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The electrochemical behaviors of perfluorinated anionic (FSA and FC-99), cationic (FSD), and nonionic (FSN) surfactants at Hg and Pt electrodes were s...
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Langmuir 1998, 14, 6280-6286

Behavior of Perfluorinated Surfactants at the Electrode/ Solution Interface Chuan-sin Cha* and Yan-bing Zu Department of Chemistry, Wuhan University, Wuhan 430072, People’s Republic of China Received April 16, 1998. In Final Form: July 28, 1998 The electrochemical behaviors of perfluorinated anionic (FSA and FC-99), cationic (FSD), and nonionic (FSN) surfactants at Hg and Pt electrodes were studied by using the interfacial capacitance measurement and cyclovoltammogram techniques. Their behaviors were also compared with that of the analogous hydrogenated surfactants. In the potential region near the potential of zero charge, both types of surfactants were found to be adsorbed at the Hg/solution interface with their hydrophobic segments oriented toward the electrode surface. However, the surface activity of the perfluorinated surfactants is in general weaker and the saturated adsorption layer is less compact than that of the hydrogenated surfactants at the weakly charged Hg/solution interface. On a more hydrophilic Pt electrode surface and on a strongly charged surface of Hg, both types of surfactants were found to be adsorbed at the electrode/solution interface with their hydrophilic end-groups or segments situated at the interface. On the basis of these results, basic principles for application of perfluorinated surfactants in electrochemical systems were postulated.

Introduction The perfluorinated surfactants (RF SAS) are nowadays commercially available.1,2 These surfactants (SAS) seem to possess a number of properties superior over the analogous hydrogenated surfactants (RH SAS). For example, by application of the RF SAS, usually a lower surface tension can be achieved at the liquid/air interface, especially in cases of aqueous and polar nonaqueous systems. The applications of RF SAS in electrochemical systems have been reported in some relatively recent publications3-5 and patent literature.6,7 However, reports on the basic electrochemical behavior of the RF SAS are relatively few. Comparison of the adsorptional behaviors of perfluorinated compounds such as perfluorobutyl alcohol at the solution/air and the Hg/solution interfaces has been summarized in ref 8. The behaviors of various types of commercial RF SAS at the gold electrode were reported in refs 9-11. We reported recently our preliminary results of study on the behaviors of RF SAS at the Pt/solution interface.12 In this paper we present results of comparative study of the behaviors of the RF and RH SAS at the Hg/solution and Pt/solution interfaces, and on the basis of these results, we also try to postulate the basic principles for the application of RF SAS in electrochemical systems. The structures of the main constituents of a number of commercial RF SAS of the Zonyl series from Du Pont Co.1 (1) Du pont Company, Zonyl Fluorosurfactants. (2) 3M Company, Fluorad Flurochemical Surfactants. (3) Juhel, G.; Boden, B.; Lamy, C.; Leger, J. M. Electrochim. Acta 1990, 35, 479. (4) Cachet, C.; Chami, Z.; Wiart, R. Electrochim. Acta 1987, 32, 465. (5) Cachet, C.; Saidani, B.; Wiart, R. J. Electrochem. Soc. 1991, 138, 678. (6) Tsuchiya, S.; Inoue, T. Japanese Patent 02,236,967 (90,236,967); Chem. Abstr. 1993, 114, 85389d. (7) Myake, S. Japanese Patent 05,121,089 (93,021,089); Chem. Abstr. 1993, 119, 99935u. (8) Frumkin, A.; Damaskin, B. B. Pure Appl. Chem. 1967, 15, 263. (9) Cachet, C.; Keddem, M.; Mariotte, V.; Wiart, R. Electrochim. Acta. 1992, 37, 377. (10) Cachet, C.; Keddem, M.; Mariotte, V.; Wiart, R. Electrochim. Acta 1993, 38, 2203. (11) Cachet, C.; Keddem, M.; Mariotte, V.; Wiart, R. Electrochim. Acta 1994, 39, 2743. (12) Cha, C. S.; Zu, Y. B. Russ. J. Electrochem. 1995, 31, 796.

Table 1. Typical Structures of the Main Constituents of Commercial Perfluorinated Surfactants Zonyl FSN Zonyl FSD Zonyl FSA Fluorad FC-99 Fluorad FC-135

F(CF2CF2)3-8CH2CH2O(CH2CH2O)xH F(CF2CF2)3-10CH2CH2SCH2CH2N+(CH3)3ClF(CF2 CF2)3-8SCH2CH2COOLi F(CF2)nSO3H, n ) 4-8, compounded with diethanolamine (DEA) in 1:1 molar ratio to improve its solubility in water F(CF2)nSO2NHC3H6N+(CH3)3I -, n ) 4-8

and the Fluorad series from 3M Company2 are listed in Table 1. Comparing them with the structures of analogous RH SAS, we can see that the structural peculiarities of the RF SAS are mainly the following: (1) the perfluorinated hydrophobic carbon chain; (2) the hydrophilic part of the RF SAS molecule is not fluorinated, i.e., usually the same as or very similar to the hydrophilic part of analogous RH SAS; (3) occasional inclusion of segments, such as -S-, -SO2- or -SO2NH-, which are usually not found in the structure of RH SAS. Experimental Section RF SAS used in this work include various types of commercial products listed in Table 1. The commercial products may also contain organic solvents and inorganic salts. All commercial SAS were subjected to prolonged heating at ca. 80 °C before use to remove volatile solvent. Hexylene glycol in FSD was extracted with ethyl acetate. The chloride ions in FSD solution were removed by passing the solution through a packed column of freshly precipitated silver hydroxide. However, no effort has been made to fractionize the commercial RF SAS according to their molecular weight. The RH SAS used in this work were either pure reagents or commercial products from well-known producers. All solutions were prepared with reagents of analytical grade and doubly distilled water. The main electrochemical techniques employed in this work include measuring of interfacial differential capacitance (Cd) and electrocapillary curve at the dropping mercury electrode,13,14 and cyclic voltammetry with the Pt microdisk electrode. The frequency of ac signal used in the capacity measurement was 1 kHz. (13) Grahamn, D. C. J. Am. Chem. Soc. 1941, 63, 1207; 1946, 68, 301; 1949, 71, 2975; J. Phy. Chem. 1957, 61, 701. (14) Damaskin, B. B. Zh. Fiz. Khim. (Russ. J. Phys. Chem.) 1958, 32, 2119 (in Russian).

S0743-7463(98)00436-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/19/1998

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Figure 2. Differential capacity curves of a Hg electrode in solution: 1, 0.1 M NaOH; 2, 0.1 M NaOH + 0.1 wt % FSA; 3, 0.025 M NaOH + 0.025 M C9H19COOH (from: Eda, K. J. Chem. Soc. Jpn. 1959, 80, 461 (in Japanese)).

Figure 1. (a) Differential capacity curves of a Hg electrode in 0.1 M NaOH + x wt % FSA: 1, x ) 0; 2, x ) 0.01; 3, x ) 0.02; 4, x ) 0.05; 5, x ) 0.1. The cut shows the change of potential of desorption with concentration of FSA. (b) Differential capacity curves of a Hg electrode in solution: 1, 0.1 M NaOH + 0.1 wt % FSA; 2, 0.1 M Na2SO4 (pH ) 7) + 0.1 wt % FSA; 3, 0.1 M Na2SO4 (pH ) 3, adjusted with H2SO4) + 0.1 wt % FSA. Dropping mercury electrodes (dme) with dropping times longer than 5 s were used in capacity and electrocapillary curve measurement to ensure better precision of experimental results. The Pt microdisk electrode (100 µm diameter) was polished before each measurement with grit carborundum paper and alumina powders down to 0.05 µm grade to obtain a mirror surface, then ultrasonicated, and thoroughly rinsed with doubly distilled water. Before each experiment, the Pt microelectrode was subjected to repeated scanning in the potential range -0.3 to 1.2 V (vs SCE) at a scan rate 100 mV/s in 0.1 M H2SO4 until the standard voltamogram was obtained. The reference electrode was a saturated calomel electrode, and a Pt gauze was used as the counter electrode.

Results and Discussions (1) Adsorptional Behaviors of Perfluorinated SAS at the Hg/Solution Interface. Mercury is usually considered to be a relatively hydrophobic metal. The Hg/ solution interface behaves as an ideally polarized electrode in the potential range from ca. 0.0 to -1.8 V (varies somewhat with pH and anion species). In this potential range the surplus of electric charge at the surface of Hg electrode changes from positive to negative as the electrode potential shifts in the negative direction. Therefore, it is possible to study the adsorptional behaviors of SAS on strongly positively charged, weakly charged or uncharged, and strongly negatively charged Hg electrode surface. In alkaline solutions FSA exists as organic anions. Figure 1a shows that after the addition of FSA the differential capacitance of the dme decreases in the

potential region around the potential of zero charge (pzc, ca. -0.5 f -0.6 V vs SCE in most dilute solutions of inorganic electrolytes), characteristic for the adsorption of organic molecules.15 On the left side of the adsorption region we can see capacity rise due to specific adsorption of inorganic anions, and on the right side the capacity peak due to desorption of organic molecules. The minimum capacitance decreases with increase of FSA concentration until a steady value is reached, indicating the saturation of Hg electrode surface with monolayer of adsorbed molecules. The potential of cathodic desorption peak varies with the logarithm of FSA concentration (see cut in Figure 1a). In acidic solutions, in which the anions are converted to neutral molecules, the desorption peak shifts to more negative potential, as indicated in Figure 1b. All these experimental results indicate that, qualitatively speaking, the adsorptional behaviors of FSA are quite similar to that of its alkyl analogues,15 so it seems reasonable to assume that the adsorbed FSA molecules are oriented with their hydrophobic segments toward the Hg electrode surface, as in the case of alkyl carboxylic acid adsorption. In Figure 2 the behavior of a saturated adsorption layer of FSA is compared with the typical behaviors of the longchain n-alkyl carboxylic acid. The concentrations of FSA and C9H19COOH are not exactly the same, and the concentration of NaOH solution is not exactly the same also. However, in both cases the concentrations of SAS are high enough and the Cd curves represent the limiting shapes not affected by minor changes of SAS and NaOH concentrations. The differences between the adsorptional behaviors of these two types of SAS are evident. The alkyl carboxylic acid can not only be adsorbed in a wider range of potential (as indicated by the position of the desorption peak) but also decrease the interfacial capacitance to a significantly lower value. It is a well-established fact that in the presence of saturated monolayer of an alkyl compound at the Hg electrode surface, the interfacial capacitance can be reduced to 4.5-5 µF cm-2.15,16 On the contrary, upon addition of FSA the minimum capacitance was found to be about 10 µF cm-2. The higher minimum capacitance observed in the case of FSA seems to indicate weaker interaction between the perfluorinated carbon chains and the Hg surface. Weaker interaction is also suggested by the more positive FSA desorption peak, demonstrating that it is easier to expel the adsorbed FSA (15) Damaskin, B. B.; Kazarinov, V. E. Compr. Treatise Electrochem. 1980, 1, 359. (16) Chizov, A V.; Pirozkov, S. D.; Damaskin, B. B. Elektrokhimiya 1969, 5, 1227 (in Russian).

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Figure 3. Differential capacity curves of a Hg electrode in solution: 1, 0.1 M Na2SO4; 2, after addition of 0.1 wt % DSS; 3, after addition of 0.1 wt % of FC-99; 4, after addition of 0.2 wt % of FC-99.

molecules from the electrode surface when the electric field strength of the double layer increases. The results of a self-assembled monolayer (SAM) study on a Au electrode17 showed that the interfacial Cd value of the well-constructed C10 RF-thiol SAM (ca. 1.6 µF cm-2) is lower than that of the analogous C10 RH-thiol SAM (ca. 1.8 µF cm-2), an effect that can be explained by the difference between the dielectric constants () of the RH and RF chains. The  value of agglomerated -(CF2)nchains (for example,  ) 2.1 for poly(tetrafluoroethylene)) is smaller than that of agglomerated -(CH2)n- chains (for example,  ) 2.26 for polyethylene). Therefore, if the structures of saturated adsorption layers of RH and RF SAS were the same, the minimum Cd value of the Hg/ solution interface should be lower in the case of RF SAS; however it is in contradiction with the experimental findings. In ref 8 Frumkin had mentioned that on an uncharged Hg surface the maximum coverage of butyric acid and perfluorobutyric acid is practically the same (ca. 5 × 10-10 M cm-2, or 30 Å2 per molecule), but the surface activity of perfluorobutyric acid is significantly lower. Apparently our experimental data can also be explained by lower surface activity of RF SAS at the Hg/solution interface, which gives rise to less compact and less ordered absorption layer of RF SAS molecules and higher value of minimum interfacial capacitance. The reason for lower surface activity of RF SAS might be the larger cross section of the perfluorinated carbon chain and the higher electronegativity of the fluorine atoms, so more water molecules have to be replaced from the surface during the adsorption of the RF SAS molecules. However, more work should be done before a concrete conclusion can be ascertained. When FC-99 is dissolved in solution, the active compound is dissociated into perfluorinated sulfonic acid anions and diethanolamine cations. In Figures 3 and 4 the behaviors of anionic RF SAS FC-99 are compared with that of the analogous anionic RH SAS C12H25SO3Na (DSS). These experimental results clearly show that DSS is more surface active than FC-99 at the Hg/solution interface. DSS can reduce the surface tension of the Hg/solution interface and the dropping time of the dme to significantly lower values in a wider range of potential than FC-99 (Figure 4). Besides, the shift of pzc, estimated from the shift of potential of maximum dropping time of the dme, is more significant in the case of DSS addition. The effects of these two types of anionic SAS on Cd of the Hg/solution interface are quite different, as shown in Figure 3. After (17) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682.

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Figure 4. The dropping time (td)-E curves of a dme in solution: 1, 0.1 M Na2SO4; 2, after addition of 0.05 wt % of FC-99; 3, after addition of 0.05 wt % DSS.

Figure 5. Differential capacity curves of a Hg electrode in solution: 1, 0.1 M Na2SO4; 2, after addition of 0.1 wt % of FSN; 3, after addition of 0.1 wt % of Peregal O.

addition of DSS the Cd values can be reduced to 4-5 µF cm-2 in potential range around pzc, and a distinct peak appears in the cathodic region -1.1 f -1.2 V, characteristic for the adsorption of alkyl SAS with their carbon chains oriented toward the electrode surface. On the other hand, after the addition of FC-99 the interfacial Cd values were found to be lowered mainly in potential region around pzc and in the region where the electrode surface is positively charged. The minimum Cd values are about 12 µF cm-2 in the vicinity of pzc in a relatively concentrated solution of FC-99 (0.2%), and a small peak is visible at ca. -0.7 V. The lowering of Cd in potential region more negative than -0.7 V seems to be due to diethanolamine cations, since addition of DEA to a slightly acidified solution of Na2SO4 shows a similar effect on capacitance in this potential region. The narrower potential range of adsorption and higher value of minimum capacitance in the case of FC-99 addition (compare with DSS addition) make it clear again that the surface activity of the perfluorinated SAS is lower and the adsorbed monolayer is less compact than that of the analogous RH SAS at the Hg/solution interface. In Figure 5 the effect of addition of nonionic RF SAS FSN on the interfacial capacitance is compared with that of the RH SAS Peregal O (from BASF), the main constituent of which is CmH2m+1 (OC2H4)nOH, in which m and 2n are approximately equal and both are in the range 8-10. In potential region near pzc, Cd can be decreased to ca. 5 µF cm-2 by addition of Peregal O, but only to ca. 10 µF cm-2 by addition of FSN, indicating once more that the perfluorinated carbon chain is less effective than the hydrogenated alkyl chain in blocking the uncharged and weakly charged Hg surface. In the potential range -1.0

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Figure 6. Adsorption of poly(ethylene oxide) chain at negatively charged electrode surface.

Figure 8. Polarogram of Pb2+ reduction at a dme in solution: 1, 0.1 M Na2SO4 + 1 mM Pb(NO3)2; 2, after addition of 0.05 wt % of FSN; 3, after addition of 0.01 wt % of Triton X-100.

Figure 7. Differential capacity curves of a Hg electrode in solution: 1, 0.1 M Na2SO4; 2, after addition of 0.1 wt % of FSD; 3, 0.1 M ((CH3)4N)2SO4. (From: Damaskin, B. B.; NikolajevaFedorovich, N. V. Zh. Fiz. Khim. 1961, 35,1279 (in Russian)).

T -1.6 V, in which the surface of Hg is strongly negatively charged, the behaviors of Peregal O and FSN are quite similar. The corresponding Cd values of the SAS blocked surface were found to be ca. 8-9 µF cm-2 in both cases. Similar capacitance values can also be observed after addition of poly(ethylene glycol), H(OC2H4)nOH.18 Therefore, it seems probable that in the negative potential region both RH and RF nonionic SAS molecules are adsorbed with their hydrophilic segments situated at the interface, as schematically shown in Figure 6. Since the hydrophilic segments of Peregal O and FSN have very similar structure, it seems reasonable that their effects on interfacial capacitance are also quite similar. Cd-E curves measured with dme in solutions containing RF SAS FSD are depicted in Figure 7. In the potential region around pzc, the limiting Cd value is again 9.5-10 µF cm-2, characteristic for Hg surface blocked with a saturated layer of perfluorinated carbon chains. In a region more negative than -1.2 V, Cd values were found to be decreased to 13-14 µF cm-2 within a wide potential range. Such an interfacial capacitance value is characteristic for a negatively charged surface blocked with a saturated layer of (CH3)4N+ cations, as indicated by the Cd-E curve obtained in a solution of di-tetramethylammonium sulfate (curve 3 in Figure 7). The coincidence of curves 2 and 3 in the far negative potential region clearly shows that the FSD molecules are adsorbed in this potential range with their hydrophilic end groups (the quaternary ammonium cation groups) oriented toward the electrode surface. From the above-stated experimental results it is clear that in the potential region not far from pzc, all types of RF SAS molecules, just as the analogous RH SAS, are adsorbed at the Hg/solution interface with their hydrophobic segments (the perfluorinated carbon chains) oriented toward the electrode surface. However, since the interaction between the electrode and the perfluorinated (18) Hwang, Q. A.; Zhou, Y. H.; Cha, C. S. Mater. Prot. 1965, no. 2, p 11 (in Chinese).

carbon chains is in general weaker than that between electrode and the hydrogenated carbon chains, the saturated surface layers of perfluorinated chains are usually less compact and characterized by higher Cd values. The inhibiting effects of adsorption layers of RF SAS on kinetics of electrode processes in the potential region around pzc were also found to be usually less severe than that of the analogous RH SAS. An example is shown in Figure 8. The addition of 0.01% nonionic hydrogenated SAS Triton X-100 makes the half-wave potential (E1/2) of Pb2+ reduction to shift in the negative direction, while the addition of 0.05% FSN has almost no effect on the polarographic wave, except the suppression of the polarographic maximum. Therefore, the perfluorinated SAS FSN may be considered to be a better maximum suppresser, since its inhibiting effect on kinetics of electrode processes is probably less. In a region far negative from pzc, RF SAS FSN and FSD molecules are adsorbed at the electrode/solution interface with their hydrophilic segments or end groups situated at the interface, just as their analogous RH SAS. (2) Adsorptional Behaviors of Perfluorinated SAS at the Pt/Solution Interface.19 Platinum is a relatively hydrophilic metal. The potential range between the hydrogen and oxygen evolution potentials can be subdivided into three regions: the hydrogen atom adsorption/ desorption region, the double layer region, and the oxygen adsorption region. Figure 9 shows the effects of the addition of various types of perfluorinated SAS on the cyclic voltammetry (CV) curves obtained with Pt microelectrode in dilute acid solutions. In the hydrogen region (ca. 0.0 to -0.3 V vs SCE), the main effect of the added SAS is the partial suppression of the adsorption/desorption peaks, indicating the partial blockage of the adsorption sites for hydrogen atoms at the surface of the Pt electrode. The blocking effect of the anionic SAS FC-99 is not much different from that of the cationic surfactant FDS, suggesting that the electrostatic interaction between the surplus charges at the surface of electrode and the ionic charges carried by the SAS molecules is not an important factor in determining the magnitude of inhibition of the of hydrogen atom adsorption sites. The inhibition effect of nonionic SAS FSN seems to be stronger than that of FC-99 and FSD. However, since FSN can be decomposed in the more positive potential region (see following), it is difficult to justify whether the FSN molecule or its decomposition product is responsible for the stronger inhibition. (19) The basic contents of this section and curves shown in Figures 9 and 10 were first published by the authors in ref 12, an abridged and revised version of which is presented here to facilitate comparison of behaviors of SAS on Hg and Pt electrodes.

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Figure 10. Cyclic voltammograms obtained with progressively increasing anodic switching potential. The arrows indicate the curves corresponding to the start of Oad formation at the surface of Pt electrode in: (a) deaerated 0.1 M H2SO4 and after addition of 0.01 wt % (b) FC-99, (c) FSN, and (d) FSD.

Figure 9. Cyclic voltammograms of the Pt microdisk electrode measured in deaerated solution of 0.1 M sulfuric acid. Dashed lines represent the effect of addition of 0.01 wt % of perfluorinated SAS: a, FC-99; b, FSN; c, FSD. Dotted line in (c) is obtained when the anodic switching potential is limited to 0.8 V.

In the double layer region (ca. 0.0 to 0.3 V vs SCE in dilute acid solutions), the adsorption of the SAS usually manifests itself as the lowering of charging current (lowering of interfacial capacitance). Despite the distorting effect of the cathodic current due to the reduction of oxidized surface and product of oxidation of SAS generated at more positive potential, we can still see clearly from Figure 9 that all the added SAS show only a very slight affect on the interfacial capacitance in the double layer region. It should be pointed out that in Figure 9c the positive limit of potential scan is only 0.8 V after addition of FSD (dotted curve). If the potential scan is extended to 1.0 V, significant anodic current appears and the whole CV curve shrinks progressively to a very narrow loop (dashed curve in Figure 9c), indicating a very strong inhibiting effect in the whole range of potential scan. A similar effect can be observed upon the addition of FSA, which also contains the -S- linkage in its structure formula. Cottrell and Mann20 had studied the oxidation of thioethers at a Pt electrode. They found that the main products of oxidation are the sulfones, but thiols can also be produced in the presence of water. Our results indicate that some of the oxidation products of FSD and FSA behave like thiols, which are known to react with surface of Pt to give a very strongly inhibiting layer.21-23 (20) Cottrell, P. T.; Mann, C. K. J. Electrochem. Soc. 1969, 116, 1449. (21) Shindo, H.; Kaise, M.; Kondoh, H.; Nishibara, C.; Hayakawa, H.; Oho, S.; Nozoye, H. Langmuir 1992, 8, 357. (22) Dong, X. D.; Lu, J. T.; Cha, C. S. Bioelectrochem. Bioenerg. 1995, 36, 73.

In the oxygen region (>0.4 V vs SCE), adsorbed oxygen atoms and probably also other oxygen-containing surface species are formed at the Pt electrode surface. The addition of perfluorinated SAS can affect both the amount of oxygen-containing species formed (as measured by the area of the cathodic peak of their reduction on the cathodic branch of the CV curve) and the potential range in which surface oxidation takes place. Figure 10 shows CV curves obtained with stepwise increase of the switching potential. The addition of FC-99 has only a slight effect on both the anodic and cathodic branches of the CV curves in the oxygen region (compare Figure 10a and Figure 10b). The potential range of formation of adsorbed oxygen-containing species shifts slightly to more positive potential, and the area of the cathodic peak reduces some 10% after addition of FC-99, showing that the perfluorinated SAS anions are only slightly more effective than the sulfate anions in inhibiting surface adsorption of the oxygen-containing species. The addition of FSN, on the other hand, results in far more significant inhibition of surface oxidation, as indicated by marked suppression of both the anodic and cathodic branches of the CV curves (Figure 10c). Adsorption of oxygen-containing species is also seriously inhibited by addition of FSD, as shown in Figures 9c and 10d. The fact that RF SAS have almost no effect on the double layer section of CV curves but they can significantly affect the formation of surface oxygen species suggests that the later process is more sensitive to the presence of SAS at the Pt/electrolyte interface. The sharp rise of anodic current in potential range >1.0 V in Figures 9b and 10c is caused by adsorption of oxygencontaining species and the oxidation of FSN molecules, probably the oxidation of the poly(ethylene oxide) segment of the chain. Most nonionic RH SAS, just as FSN, show oxidation current in this potential region. In general, the effects of analogous perfluorinated and hydrogenated SAS are very similar on the hydrogen region and double layer region of the CV curves. Their insignificant effects on (23) In an earlier paper (ref 12), we had interpreted the strong inhibiting action of FSD as the effects of the perfluorinated cations. However, more recent data reveal that the effect is most probably due to the product of their anodic decomposition.

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Figure 11. (a) Effects of RF SAS on the reduction of dissolved oxygen at Pt electrode in 0.1 M H2SO4: (1) no addition, and addition of 0.01 wt % of FC-99 (2), FSD (3), and FSN (4). (b) Effects of RH SAS on the reduction of dissolved oxygen at Pt electrode in 0.1 M H2SO4: (1) no addition, and addition of 0.01 wt % of sodium octylbenzylsulfonate (2), C14-trimethylammonium bromide (3), and Peregal O (4).

hydrogen atom adsorption and double layer capacitance seem to indicate that the surface activity of both types of SAS originates from the hydrophilic sections of the molecules, which are the same for both types of SAS. The effects of perfluorinated SAS on chloride ion adsorption, oxygen reduction and electro-oxidation of methanol at Pt electrode had been reported by us previously in ref 12. They were found to be very similar to that of the analogous RH SAS.24 An example is given in Figure 11. (3) General Behaviors of the Perfluorinated SAS at the Electrode/Solution Interface and Proposed Guidelines for the Application of Perfluorinated SAS in Electrochemical Systems. The above-stated experimental results and discussions clearly indicate that the behaviors of perfluorinated SAS at the electrode/ solution interface are quite different from that at the air/ solution interface. Both RH and RF SAS are known to be adsorbed at the air/solution interface with the hydrophobic segments of their molecules oriented toward the air phase, and the adsorption isotherms of RF SAS and their analogous RH SAS are quite similar.8 However, the limiting reduction of surface tension is more significant in the cases of RF SAS. This phenomenon is generally interpreted as the result of weaker interaction between the hydrophobic segments of the oriented RF SAS molecules in the interfacial adsorption layer. When the electrode material is relatively hydrophobic and the surface charge density relatively low, as in the case of Hg electrode in potential range near pzc, the RH SAS are usually adsorbed at the electrode/solution interface with their hydrophobic segments oriented toward the electrode.15 Our results indicated that RF SAS behave very similarly. Nevertheless, their surface activity is lower than that of the analogous RH SAS, as judged from the lowering of interfacial tension, the value of minimum interfacial capacitance, the potential range of adsorption, (24) Zu, Y. B. Ph.D. Dissertation, Wuhan University, 1995.

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and the inhibiting effect of the adsorption layer on kinetic of electrode reactions. The differences between the behaviors of RH and RF SAS at the air/solution and electrode/solution interfaces are most probably caused by the different behaviors of water molecules at these interfaces. At the air/solution interface there is no interaction between the air phase and the surface water molecules, so the surface water molecules possess almost no preference of orientation. On the other hand, as the result of chemical and/or electrostatic (including effect of image force) interaction between the electrode surface and water molecules, the later are usually adsorbed at the electrode/solution interface with more or less definite orientation,25 even in the case when the electrode is relatively hydrophobic and the surface charge density is low. Therefore, upon adsorption of SAS at the electrode/ solution interface, a certain amount of the adsorbed water molecules must be replaced by the SAS molecules. Apparently, the RF SAS are less effective in “squeezing out” oriented water molecules from the Hg/solution interface. When the electrode material is more hydrophilic or the surface charge density is high, as in the cases of Pt electrode and strongly negatively charged Hg electrode, the interaction between electrode and adsorbed water molecules is definitely stronger. At such electrode/solution interfaces, the hydrophobic segments of both RH and RF SAS are apparently unable to displace the adsorbed water molecules, so only slight changes of interfacial capacitance were observed. In cases of FSN, FSD, and their hydrogenated analogues, which contain alkyl derivatives of polar functional groups as their hydrophilic segments, the SAS can be adsorbed at a more hydrophilic electrode with their hydrophilic segments situated at the interface. However, in such a mode of adsorption these SAS could influence only marginally the interfacial capacitance. Since the structures of hydrophilic segments of FSN and FSD are very similar to those of their analogous RH SAS, the behaviors of both types of SAS are also quite similar at more hydrophilic electrode surfaces. The electrochemical stability of perfluorinated carbon chains against oxidation is definitely higher than that of the hydrogenated carbon chains, as indicated by the higher stability of PTFE over PE. However, the electrochemical stability of SAS is mainly determined by the stability of the hydrophilic segments of the SAS molecules, not that of the carbon chains, since the former are usually more vulnerable to oxidation. The fluorination of the carbon chain may have some effect on the oxidation-resisting property of the polar functional groups, but in general we should not expect that RF SAS can be significantly more stable than the analogous RH SAS, since the structures of the hydrophilic segments of RH SAS and analogous RF SAS are practically the same. The perfluorinated alkyl sulfonic acid type SAS (for example FC-99) is perhaps the most oxidation resistant, since this type of SAS molecule has a totally perfluorinated carbon chain and contains no easily oxidizable organic polar group. Fuel cell researches had proved that a polymer membrane of the perfluorinated alkyl sulfonic acid type (such as Nafion) is far more stable than the polystyrenesulfonic acid membrane, and the perfluorinated sulfonate type SAS has already been successfully employed in the chromium plating industry, so it seems reasonable to assume that FC-99 is also more oxidation resistant than its analogous RH SAS. However, it is not (25) Bockris, J.O’M; Reddy, A. K. N. Modern Electrochemistry, MacDonald 1970; Chapter 7.

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possible to detect the stability difference between the alkyl and the perfluorinated alkyl sulfonic acids from the voltammogram curves. As general principles for application of RF SAS in electrochemical systems we would suggest the following: (1) Use of perfluorinated sulfonate type SAS should be considered in those cases, in which highly oxidative electrolyte and/or highly positive electrode potential are employed. Sulfonate type RF SAS are not only highly stable in strongly oxidative environment but also inhibiting the kinetics of electrode reactions less than most other SAS. (2) Application of RF SAS can be tried to substitute RH SAS if the main purpose of SAS application is to control interfacial tension, such as to improve the wettability of electrode surface or to reduce the size of gas bubbles. RF SAS are definitely more effective in reducing the surface tension of the air/solution interface, and their inhibiting effects on electrode kinetics are often less severe than their RH SAS analogues.

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(3) In some cases the inhibiting effects of SAS should be carefully tuned, such as the control of the relative growth rates of different crystal planes in electroplating. Application of RF SAS may provide new opportunity to achieve the desired purpose. Conclusions RF SAS are in general not more surface active and not significantly more oxidation resistant than their RH SAS analogues at the electrode/solution interface. Nevertheless, the application of RF SAS may still provide new opportunities to improve the characteristics of electrochemical systems. Acknowledgment. The authors sincerely thank the Chinese State Education Committee (CEdC) for financial support of this work. LA980436G