Non-aqueous emulsions

KH Bauer, B Dortunc. Non-aqueous emulsions as vehicles for capsule filling. Drug Devel Ind Pharm 10 699-712, 1984. 

Alexander Florence is Dean of The School of Pharmacy at the University of London he was previously James P. Todd Professor of Pharmaceutics at the University of Strathclyde. His research and teaching interests are drug delivery and targeting, dendrimers, nanoparticles, non-aqueous emulsions, novel solvents for use in pharmacy and general physical pharmaceutics. He co-authored the book Surfactant Systems their Chemistry, Pharmacy and Biology with David Attwood. 

The ability of SPEs to form vesicles, which are strueturally comparable to liposomes, has opened a wide range of possibilities for the incorporation of active ingredients. Silicone vesicles can have a diameter from about 0.05 to 1 micron and an internal volume of lO pm. The membrane thickness of these vesicles is about 3 to 4 nm [56]. The main application of siloxane-based surfactants vesicles is in eosmetics. For example, using silicone vesicles, hydrophilic and hydrophobic active substances can be separated and protected from each other, thus reducing for example skin irritancy. Active delivery systems include non-aqueous emulsions of polyols in silicone fluids, multiple-phase emulsions, and polar solvent-in-oil emulsions. 

It should be noted that the same equations may be used to calculate the swelling of particles with a mixture of a Z compound andaL compound. This situation will be met in the case of non-aqueous emulsions, in which case the equilibrium between phases (a) and (c) is considered, and likewise for the swelling of particles with a monomer when the presence of water in the two phases is taken into consir deration. 

Although most of the reports deal with the preparation of microparticles, nanosized particles and capsules are also accessible, usually by employing ultrasonication to form very small droplets [12] from which the solvent is evaporated. Usually, the continuous phase is an aqueous solution. Inverse systems in which water is the solvent have been reported [13, 14] as well as non-aqueous emulsions [15] such as dimethylformamide-in-paraffin [16], dichoromethane-in-fluorinated solvent for microparticles [17], and formic acid-in-paraffin for... 

In mass polymerization bulk monomer is converted to polymers. In solution polymerization the reaction is completed in the presence of a solvent. In suspension, dispersed mass, pearl or granular polymerization the monomer, containing dissolved initiator, is polymerized while dispersed in the form of fine droplets in a second non-reactive liquid (usually water). In emulsion polymerization an aqueous emulsion of the monomer in the presence of a water-soluble initiator Is converted to a polymer latex (colloidal dispersion of polymer in water). 

In latex technology, concentrated latex is first blended with the different additives required. To prevent premature destabilisation the powders are added as dispersions and non-aqueous liquids are generally added as emulsions. Care must be taken to avoid destabilisation, which can be brought about in different ways such as... 

Non-aqueous liquid content Design of treatment or Breaking emulsion... 

The substitution of water-borne versions of these primers is increasing as environmental restrictions on the use of organic solvents become stricter. These are generally aqueous emulsions of epoxy novolac or phenolic based resins stabilized by surfactants [34]. Non-ionic surfactants are preferred, as they are non-hygroscopic in the dried primer films. Hygroscopic ionic surfactants could result in excessive water absorption by the primer film in service. 

Heterogeneous polymerization processes (emulsion, miniemulsion, non-aqueous dispersion) offer another possibility for reducing the rate of termination through what are known as compartmcntalization effects. In emulsion polymerization, it is believed that the mechanism for chain stoppage within the particles is not radical-radical termination but transfer to monomer (Section 5.2.1.5). These possibilities have provided impetus for the development ofliving heterogeneous polymerization (Sections 9.3.6.6, 9.4.3.2, 9.5.3.6). 

Polyvinylacetate aq emulsion for use as a binder in non-metallic cartridge cases is covered by US Mil Spec, Polyvinyl Acetate Aqueous Emulsion (PAAE) (For use in Ammunition) , MIL-P-50855(MU) (31 March 1971). The requirements and criteria are in Table 1... 

Dispersible silicone emulsions are generally preferred for aqueous systems, whereas silicone fluids and compounds are preferred for non-aqueous systems. Silicones are widely employed in cooling water treatment programs, less so in boiler plants because of higher operating costs than available alternatives, but also because of sometimes questionable emulsion stability at higher temperatures. 

In normal emulsion polymerization the diffusion of monomers from droplets allows particles to grow. The polymerization is usually initiated in the aqueous phase and the oligomeric radicals either enter micelles or merge with other growing species. In the crosslinking ECP of EUP the ratio EUP/comonomer and the solubility or insolubility of both components and the initiator in the aqueous and non-aqueous phases respectively are parameters which decide whether diffusion of the reactants in the aqueous phase plays a role and where the initiation takes place. 

Emulsion copolymerization of EUP and comonomers may be initiated in the aqueous (persulfate) or in the non-aqueous phase (AIBN). On the decomposition of persulfates, sulfate and hydroxyl groups are introduced into macromolecules and microgels, thus influencing their surface properties [118,123-125]. By using AIBN as initiator a change of the chemical character of the surface and of the properties of the microgels is avoided. 

Crosslinked acrylic microgels in aqueous and non-aqueous media were patented as paint constituents in 1979 to improve the orientation of aluminum flake pigments, restrict the flow of the liquid coating on the substrate and restrict sagging [324]. As the patent speaks of emulsions, insolubility of the microgels and particle sizes up to 200 nm, it is questionable whether these polymers consisted of microgels only. 

Non-aqueous (or-oil-in-oil) emulsions, where the phases are two immiscible organic liquids, have received relatively little attention in the literature. Riess et al. [116-119] have studied the stabilisation of waterless systems with block and graft copolymers, where one of the liquids is a good solvent for one of the blocks and a non-solvent for the other, and vice versa. Thus, poly(styrene-b-methylmethacrylate) copolymers could emulsify acetonitrile/cyclohexane mixtures, and poly(styrene-b-isoprene) was effective for DMF/hexane systems [116]. These, however, are not HIPE systems. 

Non-aqueous HIPEs have received even less attention indeed, to date, there have been only two publications dealing with this subject, to the authors knowledge [124,125]. These describe the preparation of highly concentrated emulsions of jet engine fuel in formamide, for use as safety fuels in military applications. The emulsifier system used was a blend of two nonionics, with an optimal HLB value of 12. 

The non-aqueous HIPEs showed similar properties to their water-containing counterparts. Examination by optical microscopy revealed a polyhedral, poly-disperse microstructure. Rheological experiments indicated typical shear rate vs. shear stress behaviour for a pseudo-plastic material, with a yield stress in evidence. The yield value was seen to increase sharply with increasing dispersed phase volume fraction, above about 96%. Finally, addition of water to the continuous phase was studied. This caused a decrease in the rate of decay of the emulsion yield stress over a period of time, and an increase in stability. The added water increased the strength of the interfacial film, providing a more efficient barrier to coalescence. 

There exists, in the literature on high internal phase emulsions, a small number of publications on possible applications of HIPEs, involving a diverse range of topics. The production of petroleum gels as safety fuels is one such example [124,125] this was mentioned in the section on non-aqueous HIPEs. The main advantage over conventional fuels is the prevention of spillage, which reduces the risk of fire in an accident. Also, studies on the flash-point of emulsified fuels [127] showed a considerable increase, compared to the liquid state, for commercial multicomponent fuels. In addition, there may be an enhancement of the efficiency of combustion of the fuel on emulsification, as it is known that a small amount of water in fuel can improve its performance [19]. 

Patents have been granted for innovations involving the preparation and activities of broad-spectrum antimicrobial emulsions from 1977 (Sippos) to 2000 (Baker). All of these patents claim antibacterial activity, but all involve additives in the non-aqueous phase of the emulsion that are known to be antibacterial alone and before emulsification. Wide spectrum applications for these nanoemulsions have been claimed with positive results for bacteria, fungi, and viruses. The term nanoemulsion is used in US patents discussed below, but the generic term for the product of an emulsification (Gooch 2002, 1980) of a liquid within a liquid is an emulsion. United States patents 6,015,832 and 5,547,677 were examined and formulations in key claim statements were reproduced, and tested using standard methods for effectiveness. Additional patents listed in the reference section were reviewed as part of this study. 

Particle Structure - So far, the discussion of non-uniform emulsion polymers has centered around mechanical properties, interpreted in terms of gradations of sequences between set compositional limits. But latexes are aqueous dispersions, and an exploration of particle morphology is equally interesting and important. For instance, can polymer particles be constructed such that particle sequence compositions are located in a desired region of the particle, and what evidence exists that such particle morphologies have been realized ... 

Device Any device that is used to break emulsions. Such devices may use chemical, electrical, or mechanical means, or a combination, to break an emulsion and cause separation into its constituent liquid phases. (DNAPL) See Non-aqueous Phase Liquid. 

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

As regards catalysts not containing preformed metal carbon bonds such as rhodium salts active in butadiene polymerisation in aqueous emulsions or in protic solvents as well as other catalysts of this type (used in non-polar hydrocarbon media), there is a theoretical rather than practical interest paid to such catalysts. However, a few of them have activity and stereospecificity comparable with Ziegler Natta catalysts [27-35],... 

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