Optical determination of ionophore diffusion coefficients in plasticized poly(vinyl chloride) sensing films

Abstract

The knowledge of accurate diffusion coefficients of electrically neutral ionophores in solvent polymeric membranes is important in view of understanding and optimizing important sensor characteristics of ion-selective electrodes (ISEs) and their corresponding optical sensors. A spectroscopic imaging technique is introduced here to determine the diffusion coefficient of the chromoionophore ( N, N-diethyl-5-(octadecanoylimino)-5 H-benzo[a]phenoxazine-9-amine, ETH 5294) in solvent polymeric membranes with different types of plasticizers and varying polymer–plasticizer ratio. This method is based on following the changes in the absorbance profiles of the chromoionophore as a function of space and time. The desired membrane composition is solvent cast onto a glass slide and placed under a microscope objective. The membrane is then exposed to light from a mercury arc lamp which photobleaches the dye in a circular area, providing an interface from which the time rate of absorbance change is studied. Spectral images are acquired over fixed time intervals with a microscope equipped with a charge-coupled device (CCD) camera providing 0.41 μm nominal resolution. Two plasticizers, ortho-nitrophenyl octyl ether ( o-NPOE) and bis(2-ethylhexyl) sebacate (DOS), are examined in poly(vinyl chloride) (PVC) by varying the ratio of polymer to plasticizer content and measuring the diffusion coefficient of the chromoionophore. Corresponding membrane compositions containing 10 wt.% of an inert lipophilic salt (ETH 500) are also examined. Near linear relationships between the logarithmic diffusion coefficients and the PVC content are observed that obey the following relationships: log D=−6.17 –0.0459 wt.% PVC (for PVC–DOS), log D=−6.46 –0.0468 wt.% PVC (for PVC–DOS–ETH 500), log D=−5.89 –0.0508 wt.% PVC (for PVC–NPOE), log D=−6.22 –0.0473 wt.% PVC (for PVC–NPOE–ETH 500).