Air Interfaces: Molecular Ordering

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Long Chain n-Alkanes at SiO2/Air Interfaces: Molecular Ordering, Annealing, and Surface Freezing of Triacontane in the Case of Excess and Submonolayer Coverage H. Schollmeyer,† B. Struth,‡ and H. Riegler*,† Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Am Mu¨ hlenberg, D-14476 Golm/Potsdam, Germany, and ESRF, Av. des Martyrs - B. P. 220, 38043 Grenoble Cedex 09, France Received December 12, 2002. In Final Form: April 2, 2003 We present a comprehensive study on the interfacial molecular ordering of an n-alkane, triacontane, at the SiO2/air interface, for submonolayer and excess coverage. The molecular ordering was studied by X-ray diffraction and reflectivity at temperatures from far below bulk melting to above the surface freezing temperature. It is found that the phase behavior of bulk and that of interfacial triacontane are quite different. From the literature it is known that bulk triacontane has three solid phases: one crystalline phase (monoclinic) and two rotator phases (RIII and RIV) with solid/solid transitions at ≈61 °C and close to the melting point into the liquid phase (≈67 °C), respectively. All solid bulk phases have inclined molecules. We show that interfacial triacontane has only two ordered solid phases: a crystalline phase (orthorhombic) and a rotator phase. In both phases the molecules are oriented normal to the interface. At the interface, the temperature of the transition from the crystalline phase to the rotator phase can be as low as 40 °C. For both submonolayer and excess coverage, the interfacial rotator phase can persist up to ≈70 °C. This means that above 67 °C, for excess coverage, the solid rotator phase coexists with liquid bulk triacontane (“surface freezing”). The different phase behavior in bulk and at the interface is explained with different interlayer interactions. We also find that, directly after solidification, the interfacial layer is usually in a highly amorphous, nonequilibrium state. This we attribute to rapid solidification because of the efficient cooling at the interface. Depending on thermal treatment and time, the interfacial layer can anneal to various degrees of molecular ordering, which is reflected in the phase behavior.

1. Introduction Long chain hydrocarbons are a main constituent of many molecules with industrial and biological relevance. Therefore, it is important to understand their properties and phase behavior. Especially the interfacial molecular ordering of long chain n-alkanes has been the subject of many experimental1-21 and theoretical22-30 studies, last † ‡

Max-Planck-Institut fu¨r Kolloid- und Grenzfla¨chenforschung. ESRF.

(1) Earnshaw, J. C.; Hughes, C. J. Phys. Rev. A 1992, 46, R4494. (2) Wu, X. Z.; et al. Phys. Rev. Lett. 1993, 70, 958. (3) Wu, X. Z.; et al. Science 1993, 261, 1018. (4) Wu, X. Z.; et al. Physica A 1993, 200, 751. (5) Sefler, G. A.; et al. Chem. Phys. Lett. 1995, 235, 347. (6) Wu, X. Z.; et al. Phys. Rev. Lett. 1995, 75, 1332. (7) Pfohl, T.; Beaglehole, D.; Riegler, H. Chem. Phys. Lett. 1996, 260, 82. (8) Ocko, B. M.; et al. Phys. Rev. E 1997, 55, 3164. (9) Hayami, Y.; Findenegg, G. H. Langmuir 1997, 13, 4865. (10) Doerr, A. H.; et al. Colloids Surf., A: Physicochem. Eng. Aspects 1997, 128, 63. (11) Merkl, C.; Pfohl, T.; Riegler, H. Phys. Rev. Lett. 1997, 79, 4625. (12) Gang, H.; et al. Europhys. Lett. 1997, 43, 314. (13) Maeda, N.; Christenson, H. K. Colloids Surf., A: Physicochem. Eng. Aspects 1999, 159, 135. (14) Yamamoto, Y.; et al. Chem. Phys. Lett. 1999, 304, 231. (15) Holzwarth, A.; Leporatti, S.; Riegler, H. Europhys. Lett. 2000, 52, 653. (16) Colussi, A. J.; Hoffmann, M. R.; Tang, Y. Langmuir 2000, 16, 5213. (17) Maeda, N.; Yaminsky, V. V. Phys. Rev. Lett. 2000, 84, 698. (18) Maeda, N.; Kohonen, M. M.; Christenson, H. K. J. Phys. Chem. B 2001, 105, 5906. (19) Maeda, N. N.; Kohonen, M. M.; Christenson, H. K. Phys. Rev. E 2000, 61, 7239. (20) Volkmann, U. G.; et al. J. Chem. Phys. 2002, 116, 2107. (21) Schollmeyer, H.; Ocko, B.; Riegler, H. Langmuir 2002, 18, 4351. (22) Xia, T. K.; Jiang, O.; Ribarsky, M. W.; Landman, U. Phys. Rev. Lett. 1992, 69, 1967. (23) Xia, T. K.; Landman, U. Phys. Rev. B 1993, 48, 11313.

but not least because of the peculiar surface freezing behavior of these molecules. In the case of surface freezingsa rare phenomenon (most substances show surface melting)sthe melting temperature of the molecules at interfaces is higher than that in the bulk. This means that, in a temperature range between the bulk freezing temperature Tb and the surface freezing temperature Tsf (Tsf > Tb), a solid interfacial alkane layer can coexist (in thermodynamical equilibrium) with liquid alkane bulk. The thickness of this “frozen” layer is typically about one all-trans molecular length, which indicates that it is a monolayer with the molecules oriented normal to the interface. It can be expected that the molecular ordering and phase sequence are different at the interface and in the bulk. The interfacial molecules may even order in phases which are not even observed for the same substance in bulk. This has already been shown for the interface between liquid alkane melt and air. Bulk C20, for instance, has only one rotator phase, RI (untilted, rectangularly distorted hexagonal lattice),43 whereas a C20 surface frozen mono(24) Tkachenko, A. V.; Rabin, Y. Phys. Rev. Lett. 1996, 76, 2527. (25) Leermakers, F. A. M.; Cohen Stuart, M. A. Phys. Rev. Lett. 1996, 76, 82. (26) Tkachenko, A. V.; Rabin, Y. Phys. Rev. E 1997, 55, 778. (27) Tkachenko, A. V.; Rabin, Y. Phys. Rev. Lett. 1997, 79, 531. (28) Tkachenko, A. V.; Rabin, Y. Langmuir 1997, 13, 7146. (29) Smith, P.; Lynden-Bell, R. M.; Earnshaw, J. C.; Smith, W. Mol. Phys. 1999, 96, 249. (30) Shimizu, T.; Yamamoto, T. J. Chem. Phys. 2000, 113, 3351. (31) Shimizu, T.; Yamamoto, T. J. Chem. Phys. 2001, 114, 5774. (32) Strobl, G.; Ewen, B.; Fischer, E. W.; Piesczek, W. J. Chem. Phys. 1974, 61, 5257. (33) Ewen, B.; Fischer, E. W.; Piesczek, W.; Strobl, G. J. Chem. Phys. 1974, 61, 5265. (34) Doucet, J.; Denicolo, I.; Craievich, A.; Collet, A. J. Chem. Phys. 1981, 75, 5125.

10.1021/la026989f CCC: $25.00 © 2003 American Chemical Society Published on Web 05/14/2003

Long Chain n-Alkanes at SiO2/Air Interfaces

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Figure 1. (a) Schematics of the topology for “submonolayer” (left) and “excess” (right) alkane coverages for temperatures above the surface freezing temperature (Tsf), between surface freezing and bulk melting (Tb), and below bulk melting. (b) Possible scenarios of thin layers of individual, disordered alkane molecules suggested from monolayer thicknesses in excess of the molecular all-trans length11 and ellipsometric data.20

layer at the interface between liquid alkane bulk and air is ordered in a hexagonal packing with the molecules untilted (RII).8 Alkane surface freezing was first observed at the interface between a bulk alkane melt and air.1,2 Meanwhile, it has also been detected for alkanes at solid/air interfaces.11,13-15,17,18,20,21 Figure 1 presents the current knowledge (including some still open questions) on the molecular ordering and wetting topologies at solid/air interfaces. It is useful to distinguish between overall alkane coverages that are sufficient for either less (“submonolayer coverage”) or more than one monolayer (“excess coverage”). Historically, at first, samples with excess coverage were investigated.11 It was found that, above Tsf, an all-liquid alkane film completely wets the SiO2/air interface. Upon cooling, a wetting transition is observed. Between Tsf and Tb, liquid alkane droplets partially wet (coexist with) a closed solid alkane monolayer at the solid/air interface. Data from X-ray reflectivity,11 X-ray diffraction, and surface force microscopy15,21 show that below Tsf the interfacial alkane molecules are oriented (35) Denicolo, I.; Doucet, J.; Craievich, A. F. J. Chem. Phys. 1983, 78, 1465. (36) Craievich, A. F.; Denicolo, I.; Doucet, J. Phys. Rev. B 1984, 30, 4782. (37) Maronelli, M.; Strauss, H. L.; Snyder, R. G. J. Chem. Phys. 1985, 82, 2811. (38) Craievich, A.; Doucet, J.; Denicolo, I. Phys. Rev. B 1985, 32, 4164. (39) Ungar, G.; Masic, N. J. Chem. Phys. 1985, 89, 1036. (40) Guillaume, F.; Doucet, J.; Sourisseau, C.; Dianoux, A. J. J. Chem. Phys. 1989, 91, 2555. (41) Sirota, E. B.; King, H. E., Jr.; Singer, D. M.; Shao, H. H. J. Chem. Phys. 1992, 98, 5809. (42) Sirota, E. B.; King, H. E., Jr.; Hughes, G. J.; Wan, W. K. Phys. Rev. Lett. 1992, 68, 492. (43) Sirota, E. B.; Singer, D. M. J. Chem. Phys. 1994, 101, 10873.

normal to the interface in their all-trans configuration. The wetting transition seems to be coupled to the solidification of the interfacial alkane. For submonolayer coverage, surface freezing, that is, a solidlike molecular ordering at the interface above Tb without contact with bulk material, has been proven only recently.15,20,21 For submonolayer samples it is important to realize that the surface coverage is an additional variable which may affect the molecular ordering on various length scales. On the micrometer scale this has already been shown. The solidification conditions (e.g. cooling rate) and the surface coverage affect the morphology of the solid domains. This has been explained through different migration distances for the molecules on their way from the fluid film reservoir to the solidification front.20,21 The observed fractal morphology of the domains shows that the interfacial alkane layers solidify into nonequilibrium aggregates. This nonequilibrium aggregation may also occur on a much smaller, molecular scale. The surface coverage and the sample history may influence the positional ordering and the phase behavior. Experiments at solid/air interfaces have the advantage that they allow the investigation of the interfacial molecular ordering above and below the bulk melting point, andsin the case of submonolayer coverageseven without interference of bulk alkane. For the interface between bulk alkane and air, this is usually not possible because solid bulk alkane is normally polycrystalline; its solid/air surface is not smooth enough for diffraction studies. In the following we will present experimental data on the molecular ordering of triacontane (C30) at SiO2/air interfaces for both excess and submonolayer coverage in a wide temperature range. Triacontane was selected because it is commercially available, is nonvolatile, and

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has a melting temperature (≈67 °C) that is experimentally convenient. Its bulk phase behavior is known from the literature and is typical for long chain alkanes.32-47 Continuing earlier investigations, we will focus especially on the interfacial phase transitions below the surface freezing range. Additionally, we will, for the first time, investigate hysteresis and annealing effects with heating and cooling cycles. 2. Materials and Methods The triacontane (C30) was obtained from Aldrich with a purity of 99+% and used as received. Pieces of silicon wafers (typically 1 × 2 cm2) from Wacker Siltronic GmbH (Burghausen, Germany) with a natural, amorphous oxide layer (≈15 Å thick) served as solid substrates. The surfaces (SiO2) were cleaned with a modified RCA-cleaning process (SC-1) as described elsewhere.48 Onto these smooth surfaces (roughness in the range of angstroms) molecularly thin alkane films were deposited by spin-coating of toluene solutions of C30. The overall alkane coverage was adjusted by the alkane solution concentration and the spin-coating conditions.49 For example, 50 µL of an alkane/toluene solution (alkane solution concentration ) 3 × 10-3mol/L) was deposited on the substrate. Immediately afterward the substrate was accelerated to 3000 rpm. The originally deposited solution volume is sufficient for an alkane film of about 600 Å thickness. Through the rotation, most of the solution is spun off and a smooth solution film is left. With these preparation specifications, the film contains alkane sufficient for only about one monolayer. After the solvent had evaporated, the samples were heated to 80 °C for a short time. At 80 °C all alkane is molten and forms a liquid film which wets the surface (the bulk melting temperature of C30 is ≈67 °C; the surface freezing temperature is ≈70 °C). After the short heating step the samples were cooled to room temperature at a typical rate of ≈2 °C/s. Then they were ready for the investigations. Most of the preparation and treatment of the alkane films was performed under ambient air. Only the X-ray diffraction (GIXD) experiments, which were performed at the ESRF (TROIKA beamline, Grenoble, France) and at the BNL (beamline X22, Brookhaven, USA), were done under helium atmosphere. The average alkane coverage was derived from the liquid film thickness measured at 80 °C by X-ray reflectivity (SAXS) with a conventional Θ/Θ-reflectometer (STOE, Darmstadt, Germany).

3. Experimental Results 3.1. Molecular Ordering of Submonolayer and Excess Samples at Room Temperature. Figures 2 and 3 present diffraction data for submonolayer and excess coverages, respectively, at room temperature. Figure 2a shows a contour plot (diffracted intensities in qxyz-space, darker areas ) higher intensity, logarithmic gray scale shading) for submonolayer coverage. The intensity in qz, integrated from qz ) 0.0 to 0.8 Å-1, is presented in Figure 2b for qxy from 1.425 to 1.750 Å-1. Parts c and d of Figure 2 show the diffraction intensities in qz (“rod scans”) for qxy ) 1.51 Å-1 and qxy ) 1.68 Å-1, respectively. Figure 3 presents equivalent data for excess coverage. The rod scans of the submonolayer sample (Figure 2c and d) show that the diffraction maxima are at qz ≈ 0.0 Å-1; that is, the molecules are oriented upright. This and the two peaks with qxy1 ) 1.51 Å-1 and qxy2 ) 1.67 Å-1 indicate a distorted hexagonal (orthorhombic) ordering with spacings d11 ) 2π/1.51 Å-1 ) 4.16 Å and d20 ) 2π/1.68 Å-1 ) 3.74 Å. The (44) Sirota, E. B.; Singer, D. M.; King, H. E., Jr. J. Chem. Phys. 1994, 100, 1542. (45) Sirota, E. B.; King, H. E., Jr.; Shao, H.; Singer, D. M. J. Phys. Chem. 1995, 99, 798. (46) Sirota, E. B. Langmuir 1997, 13, 3849. (47) Gang, H.; et al. J. Phys. Chem. B 1998, 102, 2754. (48) Graf, K.; Riegler, H. Colloids Surf., A: Physicochem. Eng. Aspects 1998, 131, 215. (49) Bornside, D. E.; Macosco, C. W.; Scriven, L. E. J. Imaging Technol. 1987, 13, 123.

Figure 2. X-ray diffraction data from a sample with submonolayer coverage at 30 °C.

distortion D ) |1 - {[(2qxy1/qxy2)2 - 1]/3}0.5| (ref 41) is ≈0.130 (for qxy2 ) 1.68 Å-1, D ) 0.138). The area per molecule A orth ) 8π2/qxy2(4qxy12 - qxy22)0.5 (ref 41) is A orth ≈ 18.79 Å2. t t The full width at half-maximum peak intensity (fwhm), ∆qxy, is less than 0.01 Å-1, corresponding to a quasi-longrange order with a correlation length of several hundred angstroms.8 The rod scan at qxy ) 1.51 Å-1 (Figure 2c) shows a weak minimum at qz ≈ 0.17 Å-1. This indicates a film thickness of d ) 2π/0.17 Å-1 ≈ 37 Å, which is reasonably consistent with an expected film thickness of one alkane all-trans length (41 Å), taking into account the weak and not so well-defined peak. The comparison between the diffraction data of Figures 2 and 3 reveals the contributions from the additional bulk material in the case of excess coverage. Most prominently, one observes three pronounced Debye-Scherrer rings with qxy ≈ 1.51 Å-1, qxy ≈ 1.53 Å-1, and qxy ≈ 1.67 Å-1 at qz ) 0 Å-1, respectively. Additionally, there are several strong maxima at qz * 0 Å-1, which are not on the DebyeScherrer rings. The two Debye-Scherrer rings with qxy ≈ 1.51 Å-1 and qxy ≈ 1.67 Å-1 at qz ) 0 Å-1 stem from polycrystalline bulk material with the same orthorhombic ordering as that of the (sub)monolayer. The third, weaker Debye-Scherrer ring with qxy ≈ 1.53 Å-1 at qz ) 0 Å-1 may be attributed to a small portion of polycrystalline bulk material in a hexagonal ordering with an intermolecular spacing of d ) 4.11 Å-1, corresponding to a ) 2d2/30.5 ≈ 19.50 Å2 per molecule. molecular area8 of A hex t The strong maxima out of the qz ) 0 Å-1 plane, which are obviously not on the Debye-Scherrer rings, may originate from thick lamellar multilayer platelets on top of the monolayer. Estimated peak widths of ∆qxy ≈ 0.002 Å-1


Figure 3. X-ray diffraction data from a sample with excess coverage at 30 °C.

correspond to multilayer thicknesses of ≈1000 Å, which is consistent with observations by optical microscopy.11 There are also indications of minima (for qz ) 0 Å-1) at qxy ≈ 0.17 Å-1, at qxy ≈ 0.3 Å-1, and, possibly, at qxy ≈ 0.45 Å-1 (see Figure 2c and d), corresponding to spacings of ≈37 and 42 Å, which might originate from the monolayer. 3.2. Molecular Ordering of Submonolayer and Excess Samples between 30 and 55 °C. Figures 4-7 show the change of the diffraction data of submonolayer and excess coverage samples when the temperature is increased from 30 to 55 °C. Figure 4 presents contour plots for a submonolayer sample, and Figure 5 the diffracted intensity integrated over qz as a function of qxy. Figures 6 and 7 show analogous data for samples with excess coverage. The intensity plots in the contour plots are split into regions of two different resolutions in qxy. The region between qxy ) 1.535 Å-1 and qxy ) 1.625 Å-1 contains no peaks, and the resolution is reduced to 0.014 Å-1. Elsewhere, the resolution is 3.5 × 10-3 Å-1. The reduced (vanishing) background noise above the strong peaks in the qz-direction is an artifact caused by the detector. The high intensity around qz ) 0 Å-1 slightly reduces the sensitivity (background noise) of the detector at higher qz values. This reduction of the sensitivity is highly exaggerated by the logarithmic grading of the gray scale plot. It is not a serious saturation effect of the detector. If there is a significant peak in the range of higher qz values, it is observed, as can be seen from the lowtemperature data of Figure 6. All plots from Figures 4-7 show that the peak at qxy ≈ 1.51 Å-1 remains up to 55 °C with a slight shift to lower qxy values at elevated temperatures (qxy ≈ 1.50 Å-1 at 55 °C). The intensity decreases slightly between 30 and 40 °C. Between 40 and 50 °C it decreases significantly for the submonolayer

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Figure 4. X-ray diffraction data (contour plots) from a sample with submonolayer coverage at various temperatures below bulk melting.

Figure 5. X-ray diffraction intensity integrated over qz from a sample with submonolayer coverage at various temperatures below bulk melting.

coverage. For the excess coverage the beginning of the sharp decrease is shifted toward slightly higher temperatures. It occurs between 50 and 55 °C. Concomitant with the intensity decrease, the peaks also broaden by roughly one magnitude to ∆qxy ≈ 0.04 Å-1 at 55 °C. It is clearly observed that the intensity decrease for qxy ≈ 1.51 Å-1 around 50 °C occurs simultaneously with the complete disappearance of the peaks at qxy ≈ 1.66 Å-1. This indicates that the monolayer undergoes a phase transition from an orthorhombic to a hexagonal phase with A hex ≈ 20 Å2 per molecule and a fairly short t

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Figure 6. X-ray diffraction data (contour plots) from a sample with excess coverage at various temperatures below bulk melting.

Figure 7. X-ray diffraction intensity integrated over qz from a sample with excess coverage at various temperatures below bulk melting.

positional correlation length of only several tens of angstroms. With the excess sample some residual DebyeScherrer ring is still clearly visible at 55 °C for the larger spacing (qxy ≈ 1.50 Å-1 at qz ) 0 Å-1). For the smaller spacing (qxy ≈ 1.65 Å-1 at qz ) 0 Å-1) the Debye-Scherrer ring is very weak at 55 °C. It must be assumed that up to 50 °C most of the bulk is in the crystalline state. At 55 °C most of the bulk material has changed to the rotator phase. 3.3. Molecular Ordering of Submonolayer Samples upon Temperature Increase from 55 to 74.5 °C.

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Figures 8 and 9 show diffraction data of submonolayer samples obtained while heating from 55 to 74.5 °C. Figure 8 is a contour plot, and Figure 9 shows the intensity at qz ) 0 Å-1 as a function of qxy. According to Figure 8 the intensity decreases substantially between 55 and 60 °C. Upon further heating, the decrease is less pronounced and there is still a weak diffraction peak at 68 °C. The intensity maximum is around qz ) 0 Å-1, which means the molecules are preferentially standing upright. Figure 9 shows that the molecular order disappears somewhere between 68.9 and 74.5 °C, which is consistent with a surface freezing temperature of ≈70 °C. Figure 9 also shows that the peak width ∆qxy is largely constant between 62.1 and 68.9 °C, whereas the intensity decreases by about a factor of 2 and the peak position is shifted to lower q values. Presumably this is due to an increase of thermal agitation (Debye-Waller). 3.4. Layer Thicknesses of Submonolayer Samples upon Temperature Decreases from 75 °C to between 70.6 and 65.4 °C. The data of Figures 10 and 11 were taken from one submonolayer sample during multiple heating/cooling cycles. Figure 10 shows X-ray reflectivity data at temperatures close to and below the surface freezing temperature. Figure 11 shows X-ray reflectivity data at T ) 75 °C, that is, when the triacontane is all liquid. The dots represent the experimental data, and the lines are fits based on a simple one-box model for the alkane film. While the reflectograms were recorded (which typically took a few minutes), the sample temperature was kept constant at one of the indicated temperatures. The sequence of sample temperatures switched between 75 °C and the lower temperatures of Figure 10, with a random sequence of the lower temperatures. This assured identical starting conditions for each cooling/solidification step and avoids artificial falsifications due to systematic temperature shifts. The data of Figure 11 demonstrate that in the course of the experiment the thicknesses of the liquid film decreased continuously from ≈36 to ≈31 Å. Obviously, a small amount of alkane evaporated. The reflectograms of Figure 11 also prove that the alkanes always formed an all-liquid, smooth film at 75 °C. In Figure 10 the diffractograms are arranged according to decreasing film thickness from top to bottom (which, as mentioned above, does not reflect the real time sequence of the recording). At and below the bulk freezing temperature (≈67 °C) all samples had the same layer thickness of ≈43 Å. When cooling to 70.6 °C, that is, slightly above the surface freezing temperature, the reflectogram is very similar to those from Figure 11. The film is probably allliquid with a thickness of ≈33 Å. For those cases when the sample was cooled to temperatures in the surface freezing range, reflectograms are found, which originate from films of various thicknesses (from thin (all-liquid) films to thicker (all-solid) films). The main change in the film thickness seems to occur around 69.8 °C. At this temperature, the weak minimum at q ≈ 0.11 Å-1 indicates some filmlike layering at the surface, whereby the noise at q > 0.11 Å-1 shows that this layer is quite rough. At 69 °C the curve is much smoother and the minimum at q ≈ 0.11 Å-1 is fairly well developed. Yet, a second minimum is missing, and the reflectogram is indicative of a rough layer which is somewhat thicker than the allliquid films. At 68.2 °C, two minima are observed with positions indicating a layer thickness of ≈43 Å. At this temperature the alkane film is very similar to the ones at temperatures below bulk melting, although the still somewhat shallower minima hint toward a somewhat higher surface roughness. In addition to the heating/ cooling cycles presented in Figures 10 and 11, Figure 12


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Figure 8. X-ray diffraction data (contour plots) from a sample with submonolayer coverage at various temperatures in the vicinity of bulk melting.

Figure 9. X-ray diffraction intensity at qz ) 0 Å-1 from a sample with submonolayer coverage at temperatures from below bulk melting to above surface freezing.

shows the decrease of alkane film thickness in the allliquid state at 75 °C due to evaporation. The film thicknesses were calculated from X-ray reflectivity data for two different samples with different alkane surface coverages after preparation. The thickness decrease ∂d/∂t is typically ≈0.1 Å/min. 3.5. Molecular Ordering in Submonolayer Samples upon Temperature Decrease from 75 °C to between 30.0 and 68.9 °C. Figures 13 and 14 show reflectivity and diffraction data from submonolayer samples which were recorded in the course of cooling from the all-liquid state (at 75 °C) to 60 °C (Figure 13), and from 75 to 30 °C (Figure 14), respectively. The intensity integrated over qz as a function of qxy (Figure 13) shows that, upon cooling into the surface freezing regime between ≈70 and ≈67 °C,

Figure 10. X-ray reflectivity data from a sample with submonolayer coverage at temperatures from below bulk melting to above surface freezing. The data were recorded after cooling the sample from 75 °C (see Figure 11) to one of the temperatures shown in the plot. The temperature sequence of the measurements was random, whereas the reflectograms are sorted according to increasing temperature.

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Figure 11. X-ray reflectivity data from the same sample as shown in Figure 10 with submonolayer coverage at 75 °C. The reflectograms are sorted according to increasing liquid film thickness, which reflects also the original measurement sequence which started with the thickest film (lowest curve). The film thinning is due to evaporation. The thin lines at q ) 0.10 Å-1 and q ) 0.275 Å-1 are guides to the eye to show the film thinning due to evaporation.

Figure 12. Film thicknesses d as a function of heating time of thin liquid alkane films at 75 °C. The film thicknesses decrease with a rate of ∂d/∂t ≈ 0.1 Å/min. The film thicknesses were derived from X-ray reflectivity data from two different samples. One sample had an original thickness of ≈45 Å, and the other one ≈25 Å.

there is no solidlike molecular order. Peaks appear only after cooling to and below 67 °C. As Figure 14 demonstrates, the intensities are not sharply focused at qz )

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Figure 13. X-ray diffraction intensity data (at qz ) 0) from a sample with submonolayer coverage subject to cooling from 75 °C to the given temperatures without any intermediate heating or cooling.

0 Å-1. The intensity maximum is around qz ≈ 0.2 Å-1, which corresponds to a tilt angle of Θ ≈ 8° (tan Θ ≈ qz/ qxy).8 The molecular ordering increases significantly and the peak is shifted closer to qz ) 0 Å-1 when the sample is cooled to below 60 °C. 3.6. Molecular Ordering in Submonolayer Samples Which Were Reheated from Temperatures Close to Bulk Melting. Figure 15 demonstrates the difference in the temperature-dependent molecular ordering upon reheating to 68.5 °C between those submonolayer samples which were cooled to only a few degrees below the bulk melting temperatures and those which were cooled to lower temperatures. If the sample is cooled from the all-liquid state at 75 °C to only 50 °C and then reheated to 68.5 °C, the solidlike molecular ordering which is observed at temperatures below 67 °C (see Figure 13) disappears. If the sample is cooled to 45 °C and then reheated to 68.5 °C, a diffraction peak remains. 3.7. Molecular Ordering in Submonolayer and Excess Samples Subjected to Heating/Cooling/ Heating Cycles from Temperatures below Bulk Melting to above Surface Freezing. Figures 16 and 17 present the diffraction peak intensities at qxy ) 1.503 Å-1 for qz ) 0 Å-1 during heating/cooling/heating cycles for submonolayer and excess coverage samples, respectively. For each run the samples were shifted laterally by about twice the width of the X-ray beam to measure a fresh footprint area (to reduce possible radiation damage). The first heating run started at room temperature with the samples kept there for some time (hours). For the first heating run the peak intensity of the submonolayer sample


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Figure 14. Contour plots from a sample which was cooled from 75 °C to the given temperature without any intermediate heating or cooling.

Figure 15. X-ray diffraction intensities integrated over qz from samples with submonolayer coverage at 68.5 °C. One sample was cooled from 75 to 50 °C and then heated to 68.5 °C. The other sample was cooled from 75 to 45 °C and then heated to 68.5 °C.

remains constant up to 69 °C. Then it decreases and, at 71.5 °C, all the solidlike molecular ordering is gone. For the excess coverage sample (Figure 17) the decrease starts at slightly lower temperatures (69 °C) and at 70 °C all the intensity has dissappeared. Upon cooling after this first heating run, solidlike molecular ordering reappears for both samples between ≈69 and 68 °C. Around 60 °C the peak intensity is comparable to that of the first heating run, which started at room temperature. In the second heating run, which now started at 60 °C, the solid ordering

Figure 16. X-ray diffraction data for qz ) 0 from a sample with submonolayer coverage during a heating (#1), a cooling (#2), and a second heating run (#3). To avoid unnecessary beam damage, the sample was shifted between the runs laterally by about twice the width of the footprint of the X-ray beam.

disappears already at ≈67 °C (submonolayer) or ≈67.5 °C (excess). Also, for the submonolayer sample, the intensity below 67 °C is lower than that for the two preceding measurements. 4. Discussion This report focuses on the molecular ordering of C30 at a planar solid/gas interface as function of the temperature, the surface coverage, and the thermal history. To draw conclusions on the specific influence of the interface, we

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Figure 17. X-ray diffraction data for qz ) 0 from a sample with excess coverage during a heating (#1), a cooling (#2), and, again, a heating run (#3). To avoid unnecessary beam damage, the sample was shifted between the runs laterally by about twice the width of the footprint of the X-ray beam.

will first survey briefly what is known about the bulk phase behavior of long chain alkanes and alkane mixtures with a focus on the various rotator phases and their temperature range. We will then evaluate our experimental results on the interfacial ordering in relation to the bulk phase behavior. Finally we will discuss the annealing and hysteresis of the phase transitions of the interfacial layers. In thermodynamic equilibrium, bulk C30 has three solid phases, a crystalline phase (X) and two so-called “rotator phases” (R) below the isotropic liquid phase (L). For long chain alkanes, up to five different bulk rotator phases have been identified.42-46 The rotator phases are lamellar with long range positional order whereas the rotational order along the long molecular axis is only short ranged or even nonexisting. The bulk phase sequence of C30 upon heating is as follows:41 X-RIV (phase transition temperature TX-RIV ≈ 63.5 °C), RIV-RIII (TRIV-RIII ≈ 65.5 °C), and RIII-L (TRIII-L ) Tm ≈ 66.9 °C). In the crystalline phase the molecules are organized in lamellar superlattices, with a monoclinic symmetry (tilt angle ≈ 27°) within the lamellae. The symmetry of the rotator phase RIV is slightly distorted hexagonal with the molecules tilted in a direction between nearest neighbors. For RIII the symmetry is triclinic, and the molecular tilt is not along any symmetry direction of the lattice. For both bulk rotator phases, between the lamellae, the molecules are stacked end to end. As is the case for other alkanes with similar chain length, the temperature range of the rotator phases of C30 is quite narrow, being over only a few degrees centigrade.41 It has been suggested45 that the tendency of the alkanes to form lamellae is dominated by the lateral intermolecular interactions whereas interlayer interactions mainly have an influence on the packing symmetry and the molecular tilt. This can, for example, be concluded from experiments with bulk alkanes under high pressure44 or from experiments with alkane mixtures.45 Alkane mixtures do not phase separate in the solid state if the chain lengths differ by less than about 20%. Like the pure substances, they form homogeneous lamellar phases. It is observed45 that, compared to the case of systems with uniform chain length, the temperature range of the rotator phases of mixtures is increased substantially and the molecules are oriented preferentially normal to the layer plane. This has been attributed to the reduced interlayer

Schollmeyer et al.

coupling for mixtures due to the dispersion of the chain lengths. To give an example, pure C24 has a phase sequence (upon heating) X-RV-RI-RII-L, where RV is a hexagonally distorted rotator phase with the molecules tilted toward the next-nearest neighbors. The entire temperature range of the three rotator phases extends only over a range of about 6 °C below bulk melting. For a binary mixture of 50% C23 and 50% C25, that is, for an average chain length comparable to that of C24, the temperature range of the rotator phases has increased to 17 °C and only RI and RII rotator phases, both with the molecules untilted, are observed. The RV phase, with the tilted molecules, has disappeared. This is explained by weaker interlayer interactions for mixtures because the interface between the lamellae is envisioned as being “rougher” due to the mixture of chain lengths. There are no literature data available for mixtures equivalent in chain length to C30. But it is reasonable to expect a behavior analogous to that of the shorter chains. For C30 at SiO2/air interfaces, the interlayer interaction is most likely also reduced compared to the situation in bulk. Toward air the interaction is negligible. Toward the SiO2 surface the interaction will be weaker because the surface is rougher than that of an adjacent alkane lamellae within the bulk. The SiO2 surface is amorphous; thus, any epitaxial-like layer growth, which leads to the bilayer and trilayer stackings of some rotator phases within solid bulk C30, is excluded. Henceforth, analogous to the case for mixtures, for the SiO2 surface one can expect an extension of the temperature range of the rotator phase(s) and a preferentially upright molecular orientation. This is indeed observed. For submonolayer coverage, upon heating, a transition into a hexagonal phase analogous to bulk RII occurs already between 40 and 50 °C (Figure 5) for submonolayer coverage; for excess coverage it occurs between 50 and 55 °C (Figure 7). The transition takes place between 10 and 25 °C below the bulk melting temperature compared to ≈6 °C for bulk C30. The symmetry of the low-temperature phase at the interface is orthorhombic. This means it could be either a crystalline phase or an orthorhombic rotator phase RI. In fact, in bulk, all alkanes (pure material and mixtures), which show a high-temperature hexagonal rotator phase RII, also have always a lower temperature orthorhombic rotator phase RI (or RV) before they pack in the crystalline state (X) at even lower temperatures. Because C30 at the SiO2/air interface has also a packing analogous to RII, a similar phase sequence seems conceivable. However, the measured hexagonal distortion of D ≈ 0.13 for the lowest temperature phase is typical for an untilted orthorhombic crystalline (X) phase. The thermal expansion coefficient is (dA/dT)/A ≈ 3 × 10-4 °C-1, which also agrees reasonably well with the literature data46 for a crystalline bulk phase (5 × 10-4 °C-1). The expansion coefficient is estimated from the molecular areas at 30 °C (≈18.79 Å2) and 40 °C (≈18.85 Å2), respectively. Thus, it is assumed that the interfacial C30 has a low-temperature phase similar to an untilted orthorhombic bulk crystalline phase. At intermediate temperatures a hexagonal phase analogous to a bulk rotator phase RII is observed, which melts directly into an isotropic liquid. First-order phase transitions always show hysteresis due to the interplay between interfacial and bulk energies in small clusters.50 This could explain the observed, pronounced hysteresis effects for the molecular ordering (see, for example, Figures 16 and 17). But for the interfacial (50) Kurz, W.; Fisher, D. J. Fundamentals of Solidification; Trans Tech Publications: Uetikon-Zuerich, Switzerland, 1998.


alkane monolayer the nucleation barrier should be quite low because heterogeneous nucleation with a low energetic barrier should prevail. Therefore, nucleation is probably not the reason for the hysteresis. An analysis of the diffraction data for the first and second temperature runs as shown in Figures 16 and 17 in combination with the reflectivity data of Figure 10 reveals another, more likely explanation. The reflectivity data indicate that there seems to be a transition from the smooth, all-liquid film at 70.6 °C to a topology with a thicker, quite rough (heterogeneous) layering at 69.8 °C. At 69.0 °C the topology is still not very smooth but the layering has again increased in thickness. Finally, at 68.2 °C, we find a much smoother layering with yet again increased thickness. Upon further cooling, the changes are minor. It can be assumed that the film is probably not liquid anymore at 69.8 °C; certainly it is not at 69.0 °C. On the other hand, diffraction shows no crystallinity at this temperature. It seems that the solid film is quite amorphous directly after solidification with the molecules oriented preferentially normal to the interface (the film thickness is roughly equal to the alltrans molecular length) but with little positional order. Presumably, upon cooling, the alkane molecules are kinetically trapped in an amorphous solid state. Indeed, because the heat flux through the substrate is quite efficient, the interfacial alkane film is cooled quite rapidly although the cooling rate (≈1-2 °C/s) is not really high. Thus, many alkane molecules may be undercooled sufficiently to solidify in nonequilibrium (amorphous) molecular packings before they can arrange themselves in their equilibrium state (“rapid solidification”50). For submonolayer coverages this nonequilibrium solidification is enhanced additionally by the specific molecular transport conditions. Because the solid film sections are thicker than the liquid film, a solidification front creates and grows into a liquid depletion zone. Thus, the liquid molecules have to migrate over large distances (on a molecular scale) before they reach the solidification front. This effectively increases the undercooling. In fact, the observed seaweedlike fractal domain shapes for the submonolayer coverage15,21 are a clear indication for the nonequilibrium solidification process. The diffraction data show the appearance of positional molecular ordering upon cooling to 67.5 °C (Figure 14) with hexagonal symmetry and a range of molecular tilts close to normal orientation. That this occurs close to the bulk melting temperature may be pure coincidence. It is not clear whether the appearance of the molecular ordering is coupled to a specific temperature or rather just a combined result of annealing time and temperature. At much lower temperatures (i.e. after longer time) the system finally relaxes into the orthorhombic crystalline phase. If the solid alkane layer is reheated after only insufficient annealing, that is, from the hexagonal phase, then the weak ordering is quickly lost in the surface freezing temperature range (see Figures 15-17). If the layer has been annealed to crystallinity before reheating, the rotator phase is more persistent and positional ordering is observed up to at least 70 °C. This holds for both submonolayer and excess coverage. One can speculate that the melting proceeds in a scenario along the defect mediated melting behavior according to Kosterlitz, Thouless, Halperin, Nelson, and Young.51 5. Summary and Conclusion We present experimental results on the interfacial molecular ordering of triacontane (C30) at SiO2/air interfaces in the temperature range from the crystalline (51) Strandburg, K. J. Rev. Mod. Phys. 1988, 60 (1), 161.

Langmuir, Vol. 19, No. 12, 2003 5051

phase to the all-liquid state and discuss the findings in comparison to the bulk behavior of C30. The samples were investigated by X-ray reflectivity and diffraction as a function of the surface coverage and the thermal history. It is found that there is a substantial difference in the molecular ordering in the bulk and at the interface. The low temperature, crystalline phase of interfacial C30 is orthorhombic with the long molecular axis oriented upright whereas the crystalline bulk phase of C30 is monoclinic, that is, with tilted molecules. At the SiO2/air interface the transition between the crystalline phase and the rotator phase(s) occurs at much lower temperatures than in the bulk. For the interfacial molecules the transition from the crystalline phase to a rotator phase with hexagonal symmetry and with the molecules oriented upright is already at ≈50 °C for the submonolayer coverage (at ≈55 °C for excess coverage). The interfacial rotator phase is similar to the bulk rotator phase RII which has been observed for alkanes with 21-27 carbon atoms but not at any temperature for bulk C30. Bulk C30 melts into a rotator phase, RIII, at ≈61 °C and into another rotator phase, RVI, just below the bulk melting temperature at ≈67 °C. Both of these bulk rotator phases have a hexagonal symmetrysjust like the interfacial rotator phasesbut the molecules are inclined. We explain both the wider temperature range of the rotator phase and the different molecular orientation for the interfacial molecules with the weaker (and nonepitaxial) interlayer interactions for the interfacial alkane molecules. It has been shown already some time ago for bulk systems that interlayer interactions stabilize the temperature range of the crystalline phase and enforce a denser packing with tilted molecules. The phase transition behavior depends on the preparation history. The samples can be far from thermodynamic equilibrium. Immediately after solidification/cooling, the interfacial alkane layers are quite amorphous. X-ray diffraction shows no positional ordering although the X-ray reflectivity data strongly suggest that the interfacial alkanes are solid with a molecular orientation preferentially normal to the interface. It is assumed that the efficient heat flux through the substrate and, for the submonolayer coverage, the large molecular transport distances through the depletion zone ahead of the solidification front lead to an efficient and fast cooling although the externally applied cooling rate is not very high. This “rapid solidification” leads to the amorphous, nonequilibrium packing. Time and/or cooling anneals the layers and induces positional ordering observable by X-ray diffraction. If the samples are cooled into the crystalline phase before they are reheated, then the solid molecular ordering persists up to ≈70 °C. If the samples are only cooled into the rotator phase and instantaneously heated again, then the positional order only persists to ≈67-68 °C. Acknowledgment. We thank Wacker Siltronic GmbH Burghausen for grateful donation of the silicon wafers. The work was supported by the Deutsche Forschungsgemeinschaft through the Schwerpunktprogramm “Benetzung und Strukturbildung an Grenzfla¨chen”. We gratefully acknowledge fruitful discussions with Helmuth Mo¨hwald and Ben Ocko. The experiments at Brookhaven National Laboratory were supported by the U.S. Department of Energy. LA026989F

Air Interfaces: Molecular Ordering

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