A new standardized lipolysis approach for characterization of emulsions and dispersions

A new standardized lipolysis approach is presented where the focus is on the initial rate of lipolysis. An advantage is that data obtained in this way reflect degradation before growing amounts of lipolysis products retard the process. The method can be used to rank different lipase substrates. In particular, the method can be used to obtain information about the susceptibility to degradation of various emulsions and dispersions that are used in technical applications. We present how the method is standardized to facilitate comparison of various substrates. This involves (i) lipase substrate in excess, i.e., the amount of lipase is rate limiting, and (ii) expressing rate of degradation relative to that of a reference substrate, tributyrin. Under such conditions, with the amount of lipase substrate held constant, an increase in enzymatic activity will generate a proportional increase in the lipolysis rate. This enables comparison of results obtained from different enzyme batches and corrects for day-to-day variability. Examples illustrating the potential of the method to discriminate and rank different lipase substrates with regard to enzymatic degradation are presented.     

Multiple emulsions for food use

Numerous investigations were done to describe the effects of different food-grade components (e.g. lipid phases, emulsifiers, electrolytes, biopolymers, sugars) on the stability of multiple emulsions (type W/O/W and O/W/O). In addition to the emulsion composition, the stability of such systems also depends heavily on the emulsion matrix, influenced by the dispersing methods used. This review highlights recently published results of research done in the preparation of such dispersed systems and in the manipulation of their emulsion stability as well as encapsulation behaviour.     

Biocompatible surfactants for water-in-fluorocarbon emulsions

Drops of water-in-fluorocarbon emulsions have great potential for compartmentalizing both in vitro and in vivo biological systems; however, surfactants to stabilize such emulsions are scarce. Here we present a novel class of fluorosurfactants that we synthesize by coupling oligomeric perfluorinated polyethers (PFPE) with polyethyleneglycol (PEG). We demonstrate that these block copolymer surfactants stabilize water-in-fluorocarbon oil emulsions during all necessary steps of a drop-based experiment including drop formation, incubation, and reinjection into a second microfluidic device. Furthermore, we show that aqueous drops stabilized with these surfactants can be used for in vitro translation (IVT), as well as encapsulation and incubation of single cells. The compatability of this emulsion system with both biological systems and polydimethylsiloxane (PDMS) microfluidic devices makes these surfactants ideal for a broad range of high-throughput, drop-based applications.     

Microgels as stimuli-responsive stabilizers for emulsions

Temperature- and pH-sensitive microgels from cross-linked poly(N-isopropylacrylamide)-co-methacrylic acid are utilized for emulsion stabilization. The pH- and temperature-dependent stability of the prepared emulsion was characterized. Stable emulsions are obtained at high pH and room temperature. Emulsions with polar oils, like 1-octanol, can be broken by either addition of acid or an increase of temperature, whereas emulsions with unpolar oils do not break upon these stimuli. However, complete phase separation, independent of oil polarity, can be achieved by successive acid addition and heating. This procedure also offers a way to recover and recycle the microgel from the sample. Interfacial dilatational rheology data correlate with the stimuli sensitivity of the emulsion, and a strong dependence of the interfacial elastic and loss moduli on pH and temperature was found. The influence of the preparation method on the type of emulsion is demonstrated. The mean droplet size of the emulsions is characterized by means of flow particle image analysis. The type of emulsion [water in oil (w/o) or oil in water (o/w)] depends on the preparation technique as well as on the microgel content. Emulsification with high shear rates allows preparation of both w/o and o/w emulsions, whereas with low shear rates o/w emulsions are the preferred type. The emulsions are stable at high pH and low temperature, but instable at low pH and high temperature. Therefore, we conclude that poly(N-isopropylacrylamide)-co-methacrylic acid microgels can be used as stimuli-sensitive stabilizers for emulsions. This offers a new and unique way to control emulsion stability.     

Polyoxyethylenated bisphenol-A for breaking water-in-oil emulsions

Polyethylene glycol (PEG) of different molecular weights, namely, 600, 1000 and 4000 g/mol was reacted with bisphenol A to form compounds having different hydrophile–lipophile balances and hence different surface activities. The interfacial tension at the aqueous/benzene interface was determined. It was found that the concentrations of demulsifiers required to cause a minimum interfacial tension are always less than those inducing a maximum demulsification efficiency. The demulsification efficiency of the prepared surfactants in breaking synthetic water in benzene emulsions stabilized by petroleum asphaltenes was evaluated. The data revealed that the demulsification efficiency increases with increasing demulsifier concentration, contact time and hydrophilicity.     

Structured emulsions

Recent investigations have allowed a better understanding of interdroplets interactions, of the impact of protein–polysaccharide interactions on emulsion stability and of the mechanisms involved in the production of emulsion gels. Owing to the possibility of controlling the structural properties, the design of food emulsions with improved kinetic stability and/or novel functional properties becomes possible.     

A versatile coating approach to fabricate superwetting membranes for separation of water-in-oil emulsions

Separation of oil/water mixtures, especially for the emulsified oil/water mixtures, is important because of the frequent occurrence of oil spill accidents. Utilizing superwetting porous membrane has become a promising approach to separate either surfactant-free or surfactant-stabilized emulsions. Herein we report a facile and versatile strategy for preparing hydrophobic/under-oil superhydrophobic membranes by coating the skeletons of the membranes with the poly[(3,3,3-trifluoropropyl)methylsiloxane] (PTFPMS) nanoparticles. The obtained membranes could be used to separate various waterin- oil emulsions with high flux and separation efficiency. In addition, owning to the outstanding resistance of PTFPMS to the most organic solvents or oils, the modified membranes exhibited the excellent reusability and the antifouling properties that were critical in the practical applications. Many commercially available membranes can be modified by such a simple method.     

A microfluidic electrochemical detection technique for assessing stability of thin films and emulsions

An experimental technique is developed for assessing stability of thin liquid films by application of electric potential to compress the liquid film and to simultaneously measure the electrical properties of the system. The concept involves creating a thin film at the intersection of two microchannels etched onto a glass substrate. A ramped DC potential difference is applied across the film, which develops an electrical stress across the film. Increasing the potential to a critical value leads to the rupture of the film. The critical potential is used to assess the stability of the liquid film. Small channel dimensions in this microfluidic platform allow characterization of thin films formed between micron-sized droplets representing systems with high capillary pressures, analysis of which are typically beyond the scope of conventional thin film characterization techniques. The results of DC potential breakdown of films show that critical potential can be considered as a measure of thin film stability.     

Solid-stabilized emulsions

The comprehension of bulk properties of solid-stabilized emulsions (stability, compressibility, elasticity) in relation with interfacial properties has progressed. The association of oil, water and particles allows a large set of materials to be obtained, where emulsions are used either as intermediate or end products. The efficiency of some stimulus-responsive particles to stabilize or destabilize emulsions “on demand” has been experimentally evidenced.     

A study of oil-in-water emulsions stabilized by whey proteins

Like many other emulsifiers, whey protein concentrates stabilize oil-in-water emulsions. However, the emulsifying capacity of whey proteins is affected by several factors, e. g., type of homogenizer, degree of homogenization, protein concentration, oil volume fraction, pH and ionic strength of the aqueous phase. For the present study, oilin-water emulsions were made by homogenizing known amounts of whey protein concentrate with a vegetable oil (i. e. grapeseed oil) at different pH. The emulsifying properties of whey proteins are expressed as a function of the particle size and size distribution of oil droplets as measured by light scattering, and of the surface charge density derived from the electrophoretic mobility.The whey protein concentrate was shown to have an isoelectric point at pH 4.4. Near this pH value, the oil-in-water emulsions exhibited poor stability as expected from the low surface coverage.     

Particles with tunable wettability for solid-stabilized emulsions

We demonstrate that the wettability of cosmetic grade, silica-coated titanium dioxide nanoparticles may be tuned by simply soaking them in a cyclic silicone oil. This allows for tuning the type of emulsion that they stabilize, from oil-in-water to water-in-oil. By analyzing the sedimentation of water-in-oil emulsions, the effect of the soaking time (wettability) on drop size and drop-drop interactions was investigated. From centrifugation experiments performed up to emulsion breakage, we obtain an effective water-oil interfacial tension for the particle-loaded interfaces, which indicate lateral particle-particle interactions. Finally, we demonstrate that the proposed particle functionalization is terminated upon addition of water followed by emulsification. In addition, it is irreversible and can be accelerated through heating.GRAPHICAL ABSTRACT     

Ideal limiting fluxes in ultrafiltration: comparison of various theoretical relationships

The ultrafiltration of macromolecules is characterised by a limiting flux at high transmembrane pressures. There is also some evidence that at high pressures and low crossflow velocities the flux decreases slightly with increasing pressure. It is confirmed from a theoretical viewpoint that this can only be caused by a decrease in the average mass-transfer coefficient due to concentration increases in the boundary layer. At the practical level, we propose an expression which, for a given system, enables the ideal flux to be estimated a priori as a function of the transmembrane pressure. The ideal flux is defined as that flux which would occur in the absence of fouling and gelation. The model includes the influence of both osmotic pressure and the variation in viscosity due to concentration polarisation. Thus for predictive purposes knowledge of osmotic pressure and viscosity as a function of concentration is required. The only membrane parameter that has to be experimentally determined is the membrane permeability. In the absence of adsorption (which is the ideal case) this is the permeability to the pure solvent. The model has been tested against Jonsson's data for the ultrafiltration of dextran solutions. The results are most encouraging.     

Chemical destabilization of crude oil based emulsions and asphaltene stabilized emulsions

A comparison of low and high molecular weight demulsifiers and their effect on both crude oil and asphaltene based water-in-oil emulsions is performed. Physical characteristics are given for crudes and for the chemicals. These parameters were then correlated with the demulsifier performance. Results indicate that a significant lowering of interfacial tension is required, but not sufficient for an efficient demulsification. Addition of the chemicals directly to the oil phase prior to emulsification, i.e., as inhibitors, increased the performance of the chemicals significantly. Received: 3 May 2000 Accepted: 10 July 2000     

Deaggregation of acacia emulsions

Summary Emulsions aggregated by the hydrocolloid potassium arabate through interparticular bridging, are deaggregated by shear and added surface active agent but are unaffected by simple electrolyte. The minimum shear rate for almost complete deaggregation is in the region of 550 sec⁻¹ and the process is irreversible. In terms of critical micelle concentrations (c.m.c), potassium laurate (anionic) is more effective than hexadecyl-trimethylammonium bromide (cationic) which in turn is very much more effective than Tween 20 (non-ionic). The rate of deaggregation by surface active agent is rapid.