Langmuir 1995,11, 2125-2130
2125
Examination of Vapor Sorption by Fullerene, Fullerene-Coated Surface Acoustic Wave Sensors, Graphite, and Low-Polarity Polymers Using Linear Solvation Energy Relationships J a y W. Grate* Environmental Molecular Sciences Laboratory, Pacific Northwest Laboratory, Richland, Washington 99352
Michael H. Abraham* and Chau My Du Chemistry Department, University College London, London WCIH OAJ, United Kingdom
R. Andrew McGill Geo-Centers, Inc., 10903 Indian Head Highway, Fort Washington, Maryland 20744
Wendel J. Shuely SCBRD-RT, U.S. Army Edgewood Research, Development and Engineering Center, Aberdeen Proving Ground, Maryland 21010 Received November 1, 1994. I n Final Form: February 27, 1995@ The sorption of vapors by fullerene is compared with the sorption of vapors by an assembled fullerene thin film on a surface acoustic wave vapor sensor. A linear solvation energy relationship derived for solid fullerene at 298 K was used to calculate gaslsolid partition coefficients for the same vapors as those examined using the vapor sensor. This relationship correctly predicted the relative vapor sensitivities observed with the vapor sensor. Anew linear solvationenergy relationship for vapor adsorptionby graphite at 298 K has been determined, and solid fullerene and solid graphite are found to be quite similar in their vapor sorption properties. Comparisons have also been made with linear organic and inorganic polymers, including poly(isobutylene), poly(epichlorohydrin),OV25, and OV202. In all cases, sorption is driven primarily by dispersion interactions. The assembled fullerene material is generally similar in vapor selectivityto the other nonpolar sorbent materials considered but yields less sensitive vapor sensors than linear organic polymers.
Introduction The properties and potential applications of fullerenes have attracted widespread interest since the discovery that these molecular solids could be extracted easily from soot.lt2 Recently, Li and Swanson described thin multilayer films of a fullerene derivative that were deposited ~ combinaon a surface acoustic wave (SAW)d e v i ~ e .This tion yielded a chemical sensor where the role of the fullerene layer was to collect and concentrate vapor molecules on the SAW device surface by sorption. The vapor sorption process plays a critical role in determining the sensitivity, selectivity, response time, and reversibility ofthis type of The fullerene layer was assembled by reacting fullerene CEO with (3-aminopropy1)trimethoxysilane to form a functionalized adduct that polymerizes via the formation of siloxane bridges. This process yields a network polymer that can covalently bond to silica surfaces. Sensors were coated by a dipping procedure. In independent studies, we described the adsorption of organic vapors by solid fullerene as determined by Abstract published in Advance A C S Abstracts, May 1, 1995. (1) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, R. R. Nature 1990,347, 354-358. (2) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685-693. (3) Li, D.; Swanson, B. I. Langmuir 1993,9, 3341-3344. (4) Wohltjen, H. Sens. Actuators 1984, 5, 307-325. ( 5 ) Grate, J. W.; Abraham, M. H. Sens. Actuators B 1991,3,85-111. (6) Grate, J. W.; Martin, S. J.;White, R. M. Anal. Chem. 1993, 65, 940A-948A. (7) Grate, J . W.: Martin, S. J.: White, R. M. Anal. Chem. 1993, 65, 987A-996A. @
chromatographic measurements.8 A sample of sieved crushed fullerene granules packed into a column served as the stationary phase, and retention data for a series of diverse probe vapors were collected at 298 K. Gassolid partition coefficients, K,, defined according to eq 1,
K,= CJC,
C,-0
(1)
were then calculated by the method of elution by characteristic point, ECP.g K, is the ratio of the concentration of the vapor sorbed by the solid adsorbent, C,, in grams of vapor per gram of adsorbent, divided by the concentration of the vapor in the gas phase, C,, in grams per liter. These partition coefficients were regressed against solvation parameters for the probe vapors by the method of multiple linear regression to generate a linear solvation energy relationship (LSER) of the form in eq 2. In this
log K , = c
+ rR2 + s 4 + aCa: + bCPf + I log
L16
(2)
equation, Rz,#, Caf, C p f , and log LI6 are solvation parameters that characterize the solubility properties of the probe vapors. The parameters Caf and Cpf measure the vapor overall or effective hydrogen-bond acidity and (8) Abraham, M. H.; Du,C. M.; Grate, J. W.; McGill, R. A.; Shuely, W. J. J. Chem. SOC.,Chem. Commun. 1993,1863-1864. (9) Conder, J. R.; Young, C. L.Physicochemical Measurements by Gas Chromatography; John Wiley & Sons: New York, 1979.
0743-746319512411-2125$09.00/0 0 1995 American Chemical Society
2126 Langmuir, Vol. 11, No. 6, 1995 basicity, respectively.10-12 The parameter $ is a dipolarity/polarizability parameter that measures the ability of a molecule to stabilize a neighboring charge or dip01e.l~ Values of this parameter are approximately proportional to molecular dipole moments for nonprotonic, aliphatic solutes with a single dominant dipole. L16 is the gasliquid partition coefficient of the solute on hexadecane at 298 K determined by gas-liquid chromatography.14 Hexadecane is a completely nonpolar sorbent, precluding the possibility of any dipole-dipole or hydrogen-bonding interactions with solute vapors. logL16is therefore related to dispersion interactions. R2 is a calculated excess molar refraction parameter that provides a quantitative indication of polarizable n and z electrons.16 All of these parameters except RZ are derived from thermodynamic measurements of partitioning and/or complexation equilibria. These solvation parameters are available for hundreds of organic compounds and have recently been reviewed.12 The coefficients in eq 2 characterize the solubility properties of the sorbent material. The a and b coefficients, being complementary to the vapor hydrogen-bond acidity and basicity, represent the sorbent phase hydrogen-bond basicity and acidity, respectively. The s coefficient is related to the sorbent phase dipolarity/polarizability. The I coefficient is related to dispersion interactions. Larger values of I indicate that differences between the partition coefficientsfor a series of homologous vapors will be larger (compared to a material with a smaller value of the 1 coefficient). The r coefficient refers to the ability of the phase to interact with solute n and 7~ electron pairs and provides an indication of polarizability. The constant, c, arises from the method of multiple linear regression used to obtain eq 2. The LSER method has been applied to the characterization of the solubility of gaseous solutes in polymers,16 partitioning into gas-liquid chromatographic stationary phases,15z17-19adsorption on solid sorbents,20toxicity of gases and vapors,12and the sorption of vapors by blood and tissue.21 The application of this method to sorbent materials for acoustic wave sensors has been described in detai1.5*22-29 Applying this now well-established approach
Grate et al. Table 1. Vapor Solvation Parameters and Observed log K, Values for Graphite and Fullerene log Kc x,5'Bf: graphite fullerene Rz 4 0.0000.000 0.0000.000 2.668 0.36 n-hexane n-decane 0.000 0.000 0.000 0.0004.686 1.14 0.55 0.85 n-undecane 0.0000.0000.000 0.0005.191 1.53 1.21 n-dodecane 0.0000.0000.000 0.0005.696 1.82 norbornane 0.3700.1500.000 0.0003.187 0.73 decalin 0.505 0.2500.000 0.0005.077 1.77 a pinene 0.446 0.2000.000 0.1004.030 1.05 2-norbornene 0.4600.2500.000 0.100 3.242 1.05 dichloromethane 0.3870.5700.100 0.050 2.019 0.35 trichloromethane 0.4250.4900.1500.0202.480 0.67 tetrachloromethane 0.4580.3800.000 0.0002.823 0.65 l,l,2,2-tetrachloro- 0.5950.7600.1600.1203.803 1.33 0.96 ethane trichloroethene 0.5240.3700.0800.0302.997 0.81 tetrachloroethene 0.639 0.4400.000 0.0003.584 0.83 0.57 diiodomethane 1.453 0.6900.050 0.2303.857 1.20 0.54 di-n-butyl ether 0.0000.2500.000 0.4503.924 1.11 0.54 tetrahydrofuran 0.2890.5200.000 0.4802.636 0.71 propanone 0.1790.7000.040 0.490 1.696 0.76 octan-2-one 0.1080.6800.000 0.5104.257 1.50 0.93 decan-2-one 0.1080.6800.000 0.5105.245 2.19 1.51 norcamphor 0.4700.9100.000 0.5604.176 1.74 n-butyl propanoate 0.0580.5600.000 0.4703.833 1.21 0.54 methanol 0.278 0.4400.4300.4700.970 0.35 octan-1-01 0.1990.4200.3700.4804.619 2.17 1.22 dimethyl sulfoxide 0.5221.7400.000 0.8803.459 2.17 1.16 triethyl phosphate 0.0001.000 0.000 1.0604.750 2.13 1.49 benzene 0.6100.5200.000 0.1402.786 0.75 toluene 0.601 0.5200.000 0.1403.325 1.05 n-propylbenzene 0.6040.5000.000 0.1504.230 1.13 0.49 1,2-dichlorobenzene 0.8720.7800.000 0.0404.518 1.55 0.82 4-chlorotoluene 0.7050.6700.000 0.0704.205 1.48 0.52 iodobenzene 1.1880.820 0.000 0.120 4.502 1.53 0.89 nitrobenzene 0.871 1.1100.000 0.2804.557 2.13 1.17 m-cresol 0.8220.8800.570 0.3404.310 2.13 1.51 2-chlorophenol 0.8530.8800.320 0.3104.178 1.81 1.24 benzyl alcohol 0.8030.8700.3900.5604.221 2.12 1.20 0.53 Pyrrole 0.6130.7300.4100.2902.865 1.14
xG
vapor
!8
to the sorption of 22 vapors by fullerene at 298 K yielded eq 3.8The number of vapors is given by n , e is the
log K, = -1.58 - 0.24R2
+ 0 . 7 2 4 + 1.04Ca: + 0.48 log L16
(10)Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem. SOC.,Perkin Trans. 2 1989,699-711. ( 11)Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.;Taylor, P. J.J. Chem. SOC.,Perkin Trans. 2 1990,521-529. Rev. 1993,22,73-83. (12)Abraham, M. H. Chem. SOC. (13)Abraham, M. H.; Whiting, G. S.; Doherty, R.M.; Shuely, W. J. J. Chromatogr. 1991,587,213-228. (14)Abraham, M. H.; Grellier, P. L.; McGill, R. A. J. Chem. SOC., Perkin Trans. 2 1987,797-803. (15)Abraham, M. H.; Whiting, G. S.; Doherty, R.M.; Shuely, W. J. J. Chem. SOC..Perkin Trans. 2 1990. 1451-1460. (16)Abraham, M. H.; Whiting, G.'S.; Dohherty, R. M.; Shuely, W. J.; Sakellariou, P. Polymer 1992,33,2162-2167. (17)Abraham, M. H.; Whiting, G. S.; Doherty, R.M.; Shuely, W. J. J. Chromatogr. 1991,587,229-236. (18)Abraham, M.H.; Whiting, G. S.; Andonian-Haftvan, J.; Steed, J. W.; Grate, J. W. J . Chromatogr. 1991,588,361-364. (19)Abraham, M. H.; Whiting, G. S.; Doherty, R. M.; Shuely, W. J. J. Chromatogr. 1990,518,329-348. (20)Abraham, M. H.; Walsh, D. P. J. Chromatogr. 1992,627,294299. (21)Abraham, M. H.; Weathersby, P. K. J. Pharm. Sci., in press. (22)Grate, J.W.; Snow, A.; Ballantine, D. S.;Wohltjen, H.;Abraham, M. H.; McGill, R. A.; Sasson, P. Anal. Chem. 1988,60,869-875. (23)Grate, J. W.; Klusty, M.; McGill, R. A.; Abraham, M. H.; Whiting, G.; Andonian-Haftvan, J.Anal. Chem. 1992,64,610-624. (24)Abraham, M. H.; Hamerton, I.; J. B. Rose; Grate, J. W. J . Chem. SOC..Perkin Trans. 2 1991. 1417-1423. ~~~(25) Grate, J. W.; McGill, R.A.; Abraham, M. H.Proc. IEEE Ultrason. Symp. 1992,275-279. (26)Abraham, M. H.; Andonian-Haftvan, J.; Du, C. M.; Diart, V.; Whiting, G.; Grate, J. W.; McGill, R. A. J. Chem. SOC., Perkin Trans. 2 1996,369-378. (27)Grate, J. W.; Abraham, M. H.; McGill, R. A. In Polymer Films a n d Sensor Applications; Harsanyi, G., Ed.; Technomic Publishing Co.: Lancaster, PA, 1995;pp 136-149. I
~~~
n=22
e=0.951
sd=0.12
(3)
F=40
correlation coefficient, sd is the standard deviation, and
F is the Fisher F-statistic. The experimental logK, values used to derive eq 3 are given in Table 1. This equation has a number of useful features, including the fact that the coefficients characterize the properties of the fullerene as a sorbent and indicate the types of vapors that will be sorbed; the terms in the equation can be used to evaluate the relative contributions of various interactions to the sorption process; and partition coefficients can be calculated for vapors whose experimental values have not been determined. In this paper we describe the full experimental details and results for the sorption of organic vapors by fullerene and report new results on the sorbent properties of graphite for comparison. In addition, we derive a relationship between SAW vapor sensor responses and K, values and compare the fullerene-coated SAW vapor sensor responses reported by Li and Swanson with K, values for fullerene calculated using eq 3. Comparisons with linear organic and inorganic polymers are also made, ~
(28)Grate, J. W.; Abraham, M. H.; R. A. McGill In Handbook of Biosensors: Medicine, Food, a n d the Environment; Kress-Rogers, E., Nicklin, S. Eds.; CRC Press: Boca Raton, FL, in press. (29)McGill, R. A,; Abraham, M. H.; Grate, J. W. CHEMTECH 1994, 24 (9),27-37.
Vapor Sorption by Fullerene
Langmuir, Vol. 11, No. 6, 1995 2127
and the merits of the assembled fullerene multilayer as a SAW vapor sensor coating are discussed.
of the mass of sorbed vapor to the mass of sorbent in eq 5 is identical to C , as defined in connection with eq 1.
Materials and Methods The method used to measure the gas-solid partition coefficient, K,, was elution by characteristic point at infinite d i l ~ t i o n .The ~ instrument used was a Perkin Elmer F11 gas chromatograph with a flame ionization detector. A column temperature of 298 K was maintained by immersing the column in a thermostatted water bath. The carrier gas flow rate was measured using a soap bubble meter. The inlet carrier gas pressure was measured with a mercury U-tube, and the outlet gas pressure was taken as atmospheric, as measured on a mercury barometer in the laboratory. Corrections were made for the pressure drop across the column, the vapor pressure of water in the soap bubble flow meter, and any differencein temperature between the flow meter and the column. Graphite flakes (Aldrich)were sieved between 20 and 30 mesh and packed into a glass column of 3 mm diameter and 5 cm length. The packed column was conditioned at 420 K for 24 h under a stream of dry helium, and this conditioning was repeated from time to time. The probe compounds were mostly injected as vapors, using a gas-tight syringe, but a few were injected as neat liquids. The fullerene sample (83.9% C60 and 16.1% C70) from Polygon Enterprises, Waco, TX, was similarly sieved at 20-30 mesh and packed into a column 2 mm diameter and 9.3 cm long. The column was conditioned as for the graphite column; other details are the same as for the graphite column. The detector output was monitored by a chart recorder set to the highest sensitivity, and data were collected via an on-linepersonal computer using the software Unkelscope and an analog- digital converter. An in-house program30 was used to process the adsorption data, and from the chromatographic peak the partition coefficient was calculated via a series of areas Ah, corresponding to pen deflections h , exactly as described before.31 Values of CJC, were then calculated accordingto CJC, =AhFdh WQ, where FG is the fully corrected gas flow rate at the column temperature, W is the weight of adsorbent in the column, and Q is the chart recorder speed.
Results SAW Vapor Sensor Responses and&. The surface waves generated by SAW devices are exquisitely sensitive to changes in the physical properties of films on the device s ~ r f a c e . ~ A, ~n increase , ~ ! ~ ~ in mass, for example, decreases the wave velocity, and this can be detected as a shift in the frequency of a n oscillator circuit containing the SAW device a s the resonant element. Depending on the film material used, frequency shifts can also be observed in response to changes in polymer viscoelastic properties or changes in the sheet conductivity of a semiconducting material. For the case of mass-loading, the frequency shift, Afs, due to the deposition of the film material can be expressed according to eq 4.4In this equation, F is the fundamental
Afs = ( k ,
+ k2)F2mJA
(4)
resonant frequency of the oscillator, the constants kl and kz are material constants for the piezoelectric substrate, and m$A is the film mass per unit area. If vapor is sorbed by a coating on a SAW device, the frequency shift Af, due to the mass per unit area of vapor sorbed, m,lA, can be expressed according to eq 5.22,33 The relationship of K, to
Af, is readily obtained from eqs 1 and 5, since the ratio (30) Walsh, D. P. Ph.D. Thesis, University of London, 1993. (31)Abraham, M. H.; Buist, G. J.; Grellier, P. L.; McGill, R. A.; Doherty, R. M.; Kamlet, M. H.; TaR, R. W.; Maroldo, S.G. J.Chromatogr. 1987,409,15-27. (32) Frye, G. C.; Martin, S. J. Appl. Spectrosc. Rev. 1991,26,73149. (33)King, W. H. Anal. Chem. 1964,36, 1735-1739.
In the report by Li and Swanson, SAW vapor sensor responses were expressed in terms of Af, divided by the saturated vapor pressure, Plat,of the test vapor.3 Psatand C, (as defined in connection with eq 1)are related by eq 7, assuming the ideal gas law. MWis the molecular weight,
C, = P,,,MW/RT
(7)
R is the gas constant, and T is absolute temperature. Dividing both sides of eq 6 by Peat, and substituting for C , according to eq 7, yields eq 8. This equation shows that
vapor sensor responses expressed according to AfvlPsat should be proportional to K,MW. The factors Afs, R , and Tare constant in measuring the responses of a given coated SAW sensor to a series of vapors. Equations 6 and 8 are derived for the case where a vapor is adsorbed on the surface of a solid. When vapors are absorbed into the bulk of a rubbery polymer the gas1 polymer partition coefficient, K, can be defined according to eq 9. K is the ratio of the concentration of the vapor
K=CdC,
C,-0
(9)
absorbed by the polymer, C,, in grams of vapor per liter of absorbent, divided by the concentration of the vapor in the gas phase, C,, in grams per liter. The mass-loading response of a polymer-coated SAW vapor sensor where absorption is the dominant sorption process is then given by eqs 10 and 11.22 Equation 11is similar to eq 8 with the
Af, = Af,C&/d AfJPsat = Af,MWKJdRT
(10) (11)
additional factor of the density of the sorbent polymer material, d . This factor arises simply from the difference in units ofK, and K, (g/g)/(g/L)and (g/L)/(g/L),respectively. Responses from a polymer-coated vapor sensor expressed according to AfJPsatare therefore proportional to KMWI d. Comparison of Predicted and Experimental Relative Vapor Sensitivities. In order to compare relative vapor sensitivities of the fullerene-coated SAW sensor, Li .~ and Swansonplottedvalues of hfvlPsatfor l l ~ a p o r sThese values were derived from measurements of SAW sensor frequency shifis in response to vapors a t their saturated concentration a t room temperature. We have replotted these data in Figure l a . The black bars show the data for all the vapors on the same vertical scale, while the gray bars replot the data for all vapors except decalin. We have also calculated K, values at 298 K for the same set of vapors on solid fullerene using eq 3 and the solvation parameters in Table 1. K,MW values for these vapors are plotted in Figure lb. Just by chance, experimental K, values on fullerene had been determined for only one of the 11 vapors plotted by Li and Swanson (tetrachloroethene). Therefore, the predictions in Figure l b are derived from a n equation that included only one of these vapors in the training set.8 It can be seen that eq 3 is successful in estimating the relative sensitivity trends among the various vapors. Decalin is correctly predicted to be by far the most strongly sorbed, with tetrachloroethene and toluene being next most strongly sorbed, in that order. The most notable
Grate et al.
2128 Langmuir, Vol. 11, No. 6, 1995
3000
ASSEMBLED FULLERENE ON SAW SENSOR
2500
5 2000 1I5Oo n
1000
1 :.:.:.:.:
..... ..... ..... .....
500 0
6
5 0
0
Figure 1. (a) Relative vapor sensitivities, Afv/Psat(kHdatm), of a fullerene-coated SAW vapor sensor as reported by Li and calculated for solid fullerene using LSER eq 3. (c) Solubilities S ~ a n s o n(b) . ~ Relative vapor sorption,K,MW (((g/g)/(g/L))*(g/mol)), of fullerene in organic solvents. Data are from Ruoff et al.36(d) Relative vapor sorption, K,MW, for graphite using experimental K, values from Table 1.In all graphs, black bars refer to the vertical scale on the left. Gray bars show the data for all vapors excluding decalin, expanded to fill the plot. Table 2. Interaction Terms for the Sorption of Vapors by Fullerene vapor
decalins
tetrachloroethene toluene tetrachloromethane trichloromethane benzene tetrahydrofuran dichloromethane n-hexane propanone
methanol
rRz -0.12 -0.15 -0.14 -0.11 -0.10 -0.14 -0.07 -0.09 0.00 -0.04 -0.07
SJ$
ax:
110gL16
0.18
0.00 0.00 0.00 0.00
2.42 1.71 1.59 1.35 1.18 1.33 1.26 0.96 1.27 0.81 0.46
0.32 0.37 0.27 0.35 0.37 0.37 0.41 0.00 0.50 0.32
0.16
0.00 0.00 0.10 0.00 0.04 0.45
difference between the predictions and the SAW sensor results is the relative sorption of methanol, which is predicted to be less strongly sorbed than was observed in the SAW measurements. However, this difference is actually rational because the fullerene layer on the SAW device contained amino functionalities essential to the assembly process employed. (The number of amino groups per fullerene molecule is estimated to be 6.3)These amino groups should confer basic properties to the assembled thin film that are not present in the parent material. Consequently, methanol, the most hydrogen-bond acidic vapor in this set of vapors, should be more strongly sorbed by the assembled fullerene layer than by underivatized fullerene. Factors Governing Sorption by Fullerene. Using eq 3 it is possible to examine in more detail the interactions that govern the sorption ofvapors by fullerene. Interaction terms in this equation have been calcluated for each of the test vapors and the results are given in Table 2. Dispersion interactions are shown to be the predominant factor influencing the sorption of vapors by fullerene. Indeed for the most strongly sorbed vapors, decalin, tetrachloroethylene, and toluene, the 1 logL16is by far the
largest term. The values of this term range from 0.47 to 2.47 over the set of 11 test vapors. The dipolarityl 2 ~makes a contribution for polarizability term s 7 ~ also most vapors, but it is not dominating. Moreover, since the value of this term is between 0.27 and 0.41 for most of the vapors, it is not a particularly discriminating factor. In general, excepting cases where particularly strong hydrogen-bonding or dipolar interactions are set up, dispersion interactions are the main driving force for sorption of monomeric organic vapors from the gas phase to a condensed phase, and the fullerene results are consistent with this reality. The role of dispersion interactions in the sorption of vapors by fullerene has also been emphasized in recent studies by Golovnya et al., using fullerene as the stationary phase in a capillary gas chromatography column.34 These authors found that fullerene was comparable to nonpolar liquid phases such as squalane and Apiezon L in the retention of n-alkanes. It was suggested by Li and Swanson that the relative vapor sensitivities of their fullerene-coated SAW sensor were expected on the basis of the solubility of fullerene in various organic solvent^.^ We have plotted the solubilities of fullerene in organic solvents in Figure IC,using data reported by Ruoff et al.35 The substantial differences between this plot and those in Figure 1, a and b, show that this approach is not a consistently reliable predictor of relative vapor sensitivities. Using these published solubilities, for example, toluene is predicted to be more strongly sorbed than tetrachloroethene, and benzene is predicted to be more strongly sorbed than tetrachloroethene, carbon tetrachloride, and chloroform. The increase in dispersion interactions on transfer of a mono(34) Golovna, R. V.; Terenina, M. B.; Ruchkina, E. L.; Karnatsevich, V. L. Mendeleev Commun. 1993,231-233. (35) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993,97,3379-3383.
Vapor Sorption by Fullerene
Langmuir, Vol. 11, No. 6, 1995 2129
Figure 2. Relative vapor sorption,KMW/d, estimated for (a) poly(isobuty1ene)using LSER eq 14, (b) poly(epich1orohydrin)using LSER eq 15, (c) OV25 using LSER equation 16, and (d) OV202 using LSER eq 17. In all graphs, black bars refer to the vertical scale on the left. Gray bars show the data for all vapors excluding decalin, expanded to fill the plot.
meric vapor molecule from the gas phase to a sorbent phase is a key driving force for sorption, and this factor is not modeled by the dissolution of a solid sorbent such as fullerene into a bulk liquid solvent. Comparisons with Graphite. We were curious to see if the sorbent properties of fullerene were in any way unusual compared to graphite and other low-polarity organic materials. Experimental log K, values for the sorption of organic vapors by graphite a t 298 K are given in Table 1. For a set of 22 vapors identical to those used to characterize fullerene,8we obtained eq 12 for graphite. log& = -1.55 - 0.26R2
~ ~ 0 . 9 7 1s d = 0 . 1 2
F=71
n=36
Q=0.970
sd=0.15
F = 124
e = 0.996
log K = -0.75
(13)
The coefficients in these equations for graphite are essentially the same as those for fullerene in eq 3, indicating that the two primary components of soot, fullerene and graphite, are not substantially different from one another in their properties as sorbents. Using experimental values of K,, we have calculated K,Mw for the test vapors considered in Figure la,b, and these results are plotted in Figure Id. Comparisons with Figure 1,a and b, confirm that graphite is very similar to fullerene. Interestingly, the idea of using soot extracts as the sorbent layer on a n acoustic wave chemical sensor was reported as long ago as 1978 by Guilbault and c o - w o r k e r ~ . ~ ~
sd = 0.07
n = 50
e = 0.996
log K = -0.85
n=40
(14)
sd = 0.07
+ 0.83 log L16
F = 1209 (15)
+ 0.18R2 + 1.29$ + 0.56Zay + 0.44Cpy + 0.88 log
e=0.993
l o g K = -0.39 - 0.48R2
n=50
F = 942
+ 0.10R2 + 1 . 6 3 4 + 1.45Caf + 0.71Cpf
+ 0 . 8 6 4 + 0.94Caf + 0.46 log
+ 0.18Ca; + 1.02 log L16
(12)
For a larger set of vapors including the 22 in the previous equation, the 11 vapors considered in the SAW sensor work, and a few others, we obtained eq 13. log K, = -0.86 - 0.27R2
+
l o g K = -0.77 - 0.08R2 0 . 3 7 e
n = 36
+ 0 . 9 9 4 + 1.11Caf + 0.59 log L16
n=22
Comparison with Linear Polymers. We have characterized 14 organic and inorganic linear polymers by the LSER method through the measurement ofKvalues by gas-liquid chromatography.26 We selected four of these for comparison with fullerene and graphite. The LSER equations for poly(isobutylene), poly(epichlorohydrin), OV25, and OV202 are given in eqs 14-17, respectively.
Q=O.997
sd=0.10
F=495
(16)
+ 1 . 3 0 4 + 0.44CG + 0.71Cp: + 0.81 log L16 sd=0.07
F = 1319 (17)
These are all low-polarity polymers. OV25 and OV202 are gas chromatographic stationary phases consisting of siloxanes with phenyl (OV25)and trifluoropropyl groups (OV202) in addition to methyl groups. K values for the test vapors considered above were calculated using these equations and the parameters in Table 1. Plots of KMwId are shown in Figure 2a-d for comparison with the previous figures, in accord with the
2130 Langmuir, Vol. 11, No. 6, 1995 proportionality in eq 11. For all four polymers the sorption process is dominated by dispersion interactions, and decalin is therefore the most strongly sorbed, followed by tetrachloroethene and toluene. The overall appearances of these plots are similar to those shown for fullerene, graphite, and the SAW vapor sensor coated with the assembled fullerene layer. These comparisons are related to relative vapor sensitivities, or selectivity. The absolute sensitivities can be compared using published SAW vapor sensor measurements. Toluene vapor is common to a number of studies. The fullerene-coated SAW vapor sensor responded to toluene vapor saturated a t room temperature with frequency shifts (Afv) of 6000-8000 H z . ~The Afs value for this assembled fullerene film, indicating the amount of sorbent materials on the sensor's surface, was 233 kHz. Saturated toluene vapor a t 298 K has a vapor pressure of 27 Torr yielding a concentration of 135 000 mg/m3,37 Working with diluted vapor streams, we reported that a poly(isobuty1ene)-coated SAW vapor sensor with a 280 kHz film gave a response of 17 770 Hz to toluene vapor a t 21 200 mg/m3.23 Similarly a poly(epich1orohydrin)coated SAW vapor sensor with a 254 kHz film gave a response of 13 670 Hz to toluene a t the same concentrationSz3Patrash and Zellers have also examined responses of a poly(isobuty1ene)-coatedSAW vapor sensor to toluene and observed responses of 1680 Hz at 3110 mg/m3toluene vapor concentration with 199 kHz film.38,39Although comparing the response of a sensor to a vapor a t saturated concentration with the responses of other sensors tested a t diluted vapor concentrations is not optimal because of the possibility of nonlinear sorption isotherms, especially a t high vapor concentrations, this is the best that can be done in this case because no fullerene-coated sensor responses were reported a t any vapor concentrations less than saturated. Nevertheless, the results reported a t 21 200 mg/m3toluene demonstrate that the SAW sensors coated with linear organic polymers gave responses 2-3 times greater than fullerene-coated SAW sensor exposed to a vapor concentration that was over six times higher.
Discussion Our results show that the relative vapor sensitivities ofthe fullerene-coated SAW sensor can be estimated using the LSER equation determined for fullerene by inverse gas chromatographic measurements. Such predictions (36) Webber, L. M.; Karmarker, K. H.; Guilbault, G. G.AnaZ. Chim. Acta 1978, 97, 29. (37) Schlessinger, G. G. In Handbook of Chemistry and Physics, 54th ed.; Weast, R. C., Ed.; CRC Press: Cleveland, OH, 1973; pp D162nlR2 (38) Patrash, S. J. Ph.D. Thesis, University of Michigan, 1994. (39) Patrash, S. J.;Zellers, E. T. Anal. Chem. 1993,65,2055-2066.
Grate et al. could be made for literally hundreds of vapors using published solvation parameters and eq 3.12 In addition, these studies illustrate the value of the LSER approach in facilitating comparisons of one material to another. This can be done by comparing the coefficients in the LSER equations (eqs 3 and 12-17) or through predictions of vapor sorption for common sets of vapors (Figures 1and 2). This approach transcends the difficulties that arise in comparing materials or sensors characterized in different laboratories using different sets of vapors. Thus, we have been able to compare fullerene with graphite, linear organic polymers, linear inorganic polymers, and a n assembled fullerene thin film material. With regard to vapor sensing our comparisons show that the assembled fullerene layer is similar in selectivity to other nonpolar organic sorbents such as graphite and low-polarity polymers, and thus could serve as a sensor for nonpolar vapors in a sensor array. Compared to graphite, fullerene has the advantage that it can be assembled into a thin film starting with molecular precursers. Compared to the linear polymers, however, the assembled fullerene layer yields less sensitive SAW vapor sensors. The assembled fullerene layer is noteworthy in that the film is immobilized by being networked with siloxane bridges and covalently bound to the surface. This could produce a more durable film than simple linear polymers bound to the surface only by physisorption. At the same time, the assembled fullerene layer illustrates one of the challenges faced when designing sensing layers that can be covalently immobilized: the chemistry used may introduce properties other than those present in the parent material and thus decrease the selectivity of the material. The assembly of the fullerene layer introduced amine functionalities that afforded greater sensitivity to hydrogen-bond acidic vapors than fullerene alone, thus decreasing the overall selectivity for nonpolar vapors such as chlorinated and petroleum hydrocarbons.
Acknowledgment. We are grateful to the U.S.Army for support under contract DAJA45-93-C-0100. We thank Dr. David Venezky of the Naval Research Laboratory for the funds to purchase the fullerene, and Dr. John H. Callahan, who kindly obtained a mass spectrum of the fullerene sample. Data analysis a t PNL was funded in part by the Office of Technology Development, within the Department of Energy's Office of Environmental Management, under the Characterization, Monitoring and Sensor Technology Cross-cutting Program. The Pacific Northwest Laboratory is a multiprogram national laboratory operated for the U S . Department of Energy by Battelle Memorial Institute under Contract DE-ACO676RLO 1830. LA9408580
