4010
Langmuir 2002, 18, 4010-4013
Mixing Behavior at the Solid/Liquid Interface: Binary Monolayers of Linear Alcohols Adsorbed on Graphite Loic Messe and Stuart M. Clarke* BP Institiute and Department of Chemistry, University of Cambridge, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, U.K.
Thomas Arnold, Chucuan Dong, and Robert K. Thomas Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, U.K.
Akira Inaba Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560, Japan Received December 31, 2001. In Final Form: February 15, 2002 Differential scanning calorimetry combined with neutron diffraction has been used to characterize the mixing behavior of crystalline monolayers of binary mixtures of linear alcohols adsorbed from solution to a graphite surface. The results indicate that for pairs of alcohols with alkyl chain lengths of C5- C11 and which differ in length by two or more methylene groups there is phase separation in the mixed monolayers. These results are shown to be in agreement with a recent quantitative model of phase separation in two-dimensional physisorbed monolayers.
Introduction Adsorption of solid monolayers from multicomponent solutions is of great academic and industrial importance. This is because many systems of interest are mixtures either through design, to incorporate a synergy of action of two components, or because pure compounds are too expensive. In both cases it is clearly essential to understand how the two or more components behave on the surface; in particular, do they mix or phase separate (eutectic behavior)? However, although important, the study of such “buried” monolayers is technically extremely difficult largely due to the presence of the two bulk phases either side of the monolayer of interest. Only recently have techniques sensitive enough to probe these adsorbed layers become available.9,17 * Address correspondence to this author. (1) Castro, M.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. Phys. Chem. Chem. Phys. 1999, 1, 5017-5023. (2) Castro, M.; Clarke, S. M.; Inaba, A.; Thomas, R. K.; Arnold, T. Phys. Chem. Chem. Phys. 2001, 3, 3774-3777. (3) Castro, M. A.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. J. Phys. Chem. 1998, B102, 10528-10534. (4) Castro, M. A.; Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. Phys. Chem. Chem. Phys. 1999, 1, 5203-5207. (5) Castro, M. A.; Clarke, S. M.; Inaba, A.; Thomas, R. K. Physica B 1998, 241-243, 1086-1088. (6) Castro, M. A.; Clarke, S. M.; Inaba, A.; Thomas, R. K.; Arnold, T. J. Phys. Chem. 2001, 105, 8577-8582. (7) Clarke, S. M.; Inaba, A.; Arnold, T.; Thomas, R. K. J. Therm. Anal. Calorim. 1999, 57, 641-651. (8) Clarke, S. M.; Messe, L.; Whitehead, C.; Inaba, A.; Thomas, R. K.; Arnold, T. Linear alcohols adsorbed on graphite from the liquid. Presented at the ICNS 2001, Munchen, Germany, 2001. (9) Gilbert, E. P.; Reynolds, P. A.; Thiyagarajan, P.; Wozniak, D. G.; White, J. W. Phys. Chem. Chem. Phys. 1999, 1, 2715-2724. (10) Gilbert, E. P.; Reynolds, P. A.; White, J. W. Colloids Surf., A 1998, 141, 81-100. (11) Groszek, A. J. Proc. R. Soc. London 1970, A314, 473. (12) Inaba, A.; Clarke, S. M.; Arnold, T.; Thomas, R. K. Chem. Phys. Lett. 2001, in press.
It has been demonstrated that simple linear alkanes adsorbed to a graphite surface may show a rich variety of phase behavior.2,4,6,9,10 Although complex, it has been possible to identify the some of the key factors that determine the mixing behavior. For example, if the space groups of the monolayers of the two pure components are the same, then mixing, rather than phase separation, is possible.6,12 However, even if the space groups are the same, the unit cells must be quantitatively similar, as expressed in terms of a monolayer “isomorphism” coefficient,6,18 mi,
mi ) 1 -
∆m Γm
where ∆m is the area due to the nonoverlapping parts (when the two unit cells are positioned so as to achieve maximum overlap) and Γm is the area due to the overlapping parts. For a pair of alkanes mi is expected to be approximately 0.88 or above to be completely miscible. Recently, it was reported that all the simple linear alcohols with alkyl chain lengths between C5 and C20 form solid monolayers when adsorbed at submonolayer coverages or from the liquid to a graphite surface.8,14-16 In this work we present a combination of calorimetry and neutron diffraction data on the phase behavior of solid monolayer formed binary mixtures of simple linear alcohols adsorbed on graphite. Here we investigate (13) Kjems, J. K.; Passell, L.; Taub, H.; Dash, J. G.; Novaco, A. D. Phys. Rev. 1976, B13, 1446. (14) Morishige, K.; Kato, T. J. Chem. Phys. 1999, 111, 7095-7102. (15) Morishige, K.; Sakamoto, Y. J. Chem. Phys. 1995, 103, 2354. (16) Morishige, K.; Takami, Y.; Yokota, Y. Phys. Rev. 1993, B48, 8277-8281. (17) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (18) Rajabalee, F.; Espeau, P.; Haget, Y. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 269, 165-173.
10.1021/la0118762 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/19/2002
Mixing Behavior at the Solid/Liquid Interface
Langmuir, Vol. 18, No. 10, 2002 4011
combinations of all the linear alcohols from C5 to C11, which are liquids at room temperature. The calorimetry data provide a convenient means of sweeping the phase diagrams of these mixed adsorbed layers and indicates the phase behavior in the layers. Neutron diffraction data are used to support the conclusions drawn from the calorimetry results. These results are discussed in terms of the quantitative model of mixing in two dimensions outlined above. Experimental Section Calorimetry. A full description of use of differential scanning calorimetry (DSC) for the study of phase behavior in adsorbed layers has been given elsewhere.7 The DSC measurements were performed on two Pyris 1 Power compensation systems at the BP Institute and the Polymers and Colloids Group, Cavendish Laboratory, both at the University of Cambridge, as discussed previously. The minimum temperatures accessible with these instruments were -130 and -65 °C, respectively. The heating rate used was 10 °C/min.3 We have demonstrated previously that the composition dependence of the monolayer melting transition is an excellent indicator of surface phase behavior.1 The melting temperatures of first-order phase transitions from normal bulk materials are usually obtained from the extrapolated “onset” temperature of the peak. This extrapolated onset approach has been used here to extract the melting temperature of the adsorbed layers. However, it must be noted that melting in adsorbed monolayers may not be first order and the peak maximium position may be a more appropriate measure. For the monolayers discussed here the peak maximium position is simply displaced by a reasonably constant amount, 2 °C, to higher temperature from the onset temperature. Neutron Diffraction. The apparatus and procedures for such experiments have been described elsewhere.5 The instrument, OSIRIS, at the ISIS Spallation Source, Rutherford Appleton Laboratory, Oxford, U.K., was used for the neutron diffraction measurements. This is a high-resolution backscattering device. Scattering from crystalline two-dimensional adsorbed layers gives rise to diffraction peaks13,19 with a characteristic saw-tooth line shape. At the higher coverages of interest here there is the solid monolayer coexisting with the bulk fluid phase of the adsorbate. The sharp peaks of the adsorbed layer are readily distinguished from the very broad peak characteristic of the fluid phase. For the neutron experiments deuterated alcohols were used to minimize the incoherent scattering background that would have arisen from protonated samples. To optimize the sensitivity to the scattering from the adsorbed monolayers the amount of bulk fluid was kept to a minimium. The mixing behavior in an adsorbed layer is evident from the composition dependence of the diffraction patterns. If the two components do not mix (e.g. eutectic behavior), then the diffraction pattern of the mixture is simply a sum of diffraction patterns from both pure compounds.6 If the materials mix on the surface, then new peaks are evident in the diffraction pattern. In the experiments presented here, our focus is on obtaining the mixing behavior of the adsorbed layers and the instrument OSIRIS is ideal providing diffraction patterns from the adsorbed layers with high resolution but only over a restricted q range. The detailed crystallographic structures for some of the pure adsorbed alcohols and for some adsorbed from solution are presented elsewhere.14-16 The adsorbent used was recompressed exfoliated graphite Papyex (Le Carbone Lorraine) characterized by adsorption of nitrogen and found to have a specific surface area of 31.6 m2 g-1. The deuterated alcohols were prepared by us by isotope exchange of the corresponding carboxylic acid and reduction to the alcohol. The deuteration levels are approximately >98%. The graphite substrates were outgassed under vacuum in an oven at 350 °C before known quantities of alcohol mixtures were added as liquid by microsyringe. When dosing the graphite, it is convenient to know the approximate number of equivalent monolayers adsorbed. This can be estimated from the areas/molecule, taken from the work (19) Warren, B. E. Phys. Rev. 1941, 59, 693.
Figure 1. Experimental DSC thermogram from a mixture of pentanol and hexanol on graphite at a mole fraction of hexanol of 0.88 and a total coverage of approximately 40 layers. of Groszek,11 and the specific surface area of the graphite. In the model of Groszek, each CH2 unit of the alkyl chain is taken to occupy one graphite hexagon (0.524 nm2). Chain end methyls, CH3, and hydroxyl groups are estimated to occupy 2 graphite hexagons (1.048 nm2). In this way the area/molecule can be estimated. From the specific surface area of the graphite adsorbate, determined by nitrogen adsorption, we can now estimate the number of molecules required to form a complete, “equivalent” monolayer. Any quantity of adsorbate added to the substrate can now be expressed in terms of the number of equivalent monolayers added.
Results Calorimetry. Figure 1 presents the experimental DSC thermogram from the binary mixture of pentanol (C5OH) and hexanol (C6OH) on graphite at a mole fraction of hexanol of 0.88. These alcohols differ in alkyl chain length by two carbon atoms, ∆n ) 2. The large peak at a temperature of approximately -55 °C also found in the thermogram of the alcohol mixture in the absence of graphite is therefore assigned to a transition of the “bulk” material. Significantly, however, at the higher temperatures of -3 °C, well above the bulk melting point, Figure 1 shows an additional peak that is not evident in the thermogram either from the mixture without the graphite or from the graphite alone. This peak is therefore attributed to the adsorbed monolayer, as described previously for the pure alcohol and alkane monolayers.1,7 Figure 2 presents the composition dependence of the monolayer melting peaks identified in the DSC thermograms of the binary mixtures of alcohols differing by 2 methylene groups from C5 to C11. The composition dependences of the monolayer melting peaks are very similar for all these combinations showing a pronounced and sharp minimium indicative of eutectic phase separation, as discussed previously.1 As discussed previously, the very weak eutectic invariant is not observed with the DSC technique but can be seen with the more sensitive method of adiabatic calorimetry.1,2 Figure 3 presents the composition dependence of the monolayer melting points for combinations of alcohols which differ by 3, 4, and 5 methylene groups. All of these figures again show the pronounced minimum indicative of phase separation in the adsorbed layer. Neutron Diffraction. Figure 4 presents neutron diffraction data from (top) pure heptanol on graphite, (bottom) pure nonanol on graphite, and (middle) a mixed monolayer of heptanol and nonanol on graphite. The bulk
4012
Langmuir, Vol. 18, No. 10, 2002
Messe et al.
Figure 2. Phase diagrams from binary mixtures of alcohols which differ by two methylene groups, ∆n ) 2: (9) hexanol/ octanol; (0) heptanol/nonanol; (b) octanol/decanol; (O) nonanol/ undecanol. Total coverage in all cases is approximately 40 layers. Figure 4. Experimental neutron diffraction patterns from monolayers of (top) pure heptanol, (bottom) pure nonanol, and (c) a binary mixture of heptanol and nonanol. See main text for composition. All patterns are taken at temperatures where the bulk adsorbate is liquid.
Figure 3. Phase diagrams from binary mixtures of alcohols which differ by (b) three methylene groups (hexanol/nonanol), (O) four methylene groups (hexanol/decanol), and (9) five methylene groups (hexanol/undecanol). Total coverage in all cases is approximately 40 layers.
mixture composition, Xnonanol ) 0.1, corresponds to the monolayer eutectic composition in the 2D phase diagram determined by DSC. The total coverage was approximately 5 equivalent monolayers for all the patterns. All these neutron patterns were collected at temperatures above the bulk melting point and below the monolayer melting point. The peaks in these figures are therefore the diffraction from the crystalline adsorbed monolayers and have the characteristic asymmetric line shape expected from two-dimensional layers. The pattern from the pure heptanol on graphite shows three peaks in this region at d spacings of approximately 4.0, 4.45, and 4.78 Å. These data clearly support the conculsion that this alcohol forms a solid adsorbed layer above the bulk melting point. Similarly the diffraction pattern from the pure nonanol of graphite shows three peaks at 4.0, 4.25, and 4.58 Å again indicating the presence of a solid monolayer coexisting with the bulk liquid. The pattern from the mixture also contains sharp peaks indicating that there is a solid monolayer formed from the mixture. The peak positions in the mixed layer are
the same as those of both the pure heptanol and pure nonanol (as indicated by the vertcal lines in the figure). This result clearly supports the conclusion that these two alcohols that differ by 2 methylene groups phase separate on the graphite surface. This is in excellent agreement with the calorimetry data given above. Some adsorbed alkane mixtures show some limited solubility at the extremes of composition. However, the diffraction patterns presented here indicate that these two alcohols, heptanol and nonanol, have essentially no mutual solubility at all. In this work we focus on the mixing behavior of binary alcohol mixtures. Detailed analysis of the relative intensities of these patterns to determine the full crystallographic structure of the adsorbed layers is impossible with the limited q range of data available with OSIRIS. For these types of monolayers the domain size, which may be estimated from the peak width, is approximately 20 nm. Because the widths of peaks in the pure monolayers and the binary mixtures are not significantly different, we conclude that the patch size in the mixtures is associated with surface defects. Recent work has used time-resolved small angle scattering to look at the kinetics of phase separation in adsorbed alkane layers.9 It is this region of scattering that may be able to probe the length scale of the phase-separated domains. Discussion and Conclusions The major conclusion of this work is that binary mixtures of simple linear alcohols which differ in alkyl chain length by 2 or more methylene groups form solid adsorbed monolayers above the melting point of the bulk mixtures and that the two different alcohols do not mix to form a single phase on the surface but phase separate into patches of essentially pure components. In assessing these conclusions, we now consider the closely related systems of binary mixtures of adsorbed alkanes. As discussed above, similar phase separation in mixed alkane monolayers could be understood in terms of the plane groups of the layers and the isomorphism
Mixing Behavior at the Solid/Liquid Interface
Langmuir, Vol. 18, No. 10, 2002 4013
Table 1. Calculated Isomorphism Coefficient for the Binary Alcohol Mixtures with ∆n ) 2 Used in This Studya mixture
isomorphism coeff
C5OH/C7OH C6OH/C8OH C7OH/C9OH
0.70 0.74 0.77
mixture
isomorphism coeff
C8OH/C10OH C9OH/C11OH
0.79 0.81
a Mixtures with isomorphism coefficient values of 0.88 or more are expected to mix.
coefficient, representing the quantitative similarity of the sizes of the unit cells of the pure monolayers. To make a similar calculation for the alcohols we use the data of Morishige and others,14-16 who have presented some structural analysis of adsorbed alcohol monolayers at both low and high coverages on the basis of X-ray diffraction. In using these data for C10, C12, and C18, we observe that the b axis remains approximately constant at 0.5 nm while the a axis increases by 0.55 nm as the alkyl chain
length is increased by two methylene groups, ∆n ) 2. The estimated isomorphism coefficients for the alcohol mixtures of this study are given in Table 1. We see that, in excellent agreement with our DSC and neutron diffraction results, all the isomorphism coefficients for the alcohol mixtures of this study are below 0.88 and are therefore not expected to mix in the adsorbed monolayers, supporting the reasonableness of our conclusions. The experimentally observed phase behavior is therefore consistent with the same isomorphism model that applies to adsorbed binary mixtures of alkanes. Acknowledgment. The authors thank the U.K. EPSRC (T.A.), the Spanish DGICYT, the Leverhulme Trust (L.M.), and the Yamada Science Foundation (A.I.) for financial support and the staff and scientists at ISIS for beam time and technical assistance. LA0118762
