Classical Emulsion Research Articles

Nimesulide PLA microspheres as a potential sustained release system for the treatment of inflammatory diseases

Polylactic acid (PLA- l) microspheres were prepared as a biodegradable polymeric carrier for a non-steroidal anti-inflammatory drug, nimesulide. The preparation of this system was performed by the classical emulsion solvent-evaporation method. Size analysis of the microparticulate system showed that unloaded and loaded nimesulide-PLA microspheres had average diameters of about 42.9 nm and 2.1 μm, respectively. Scanning electron microscopy (SEM) of loaded and unloaded microsphere samples showed that the particles shape were perfectly spherical, the loading efficiency of nimesulide in PLA microspheres was 70%; Thus, the microparticle system evaluated in this work showed the potential to act as a sustained release system for nimesulide: in vitro dissolution profiles showed the PLA- l microparticles were able to sustain the release of the drug for a considerable period of time (28.7% within 108 h).

Colloidal inclusions in liquid crystals: Phase separation mechanisms and some dynamical aspects

This article reviews recent findings on the formation of ordered colloidal structures from bulk demixing in liquid crystalline materials. In contrast to classical phase separations, which lead to randomly distributed macrodomains of various sizes, demixing in liquid crystals leads to remarkably uniform droplets that form ordered arrays of either infinitely long straight chains (nematic phase) or highly curved chains (cholesteric phase). This distinctive behaviour arises from the presence of topological defects and elastic distortions around the inclusions formed during the separation. These distortions induce long‐range attractions and short‐range repulsions. These forces direct the ordering of microdomains and stabilize them against coalescence, thereby limiting the coarsening mechanism of the separation. We show that the ordering can be controlled on a large scale by simply controlling the macroscopic alignment of the liquid crystal. We also discuss the influence of an electric field and demonstrate marked differences with classical emulsions and colloids in isotropic fluids. Finally, we analyse the Brownian fluctuations of an isolated droplet immersed in a nematic phase and derive a quantitative determination of the Stokes drag coefficients and their anisotropy.

Atom transfer radical polymerization of n‐butyl methacrylate in an aqueous dispersed system

AbstractAtom transfer radical polymerization of n‐butyl methacrylate (BMA) was conducted in an aqueous dispersed system with different kinds of copper complexes. The partitioning behavior of the copper complexes, including CuCl/4,4′‐di(5‐nonyl)‐2,2′‐bipydine (dNbpy), CuCl2/dNbpy, CuCl/2,2′‐bipydine (bpy), CuCl2/bpy, CuCl/bis(N,N′‐dimethylaminoethyl)ether (bde), and CuCl2/bde between the monomer (BMA), and water was studied in detail with ultraviolet‐visible spectroscopy. The results show that with a less hydrophobic ligand, such as bpy or bde, most of the Cu(I) or the Cu(II) complexes migrated from the BMA phase to the aqueous phase, the atom transfer equilibrium was destroyed, and the polymerization was nearly not controlled; it converted to classical emulsion polymerization. As to the very hydrophobic ligand dNbpy, although the partitioning study of the copper complexes indicated that not all the copper species were restricted to the organic phase, the linear correlation between the molecular weight and the monomer conversion and the narrow polydispersities confirmed that the polymerization was still quite well controlled. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3175–3179, 2003

A study of chemical micromechanism governing detonation initiation of condensed explosive mixtures by means of differential thermal analysis

Thermal reactivities of 11 commercial explosives with different nitric ester contents, oxidizing systems of 10 classical emulsion explosives (water in oil (W/O) type), four of these fortified by 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) or pentaerythritol tetranitrate (PETN), and three samples of W/O emulsion explosives were examined by means of differential thermal analysis (DTA) and data were analyzed according to the Kissinger method. The reactivities, expressed as the E a R −1 slopes of the Kissinger relationship, correlate with the squares of the detonation rates of the corresponding explosive mixtures in the form of the modified Evans–Polanyi–Semenov ( E–P–S) equation. In explosive mixtures, the thermal reactivity of the oxidizing system and/or its mixture with a high explosive replaces the primary thermal reactivity of explosophore groups in the individual energetic materials. Addition of any inorganic nitrate to ammonium nitrate in a classical W/O, emulsion explosive was found to result in a decrease in the detonation parameters of the resulting mixture.

Radical polymerization in homogenized butyl acrylate emulsion

The addition of a small amount of monomer strongly decreased the clouding temperature of nonionic emulsifier (Tween 20). The clouding temperature of the Tween 20 aqueous solution was independent of emulsifier concentration but it strongly varied in the presence of monomer. The decreased cloud temperature was attributed to the penetration of monomer molecules into the interfacial layer that increased the flocculation of microdroplets (monomer-swollen micelles). The surface tension of homogenized ((mini)emulsion) butyl acrylate aqueous emulsion was much smaller than that estimated at or above CMC of Tween 20. The polymerization rate vs. conversion curve of the (mini)emulsion deviates from the three rate intervals typical for the emulsion polymerisation. The shape of the rate-conversion curve reminds more the four rate intervals curve. Interval 2 is overlapped with the initial maximal rate and rate shoulder at higher conversion. The initial maximal polymerization rate (Rp,max,1) is attributed to the abrupt increase in polymer particles, the polymerization under monomer saturated condition and emulsifier containing peroxide groups (Twperoxid 20). The rate of emulsion polymerization of BA initiated by ammonium peroxodisulphate (APS) is ca. by one order of magnitude larger than that of blank polymerization (without APS). The second maximal rate (rate shoulder) can result from the gel effect. The more pronounced increase in Rp,max,1 with Tw 20 concentration supports the presence of peroxide groups. The slight dependence of Rp,max,2 on [Tw 20] for both APS and DBP (dibenzoyl peroxide) is discussed in terms of the depressed radical entry rate into the close packed surface later of polymer particles. The low activation energy is attributed to the decreased barrier for entering radicals into the polymer particles with increasing temperature. This is more pronounced with the accumulation of covalently bound emulsifier moieties (resulting from Twperoxid 20) at the particle surface. The ratio of the final number of polymer particles to the initial number of monomer droplets (Np/Ndrop) promotes the partial monomer droplet nucleation. The dye approach indicates that the degree of depletion of monomer droplets decreases from the classical emulsion polymerization to the polymerization in pre-homogenized emulsions and the emulsion polymerization with a prolonged-emulsification interval.

On the Stability of Liquid Nanodroplets in Polymerizable Miniemulsions

ABSTRACT Liquid nanodroplets within a size range of 50 to 500 nm can easily be prepared by shearing a system containing oil, water and a surfactant. The growth of the nanodroplets can effectively be suppressed by using a strong hydrophobe as an additivie to the oil and an effective surfactant. The hydrophobe acts as an osmotic agent which stabilizes the system against Ostwald ripening. The growth of the droplets by collision is controlled by the density of the surfactant layer. Freshly prepared miniemulsions are “critically stabilized” and show a slow, but pronounced growth, whereas a miniemulsion in “equilibrium” exhibits constant droplet size on longer time scales. Polymerization of the oil droplets of such miniemulsions turns out to be very promising and extends the possibilities of classical emulsion polymerization. Since each droplet can be considered a small reactor in which polymerization reactions take place, the process allows one to create new particle structures, e.g. polyaddition reactions can take advantage of unusual monomers, the incorporation of materials which are not soluble in the continuous phase, and the formation of nanocapsules.

Liquid Crystal Emulsions

ABSTRACT We review recent findings about the behavior of emulsions made of droplets suspended in liquid crystalline materials. By contrast to classical emulsions, which are usually made of isotropic oils and water, liquid crystal emulsions exhibit a variety of structures result in the ordering of the continuous phase. The droplets induce the formation of topological defects and distortions that lead to strong and anisotropic elastic forces between the particles. These elastic forces govern the stability and the ordering of the particles. This is observed in aqueous emulsions as well as in non-aqueous emulsions obtained from phase separation phenomena. It is shown that phase separations in liquid crystals can lead to the formation of highly ordered arrays of uniformly sized droplets. More generally, ordered structures seen in liquid crystal emulsions are of interest as examples of topologicallv-controlled organizations; they are also of potential practical importance as a novel way to control both the stability and the structures of colloidal particles.

Measurement of average drop size in aqueous mixtures of Na-alginate and Na-caseinate by linear oscillatory tests

The linear viscoelastic behaviour of two-phase immiscible systems is characterised by an increased elasticity at low frequencies, which can be quantitatively related to the average size of the dispersed phase droplets and to the interfacial tension between the two phases (Rheol. Acta 29 (1990) 204). In the case of aqueous biopolymer mixtures, however, the application of classical emulsion models, such as the one of Palierne, is not trivial, due to sedimentation of the dispersed phase and to the extremely low values of interfacial tension which are typical of these systems. In this work, we investigated the application of Palierne model to aqueous mixtures of Na-alginate and Na-caseinate. The volume average drop radius was calculated by fitting the Palierne model to experimental data of elastic modulus vs frequency right after cessation of shear flow. A comparison with direct drop size measurements was performed by shearing the mixtures in a parallel-plate apparatus coupled with an optical microscope. In this apparatus, the mixtures were subjected to the same flow history as in the rheometer and drop size was measured by image analysis. Good agreement was found in all the four mixtures investigated, covering two volume fractions of dispersed phase, i.e. 5 and 10%, and two values of viscosity ratio (drop viscosity/continuous phase viscosity), i.e. 0.34 and 3.0.

Nitroxide-Mediated Living Free Radical Miniemulsion Polymerization of Styrene

The miniemulsion technique was applied to the controlled/living free radical polymerization of styrene utilizing TEMPO-terminated oligomers of polystyrene (TTOPS) as a macroinitiator species to initiate the polymerization at 125 °C. Miniemulsion polymerizations of styrene using different amounts of TTOPS were studied. It was found that the polymerization rate of a miniemulsion containing no TTOPS was significantly faster than those containing TTOPS. In the latter reactions, the molecular weight of the polymer increased almost linearly with conversion up to 75% conversion. The slope of the molecular weight vs conversion curve increased with decreasing TTOPS concentration as expected. Particle size distributions were measured by both TEM and CHDF and were found to be broad. Using the kinetics and particle size data, the average number of active free radicals per particle (n in classical emulsion polymerization systems) was estimated and found to be low (≤0.005).

The polymerization of acrylonitrile in miniemulsions: “Crumpled latex particles” or polymer nanocrystals

Stable dispersions of polyacrylonitrile (PAN) nanoparticles in the size range between 100 nm < d < 180 nm were made by polymerization in miniemulsion and characterized by dynamic light scattering, transmission electron microscopy (TEM), and wide angle X-ray scattering (WAXS). Due to the insolubility of the polymer in its monomer, such particles are not accessible by classical emulsion polymerization. The pure PAN particles are composed of ca. 10 nm large polymer nanocrystals, i. e., the formed polymer precipitates and crystallizes direct after formation. As a consequence, the final latexes do not adopt spherical shape, but show a well defined, narrowly distributed boulder-like phenotype which is called “crumpled latexes”. Copolymerization with styrene results in a continuous transition between the crumpled and a smooth spherical morphology, which is again related to a decreased degree of crystallinity.

Effect of temperature on styrene emulsion polymerization in the presence of sodium dodecyl sulfate. II

The batch emulsion polymerization kinetics of styrene initiated by a water-soluble peroxodisulfate at different temperatures in the presence of sodium dodecyl sulfate was investigated. The curves of the polymerization rate versus conversion show two distinct nonstationary-rate intervals and a shoulder occurring at a high conversion, whereas the stationary-rate interval is very short. The nonstationary-state polymerization is discussed in terms of the long-term particle-nucleation period, the additional formation of radicals by thermal initiation, the depressed monomer-droplet degradation, the elimination of charged radicals through aqueous-phase termination, the relatively narrow particle-size distribution and constant polydispersity index throughout the reaction, and a mixed mode of continuous particle nucleation. The maximum rate of polymerization (or the number of polymer particles nucleated) is proportional to the rate of initiation to the 0.27 power, which indicates lower nucleation efficiency as compared to classical emulsion polymerization. The low activation energy of polymerization is attributed to the small barrier for the entering radicals. The overall activation energy was controlled by the initiation and propagation steps. The high ratio of the absorption rate of radicals by latex particles to the formation rate of radicals in water can be attributed to the efficient entry of uncharged radicals and the additional formation of radicals by thermally induced initiation. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1477–1486, 2000

Polymerizable microemulsions: some criteria to achieve an optimal formulation

Two types of water-soluble polymers have been prepared via a microemulsion polymerization route: (i) copolymers of acrylamide with sodium-2-acrylamido-2-methylpropanesulfonate and (ii) copolymers of acrylamide and sodium acrylate hydrophobically modified with an amphiphilic monomer. The formulation of the polymerizable microemulsions was optimized by using cohesive energy ratio (CER) and hydrophile-lipophile balance (HLB) concepts. In addition, the role played by the nature of the surfactants and the monomers was thoroughly investigated. The best formulation was obtained for high HLB values (8–11), in favor of microemulsions with a bicontinuous structure. The process was shown to yield remarkably stable, clear microlatexes of moderate particle size containing up to 25% high molecular weight polymer in the medium, in contrast to inverse latexes prepared by using classical emulsions which are unstable and turbid. These materials are of great interest for industrial applications.

Compositional Heterogeneity in Parenteral Lipid Emulsions after Sedimentation Field Flow Fractionation

This study examines the size and compositional heterogeneity of particles in a commercial lipid emulsion (Intralipid) before and after equilibration with penclomedine, a highly lipophilic cytotoxic agent. Emulsions were fractionated by sedimentation field-flow fractionation (sedFFF), and particle sizes of the monodisperse fractions were determined by photon correlation spectroscopy. The triglyceride (TG), phosphatidylcholine (PC), and penclomedine (in drug loaded emulsions) contents in each fraction were determined by HPLC. The aqueous-entrapped volume within Intralipid was determined to be≈10% by size-exclusion chromatography using [3H]mannitol. Thirteen sedFFF fractions collected from the drug free emulsions yielded particles ranging in size from 154 to 423nm. Total channel recoveries were 89% and 95% for TG and PC, respectively. Apparent particle densities varied significantly with size, suggesting heterogeneity in composition as confirmed by PC/TG mass ratios which varied dramatically. Computer fits of the distribution profiles suggested populations of phospholipid vesicles and oil droplets containing excess phospholipid in addition to classical emulsion droplets. Drug loading induced a significant shift of the predominant triglyceride containing population to a larger particle size. The penclomedine distribution profile closely mimicked that of the TG rather than the PC fraction. These studies suggest the need to consider not only size distribution but also compositional distribution in characterizing parenteral emulsions.

Diblock copolymers as emulsifying agents in polymer blends: Influence of molecular weight, architecture, and chemical composition

Interfacial agents used in the compatibilization of immiscible polymer blends often consist of block copolymers containing at least one segment compatible with each of the two phases of the blend. This work examines the influence of the molecular weight, architecture, and chemical composition of the interfacial agent on its ability to emulsify a polymer blend. The system chosen is a blend containing 80% polystyrene and 20% ethylene-propylene rubber, compatibilized by diblock copolymers of poly(styrene-hydrogenated butadiene). The emulsification curve, which relates the dispersed phase particle size to the concentration of interfacial agent added to the system, was used as a tool to characterize the efficacy of the different interfacial agents. The observed behavior is similar to that of classical emulsions: a rapid drop in phase size at low concentrations of interfacial modifier, followed by a levelling off to an equilibrium diameter value once a “critical” concentration has been reached. For systems compatibilized by symmetrical diblocks (i.e., containing approximately 50% styrene by weight), the volume average particle diameter decreased from 2.7 μm for the unmodified system to about 0.4 μm once interfacial saturation is reached. The critical concentration for emulsification decreased with increasing interfacial agent molecular weight, due to the higher interfacial area occupied by longer molecules; however, this parameter did not affect the equilibrium particle diameter. The asymmetrical diblock copolymer (30% styrene) was found to be less effective than the symmetrical ones over the entire range of concentrations studied (5 to 35% modifier, based on the volume of the minor phase). Asymmetrical diblock copolymers would tend to form micelles, whereas symmetrical copolymers are less constrained at the interface. No significant difference was observed between the emulsifying capability of tapered and pure diblocks of similar composition and molecular weight. © 1996 John Wiley & Sons, Inc.

Predominant droplet nucleation in emulsion polymerization

Emulsions stabilized against diffusional degradation by incorporating a polymeric cosurfactant have been produced and polymerized. The presence of large numbers of small droplets shifts the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. When an efficient cosurfactant is used, this process is referred to as miniemulsion polymerization. Polymer, however, is known to be a poor cosurfactant. Its advantage is that, unlike most cosurfactants, it is innocuous in the recipe. Results indicate that even a poor cosurfactant (polymer) is adequate to stabilize small droplets against diffusional degradation long enough to nucleate them into polymer particles. The dependence of the concentration and molecular weight of the cosurfactant on the droplet size and distribution is investigated. Droplet diameters range from 19.5 to 141.2 nm with polydispersities of about 1.023. The polymeric cosurfactant is found to affect the mechanism of nucleation. On-line conductance measurements are used to successfully differentiate between nucleation mechanisms. The observed reaction rates are dependent on the amount of polymeric cosurfactant present. In addition, the latexes prepared with the polymeric cosurfactant have lower polydispersities (1.006) than either latexes prepared from classical emulsions (1.049) or from alkane-stabilized miniemulsions (1.037). © 1996 John Wiley & Sons, Inc.

Microemulsions of Perfluorinated and Semifluorinated Compounds

Perfluorocarbons combine high gas dissolving capacities with extreme chemical and biological inertness: they are good oxygen carriers in artificial blood and in liquid breathing. However fluorocarbons are highly hydrophobic molecules. To solve the problem of their transport, it is necessary to use the perfluorocarbons as an oil-in water emulsion (O/W). To avoid harsh treatment to form such emulsions and in order to have injectable "blood substitutes", microemulsions seem particularly attractive since they are fluid, transparent, thermodynamically stable microheterogeneous systems. Microemulsions, contrarily to classical emulsions, are formed spontaneously by adding suitable surfactants (or a surfactant+a cosurfactant) in appropriate proportions to a non miscible mixture of water and oil. Biocompatible O/W microemulsions are difficult to obtain because of: 1) the existing segregation between perfluorinated and hydrogenated chains, resulting, in some cases in heterogeneities or gelation phases; 2) the toxicity of some components; 3) the possible harmfulness of the dispersed system, etc... We'll discuss all the parameters involved of the microemulsification process, the nature of products, the phase diagrams, and the phase behaviors. This study will outline certain guidelines necessary for the formation of microemulsions of perfluorinated (or almost completely fluorinated) oils with perfluorinated (or partially fluorinated) surfactants.

Emulsion polymerization of styrene in the presence of poly(oxyethylene)‐poly(oxypropylene) block copolymers

AbstractPoly(oxyethylene)‐block‐poly(oxypropylene)‐block‐poly(oxyethylene) copolymers were applied as nonionic surfactants in styrene emulsion polymerization. The clouding phenomenon of the surfactant in the aqueous phase and the resulting characteristic partition of the block copolymer between aqueous and organic phase result in two particularities of this system compared with classical emulsion polymerization: (i) bimodal particle size distribution, and (ii) unusual dependence of reaction rate on temperature.

Scanning electron microscopy radioautography by emulsion and polymer track detectors

ABSTRACT The paper deals with introdusing SEM in radioautography by classical nuclear emulsion and possibility to use PTD. We have observed individual AgBr grains by SEM and alpha tracks on the developed emulsion, as well. L cells line as free cells labelled with 3 H-thymidine for SEMR by emulsion was used. PTD are suitable for SEMR as reverse side etched. Electron beam response of thermoset plastics CR 39 and thin layer monomer polymerisation over specimen indicated application in SEM alpha and fragment radioautography.

Characteristics of inclusions in the dispersed phase of liquid‐liquid suspensions

AbstractThe classical emulsion is made up of drops of one liquid dispersed in a containing liquid. Under certain circumstances, however, some of the containing phase may form drops within the drops of the dispersed liquid, giving rise to a structure known as a double‐multiple emulsion. This study was an investigation of the hydrodynamic formation of these structures.Double‐multiple emulsions were produced by using both transient and steady state liquid injection, with a cylindrical jet configuration. Analysis of the spray was by means of high‐speed photography.The distribution of the large drops and that of the included drops were both found to be represented satisfactorily by a log‐normal probability function. A simplified model based on the use of nozzle parameters satisfactorily correlated percent included area and volume as a function of Reynolds number and Weber number. Large dispersed phase drops contained proportionally many more inclusions than did the smaller sizes.The mechanism of inclusion formation was found to involve the drawing out of a ligament or sheet from the flowing jet, followed by the recoalescence of the free end of this ligament or sheet with the main jet body, entraining in the process a portion of the containing phase. Thus, the inclusions predate the formation of the drops of the dispersed phase from the jet disruption.