Quantitative results using our technique can be achieved by adding another metal as an internal standard and removing both metals from solution with the treated fiber glass disk. For example, cadmium can be added as an internal standard for lead when using the dithiocarbamate glass. Another convenient internal standard is the use of silicon lines from the glass. In a variety of studies involving glass fiber we have found these to be useful intensity references. An advantage of the ESCA-glass fiber disk method is that conventional chelate chemistry can be used t o control selectivity; chelates such as dithiocarbamate can be used to scavenge a single metal or several metals of a group. Application of various chelating func-
tional groups to glass fiber surfaces can be accomplished by conventional synthetic organic techniques once the initial silylizing function has been attached. ESCA sensitivities are comparable for most elements ( 5 ) . Therefore, combining the scope of conventional chelate chemistry, the ease of synthesis of chelating functional groups and the wide applicability of ESCA as a measuring device, the ESCA-glass fiber disk technique appears to be one of very general utility. Received for review March 30, 1973. Accepted June 6, 1973. This work was supported by the National Science Foundation under Grant GP-32484.
I CORRESPONDENCE Conductance of Quaternary Ammonium Salt Dispersions in Polymeric Films Sir: The recently developed coated wire ion selective electrodes (1-3) for inorganic and organic anions and cations, consisting of a film of a polymeric composition incorporating the exchange material, which is directly coated on a metallic conductor, without the conventional internal reference solution, pose some very puzzling questions concerning their underlying functioning mechanism. Why are super-Nernstian slopes sometimes observed ( I ) ? Why are the selectivities sometimes greater than those of the corresponding barrel type liquid membrane electrodes (1-3)? How is a well-defined, reversible potential established in such a system? In elucidating the mode of behavior of the new electrodes, one important question that arises is the nature and extent of electrical conductivity of the polymeric films. This study is addressed t o examining this question. EXPERIMENTAL Materials. A number of quaternary ammonium salts were prepared from Aliquat 3365 (methyltricaprylylammonium chloride) (General Mills) and a series of tetraalkylammonium halides, from butyl to heptyl, (Eastman Organic) by repeated shaking of the material as received with 1.OM aqueous solutions of the appropriate salt. Specimens of unplasticized polymers were dissolved in minimal amounts of appropriate solvents; polyethylene in formic acid, polystyrene in CHCls, Nylon 66 (Du Pont) in CsHs, and poly(methylmethacrylate) in methylacetate. Epoxy resin, of the ordinary household type, was mixed with an equal amount of curing agent (diethylene triamine). Quaternary ammonium salt-polymer solutions were mixed in a 1:6 weight ratio and cast either in thin disks or as ca. 2.5-mm beads in which were embedded two fine (No. 30) platinum wire electrodes spaced ca. 0.17 mm apart. Samples thus prepared were dried in uacuo for 12 hr and then placed in a dry argon atmosphere for storage and measurement. (1) R. W. Cattrall and H. Freiser,AnaL Chem., 43, 1905 (1971). (2) H. J. James, G. D. Carmack, and H. Freiser, Anal. Chem., 44, 856 (1973). (3) 6.M. Kneebone and H . Freiser, Anal. Chem., 45,499 (1973). '
Electrical Conductivity Measurements. Direct current conductivity was determined by applying a potential across the samples with a 0-600 V power supply and measuring the current using a Keithley Model 160 multimeter in the nA mode. The input resistance of the measuring circuit was ca. 1012 by standardization against known resistors. Absolute conductivities of the materials tested were determined according to ASTM Procedure 257-61 ( 4 ) using the disks with steel and mercury electrodes. Samples with the embedded wire electrodes, enclosed in argonfilled capillaries, were used in the examination of the temperature dependence of conductivity. These were thermostated in an insulated water bath with both current and temperature measurements made at regular intervals. Approximately 5 minutes was necessary for thermal equilibrium t o be reached at each new temperature. Upon application of voltage across the sample, the current was observed to decrease with time. Instead of using an extrapolation to zero time, the value after 10 seconds was used. This procedure gave results that were reproducible to better than 3%. If the imposed voltage was removed from the sample for a few minutes, the identical current-time relationship was observed.
DISCUSSION OF RESULTS The most surprising feature of the conductivity behavior of the polymer dispersed salts is the unusually high temperature coefficient. From the plots shown in Figure 1, some of these materials compare favorably with commercially available semiconductor thermistors. Indeed the temperature dependence of the conductivity follows the operational semiconductor expression cr = cro exp(--E,/")
(1)
As may be seen from Table I, listing the values of E, calculated for a series of Aliquat salts in various polymeric matrices using Equation 1, the activation energies vary from 0.6 to almost 3 electron volts (or 15-65 Kcal/mol). This is in sharp contrast to the behavior of cellulose acetate films impregnated with alkali metal salts which was (4) American Society for Testing and Materials, Philadelphia, Pa., Method D 257-61.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
1975
IL.
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-7
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2.4
2.6
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2.8
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Figure 1. Temperature dependence of conductivity of quat+-C104- in various polymeric matrices
0 . 3 M Ali-
1 . Polyethylene. 2. polystyrene, 3. polymethylmethacrylate, 4. Nylon-66, 5. epoxy
-IZ
t 2.4
2.6
2.8
3.0
3.1
3.4
3.6
3.8
lOOO/T
Figure
4.
Temperature dependence of conductivity of Aliquat+-
clod- in epoxy resin as a function of concentration 1) 1.2M, 2) 0.92M, 3) 0.20M, 4) 0.090M
1.0
2.0 3.0 4.0 A C T I V A T I O N t N E R G Y (GV)
5.0
Figure 2. Conductivity-activation energy relationships for various organic systems A . Aromatics, 6.proteins, C. Aliquat-CIOa
Table I. Conductance Activation Energies for Aliquat Salt-Polymer Films Aliquat salt
Polystyrene
Polymethylmethacrylate
Polyvinylchloride
EPOXY
C104-
0.96 (22.1)
1 . 0 3 (23.8)
2.05 (47.3)
2.78 (64.1)
NOS-
0.88 ( 2 0 . 3 ) 0.60 (13.8)
0.66 ( 1 5 . 2 )
1.53 (35.3)
0.70 ( 1 6 . 1 ) 0.50 (1 1 . 5 ) 0.74 (17.1)
0.67 (15.5) 0.67 ( 1 5 . 5 ) 0.64 ( 1 4 . 8 )
1.33 (30.7)
eV (Kcal) s04'-
OAcCNSCI-
1976
2'56 ( 5 9 ' 0 )
1 . 1 0 (25.4)
2 . 1 3 (49.1) 2.08 (48.0)
characterized not only by low conductivity ( - 10-14 ohmcm-1) but also by low activation energies (
