Colloids Interfaces, Volume 2, Issue 1 (March 2018) – 12 articles

Clays consist of fine particles (clay minerals) which under certain conditions can be dispersed in water, forming a fairly stable dispersion. The stability of the clay mineral dispersions depends on the pH, solid/liquid ratio, and most of all on the type and concentration of dissolved ions. The difference between the dispersibility of different clays is not yet understood. In the present study allophane and smectite (main constituent of bentonite) are compared. Allophane surfaces are dominated by aluminol groups. Therefore, they can be used to investigate the properties of the edges of platy dioctahedral clay minerals. Dispersion and coagulation of allophane was understood based on the charge of the surface functional groups. Dispersion of allophane was only possible at pH < 5 where the aluminol groups became positive and at low ionic strength (critical coagulation concentrations (CCC) < 10 mmol/L NaCl). Dispersion at high pH may also be possible but this was not tested in the present study. In contrast to allophane, bentonite dispersions coagulated when the edges of the smectites became positive. This is probably caused by heterocoagulation leading to edge (+)/face (−) aggregation. CCC values of different bentonite dispersions ranged from 10 to 750 mmol/L. Most of the Wyoming bentonite dispersions were more stable and even above the CCC, sedimented much slower compared to other bentonites. The reason for this phenomenon could not be identified and needs further research. Full article

Plant Acacia gum exudates are composed by glycosylated hydroxyproline-rich proteins, which have a high proportion of heavily branched neutral and charged sugars in the polysaccharide moiety. These hyperbranched arabinogalactan-proteins (AGP) display a complexity arising from its composition, architecture, and conformation, but also from its polydispersity and capacity to form supramolecular assemblies. Flexibility and hydration partly determined colloidal and interfacial properties of AGPs. In the present article, these parameters were estimated based on measurements of density and sound velocity and the determination of volumetric parameters, e.g., partial specific volume (v_s°) and coefficient of partial specific adiabatic compressibility coefficient (β_s°). Measurements were done with Acacia senegal, Acacia seyal, and fractions from the former separated according to their hydrophobicity by Hydrophobic Interaction Chromatography, i.e., HIC-F1, HIC-F2, and HIC-F3. Both gums presented close values of v_s° and β_s°. However, data on fractions suggested a less hydrated and more flexible structure of HIC-F3, in contrast to a less flexible and more hydrated structure of HIC-F2, and especially HIC-F1. The differences between the macromolecular fractions of A. senegal are significantly related to the fraction composition, protein/polysaccharide ratio, and type of amino acids and sugars, with a polysaccharide moiety mainly contributing to the global hydrophilicity and a protein part mainly contributing to the global hydrophobicity. These properties form the basis of hydration ability and flexibility of hyperbranched AGP from Acacia gums. Full article

In the present work, the use of Cyclodextrins (CDs) as wettability modifiers for enhanced oil recovery (EOR) was evaluated. Cyclodextrins (CDs) are cyclic oligosaccharides that form inclusion complexes with various organic molecules, including n-alkanes. Wettability was evaluated through the contact angle (θ) of an n-dodecane drop in contact with a quartz surface and immersed in a 0.6 M NaCl aqueous solution containing the CDs. The quartz surface was functionalized with octadecyltrichlorosilane (OTS), rendering the surface oil-wet (C₁₈-quartz). Here, the n-dodecane, the saline solution and the C₁₈-quartz represent the oil, the reservoir brine and an oil-wet rock surface, respectively. In the absence of CDs, the n-dodecane drops spread well over the C₁₈-quartz, showing that the surface was oleophilic. In the presence of CDs, remarkable effects on the wettability were observed. The most dramatic effects were observed with α-cyclodextrin (α-CD), in which case the C₁₈-quartz surface changed from oil-wet (θ = 162°) in the absence of CD to water-wet (θ = 33°) in the presence of 1.5% (w/v) α-CD. The effects of the CDs can be explained by the formation of surface-active inclusion complexes between the CDs and n-dodecane molecules. The CD inclusion complexes can be regarded as pseudo-surfactants, which are less harmful to the environment than the traditional surfactants employed by the petroleum industry. Full article

Polyhedral oligomeric silsesquioxanes (POSSs) with nano-size cage structures have been conventionally incorporated into polymers to improve the polymers’ physical properties. In this work, POSS films formed by using POSS nanomaterials with different thermal treatments have been implemented as liquid crystal (LC) alignment films instead of using conventional polyimide alignment films adopted in the LC displays industry. The homeotropic alignment of LCs anchored on POSS films was observed. The morphology and surface energy of POSS films were measured to study their effects on LC orientation anchored on the POSS films. Full article

Liquid flow in foams mostly proceeds through Plateau borders where liquid content is the highest. A sufficiently thick (~180 µm) free liquid film is a reasonable model for understanding of electrokinetic phenomena in foam Plateau borders. For this purpose, a flow cell with a suspended free liquid film has been designed for measurement of electrokinetic flow under an imposed electric potential difference. The free liquid film was stabilised by either anionic (sodium lauryl sulfate (NaDS)) or cationic (trimethyl(tetradecyl) ammonium bromide (TTAB)) surfactants. Fluid flow profiles in a stabilised free liquid film were measured by micron-resolution particle image velocimetry (µ-PIV) combined with a confocal laser scanning microscopy (CLSM) setup. Numerical simulations of electroosmotic flow in the same system were performed using the Finite Element Method. The computational geometry was generated by CLSM. A reasonably good agreement was found between the computed and experimentally measured velocity profiles. The features of the flow profiles and the velocity magnitude were mainly determined by the type of surfactant used. Irrespective of the surfactants used, electroosmotic flow dominated in the midfilm region, where the film is thinnest, while backflow due to pressure build-up developed near the glass rods, where the film is thickest. Full article

This paper reviews two separation methods applying two-step two-liquid flotation for recovering ultra-fine rare earth fluorescent powders (i.e., red, green, and blue). The paper aims to extract the science behind separation by two-liquid flotation, and to provide resulting engineering tips for material recycling. Two-liquid flotation, also called liquid-liquid extraction, involves two solvents (i.e., non-polar and polar solvents) to capture hydrophobic/hydrophobized particles at their interface, and a surfactant to selectively modify the surface property of the target powder(s). For separating a three powder mixture, two different developed flowsheets, composed of two-step separation are discussed. The major difference found was the polar solvents used. The first flowsheet (called the aqueous-organic system) employed water as a polar solvent while the second flowsheet (called the organic-organic system) utilized N,N-dimethylformamide, DMF as a polar solvent. The organic-organic system at the optimized conditions achieved both the grade and recovery of all the separated fluorescent powders at greater than 90% while the aqueous-organic system did not satisfy these criteria. This paper also reviews the mechanism behind the separation, as well as performing a cost comparison between the two methods. The cost comparison indicates that the organic-organic system is a more cost effective method for recovering rare earth fluorescent powders than the aqueous-organic system. Since the size of powders (i.e., several µm) is too small for the application of conventional separation technologies (e.g., froth flotation), two-liquid flotation is a unique pathway for the material recycling of ultra-fine rare earth fluorescent powders. Full article

To understand the behavior of cellular interfaces, it is important to clarify the effect of chemical compounds on artificial cell membranes. In this study, an aqueous acetonitrile solution was mixed with a suspension of lipid vesicles, and the changes in vesicle behavior arising as a result of acetonitrile application were observed. The fast Fourier transformations (FFTs) of the membrane waviness/crinkliness of the vesicles were carried out, and the membrane thermal fluctuations were analyzed. The experimental results show that the addition of acetonitrile molecules enhances the fluctuation of lipid membranes. In particular, the $k=2$ mode fluctuation was significantly enhanced. This finding is expected to lead us to a further understanding of the fundamental properties of living cells. Full article

The dynamic surface tension (DST) data and biochemical parameters (BCP) of animal blood have been obtained. A strong positive correlation was found for goats between σ₁, σ₂, σ₃ (DST) and sodium levels (BCP), λ_z (DST)—with the levels of lipid and sodium (BCP); whereas a strong negative correlation was found for goats between σ₃ (DST) and the level of total protein and chloride (BCP), λ_z (DST)—with the level of albumin (BCP). A strong positive correlation was found for horses between σ₁, σ₂, σ₃ (DST) and lipid levels (BCP); λ₀ (DST)—the level of chloride (BCP); λ_z (DST)—the level of albumin and chloride (BCP). A strong negative correlation was found for horses between σ₁ (DST) and sodium level (BCP); σ₂, σ₃ (DST) and chloride level (BCP); λ₀ (DST) and lipid level (BCP). Some moderate and weak correlations of different types were also found, but these have less importance for practical usage. The particular correlations between BCP and DST data of the same serum samples allowed DST data to be checked directly at the farm, and requiring the use of more expensive and time-consuming biochemical analysis only in cases of high necessity for a particular animal. Full article

Energy storage in porous capacitor materials, capacitive deionization (CDI) for water desalination, capacitive energy generation, geophysical applications, and removal of heavy ions from wastewater streams are some examples of processes where understanding of ionic transport processes in charged porous media is very important. In this work, one- and two-equation models are derived to simulate ionic transport processes in heterogeneous porous media comprising two different pore sizes. It is based on a theory for capacitive charging by ideally polarizable porous electrodes without Faradaic reactions or specific adsorption of ions. A two-step volume averaging technique is used to derive the averaged transport equations for multi-ionic systems without any further assumptions, such as thin electrical double layers or Donnan equilibrium. A comparison between both models is presented. The effective transport parameters for isotropic porous media are calculated by solving the corresponding closure problems. An approximate analytical procedure is proposed to solve the closure problems. Numerical and theoretical calculations show that the approximate analytical procedure yields adequate solutions. A theoretical analysis shows that the value of interphase pseudo-transport coefficients determines which model to use. Full article

The aim of our work was to study the structure of a typical modern cosmetic oil-in-water emulsion (o/w-emulsion), based on the emulsifier polyglyceryl-3 dicitrate/stearate and glycerylstearate/stearyl alcohol as consistency enhancer. We have used a systematic approach building up the cosmetic emulsion step by step, characterizing all systems by using Small Angle Neutron Scattering (SANS), Differential Scanning Calorimetry (DSC), Freeze Fracture Transmission Electron Microscopy (FF-TEM), light microscopy and rheology. The starting point was the pure emulsifier in water, which was shown to form lamellar stacked bilayers with a spacing of 7 nm, coexisting with polydisperse unilamellar vesicles in the sub-µm range. Upon addition of consistency enhancer, also multilamellar vesicles could be obtained. Then, oil has been added stepwise, until finally a complete cosmetic o/w-emulsion was obtained. In the final emulsion, oil droplets with sizes in the µm range are surrounded by multiple, irregularly spaced bilayer structures and vesicles. Approximately 30% of the water present in the system shows a restricted mobility (“encapsulated water”) according to PFG-NMR. Crucial for the viscosity build-up is the presence of the oil droplets; a cream-like consistency is obtained by steric interaction of oil droplets and crystalline bilayer structures in the aqueous phase of the emulsion. Full article

Today, naturally occurring foam constituents and surface-active proteins with intriguing structures and functions are being identified from a variety of biological and chemical sources. In this paper we studied the colloid chemical properties of high-molecular natural surfactants such as keratin hydrolyzate (1.5–15%), gelatin (0.1–1%), and egg albumin (0.1–1%) in a wide concentration range. The foaming ability and foam-stabilizing properties of mixtures of these proteins were established. The high stability of foams obtained from mixtures of surfactants can be explained by the formation of mixed structured layers from the surface-active associates, promoting the thickening of foam films. The ratio of polymer mixtures was optimized (keratin (15%)-albumin (1%) (1:1)) to produce high-quality foaming agents. The foam parameters such as surface tension, capillary pressure of the Plateau-Gibbs channels, radii of curvature, critical micelle concentration, and relative viscosity were defined. The high surface activity and foam stability corresponds to a pH close to the isoelectric state of the proteins. This occurs due to the conformational changes of macromolecules of the protein at the liquid-gas interface, forming particles of colloidal size. Full article

Microbial Enhanced Oil Recovery (MEOR) is one of the tertiary recovery methods. The high viscosity and low flow characteristics of heavy oil makes it difficult for the extraction from oil reservoirs. Many spore-forming bacteria were isolated from Oman oil fields, which can biotransform heavy crude oil by changing its viscosity by converting heavier components into lighter ones. Two of the isolates, Bacillus firmus BG4 and Bacillus halodurans BG5, which showed maximum growth in higher concentrations of heavy crude oil were selected for the study. Gas chromatography analysis of the heavy crude oil treated with the isolates for nine days showed 81.4% biotransformation for B. firmus and 81.9% for B. halodurans. In both cases, it was found that the aromatic components in the heavy crude oil were utilized by the isolates, converting them to aliphatic species. Core flooding experiments conducted at 50 °C, mimicking reservoir conditions to prove the efficiency of the isolates in MEOR, resulted in 10.4% and 7.7% for B. firmus and B. halodurans, respectively, after the nine-day shut-in period. These investigations demonstrated the potential of B. firmus BG4 and B. halodurans BG5 as an environmentally attractive approach for heavy oil recovery. Full article