Colloids and Surfaces最新文献

Two surfactants, bis[2-(dodecyloxycarbonyl)ethyl](p-vinylbenzoyl)methylammonium chloride (SI) and bis[2-(10-undecenoyloxycarbonyl)ethyl](p-methylbenzoyl)methylammonium chloride (SII) were synthesized and copolymerized with polar and non-polar monomers to form regularly layered structures.

The results showed a pronounced difference between the two surfactants. SI with the double bond close to the polar group accepted a large number of monomers, in contrast to the behavior of SII. For SI the polymerization changed the conformation of the p-vinylbenzoyl group depending on the location of the added monomer while copolymerization of the monomers with SII gave no significant change.

A comparison with earlier results from bis[2-(10-undecenoyloxycarbonyl)ethyl](p-vinylbenzoyl)methylammonium chloride provided proof of copolymerization of all the groups.

It is demonstrated that the depth of the potential well between two colloidal particles which interact via van der Waals interactions can be minimized if they are covered with a shell of adsorbed surfactant molecules whose Hamaker constant is near to that or the liquid medium. This result is used to explain some experimental observations (I. Sushumna, R.P. Gupta and E. Ruckenstein, J. Mater. Res., 7 (1992) 2884) which indicate that long-chain fatty acids with an alkyl side chain of 5–10 carbon atoms located far from the head group of the surfactant lead to lower paste viscosities than those without side chains or with longer side alkyl chains. It is also suggested that, in concentrated suspensions, the collective interactions when the pair interaction is attractive can lead to an effective repulsion component between two neighboring particles, and that this effect may contribute to the stability of the concentrated dispersions.

The dynamic surface of sodium tetradecylsulphate and sodium bexadecylsulphate solutions in water and also in Triton X-100 solutions was measured by the maximum bubble-pressure method, using modern computerized instrumentation, for a wide range of surface lifetimes (from 0.001 to 10 s), temperatures (from 30 to 80°C) and surfactant concentrations (from 1 to 200 CMC). On the basis of a previously suggested adsorption kinetics theory for micellar solutions of ionogenic surfactants (V.B. Fainerman, Colloids Surfaces, 62 (1992) 333) a method was developed for the calculation of the micellar dissociation rate constant k. For the surfactants studied, k increases with increasing concentration. Moreover, for ionic surfactants the dependence of k on concentration (C) becomes more striking for C> (10–30) CMC. This can be explained by a micelle shape transition and by a strengthening of the intermolecular repulsion in micelles. In solutions of the ionic surfactants the constant k increasing with increasing temperature, whereas in Triton X-100 solutions a temperature dependence is absent. This phenomenon is associated with the different nature of the molecular interactions for ionogenic and non-ionogenic surfactants in micelles. The k values, obtained from results of dynamic surface tension measurements, are in satisfactory agreement with the results of a study of the relaxation of micellar solutions published previously.

Contact angle kinetics of sessile drops of albumin solution on hydrophilic acetal and hydrophobic FC 721 surfaces were measured using axisymmetric drop shape analysis. Young's equation is used to calculate the solid/liquid interfacial tension from measured contact angles and surface tensions as a function of time. The change in solid/liquid interfacial tension is a result of protein adsorption. It indicates that at the hydrophilic acetal surface the albumin molecules, interact only weakly, whereas the interaction with the hydrophobic FC 721 surface is quite strong.

The open circuit potential (OCP) vs pH response of Ti/TiO₂ electrodes prepared by thermal and electrochemical oxidation of metallic titanium was measured in KNO₃ aqueous solutions in order to establish whether the slopes of the OCP-pH curves can be used as a measure of the variation of surface potential (ψ_o) of TiO₂ with the pH of the aqueous solution. For comparison purposes, ψ_o—pH slopes were also evaluated from surface charge-pH data obtained by acid-base potentiometric titrations of TiO₂ dispersions.

Ti/TiO₂ electrodes showed a linear OCP-pH response in the range of ph 5–10 with slopes of −0.039 ± 0.005 V ph⁻¹. The OCP-pH dependence of Ti/TiO₂ electrodes was lower than that predicted thermodynamically. According to the triple-layer model, used to describe the oxide/aqueous solution interface, the OCP-pH slopes obtained for Ti/TiO₂ electrodes cannot be identified with the variation of ψ_o with the pH. Calculations with this model indicated that neither the single nor the double extrapolation methods used to evaluate the intrinsic ionization constant of oxide surface sites render realistic values of these parameters for the TiO₂ surface.

The analysis of surface charge—pH data and the use of the triple-layer model to predict experimental values indicated that the ψ_o—pH slopes of TiO₂ surfaces must be greater (in absolute magnitude) than 0.041 V pH⁻¹.

The thermodynamics of the adsorption of phosphamidon on antimony(V) phosphate cation exchanger has been studied at 30, 45 and 50°C and the thermodynamic equilibrium constant (K₀), standard free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) have been calculated to predict its adsorption behaviour. All the data are adequately represented by the Freundlich isotherms.

Axisymmetric drop-shape analysis is used to investigate the relaxation of surfactant adsorption layers after transient interfacial area changes, e.g. square-pulse or step-type disturbances. The response functions for sodium dodecyl sulfate solutions of different origin and for sodium tetradecyl sulfate are measured and interpreted in terms of a diffusion-controlled matter-exchange model. For both surfactant systems the data can be explained by considering small amounts of surface-active contaminations.

Dynamic and static light scattering studies are reported on a series of alkyl dimethyl betaines C_mDMB (m = 12, 16, 18) in aqueous solutions as functions of surfactant concentration, salt concentration and temperature. The characteristics of the micelles change dramatically as the alkyl chain length is increased from 12 to 18. In the case or C₁₈DMB, large micelles form at very low surfactant concentrations (about 0.2 wt%) and grow rapidly to become entangled at slightly higher concentrations (about 1 wt%). Upon addition of salt, C₁₈DMB micelles become very large and semiflexible. At 40.3°C the persistence lengths of C₁₈DMB micelles in 0.01 and 0.1 M NaCl aqueous solutions are approximately 1400 ± 200 Å and 1600 ± 250 Å, respectively. The persistence length of C₁₈DMB micelles in 0.1 M NaCl solution at 51.5°C is 1650 ± 250 Å. The dynamic light scattering data are consistent with estimated persistence lengths. Quantitative agreement is obtained with the theory developed by Fujime and co-workers (S. Fujime and M. Maruyama, Macromolecules, 6 (1973) 237; T. Maeda and S. Fujime, Macromolecules, 14 (1981) 809) for semiflexible, rodlike particles. Micelles of C₁₆DMB in water are very small compared to C₁₈DMB. The micelles are polydisperse and the size increases with increasing surfactant concentration. In 1.0 M NaCl aqueous solutions, the C₁₆DMB micelles are larger than in pure water and become semiflexible. After accounting for polydispersity, the persistence length is calculated to be roughly 600 ± 100 Å at 25°C. Therefore, micelles of C₁₆DMB are less stiff than those of C₁₈DMB. In contrast to C₁₈CMB and C₁₆DMB systems, C₁₂DMB micelles remain small and spherical over a wide range of surfactant concentration, in agreement with previous studies.