8878
J. Phys. Chem. B 1997, 101, 8878-8882
Solid Monolayers Adsorbed at the Solid-Liquid Interface Studied by Incoherent Elastic Neutron Scattering Miguel A. Castro Instituto de Ciencia de Materiales de SeVilla, AVda. Americo Vespucio, SeVilla, Spain
Stuart M. Clarke* CaVendish Laboratory, Madingley Road, Cambridge, CB3 0HE, U.K.
Akira Inaba Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560, Japan
Robert K. Thomas Physical Chemistry Laboratory, South Parks Road, Oxford, U.K. ReceiVed: April 16, 1997; In Final Form: June 24, 1997X
Incoherent elastic neutron scattering has been used to demonstrate the presence of an immobile monolayer adsorbed at the interface between liquid alkanes (methane, heptane, dodecane), an alkanol (dodecanol), and a carboxylic acid (dodecanoic acid) and a graphite substrate. The derived areas per molecule in the solid layers are consistent with the molecules lying with their long axis parallel to the surface. In every case studied the layers were found to melt approximately 10% higher than the melting points of the bulk materials. Diffraction data are used to confirm the two-dimensional nature of the solid monolayer in the case of methane. In every case there is also evidence for premelting of the bulk materials as the melting points are approached from below.
Introduction It has been suggested for some time that organic molecules such as alkanes, alkanols, and carboxylic acids form solid adsorbed monolayers at a graphite surface when the bulk hydrocarbon is liquid.1,2 These molecular monolayers have an important role in many interfacial phenomena such as wetting, lubrication, and adhesion. Early work employed volumetric measurements to characterize these layers but provided no structural information. More recently, scanning tunneling microscopy (STM) and diffraction techniques have provided some structural information on these types of systems.3,4 However, due to the potential artifacts introduced by the presence of the STM probe and the transmission problems using X-ray diffraction techniques, there is still considerable debate over the state and structure of any adsorbed layers. Neutrons, as used in this work, are only weakly absorbed by matter and can provide unambiguous information on the state of any adsorbed layer as well as structural information. Isotherm work on the adsorption of pure alkanes, alkanols, and acids onto a graphite interface1,2 has indicated both monoand multilayer adsorption depending upon the system. Ordered monolayers were reported for alkanols and acids and alkanes longer than C22, but not for short-chain alkanes. Intermolecular hydrogen bonding in the acids and alkanols, which is not possible in the alkanes, was suggested as an explanation of these differences. There was some suggestion of multilayer formation, particularly for the alkanols. Some trends in behavior with chain length were also reported. For example, hexanol does not form an ordered monolayer although octanol and decanol do, with dodecanol forming a trilayer. Additionally, other workers have * Address correspondence to this author. X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5647(97)01322-9 CCC: $14.00
reported a marked odd-even variation with chain length in adsorption of the diacids.5 The difficulty of applying scattering techniques to the solid/ liquid interface is partly one of adequate penetration to observe the buried interface and partly one of distinguishing the surface layer from molecules forming the bulk liquid. This makes it desirable to reduce the quantity of bulk liquid phase to a minimum, but the question then arises as to how much material constitutes this minimum. The answer to this question is not known, but it seems reasonable in experiments on pure materials focused around the bulk melting point that the overall thickness of the layer should be at least 3-10 molecular layers. In this range of coverage and temperature most physisorbed liquids will be indistinguishably close to their bulk vapor pressure. Our interest here is purely in distinguishing the molecular layer adjacent to the surface from liquid material. This is quite different from the aims of those who study wetting phenomena, for whom the key interest is the propagation of the effects of the surface through successive layers, usually at temperatures below the triple point. Wetting has been extensively studied by many physical techniques (see, for example, several articles in ref 6 ). In this work the adsorption of three simple alkanes, methane (C1), n-heptane (C7), and n-dodecane (C12), on a graphite surface is presented. The adsorption behavior of an alkanol and a carboxylic acid, 1-dodecanol (C12) and dodecanoic acid (C12), is also described. In this manner any effects of chain length can be distinguished from those arising from the nature of the chemical group. Neutron incoherent elastic scattering is used to identify the fraction of solid adsorbate present as a function of temperature. Additionally, structural identification and characterization of the two-dimensional solid layer of adsorbed methane is made with coherent neutron diffraction. © 1997 American Chemical Society
Solid Monolayers Adsorbed at Interfaces
Figure 1. Schematic diagram of the incoherent quasielastic neutron scattering from a sample containing both solid and liquid. The broad peak is from the fluid phase, and the narrow peak is from the “solid” phase. The contribution from the broad fluid peak to the narrow elastic peak is indicated by the shaded area.
Experimental Section Two techniques, incoherent elastic neutron scattering and neutron diffraction, were used. The apparatus and procedures for the neutron diffraction experiments have been described elsewhere.7,8 The wavelength of the neutrons was 0.252 nm, and the instrument used was D1B at the Institut Laue-Langevin, Grenoble.9 Scattering from crystalline two-dimensional adsorbed layers can give rise to diffraction peaks10,11 which can be used to determine the structure of the layer in a manner analogous to diffraction from three-dimensional crystals. Diffraction from two-dimensional crystalline structures gives rise to peaks with a characteristic “saw-tooth” line shape12,13 while fluid phases give rise to very broad diffraction peaks. The diffraction pattern from adsorbed material is obtained by subtraction of the scattering from the substrate alone from that of the substrate and adsorbed material together. Full details of the technique of incoherent quasielastic neutron scattering can be found elsewhere.14 However, here we use a modification of the usual quasielastic technique which, to our knowledge, is the first application to the study of adsorbed layers. When neutrons are scattered by nuclei of a solid phase, the majority of neutrons are scattered with no exchange of energy; i.e., they are elastically scattered. If the nuclei of the sample are not “static” but are undergoing some motion, such as the translational motion of a fluid, the vast majority of the neutrons will gain or lose energy. The fraction of solid material present in the sample is therefore related to the proportion of elastically scattered neutrons. Figure 1 schematically illustrates a typical quasielastic spectrum.14 The neutrons can both gain and lose energy, and the center of the spectrum corresponds to zero energy exchange. The broad peak in Figure 1 represents neutrons that have been scattered by the fluidlike motion of nuclei in the sample. The sharper peak represents the elastically scattered neutrons from “static” nuclei in the sample. In practice, the range of energies involved in translational diffusion is such that the broad peak would be much broader than represented in the figure. In these circumstances the contribution of the broad peak to the intensity of the elastic peak, the shaded area in Figure 1, will be negligible. The width of the elastic peak is determined by the instrumental resolution. The width of the broad peak depends on the time scale of the particular motion of the molecules. For
J. Phys. Chem. B, Vol. 101, No. 44, 1997 8879 fluid phases of the hydrocarbons used in this work, with diffusion constants15.16 of the order 5 × 10 -5 cm2 s-1, the contribution under the elastic peak will be only about 1%. Under these conditions the intensity of the elastic peak is directly proportional to the amount of static material present. The intensity of the elastic peak is dominated by the switch from a solid to the translational motion accompanying melting, but there are some additional factors that reduce the elastic peak intensity, although to a much lesser extent. Thus, both vibrational motion, through the Debye-Waller factor, and rotational motion cause reductions in the elastic peak intensity. However, these, although significant, are small in comparison to the effect of the onset of translational motion. For example, methane is the only one of the molecules studied here that has a rotator phase in the bulk. The onset of rotational diffusion in solid methane at 35 K is below all temperatures used in this study and will therefore not be expected to give rise to significant effects in this work. Rotational diffusion would not be anticipated in the larger molecules studied. The narrow energy resolution required for these experiments was obtained with the high-resolution backscattering instrument IN10 at the Institut Laue-Langevin, Grenoble,9 with an energy resolution of approximately (1.5 µeV. The incident wavelength was 0.6275 nm with both monochromator and analyzer crystals of Si(111). The Doppler drive was turned off such that only elastically scattered neutrons reached the detectors. The detectors were placed at Q values of 5.97, 8.62, 14.16, and 16.40 nm-1 to avoid coherent scattering features, small-angle scattering and diffraction peaks, from the samples. The momentum transfer, Q, is defined as (4π sin θ)/λ, with λ the wavelength of the neutrons and θ half the scattering angle. In the results that follow only data from the detector at Q ) 16.40 nm-1 will be presented; the data from all of the detectors were similar. Two adsorbents were used in these experiments, both of which were recompressed exfoliated graphites, Grafoil MAT (Union Carbide) and Papyex (Le Carbone Lorraine), whose specific surface areas of 24 and 20 m2 g-1, respectively, were determined by adsorption isotherm measurements using nitrogen.17 The hydrocarbons used were methane, n-heptane, n-dodecane, 1-dodecanol, and dodecanoic acid obtained from Aldrich. For the neutron diffraction experiments the methane was deuterated to reduce the background from incoherent scattering of protonated samples. Deuterated methane was obtained from Merck, Sharp and Dohme. For the incoherent elastic experiments the incoherent scattering of protonated samples is an advantage because it increases the contribution from the adsorbed layer relative to the substrate. The graphite substrates were outgassed under vacuum in an oven before known quantities of the adsorbates were added, either directly from the vapor phase (methane) or under an inert atmosphere (He) drop by drop using a microsyringe (heptane, dodecane, and dodecanol) or addition of a known mass of solid (dodecanoic acid). The accuracy in the absolute coverage for methane was about 2% and of the nonvolatile materials about 10%. The graphite samples were contained in aluminum cans with which they were in excellent thermal contact, inside a helium/nitrogen cryostat. The accuracy of the temperature settings of the cryostat was (0.1 K, and previous tests on similar samples indicate that there is no temperature gradient in the sample can. All samples were annealed well above their bulk melting temperature, where the significant vapor pressure and the uniformity of sample temperature ensure that equilibrium is attained. They were then cooled to below their bulk melting point.
8880 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Figure 2. Incoherent elastic scattering as a function of temperature for approximately 5.5 monolayers of methane adsorbed on graphite MAT. The vertical line indicates the bulk melting point and the horizontal dashed line the background level.
Castro et al.
Figure 3. Incoherent elastic scattering as a function of temperature for 2.4 mmol of n-heptane adsorbed on Papyex. The vertical line indicates the bulk melting point and the horizontal dashed line the background level.
Results A. Incoherent Elastic Neutron Scattering. Figure 2 presents the intensity of the elastic peak as a function of temperature for 7.58 mmol of methane, CH4, adsorbed on 4.66 g of Grafoil MAT. This corresponds to 5-6 monolayers. At the lowest temperatures in the figure all the methane is solid, giving a strong signal. As the bulk melting point (90 K) is approached, the intensity of the elastic peak begins to fall, falling particularly rapidly over the range 70-90 K. However, just above the bulk melting point not all the elastic intensity has disappeared. The signal reaches the background at about 100 K. The onset of rotational diffusion in bulk methane is below the lowest temperature in Figure 2, and therefore none of the decline in intensity below 90 K can be attributed to rotational motion. Similarly, the effect of the known Debye-Waller factor18 can only account for about 10% of the decrease in intensity. This strongly suggests that the main cause of the intensity decrease is the onset of translational motion and that above 90 K there is a fraction of solidlike material remaining. An estimate of the amount of solid at just above the bulk melting point can be made from the ratios of intensities of the elastic scattering at this temperature and at the lowest temperature where all the methane is solid. The amount of solid methane just above the bulk melting point is determined to be about 1.1 mmol. Considering the assumptions above concerning the motion of the methane molecules, this is remarkably close to the value expected for a single monolayer.19 This material survives up to a temperature of approximately 98 K, and then it too melts with the scattering falling to the level of the graphite background. These features are reproduced at a higher coverage of about 7.5 equivalent monolayers with the same absolute amount of residual methane present above the bulk melting point. Similar data for the other larger molecules adsorbed onto approximately 3.24 g of Papyex are presented in Figures 3, 4, 5, and 6 for n-heptane, n-dodecane, 1-dodecanol, and dodecanoic acid, respectively. Details of the quantities of each species in the sample cell are given in Table 1. The elastic scattering intensity in all these figures exhibits features similar to those of methane in Figure 2 with all the material being solid at the lowest temperatures, loss of solid material as the bulk melting point is approached from below, with a fraction of solid material remaining above the bulk melting point and final melting of this residual fraction. These results are highly suggestive of a monolayer of immobile material persisting above the bulk
Figure 4. Incoherent elastic scattering as a function of temperature for 0.674 mmol of n-dodecane adsorbed on Papyex. The vertical line indicates the bulk melting point and the horizontal dashed line the background level.
Figure 5. Incoherent elastic scattering as a function of temperature for 0.429 mmol of 1-dodecanol adsorbed on Papyex. The vertical line indicates the bulk melting point and the horizontal dashed line the background level.
melting point in every case. For this reason we present these residual amounts in terms of an area per molecule, and the values of these, just above the bulk melting point and estimated in the same way as for methane, are given in Table 1. Table 2 gives the values of the bulk melting points of the materials and of the melting points of the adsorbed monolayers, the melting
Solid Monolayers Adsorbed at Interfaces
J. Phys. Chem. B, Vol. 101, No. 44, 1997 8881
Figure 6. Incoherent elastic scattering as a function of temperature for 0.4 mmol of dodecanoic acid adsorbed on Papyex. The vertical line indicates the bulk melting point and the horizontal dashed line the background level.
TABLE 1: Quantities of the Different Materials in the Sample Cells for the Incoherent Elastic Scattering Experiments adsorbate
amt adsorbed (mmol)
equiv monolayers
7.58 2.40 0.674 0.429 0.40
5.5 10 5 3 3
methane n-heptane n-dodecane 1-dodecanol dodecanoic acid
area per molecule (nm2) expta Groszekb 0.14 0.46 0.75 0.78 0.86
Figure 7. Experimentally determined neutron diffraction pattern of methane adsorbed on Grafoil MAT. The coverage is 5.5 monolayers, wavelength 0.252 nm, and the temperature 92 K. The solid lines are fits to the data based on a simple model of coexisting fluid and twodimensional solid. The structural parameters obtained in the fit are discussed in the text.
the 2-D commensurate phase, observed at low coverages (0.426 nm).19 The area per molecule is calculated to be 0.149 nm2, which compares well with the estimate determined from the incoherent elastic scattering of 0.143 nm2. The broad peak characteristic of fluid was also found to persist over a significant temperature range (several degrees) below the bulk melting point, depending on coverage. Discussion
0.472 0.734 0.786
a Experimentally determined areas per molecule. b Predicted areas per molecule based on the model of Groszek.
TABLE 2: Melting Points of Bulk and Adsorbed Monolayers of the Materials Determined by Incoherent Elastic Neutron Scattering adsorbate
T3D (K)
T2D (K)
T2D/T3D
methane n-heptane n-dodecane 1-dodecanol dodecanoic acid
90 183 263 297 317
98 206 285 335 342
1.09 1.12 1.08 1.13 1.08
point here being defined as the temperature at which the elastic peak disappears into the background. B. Neutron Diffraction. The neutron diffraction pattern from the same relative amount of CD4 on graphite MAT as used for Figure 2, i.e., about 5.5 monolayers, at 92 K is shown in Figure 7. The graphite background has already been subtracted, and the feature at θ ) 22° arises from imperfect graphite subtraction. There are two main features from the methane: a broad peak, characteristic of fluid, and the typical saw-tooth line shape, characteristic of diffraction from an adsorbed monolayer.12,13 The presence of the latter feature confirms the two-dimensional nature of the “residual solid material” identified in the incoherent elastic measurements above. A quantitative fit of the data to the 2-D peak and to a simple approximation of a Gaussian peak for the fluid is shown by the solid lines in the figure. It is worth noting that this diffraction pattern was accurately calibrated with the positions of the (111) and (200) reflections of bulk methane, evident at lower temperatures and higher coverages, with d spacings of 0.344 and 0.298 nm, respectively. The experimentally determined position of the two-dimensional reflection corresponds to an intermolecular spacing in the layer of 0.415 nm, which is about 1% more compressed than in bulk methane (0.421 nm) and nearly 3% more compressed than in
Given the discussion of rotational motion above, the incoherent elastic scattering data for methane clearly indicate premelting as the bulk melting point is approached from below. None of the other alkanes used here are known to have a rotator phase in the bulk. Thus, for these materials also, the falloff must be associated with premelting, where we explicitly identify premelting with the onset of translational motion. Premelting of the surfaces of crystals of bulk lead has been observed previously.20 Similar considerations may apply here where there are many tiny crystals of hydrocarbon with a large surface-tovolume ratio. Without either further information or assumptions, it is not possible to determine accurately the fraction of fluid material at any temperature below the bulk melting. All three alkanes form a solid monolayer above their bulk melting points. The diffraction evidence for methane indicates that this is a crystalline monolayer coexisting with fluid. The area per molecule (0.145 nm2) determined from both the incoherent and diffraction measurements indicates an incommensurate monolayer of methane compressed by about 1% with respect to the most dense plane of bulk methane. In his model Groszek21 estimated the area per molecule of hydrocarbons adsorbed on graphite using commensurate structures with each CH2 group occupying a graphite hexagon (0.0524 nm2) and CH3, Br, and OH groups occupying two hexagons. Table 1 gives the predictions of the surface areas of heptane, dodecane, and dodecanol calculated on the basis of Groszek’s model, and they are in good agreement with the experimentally determined estimates of the surface areas adopted by n-heptane and n-dodecane molecules (0.46 and 0.75 ( 0.10 nm2). These values strongly suggest the adsorbed hydrocarbons are lying with the long axis of the molecule parallel to the surface. The uncertainty in our derived areas per molecule is not sufficient to distinguish between commensurate and incommensurate layers for heptane and dodecane. The solid monolayers of all three alkanes melt at approximately 1.1 times their bulk melting point. It is usually the case that adsorbed monolayers with a coverage at or less than unity melt below their bulk melting point,7 although there are a few exceptions,22 but solid monolayers that exist above
8882 J. Phys. Chem. B, Vol. 101, No. 44, 1997 the melting point of the bulk hydrocarbon have been reported.1,2,23,24 The results of this work, which indicate that all three alkanes, C1, C7, and C12, form solid monolayers, are in marked contrast to the work of Findenegg1,2 where isotherm measurements with closely related alkanes showed no evidence of ordered monolayers, although longer chain alkanes >C22 were found to form ordered monolayers. However, in comparison with incoherent elastic scattering, the sensitivity of isotherm measurements is expected to be low. The incoherent elastic scattering from dodecanol and dodecanoic acid indicates that these materials exhibit premelting similar to that of the alkanes discussed above. There is evidence of solid monolayers above the bulk melting points in both cases. The estimated surface area per molecule for dodecanoic acid (0.86 nm2) is in reasonable agreement with other workers25,26 (0.79 and 0.80 nm2), although they studied dodecanoic acid adsorbed from solution. The values confirm that adsorption is in the form of a monolayer and are also consistent with the molecules lying with their long axes parallel to the graphite surface. Recent STM measurements of dodecanol adsorbed on graphite3 report two solid monolayer phases above the bulk melting point (297 K), with the solid layer finally melting at 333 K, in reasonable agreement with the temperature by which all the solid dodecanol has disappeared in this work (340 K). This STM work also indicates that the molecules are lying with their long axes parallel to the surface. The estimate of the surface area per molecule in the present work (0.78 nm2) is in good agreement with that expected for dodecanol molecules on the surface based on the model of Groszek21 (0.786 nm2). The transition from the solid layer to a fluid is generally quite sharp, particularly for dodecanoic acid. However, from the data shown in the figures and that obtained at the three other angles the transition for dodecanol appears to be very gradual. We do not know the reason for this behavior, but it suggests that the monolayer of dodecanol differs in some respects from the others. Findenegg1,2 reported the formation of an adsorbed solid trilayer of dodecanol. However, no evidence for such a trilayer was found in the incoherent scattering data presented here. Experiments with approximately 3 monolayers (data given in Figure 5) and 10 monolayers (not shown) of dodecanol both show the same behavior with at most a solid monolayer above the bulk melting point. Conclusions Incoherent elastic neutron scattering has been shown to be a sensitive means for probing immobile layers adsorbed at the solid surface in the presence of significant amounts of liquid. The experiment is fairly quick to perform relative to conventional quasielastic scattering measurements and, although not fully quantitative at this stage, could be developed further to give more detailed structural information. Liquid alkanes with different chain lengths, C1, C7, and C12, are all found to adsorb as solid monolayers on graphite. In
Castro et al. every case studied the melting point of the layer is approximately 1.1 times the bulk melting point, and there is evidence of premelting of bulk material below the bulk melting point. The molecules in the monolayers of the C7 and C12 alkanes are probably lying with their long axes parallel to the surface of the substrate. In the case of methane the molecules in the monolayer are incommensurate with respect to the underlying graphite and slightly compressed relative to the bulk material. Although there is reasonable agreement between the area per molecule for the C7 and C12 alkanes with the commensurate model of Groszek, the incoherent scattering data are not sufficiently accurate to establish that the layers are commensurate or incommensurate. The alkane, alkanol, and carboxylic acid with the same carbon skeleton all form solid layers above the bulk melting points. The areas of each of the molecules in the monolayers suggest that the molecules are lying with their long axes parallel to the surface of the substrate. Acknowledgment. The authors thank EPSRC (S.M.C.), the spanish DGICYT, PB94-1426, and The Yamada Science Foundation (A.I.) for financial support and Dr. O. Randl at The Institut Laue-Langevin for technical assistance. References and Notes (1) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1799. (2) Findenegg, G. H. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1069. (3) Yeo, Y. H.; McGonigal, G. C.; Thompson, D. J. Langmuir 1993, 9, 649. (4) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (5) Wright, E. H. M. J. Chem. Soc. B 1996, 355. (6) Hess, G. In Phase Transitions in Surface Films; Taub, H., Torzo, G., Lauter, H. J., Eds.; Plenum Press: New York, 1991. (7) Clarke, S. M. D. Phil. Thesis, University of Oxford, 1986. (8) Bucknall, R. A.; Clarke, S. M.; Shapton, R. A.; Thomas, R. K. Mol. Phys. 1989, 67, 439. (9) Neutron Research Facilities at the ILL High Flux Reactor, Institut Laue-Langevin, Grenoble, France. (10) Clarke, S. M.; Thomas, R. K. Mol. Phys. 1991, 72, 413. (11) Knorr, K. Phys. Rep. 1992, 214, 113. (12) Warren, B. E. Phys. ReV. 1941, 59, 693. (13) Kjems, J. K.; Passell, L.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. ReV. 1976, B13, 1446. (14) Bee, M. Quasielastic Neutron Scattering; Adam Hilger: Bristol, 1988. (15) Coulomb, J.-P.; Bienfait, M.; Thorel, P. Faraday Discuss. Chem. Soc. 1985, 80, 79. (16) Coulomb, J.-P.; Bienfait, M.; Thorel, P. J. Phys. (Paris) 1981, 42, 293. (17) Inaba, I.; Koga, Y.; Morrison, J. A. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1635. (18) Press, W. J. Chem. Phys. 1972, 56, 2597. (19) Vora, P.; Sinha, S. K.; Crawford, R. K. Phys. ReV. Lett. 1979, 43, 704. (20) Kristensen, J. K.; Cotterill, R. M. J. Philos. Mag. 1977, 36, 437. (21) Groszek, A. J. Proc. R. Soc. London 1970, A314, 473. (22) Morishige, K.; Kawai, N.; Shimizu, M. Phys. ReV. Lett. 1993, 70, 3904. (23) Herwig, K. W.; Trouw, F. R. Phys. ReV. Lett. 1992, 69, 89. (24) Bienfait, M.; Gay, J. M. Surf. Sci. 1988, 204, 331. (25) Liphard, M.; Glanz, P.; Pilarski, G.; Findenegg, G. H. Prog. Colloid Polym. Sci. 1980, 67, 131. (26) Kipling, J. J.; Wright, E. H. M. J. Chem. Soc. 1963, 3382.
