Dielectric characterization of colloidal solutions of retinoic acid embedded in microspheres of polyvinyl alcohol

We have previously used dielectric spectroscopy to investigate the cis-trans photoisomerization process in Langmuir-Blodgett films of retinal derivatives. In this work we report the dielectric response of retinoic acid molecules contained in an emulsion of colloidal particles. Under continuous agitation, a solution of (13-cis or all-trans) retinoic acid (RA) molecules dispersed in chloroform was mixed to an aqueous solution of polyvinyl alcohol (PVA), until a homogeneous micro-emulsion is obtained. The spherical particles in suspension were composed by an outer shell of PVA containing in its interior a solution of RA molecules dispersed in chloroform. The dielectric characteristics of the emulsion were investigated in the 1 Hz to 1 MHz range of frequencies. For this, it was shown that the PVA microspheres provide a percolation path for the charge transfer throughout the emulsion, reducing the global impedance to levels at which the dielectric signatures of both cis and trans RA isomers can be individually identified. We have implemented a chemometric analysis of the different parameters involved in the preparation of the emulsions, to better understand the factors affecting either the final levels of impedance or the average size of the microspheres; we were then able to establish how the real part of the impedance at low frequencies depends on the relative amounts of chloroform/chromophore and PVA in the emulsions and that the stirring time is the most crucial single factor determining the total number and size of colloidal particles. At present, work is directed towards both a better control of the size of the spherical RA particles and to their immobilization in solid thin films.