Colloids Interfaces, Volume 4, Issue 1 (March 2020) – 13 articles

A closed form expression capable of predicting the evolution of the shape of liquid capillary bridges and the resultant force between parallel platens is derived. Such a scenario occurs within many micro-mechanical structures and devices, for example, in micro-squeeze flow rheometers used to ascertain the rheological properties of pico- to nano-litre volumes of complex fluids, which is an important task for the analysis of biological liquids and during the combinatorial polymer synthesis of healthcare and personal products. These liquid bridges exhibit capillary forces that can perturb the desired rheological forces, and perhaps more significantly, determine the geometry of the experiment. The liquid bridge has a curved profile characterised by a contact angle at the three-phase interface, as compared to the simple cylindrical geometry assumed during the rheological analysis. During rheometry, the geometry of the bridge will change in a complex nonlinear fashion, an issue compounded by the contact angle undergoing hysteresis. Owing to the small volumes involved, ascertaining the bridge geometry visually during experiment is very difficult. Similarly, the governing equations for the bridge geometry are highly nonlinear, precluding an exact analytical solution, hence requiring a substantial numerical solution. Here, an expression for the bridge geometry and capillary forces based on the toroidal approximation has been developed that allows the solution to be determined several orders of magnitude faster using simpler techniques than numerical or experimental methods. This expression has been applied to squeeze-flow rheometry to show how the theory proposed here is consistent with the assumptions used within rheometry. The validity of the theory has been shown through comparison with the exact numerical solution of the governing equations. The numerical solution for the shape of liquid bridges between parallel platens is provided here for the first time and is based on existing work of liquid bridges between spheres. Full article

Herein, we synthetized and characterized polysaccharide-based complex coacervates starting from two water-soluble biopolymers, i.e., hydrochloride chitosans and sodium hyaluronan. We used chitosans encompassing a range of molecular weights from 30,000 to 400,000 and showing different fraction of acetylated units (i.e., F_A = 0.16, 0.46, and 0.63). This set of chitosans was mixed with a low molecular weight hyaluronan to promote electrostatic interactions. Resulting colloids were analyzed in terms of size, polydispersity and surface charge by Dynamic Light Scattering. The weight ratio between the two polyelectrolytes was studied as additional parameter influencing the liquid-liquid phase separation. Main results include the following: the polymers weight ratio was fundamental in dictating the colloids surface charge, whereas chitosan physical-chemical features influenced the dimension and homogeneity of colloids. This contribution presents additional understanding of the complex coacervation between these two oppositely charged polysaccharides, with the potential translation of present system in food and biomedical sectors. Full article

Bitter taste is a positive sensory attribute that correlates with the concentration of phenols in olive oil. However, excessive bitterness can be perceived by consumers as a negative attribute. The aim of this investigation was to improve the process of debittering Don Carlo extra virgin olive oil (EVOO), which is rich in phenols, through blending with newly produced Leccino EVOOs, which can provide high oleuropeinolytic activity. The debittering process of blending Don Carlo EVOO with two types of Leccino EVOOs (decanter and settled EVOO), was carried out during three months of storage in canisters placed in fixed positions, or periodically inverted to prevent sedimentation. The reduction in phenolic concentration and bitterness index (K225 value) reached maximum values of 51% and 42% respectively in Don Carlo EVOO mixed with Leccino settled EVOO after three months of storage in periodically inverted containers. Analytical indices and sensory analysis, in accord with bitterness index (K225) results, confirmed a reduction or elimination of bitter taste in the oil samples depending on the type of Leccino EVOO added, and the sample storage method. All analytical results remained within parameters established by the European Community regulations for commercial merceological class EVOO. Full article

We confirmed that mannosylerythritol lipid B (MEL-B), a biosurfactant, enhances the skin permeability of the model water-soluble compound calcein. MEL-B liposomes were prepared by the thin-layer evaporation technique, and then applied to the skin. Although we attempted to adjust the size by extrusion, we could not control the particle diameter of the liposomes. However, the MEL-B liposome particle diameter remained the same over the 7-day study period. We observed an endothermic peak, with 74.7 °C as the transition temperature by differential scanning calorimetry. We also performed a fusion experiment with a fluorescence resonance energy transfer. A high amount of fusion of intercellular lipid liposomes and MEL-B liposomes occurred in a short period of time. After applying the MEL-B liposomes containing calcein to the skin, we measured the degree of calcein permeation and the amount of calcein within the skin. The resulting values were higher than those of an aqueous solution. The results obtained using a confocal laser scanning microscope suggested that calcein had been delivered deeply into the skin. Using the attenuation of total reflectance Fourier-transform infrared spectrometry, we observed that the OH stretching vibration had shifted to a higher wavenumber; however, this did not affect the CH stretching vibration. The measurement of transepidermal water loss after four days of continuous application of 1% MEL-B to animals revealed no changes. Our results suggest that MEL-B increases the skin permeability of compounds (calcein) that are difficult to deliver transdermally by changing the OH stretching vibration, which shifts to a higher wavenumber. Full article

This work is devoted to the influence of NaCl salt concentration on the formation and stability of colloidal gas aphrons (CGA) produced by the anionic surfactant sodium dodecyl sulfate (SDS) and zwitterionic surfactant coco amido propyl betaine (CAPB) in the presence of xanthan gum (XG) as a stabilizer. Dynamic surface tension measurements as well as volume and half-life time of the produced foams are considered for stability analysis. A sharp decrease of the half-life time and volume of the CGAs at NaCl concentrations larger than 20,000 ppm was observed, which was attributed to the precipitation of SDS in the solution. The mentioned SDS precipitation altered the dynamic surface tension behavior, dilational surface elasticity, and turbidity of the solution. The main reason for the precipitation of SDS is the increased Krafft point caused by the addition of salt. However, for the zwitterionic surfactant CAPB, the effects of added NaCl on the interfacial properties required for CGAs production was negligible due to the simultaneous effects on the cationic and anionic head groups in the CAPB leading to negligible changes in the net repulsion forces. Yet, an overall reduction in the half-life time of CGAs with increasing salt concentration, even when we have no precipitation, was observed for both surfactants, which could be explained by the reduction in the ability of XG to increase the viscosity with increasing salt concentration. Full article

Double emulsions are very promising for various applications in pharmaceutics, cosmetics, and food. Despite lots of published research, only a few products have successfully been marketed due to immense stability problems. This review describes approaches on how to characterize the stability of double emulsions. The measurement methods are used to investigate the influence of the ingredients or the process on the stability, as well as of the environmental conditions during storage. The described techniques are applied either to double emulsions themselves or to model systems. The presented analysis methods are based on microscopy, rheology, light scattering, marker detection, and differential scanning calorimetry. Many methods for the characterization of double emulsions focus only on the release of the inner water phase or of a marker encapsulated therein. Analysis methods for a specific application rarely give information on the actual mechanism, leading to double emulsion breakage. In contrast, model systems such as simple emulsions, microfluidic emulsions, or single-drop experiments allow for a systematic investigation of diffusion and coalescence between the individual phases. They also give information on the order of magnitude in which they contribute to the failure of the overall system. This review gives an overview of various methods for the characterization of double emulsion stability, describing the underlying assumptions and the information gained. With this review, we intend to assist in the development of stable double emulsion-based products. Full article

The aim of this study is the development of an emulsification device for two immiscible liquids with a total volume of approximately 3 mL. The heart of the device is a piston, with an aluminum plate fixed at its tip, which moves periodically up and down inside a rectangular cell. The plate geometry (uniform or non-uniform height) affects significantly both the emulsions stability and the size of the droplets of the prepared emulsions. Five parameters are examined during testing (surfactant type, surfactant concentration, proportion of immiscible liquids, piston stroke frequency, duration of emulsification) and all of them appear to have an important role in the resulting droplet size distribution. A macroscopic theoretical model is developed for the determination of the main hydrodynamic parameters of the innovative device. It is shown that the non-uniform height plate achieves higher shear rates when compared to the uniform height plate because of the smaller gap between the plate and the cell walls. However, the benefits of the higher shear rate are overturned by the larger effective breakage time encountered in the uniform height plate resulting from the larger surface area of its sides. The results of the emulsification experiments are analyzed using the parameter values derived by the developed model. Full article

Limonene as an interesting bioactive material that has great benefits due to its antimicrobial and anti-carcinogen properties. However, it has several limitations such as its oxidative and oily nature. In order to overcome these limitations, a high-pressure homogenizer (HPH) was utilized to produce limonene nanoemulsion, which enhances its dispersibility while preventing oxidation with great stability. Limonene was pre-mixed with soybean oil as carrier oil prior to emulsification. The effect of soybean oil to limonene ratio, number of pass, homogenization pressure, emulsifier concentration and homogenization method were observed. A stability test was also conducted for 28 days at room temperature. The result revealed that soybean oil and limonene demonstrated a certain ratio to produce the most stable nanoemulsion. Meanwhile, emulsion size could be reduced from 327.8 nm to 55.5 nm in five passes at 1000 bar. Increasing the emulsifier concentration could reduce the droplet size to 40 nm. A comparison with other emulsification method showed that HPH was the best emulsification technique due to its intense emulsification power resulted from shear, cavitation, and droplet impacts. This study reveals that HPH is a great and simple way to produce stable limonene nanoemulsion for the cosmetic, pharmaceutical, and food industries. Full article

This paper reports the use of a numerical solution of nanofluid flow. The boundary layer flow over a stretching sheet in combination of two nanofluids models is studied. The partial differential equation that governs this model was transformed into a nonlinear ordinary differential equation by using similarity variables, and the numerical results were obtained by applying the shooting technique. Copper (Cu) nanoparticles (water-based fluid) were used in this study. This paper presents and discusses all numerical results, including those for the local Sherwood number and the local Nusselt number. Additionally, the effects of the nanoparticle volume fraction, Brownian motion Nb, and thermophoresis Nt on the performance of heat transfer are discussed. The results show that the stretching sheet has a unique solution: as the nanoparticle volume fraction φ (φ = 0), Nt (Nt = 0.1), and Nb decrease, the rate of heat transfer increases. Furthermore, as φ (φ = 0) and Nb decrease, the rate of mass transfer increases. The data of the Nusselt and Sherwood numbers were tested using different statistical distributions, and it is found that both datasets fit the Weibull distribution for different values of Nt and rotating φ. Full article

The effects of unmodified and modified bentonite nanoclays (with various degrees of surfactant modification) on the catastrophic phase inversion from water-in-oil (W/O) emulsion to oil-in-water (O/W) emulsion were determined experimentally. The bentonite nanoclay (NC-Bt) was suspended in the aqueous phase, and the critical volume fraction of water where phase inversion from W/O to O/W emulsion took place was determined through conductivity measurements. Cetyltrimethyl ammonium bromide (CTAB) was used as a surfactant to modify the nanoclay. The adsorption of CTAB onto nanoclay had a strong influence on the contact angle and the critical volume fraction of water where phase inversion took place. The modification of the nanoclay brought about by the adsorption of CTAB increased the three-phase contact angle (measured through the aqueous phase), thereby making it more hydrophobic, and prolonged the phase inversion point. CTAB alone and CTAB-modified nanoclay delayed the phase inversion process in a similar manner, showing a strong dependence on the CTAB concentration. Full article

The van der Waals equation is well known for the description of two-dimensional monolayers. The formation of a monolayer is the result of a compromise between the process of self-organization on the surface and the probabilities of spatial configurations of adsorbate molecules near the surface. The main reasons for the geometric heterogeneity of the monolayer are the geometric disorder and the energy inhomogeneity of the surface profile. A monolayer is a statistically related system and its symmetry causes correlations of processes at different spatial scales. The classical van der Waals equation is written for the two-dimensional, completely symmetric Euclidean space. In the general case, the geometry of the monolayer must be defined for the Euclidean space of fractional dimension (fractal space) with symmetry breaking. In this case, the application of the classical van der Waals equation is limited. Considering the fractal nature of the monolayer–solid interface, a quasi-two-dimensional van der Waals equation is developed. The application of the equation to experimental data of an activated carbon is shown. Full article