Surface characterization of oil-containing polyterephthalamide microcapsules prepared by interfacial polymerization

Oil-containing polyterephthalamide microcapsules were synthesized by the interfacial polymerization of an oil-soluble monomer (terephthaloyl dichloride, TDC) and a mixture of two water-soluble monomers (diethylenetriamine, DETA, and 1, 6-hexamethylenediamine, HMDA). The influence of several synthesis parameters (e.g. concentration ratio of the two amine monomers, stirring rate, concentration of the steric stabilizer PVA) on the size distribution, the membrane morphology and the electrokinetic properties of the microcapsules, was thoroughly investigated. Morphological analysis by electron microscopy showed a strong dependence of the membrane external morphology on the functionality of the water-soluble amine monomer. High stabilizer concentrations and agitation rates during emulsification favoured the production of smaller microcapsules with non-porous and rigid membranes. The electro-chemical interfacial properties of the microcapsules were investigated using a combination of potentiometric, conductimetric and electrokinetic measurements. The dependence of the mean surface charge density on pH revealed the presence of essentially two kinds of chemical groups (e.g. amino and carboxylic groups) on the microcapsule external surface. The total concentration of surface chemical groups and the isoelectric pH were measured as a function of the microcapsule synthesis conditions. Using the experimental data and an appropriate interfacial ionization model, the ratio of the surface groups densities, R = (S - COOH)/(S NH3(+) ), was evaluated and rationalized with respect to the microcapsules synthesis parameters.