Colloids Interfaces, Volume 3, Issue 4 (December 2019) – 9 articles

The interest to monophasic liquid capillary bridges (CB) has a long history. These shapes are attractive not only because of their interesting surface properties but also because of the possibility of their behavior to be analytically predicted by the equations of differential geometry. In the current paper we extend our previous studies by implementation of an approach for prediction of liquid gravityless CB behavior during their quasi-static stretching. It was found, that a simple linear relation, $\frac{h}{r_m}~\ln \frac{R}{r_m}$ , is valid the case of good wetting, 0° ≤ θ ≤ 90°, where h is the height of CB, R is the radius at the contact surface, r_m is the CB waist radius, and θ is the solid/liquid (static, receding) contact angle. We experimentally studied the geometrical properties evolution of monophasic cedar oil and water CBs between two glass plates during their quasi-static (stepwise with equilibration after each step for 1–2 min.) stretching. In addition, we investigated a binary CB of a new type, resembling “sandwich”. There, due to the stronger glass wetting by the water, the oil phase is adhered at the water/gas interface, partially engulfed with a tendency to stand in the zone around the waist (minimal surface energy). During the stretching, it tends to replace the water in the CB waist region. A simple mechanism for interaction of the two immiscible liquids leading to creation of “sandwich” like binary structures, is proposed. Experiments of capillary bridges (CB) stretching between two flat surfaces have been carried for all liquids at different volume proportions. The investigation is extended also to identification of CB profile generatrix shape. We experimentally found that for monophasic CB, it can be described by a circle during the quasi-static stretching. If the CB height is increased, before the rupture, the shape evolves consecutively to an ellipse, parabola, or possibly to a hyperbola. The investigated binary CB evolves a similar way. Conclusions are drawn and directions for further investigations are given. Full article

In the present work, the properties of dodecyl dimethyl phosphine oxide (C₁₂DMPO) at the water/decane interface are studied and compared with those obtained earlier at the interface to hexane. To simulate the interfacial behavior, a two-component thermodynamic model is proposed, which combines the equation of state and Frumkin isotherm for decane with the reorientation model involving the intrinsic compressibility for the surfactant. In this approach, the surface activity of decane is governed by its interaction with C₁₂DMPO. The theory predicts the influence of decane on the decrease of the surface tension at a very low surfactant concentration for realistic values of the ratio of the adsorbed amounts of decane and surfactant. The surfactant’s distribution coefficient between the aqueous and decane phases is determined. Two types of adsorption systems were used: a decane drop immersed into the C₁₂DMPO aqueous solution, and a water drop immersed into the C₁₂DMPO solution in decane. To determine the distribution coefficient, a method based on the analysis of the transfer of C₁₂DMPO between water and decane is also employed. Full article

In this communication, the single element version of the fractional Maxwell model (single-FMM or Scott–Blair model) is adopted to quantify the observed behavior of the linear interfacial dilational viscoelasticity. This mathematical tool is applied to the results obtained by capillary pressure experiments under low-gravity conditions aboard the International Space Station, for adsorption layers at the hydrocarbon/water interface. Two specific experimental sets of steady-state harmonic oscillations of interfacial area are reported, respectively: a drop of pure water into a Span-80 surfactant/paraffin-oil matrix and a pure n-hexane drop into a C₁₃DMPO/TTAB mixed surfactants/aqueous-solution matrix. The fractional constitutive single-FMM is demonstrated to embrace the standard Maxwell model (MM) and the Lucassen–van-den-Tempel model (L–vdT), as particular cases. The single-FMM adequately fits the Span-80/paraffin-oil observed results, correctly predicting the frequency dependence of the complex viscoelastic modulus and the inherent phase-shift angle. In contrast, the single-FMM appears as a scarcely adequate tool to fit the observed behavior of the mixed-adsorption surfactants for the C₁₃DMPO/TTAB/aqueous solution matrix (despite the single-FMM satisfactorily comparing to the phenomenology of the sole complex viscoelastic modulus). Further speculations are envisaged in order to devise combined FMM as rational guidance to interpret the properties and the interfacial structure of complex mixed surfactant adsorption systems. Full article

Bubble measurement has been widely discussed in the literature and comparison studies have been widely performed to validate the results obtained for various forms of bubble size inferences. This paper explores three methods used to obtain a bubble size distribution—optical detection, laser diffraction and acoustic inferences—for a bubble cloud. Each of these methods has advantages and disadvantages due to their intrinsic inference methodology or design flaws due to lack of specificity in measurement. It is clearly demonstrated that seeing bubbles and hearing them are substantially and quantitatively different. The main hypothesis being tested is that for a bubble cloud, acoustic methods are able to detect smaller bubbles compared to the other techniques, as acoustic measurements depend on an intrinsic bubble property, whereas photonics and optical methods are unable to “see” a smaller bubble that is behind a larger bubble. Acoustic methods provide a real-time size distribution for a bubble cloud, whereas for other techniques, appropriate adjustments or compromises must be made in order to arrive at robust data. Acoustic bubble spectrometry consistently records smaller bubbles that were not detected by the other techniques. The difference is largest for acoustic methods and optical methods, with size differences ranging from 5–79% in average bubble size. Differences in size between laser diffraction and optical methods ranged from 5–68%. The differences between laser diffraction and acoustic methods are less, and range between 0% (i.e., in agreement) up to 49%. There is a wider difference observed between the optical method, laser diffraction and acoustic methods whilst good agreement between laser diffraction and acoustic methods. The significant disagreement between laser diffraction and acoustic method (35% and 49%) demonstrates the hypothesis, as there is a higher proportion of smaller bubbles in these measurements (i.e., the smaller bubbles ‘hide’ during measurement via laser diffraction). This study, which shows that acoustic bubble spectrometry is able to detect smaller bubbles than laser diffraction and optical techniques. This is supported by heat and mass transfer studies that show enhanced performance due to increased interfacial area of microbubbles, compared to fine bubbles. Full article

Essential oil compounds (EOCs) are molecules with well-known antimicrobial and antipest activity. However, such molecules possess limited solubility in water, making their handling difficult. This work aimed to enhance the distribution of a solid essential oil compound, thymol, using oil-in-water (o/w) microemulsions for its solubilization. The use of mixtures formed by an alkyl polyglucoside (APG) and soybean lecithin (SL) allowed for stabilization of the o/w microemulsions in a broad range of compositions, with the total concentration of the mixture of the two surfactants (APG+SL) and the APG:SL ratio both being essential for controlling the nature of the obtained dispersions. The microemulsions obtained using oleic acid as the oil phase and with compositions far from those corresponding to the onset of the emulsion region showed a good efficiency for thymol solubilization. This is an advantage from a stability point of view, as well as for ease of thymol preparation. The present work opens new alternatives for designing eco-sustainable formulations for EOC solubilization, with the possibility of preparing the formulations at the place of use, thereby saving transport costs and reducing the emission of pollutants. Full article

The heterogeneous interactions of colloidal U particles with organophosphates, leading to the formation of U-phosphate minerals, can retard the migration of U in contaminated sites. Here, we studied the hydrolytic mechanism of p-nitrophenyl phosphate (NPP) on the surfaces of tetravalent uranium nanoparticles (U(IV)_NPs), resulting in the formation of U-phosphate precipitates. Our study shows that the reaction rate of NPP hydrolysis is significantly enhanced by U(IV)_NPs through a multi-step heterogeneous reaction on the particle surfaces. The end products of the reaction were identified as U(IV)_NPs-aggregates with surface-bound phosphates. Colloidal properties, such as high positive values of the zeta-potential (>+30 mV) and large surface areas of U(IV)_NPs due to their unique cluster structures consisting of relatively small primary UO₂(cr)-particles, are correlated with their reactivity towards hydrolysis reaction. Reaction kinetic modeling studies using spectrophotometric data indicated the presence of two distinct reaction intermediates as the surface complexes of NPP on U(IV)_NPs. We suggest the involvement of the NPP inner-sphere complexes in the rate-determining step based on the results obtained by analyzing the ATR-FTIR spectra and the surface-enhanced infrared absorption of NPP bound to substrate surfaces. Full article

The influence of different types of salts (NaCl, CaCl ${}_2$ , MgCl ${}_2$ , NaHCO ${}_3$ , and Na ${}_2$ SO ${}_4$ ) on the surface characteristics of unconditioned calcite and dolomite particles, and conditioned with stearic acid, was investigated. This study used zeta potential measurements to gain fundamental understanding of physico-chemical mechanisms involved in surface charge modification of carbonate minerals in the presence of diluted salt solutions. By increasing the salt concentration of divalent cationic salt solution (CaCl ${}_2$ and MgCl ${}_2$ ), the zeta potential of calcite particles was altered, resulting in charge reversal from negative to positive, while dolomite particles maintained positive zeta potential. This is due to the adsorption of potential-determining cations (Ca ${}^{2+}$ and Mg ${}^{2+}$ ), and consequent changes in the structure of the diffuse layer, predominantly driven by coulombic interactions. On the other hand, chemical adsorption of potential-determining anions (HCO ${}_3^{-}$ and SO ${}_4^{2-}$ ) maintained the negative zeta potential of carbonate surfaces and increased its magnitude up to 10 mM, before decreasing at higher salt concentrations. Physisorption of stearic acid molecules on the calcite and dolomite surfaces changed the zeta potential to more negative values in all solutions. It is argued that divalent cations (Ca ${}^{2+}$ and Mg ${}^{2+}$ ) would result in positive and neutral complexes with stearic acid molecules, which may result in strongly bound stearic acid films, whereas ions resulting in negative mineral surface charges (SO ${}_4^{2-}$ and HCO ${}_3^{-}$ ) will cause stearic acid films to be loosely bound to the carbonate mineral surfaces. The suggested mechanism for surface charge modification of carbonates, in the presence of different ions, is changes in both distribution of ions in the diffuse layer and its structure as a result of ion adsorption to the crystal lattice by having a positive contribution to the disjoining pressures when changing electrolyte concentration. This work extends the current knowledge base for dynamic water injection design by determining the effect of salt concentration on surface electrostatics. Full article

As the demand for uranium production-based energy worldwide has been increasing in the last decades to maintain nuclear growth for electricity production, there are great efforts towards developing an easy and inexpensive method for uranium extraction and separation from its ores. For this purpose, mesoporous inorganic cation exchangers provide an efficient separation technology that can help streamline production and lower overall cost. This study describes the development of nano-structured mesoporous sodium zirconium phosphate (NaZrP-CEX) for separation and extraction of uranyl ions from real samples. The fabricated NaZrP-CEX was well characterized by various techniques such as X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), Scanning Electron Microscope (SEM), N₂ adsorption/desorption, Dynamic light scattering (DLS) and zeta potential). The kinetics/thermodynamic behaviors of uranyl ion adsorption into NaZrP-CEX from an aqueous solution were minutely studied. The kinetic studies showed that the pseudo-second order model gave a better description for the uptake process. The negative value of ΔG indicate high feasibility and spontaneity of adsorption. Finally, mesoporous NaZrP-CEX can be regenerated using both of HNO₃ (0.05 M) or HCl (1 M) up to seven cycles of operation. Full article

In real life, sessile droplets usually have a three-dimensional shape, making it difficult to understand their forced wetting behavior, both from an experimental and a theoretical perspective. Even in the case of spreading under quasi-static conditions, where the droplet shape is described by the Young–Laplace equation, there is no fundamental approach to describe the contact line evolution. In the present work, a few existing approaches on this issue are analyzed and assessed. It is shown that an experimentally inspired fixed shape for the contact line of droplets that are spreading under the action of tangential forces can be considered equivalent to a theory for contact line motion. There is a lack of experimental data for contact line evolution under arbitrary scenarios of forces. Such data will be very helpful for the further development of the suggested approach to contact line motion. Of particular interest is the case of small contact angle droplets, for which a top view can clearly indicate the contact line location. On the contrary, in such droplets, the direct experimental measurement of contact angle profile is very difficult. This must be estimated theoretically; thus, a special approach has been developed here for this purpose. Full article