Colloids and Interfaces — Open Access Journal

Flow field-based methods are becoming increasingly popular for the analysis of interfacial shear rheology data. Such methods take properly into account the subphase drag by solving the Navier–Stokes equations for the bulk phase flows, together with the Boussinesq–Scriven boundary condition at the fluid–fluid interface and the probe equation of motion. Such methods have been successfully implemented on the double wall-ring (DWR), the magnetic rod (MR), and the bicone interfacial shear rheometers. However, a study of the errors introduced directly by the numerical processing is still lacking. Here, we report on a study of the errors introduced exclusively by the numerical procedure corresponding to the bicone geometry at an air–water interface. In our study, we set an input value of the complex interfacial viscosity, and we numerically obtained the corresponding flow field and the complex amplitude ratio for the probe motion. Then, we used the standard iterative procedure to obtain the calculated complex viscosity value. A detailed comparison of the set and calculated complex viscosity values was made in wide ranges of the three parameters herein used, namely the real and imaginary parts of the complex interfacial viscosity and the frequency. The observed discrepancies yield a detailed landscape of the numerically-introduced errors. Full article

Using neodymium ring magnets (0.5–0.65 T), the experiments on the magnetic field (MF) effects on water evaporation rate and surface tension were performed at room temperature (22–24 °C). In accordance with the literature data, the enhanced evaporation rates were observed in the experiments conducted in a period of several days or weeks. However, the evaporated amounts of water (up to 440 mg over 150 min) in particular experiments differed. The evaporated amounts depended partially on which pole of the ring magnet was directed up. The relatively strong MF (0.65 T) caused a slight decrease in surface tension (−2.11 mN/m) which lasted longer than 60 min and the memory effect vanished slowly. The surface tension data reduced by the MF action are reported in the literature, although contrary results can be also found. The observed effects can be explained based on literature data of molecular simulations and the suggestion that MF affects the hydrogen bonds of intra- and inter-clusters of water molecules, possibly even causing breakage some of them. The Lorentz force influence is also considered. These mechanisms are discussed in the paper. Full article

The synthesis and properties of submicrocontainers with a shell of nanoparticles of silicon dioxide and a core of polymerized 3-(Trimethoxysilyl) propyl methacrylate loaded with 5Dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) are considered. The resulting containers were characterized by scanning electron microscopy SEM, laser correlation spectroscopy and thermogravimetric analysis. The obtained submicrocontainers show low polydispersity with a small increase in size in comparison with the initial droplet size of the Pickering emulsion. The Zeta potential of the final containers was sufficiently negative at pH7 to be stable. The maximum release of encapsulated biocide was observed over approximately 24–27 h with a lease of about 78% of the encapsulated biocide during 3.5 h. The effectiveness of the encapsulated biocide by the Pickering emulsion technique was studied by tests on the growth rate of a microfungi colony (Aspergillus niger, Aspergillus awamori) and the growth rate of the bacteria Bacillus cereus. The test shows that the submicrocontainers of DCOIT facilitate a growth inhibition of 70% against 52% for the free biocide after 5 days; this is due to the fact that free biocide loses its activity promptly, while the encapsulated biocide is released gradually, and thus retains its effectivity for a longer time. Full article

Mesoporous carbon aerogels (MCA) synthesized via aqueous polymerization of resorcinol and formaldehyde were modified by copper chloride and applied for adsorption removal of elemental mercury (Hg⁰) at a low temperature. The sorbents were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), nitrogen adsorption/desorption isotherms, and X-ray photoelectron spectroscopy (XPS) techniques. The results show that CuCl₂-modified MCA exhibited good ability for Hg⁰ adsorption in the 40–160 °C temperature range, with Hg⁰ removal efficiency all above 95.0%. The Hg⁰ removal efficiency first increased and then decreased with the elevation of reaction temperature. It performed optimally at 80 °C with the highest Hg⁰ removal efficiency of 98.7%. XPS results indicate that covalent chlorines (C-Cl groups) play an important role in elemental mercury adsorption process. Hg⁰ is firstly captured in the form of oxidized mercury (Hg²⁺) and then reacts with C-Cl groups to form HgCl₂. Full article

The colloidal probe technique was used to measure the inner layer capacitance of an electrical double layer. In particular, the forces were measured between silica surfaces and sulfate latex surfaces in solutions of monovalent salts of different alkali metals. The force profiles were interpreted with Poisson-Boltzmann theory with charge regulation, whereby the diffuse layer potential and the regulation properties of the interface were obtained. While the diffuse layer potential was measured in this fashion in the past, we are able to extract the regulation properties of the inner layer, in particular, its capacitance. We find systematic trends with the type of alkali metal ion and the salt concentration. The observed trends could be caused by difference in ion hydration, variation of the binding capacitance, and changes of the effective dielectric constant within the Stern layer. Our results are in agreement with recent experiments involving the water-silica interface based on a completely independent method using X-ray photoelectron spectroscopy in a liquid microjet. This agreement confirms the validity of our approach, which further provides a means to probe other types of interfaces than silica. Full article

The effect of aqueous phase content and temperature North Sea crude oil emulsion viscoelastic behavior has been studied. Heavy crude oil from the North Sea is of high viscosity and is capable of forming stable water-in-crude oil (w/o) emulsions without introducing any synthetic surfactants. The aqueous phase volume content was varied from 1 to 40%, and the temperature was varied from 0 to 30 °C. The w/o emulsion viscosity increased sharply when the aqueous phase content exceeds 20%, being more pronounced at the lower temperatures. The viscosity flow curves for emulsions containing more than 20% aqueous phase demonstrate non-Newtonian behavior, in contrast to crude oil, which is Newtonian. The coefficients in the master curve describing the viscosity-temperature dependence were determined. Oscillatory rheological tests showed that the loss modulus substantially exceeds the storage modulus which indicates the liquid-like state of the emulsions. Full article

This study aimed to evaluate the production of a surfactant by Cunninghamella echinulata, using agro-industrial residues, corn steep liquor (CSL), and soybean oil waste (SOW). The study had a factorial design, using as a variable response to the reduction of surface tension. C. echinulata was able to produce biosurfactant in assay, CSL (8.82%) and SOW (2%). The results showed that the biosurfactant was successfully produced by C. echinulata and had attractive properties, such as a low surface tension (31.7 mN/m), a yield of 5.18 g/L at 120 h of cultivation, and an anionic profile. It also achieved a reduction in surface tension stability in a wide range of pH values, temperatures, and salinity values. The biosurfactant produced by C. echinulata showed an absence of toxicity to Artemia salina. The influence of the biosurfactant on the viscosity of engine oil, burnt engine oil, diesel, soybean oil post-frying, canola oil, and water was investigated. The results reveal a mechanism for the decrease of the viscosity using hydrophobic substrates and the new biosurfactant solution at 1.5% of the (CMC). This enables the formulation of a low-cost culture medium alternative, based on corn steep liquor and the reuse of soybean oil after frying to produce a biosurfactant. Additionally, performance of the biosurfactant isolated from C. echinulata showed an excellent ability to remove spilled oil, such as diesel (98.7%) and kerosene (92.3%) from marine sand. Full article

Humidity-dependent closing and reopening slit pores can produce hysteresis loops in sorption diagrams even at low relative vapor pressure. Pore closing is supported by adhesion of the slit wall surfaces. In a macroscopic model for sorption hysteresis in narrow slits, the adhesion energy jumps by a finite value when touching slit walls are separated from each other. We consider a more realistic adhesion model by introducing a smoothly-varying adhesion force, which depends on the distance between the slit walls. The range of the attraction between the slit walls is found to have a pronounced influence on the shape of hysteresis loops at low vapor pressure. A large interaction range avoids an extraordinarily small relative vapor pressure necessary for pore closing, which is a precondition for low pressure hysteresis. Our extended model allows us to describe a discontinuity, which can appear in the desorption branch of swelling/shrinkage diagrams for hardened cement paste. Full article

Manganese oxide forms prepared by different methods differ by their compositions, phase ratios in polyphase samples, and crystallite sizes (XRD and TEM characterization). Among the phases, tunnel-structured β-MnO₂ (pyrolusite), α-MnO₂ (cryptomelane), ε-MnO₂ (akhtenskite), and β-Mn₂O₃ (bixbyite) have been identified. Water vapor sorption isotherms showed substantial differences in the affinities of water molecules to oxide surfaces of the manganese oxide forms under study. The parameters of the BET equation and pore size distribution curves have been calculated. The manganese oxide forms have mesoporous structures characterized by uniform and non-uniform pore sizes as well as by moderate hydrophilic behavior. Full article

We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P_6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact. Full article

Modification of SBA-15 with non-noble metal leads to functional materials, which can be applied as gas sensors, adsorbents, and catalysts of various reactions. The new materials contain up to four various metals, which are deposited consecutively or simultaneously at various concentrations ranging from a fraction of 1% to an amount that is comparable with the mass of silica-support. These materials contain metals at various oxidation levels, usually as oxides, which occur in crystalline form (a typical crystallite size of about 10 nm matches the width of the SBA-15 channels), but in a few other materials, crystalline metal compounds have not been detected. Many researchers have provided detailed physico- chemical characteristics of SBA-15 modified with non-noble metals by the means of various microscopic and spectroscopic techniques. Full article

In this work, we investigated the potential of Bacillus subtilis UCP 0146 in the bioconversion of a medium containing 100% cassava flour wastewater to obtain a bioemulsifier. The evaluation of the production was carried out by the emulsification index (IE₂₄) and the surface tension (ST). The ionic charge, stability (temperature, salinity, and pH measured by IE₂₄ and viscosity), and ability to remove and disperse oil and textile dye were investigated. B. subtilis produced an anionic bioemulsifier in the medium containing 100% cassava wastewater under Condition 4 of the factorial design (inoculum 9% at a temperature of 35 °C and shaken at 100 rpm), and showed a surface tension of 39 mN/m, an IE₂₄ of 95.2%, and a yield of 2.69 g·L⁻¹. The bioemulsifier showed stability at different pH (2–8), temperatures (0–120 °C), and NaCl concentrations, a dispersion oil displacement area (ODA) test of 55.83 cm², and a reduction of the viscosity of the burned engine oil (90.5 Cp). The bioemulsifier was able to remove petroleum (94.4%) and methylene blue azo dye (62.2%). The bioemulsifier and its synthesis from bacteria also emphasizes the role of surfactants in oil remediation. Full article

Harmonic drop surface area oscillations are performed at a fixed frequency (0.1 Hz) to measure the dilational visco-elasticity for three proteins: β-casein (BCS), β-lactoglobulin (BLG), and human serum albumin (HSA). The surface area oscillations were performed with different amplitudes in order to find the origin of non-linearity effects. The analysis of data shows that the non-linearity in the equation of state—i.e., the relation between surface pressure and surface concentration of adsorbed protein molecules—is the main source of the amplitude effects on the apparent visco-elasticity, while perturbations due to non-uniform expansions and compressions of the surface layer, inertia effects leading to deviations of the drop profile from the Laplacian shape, or convective transport in the drop bulk are of less importance. While for the globular proteins, HSA and BLG the amplitude effects on the apparent visco-elasticity are rather large, for the non-globular protein BCS this effect is negligible in the studied range of up to 10% area deformation. Full article

Wetting of deformable substrates has gained significant interest over the past decade due to a multiplicity of industrial and biological applications. Technological advances in the area of interfacial science have given rise to the ability to capture interfacial behavior between a liquid droplet and an elastic substrate. Researchers have developed several theories to explain the interaction between the two phases and describe the process of wetting of deformable/soft substrates. A summary of the most recent advances on static wetting of deformable substrates is given in this review. It is demonstrated that action of surface forces (disjoining/conjoining pressure) near the apparent three-phase contact line should be considered. Any consideration of equilibrium droplets on deformable (as well as on non-deformable) substrates should be based on consideration of the excess free energy of the system. The equilibrium shapes of both droplet and deformable substrate should correspond to the minimum of the excess free energy of the system. It has never been considered in the literature that the obtained equilibrium profiles must satisfy sufficient Jacobi’s condition. If Jacobi’s condition is not satisfied, it is impossible to claim that the obtained solution really corresponds to equilibrium. In recently published studies, equilibrium of droplets on deformable substrates: (1) provided a solution that corresponds to the minimum of the excess free energy; and (2) the obtained solution satisfies the Jacobi’s condition. Based on consideration of disjoining/conjoining pressure acting in the vicinity of the apparent three-phase contact line, the hysteresis of contact angle of sessile droplets on deformable substrates is considered. It is shown that both advancing and receding contact angles decrease as the elasticity of the substrate is increased and the effect of disjoining/conjoining pressure is discussed. Fluid inside the droplet partially wets the deformable substrate. It is shown that just these forces coupled with the surface elasticity determine the deformation of the deformable substrates. Full article

Rubber materials play an important role in robotics, due to their sensing and actuating abilities, that are exploited in soft smart materials endowed with shape-adaptive and electroadhesive properties. The application of an electric field produces non-linear deformation that has been extensively modelled, but is not understood at the molecular level. The symmetric effect (the production of an electric field due to rubber deformation) was recently discovered and explained as follows: rubber surface chemical composition and adsorptive properties change during rubber deformation, allowing the surface to exchange charge with the atmosphere. The present work describes the complex surface morphology and microchemistry of tubing made from vulcanized natural rubber, showing that it is rough and made from two domain types: stiffer elevations containing Br or Al (depending on the sample used) and O, that rise above an elastic base that is exempt of elements other than C and H. The surface area fraction occupied by the elastic base is higher in the strained rubber than when it is relaxed. Electrostatic potential on rubber surfaces was measured as a function of the stretching frequency, using Kelvin electrodes and showing frequency-dependent potential variation. This is explained considering charge exchange between the atmosphere and rubber surface, mediated by water vapor adsorbed in the stretched rubber and trapped when it relaxes. Full article

Capillary pressure experiments are performed in microgravity conditions on board the International Space Station to quantify the dynamic interfacial behavior of mixed adsorption layers of TTAB and C₁₃DMPO at the water/hexane interface. While the non-ionic surfactant C₁₃DMPO is soluble in both bulk phases, water and hexane, the cationic surfactant TTAB is only soluble in the aqueous phase. The interfacial layer is thus formed by TTAB molecules adsorbing from the aqueous phase while the C₁₃DMPO molecules adsorb from the aqueous phase, and transfer partially into the hexane phase until both the equilibrium of adsorption and the distribution between the two adjacent liquid phases is established. The experimental constrains as well as all possible influencing parameters, such as interfacial and bulk phase compressibility, interfacial curvature, calibration of pressure and absolute geometry size, are discussed in detail. The experimental results in terms of the dilational interfacial viscoelasticity of the mixed adsorption layers in a wide range of oscillation frequencies show that the existing theoretical background had to be extended in order to consider the effect of transfer of the non-ionic surfactant across the interface, and the curvature of the water/hexane interface. A good qualitative agreement between theory and experiment was obtained, however, for a quantitative comparison, additional accurate information on the adsorption isotherms and diffusion coefficients of the two studied surfactants in water and hexane, alone and in a mixed system, are required. Full article

A series of Eu³⁺ doped CaSiO₃/SiO₂ nano-phosphor powder of controlled grain size, crystalline structure, and chemical composition were synthesized using the microemulsion technique. The morphology, size, and shape of the synthesized nanophosphorous powder were investigated using transmission electron microscopy and X-ray diffraction (XRD) analysis. XRD profiles of samples sintered over 600 °C, suggested phase shift from amorphous powder grain to more ordered polycrystalline powder of triclinic type wollastonite, CaSiO₃, with preferred crystal phase orientation of (112) and tetragonal type cristobalites of SiO₂. The grain size, crystallinity, and chemical composition of the host matrix, activator and sensitizer strongly affected both the absorption and emission bands of these samples. The amplitude of both the orange and red emission bands significantly increased with sintering temperature. The emission band is red-shifted with decreasing grain sizes. These bands displayed good sensitivity to ionic concentration of the Si⁴⁺, Ca²⁺, and Eu³⁺. With increasing Ca²⁺ ion concentration both the intensity of the red photoluminescence (PL) band increased and a concentration quenching observed. Increase in Si⁴⁺ ion concentration led to quenching in PL intensity of both the orange and red bands, whereas the amplitude of the blue-band slightly increased. With increasing Eu³⁺ ion concentration the red-band initially increased whereas it started decreasing at higher sample concentration. In the presence of Ca²⁺ ion as a sensitizer, the sample showed a remarkable PL property-including–about 100% photon conversion efficiency and a two-fold increase in excitation and emission photons. Full article

Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone of colloid stability. Electrostatic interactions dominate van der Waals attractions at large colloid-colloid separations h, unless strongly screened. Under these conditions, the potential $U\left(h\right)$ between charged colloids is expected to be exponentially screened, $U\left(h\right)\sim \exp (-\kappa h)/h$, with ${\kappa }^{-1}={\lambda }_{\mathrm{D}}$ where ${\lambda }_{\mathrm{D}}$ is the classical Debye-Hückel screening length. By measuring the force between individual charged particles at dilute electrolyte concentrations (<mM) using optical tweezers, we tested experimentally the prediction ${\kappa }^{-1}={\lambda }_{\mathrm{D}}$ in a nonpolar solvent. At low salt concentrations, we found close agreement between the directly-measured decay length ${\kappa }^{-1}$ and Debye-Hückel predictions. However, above a critical electrolyte concentration (≈450 $\mathsf{\mu }$M), we obtained significant discrepancies between measured and predicted screening lengths, with ${\kappa }^{-1}\gg {\lambda }_{\mathrm{D}}$. In marked contrast to expectations, we found that the measured screening length ${\kappa }^{-1}$ appears to grow as the ionic strength of the solution is increased. The origin of this discrepancy is discussed and the importance of considering the surface is highlighted. Full article