Nanocapsules with liquid cores

The influence of surfactant concentration on particle size and stability of nanocapsules with liquid cores, synthesized by an in-situ miniemulsion polymerization process, was investigated by van Zyl et al. [74]. 

Nanocapsules are reservoir type systems comprising an oily liquid core surrounded by a polymeric shell [27]. The drug is usually dissolved in this liquid core but may be more closely associated with the shell polymer and the exposed surface, as illustrated in Figure 2b. 

The synthesis of nanocapsules can best be obtained in miniemulsion using different approaches [107], One possibility is based on the phase separation process within a droplet during the polymerization [108], Here, vinyl monomers were polymerized in the presence of a hydrophobic oil. During the polymerization, the polymer becomes insoluble in the oil, leading to a phase separation. With properly chosen physicochemical properties of monomer and encapsulated material, a polymeric shell surrounding the liquid core can be formed. 

In another approach, the polymer is precipitated from the continuous phase onto on stable nanodroplets in an inverse miniemulsion [109], In this case, a miniemulsion with the liquid core material is formed in a continuous phase that consists of a mixture of a solvent and a nonsolvent for the polymer. That way, PMMA nanocapsules encapsulating an antiseptic agent could be produced. 

Fig. 1 Schematic representation of a nanosphere (A) and of a nanocapsule (B). In nanospheres, the whole particle consists of a continuous polymer network. Nanocapsules present a core-shell structure with a liquid core surrounded by a polymer shell.

Fig. 16 Capsule formation by phase separation, (a) Scheme a solution of monomer and hydrophobic oil (ie/t) is dispersed in an aqueous surfactant solution middle). Phase separation between the growing polymer and the oil occurs, leading to core shell morphology with encapsulated liquid, (b) Nanocapsules with hexadecane by phase separation [35]. (c) Encapsulation of Lucirin TPO [173] and (d) the fragrance 1,2-dimethyl-1-phenyl-butyramide [174]...

Miniemulsion is a special class of emulsion that is stabilized against coalescence by a surfactant and Ostwald ripening by an osmotic pressure agent, or costabilizer. Compared with conventional emulsion polymerization process, the miniemulsion polymerization process allows all types of monomers to be used in the formation of nanoparticles or nanocapsules, including those not miscible with the continuous phase. Each miniemulsion droplet can indeed be treated as a nanoreactor, and the colloidal stability of the miniemulsion ensures a perfect copy from the droplets to the final product. The versatility of polymerization process makes it possible to prepare nanocapsules with various types of core materials, such as hydrophilic or hydrophobic, liquid or solid, organic or inorganic materials. Different techniques can be used to initiate the capsule wall formation, such as radical, ionic polymerization, polyaddition, polycondensation, or phase separation from preformed polymers. 

Nanoparticles, i.e. particles with a size usually in the range 50-1000 nm, have drawn the attention of researchers designing drug-delivery systems that can be injected intravenously owing to their small size. Nanoparticle is a general name for nanospheres and nanocapsules. Nanospheres have a matrix-type structure, whereas nanocapsules are hollow and have a liquid core surrounded by a polymeric wall, as illustrated in Figure 4.34. 

Nanoparticles consisting of different molecular weight PS-6-PMMA copolymers and nanocapsules made of the same copolymers, but additionally with hexadecane as liquid core material, were prepared by using a combined miniemulsion and solvent evaporation technique [220]. The morphology of block copolymer assemblies was investigated in dependence of the nanoconfinement. We introduced two nanoconfinement parameters the diameter D of the droplet throughout the synthesis and the shell thickness S of the nanocapsules with a liquid as core. D was controlled by varying the concentration of the surfactant in the miniemulsion, while 5 was controlled by the ratio of hexadecane to copolymer. 

Fig. 2 Scheme showing the versatility of the emulsion-solvent evaporation technique for the preparation of nanocapsules. Polymers with completely different properties could be used to build the shell (left) while monomers for self-healing reactions based on various types of polymerization could be encapsulated as liquid core (right). PLLA poly(L-lactide), PVF poly(vinyl framal), PPO poly(phenylene oxide), PMMA poly(methyl methacrylate), PVCi poly(vinyl cinnamate), PVAc poly(vinyl acetate), OMCTS octamethylcyclotetrasiloxane, PDMS-DE polydimethylsiloxane diepoxy terminated [31]... 

Nanocapsules consisting of the block copolymer as shell and with a liquid as core could be obtained by adding hexadecane, which is a nonsolvent for the copolymer. The block copolymer was dissolved in a mixture of chloroform and hexadecane. During evaporation of the chloroform there is a phase separation of the block copolymer and the hexadecane, and a microphase separation of the block copolymer itself. For the successful formation of nanocapsules, the interfacial tensions for the polymer/water and water/hexadecane interfaces are important. 

It has been shown that a power law type can be effective in relating the particle size to the main operating variables, both for solid nanospheres and nanocapsule either with a liquid or fatty core. 

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