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Adsorption of Normal Pentane on the Surface of Rutile. Experimental Results and Simulations G. U. Rakhmatkariev,† A. J. Palace Carvalho,*,‡ and J. P. Prates Ramalho§ Institute of General and Inorganic Chemistry, Academy of Sciences of Uzbekistan, 700170 Tashkent, Uzbekistan, Centro de Quı´mica de EÄ Vora, UniVersity of EVora, 7000-671 EVora, Portugal, and Department of Chemistry, UniVersity of EVora, 7000-671 EVora, Portugal ReceiVed October 24, 2006. In Final Form: March 28, 2007 Adsorption isotherms and differential heats of normal pentane adsorption on microcrystalline rutile were measured at 303 K. The heat of adsorption of n-pentane on rutile at zero occupancy is 64 kJ/mol. The differential heats have three descending segments, corresponding to the adsorption of n-pentane on three types of surfaces. At low coverage (first segment), the adsorption is restricted to the rows A of the (110) faces along the 5-fold coordinatively unsaturated (cus) Ti4+ ions with differential heat showing a linear decrease with increasing occupancy. The second segment is attributed to bonding with atoms of the rows along the remaining faces exposed, (101) and (100). The third segment is related to a multilayer adsorption. The mean molar adsorption entropy of n-pentane is ca. -25 J/mol K less than the entropy of the bulk liquid, thus revealing a hindered state of motion of the n-pentane molecules on the surface of rutile. Simulations of the adsorption of n-pentane on the three most abundant crystallographic faces of rutile were also performed. The adsorption isotherm obtained from the combination of each face’s isotherm weighted by the respective abundance was found to be in a good agreement with the experimental data. A structural characterization of n-pentane near the surface was also conducted, and it was found that the substrate, especially for the (110) face, strongly perturbs the distribution of n-pentane conformations, compared to those found for the gas phase. Adsorbed molecules are predominantly oriented with their long axes and their backbone zigzag planes parallel to the surface and are also characterized by fewer gauche conformations than observed in the bulk phase.
1. Introduction The adsorption of specific interacting organic and inorganic dipole and quadrupole molecules is the most extensive area of study in the surface science of TiO2.1 Much less attention is paid to the interaction of nonspecific interacting molecules such as hydrocarbons. There are two main types of interactions responsible for hydrocarbon adsorption: the short-range van der Waals interactions with the surface of TiO2 and the long-range electrostatic interactions with the coordinatively unsaturated (cus) Ti4+ ions (here, an induced effect). The contribution of the induced effect to the total energy of nonpolar hydrocarbon molecules is usually small. For instance, for n-pentane adsorbed in NaX zeolite, such contribution is 3.7 kJ/mol, being 1 order of magnitude less than the van der Waals component.2 A great number of examples show the effectiveness of the adsorption calorimetric method with Tian-Calvet calorimeters for the investigation of the crystallochemistry and adsorption thermodynamics on the pore adsorbents-zeolites [for instance, refs 3-10]. This method proved to be very successful for studying * Corresponding author. E-mail:
[email protected]. † Academy of Sciences of Uzbekistan. ‡ Centro de Quı´mica de E Ä vora, University of Evora. § Department of Chemistry, University of Evora. (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. (2) Dubinin, M. M.; Isirikyan, A. A.; Regent, N. I. IzV. Akad. Nauk SSSR, Ser. Khim. 1976, 2, 288-294. (3) Akhmedov, K .S.; Rakhmatkariev, G. U.; Dubinin, M. M.; Isirikyan, A. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1987, 8, 1717-1721. (4) Boddenberg, B.; Rakhmatkariev, G. U.; Greth, R. J. Phys. Chem. B 1997, 101, 1634-1940. (5) Boddenberg, B.; Rakhmatkariev, G. U.; Viets, J. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 177-182. (6) Boddenberg, B.; Rakhmatkariev, G. U.; Hufnagel, S.; Salimov, Z. Phys. Chem. Chem. Phys. 2002, 4, 4172-4180. (7) Boddenberg, B.; Rakhmatkariev, G. U.; Wozniak, A.; Hufnagel, S. Phys. Chem. Chem. Phys. 2004, 6, 2494-2501. (8) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1989, 12, 2862-2864.
the energetics of the adsorption processes on clay mineral muscovite11 and also on the nonporous surface of microcrystalline rutile.12 Differing from zeolites, which possess a big adsorption capacity, the specific surface of TiO2 is 11 m2/g. Despite the small specific surface, we managed to obtain quite reproducible energetic characteristics of n-pentane adsorption on this specimen. In this paper, we present experimental and computer simulation results of n-pentane adsorption on rutile. Experimental isotherm adsorption data is compared with simulation results, and the structural properties of the adsorbed molecules on the different crystalline faces of rutile are analyzed and discussed. 2. Experimental The rutile sample was prepared by hydrolysis of redistilled titanium tetrachloride followed by heating in the air at 773 K. After calcination at 973 K in air, the rutile specimen was repeatedly washed with water and dried at 423 K. The chloride content was 0.007%. The rutile structure of the sample was established from X-ray diffraction. It was pure rutile, as evidenced by the excellent agreement of both the Bragg angles and peak intensities of the Debye-Scherrer pattern with literature data and by the complete absence of the anatase and brookite lines. The chemical analysis showed 98% rutile, the remaining being water and traces of impurities. Specific surface areas, S, of 11 and 9.5 m2/g were obtained by the Brunauer-EmmettTeller (BET) method with nitrogen and argon, respectively. The rutile powder was compacted in a cylindrical glass tube. For further experiments, rutile was subjected to the following pretreatment. To avoid the change of the surface properties during dehydroxylation, the specimen was additionally oxidized in hot (9) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1989, 11, 2633-2635. (10) Dubinin, M. M.; Rakhmatkariev, G. U.; Isirikyan, A. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1989, 11, 2636-2638. (11) Rakhmatkariev, G. U. Clay Clay Miner. 2006, 54, 405-411. (12) Rakhmatkariev, G. U. Uzb. Khim. Zh. 1987, 4, 40-42; 1988, 4, 32-35.
10.1021/la063113q CCC: $37.00 © 2007 American Chemical Society Published on Web 06/09/2007
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Figure 1. (110), (101), and (100) cleavage planes of rutile. The lighter atoms represent the outmost oxygens, the gray atoms represent oxygens, and the black balls are the titanium atoms. conditions (to restore stoichiometry), as the specimen loses oxygen ions under high vacuum conditions at temperatures higher than 470 K. Evacuation at 723 K for 15 h (p < 10-5 Torr) was followed by oxygen treatment (200 Torr) for 1 h and subsequent evacuation for 1 h at the same temperature. After the procedure had been repeated, the sample was again contacted with oxygen at 723 K and then cooled slowly to 423 K, at which temperature the oxygen was pumped off for 5 h. Finally, the sample was cooled to ambient temperature under high vacuum conditions. The details of measuring the adsorption isotherms as well as the differential heats of adsorption using a differential microcalorimeter of the Tian-Calvet type have been described elsewhere.11 After each admission of a small amount of n-pentane, the heat flux was monitored until thermodynamic equilibrium was achieved. The attainment of this state was defined to be the time at which the measured heat flux was just below the sensitivity of the instrument (1 µW).
3. Simulation Details Gas adsorption processes are frequently simulated by the grand canonical Monte Carlo (GCMC) method because, during the simulation in the grand canonical ensemble, the volume, the temperature, and the chemical potential of the system are held constant while the number of atoms is allowed to vary. The chemical potential can be related to the pressure of the gas in contact with the surface by an equation of state. Another technique commonly used is the Gibbs ensemble Monte Carlo method, where two simulation cells in equilibrium are studied simultaneously, one containing the adsorbing surface with adsorbed gas and the other containing the coexisting bulk fluid.13 In order to apply the simulation method, it is necessary to know the adsorption potential surface of an n-pentane molecule in the different rutile crystallographic faces. In the rutile case, 98% of the total surface is composed of three crystallographic planes: the (110), (101), and (100).14 These cleavage planes are shown in Figure 1. The non-shaded oxide ions lie above the plane defined by the titanium ions. In the case of the (110) face, the shaded oxide ions are coplanar with the cations, while for the (101) and (100) faces they lie beneath the plane containing the cations. The Monte Carlo method was used in the simulation of n-pentane adsorption on the three most important crystallographic facess(110), (101), and (100)sof rutile, and comparisons were made with experimental data. In the simulations, the GCMC method was employed, using the MCCCS Towhee package, version 4.7.715 to perform the calculations. In the simulations of n-pentane adsorption on rutile there are five types of trial moves involved, which are selected stochasti(13) Panagiotopoulos, A. Z. Mol. Phys. 1987, 61, 813-826. (14) Jones, P.; Hochey, J. A. Trans. Faraday Soc. 1971, 67, 2679-2685. (15) Martin, M. G. MCCCS Towhee. http://towhee.sourceforge.net.
cally: (i) displacement of the center of mass of an n-pentane molecule chosen at random, (ii) rotation of a randomly chosen n-pentane molecule, (iii) regrowth of a randomly chosen n-pentane molecule using the configurational-bias algorithm,16 (iv) insertion of a new n-pentane molecule in the simulation box using the configurational-bias algorithm, and (v) deletion of an n-pentane molecule chosen randomly. The intermolecular interactions between all the particles that compose the system of interest were modeled as a pairwise summation of 12-6 Lennard-Jones (LJ) potentials
[( ) ( ) ]
uLJ(rij) ) 4ij
σij rij
12
-
σij rij
6
where rij is the distance between the ith and jth interacting sites, and ij and σij are the corresponding LJ parameters. The transferable potentials for phase equilibria (TraPPE) united atom (UA) model17 was employed for the pentane-pentane interactions. In the TraPPE-UA model, each CH3 and CH2 group is represented by a single site located at the carbon atom positions. The pentane-rutile interactions were calculated as the summation of LJ potentials over all the atoms that compose the crystalline surfaces, and the cross-interaction LJ parameters were obtained by applying the Lorentz-Berthelot mixing rules using the values given by Bakaev and Steele18 as the pure LJ parameters of the Ti and O sites. The parameters are presented in Table 1. All the interactions with other sites outside a sphere of 15 Å radius around each site were neglected. For the n-pentane intramolecular interactions, a fixed bond length was assumed between the CHn-CHn groups. The bond bending and torsional potentials are respectively given by
ubend(θ) ) k(θ - θ0)2 utorsion(φ) ) c1[1 + cos(φ)] + c2[1 + cos(2φ)] + c3[1 + cos (3φ)] with the parameters listed in Table 2 (note that the intramolecular parameters have identical values for both the CH3 and CH2 groups). For all the simulations, the temperature was set at 303 K. The system was allowed to equilibrate for 3 × 106 steps, after which data were collected for another 6 × 106 steps. The simulation (16) Martin, M. G.; Siepmann, J. I. J. Phys. Chem. B 1999, 103, 4508-4517. (17) Martin, M. G.; Siepmann, J. I. J. Phys. Chem. B 1998, 102, 2569-2577. (18) Bakaev, V. A.; Steele, W. A. Langmuir 1992, 8, 1372-1378. Bakaev, V. A.; Steele, W. A. Langmuir 1992, 8, 1379-1384. Bakaev, V. A.; Steele, W. A. J. Chem. Phys. 1993, 98, 9922-9932.
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Figure 2. Experimental and simulated adsorption isotherms of n-pentane on rutile at the temperature 303 K.
Figure 3. Differential heats of adsorption, Qd, of n-pentane at 303 K on rutile.
Table 1. Parameters of Intermolecular Interactions
Table 2. Parameters of Intramolecular Interactions
Lennard-Jones parameters CH3 CH2 Ti O
/K
σ/Å
98 46 48.85 213.69
3.75 3.95 1.416 3.096
boxes were constructed with a realistic atomistic model of the surface on the base, modeling each of the three most important crystallographic faces of rutile, notably the (110), (101), and (100) faces, and a hard wall boundary at the top located at 32 Å from the base. The base area of the boxes were 45.478 × 41.426 Å for the (110) surface (7 × 14 surface units cells), 43.716 × 41.346 Å for the (101) surface (8 × 9 surface units cells), and 42.793 × 42.876 Å for the (100) surface (9 × 14 surface units cells). Periodic boundary conditions, together with the minimum image convention, were applied in the directions parallel to the surface plane.
4. Results and Discussion Figure 2 shows the experimental adsorption isotherm of n-pentane on rutile at 303 K. From the linear BET plots in the relative pressure range 0.05 e p/po e 0.2, the monolayer capacity nm ) 33.3 µmol/g, C ) 107, and the specific surface area S ) 9 m2/g were obtained by using the conventional n-pentane molecule cross-section, ω ´ C5H12 ) 0.45 nm2. This value is close to the S determined from Ar adsorption, 9.5 m2/g. Figure 3 shows the experimental differential heats of adsorption, Qd, at 303 K of n-pentane on rutile as a function of the amount adsorbed. The differential heats have three descending segments, corresponding to the adsorption of n-pentane on three types of surfaces. Qd starts at about 64 kJ/mol and decreases almost linearly to 42 kJ/mol at 23.4 µmol/g (first segment). Subsequently, Qd decreases slowly, forming two slight steps at ∼41 kJ/mol, in the interval 23.4-33 µmol/g (second segment), and at ∼37.5 kJ/ mol, in the interval 33-41 µmol/g (third segment). The surface of dehydrated rutile predominantly exhibits Lewis acidic properties. This is explained as being due to the exposure of the cus Ti4+ ions that occur in different stereochemical environments. These different environments were attributed to the exposure of a limited number of crystallographically well-
intramolecular parameters bond terms angle terms torsion terms
r0 k θ0 c1 c2 c3
1.54 Å 31 250 K/rad2 114° 355.03 K -68.19 K 791.32 K
defined cleavage faces, of which (110) is the most frequent and active among them.14,19 In order to determine the Ti4+ density on the different faces, in a previous work one of us (G.R.) performed water adsorption studies on the presently studied rutile by means of adsorption calorimetry and single-time IR spectroscopic and gravimetric measurements.12 The curve of differential heats (Qd) of water adsorption on dehydroxylated rutile (pumped out at 720 K) showed three high-energy domains corresponding to three types of mechanisms of water adsorption. The first domain from 0 to 36 µmol/g corresponds to a dissociative chemisorption with the energy changing from 95 to 85 kJ/mol. The verification of the water dissociation in this domain of filling is supported by IR spectroscopic data by an increase in the intensity of the absorption bands of the OH groups stretching vibration (νOH ) 3700, 3690, 3670 cm-1) in the interval of adsorption from 0 to 36 µmol/g. In this domain, water molecules are chemisorbed on each second cus Ti4+ ion of A rows. When water adsorbs, only 50% of the surface of the (110) faces is covered with OH groups. The remaining 50% of the (110) faces is covered with water molecules coordinated with cus Ti4+ in molecular form (second domain, next 36 µmol/g). This mechanism of adsorption proceeds with an energy of 84-67 kJ/mol. Therefore, the total amount of cus Ti4+ on (110) faces is 72 µmol/g. The coordinative mechanism is confirmed by an increase in band intensity of the deformation vibration of the coordinatively bonded water, νH2O ) 1610 cm-1, in the interval of occupancy from 36 to 72 µmol/g. A coordinative mechanism also develops with the cus Ti4+ of the (100) and (101) faces with an energy of 67-58 kJ/mol (third domain, 26 µmol/g). Similar data have been obtained with methanol and ammonia adsorption studies on the presently studied rutile.12 These quantities were the basis for the considerations of n-pentane adsorption on rutile. The distance between nearest neighbor adsorption sites on (110) is d(Ti-Ti) ) 0.297 nm, and these are arranged in parallel (19) Jones, P.; Hockey, J. A. J. Chem. Soc., Faraday Trans. 1 1972, 68, 907913.
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Figure 4. Differential entropy of adsorption, ∆S, of n-pentane at 303 K, on rutile. The entropy of the normal liquid was taken as being equal to zero.
rows separated 0.650 nm from each other by superficial oxygen ions.20 Considering the length of the n-pentane molecules and the distance between the Ti4+ sites on (110), on average, each species can cover three cus Ti4+ sites. According to this model, the number of n-pentane molecules for saturation of the (110) faces of the presently used rutile are 3 times less than water, that is, 24 µmol/g. On the (101) and (100) faces, the cus Ti4+ ions are arranged in zigzag (d(Ti-Ti) ) 0.357 nm) and linear (d(TiTi) ) 0.296 nm) rows, respectively, separated 0.485 nm from each other. The average Ti4+ distance on (101) + (100) in the ratio 1:1 is d(Ti-Ti) ) 0.326 nm. Each n-pentane molecule covers 2.76 Ti4+ sites on average. For full occupancy of these faces, it is necessary to adsorb 9.4 µmol/g. Therefore, calculated monolayer capacities are nm ) 24 + 9.4 ) 33.4 µmol/g. As can be seen, these values for the total monolayer capacity (33.4 µmol/ g) are in good agreement with the n-pentane BET nm (33.3 µmol/ g) and with the nm determined from Qd (33 µmol/g, first + second segments). The agreement between the calculated amount of n-pentane molecules for the monolayer completion of the (110) faces and the extension of the first segment at low coverage on the Qd curve means quite obviously that adsorption on this surface is restricted to the rows A of the (110) faces along the 5-fold cus Ti4+ ions. The preference of n-pentane for these sites is reasonable considering that, in these locations, the dispersive interactions with the surface are maximized. The second segment is attributed to adsorption over the rows of the Ti ions along the remaining faces exposed, (101) and (100). At higher coverage (the third segment), most likely, adsorption begins along the rows of the exposed faces between previously adsorbed molecules. For a more precise study of this domain by the adsorption calorimetric method, it will be more suitable to use n-alkanes with lower vapor pressures, for instance, n-heptane (such data will be presented in a future paper). Figure 4 shows the differential adsorption entropy, ∆S, at 303 K, of n-pentane adsorbed on the rutile sample as a function of the amount adsorbed. The reported entropy values refer to the liquid state at the measuring temperature, ∆S ≡ Sh - Sliq, where S h is the differential entropy of the adsorbed phase. These data were calculated from the corresponding measured heat values and the adsorption isotherm according to ∆S ) - (Qd - ∆vH)/T (20) Hippel, A.; Kalnajs, J.; Westphal, W. B. J. Phys. Chem. Solids 1962, 23, 779-796. .
Figure 5. Distributions of the pentane’s CH3 and CH2 groups and the center of mass as functions of the distance to the rutile surface.
- R ln(p/p0). The whole curve is below the level of the entropy of bulk n-pentane. The mean molar integral adsorption entropy of n-pentane is ca. -25 J/mol K less than the molar entropy of the bulk liquid, thus indicating the hindered state of motion of the n-pentane molecules on the surface of rutile. Due to the influence of the surface, the properties of the adsorbed molecules can be very different from those on the bulk.
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Figure 6. Adsorption isotherms at single crystallographic faces and the isotherm resulting from the combination of 70% (110) 15% (101) - 15% (100).
Molecular simulation methods provide diverse theoretical means to investigate the adsorption phenomena on a molecular scale. For instance, the plots of various atom distributions obtained in the simulations can give some insight on the properties of the adsorbed molecules. The distributions of the carbon groups and the center of mass of the adsorbed molecules relative to the surface are plotted in Figure 5. The distributions were obtained for surface coverages where the monolayer was already formed. For all surfaces, the distributions of CH3 groups are closer to the surfaces than the CH2 groups, reflecting their stronger interaction with the surfaces and the smaller value of their σ parameter for the LJ potential. For the center of mass distributions, at small distances, one peak appears, corresponding to the adsorption of n-pentane on the more energetic sites. For the (110) case, the first peak of the center of mass distribution is split. In this case, the smaller shoulder appearing later corresponds to molecules covering the protruding oxygen atoms on the surface. After a minimum, another broader peak appears, corresponding to the formation of a second layer of adsorbed molecules. Integrating the distributions up to the minima yields the monolayer coverage for each surface. For the (110), (101), and (100) surfaces, the monolayer amounts obtained were 3.70, 3.67, and 4.08 µmol/m2, respectively. Using the experimental percentages of 70% for the adsorption on the (110) face and 15% for the adsorption on each one of the (101) and (100) faces (obtained from the comparison of the Qd segment corresponding to the adsorption on the (110) face only, 23.4 µmol/g, obtained from the first segment, and the total monolayer adsorption of n-pentane, 33 µmol/g, with the (101) and (100) faces assumed to be in a 1:1 proportion) and the experimental value of the specific surface area S ) 9 m2/g, one obtains the monolayer capacity value of 33.76 µmol/g, which compares well with the monolayer capacity of 33.3 µmol/g determined experimentally.
Figure 7. Scatterplots of the CH3 and CH2 density distribution over the three most important faces of rutiles(110), (101), and (100)sat a surface coverage of 0.9.
In Figure 6, one can see the GCMC-simulated adsorption isotherms corresponding to the three most abundant faces. Each isotherm shows some remarkable differences from the others. The isotherm corresponding to the (110) face has the steepest rise in the low-pressure region. On the other hand, the (100) face exhibits the lowest coverage of the three on the whole pressure range. At higher pressures, all the isotherms eventually display
a parallel profile. This order in the isotherms’ rise at low pressures reflects the strength of the interactions between the adsorption sites and the n-pentane molecules. The sites in the A rows of the (110) faces are the strongest ones, followed by the (101) sites. The (100) sites have the weakest interactions and start being occupied when the most energetic sites in the (110) face are almost completely covered.
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Figure 8. Distribution of the tilt angles between n-pentane’s endto-end vector and the rutile surface’s normal.
For a crystalline material with a finite number of crystalline faces, the total adsorption isotherm should be written as f
nj(p) )
xini(p) ∑ i)1
where ni represents the isotherm of the ith face, xi is its contribution to the total isotherm, and the sum extends over all crystallographic faces. For the combination of the face isotherms using the equation above, we used experimental abundances of 70%, 15%, and 15% for the (110), (101), and (100) crystalline faces, respectively.
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Figure 9. Distribution of the n-pentane’s end-to-end lengths (gasphase distribution also included).
Figure 2 shows the combination of the three calculated isotherms compared with the experimental one. The comparison shows a very good agreement between both isotherms, considering that no fitting has been made, and no empirical information other than the face abundances and the surface specific area (9 m2/g) was used. In Figure 7 scatterplots of the residence probabilities of the CH3 and CH2 groups of absorbed n-pentane molecules are depicted for the three more important cleavage planes of rutile. These densities were calculated for relative pressures corresponding, in each case, to a surface coverage of ca. 90% of the monolayer capacity. It is clear from the plots that, for all the
Adsorption of Normal Pentane on Rutile
faces, the carbon groups adsorb preferably in the rows along the Ti4+ cations forming distinct lines. In particular, for the (101) face, one can even discern the zigzag-shaped rows of the Ti4+ cations. Just a small amount of CH3 and CH2 groups can be seen over the protruding oxygens. The tilt angle, defined as the angle between the end-to-end unit vector e of the n-pentane molecule and the unit vector r normal to the surface, is shown in Figure 8 for each of the rutile faces, for different surface coverages. For all the faces, the preferred orientation lies about 90°, showing that the n-pentane backbone lies parallel to the surface, maximizing the interactions with it. This preferred orientation is more evident in the sharper peak exhibited by the (110) face, owing to a stronger interaction between this face and the n-pentane. For higher coverages, the relative amount of molecules parallel with the surface decreases, reflecting the weaker interaction with the surface and the competing interaction between n-pentane neighbors. For the (100) surface, this decrease in orientation preference is particularly evident, with a very flat tilt angle distribution for a coverage just above the monolayer completion. Figure 9 shows the n-pentane molecule end-to-end distance distribution f(dee), in the different faces, at different coverages, compared with the corresponding distribution for the n-pentane in the gas phase at the same temperature. The higher peak corresponds to the trans-trans configuration where the molecule is fully extended showing a longer end-toend distance. The smaller peak corresponds to trans-gauche conformations. In all the faces, there are increased populations of trans-trans molecules, relative to the gas-phase population. Again, this preference for the fully extended trans-trans conformation is due to the interaction with the surface, since the interaction is enhanced in this conformation. For the (110) face, where the interaction with the surface is stronger, this conformation preference is more clearly depicted by the higher transtrans peak. For a small coverage in this surface, almost all molecules are planar and parallel with the surface. At increasing coverages, the intensity of the peak at 5.1 Å decreases, and an increase in the smaller peak is observed. This corresponds to an increase in the trans-gauche population at the cost of the transtrans conformation population. Also as the coverage is increased, the distribution approaches the gas-phase shape and, for the case of the (100) crystallographic face, even for small coverages, the end-to-end distance distribution is remarkably similar to the gasphase correspondent.
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5. Conclusions Adsorption microcalorimetry combined with GCMC computer simulations can be a powerful set of tools to study moleculesurface interactions, as both approaches complement some of the molecular insight that is provided by each technique. The adsorption of n-pentane on microcrystalline rutile has been studied experimentally by thermodynamic techniques (adsorption isotherms, microcalorimetry), over a wide range of coverage degrees, and complemented by GCMC simulations. The differential heats of n-pentane adsorption have a stepwise appearance, and each step quantitatively delimits an interaction of n-pentane molecules with the faces of different crystallographic indexes. n-Pentane adsorbs along the 5-fold cus Ti4+ ions of the faces. Simulations of the adsorption of n-pentane on the three most abundant crystallographic faces of rutile showed that the single cleavage plane isotherms are quite different from each other. The strength of the n-pentane interactions with each face resulted in the following order: face (110) has the strongest interactions with the n-pentane molecules, followed by face (101), while face (100) has the weakest interactions and starts being occupied when the most energetic sites in the (110) face are almost completely covered. Nevertheless, the adsorption isotherm resulting from the combination of each face’s isotherm weighted by the respective face abundance was found to compare well with the experimental isotherm. It should be noted that no fitting was made in these isotherms and that no empirical information other than the face abundances and the surface specific area was used. A structural characterization of n-pentane near the most important rutile crystallographic faces was also conducted, and it was found that the substrate, especially for the (110) face, strongly perturbs the distribution of n-pentane conformations, compared to those found for the gas phase. Adsorbed molecules are predominantly oriented with their long axes and their backbone zigzag planes parallel to the surface and are also characterized by fewer gauche conformations than observed in the bulk phase. These more ordered distributions of the molecules relative to the gas phase are reflected in the negative adsorption entropies observed experimentally. Acknowledgment. This work was performed under Project INTAS-2000, Ref No. 00-505. The authors are grateful to F. Villieras for providing the argon adsorption isotherm on rutile. LA063113Q
