In situ determination of the size and polydispersity of concentrated emulsions

Droplet size distributions of concentrated, polydisperse oil-in-water emulsions have been measured using ultra small angle neutron scattering (USANS). The mean radii calculated by fitting a model for polydisperse hard spheres with excluded volume interactions to the USANS data were consistent with those derived from electroacoustics on diluted emulsions after correction for conductance behind the shear plane. The Porod radii measured by USANS were similarly consistent with the mean surface-area-weighted radii derived from electroacoustics, irrespective of the drop concentration or polydispersity.     

Critical behavior in microemulsions

Interactions in dispersions have been studied using light scattering techniques applied to microemulsions. In these systems, hard sphere interactions are dominant. The remaining interactions (van der Waals, etc.) are usually attractive and short-ranged and can be treated as perturbations. However, close to phase transitions where the microemulsion separates into two other microemulsions, the attractive part of the potential becomes large and behaves as if long range interactions were present; the characteristics of the scattered light can also be interpreted by assuming that the system is close to a critical consolute point. The low interfacial tensions (measured between the two microemulsions in equilibrium using surface light scattering techniques) and the large interfacial thicknesses (deduced from optical reflectivity) are consistent with the picture in terms of critical phenomena.     

Studies on microemulsions

The microemulsions formed in the 4-component system water-potassium oleate-hexanol-dodecane have been investigated by time-average light scattering and small angle neutron scattering. A constant volume fraction ratio water: potassium oleate of 1.44 was used and at this constant composition, which gave a pseudo 3-component system, a wide region of the microemulsion domain was examined. In order to interpret the scattering data at finite volume fractions of the dispersed phase, water, allowance had to be made for interactions between the water-in-oil microemulsion droplets. This was carried out using a hard sphere model for the interaction. It is shown that using this model self-consistent results are obtained by light scattering and neutron scattering and an estimate can be made of the size of the particles in concentrated colloidal dispersions.     

Ultrasonic attenuation spectroscopy of emulsions with droplet sizes greater than 10 microm

Ultrasonic attenuation measurement is a frequently used tool for non-destructive determination of dispersion characteristics. Useful information like particle or droplet size and their concentration can be obtained, if the relation between size and attenuation of the dispersion is known. In this work, the theoretical model by Faran for the intermediate sound wave regime (IWR) is presented in combination with experimental data. In the IWR, the acoustic behavior is governed by elastic scattering rather than by dissipative effects. Experiments with emulsion of droplet sizes greater than 10 mum were carried out. Silicone oil, sunflower oil and olive oil were selected for the disperse phase of the oil-in-water emulsions. First, emulsions having droplets in the micrometer range were created. Afterwords, attenuation measurements of different concentrated emulsion were carried out. Some adjustments reflecting concentration influence were performed to outline the agreement between calculations and measurements. The validity of the model can be confirmed, if the volume fraction of the disperse phase is considered as a variable. Finally, droplet size distributions from theoretical attenuation spectra could be calculated based on a log-normal distribution.     

Industrial applications of dispersions

In this review some industrial applications of dispersions have been discussed. After a general introduction, some specific topics have been covered. The preparation of dispersions using condensation and dispersion methods was discussed in terms of the various interfacial processes involved such as nucleation and growth, wetting, breaking of aggregates and agglomerates as well as comminution. The process of emulsification (for production of liquid/liquid dispersions) was also analyzed in terms of the interfacial processes such as reduction in interfacial tension, interfacial elasticity and viscosity. The control of the properties of dispersions was described in terms of the interaction forces between the particles or droplets in the system. These interaction forces are governed by the structure and properties of the interfacial region such as double layers, presence of adsorbed surfactant or polymer layers. Four main types of interaction forces may be distinguished : hard-sphere, electrostatic, steric and van der Waals. Combination of these forces lead to three general energy-distance curves that can be used to describe the state of the dispersion (stable, flocculated or coagulated). The various physical states of suspensions and emulsions produced on standing were schematically presented and they could be explained in terms of the energy-distance curves. The flow characteristics (rheology) of dispersions could also be accounted for in terms of the various interaction forces between the particles.Solubilization and microemulsions, which produce thermodynamically stable dispersions, could be described in terms of the balance between the interfacial energy and entropy of dispersion of the system. The driving force for producing such thermodynamically stable systems was the ultra low interfacial tension which could be achieved by using a combination of surfactants. The application of microemulsions in various fields such as solubilization, enhanced oil recovery and energy production was briefly described.The application of dispersions in microncapsulation and slow release was described in terms of interfacial polymerization, coacervation and multiple emulsion formation. These systems find application in medicine, agrochemicals and cosmetics. The application of dispersions in pharmacy and medicine was also described by quoting specific examples such as liposomes (vesicles), nanoparticles and magnetic microspheres. These systems have potential use in targeting delivery of drugs.     

Stacking structure of concentrated shear ordered dispersions by two scattering methods

In this paper, the ordering in concentrated charge stabilized colloidal dispersions is considered. Despite the impressive Bragg reflections obtained for shear ordered dispersions by light (LS), small-angle neutron (SANS), and small-angle X-ray scattering (SAXS), a number of open questions remain. Sheared dispersions are usually ordered in layers. For such systems, two questions arise: (1) What is the structure in a layer? (2) What is the stacking structure perpendicular to the layers? The second question requires a method to determine the structure perpendicular to the layers. Although originally interested only in structural aspects, we were forced to consider different methods. Two methods are treated both applicable to neutron and X-ray scattering from concentrated dispersions. One has been used by physicists and chemists for many years to determine the structure of crystals by sample rotation. In colloid science, we have used it previously in neutron and X-ray scattering. A second method is treated here which can be applied in small-angle scattering from a Couette cell. It gives the scattering intensity in a certain direction without sample rotation. Although very useful with the Couette cell, it cannot be found in any of the well-known references on colloid science. A theoretical explanation and experimental examples obtained by synchrotron X-ray scattering from a Couette cell are given in the paper.     

Highly concentrated emulsified microemulsions as solvent-free plant protection formulations

Effective plant protection agents are readily available and well implemented in industry. However, delivery to the plant and application on the leaf are processes that still need to be optimized. Up to now plant protection formulations represent either emulsion or suspension concentrates that often contain environmentally harmful organic solvents and/or adjuvants. Emulsified microemulsions are hierarchically organized systems comprising emulsion droplets that confine a water-in-oil microemulsion. In the present contribution we show that emulsified microemulsions prepared from environmentally friendly components can be loaded with the plant-protection agent Fenpropimorph® up to 48 wt.% without organic solvent. The emulsion itself is highly concentrated, containing 60 wt.% of dispersed phase, and can be readily diluted with water for spraying in farming applications. Small-angle X-ray measurements reveal the existence of a water-in-Fenpropimorph® microemulsion confined inside the emulsion droplets. Dynamic light scattering shows that the emulsions prepared are monomodal, comprising droplet radii in the hundred nanometer range.     

Synthesis,morphology, and properties of waterborne m-TMXDI-based anionic polyurethane and hybrids

In this study, waterborne polyurethane (WPU) hybrid emulsions with a weight ratio of 2/1 were prepared by emulsion polymerization using a mixture of styrene (St) and/or butyl acrylate (BA) monomers with WPU dispersion. WPU dispersion was synthesized with isocyanic acid and m-tetramethylxylene diisocyanate (m-TMXDI)-based anionic poly(urethane-urea) dispersions using the prepolymer mixing process. The structures of WPU and hybrids were characterized by FTIR spectroscopy. The size and morphology of the latex particles were investigated using dynamic light scattering and transmission electron microscopy, respectively. The stability of the emulsions was determined according to their shelf life and particle size using the dispersion analyser LUMiSizer® with STEP?-Technology. The thermal and mechanical properties of these films were examined by thermogravimetric analysis and strain-stress curves.     

Pickering Interfacial Catalysis for Biphasic Systems: From Emulsion Design to Green Reactions

Pickering emulsions are surfactant‐free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant‐stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.     

Physical-chemical conditions for production of combined alkyd-acrylic dispersion

When preparing the mixing of acrylic polymer and copolymer dispersions with alkyd oligomer emulsions it is necessary to provide agregative stability of the combined dispersions. It was established that transfer of polymer particles mass of highly dispersed systems onto particles of less dispersed systems is seen during geterocoagulation of combined dispersions. Optimal physical-chemical and hydrodynamic conditions of the emulsification of alkyd oligomers with the required dispersion degree for producing the mixed systems were established. The combined dispersion stability is determined from the ratio of electrokinetic potentials of particles of polymers and oligomers being combined as well as ratio of their isoelectric points. The zone of dispersion incompatibility was established by method of microelectrophoresis.     

Phase transitions and microstructure of emulsion systems prepared with acylglycerols/zinc stearate emulsifier

The emulsification processes, during which acylglycerols/zinc stearate emulsifier, water, and oil phase formed ternary systems, such as water-in-oil (W/O) emulsions, oil-in-water (O/W) dispersions, and unstable oil-water mixtures, were investigated in order to characterize the progressive transformations of the dispersed systems. The type, structure, and phase transitions of the systems were found to be determined by temperature and water phase content. Crystallization of the emulsifier caused the destabilization and subsequent phase inversion of the emulsions studied, at a temperature of 60-61 degrees C. The observed destabilization was temporary and led, at lower temperature, to W/O emulsions, "O/W + O" systems, or O/W dispersions, depending on the water content. Simultaneous emulsification and cooling of 20-50 wt % water systems resulted in the formation of stable W/O emulsions that contained a number of large water droplets with dispersed oil globules inside them ("W/O + O/W/O"). In water-rich systems (60-80 wt % of water), crystallization of the emulsifier was found to influence the formation of crystalline vesicle structures that coexisted, in the external water phase, with globules of crystallized oil phase. Results of calorimetric, rheological, and light scattering experiments, for the O/W dispersions obtained, indicate the possible transition of a monostearoylglycerol-based alpha-crystalline gel phase to a coagel state, in these multicomponent systems.     

Formulation and physicochemical characterization of microemulsion system using isopropyl myristate, medium-chain glyceride, polysorbate 80 and water

The phase behavior of the system isopropyl myristate/medium-chain glyceride/polysorbate 80/water forming both w/o and o/w microemulsions has been studied to develop microemulsion comprising of pharmaceutical excipients. The pseudo-ternary phase diagrams with large monophasic zones and gel formation were realized and several compositions were identified in the phase diagram for rheological, dynamic light scattering (DLS) and calorimetric measurements. The identified systems at different temperatures behaved as Newtonian fluid and the activation parameters for their viscous flow were evaluated. From DLS measurements, hydrodynamic diameter, polydispersity and diffusion coefficient of the microheterogeneous dispersions were determined. The free energy, enthalpy and entropy of solution of the w/o and o/w microemulsions were determined from calorimetric measurements.     

Stability of suspensions, emulsions, and foams studied by a novel automated analyzer

The stability of dilute and concentrated dispersions (suspensions, emulsions, and foams) has been studied by a newly developed automated dispersion stability analyzer, DiStA 24. The analyzer operates by taking consecutive images of the samples studied and analyzing them by a personal computer (PC). As a result, the vertical profile of light scattered from each sample is obtained, recorded, and analyzed at time intervals preset by the computer software. In this way, the destructive processes in dispersions (sedimentation, creaming, and coalescence) can be detected well before it is possible by the naked eye and monitored automatically in the temperature range 0-50 degrees C. The new analyzer differs from the existing instruments by its simplicity and high productivity, allowing 24 samples to be measured simultaneously.     

Characterization of a complex dispersion of multilamellar vesicles

Spherulites ^® are multilamellar vesicles made up of surfactant bilayers. These vesicles would potentially be very useful for the encapsulation and protection of molecules; however, traditional formulations of these vesicles are poor at retaining small hydrophilic molecules (below 1000 g/mol). In this study, we present new systems of Spherulites called complex dispersions. These are prepared by dispersing Spherulites in an oil medium, and then emulsifying this oily dispersion of Spherulites within an aqueous solvent. These new systems provide an additional oil barrier between encapsulated molecules and an external aqueous phase. We have used polarized light optical microscopy, X-ray diffraction and freeze–fracture electron microscopy to study a complex dispersion of Spherulites at all stages of its preparation. We first studied the sheared lamellar phase, followed by the dispersion of the multilamellar vesicles in the oily medium and finally the emulsification of the oily dispersion within the aqueous solvent. We compared our results on lamellar phases with previous results obtained with Spherulites directly dispersible in an aqueous medium. Since the formulation of our lamellar phase included a large percentage of oil as a component, we studied the localization of the oil in the lamellar structure. We also studied the influence of osmotic pressure on complex dispersions, because complex dispersions possess a double structure similar to that of water-in-oil-in-water emulsions and multiple emulsions are known to be sensitive to osmotic pressure. In conclusion, complex dispersions proved to be new potential carriers exhibiting some unique physical properties.     

Anticipating colloidal instabilities in cationic vesicle dispersions by measuring collective motions with dynamic light scattering

Vesicle dispersions are useful for many applications from medicinal to consumer products. However, using these dispersions requires some knowledge of and control over their colloidal properties. Measuring interparticle interactions between vesicles should allow framing the problem in terms of Smoluchowski kinetic models and consequently anticipating time-dependent aggregation and coalescence for the dispersions. However, this can be a difficult task for many complex mixtures. A primary goal of this paper is to show that it is possible to measure interparticle potential between small vesicles by measuring the concentration-dependent collective motion using dynamic light scattering. These measurements allow determination of the second virial coefficient for the dispersion, providing a convenient platform for summing all contributions to the interaction potential over all vesicle conformations, thus making the analysis of complex mixtures more tractable. As a verification of the approach, a comparison is made to dispersions in which the stability is governed solely by electrostatics, using existing techniques to anticipate instabilities. A second goal of this paper is to build a simple potential model in which the Smoluchowski model can be used to quantitatively anticipate the aggregation behavior of the small vesicle dispersion. Together, these observations constitute a convenient approach to anticipating the behavior of vesicle (and other) dispersions in complex mixtures.     

DIFFUSION MEASUREMENTS IN MICROEMULSIONS

We have carried out diffusion coefficient measurements in both aqueous micelles and microemulsions using the techniques of palaeography and quasielastic light scattering (QLS) The former method involves the determination of the diffusion coefficient of an electroactive oil soluble probe at a polarizable microelectrode. For high water content microemulsions, both methods yield the same diffusion coefficients, which can be identified as the self diffusion coefficient For cetyltrimethylammonium bromide (CTAB) micelles, both methods yield the same result at the salt (NaBr) concentration at which the QLS measurements are independent of CTAB concentration. In more concentrated microemulsions, QLS data gave diffusion coefficients in agreement with polarography only for a sodium cetyl sulfate (SCS) system at 65-75 wt % water. For the SCS microemulsions at 60% water, and CTAB microemulsion at 60-75% water, the QLS data yielded rapid, nonexponential decays. However, consistent polarographic diffusion coefficients could still be obtained, By using probes of varying chain length (oil solubility), it has been demonstrated that these CTAB and SCS microemulsions containing butanol and pentanol cosurfactants respectively, are not cosolubilized systems but do contain distinct hydrophilic and hydropobic regions.     

Block copolymers as stabilizers in supercritical fluids

An emerging fundamental understanding of colloid and interface science in CO₂-based systems is guiding the discovery and development of microemulsions, emulsions, inorganic suspensions and polymer latexes. Steric stabilization of these dispersions may be achieved with block copolymer surfactants that adsorb at CO₂-organic or CO₂-aqueous interfaces.     

Suppression of multiple scattering by photon cross-correlation techniques

The effectiveness of cross-correlation schemes for suppressing multiple scattering in light scattering measurements has been demonstrated convincingly. Thus, measurements on turbid samples can be analysed as though the samples were transparent, i.e. exhibiting only single scattering. The methods are now being used for new research, particularly in the study of concentrated colloidal dispersions. This article reviews the current state of the field with emphasis on the two-colour and three-dimensional dynamic light scattering techniques. Although these methods were originally designed to suppress multiple scattering in dynamic light scattering, it has recently been recognised that they are also effective in static light scattering. The cross-correlation schemes are compared briefly with other light-scattering methods for studying turbid and opaque samples such as fibre-optic probes and diffusing wave spectroscopy.     

Small angle and quasi-elastic neutron scattering studies on polymethylmethacrylate latices in nonpolar media

Poly(methylmethacrylate) particles stabilised by poly(12-hydroxystearic acid) in a hydrocarbon medium provide, as a polymer latex, a useful system for investigating the properties of a sterically stabilised concentrated dispersion. In the present work these latices were investigated by small angle coherent neutron scattering and by quasielastic incoherent scattering. The former technique was used to probe the dimensions of the surface layer of poly(12-hydroxystearic acid) and to determine the diameter of the core particle of poly(methylmethacrylate) using dilute dispersions. In concentrated dispersions it was used to determine the structure factor. The latter was compared with that expected for an interacting system of hard spheres and reasonable agreement obtained. The quasielastic technique was used to probe the translational diffusion of the poly(12-hydroxystearic acid) molecules on the surface and the diffusion of the total particles in the concentrated systems.