Application of Fluorous Polymer Matrixes in Ion-Selective Elcetrodes

Abstract

Polyperfluoro(4-vinyloxy-1-butene), which is also known as Cytop, and poly[4,5-difluoro-2,2,-bis (trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) copolymers with dioxole monomer contents of 65% or 87% (known as Teflon AF1600 and Teflon AF2400, respectively) were plasticized with four fluorous compounds. While plasticization of all polymers with perfluoroperhydrophenanthrene, perfluoro(1-methyldecalin), a perfluorotetraether with three trifluoromethyl side groups and one hydrogen atom, and a linear perfluorooligoether with an average of 14.3 ether groups per molecule was successful, these four plasticizers affected the twelve blends very differently. A threshold of plasticization beyond which further increases in the plasticizer volume fraction did not further affect the glass transition temperature, Tg, was observed for some blends. Also, the limit of miscibility ranged from as low as 20% plasticizer content to complete miscibility at all volume fractions. The blends of Teflon AF2400 or Teflon AF1600 with high contents of the linear perfluorooligoether provided Tg values as low as –114 ºC, lower than for any other fully miscible blend. The occurrence of two glass transitions in an intermediate range of plasticizer volume ratios for these two types of blends can be explained by distinct local environments rather than macroscopic phase separation, as anticipated by the Lodge-McLeish model. In spite of the widespread use of perfluorinated solvents with amino and ether groups in a variety of application fields, the coordinative properties of these compounds are poorly known. It is generally assumed that the electron withdrawing perfluorinated moieties render these functional groups rather inert, but little is known quantitatively about the extent of their inertness. This chapter reports on the interactions between inorganic monocations and perfluorotripentylamine and 2H-perfluoro-5,8,11-trimethyl-3,6,9,12-tetraoxapentadecane, as determined with fluorous liquid-membrane cation-selective electrodes doped with tetrakis[3,5-bis(perfluorohexyl)phenyl]borate salts. The amine does not undergo measurable association with any ion tested, and its formal pKa is shown to be smaller than –0.5. This is consistent with the nearly planar structure of the amine at its nitrogen center, as obtained with density functional theory calculations. The 2HPFTE interacts very weakly with Na+ and Li+. Assuming 1:1 stoichiometry, formal association constants were determined to be 2.3 and 1.5 M-1, respectively. This disproves an earlier proposition that the Lewis base character in such compounds may be non-existent. Due to the extremely low polarity of fluorous solvents and the resulting high extent of ion pair formation, a fluorophilic electrolyte salt with perfluoroalkyl substituents on both the cation and the anion had to be developed for these experiments. In its pure form, this first fluorophilic electrolyte salt is an ionic liquid with a glass transition temperature, Tg, of -18.5 ºC. Interestingly, the molar conductivity of solutions of this salt increases very steeply in the high concentration range, making it a particularly effective electrolyte salt. Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2,-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (known as Teflon AF) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis(3,5-bis(perfluorohexyl)phenyl)borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+-ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability and maintained their selectivity over at least four weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 103.5, 101.8, 106.8 and 104.4 M–1, respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride C(꞊O)F groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(꞊O)F groups can be eliminated chemically. In this work we demonstrate the remarkable stability of fluorous-based ion-selective electrode (ISE) membranes by exposing them to a cleaning-in-place treatment, CIP, as it is used in many industrial processes. The sensing membranes were made up of a linear perfluoropolyether as membrane matrix, 0.5 mmol/kg ionic sites (tetrakis[3,5-bis(perfluorohexyl)phenyl]borate), 2 mmol/kg ionophore (tris[(perfluorooctyl)propyl]amine or tris[(perfluorooctyl)pentyl]amine), and Teflon AF2400. To mimic a typical CIP treatment, the electrodes were repeatedly exposed for 30 min to 3.0% NaOH solution at 90 ºC (pH ≈12.7). After ten exposures and a total of 5 h at 90 ºC, the fluorous sensing membranes doped with the more selective ionophore still showed the ability to respond with a theoretical (Nernstian) slope without loss in selectivity. Addition of a fluorophilic electrolyte salt reduced the membrane resistance by an order of magnitude.