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Interfacial behaviour of brominated fullerene (C60Br24) and stearic acid mixed Langmuir films at air–water interface
Chemical Physics Letters 433 (2007) 317–322 www.elsevier.com/locate/cplett
Interfacial behaviour of brominated fullerene (C60Br24) and stearic acid mixed Langmuir films at air–water interface S. Shankara Gayathri, Archita Patnaik
*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India Received 15 October 2006; in final form 9 November 2006 Available online 17 November 2006
Abstract The interfacial properties of the spread Langmuir films of C60Br24 and stearic acid have been studied at air–water interface by measuring their surface pressure–area isotherms. Monolayer studies on C60Br24 suggest a strong aggregation at air–water interface which corroborated the ab initio electron density calculations. The mixed films were found to be immiscible and non-ideal with an exception of X C60 Br24 ¼ 0:5 which is the composition of greater stability. The interaction between C60Br24 and stearic acid results in the lowering of surface pressure withstood by stearic acid and hence the decrease in compressibility with the increasing X C60 Br24 , thereby indicating the domination of C60Br24 characteristics in the mixed monolayers. 2006 Elsevier B.V. All rights reserved.
1. Introduction A feature of contemporary science is the heightened interest in materials with complex molecular structure. Ordered molecular films, with thickness ranging from a few nanometers in the case of a monolayer to several hundred nanometers, show considerable technological promise. The study of molecular interactions in thin-film structures is a necessary step for a better understanding of the collective properties of the ordered arrays of conducting or semi-conducting materials incorporated in these useful devices [1]. The Langmuir trough technique provides a method of constructing simple artificial systems of molecules on a substrate. The particular physicochemical properties of fullerenes, essentially due to their singular geometry and electronic structure have become the focus of considerable interest in monomeric or aggregated forms. Thin films of C60 and its derivatives evidence attractive characteristics, like superconductivity [2–4], pharmacological properties [5,6], charge
*
Corresponding author. E-mail address: [email protected] (A. Patnaik).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.055
transfer behaviour [7–9], and non-linear optic behaviour [10–12]. An essential requirement for the systematic investigation of these properties is the incorporation of fullerenes in organized 2D arrays and 3D networks [13]. Numerous efforts are made to elaborate the well ordered monomolecular films of C60 and its derivatives since their electronic properties can be better controlled in a 2D arrangement [14–17]. Although from measurements of pressure vs. molecular area (p–A) isotherms, Obeng and Bard reported for the first time the formation of stable monolayer of fullerene (C60) at air–water interface [14], many studies have revealed that, under the most common experimental conditions, C60 does not form truly stable monolayers [15–17]. A high cohesive energy of more than 30 kcal/mol reflects the strong intermolecular attractive p–p interactions, and in turn, C60 forms 3D aggregates at air-water interface. These aggregates were experimentally confirmed by UV–Vis spectroscopy with a band centered at 450 nm [18]. A recent Langevin dynamics simulation provided a clear explanation for the above [19]. The registered limiting areas per molecule were significantly smaller ˚ 2/molecule, calcuthan the theoretical value of about 98 A lated for an ideal close-packed monolayer. The thickness of the floating layers, e.g., monolayer vs. multilayer forma-
318
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
tion, depends critically on the initial spreading conditions. Concentration of the spreading solution, nature, and volume of the spreading solvent, all impact the film thickness. To minimize the aggregation phenomenon, two strategies can be followed: (a) functionalisation of C60 by polar addends to introduce amphiphilicity thereby enhancing the film forming capability (b) to mix C60 with a good film forming agents. In the present study, p–A isotherms of C60Br24 have been registered at the air–water interface, showing the formation of multilayers at the air–water interface. Therefore, the interfacial compatibility of films of binary mixtures of C60Br24 have been studied with stearic acid (STA), which is well known to form stable monolayers at the air–water interface. 2. Experimental 2.1. Synthesis of C60Br24 C60Br24 was synthesized according to previously reported procedure [20]. C60 (MER Corporation Arizona USA, 99.5 % purity) was mixed with bromine in the composition of 0.13 mL/mg of C60 under nitrogen atmosphere. The reaction proceeded in the dark for 8–10 days and the excess bromine was removed by bubbling nitrogen gas through the solution. The product was obtained as an orange-yellow solid and was dried for 24 h under vacuum. Infrared spectrum was recorded with a Jasco-410 FT-IR spectrometer with a resolution of 4 cm1. The 3 Tu IR active modes for C@C stretch occurred at 1685, 1640 and 1610 cm1 with the C–Br stretch modes at 602 and 544 cm1.
opposite barriers (material = Delrin). The compression speed was 15 mm/min. The surface pressure was recorded by a Pt-wilhelmy balance with an accuracy ±0.05 mN/m. The trough was kept on an anti-vibrational table and was enclosed under a glass hood. 3. Results and discussion 3.1. Molecular behavior of C60Br24 at air–water interface Fig. 1 shows the p–A isotherm of C60Br24 at 13 C and 25 C. At both the temperatures molecule shows a very ˚ 2 as compared with compressed isotherm, A0 = 10 ± 2 A 2 ˚ ). This suggests a strong the monolayer of C60 (A0 = 95 A aggregation at the air–water interface which agrees with the results previously reported that the brominated derivatives of C60 do not form monolayers at the air–water interface [21–24]. C60Br24 is a symmetrical molecule belonging to the Th point group [20] with twenty four bromine atoms substituted symmetrically around the C60 sphere. Thus the molecule is still a non-amphiphilic system. As a consequence, from the energetic point of view, fullerene aggregation may be more favorable than adsorption onto the surface. The adsorption energy, pA 9 · 1022 J/molecule1 at 25 C is observed at p 17 mN/m which is much lower than the thermal energy kBT (4 · 1021 J) and therefore the aggregated state is the more stable phase for C60Br24 as a result of which, it is unable to form a good floating film at the air–water interface. Density functional theory (DFT) calculations at a moderate level with B3LYP/3-21G* basis set often yielded correct geometry and electronic structures, agreeing with experimental
2.2. Langmuir experiments 18 o
25 C o 13 C
16 14
surface pressure (mN/m)
Stearic acid (99+%) was procured from Sigma–Aldrich Co. Ltd. USA and was used without further purification. The chloroform (HPLC Grade, Merck, Germany) was used as a spreading solvent for the film preparation. Solutions (1 · 103 M) of C60Br24 and STA were used for acquiring pure monolayers. The molar fractions are expressed in terms of C60Br24 as 0.25, 0.50 and 0.75 for the mixed monolayers. The p–A isotherms were recorded using a commercial Teflon trough (772.5 cm2 KSV 5000, Standard trough). The ultrapure water of 18 MX cm resistivity and pH 5.9 (measured using Control Dynamics pH meter, C.D. Instruments Pvt., Ltd. India) was produced by a Millipore Elix A3 – MilliQ system was used as subphase after filteration through 0.5 lm nylon disk. The subphase was thermostated by Julabo F-34 (Julabo LABORTECHNIK GMBH, Germany). The p–A isotherms were recorded at two different temperatures, 13 C and 25 C after spreading 200 lL of the appropriate solutions uniformly spread over the water subphase using Hamilton micro-syringe. The initial state of the film was always chosen to have a surface pressure, p 0. After the solvent is evaporated (30 min), the film was symmetrically compressed by two
12 10 8 6 4 2 0 0
10
20
30
40
50
60
area per molecule (A2) Fig. 1. The p–A isotherms of C60Br24 on water subphase at two different temperatures. Inset: Calculated electron density (e/au3) distribution in ab initio B3LYP/3-21g* geometry optimized C60Br24, showing maximum electron density on the surface.
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
results [25,26]. In order to prove the above observation, structure optimization was done using GAUSSAIN 03 [27] suite of programs and the inset of Fig. 1 shows the calculated electron density distribution in C60Br24. It has been well established from the electron density calculations on C60 molecule [28,29] that its curvature forces electrons to outside the cage resulting in a larger electron density on the surface leading to aggregation. A similar observation is seen with C60Br24 whose maximum electron density is observed to be localized on its surface clearly indicating that bromine atoms do not impart enough polarity to avoid the aggregation of C60 moieties. 3.2. Mixed monolayers of C60Br24 with stearic acid Owing to its non-amphiphilic character C60Br24 could not form good films at air–water interface. Thus the effect of mixing of C60B24 with STA at 13 C and 25 C were
319
studied with various mole fractions of C60Br24 as 0.25, 0.50 and 0.75 as shown in Fig. 2 where the area per molecule decreases with the increasing mole fraction of C60Br24. The results of the analysis of these isotherms are summarized in Table 1. These indicate that at p = 10 mN/m, there is a negligible change in area per molecule for C60Br24. The ˚ 2/molearea occupied by a monolayer film of C60 is 98 A cule. Further it can be seen from the Table 1 that the aggregation of C60Br24 has already started in the gaseous phase ˚ 2/molecule). Also, as the mole frac(p = 0 mN/m, A 43 A tion of C60Br24 decreases, the area at gaseous phase is dominated by the STA molecules. The reminiscence of the p–A isotherms of STA molecule indicates and supports the suggestion of Williams et al. [16], that the C60Br24 molecules in such mixed films may be squeezed out of the floating fatty acid monolayer and are placed at the ends of the close packed hydrocarbon tails. 3.3. Miscibility of C60Br24/STA mixed monolayers
a
XC
60
60
Surface Pressure (mN/m)
The compression isotherm of a two-dimensional Langmuir film as a plot of surface pressure (p) as a function ˚ 2) whose surface pressure is given as of area per molecule (A
70
1 0.75 0.50 0.25 0
Br24
50
p ¼ c0 þ c
where the co and c are the surface tension of the clean and the film covered subphase, respectively. In an ideal mixed monolayer, the intermolecular attractive forces (F) between the molecules of the two components 1 and 2 is given by
40 30
F 11 ¼ F 12 ¼ F 22 20
F 11 F 12 F 22
0 0
10
20
30
40
50
60
70
2
Molecular Area (A )
ð3Þ
In terms of the molecular area of the mixed monolayer, the ideal value of the molecular area , Aideal , is calculated from the molar ratio of the two components, Aideal ¼ A1 X 1 þ A2 X 2
70
XC
60
60
Surface Pressure (mN/m)
ð2Þ
whereas in a completely immiscible layer,
10
b
ð1Þ
1 0.75 0.50 0.25 0
Br24
50 40 30
ð4Þ
where X1 and X2 are the molar fractions of the pure components in the mixture, A1 and A2 imply the monomer areas occupied by the pure components. Once the two components form an ideally mixed monolayer or they are immiscible, the Aideal will equalize A12, the actual area occupied by a monomer in the mixed layer. The excess area
Table 1 Measured area per molecule for the various mole fractions of C60Br24/ stearic acid ˚ 2/ Area at p = 10 mN/m Mole fraction of Area at p = 0 mN/m (A ˚ 2/molecule) C60Br24 molecule) (A
20 10 0 0
5
10
15
20
25
30
35
40
45
50
2
Molecular Area (A ) Fig. 2. The p–A isotherms of C60Br24/STA mixtures on water subphase at (a) 25 C and (b) 13 C
1 0.75 0.50 0.25 0
T = 13 C
T = 25 C
T = 13 C
T = 25 C
43.3 39.1 34.9 24.3 64.2
43.5 63.9 63.9 63.9 63.9
6.0 13.9 15.3 19.1 21.4
7.35 12.0 12.4 19.0 21.4
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
Aex which is the difference in molecular area between the ideal and the measured molecular areas is given by the following equation: Aex ¼ A12 Aideal
ð5Þ
Aex is a measure of interaction between the mixed components, when there is any deviation from experimentally determined molecular area in the mixed film from the value supposed for ideal mixing. If an ideal mixed monolayer is formed and the components are completely immiscible, the Aex will be zero. A plot of Aex vs. X1 will be a straight line. Any deviation from the straight line indicates miscibility and non-ideality [30,31]. The magnitude of forces that exists between the molecules in a monolayer determines Aex. For a mixed monolayer, Aex will be negative if the intermolecular forces are attractive and Aex will be positive for repulsive interactions. The plot of Aex vs. X C60 Br24 is shown in Fig. 3, where it can be clearly seen that there is less deviation from ideality at both the measured temperatures indicating that the components are immiscible. Since the area per molecule constantly varies along the isotherm it is more convenient to calculate and represent the excessive area difference [32] as a function of C60Br24 surface pressure as shown in Fig. 4. It can be seen that with an exception of 0.5 mol fraction of C60Br24 at room temperature, there exists a repulsive intermolecular interaction between the STA molecules and C60Br24 and hence these molecules are squeezed out of the fatty acid film. At a C60Br24 mole fraction of 0.5, the excess area is negative indicating that there may be an attractive interaction between the two components. But, as the surface pressure increases, the Aex tends
to become positive showing that the C60Br24 are pushed out of the fatty acid film in this case also. This is further reflected in the p–A isotherm (Fig. 2) of 0.5 C60Br24: STA that at both the temperatures, there is an ill-defined liquid phase with the increase in collapse pressure. 3.4. Thermodynamic stability of C60Br24/STA mixed monolayers When considering a surface film both the Gibbs and Helmholtz free energies may be used to describe the state of a film. Then, branches may be distinguished which are defined by successive area, temperature and surface pressure derivatives keeping each of them constant respectively. The isothermal area derivatives relate to the intrinsic properties, the isobaric temperature derivatives relate to the strong polar structure, while the isoarea pressure derivatives represent the hydrophobic structure [33]. The method developed by Goodrich and Gershfeld can be used to arrive
a
10 5 0
25% C60Br24 50% C60Br24 75% C60Br24
-5
-10 -15
a
24
15
100*Aex/Aideal
320
-20
20
0
2
4
6
8
12
b
8
12
14
16
45 40
4
0.0
0.2
0.4
0.6
0.8
1.0
b
24 20 16
35
100*Aex/Aideal
2
Area per molecule (A )
10
π mN/m
16
25% C60Br24 50% C60Br24 75% C60Br24
30 25 20
12 15
8 4
10
0.0
0.2
0.4
0.6
0.8
1.0
XC Br 60
24
Fig. 3. Area per molecule as a function of composition for mixed C60Br24/ STA monolayers on water subphase at (a) 25 C and (b) 13 C, p = 10 mN/m.
0
2
4
6
8 π mN/m
10
12
14
16
Fig. 4. The AexAideal (%) relative excess areas of mixed monolayers of C60Br24/STA mixtures as a function of surface pressure at (a) 25 C and (b) 13 C.
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
at the thermodynamic stability compared of a mixed monolayer. The DGex and the DGmix are also the indicators of the type of interaction between the components [32]. For a two-component mixed monolayer at constant p and temperature (T), the excess free energy of mixing DGex is expressed as Z p DGex ¼ ðA12 Aideal Þdp ð6Þ 0
and the free energy of mixing DGmix can be calculated from the equation, DGmix ¼ DGideal þ DGex
ð7Þ
where DGideal can be expressed as DGideal ¼ RT ðX 1 ln X 1 þ X 2 ln X 2 Þ
ð8Þ
where R is the gas constant. Positive values of DGex were obtained for the C60Br24/STA (with an exception of 0.5 mol fraction) films indicating that there is a repulsive interaction between the two components. With the increase in mole fraction of C60Br24 these repulsive interactions increased reflecting in more positive values for DGex. Thus the above results indicate that X C60 Br24 ¼ 0:5 may be the composition of greatest stability compared to the pure component monolayers. In general, the DGmix obtained were negative for all mixed monolayers at various surface pressures indicative of the absence of phase separation in the mixed monolayers i.e. the STA collapses at a much lower surface pressure than pure component due the strong p–p interactions between the C60Br24 spheres. 3.5. Compressibility of C60Br24/STA mixed films The compressibility (C) and the reciprocal compressibility (elasticity modulus, C1) of the monolayers is deduced from the pure and mixed monolayers using the equation,
C¼
1 dA A dp T
321
ð9Þ
In general, the C1 will depend also on the state of the monolayer. The higher value of C1 indicates low interfacial elasticity. This is due to the fact that at high surface pressures, the molar composition of the mixed monolayers exerts greater influence on the values of elastic modulus. The compressibility of the subjected mixed systems decreases with the increasing pressure and with decreasing STA mole fraction as shown in Fig. 5 reflecting the increasing dominance of C60Br24 molecules in the mixed layer. Further, the compressibility for 50:50 mixture is the lowest indicating that it is the composition of highest stability. 4. Conclusions Langmuir isotherms of pure C60Br24 and its mixed monolayers with Stearic acid were investigated. The characteristic features were discussed in the wide range of composition. C60Br24 does not form a stable film at the air–water interface which has also been proved from the theoretical electron density calculations. Mixed monolayers with the amphiphilic STA indicates the reminiscence of the p–A isotherms of STA molecule which can be attributed to the fact that the C60Br24 molecules in such mixed films may be squeezed out of the floating fatty acid monolayer and are placed at the ends of the close packed hydrocarbon tails. The plot of Aex vs. X C60 Br24 showed that the system was immiscible, non-ideal with pronounced deviation from ideality. Further the mixed components showed repulsive interactions with an exception of X C60 Br24 ¼ 0:5 Which may be the composition of greater stability when compared with its pure components. The compressibility of the system decreased with increasing X C60 Br24 indicating the dominance of C60Br24 over the mixed monolayers. Acknowledgement
200
This work is supported by Department of Science and Technology (DST), Government of India under Grant No. SP/SI/H-37/1.
100% C60Br24 75 % C60Br24 50 % C60Br24 25 % C60Br24
175 150
References
100
-1
C mN/m
125
75 50 25 0 0
10
20
30 o
40
50
60
2
Area (A /molecule) Fig. 5. Equilibrium elasticity modulus of pure C60Br24 its mixed films with STA as a function of surface pressure at 25 C.
[1] M.P. Hernandez et al., Langmuir 17 (2001) 3317. [2] A.F. Hebbard et al., Nature 350 (1991) 600. [3] P. Wang, R.M. Metzger, S. Bandow, Y. Maruyama, J. Phys. Chem. 97 (1993) 2926. [4] P. Wang, Y. Maruyama, R.M. Metzger, Langmuir 12 (1996) 3932. [5] S.H. Friedman, D.L. De Camp, R.P. Sijbesma, G. Srdanov, F. Wudl, G. L Kenyon, J. Am. Chem. Soc. 115 (1993) 6506. [6] M. Prato, T. Da Ros, Chem. Commun. (1999) 663. [7] N. Martı´n, L. Sa´nchez, B. Illescas, I. Pe´rez, Chem. Rev. 98 (1998) 2527. [8] N.S. Sariciftci et al., Chem. Phys. Lett. 247 (1995) 510. [9] E. Allard, J. Cousseau, J. Ordu´na, J. Garı´n, H. Luo, Y. Araki, O. Ito, Phys. Chem. Chem. Phys. 4 (2002) 5944. [10] G. Brusatin, R. Signorini, J. Mater. Chem. 12 (2002) 1964.
322
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
[11] J. Venturini, E. Koudoumas, S. Couris, J.M. Janot, P. Seta, C. Mathis, S. Leach, J. Mater. Chem. 12 (2002) 2071. [12] F. Wudl, J. Mater. Chem. 12 (2002) 1959. [13] L. Leo, G. Mele, G. Rosso, L. Valli, G. Vasapollo, D.M. Guldi, G. Mascolo, Langmuir 16 (2000) 4599. [14] Y.S. Obeng, A.J. Bard, J. Am. Chem. Soc. 113 (1991) 6279. [15] R. Back, R.B. Lennox, J. Phys. Chem. 96 (1992) 8149. [16] G. Williams, C. Pearson, M.R. Bryce, M.C. Petty, Thin Solid Film 209 (1992) 150. [17] Y. Xiao, Z. Yao, D. Jin, F. Yan, Q. Xue, J. Phys. Chem. 97 (1993) 7072. [18] S. Ravaine, F. Le Pecq, C. Mingotaud, P. Delhaes, J.C. Hummelen, F. Wudl, L.K. Patterson, J. Phys. Chem. 99 (1995) 9551. [19] A.K. Evans, J. Phys. Chem. 102 (1998) 7016. [20] S.S. Gayathri, A. Patnaik, Chem. Phys. Lett. 374 (2003) 33. [21] Y. Xiao, Solid State Commun. 94 (1995) 193. [22] Y. Xiao, Y. Ye, Z. Yao, Thin Solid Film 261 (1995) 120.
[23] Y. Li, Y. Xu, S. Liu, Z. Wu, D. Zhu, Solid State Commun. 86 (1993) 745. [24] N.I. Denisenko, S.G. Yudin, A.V., Streletskiy. O.V. Boltalina, in: Abstracts of 201st ECS Meeting, 2002. [25] F.D. Souza, M.E. Zandler, P.M. Smith, G.R. Deviprasad, K. Arkady, M. Fujitsuka, O. Ito, J. Phys. Chem. A 106 (2002) 649. [26] F.D. Souza et al., J. Phys. Chem. A 106 (2002) 3243. [27] M.J. Frisch et al., GAUSSAIN 03, Revision C.02, Gaussian Inc., Wallingford, CT, 2004. [28] A.D. Bokare, A. Patnaik, J. Phys. Chem. B 109 (2005) 87. [29] E. Teller, Mater. Sci. Forum 105–110 (1992) 161. [30] S. Ke Gong, Feng, M.L. Go, P.H. Soew, Coll. Surf. A 207 (2002) 113. [31] S.S. Feng, G.H. Zhu, M.Y. Low, Langmuir 16 (2000) 7401. [32] E. Kiss, A. Vargha, E.I. Vargha-Butler, Phys. Chem. Chem. Phys. 6 (2004) 1575. [33] J.B. Rosenholm, P. Ihalainen, J. Peltonen, Coll. Surf. A 228 (2003) 119.
Interfacial behaviour of brominated fullerene (C60Br24) and stearic acid mixed Langmuir films at air–water interface S. Shankara Gayathri, Archita Patnaik
*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India Received 15 October 2006; in final form 9 November 2006 Available online 17 November 2006
Abstract The interfacial properties of the spread Langmuir films of C60Br24 and stearic acid have been studied at air–water interface by measuring their surface pressure–area isotherms. Monolayer studies on C60Br24 suggest a strong aggregation at air–water interface which corroborated the ab initio electron density calculations. The mixed films were found to be immiscible and non-ideal with an exception of X C60 Br24 ¼ 0:5 which is the composition of greater stability. The interaction between C60Br24 and stearic acid results in the lowering of surface pressure withstood by stearic acid and hence the decrease in compressibility with the increasing X C60 Br24 , thereby indicating the domination of C60Br24 characteristics in the mixed monolayers. 2006 Elsevier B.V. All rights reserved.
1. Introduction A feature of contemporary science is the heightened interest in materials with complex molecular structure. Ordered molecular films, with thickness ranging from a few nanometers in the case of a monolayer to several hundred nanometers, show considerable technological promise. The study of molecular interactions in thin-film structures is a necessary step for a better understanding of the collective properties of the ordered arrays of conducting or semi-conducting materials incorporated in these useful devices [1]. The Langmuir trough technique provides a method of constructing simple artificial systems of molecules on a substrate. The particular physicochemical properties of fullerenes, essentially due to their singular geometry and electronic structure have become the focus of considerable interest in monomeric or aggregated forms. Thin films of C60 and its derivatives evidence attractive characteristics, like superconductivity [2–4], pharmacological properties [5,6], charge
*
Corresponding author. E-mail address: [email protected] (A. Patnaik).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.11.055
transfer behaviour [7–9], and non-linear optic behaviour [10–12]. An essential requirement for the systematic investigation of these properties is the incorporation of fullerenes in organized 2D arrays and 3D networks [13]. Numerous efforts are made to elaborate the well ordered monomolecular films of C60 and its derivatives since their electronic properties can be better controlled in a 2D arrangement [14–17]. Although from measurements of pressure vs. molecular area (p–A) isotherms, Obeng and Bard reported for the first time the formation of stable monolayer of fullerene (C60) at air–water interface [14], many studies have revealed that, under the most common experimental conditions, C60 does not form truly stable monolayers [15–17]. A high cohesive energy of more than 30 kcal/mol reflects the strong intermolecular attractive p–p interactions, and in turn, C60 forms 3D aggregates at air-water interface. These aggregates were experimentally confirmed by UV–Vis spectroscopy with a band centered at 450 nm [18]. A recent Langevin dynamics simulation provided a clear explanation for the above [19]. The registered limiting areas per molecule were significantly smaller ˚ 2/molecule, calcuthan the theoretical value of about 98 A lated for an ideal close-packed monolayer. The thickness of the floating layers, e.g., monolayer vs. multilayer forma-
318
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
tion, depends critically on the initial spreading conditions. Concentration of the spreading solution, nature, and volume of the spreading solvent, all impact the film thickness. To minimize the aggregation phenomenon, two strategies can be followed: (a) functionalisation of C60 by polar addends to introduce amphiphilicity thereby enhancing the film forming capability (b) to mix C60 with a good film forming agents. In the present study, p–A isotherms of C60Br24 have been registered at the air–water interface, showing the formation of multilayers at the air–water interface. Therefore, the interfacial compatibility of films of binary mixtures of C60Br24 have been studied with stearic acid (STA), which is well known to form stable monolayers at the air–water interface. 2. Experimental 2.1. Synthesis of C60Br24 C60Br24 was synthesized according to previously reported procedure [20]. C60 (MER Corporation Arizona USA, 99.5 % purity) was mixed with bromine in the composition of 0.13 mL/mg of C60 under nitrogen atmosphere. The reaction proceeded in the dark for 8–10 days and the excess bromine was removed by bubbling nitrogen gas through the solution. The product was obtained as an orange-yellow solid and was dried for 24 h under vacuum. Infrared spectrum was recorded with a Jasco-410 FT-IR spectrometer with a resolution of 4 cm1. The 3 Tu IR active modes for C@C stretch occurred at 1685, 1640 and 1610 cm1 with the C–Br stretch modes at 602 and 544 cm1.
opposite barriers (material = Delrin). The compression speed was 15 mm/min. The surface pressure was recorded by a Pt-wilhelmy balance with an accuracy ±0.05 mN/m. The trough was kept on an anti-vibrational table and was enclosed under a glass hood. 3. Results and discussion 3.1. Molecular behavior of C60Br24 at air–water interface Fig. 1 shows the p–A isotherm of C60Br24 at 13 C and 25 C. At both the temperatures molecule shows a very ˚ 2 as compared with compressed isotherm, A0 = 10 ± 2 A 2 ˚ ). This suggests a strong the monolayer of C60 (A0 = 95 A aggregation at the air–water interface which agrees with the results previously reported that the brominated derivatives of C60 do not form monolayers at the air–water interface [21–24]. C60Br24 is a symmetrical molecule belonging to the Th point group [20] with twenty four bromine atoms substituted symmetrically around the C60 sphere. Thus the molecule is still a non-amphiphilic system. As a consequence, from the energetic point of view, fullerene aggregation may be more favorable than adsorption onto the surface. The adsorption energy, pA 9 · 1022 J/molecule1 at 25 C is observed at p 17 mN/m which is much lower than the thermal energy kBT (4 · 1021 J) and therefore the aggregated state is the more stable phase for C60Br24 as a result of which, it is unable to form a good floating film at the air–water interface. Density functional theory (DFT) calculations at a moderate level with B3LYP/3-21G* basis set often yielded correct geometry and electronic structures, agreeing with experimental
2.2. Langmuir experiments 18 o
25 C o 13 C
16 14
surface pressure (mN/m)
Stearic acid (99+%) was procured from Sigma–Aldrich Co. Ltd. USA and was used without further purification. The chloroform (HPLC Grade, Merck, Germany) was used as a spreading solvent for the film preparation. Solutions (1 · 103 M) of C60Br24 and STA were used for acquiring pure monolayers. The molar fractions are expressed in terms of C60Br24 as 0.25, 0.50 and 0.75 for the mixed monolayers. The p–A isotherms were recorded using a commercial Teflon trough (772.5 cm2 KSV 5000, Standard trough). The ultrapure water of 18 MX cm resistivity and pH 5.9 (measured using Control Dynamics pH meter, C.D. Instruments Pvt., Ltd. India) was produced by a Millipore Elix A3 – MilliQ system was used as subphase after filteration through 0.5 lm nylon disk. The subphase was thermostated by Julabo F-34 (Julabo LABORTECHNIK GMBH, Germany). The p–A isotherms were recorded at two different temperatures, 13 C and 25 C after spreading 200 lL of the appropriate solutions uniformly spread over the water subphase using Hamilton micro-syringe. The initial state of the film was always chosen to have a surface pressure, p 0. After the solvent is evaporated (30 min), the film was symmetrically compressed by two
12 10 8 6 4 2 0 0
10
20
30
40
50
60
area per molecule (A2) Fig. 1. The p–A isotherms of C60Br24 on water subphase at two different temperatures. Inset: Calculated electron density (e/au3) distribution in ab initio B3LYP/3-21g* geometry optimized C60Br24, showing maximum electron density on the surface.
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
results [25,26]. In order to prove the above observation, structure optimization was done using GAUSSAIN 03 [27] suite of programs and the inset of Fig. 1 shows the calculated electron density distribution in C60Br24. It has been well established from the electron density calculations on C60 molecule [28,29] that its curvature forces electrons to outside the cage resulting in a larger electron density on the surface leading to aggregation. A similar observation is seen with C60Br24 whose maximum electron density is observed to be localized on its surface clearly indicating that bromine atoms do not impart enough polarity to avoid the aggregation of C60 moieties. 3.2. Mixed monolayers of C60Br24 with stearic acid Owing to its non-amphiphilic character C60Br24 could not form good films at air–water interface. Thus the effect of mixing of C60B24 with STA at 13 C and 25 C were
319
studied with various mole fractions of C60Br24 as 0.25, 0.50 and 0.75 as shown in Fig. 2 where the area per molecule decreases with the increasing mole fraction of C60Br24. The results of the analysis of these isotherms are summarized in Table 1. These indicate that at p = 10 mN/m, there is a negligible change in area per molecule for C60Br24. The ˚ 2/molearea occupied by a monolayer film of C60 is 98 A cule. Further it can be seen from the Table 1 that the aggregation of C60Br24 has already started in the gaseous phase ˚ 2/molecule). Also, as the mole frac(p = 0 mN/m, A 43 A tion of C60Br24 decreases, the area at gaseous phase is dominated by the STA molecules. The reminiscence of the p–A isotherms of STA molecule indicates and supports the suggestion of Williams et al. [16], that the C60Br24 molecules in such mixed films may be squeezed out of the floating fatty acid monolayer and are placed at the ends of the close packed hydrocarbon tails. 3.3. Miscibility of C60Br24/STA mixed monolayers
a
XC
60
60
Surface Pressure (mN/m)
The compression isotherm of a two-dimensional Langmuir film as a plot of surface pressure (p) as a function ˚ 2) whose surface pressure is given as of area per molecule (A
70
1 0.75 0.50 0.25 0
Br24
50
p ¼ c0 þ c
where the co and c are the surface tension of the clean and the film covered subphase, respectively. In an ideal mixed monolayer, the intermolecular attractive forces (F) between the molecules of the two components 1 and 2 is given by
40 30
F 11 ¼ F 12 ¼ F 22 20
F 11 F 12 F 22
0 0
10
20
30
40
50
60
70
2
Molecular Area (A )
ð3Þ
In terms of the molecular area of the mixed monolayer, the ideal value of the molecular area , Aideal , is calculated from the molar ratio of the two components, Aideal ¼ A1 X 1 þ A2 X 2
70
XC
60
60
Surface Pressure (mN/m)
ð2Þ
whereas in a completely immiscible layer,
10
b
ð1Þ
1 0.75 0.50 0.25 0
Br24
50 40 30
ð4Þ
where X1 and X2 are the molar fractions of the pure components in the mixture, A1 and A2 imply the monomer areas occupied by the pure components. Once the two components form an ideally mixed monolayer or they are immiscible, the Aideal will equalize A12, the actual area occupied by a monomer in the mixed layer. The excess area
Table 1 Measured area per molecule for the various mole fractions of C60Br24/ stearic acid ˚ 2/ Area at p = 10 mN/m Mole fraction of Area at p = 0 mN/m (A ˚ 2/molecule) C60Br24 molecule) (A
20 10 0 0
5
10
15
20
25
30
35
40
45
50
2
Molecular Area (A ) Fig. 2. The p–A isotherms of C60Br24/STA mixtures on water subphase at (a) 25 C and (b) 13 C
1 0.75 0.50 0.25 0
T = 13 C
T = 25 C
T = 13 C
T = 25 C
43.3 39.1 34.9 24.3 64.2
43.5 63.9 63.9 63.9 63.9
6.0 13.9 15.3 19.1 21.4
7.35 12.0 12.4 19.0 21.4
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
Aex which is the difference in molecular area between the ideal and the measured molecular areas is given by the following equation: Aex ¼ A12 Aideal
ð5Þ
Aex is a measure of interaction between the mixed components, when there is any deviation from experimentally determined molecular area in the mixed film from the value supposed for ideal mixing. If an ideal mixed monolayer is formed and the components are completely immiscible, the Aex will be zero. A plot of Aex vs. X1 will be a straight line. Any deviation from the straight line indicates miscibility and non-ideality [30,31]. The magnitude of forces that exists between the molecules in a monolayer determines Aex. For a mixed monolayer, Aex will be negative if the intermolecular forces are attractive and Aex will be positive for repulsive interactions. The plot of Aex vs. X C60 Br24 is shown in Fig. 3, where it can be clearly seen that there is less deviation from ideality at both the measured temperatures indicating that the components are immiscible. Since the area per molecule constantly varies along the isotherm it is more convenient to calculate and represent the excessive area difference [32] as a function of C60Br24 surface pressure as shown in Fig. 4. It can be seen that with an exception of 0.5 mol fraction of C60Br24 at room temperature, there exists a repulsive intermolecular interaction between the STA molecules and C60Br24 and hence these molecules are squeezed out of the fatty acid film. At a C60Br24 mole fraction of 0.5, the excess area is negative indicating that there may be an attractive interaction between the two components. But, as the surface pressure increases, the Aex tends
to become positive showing that the C60Br24 are pushed out of the fatty acid film in this case also. This is further reflected in the p–A isotherm (Fig. 2) of 0.5 C60Br24: STA that at both the temperatures, there is an ill-defined liquid phase with the increase in collapse pressure. 3.4. Thermodynamic stability of C60Br24/STA mixed monolayers When considering a surface film both the Gibbs and Helmholtz free energies may be used to describe the state of a film. Then, branches may be distinguished which are defined by successive area, temperature and surface pressure derivatives keeping each of them constant respectively. The isothermal area derivatives relate to the intrinsic properties, the isobaric temperature derivatives relate to the strong polar structure, while the isoarea pressure derivatives represent the hydrophobic structure [33]. The method developed by Goodrich and Gershfeld can be used to arrive
a
10 5 0
25% C60Br24 50% C60Br24 75% C60Br24
-5
-10 -15
a
24
15
100*Aex/Aideal
320
-20
20
0
2
4
6
8
12
b
8
12
14
16
45 40
4
0.0
0.2
0.4
0.6
0.8
1.0
b
24 20 16
35
100*Aex/Aideal
2
Area per molecule (A )
10
π mN/m
16
25% C60Br24 50% C60Br24 75% C60Br24
30 25 20
12 15
8 4
10
0.0
0.2
0.4
0.6
0.8
1.0
XC Br 60
24
Fig. 3. Area per molecule as a function of composition for mixed C60Br24/ STA monolayers on water subphase at (a) 25 C and (b) 13 C, p = 10 mN/m.
0
2
4
6
8 π mN/m
10
12
14
16
Fig. 4. The AexAideal (%) relative excess areas of mixed monolayers of C60Br24/STA mixtures as a function of surface pressure at (a) 25 C and (b) 13 C.
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
at the thermodynamic stability compared of a mixed monolayer. The DGex and the DGmix are also the indicators of the type of interaction between the components [32]. For a two-component mixed monolayer at constant p and temperature (T), the excess free energy of mixing DGex is expressed as Z p DGex ¼ ðA12 Aideal Þdp ð6Þ 0
and the free energy of mixing DGmix can be calculated from the equation, DGmix ¼ DGideal þ DGex
ð7Þ
where DGideal can be expressed as DGideal ¼ RT ðX 1 ln X 1 þ X 2 ln X 2 Þ
ð8Þ
where R is the gas constant. Positive values of DGex were obtained for the C60Br24/STA (with an exception of 0.5 mol fraction) films indicating that there is a repulsive interaction between the two components. With the increase in mole fraction of C60Br24 these repulsive interactions increased reflecting in more positive values for DGex. Thus the above results indicate that X C60 Br24 ¼ 0:5 may be the composition of greatest stability compared to the pure component monolayers. In general, the DGmix obtained were negative for all mixed monolayers at various surface pressures indicative of the absence of phase separation in the mixed monolayers i.e. the STA collapses at a much lower surface pressure than pure component due the strong p–p interactions between the C60Br24 spheres. 3.5. Compressibility of C60Br24/STA mixed films The compressibility (C) and the reciprocal compressibility (elasticity modulus, C1) of the monolayers is deduced from the pure and mixed monolayers using the equation,
C¼
1 dA A dp T
321
ð9Þ
In general, the C1 will depend also on the state of the monolayer. The higher value of C1 indicates low interfacial elasticity. This is due to the fact that at high surface pressures, the molar composition of the mixed monolayers exerts greater influence on the values of elastic modulus. The compressibility of the subjected mixed systems decreases with the increasing pressure and with decreasing STA mole fraction as shown in Fig. 5 reflecting the increasing dominance of C60Br24 molecules in the mixed layer. Further, the compressibility for 50:50 mixture is the lowest indicating that it is the composition of highest stability. 4. Conclusions Langmuir isotherms of pure C60Br24 and its mixed monolayers with Stearic acid were investigated. The characteristic features were discussed in the wide range of composition. C60Br24 does not form a stable film at the air–water interface which has also been proved from the theoretical electron density calculations. Mixed monolayers with the amphiphilic STA indicates the reminiscence of the p–A isotherms of STA molecule which can be attributed to the fact that the C60Br24 molecules in such mixed films may be squeezed out of the floating fatty acid monolayer and are placed at the ends of the close packed hydrocarbon tails. The plot of Aex vs. X C60 Br24 showed that the system was immiscible, non-ideal with pronounced deviation from ideality. Further the mixed components showed repulsive interactions with an exception of X C60 Br24 ¼ 0:5 Which may be the composition of greater stability when compared with its pure components. The compressibility of the system decreased with increasing X C60 Br24 indicating the dominance of C60Br24 over the mixed monolayers. Acknowledgement
200
This work is supported by Department of Science and Technology (DST), Government of India under Grant No. SP/SI/H-37/1.
100% C60Br24 75 % C60Br24 50 % C60Br24 25 % C60Br24
175 150
References
100
-1
C mN/m
125
75 50 25 0 0
10
20
30 o
40
50
60
2
Area (A /molecule) Fig. 5. Equilibrium elasticity modulus of pure C60Br24 its mixed films with STA as a function of surface pressure at 25 C.
[1] M.P. Hernandez et al., Langmuir 17 (2001) 3317. [2] A.F. Hebbard et al., Nature 350 (1991) 600. [3] P. Wang, R.M. Metzger, S. Bandow, Y. Maruyama, J. Phys. Chem. 97 (1993) 2926. [4] P. Wang, Y. Maruyama, R.M. Metzger, Langmuir 12 (1996) 3932. [5] S.H. Friedman, D.L. De Camp, R.P. Sijbesma, G. Srdanov, F. Wudl, G. L Kenyon, J. Am. Chem. Soc. 115 (1993) 6506. [6] M. Prato, T. Da Ros, Chem. Commun. (1999) 663. [7] N. Martı´n, L. Sa´nchez, B. Illescas, I. Pe´rez, Chem. Rev. 98 (1998) 2527. [8] N.S. Sariciftci et al., Chem. Phys. Lett. 247 (1995) 510. [9] E. Allard, J. Cousseau, J. Ordu´na, J. Garı´n, H. Luo, Y. Araki, O. Ito, Phys. Chem. Chem. Phys. 4 (2002) 5944. [10] G. Brusatin, R. Signorini, J. Mater. Chem. 12 (2002) 1964.
322
S.S. Gayathri, A. Patnaik / Chemical Physics Letters 433 (2007) 317–322
[11] J. Venturini, E. Koudoumas, S. Couris, J.M. Janot, P. Seta, C. Mathis, S. Leach, J. Mater. Chem. 12 (2002) 2071. [12] F. Wudl, J. Mater. Chem. 12 (2002) 1959. [13] L. Leo, G. Mele, G. Rosso, L. Valli, G. Vasapollo, D.M. Guldi, G. Mascolo, Langmuir 16 (2000) 4599. [14] Y.S. Obeng, A.J. Bard, J. Am. Chem. Soc. 113 (1991) 6279. [15] R. Back, R.B. Lennox, J. Phys. Chem. 96 (1992) 8149. [16] G. Williams, C. Pearson, M.R. Bryce, M.C. Petty, Thin Solid Film 209 (1992) 150. [17] Y. Xiao, Z. Yao, D. Jin, F. Yan, Q. Xue, J. Phys. Chem. 97 (1993) 7072. [18] S. Ravaine, F. Le Pecq, C. Mingotaud, P. Delhaes, J.C. Hummelen, F. Wudl, L.K. Patterson, J. Phys. Chem. 99 (1995) 9551. [19] A.K. Evans, J. Phys. Chem. 102 (1998) 7016. [20] S.S. Gayathri, A. Patnaik, Chem. Phys. Lett. 374 (2003) 33. [21] Y. Xiao, Solid State Commun. 94 (1995) 193. [22] Y. Xiao, Y. Ye, Z. Yao, Thin Solid Film 261 (1995) 120.
[23] Y. Li, Y. Xu, S. Liu, Z. Wu, D. Zhu, Solid State Commun. 86 (1993) 745. [24] N.I. Denisenko, S.G. Yudin, A.V., Streletskiy. O.V. Boltalina, in: Abstracts of 201st ECS Meeting, 2002. [25] F.D. Souza, M.E. Zandler, P.M. Smith, G.R. Deviprasad, K. Arkady, M. Fujitsuka, O. Ito, J. Phys. Chem. A 106 (2002) 649. [26] F.D. Souza et al., J. Phys. Chem. A 106 (2002) 3243. [27] M.J. Frisch et al., GAUSSAIN 03, Revision C.02, Gaussian Inc., Wallingford, CT, 2004. [28] A.D. Bokare, A. Patnaik, J. Phys. Chem. B 109 (2005) 87. [29] E. Teller, Mater. Sci. Forum 105–110 (1992) 161. [30] S. Ke Gong, Feng, M.L. Go, P.H. Soew, Coll. Surf. A 207 (2002) 113. [31] S.S. Feng, G.H. Zhu, M.Y. Low, Langmuir 16 (2000) 7401. [32] E. Kiss, A. Vargha, E.I. Vargha-Butler, Phys. Chem. Chem. Phys. 6 (2004) 1575. [33] J.B. Rosenholm, P. Ihalainen, J. Peltonen, Coll. Surf. A 228 (2003) 119.