Quantitative Characterization of Hydrophilic− Hydrophobic Properties

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Quantitative Characterization of Hydrophilic-Hydrophobic Properties of MWNTs Surfaces Tı´mea Kanyo´,† Zolta´n Ko´nya,† A Ä kos Kukovecz,† Ferenc Berger,‡ ‡,§ Imre De´ka´ny, and Imre Kiricsi*,† Applied and Environmental Chemistry Department, University of Szeged, H-6720 Szeged, Rerrich Bela ter 1, Hungary; Colloid Chemistry Department, University of Szeged, H-6720 Szeged, Aradi Ve´ rtanuk tere 1, Hungary; and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, H-6720 Szeged, Aradi Ve´ rtanuk tere 1, Hungary Received September 12, 2003. In Final Form: December 17, 2003 We report the first direct measurement of surface hydrophilicity-hydrophobicity in multiwall carbon nanotubes (MWNTs) by analyzing the binary solvent adsorption excess isotherms of freshly oxidized as well as thermally annealed MWNT samples. Heat-induced changes in the sample morphology are monitored by XRD and N2 adsorption isotherms, the hydrophilic-hydrophobic surface ratio is calculated from ethanolcyclohexane adsorption excess isotherms. The surface fractal dimension and the dimension of the capillary condensation are also determined. Results obtained from these independent methods correlate well with each other. The character of the MWNT surface gradually turns from hydrophilic (no treatment) into hydrophobic (12 h in N2 at 1673 K) upon heating. The suggested method appears to be a feasible, reliable, fast, and economic technique of characterizing MWNT surface properties.

Introduction Surface chemistry of carbon nanotubes (discovered by Iijima in 19911) has been investigated by several laboratories. Studies focus most frequently on the ends and on the outer shell of the nanotubes. When ends of the nanotubes are opened, which is a general consequence of purification of the as-produced nanotubes, the interior of the tube becomes ready to accommodate guest materials such as single molecules,2 quasi-one-dimensional metal-3 and nonmetal crystals4 including complexes. The carbon shell (for the multiwall carbon nanotubes (MWNT) each of the carbon shells) is closed by various functional groups, most frequently by -COOH, -OH, and dCO groups. For MWNTs, the outer shell may contain, and very often contains discontinuous spots, imperfections. These local vacancies are also closed by functional groups of types mentioned above. For large-scale production of high quality carbon nanotubes, for nanoelectronic purposes the concentration of these imperfections should be minimized. However, for those applications in which the nanotubes are used to strengthen polymers the presence of functional groups is advantageous to ensure chemical bond between polymer and filler or enhances the solubility of carbon nanotubes in various solvent.5 Several methods have been reported to produce functional groups such as -F, -NH2, * Address correspondence to this author. Fax: +36-62-544619; e-mail: [email protected]. † Applied and Environmental Chemistry Department, University of Szeged. ‡ Colloid Chemistry Department, University of Szeged. § Nanostructured Materials Research Group of the Hungarian Academy of Sciences. (1) Iijima, S. Nature 1991, 354, 56. (2) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323. (3) Grobert, N.; Hsu, W. K.; Zhu, Y. Q.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M.; Terrones, M.; Terrones, H.; Redlich, P.; Ruhle, M.; Escudero, R.; Morales, F. Appl. Phys. Lett. 1999, 75, 3363-3365. (4) Koga, K.; Gao, G. T.; Tanaka, H.; Zeng, X. C. Nature 2001, 412, 802. (5) Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2002, 124, 89408948.

-COCl, -SH, and so forth on both singlewall (SWNT) and multiwall nanotubes.6,7 Methods for investigating individual nanotubes and nanotube samples as bulk matter involve TEM, Raman spectroscopy, XRD, BET, and thermal analysis (TG-DTA). These techniques are suitable to characterize the shape, geometry, and graphitization (HRTEM) of the individual nanotubes and bundles of SWNTs. The diameter of SWNTs, the graphitization of nanotube shells, the adsorption characteristics such as surface area, pore volume, pore diameter distribution, and the thermal behavior of the nanotube samples can be characterized as well. Investigation of the functional groups generated at the end and on the outer shell of tubes is a rather complex problem. Though IR spectroscopy is a suitable tool for monitoring functional groups in molecules, it can hardly be used for carbon nanotubes. Better results were reported from XPS8 and Raman spectroscopic9 measurements. These technologies require highly sophisticated instruments not available in every laboratory. The titration method generally used for determination of acidity of various carbon samples can be used in carbon nanotubes;10 however, its reproducibility in the low concentration range characteristic of carbon nanotubes is not satisfactory. In this contribution, we report on a simple method to characterize the hydrophilic-hydrophobic properties of carbon nanotubes. The binary solvent adsorption measurements11 can be performed easily and characterize the nanotube samples as bulk material resulting in quantita(6) Ko´nya, Z.; Vesselenyi, I.; Niesz, K.; Kukovecz, A.; Demortier, A.; Fonseca, A.; Delhalle, J.; Mekhalif, Z.; B. Nagy, J.; Koo´s, A. A.; Osva´th, Z.; Kocsonya, A.; Biro´, L. P.; Kiricsi, I. Chem. Phys. Lett. 2002, 360, 429-435. (7) Niesz, K.; Siska, A.; Vesselenyi, I.; Hernadi, K.; Mehn, D.; Galbacs, G.; Ko´nya, Z.; Kiricsi, I. Catal. Today 2002, 76, 3-10. (8) Ko´nya, Z.; Kiss, J.; Oszko´, A.; Siska, A.; Kiricsi, I. Phys. Chem. Chem. Phys. 2001, 3, 155. (9) Dresselhaus, M. S.; Eklund, P. C. Adv. Phys. 2000, 49, 705. (10) Hu, H.; Bhowmik, P.; Zhao, B.; Hamon, M. A.; Itkis, M. E.; Haddon, R. C. Chem. Phys. Lett. 2001, 3, 45, 25. (11) Dekany, I.; Szanto, F.; Nagy, L. G.; Foti, G. J. Colloid Interface Sci. 1975, 50, 265. Dekany, I.; Szanto, F.; Nagy, L. G. J. Colloid Polym. Sci. 1978, 256, 150.

10.1021/la035702r CCC: $27.50 © 2004 American Chemical Society Published on Web 01/27/2004

Hydrophilic-Hydrophobic Properties

tive values suitable to comparison of samples treated differently. The binary solvent adsorption excess isotherms determined can be analyzed by the procedure developed by Schay.12 It is possible to evaluate the hydrophilichydrophobic properties of solid materials from vapor adsorption experiments as well. Available experimental techniques are (i) the gravimetric method utilizing a microbalance,13 (ii) the GC-linked differential sorption bed system,14 the Harlick-Tezel concentration pulse method,15 and (iv) the static vapor adsorption system with offline GC sampling. A common drawback of these methods compared to the solvent excess isotherm method utilized in the present work is that each of them establishes the adsorption equilibrium in some rather complex equipment which is not available in multiple copies even in the best equipped laboratories. Therefore, the mentioned vapor adsorption methods are capable of measuring binary adsorption isotherms with the speed of approximately 1-2 isotherm points per day. On the other hand, the adsorption equilibrium in our solvent adsorption isotherm measurements is established in inexpensive glass test tubes at ambient conditions. Since these are generally available in large quantities everywhere, this method is at least 1 order of magnitude faster than vapor adsorption experiments. In fact, the time required to measure a full solvent excess isotherm with arbitrary good resolution is equal to the time necessary to establish one equilibrium point of this isotherm, provided that enough test tubes, solvents, and sample material are available. While in some cases analysis speed is not a key factor, in the rapidly expanding field of carbon nanotube science, being able to get results within days instead of months can make a large difference. In this paper, we discuss the quantitative hydrophilichydrophobic as well as the morphologic properties of MWNTs treated at increasing temperatures from room temperature to 1673 K. We find that the hydrophobic character of the nanotubes increases with the temperature and that the results obtained from solvent excess isotherms correlate well with the XRD patterns, specific surface area, and surface fractal characteristics of the materials. Experimental Section Materials. All chemicals were obtained from Sigma-Aldrich and used as received. Sample Preparation, Pretreatments. MWNT sample was synthesized using the CCVD procedure described elsewhere.16 Briefly, 2.5-2.5% of Co, Fe containing alumina supported catalyst was prepared by impregnation of alumina with Co- and Fe-acetate and placed in a quartz boat in thin layer.17 The degassing of catalysts was performed at 1000 K in flow nitrogen atmosphere. After 30 min, the gas stream was changed for a mixture of acetylene-nitrogen and the catalytic chemical vapor deposition was performed for 1 h. After the reaction, the system was cooled under nitrogen flow and the sample was purified. The purification was as follows. To remove the alumina support, the product was boiled in concentrated NaOH solution. After washing the sample neutral, the next step was a treatment in concentrated (12) Schay, G. Surface area determination; Proceedings of the International Symposium on surface Area Determination; Everett, D. H., Ottewill, R. H., Eds.; Bristol, U.K., 1969; Butterworth: London, 1970; p 123. (13) Buss, E. Gas. Sep. Purif. 1995, 9, 189. (14) Huang, Y.-H.; Liapis, A. I.; Xu, Y.; Crosser, O. K.; Johnson, J. W. Sep. Techn. 1995, 5, 1. (15) Harlick, P. J. E.; Tezel, F. H. Adsorption 2000, 6, 293. (16) Kukovecz, A.; Ko´nya, Z.; Nagaraju, N.; Willems, I.; Tamasi, A.; Fonseca, A.; B. Nagy, J.; Kiricsi, I. Phys. Chem. Chem. Phys. 2000, 2, 3071. (17) Ko´nya, Z. Carbon Filaments and Nanotubes: Common Origins, Differing Applications?; Biro, L. P., Ed; NATO Sci. Ser., Ser. E 2001, 372 p 85-109.

Langmuir, Vol. 20, No. 5, 2004 1657 HNO3-H2SO4 solution to remove the metal particles and the amorphous carbon matter. Finally, we dried the sample at 490 K overnight in air atmosphere. Upon these treatments, the ends of the nanotubes opened. It is also probable that the structural imperfections in the outer shells of MWNTs were closed by hydroxyl, carboxyl, or carbonyl groups. This sample is denoted as original MWNT (O-MWNT). Portions of this product were heat-treated at 670, 970, 1270, and 1670 K for 12 h to anneal the oxygen containing groups and to enhance the graphitization of the shells. We labeled these samples as treated T-MWNT-1, -2, -3, and -4, respectively. Characterization of Nanotubes. Both the parent and the treated carbon nanotubes were characterized by physicalchemical techniques. FTIR spectroscopy was utilized to monitor the functional groups and their change as the consequence of heat treatments. For this purpose, 1 mg of carbon nanotubes were homogenized in 1 g of KBr. From this master mixture, 1 mg was mixed with spectral grade KBr and a wafer was pressed. The spectrum of this wafer was recorded using a Mattson Genesis 1 FT-IR equipment. XRD features were recorded in the 15-30 2Θ range using a DRON 3 diffractometer. Peak intensity and half widths of the main reflections were compared. TEM images were taken on a Phillips CM10 type electron microscope operated at 100 kV. Methanol suspension of carbon nanotubes were prepared and dropped onto a copper grid. After drying, the sample was measured. The nitrogen adsorption isotherms were measured at 77 K with a QuantaChrome Nova 2200 surface area analyzer. Samples were outgassed at 393 K for 1 h to remove adsorbed contaminants. The specific surface area (AS) was calculated using the multipoint BET method18 on six points of the adsorption isotherm. Pore size distribution curves were calculated from the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method.19 Surface fractal dimension (DS) was calculated using the Frenkel-Halsey-Hill (FHH) method20 from adsorption data near monolayer coverage. Additionally, the Neimark-Kiselev method21 was also utilized to calculate a fractal dimension value from the capillary condensation region of the adsorption isotherm (DCC). Adsorption excess isotherm of ethanol-cyclohexane mixtures was studied in a static solvent adsorption system at 293 K and the adsorption capacity (Schay-Nagy extrapolation method12) was used for calculation of the equivalent specific surface area. For drying, the samples were placed into an oven at 423 K for 24 h. A measured amount of activated nanotube sample was placed in a vessel followed by introduction of one of the preprepared mixtures. The vessel was closed and equilibrated for 24 h while stirring manually from time to time. The change in the composition of the liquid mixture (∆x1) because of adsorption was determined by differential interferometry at 293 ( 0.1 K. The adsorption excess was calculated by the equation n1σ ) n0(∆x1/m), where n0 is the total molar amount of the liquids and m is the mass of the adsorbent.

Results and Discussion The IR spectra of the samples showed that the peak areas of T-MWNT under the bands in the OH vibration range are smaller than that of O-MWNT. In the spectrum of the former samples a band, that is, absent for O-MWNT, appeared at 1478 cm-1. Therefore, this band is assigned as structural imperfections. The spectral differences are generally very small, in some cases insignificant. The TEM images reveal well-graphitized MWNTs for each heat-treated sample (Figure 1). There was no significant change between the samples observable by the medium resolution TEM. (18) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (19) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (20) Pfeifer, P.; Wu, Y. J.; Cole, M. W.; Krim, J. Phys. Rev. Lett. 1989, 62, 1997. (21) Neimark, A. Physica A 1992, 191, 258.

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Figure 1. TEM images of the samples O-MWNT (A) and T-MWNT-1-T-MWNT-4 (B-E), respectively. The nanotubes are free of amorphous carbon and well graphitized.

Figure 2. XRD patterns graphite (a), O-MWNT (b), and samples T-MWNT-1-T-MWNT-4 (c-f).

The position of the XRD reflections obtained for MWNTs appeared at lower 2Θ anglessabout 0.5 degree lowers than that of the graphite. This can be traced back to the less ordered structure of rolled graphite sheets in the CVD produced MWNTs. Figure 2 shows full width at halfmaximum (fwhm) and intensity values calculated from the XRD spectra. The intensity of the reflections increased, while the half widths of the reflections decreased continuously with increasing heat treatment temperature. This behavior may reveal the increase of ordering of MWNTs and can be explained as follows. Upon heat treatment, the defect sites filled by functional groups recover. Carboxyl groups may decompose by releasing CO2 and hydroxyl groups may generate water molecules. The extents of these reactions increase with temperature. At higher temperatures, the graphitization of the outer shells may also occur. Nitrogen adsorption measurements resulted in Type IV isotherms with Type A hysteresis loops typical for mesoporous materials. The isotherms and the corresponding BJH pore size distribution (PSD) curves are presented in Figure 3. Because of their geometry, MWCNT

Kanyo´ et al.

samples can be assumed to contain cylindrical pores and no pore networks, thus satisfying the two main requirements of the BJH method. On the other hand, it is wellknown that in the studied d ) 2-10 nm range the BJH method underestimates the pore size by ca. 1 nm as compared to, for example, the Derjaguin-Broekhoff-de Boer method22 or the nonlocal density functional theory calculation of capillary condensation hysteresis.23,24 Nevertheless, we believe its application is justified here because (i) it does give reliable results concerning changes in the relative intensity of PSD peaks and (ii) it allows for a comparison between our PSD results and those obtained previously by the BJH method for carbon nanotubes.25 The PSD curves in the inset of Figure 3 exhibit three features: a shoulder at r ∼ 1.4 nm, a sharp peak at r ∼ 1.8 nm, and a broad peak at r ∼ 5 nm. We assign the first one to the cylindrical pores within the nanotubes and the third one to the adsorption of N2 in the channels of the intertube space. At a first glance, the second peak at r ∼ 1.8 nm could be interpreted as an indicator for a set of d ∼ 3.6 nm mesopores in the sample and indeed, this assignment has been accepted before.25 However, in a recent series of papers Groen et al. have shown (see ref 26 and references therein) that this peak is in fact an artifact arising from the tensile strength effect (TSE). The TSE occurs whenever the PSD curve is calculated with the BJH method from a N2 desorption isotherm showing a sharp drop at p/p0 ∼ 0.45. We would like to draw attention to the fact that this is very often the case when characterizing carbon nanotube samples.25,27,28 The pronounced increase of the peak corresponding to r ) 1.4 nm pores in the BJH pore size distribution curve upon heating indicates that functional groups blocking the entrance of certain MWNT pores are among the first to break loose. Therefore, the inner space of some nanotubes becomes available for N2 adsorption which is observable from the increased AS values as well. Specific BET surface area, FHH surface fractal dimension, and the dimension of the capillary condensation as calculated from the N2 adsorption isotherms are given in Table 1. The 50% increase in the BET surface area from O-MWNT to T-MWNT-1 is attributed to the combined effect of the removal of strongly physisorbed contaminants and the breaking of the least robust functional groups. The specific surface increases by an additional 20 m2/g from T-MWNT-1 to T-MWNT-2, yet there is practically no change between T-MWNT-2 and T-MWNT-3. In agreement with the XRD results, this phenomenon can be interpreted by assuming that the oxygen containing functional groups are removed from the nanotubes to a large extent at ∼1000 K rendering the MWNT surface more accessible for the N2 probe molecule. Our previous studies on single-wall carbon nanotubes confirmed that functional groups are indeed removed in this temperature range.29,30 The highest AS value was measured on the T-MWNT-4 sample, most (22) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1967, 9, 8. (23) Ravikovitch, P. I.; Neimark, A. V. Colloids Surf., A 2001, 187188, 11. (24) Ravikovitch, P. I.; Wei, D.; Chueh, W. T.; Haller, G. L.; Neimark, A. V. J. Phys. Chem. B 1997, 101, 3671. (25) Ruckenstein, E.; Hu, Y. H. Carbon 1998, 36, 269. (26) Groen, J. C.; Peffer, L. A. A.; Pe´rez-Ramı´rez, J. Microporous Mesoporous Mater. 2003, 60, 1. (27) Lenardi, C.; Barborini, E.; Briois, V.; Lucarelli, L.; Piseri, P.; Milani, P. Diamond Relat. Mater. 2001, 10, 1195. (28) Inoue, S.; Ichikuni, N.; Suzuki, T.; Uematsu, T.; Kaneko, K. J. Phys. Chem. B 1998, 102, 4689. (29) Kukovecz, A.; Kramberger, C.; Holzinger, M.; Kuzmany, H.; Schalko, J.; Mannsberger, M.; Hirsch, A. J. Phys. Chem. B 2002, 106, 6374. (30) Georgakilas, V.; Voulgaris, D.; Vazquez, E.; Prato, M.; Guldi, D. M.; Kukovecz, A.; Kuzmany, H. J. Am. Chem. Soc. 2002, 124, 14318.

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Figure 3. N2 adsorption-desorption isotherms of the samples O-MWNT and T-MWNT-1-T-MWNT-4. The inset shows the corresponding pore size distribution curves as calculated by the BJH method. The second peak in the PSD curve is an artifact caused by the so-called tensile strength effect. Table 1. Selected Morphological Characteristics of the Samples as Determined from N2 Adsorption Measurementa sample

AS (m2/g)

DS

DCC

O-MWNT T-MWNT-1 T-MWNT-2 T-MWNT-3 T-MWNT-4

137.3 195.3 215.6 217.4 245.0

2.532 2.508 2.526 2.514 2.493

3.87 3.912 3.86 3.747 3.02

a BET surface (A ), FHH surface fractal dimension (D ), and S S dimension of the capillary condensation as calculated according to Neimark from adsorption data in the region of capillary condensation (DCC).

probably because of the very high temperature of the treatment which enhanced active healing of the nanotube defect sites through graphitization. Fractal dimension analysis was performed on all N2 adsorption isotherms. In Figure 4, we demonstrate two representative fits (for the samples O-MWNT and TMWNT-4) both for the FHH equation (part A) and for the Neimark-Kiselev equation (part B). Fit parameters for the other MWNT samples are available on request. Following the suggestion of Tang et al.,31 the FHH method was applied only to the near-monolayer coverage section of the adsorption isotherm when calculating the surface fractal dimension (DS) of the samples. In agreement with the data reported by Staszczuk et al.,32 the surfaces of our MWCNT samples also show self-similar characteristics as evident from the DS values fluctuating around 2.5. While thermal treatment does not seem to affect surface fractal characteristics, the dimensionality of the capillary condensation (DCC) decreases monotonically with increasing temperature. The phenomenological interpretation of this quantity, as discussed recently by Armatas et al.,33 is that higher DCC values correspond to more uniform pore size distributions (e.g., DCC ∼ 10 for MCM materials). In our case, the dimensionality of the capillary condensation is originally low in agreement with the spread BJH PSD curve. The additional decrease of DCC with temperature (31) Tang, P.; Chew, N. Y. K.; Chan, H.-K.; Raper, J. A. Langmuir 2003, 19, 2632. (32) Staszczuk, P.; Matyjewicz, M.; Kowalska, E.; Radomska, J.; Byszewski, P.; Kozlowski, M. Proc. SPIE 2003, 5118, 245. (33) Armatas, G. S.; Salmas, C. E.; Louloudi, M.; Androutsopoulos, G. P.; Pomonis, P. J. Langmuir 2003, 19, 3128.

Figure 4. FHH plots used for calculating the surface fractal dimension (Ds) (part A) and Neimark-Kiselev plots used for determining the dimensionality of the capillary condensation (Dcc) (part B). Vads denotes the amount of N2 adsorbed [cm3/g], rC is the radius of curvature in [Å], and AVLI is the vapor-liquid interface area in [m2/g].

is thought to originate from the heat-induced disorder introduced gradually into the intertube space. Important as this observation is from the morphologic point of view, it is believed that the extent of intertube disorder does not play a governing role in determining the hydrophilichydrophobic surface characteristics of an MWNT sample.

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Kanyo´ et al. Table 2. Binary Solvent Adsorption Equilibrium Data and XRD Data of MWNT Samples Treated at Different Temperatures

sample

T [K]

O-MWNT 293 T-MWNT-1 673 T-MWNT-2 973 T-MWNT-3 1273 T-MWNT-4 1673

Figure 5. Raw adsorption excess isotherm data measured on the samples O-MWNT and T-MWNT-1-T-MWNT-4 in the ethanol-cyclohexane system. The nanotube surface turns from hydrophilic into hydrophobic as the annealing temperature is increased. (The continuous curves are guides for the eye.)

When the components in a binary liquid mixture are very different in polarity as they are in cyclohexanesethanol mixtures, the polarity of the surface can be characterized through the shape of the excess isotherms and the azeotropic composition. The binary solvent adsorption isotherms obtained for the nanotube samples show a special S shape and belong to type IV of the SchayNagy classification, of which the common feature is the linear part extending over a wide composition range. From this linear part n1s, n2s, and n1,0s can be determined by graphical extrapolation. Since the apolar compound is adsorbed on the hydrophobic parts and the polar compound is adsorbed on the hydrophilic parts, the volume fraction (Θ1) of compound 1 fixed in the adsorption layer can be described by the equation Θ1 ) n1sVm,1/(n1sVm,1 + n2sVm,2), where Vm,1 and Vm,2 denote the molar volume of the components 1 and 2, respectively. In other words, the positive range of the adsorbed excess belongs to the hydrophilic surface portion, while the negative one is due to the hydrophobic counterpart. The negative values of the n1σ(n) simply means that in this concentration range

xa1 [mmol/g]

Θ1 (%)

Θ2 (%)

0.74 0.37 0.35 0.24 0.09

62.2 25.9 23.5 15,7 5.7

37.8 74.0 76.5 83.3 94.3

height of half-width of XRD peak XRD peak [au] [degree] 68 74 80 89 105

2.79 2.62 2.35 2.18 1.92

there is a preferential adsorption of the ethanol relative to the cyclohexane by the sample. The isotherm of O-MWNT is characteristic of a material having rather hydrophilic character with an azeotropic point at x1a ) 0.74, as Figure 5 shows. The adsorption excess of ethanol (n1σ(n)) is in the positive range and only a small hydrophobic part is seen at high ethanol concentrations, near to 1 mole fraction of ethanol. In other words, this sample adsorbs preferentially ethanol from almost any ethanol-cyclohexane mixture. This behavior shows that the assynthesized and purified sample contains hydrophilic functional groups, as this sample was not subjected to any heat treatment, but dried at 390 K. The adsorption excess isotherms of heat-treated MWNTs show both hydrophilic and hydrophobic surface portions. With increasing temperature, the hydrophobic part increases at the expense of hydrophilic. This is reflected in the enlargement of the negative parts of the isotherms. On the sample treated at 1670 K, only a very small hydrophilic surface portion was found. The explanation of the growing hydrophobic portion at higher temperatures is that with increasing temperature the concentration of functional groups decreases because of decomposition of the functional groups making the surface hydrophilic. Thus, the hydrophilic character of the MWNT surface decreases as well. The quantitative parameters calculated from the adsorption measurements and from the XRD spectra are summarized in Table 2. The temperature dependence of transformations leading to the change of hydrophilichydrophobic properties of MWNTs treated at different temperatures is depicted in Figure 6. The linear correlation between the temperatures and the hydrophilic-hydrophobic surface portions (Θ1, Θ2, and x1a) suggests that

Figure 6. Binary solvent adsorption equilibrium parameters determined from the adsorption measurements as a function of annealing temperature. (x1a is the azeotropic composition, Θ1 and Θ2 are the volume fractions of hydrophilic and hydrophobic surface portions, respectively.)

Hydrophilic-Hydrophobic Properties

chemical transformations are behind this behavior. Among these transformations, the decomposition of carboxyl and elimination of hydroxyl groups are the most probable. This hypothesis is further supported by the fact that the surface fractal dimension (DS) of the samples is within error bars unaffected by the thermal treatment, indicating that the change in the hydrophilic-hydrophobic behavior does not originate from a morphological, but rather, from a chemical cause. Conclusions The procedure elaborated and demonstrated in this paper is suitable to characterize quantitatively the hydrophilic and hydrophobic properties of carbon nanotubes. These features of carbon nanotubes are importat when

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we want to utilize carbon nanotubes as polymer fillers or want to solubilize the tubes for any kind of application. The measurement of binary solvent adsorption isotherm does not require expensive equipment; the procedure is simple, fast, and easily applicable. Experiments are in progress to extend this method to the quantitative characterization of efficiency of -NH2, -COCl, -SH, and so forth functionalization. Acknowledgment. The authors thank the financial support of the Hungarian Ministry of Education (OTKA T037952, F038249, NKFP 1W035, and FKFP 216/2001). Z.K. and A.K. acknowledge the support of the Magyary Zoltan postdoctoral fellowship. LA035702R