Polymeric nanoemulsions

Formation of Nanoemulsions by Low-Energy Methods and Their Use as Templates for the Preparation of Polymeric Nanoparticles... 

It is useful, for reasons which are apparent in relation to movement of nanoparticles in vivo, to divide nanosystems into two types, hard and soft, although there are obviously intermediate situations. Hard systems, for example, polymeric nanoparticles and nanocapsules, nanosuspensions or nanocrystals, dendrimers, and carbon nanotubes are neither flexible nor elastic. Hard systems can block capillaries and fenestrae that have dimensions similar to the particles, whereas soft systems can deform and reform to varying degrees. Erythrocytes and many liposomes fall into this category and are thus better able to navigate capillary beds and tissue extracellular spaces. Soft systems include nanoemulsions (microemulsions) and polymeric micelles. 

As most nanoemulsions are prepared using nonionic and/or polymeric surfactants, it is necessary to consider the interaction forces between droplets containing adsorbed layers (steric stabilisation). As this was described in detail in Chapter 10, only a summary will be given here [15, 16]. 

This method has been used to prepare nanoemulsions of such polymers as cellulose esters and epoxy resins, similar to latexes produced by emulsion polymerization. The nanoemulsions are prepared by direct emulsification of solutions of the polymers in organic solvents, followed by removal of the organic solvent by steam distillation under reduced pressure. The nanoemulsions produced in this fashion had stabilities > 1 year. 

Molecular inclusion or conjugation complexes, micelles, microemulsions, polymeric particles, and emulsions and nanoemulsions can all be classified as matrix type encapsulation systems. 

Huynh, L., Neale, C., Pomes, R., AUen, C. Computational approaches to the rational design of nanoemulsions, polymeric micelles, and dendrimers for drug delivery. Nanomedicine 2012, 8 (1), 20-36. 

The use of oil/water (OAV) emulsions has also been employed to fabricate PU NPs [162-165]. In this method, the diisocyanate (IPDI) is first dissolved in an oil/ surfactant mixture (90/10, saturated medium chain triglyceride/polysorbate 80 [polyoxyethylene 20-sorbitan monooleate]). Addition of the aqueous phase with PEG 400 (diamine or diol) to the 0/S mixture in dropwise fashion (to obtain 90% aqueous component) occurs under mechanical stirring to obtain nanoemulsions, followed by heating to 70 °C to allow polymerization and achieve PU or PU urea NPs, which can be isolated by ultracentrifugation [165]. This method works by having IPDI present in the core of oil nanodroplets in the 0/W nanoemulsion, which react with the diols or diamines at the surface of the oil droplet, resulting in the formation of the NPs with a size distribution from 40 to lOOnm. 

Nature of nanoparticle Emulsification method Nature of nanoemulsion Monomers Surfactants Polymerization and emulsification parameters Particle size (nm) References... 

This section, which is by no means exhaustive, will deal with the following topics (i) Surfactants used in cosmetic formulations, (il) Interaction forces between particles or droplets in a dispersion and their combination, (iil) Description of stability in terms of the interaction forces, (iv) Self-assembly structures and their role in stabilization, skin feel, moisturization and delivery of actives, (v) Use of polymeric surfactants for stabilization of nanoemulsions, multiple emulsions, liposomes and vesicles. 

The inherently high colloid stability of nanoemulsions when using polymeric surfactants is due to their steric stabilization. The mechanism of steric stabilization was discussed above. As shown in Fig. 1.3 (a), the energy-distance curve shows a shallow attractive minimum at separation distance comparable to twice the adsorbed layer thickness 28. This minimum decreases in magnitude as the ratio between adsorbed layer thickness to droplet size increases. With nanoemulsions the ratio of adsorbed layer thickness to droplet radius (8/R) is relatively large (0.1 0.2) when compared with macroemulsions. This is schematically illustrated in Fig. 1.28 which shows the reduction in with increasing 8/R. 

These surfactants have many uses, in particular as colloid and nanoemulsion dispersants, wetting agents, detergents and even additive to dehydrate crude oils. However, most polymeric surfactants are graft-type, particularly synthetic products such as polyelectrolytes, which are not strictly surfactants or are not used for their surfactant properties. It is the case of hydrosoluble or hydrodispersible polyelectrolytes which are utilized for the antiredeposition, dispersant and viscosity-enhancing properties such as carboxymethyl cellulose, polyacrylic acid and derivatives. 

To obtain polymer nano-size objects [11, 12], direct synthesis is a possible way. One could perform polymerization at very high dilution, which however is difficult because of contaminations, or use micro- or nanoemulsions with small compartments to perform polymerization. In this way nano-sized polymer materials can be produced, which are either composed out of small polymer nanoparticles or contain nano-size holes. One chapter of the book describes procedures and properties. 

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