Role of Polymer Membrane Components

Another component of the polymer membrane of ion-selective electrodes is the plasticizer. It lowers the glass transition temperature of the polymer, serves as the solvent for the ionophore and other membrane additives, increases the polarity of the membrane, which facilitates the distribution of ions between two phases. The plasticizer content in the membrane is approx. 65% of weight. Due to the plasticizer, the membrane can be considered a liquid phase, since the diffusion coefficient of the dissolved, low-molecular-weight ionophores is approx. 10 - - 10 cmV [38]. Such membranes are often called liquid/polymer or solvent polymeric in the literature. 

The plasticizer is the environment in which the processes of ion distribution, from the water phase to the organic phase, as well as complex formation reactions occur. It should be physically compatible with the polymer and contribute to better analytical parameters of the electrode. In order to play its role in an optimal way, the plasticizer should be highly lipophilic and show proper viscosity and low vapor pressure. 

The plasticizer influences the selectivity of the ion-selective membrane. The selectivity of electrodes containing ion exchangers in the membrane phase is determined by the differences between the standard Gibbs energy of ions in the water phase and the organic phase. For example, long-chain esters of carboxylic acids of low permittivity prefer extraction of monvalent ions and formation of ion pairs. Whereas, nitroaromatic compounds of high permittivity prefer extraction of divalent ions [39]. 

The plasticizer can also affect the measuring range (i.e., the high and low detection limits). However, there is no linear correlation between the polarity of the plasticizer. 

The plasticizer can also significantly affect the lifetime of the sensor [40] e.g., polyester sebacate (PES), when used instead of classic plasticizers, i.e., DOS or NPOE, prolongs the lifetime of valinomycin-based, potassium electrodes. 

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