea sandstone, Indiana limestone, Baker dolomite and quartz) from three brines (a sodium chloride solution and two synthetic reservoir brines with different concentrations of total dissolved solids). This allows the assessment of the effects of surfactant type, rock type, brine salinity and the presence of divalent cations on surfactant adsorption. In order to probe into adsorption mechanisms, an attempt is made to correlate adsorption levels with rock surface charges.

Adsorption of the anionic surfactant on sandstone and dolomite is significantly lower than that of both amphoterics, while adsorption of the amphoterics on limestone is similar to or lower than adsorption of the anionic surfactant. The presence of divalent cations increases the adsorption of the anionic surfactant and the betaine on sandstone and limestone, but has very little effect on the adsorption of the sulfobetaine on all rocks, or on the adsorption of all three surfactants on dolomite. The effect of the total dissolved solids concentration in the brine on adsorption is different for different surfactant/rock combinations. A correlation of surfactant adsorption with rock surface charge shows that adsorption of the anionic surfactant is consistent with an electrostatic mechanism of adsorption. Adsorption of both amphoteric surfactants occurs through a complex interplay of mechanisms, some of which can be suggested from the observed trends.

The effects of free polymers on the physical properties of lamellar liquid-crystalline dispersions as model systems for active structured liquid detergents have been investigated. Addition of free polymers to lamellar dispersions may lead to osmotic compression (decrease in the volume fraction or lamellar phase), depletion flocculation and fusion of lamellar droplets. Osmotic compression will occur if the size of the polymer coil (e.g. molecular mass) is too large with respect to the lamellar water layer thickness. Depletion flocculation will occur, as found for normal dispersions, at lower polymer concentrations if the molecular mass of the polymer increases. Fusion of lamellar droplets, which results in an increase in lamellar droplet size, occurs at polymer concentrations lower than the polymer concentration at which depletion flocculation is observed. Extensive fusion is observed well above the critical polymer concentration for depletion flocculation. Osmotic compression and fusion result in a decrease in the viscosity of the lamellar dispersions and allow for concentration of the products based on lamellar dispersions while retaining pourability. Depletion flocculation results in physical instability and increased viscosity of the liquid detergents and has to be avoided.

The surface tension of an aqueous solution of a dodecylammonium chloride (DAC) and octylsulfinylethanol (OSE) mixture was measured as a function of the total molality of surfactants and the mole fraction of OSE in the solution. By analyzing the experimental results using thermodynamic relations developed previously, a phase diagram of adsorption representing the equilibrium between the adsorbed film and solution was obtained at a given surface tension. At a low surface tension, the mixture was found to form a negative azeotropic mixed film. It was also observed that the phase diagram of micelle formation has a distinct minimum. Such behavior was attributed to the attractive interaction between DAC and OSE molecules in the oriented states. Further, the molecular interaction was found to be more attractive in the micellar state than in the adsorbed state.

For the ¹H NMR spectrum of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in C₆D₆D₂O, the very complex H3′ signals, which have not yet been elucidated, have now been successfully explained by superimposition of the four AB parts, and have been discussed in connection with the diastereoisomers arising from the three asymmetric carbon atoms.

Discussion of the ¹³C NMR signal assignment of AOT in organic solvents has previously been focused on the signals of the head group (CH(SO⁻₃)CH₂) and on the CH₂OCO and OCH₂CH segments. In this study, the signals arising from the other segments of the two chains have been completely assigned to the individual carbon atoms of each 2-ethylhexyl chain, using pulse techniques.

The secondary splitting of the ¹³C chemical shifts was due to the chirality effect of an AOT molecule. In particular, the effect of the chiral centers was found to be concentrated on the α chain and to reach the C6′carbon.

A new method is proposed for the measurement of specific surface area of microporous carbons of superhigh BET surface area. The method (designated the subtracting pore effects (SPE) method) is based on the analysis of two upward swings from linearity, due to the pore effects, in the high resolution α_s plot. The measurement of slope of the linear α_s plot after subtraction of the swings gives a definite surface area, even in a microporous system. The surface area of activated mesophase carbon microbeads with a BET surface area of 3000–4000 m² g⁻¹ was determined by N₂ adsorption using the SPE method. The BET surface area obtained in the usual manner was an overestimate by about 40%. The overestimation was not attributed to enhanced micropore filling but to capillary condensation.

The micelle size was determined by dynamic light scattering for mixed anionic—non-ionic micelles containing solubilized organics. The surfactants used were sodium dodecyl sulfate and hexadecyl polyoxyethylene ethers (C₁₆POE_n where n = 10, 20, 30 or 40). For pure or mixed surfactant systems, the diameters of micelles which contain solubilized n-octane are almost independent of solute concentration, while micelles containing solubilized 1-octanol increase in size with increasing solute content. The change in micellar size for systems containing 1-octanol is influenced by hydrophilic group interactions between anionic and non-ionic surfactants in the mixed system.

1,3,3-Trimethylindolino-6′-nitrobenzopyrilospiran is solubilized in aqueous NaBr solutions of dodecyltrimethylammonium bromide (DTAB). The dye is soluble in solutions of DTAB having concentrations higher than the critical micelle concentration. The solubilization power of DTAB is 2.08·10⁻³ mol dye per mol DTAB at NaBr concentrations lower than 1.84 M, while it increases linearly with increasing NaBr concentration from 1.84–4.50 M. The constant solubilization power of DTAE is attributable to the spherical micelles, while its higher solubilization power in the presence of NaBr beyond 1.84 M can be ascribed to rod-like micelles. The solubilization capacity of a spherical micelle of DTAB ranges from 0.11–0.18, depending on the NaBr concentration, where the micelle aggregation number varies from 52–86. The rod-like micelle has an increasing solubilization capacity with increasing NaBr concentration, and it can solubilize the dye to such an amount that 136 DTAB molecules along its contour length incorporate one dye molecule. The solubilized dye is partly in the mecrocyanine form, and the solubilization locus has polarity corresponding to a dielectric constant ≈35.