Poly permselectivity

FIGURE 4-18 Permselective coatings flow injection response of a poly(l,2-diaminoben-zene)-coated electrode to the following a, hydrogen peroxide (1 mM) b, ascorbic acid (1 mM) c, uric acid (1 mM) d, L-cysteine (1 mM) and e, control human serum. (Reproduced with permission from reference 63.)... 

Ion exchange in LbL films has been also studied using radiolabeled + ions [104] by Schlenoff and coworkers. In that case, poly(butylviologen)/PSS multilayers showed the exchange of Ca in a stoichometric ratio with the charge passed through the electrode, an indication of cation permselectivity. 

The current state-of-the-art proton exchange membrane is Nafion, a DuPont product that was developed in the late 1960s primarily as a permselective separator in chlor-alkali electrolyzers. Nation s poly(perfluorosulfonic acid) structure imparts exceptional oxidative and chemical stability, which is also important in fuel cell applications. 

I. Pinnau, Ultrathin Ethyl Cellulose/Poly(4-methyl pentene-1) Permselective Membranes, US Patent 4,871,378 (October 1989). 

Except poly(dimethylsiloxane), all of the conventional polymers such as cellulose acetate, poly(phenylene oxide), nylon, polyethylene, and nafion are known to permeate water preferentially against ethanol111,112). However, in order to separate ethanol from dilute aqueous solution, it is favorable to use a membrane which permeates ethanol preferentially (ethanol-permselective membrane). 

As seen in Fig. 9, poly[l-(trimethylsilyl)-l-propyne] is ethanol-permselective, while other substituted polyacetylenes are hardly permselective or water-permselective, 13 U5>. The oc 1 and R values of poly[l-(trimethylsilyl)-l-propyne] and poly(dimethylsiloxane) are practically the same. This is rather astonishing, if it is taken into account that the former is glassy and the latter is rubbery around room temperature. 

It is important to clarify why only poly[l-(trimethylsilyl)-l-propyne] is ethanol-permselective among substituted polyacetylenes. Further investigation of pervaporation through substituted polyacetylenes is desired. 

Figure 11 is a plot of a versus R for ethanol-water pervaporation through substituted polyacetylenes. If the a value is larger than unity, the membrame permeates ethanol preferentially. Previously, with the exception of poly(dimethylsiloxane), no polymer had exhibited ethanol permselectivity. Now, we know that poly(4a) permeates ethanol preferentially (30). The a and R values of poly(4a) are close to those of poly(dimethylsiloxane). In contrast, other polyacetylenes exhibit practically no permselectivity or allow water to permeate preferentially. 

FIGURE 9.10 Dependence of ideal hydrocarbon/nitrogen permselectivity on the boiling temperature of the hydrocarbons for a poly-organosihcon membrane produced by GKSS [39-43]. (From Semenova, S.I., J. Membr. Set, 231, 189, 2004. With permission.)... 

The formation of microcapsules Is only one example of the use of polyelectrolyte complexes. Another example Is the formation of a permselective membrane around live cells by a complex formed between alginate and poly(L-lys1ne)(55). This complex 1s formed under mild conditions so as not to harm the cells, and the membrane can be tailored so that It 1s permeable to cell nutrients, but Impermeable to the cellular products. This process 1s of interest to those concerned with large-scale cell cultures. 

Liquid membrane electrodes are three (or four) component membrane based devices with positively charged, negatively charged, or electrically neutral ionophores. The main component of the liquid membrane is the ionophore, or mobile carrier—a compound with binding properties to different ions and responsible for the ion selectivity. The carriers are incorporated into membrane matrices, most commonly into plasticized poly(vinyl chloride). Liquid membranes often contain lipophilic salt additives also to enhance the permselectivity of hte membrane. 

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