Interfacial polymerization occurs between two partners of a polymerization reaction that are soluble in two different immiscible solvents. The classical interfacial polymerization partners are hexamethylene amine, which is soluble in water, and a bifunctional acid chloride, for instance sebacyl chloride dissolved in chloroform or another organic solvent. The two bifunctional components react at the interface between the two liquids under formation of HCl yielding a polymer film, in the case of the above partners the producing Nylon®
6-10. This process was one of the first used to make microcapsules .
In order to make nanocapsules this process has to be modified. Significant amounts of a water soluble organic solvent—mainly ethanol is used—have to be present in the organic phase to help the dispersion of the organic phase in the aqueous medium. Poly(alkyl butyl cyanoacry-late) nanocapsules thus are produced by dissolution of the cyanoacrylate monomers in a mixture of ethanol and Miglyol® 812 or 829, or benzyl alcohol [101-107] plus drug. This mixture then is added slowly through a thin tube to an aqueous solution of a surfactant leading to the formation of a nanocapsule with a wall. Wall formation was demonstrated by electron microscopy [102, 105]. It is conceivable that this wall forms by polymerization of the cyanoacrylate monomer present in the organic microemulsion droplets at the interface to the aqueous medium upon contact with the OH- ions present in the surrounding water.
Chouinard et al. , however, argue that the polymer is already formed in the organic phase due to the nucleophilic character of ethanol and is then precipitated at the interface after emulsification in the aqueous phase similarly to the solvent deposition process described in Section 4.4.
Interfacial polymerization also can be carried out in an outer organic phase yielding nanocapsules with an aqueous core [111, 112]. In this case a 20% aqueous ethanol solution containing the drug is added to a surfactant containing (20%) Miglyol® 812 solution and finely dispersed by stirring. Isobutyl cyanoacrylate monomer is then added leading to the formation of nanocapsules within 4 h by interfacial polymerization. The nanoparticles are then separated from the organic phase by ultracentrifugation and resuspended in an aqueous medium.
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