Ammanuel Mehreteab

Spontaneous detachment of oil drops from solid substrates: governing factors

We carried out experiments on detachment of oil drops from glass substrates in solutions of an anionic surfactant. The three-phase contact line shrinks spontaneously, and eventually the oil drop detaches from the substrate. Consecutive video frames of such drops are digitized, and the time dependencies of the contact radius and angle are determined. Three stages of detachment of a drop, situated above a horizontal substrate, can be distinguished. They correspond to three different driving factors: (1) the interfacial tension decrease because of surfactant adsorption, (2) the aqueous meniscus spontaneously advances owing to the penetration of water between the oil and solid phases, and (3) at sufficiently small contact radius the shape of the oil-water interface becomes unstable and the drop detaches under the action of buoyancy. Analyzing the experimental data, we identified two important characteristics of the drop-detachment process: the velocity of spontaneous advance of the contact line and the line drag coefficient. In the case of moving contact line, a dynamic Young equation must be used, which takes into account the line drag force. The latter is proportional to the velocity of contact-line motion. The experimental data agree with the latter dependence, from whose slope the line drag coefficient is determined.

GROWTH OF ROD-LIKE MICELLES IN ANIONIC SURFACTANT SOLUTIONS IN THE PRESENCE OF CA2+ COUNTERIONS

The experimental data for the growth of rod-like surfactant micelles in the presence of a 2:1 electrolyte (CaCl2) do not comply with the available theory, which was originally developed for a 1:1 electrolyte. To solve the problem we undertook experimental and theoretical investigations with micellar solutions of the anionic surfactant sodium dodecyl polyoxyethylene-2 sulfate. Independent dynamic and static light scattering measurements of micelle size demonstrate that the effect of micelle–micelle interactions is negligible for the solutions investigated. Ultrafiltration experiments reveal that a considerable part of the Ca2+ ions are associated with micelles. Since our experiments are carried out at a fixed surfactant-to-Ca2+ ratio, the parameter of micelle growth (the equilibrium constant of micellization) indirectly depends on the surfactant concentration through the electrolyte concentration. That dependence is derived theoretically. The model of micelle growth, extended in this way, compares well with the experimental data. The model provides a quantitative description of the micelle size and charge as functions of the surfactant and electrolyte concentrations. The rod-like micelles have lower surface charge density than the spherical micelles; this makes their growth energetically favorable.

Formation of pseudo-nonionic complexes of anionic and cationic surfactants

Abstract Mixing aqueous solutions of most anionic and cationic surfactants at 1:1 molar ratio produces turbid solutions and eventually precipitation of solids. In the present study two pairs of anionic and cationic surfactants whose 1:1 molar mixture results in clear aqueous solutions are presented. These are tetradecyltrimethylammonium bromide (TTAB) and an alkylpoly(oxyethylene) sulfate (AEOS), and tetradecyltrimethylammonium bromide and an organic alkoxy phosphate ester (APE). Experimental evidence is given that suggests the formation of anionic-cationic complexes (ion pair) in the solutions. The complexes have properties different than either of their components. Not only do they cause lower interfacial tension between water and nonpolar oil but they also exhibit cloud point behavior similar to that of polyoxyethylenated nonionic surfactants. The concept of “pseudo-nonionic” behavior for these complexes is introduced.

ADSORPTION KINETICS OF IONIC SURFACTANTS WITH DETAILED ACCOUNT FOR THE ELECTROSTATIC INTERACTIONS : EFFECT OF THE ADDED ELECTROLYTE

The problem of diffusion-controlled adsorption from a non-micellar solution of an ionic surfactant in the absence of added electrolyte is solved analytically for the case of small deviations from equilibrium. For that purpose the electro-diffusion equations of the transport of surfactant ions and counterions are combined with the Poisson-Boltzmann equation for the electrical field. The resulting set of equations is linearized and Laplace transform is applied. Analytical expression for the Laplace image of the adsorption is obtained in terms of elementary functions. Simple formulae for the short-time and long-time asymptotics of adsorption and surface tension relaxation are derived. To illustrate the effect of the electrostatic interactions we calculated the theoretical dependence of the characteristic relaxation time on the bulk surfactant concentration and surface potential for aqueous surfactant solutions in contact with various non-aqueous phases (air, heptane, decane, petroleum ether) and two surfactants: SDS and DTAB. The general trend is that the electrostatic effects decelerate the process of adsorption, as it could be expected. The derived exact analytical expressions quantifying these effects can be directly applied for the interpretation of experimental data for the kinetics of ionic surfactant adsorption. The reliability of our approach is verified through a comparison with other available theories.

Effect of the surface expansion and wettability of the capillary on the dynamic surface tension measured by the maximum bubble pressure method

The dynamic surface tension (DST) of sodium dodecyl sulfate solutions in the presence of sodium chloride is studied by the maximum bubble pressure method. The pressure oscillations are measured with a pressure transducer, while the change of the bubble area with time is determined by means of a video system. The role of the wettability of the capillary is studied by means of measurements with hydrophilic and hydrophobic capillaries. A strong effect of wettability of the capillary on the bubble growth and the DST is observed. The DST data are interpreted with a model for diffusion-controlled adsorption assuming different laws of bubble expansion. The real law of expansion is found to be important for correct interpretation in the case of the hydrophobic capillary. However, the surface expansion is not of primary importance for interpretation of the DST data obtained with the hydrophilic capillary. It is proved that the maximum pressure does not correspond to the hemispherical shape of the bubble in the presence of surfactant. Neglecting this effect does not lead to a significant error in the DST for bubbling periods smaller than several seconds.

Fluorescence technique for the determination of low critical micelle concentrations

A technique for determining low critical micelle concentrations (CMC) by means of a hydrophobic fluorescence probe has been developed. The amount of the fluorescent probe at the CMC is so small that the effect of the probe on micelle formation is negligible. The fluorescence intensity was measured at fixed dye/surfactant ratios, and it decreased with concentration. A quantity proportional to fluorescent quantum yield was calculated and found to be high for concentrations of surfactant above the CMC and almost zero below the CMC, giving a distinct break in the quantum yield vs. the concentration curve.

Rheological properties of alumina-polyacrylate gel dispersions

Rheological properties of aqueous alumina-polyacrylate gel dispersions have been investigated with a controlled stress rheometer as functions of both cross-linked polyacrylate and alumina concentrations at pH 13. These studies were conducted to understand the role of alumina-polyacrylate interactions in building the rheology of complex liquids that contain surfactants, sodium hypochlorite, alkali, and inorganic salts. Viscosity and viscoelasticity of polyacrylate gel dispersions can be enhanced by the addition of colloidal alumina. The trends in viscosity and viscoelasticity are explained on the basis of changes in the conformation of polyacrylate and repulsive interactions of negatively charged polyacrylate and alumina particles.

Automation of phase diagram recording

An automated titration system has been developed for generating data to construct phase diagrams, which are extremely useful in the development of personal and household products. In this report, we describe the system and how it can be utilized to perform the technique of dual titration. In the procedure, a clear microemulsion sample is titrated with an oil until the dispersion turns cloudy (defined to be a transmittance below 90%). The mixture is then dosed with a certain quantity of cosurfactant, more than enough to clear the mixture. The sample is again titrated with oil. This process continues until the sample no longer clear upon adding cosurfactant. The resulting measurements of oil uptake can be used to characterize the boundaries of the L1 or oil-in-water microemulsion region of the phase space. Experiments for up to sixteen samples can be performed, each having individual setup and operating instructions. Features include completely automated operation, computer-controlled two-speed mixing, viscosity detection at the end-point condition and the storage of results in a computerized format.