Microspectroscopic analyses of dye distribution characteristics in single microcapsules

The distribution characteristics of a dye in single melamine resin wall microcapsules containing a dye/toluene solution are studied by a laser trapping-absorption microspectroscopy technique. In the case of Disperse Orange 13 as a dye, the molar absorptivity determined from the slope of a dye absorbance-capsule diameter (d) plot agrees well with that observed in a homogeneous toluene solution, indicating that the dye is solubilized homogeneously in the inner toluene solution of the capsule. For tetraphenylporphyrins (MTPP:  Zn(II)TPP, Co(II)TPP, or H(2)TPP), however, the d dependence of absorbance shows a sigmoidal curve. Since absorbance of a ZnTPP/toluene droplet before polymerization of the melamine resin increases linearly with an increase in the droplet diameter, the sigmoidal d dependence of the absorbance is concluded to originate from distribution of the dye into the resin wall. Detailed analyses of the data indicate that the partitioning ratio of the dye between the toluene and resin phases is dependent on the capsule diameter. Size-dependent distribution of MTPP in single microcapsules is discussed on the basis of a proposed model and the d dependence of the dye absorbance, and it is concluded that the distribution of MTPP to the melamine resin wall is facilitated for larger microcapsules.