Anal. Chem. 1988, 58,661-662
H&(C,H,)2Pb, 3440-77-5;(CSH,),Pb, 3440-75-3; (CH,),(C,Hg)Pb, 54964-75-9; (CH3)2(CdHs)zPb,65151-01-1; (C2Hb)S(C4H9)Pb, 64346-32-3; (CzH&(C4Hg)2Pb, 65121-94-0; (C,H9)4Pb, 1920-90-7; (CH3),Pb+, 14570-16-2; (C2H&Pb+, 14570-15-1; (CH3)zPb2+, 21774-13-0; (C2H,)2Pb2+,24952-65-6; propylmagnesium chloride, 2234-82-4; water, 7732-18-5. LITERATURE CITED (1) de Mora, S. J.; Hewitt, C. N.; Harrison, R. M. Anal. froc. 1984, 27, 415-41 8. (2) Harrison, R. M.; Hewitt, C. N.; de Mora, S. J. Trends Anal. Chem. 1065. 4 . 8-11. ...., (3) Chau, Y.'K.; Wong, P. T. S.; Kramar, 0. Anal. Chim. Acta 1983, 146, 21 1-217. (4) Harrison, R. M.; RadojeviE, M. Environ. Techno/. Lett. 1985, 6 , 129-1 36.
881
(5) Chau, Y. K.; Wong, P. T. S.; Bengert, G. A.; Dunn, J. L. Anal. Chem. 1984, 5 6 , 271-274. (6) Harrison, R. M.; RadojeviC, M.; Hewitt. C. N., Papers presented at the International Conference on Heavy Metals in the Environment, C.E.P. Consultants, Athens, 1985. (7) Harrison, R. M.; Hewitt, C. N.; RadojeviE, M. Sci. Total Environ. 1985, 4 4 , 235-244. (8) Harrison, R. M.; RadojeviE, M.; Wilson, S. J. Sci. Total Environ., in press. (9) . , Birch, J.: Harrison, R. M.: Laxen. D. Sci. Total Environ. 1980, 14, 31-42. (10) Hewitt, C. N.; Harrison, R. M. Anal. Chim. Acta 1985, 167, 277-287. (11) Chakraborti, D.; de Jonghe, W. R. A.; Van Mol, W. E.; Van Cleuvenbergen, R. J. A.; Adams, F. C. Anal. Chem. 1984, 56, 2692-2697.
for review August 5, 1985* Accepted October 8,
1985.
Silane Coupling Agents for Attaching Nafion to Glass and Silica Marilyn N. Szentirmay, Leigh F. Campbell, and Charles R. Martin*
Department of Chemistry, Texas A&M University, College Station, Texas 77843 Glass and other silacious materials are frequently used as substrates for polymer coatings (1-5). For example polymer-coated silacious particles have been used as stationary phases in both gas ( 4 ) and liquid ( 5 ) chromatography. In addition, polymer films coated on glass or quartz slides have been used to investigate the spectroscopic properties of polymers or incorporated guest molecules (6). Because the adhesion of organic polymers to glass and silica is often poor, silane reagents are often used to attach polymers to these surfaces (1-3,5). For example, we have recently shown that films of a perfluorosulfonate ionomer (Nafion (7)) would not adhere to silica but would adhere to an alkylsilane-derivatized silica ( 5 ) . Because of our interest in further studying both the ion exchange ( 5 ) and spectroscopic (8) properties of these and other ionomers (9),we have attempted to identify silane derivatives that will serve as coupling agents for producing adherent Nafion films on glass and silica surfaces. Since our fundamental investigations of ionomers nearly always involve exposure of the ionomer film-coated surface to solvent ( 5 , 8 , 9), the effect of solvent on the stability of the Nafion-surface interaction is of particular interest. Because of the widespread interest in Nafion and related polymers we report the results of these studies here. EXPERIMENTAL SECTION Materials. Nafion 117 (1100 equivalent weight, proton form) was obtained from Du Pont and was dissolved in 5050 ethanol-water (IO). N-(Trimethoxysilylpropy1)-N,N,N-trimethylammonium chloride (TTACl) and octadecyltrichlorosilane (ODS) were obtained from Petrarch Systems, Inc. Five percent (v/v) solutions of these silane coupling agents were prepared in either methanol (TTAC1) or toluene (ODs). The methanol and toluene were dried over molecular sieves prior to use. Glass slides were obtained from Becton, Dickinson and Co. Silica slides were obtained from Esco Products. Ru(bpy),Cl, (bpy = 2,2'-bipyridine) was obtained from G. F. Smith; 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (PS3-)and methylviologen chloride were obtained from Aldrich. All solvents were of reagent grade or better. Distilled water was circulated through a Milli-Q water purification system (Millipore Corp.). Silanization Procedures. Glass and silica slides were washed with Alconox and water and rinsed thoroughly with water before use. Prior to reaction with ODs, all glassware was dried in an oven at 160 O C for 4 h. All steps of the ODS silanization procedure
were carried out in an Nz atmosphere glovebag. Silanizationwith TTA+ was carried out in air, although the silane reagent bottle was opened, sealed, and stored under Nz.Slides were immersed for 1 min in the 5% solution of the desired silane coupling agent, removed from the solution, and then allowed to sit undisturbed for 10-30 min. The ODs-treated slides were then rinsed with dry toluene and dried in the N2 atmosphere. TTA+-treatedslides were rinsed with water and dried in air. Nafion Film Coating. One milliliter of a 1.14% (w/v) Nafion solution was added to a cup formed by clamping a plastic ring onto a glass or silica slide (Figure 1). The solvent was evaporated in air at 40-50 "C. The plastic ring was then removed leaving a circular Nafion film on the glass or silica slide. To test whether the resultant films were strongly attached to the surfaces of the slides, the films were rinsed with water or other solvents; photographs were taken both before and after rinsing. To aid in visualization, the films were stained with a dilute solution of R~(~PY),~+. RESULTS AND DISCUSSION Silane-Treated Glass. The glass surfaces were characterized by using the anionic fluorescent dye, PS3-. When an as-received (Le., unsilanized) glass slide is exposed to a PS3solution, rinsed with water, and then exposed to UV light, the characteristic green fluorescence of PS3-is not observed; this is expected since there are no anion exchange sites on the surface and PS3-is very water soluble. When TTA+-derivatized slides were treated with PS3-,rinsed, and exposed to UV light, PS3-fluorescence is clearly seen, indicating, as expected, that derivatization with this cationic coupling agent yields a positively charged surface. This is reinforced by the fact that derivatized slides that are treated in an analogous fashion with Ru(bpy),'+ do not show R ~ ( b p y ) , ~fluorescence. + Finally, slides derivatized with ODS are rendered hydrophobic as evidenced by poor wetting of the surface by water. Nafion Films. The Nafion films obtained upon evaporation of the solvent were clear and coherent, but often showed cracks near the film edges (Figure 2). Prior to reexposure to solvent, the films adhered strongly to both the derivatized and underivatized slides; for example, tape could be applied to the film surface and then peeled away, yet the Nafion film remained attached to the slide. When rinsed with water, films cast onto underivatized slides were quickly and quantitatively removed from the surface (Figure 2A, right). In contrast, films coated on silane-treated
0003-2700/86/0358-0661$01.50/00 1986 Arnerlcan Chemical Society
662
ANALYTICAL CHEMISTRY, VOL. 58. NO. 3,MARCH 1988 Table I. Percent of Nafion Adhering to T T A + Derivatized Glass after Various Treatments % Nafion
solvent
treatment-
water
HCI NsOH water, 45 'C 1M 1M
Side V I W
CH,CN
remaining after treatment'
soak 2 h soak 7 h soak 26 h stir 16 h soak 16 h soak 16 h soak 1 h soak 2.5 h
99 98 97 98 100 0
12 75
'25 OC unless otherwise noted. '*2%.
1. coating N a h fllms:
a. @assOT silica slide: b. plastic rlng:
c. c!amp: d. Nafion solution.
Figme 2. photogaphs of Namn film on: A. untreated glass: B. ITA* derivatized glass: C. 00s derivatized glass. Tim Nafion fllms on the left of each slide were not exposed lo any sobent. The fllms on the dght W a -ked for ca. 10 min in 1 mM Ru(bpy),'+ and then rimed wkh water.
slides were not removed by water rinsing. This is demonstrated for various glass sukaces in the photographs shown in Figm 2. parta B and C. In each case. the film on the right has been stained with R u ( b p y ) p and then rinsed with water. Figure 2, parta B and C, show that the films remained intact and attached to the silane-derivatized surfaces. Results of quantitative studies of loss of Nafion from 'ITA'-derivatized surfaces are shown in Table 1. Nafion films are quite stable in the presence of 1 M HCI but are completely removed from the surface by 1 M NaOH. The removal by NaOH is undoubtedly caused by hydrolysis of the covalent bonds between the surface and the coupling agent and of the substrate glasa itself ( 2 ) . When a surfaceattached polymer fhin exposed to solvent, two distinct processes can remove the film from the surface: these processes are diasolution of the polymer in the solvent and destruction of the polymeraurface interaction. The data shown in Table I clearly show that solution-cast Nafion is not soluble in room-temperature water. Thus. the loss of Ndion from the underivatized surfaces must result from the destruction of the filmaurface interaction. Furthermore, and more importantly, the data in Figure 2 and Table I show that, in the presence of water and strong acids. derivatization of the surface produces a stronger filmaurface interaction. As-received Nafion membrane is insoluble in all solvents at temperatures below ca. 180 OC (IO). We have recently found, however. that solution.cast Nafion is soluhle in warm water and in a variety of polar organic solvents (e.g., ethanol, propanol, CH,CN, N,"-dimethylformamide) a t ambient temperatures (11). Thus, dissolution is responsible for the
logs of Nafion in the presence of CH,CN and warm water Oast 2 lines, Table I). Solution-cantNafon is not soluble in nonpolar solvents such as heptane, benzene. or xylene, nor is it soluble in methylene chloride. When exposed to xylene or heptane, Nafion films remain intact and attached to underivatized, ODS-derivatized, and 'ITA+-derivatized surfaces. Furthermore, Nafion films remain intact azrd attached to underivatized and TTA+-derivatized surfaces when exposed to methylene chloride. In contrast, films on ODS-derivatized glass become detached from the surface after 1 min of exposure to methylene chloride. Thus, of the three surfaces studied (underivatized, ODS derivatized, l T A + derivatized),only the cationic 'ITA+ surface produced strong binding to Nation in all of the nondissolving solvents studied. This is undoubtedly because 'ITA+ can interact electrostatically with the Nafion sulfonate sites producing an ionic bond between the film and the surface. Furthermore, it is important to note that we have recently developed solution processing procedures to produce solution-cast Nafion films, which, like the as-reeeived membrane, are insoluble in all solvents a t temperatures below ca. 180 "C (11). The use of these procedures in conjunction with 'ITA* surface coupling should produce Ndion film coated surfaces that are stable in the presence of any solvent. Finally, in addition to producing good adhesion in the presence of a variety of solvents, 'ITA+ has another advantage over ODS. The silanization reaction for 'ITA+ can be carried out in ambient air, whereas, in our hands, ODS silanization in air produced only partially derivatized surfaces. Because ODS derivatization requires a eontrolled atmosphere chamber, 'ITA+ is a more convenient couoline . aeent. Registry No. TTACI, 35141-36-7; ODs,112-04-9; Nafion 117, 66796-30-3; silica, 7631-86-9.
--
LITERATURE CITED (1) Pl-nn.
E. P. J . A b . 1970.2. 184. (2) Unw. K. K. "PaWSilica", Ekevier Scientific PublisMng: Amsler-
-. ... 107a .-.-. *m
(3) Arkks. B. CtEMECti 1977,788. (4) AW. w. A.; Hastings. c. R. J . ummatogr. 1969. 42.319. (5) Mwre. R. B.. 111: W l k m . J. E: Marlin. C. R. Anel. chem.1984. 56. 2572. (8) Cha. Y.; Twnooka, M.: Tanaka. M. J . PWm. Scl.. polym. chem. E .
1984.22, 2973. (7) Elsenberg. A.. Yeager. H. L.. Eds. '"Perfluorhated Ionamsr Mambram"; Anmkan Chemical Society: Washingon. DC. 1982; ACS Symp. Ssr. No. 180. (8) ~;~;lIrmay. M.. Rielo. N. Martin. C. R. J. phys. chem. 1085. 89, I",,.
(9) Szenlirmay. M.: FTWO.N.: Martln. C. R. rahm 1985. 32. 745. (101 Martin. C. R.: R b d e s . T. A.: Fer-. J. A. Anal. chem.1082.5 4 , (e10 1 1 1 " .
(11) Moore. R. B.. 111: Abamz-Roa. E. A.; colon. J. L.: Kellman. R. R.: Martin. C. R.. h preparaIbn. palm applied fa.
RFZEIVEDfor review August 22,1985. Accepted October 8, 1985. This work was supported by the Office of Naval Research and by the Robert A. Welch Foundation.
