Langmuir 1998, 14, 7245-7249
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A Grazing-Incidence X-ray Diffraction Study of Octadecanol Monolayers at High Surface Pressures R. Steitz,†,‡ J. B. Peng,† I. R. Peterson,*,§,| I. R. Gentle,† R. M. Kenn,§ M. Goldmann,§ and G. T. Barnes† Department of Chemistry, The University of Queensland, Qld 4072, Australia, and Institut Curie DPC, 11 rue Pierre & Marie Curie, F-75231 Paris Cedex, France Received February 10, 1998. In Final Form: August 28, 1998 The structure of water-surface monolayers of octadecanol is studied using grazing incidence diffraction at three distinct temperatures near room temperature and in the region of high surface pressures in the vicinity of the tilting transition and of the transition reported by Lawrie and Barnes (J. Colloid Interface Sci. 1994, 162, 36). The data were obtained in independent experiments by different groups at two different synchrotrons. The tilting transition between a low-pressure phase in which the molecules are inclined to the substrate normal and a high-pressure upright phase was observed at surface conditions reported by other authors. We conclude that the grazing incidence technique is not sensitive to the transition of Lawrie and Barnes.
1. Introduction Although water-surface monolayers have been studied for over a century, the recently developed technique of grazing-incidence X-ray diffraction (GID) has revealed new aspects of their rich behavior1. GID has shown the phase nomenclature of solid, liquid-condensed, liquid-expanded, and gas to be both incomplete and misleading. It is incomplete, because each term has been found to describe many distinct phases. It is misleading, because over most of the “solid” region the monolayer has been shown not to display long-range translational order or a shear threshold for plastic deformation but instead the correlations characteristic for liquid crystals.2 The liquid-condensed and solid regions have comparable fluidity and differ structurally in the presence or absence, respectively, of molecular tilt. The above-mentioned four regions are demarcated by transitions giving major changes in thermodynamic parameters. The changes of molecular organization at the transitions within each region are much more subtle, and the discontinuities to which they give rise in isotherms are readily smeared out by trace contaminants. However they can still be detected by the careful measurement of isotherms. Even though our present extensive understanding results from the use of GID, we owe a debt to the isotherm studies first of Stenhagen3,4 and later of Lundquist,5,6 who mapped out the monolayer phase diagrams * Corresponding author. † The University of Queensland. ‡ Current addresses: Iwan-N-Stranski Institut, Technische Universita¨t Berlin, Strasse des 17. Juni 112, D-10623 Berlin, Germany. § Institute Curie DPC. | Current address: Coventry University NES-CMBE, Priory Street, Coventry CV1 5FB, U.K. (1) Kenn, R. M.; Bo¨hm, C.; Bibo, A. M.; Peterson, I. R.; Mo¨hwald, H.; Kjaer, K.; Als-Nielsen, J. J. Phys. Chem. 1991, 95, 2092. (2) Bibo, A. M.; Knobler, C. M.; Peterson, I. R. J. Phys. Chem. 1991, 95, 5591. (3) Sta¨llberg-Stenhagen, S.; Stenhagen, E. Nature 1945, 156, 239. (4) Stenhagen, E. In Determination of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C., Eds.; Academic: New York, 1955. (5) Lundquist, M. Chem. Scripta 1971, 1, 5. (6) Lundquist, M. Chem. Scripta 1971, 1, 197.
in this way for a wide range of amphiphilic materials. GID has confirmed that all the lines shown in their phase diagrams are in fact structural changes and lie in essentially the correct positions. It is for this reason that present phase nomenclature largely reflects their usage. GID provides information both about the magnitude and direction of the molecular tilt and about the relationships of molecules to their neighbors.7 It is not seriously affected by small amounts of contaminant which may result in the coexistence of two phases over a range of surface conditions: the diffraction pattern is then the superposition of the patterns of the two phases. However GID has its own difficulties, mainly related to the limited access for any one group to the synchrotron facilities required. The phase diagrams of Stenhagen and Lundquist have allowed measurements to be made in the regions they have highlighted. While their isotherm technique is not completely reliable, for example not all the phase transitions reported for ethyl eicosanoate2 determined in this way have been confirmed by subsequent studies,8 it nevertheless considerably reduces the amount of synchrotron beam-time required. There is a transition reported by both Stenhagen and Lundquist to a phase which is not yet included in this scheme. Both found this transition at surface pressures above those of the upright hexatic rotator phase LS, in acids and esters, respectively. Stenhagen identified the higher-pressure phase as S, the upright hexatic herringbone phase, but Lundquist preferred to name it LS′. In these materials, this transition occurs at surface pressures which are so high that the monolayer collapses within minutes, too short to allow GID investigation. A high-pressure phase, the X phase, has been observed in studies of acids on different substrates,9 and its structure is closely related to that of known crystalline packings of aliphatic compounds with parallel zigzag planes. It is plausible that this local packing should occur at high surface pressures, because it can show a chain cross section (7) Kaganer, V. M.; Peterson, I. R.; Kenn,; Shih, M. C.; Durbin, M.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (8) Fischer, B.;. Teer, E.; Knobler, C. M. J. Chem. Phys. 1995, 103, 2365. (9) Steitz, R.; Mitchell, E.; Peterson, I. R. Thin Solid Films 1991, 205, 124.
10.1021/la980163p CCC: $15.00 © 1998 American Chemical Society Published on Web 11/11/1998
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Figure 1. (π, T) diagram showing the surface conditions at which octadecanol monolayers were observed in the present study, superimposed on phase boundaries consistent with the data of Harkins and Copeland, Dutta et al. and Lawrie and Barnes. Circles indicate measurements made at LURE, squares at the Photon Factory. The shaded region gives the range of monolayer metastability, where the monolayer collapses slowly.
as low as 0.182 nm2, less than the 0.1835 nm2 observed in known herringbone packings.10 However no relationship between X and LS′ has been demonstrated. The present paper reports two independent series of measurements, both of which were motivated by a recent isotherm study of octadecanol.11 The (π, T) phase diagram resulting from this study is given in Figure 1 and shows that at the LS-LS′ transition the monolayer is stable enough for extended observation. Although octadecanol has been exhaustively studied by GID over a range of surface pressures extending to above the tilting transition, the major undecided question is the nature of this novel transition. 2. Experimental Section 2.1. LURE, Saclay, France. A series of measurements was performed at Beamline D24 of the LURE synchrotron equipped with a simple bending magnet. The detector was a NaI scintillator with vertically aligned Soller collimator, giving measurements of the intensity integrated along vertical Bragg rods for a range of different values of the in-plane component of the scattering vector with a lateral resolution of better than 0.01 Å-1. The incident X-ray beam was monochromated to a wavelength of 148.8 pm by a Ge(111) focusing monochromator, and its intensity was monitored using a transmissive gas cell. The all-Teflon computer-controlled trough was constructed inhouse. The setup has been described in detail previously.12 The measurements in this series were performed at surface conditions indicated by circles in Figure 1, in most cases at a temperature of 20 °C. The measured intensities from each run were corrected for polarization and then least-squares fitted to a number of different models, involving Gaussian, Lorentzian, and exponentiated Lorentzian beam profiles superposed on a linear background. 2.2. Photon Factory, KEK, Tsukuba, Japan. Measurements were made at Beamline 16 in the Photon Factory on two separate occasions. The setup has been described previously.13 The incident collimated beam was monochromated to a wavelength of 148.8 pm. The diffracted X-ray intensity was measured using a linear position-sensitive detector with an in-plane (10) Segerman, E. Acta Crystallogr. 1965, 19, 789. (11) Lawrie, G.; Barnes, G. T. J. Colloid Interface Sci. 1994, 162, 36. (12) Saint-Jalmes, A.; Graner, F.; Gallet, F.; Nassoy, P.; Goldmann, M. Chem. Phys. Lett. 1995, 240, 234. (13) Matsushita, T.; Iida, A.; Takeshita, K.; Saito, K.; Kuroda, S.; Oyanagi, H.; Sugi, M.; Furukawa, Y. Jpn. J. Appl. Phys. 1991, 30, L1674.
Figure 2. Typical contour maps of scattered intensity. resolution of 0.009 Å-1. The all-PTFE trough was constructed in-house and used a Wilhelmy balance to measure the surface pressure. All the measurements in these series were performed at a temperature of 25 °C and at the surface pressures shown by squares in Figure 1 maintained by a feedback loop. In a first step of the data analysis, the individual intensity values resolved in both Qxy and Qz were integrated along Bragg rods. i.e., lines of constant Qxy. This procedure gives a set of data equivalent to that from LURE which will be called an integrated profile. The integrated profiles were also least-squares fitted to models with either one or two Lorentzians superimposed on a linear background. The set of wavevectors corresponding to the maxima of the integrated-profile Lorentzians will be called the principal Bragg
GID of Octadecanol Monolayers
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Table 1. Parameters for the Least-Squares Best Fit to Two Lorentzians Plus Linear Background of All the Data Sets in the Present Study data set
T/°C
π/mN m-1
03
17.5
40
59
20
20
57
20
27
17
20
30
56
20
32
55
20
35
16
20
40
8
25
8
18
25
18
22
25
22
34
25
34
40
25
40
∑I/arb u 8.58 4.29 11.59 5.79 19.5 9.77 7.70 3.85 11.3 5.67 8.80 4.40 21.5 10.8 1718 859 602 301 910 455 833 417 826 413
Qxy/Å-1
Qz/Å-1
LURE 1.4993 1.5071 1.5008 1.5193 1.5104 1.5199 1.5201 1.5170 1.5162 1.5244 1.5111 1.5216 1.5071 1.5184
0 0 0 0 0 0 0 0 0 0 0 0 0 0
0.0412 0.0134 0.0883 0.0166 0.0712 0.0114 0.0712 0.0114 0.1021 0.0140 0.0869 0.0226 0.0414 0.0139
KEK 1.5029 1.5035 1.5151 1.5187 1.5186 1.5239 1.5191 1.5263 1.5197 1.5269
0.154 0.308 0 0 0 0 0 0 0 0
0.043 0.018 0.047 0.013 0.059 0.015 0.047 0.015 0.062 0.017
rod (or rods), and the profile of intensities versus Qz along a principal Bragg rod will be called a resolved profile. In further analysis these resolved profiles were fitted to models with either one or two peak functions superimposed on a linear background. The peak function was taken to be either Gaussian or sin2(x)/x2 in shape. For all data sets, the peak positions were interpreted in terms of a 2D Bravais lattice of parallel rods. Instead of the crystallographic parameters a, b, and γ, which are ambiguous and confusing when applied to the case of monolayers, the lattice is parametrized in terms of the Sirota distortion parameters.7
3. Results and Discussion As reported by Lawrie and Barnes,11 the monolayer was stable enough up to the highest pressures investigated to allow measurement with a negligible change of area over measurement runs of many hours. Figure 2 shows typical contour plots of diffracted intensity from the three distinct phases observed at the Photon Factory (KEK). All three are qualitatively quite similar, showing just one connected region of high scattered intensity, but the reflection in the lowest pressure phase has a much greater extension in Qz. The half-maximum of the first two is approximately 0.15 Å-1 (see Table 1) above the water surface (Qz ) 0), consistent with an individual reflection from a 2.1-nm-thick monolayer centered on the surface. Considered as a single reflection above the plane of the water surface, the vertical fwhm in the lowest-pressure case is nearly 0.5 Å-1. This is consistent with the report by Dutta et al. of next nearest neighbor (NNN) tilt in this region. Figure 3 shows the curves of Bragg-integrated intensities from the Photon Factory data and the curves of best fit based on various models. Figure 4 shows the curves of best fit to the intensity profile along the Bragg rod for the value of Qxy yielding the largest integrated value. Least-squares fits of many different models were made to the data sets. In all cases the fit to two Lorentzians was significantly better than the fit using just one. Attempts were made to fit three Lorentzian peaks to the data, but this never led to a distinct improvement of the
∆Qxy fwhm/Å-1
∆Qz fwhm/Å-1
χ2 4.176 2.755 2.169 1.523 3.239 3.239 5.278
0.370 0.370 0.303 0.312 0.312 0.321 0.279 0.303 0.295 0.312
0.856 0.413 0.751 0.788 0.453
fit. Table 1 show the parameters for the least-squares fits to two Lorentzians for all measurement sets. For the values given here, the ratio of the integrated intensities under each peak, ∑I, was fixed at 2:1. The full-width at half-maximum (fwhm) for each twoLorentzian fit to the integrated profiles did not vary significantly with surface pressure. The degenerate (11) peak was always approximately 3 times broader than the nondegenerate (20) peak. Apart from the LURE data set 17, the Qxy of the degenerate peak was always smaller than that of the nondegenerate peak, corresponding to the lattice distortion azimuth7 in a nearest neighbor (NN) direction, as is the case for the common orthorhombic subcell packing of aliphatic chains.10 In the case of LURE data set 17, the distortion from hexagonal packing is small and could conceivably be due to experimental error. For each of the measurement sets, Table 2 shows the Sirota distortion parameters7 of the two-dimensional Bravais lattice corresponding to the best-fit parameters given in Table 1. These are fully equivalent to the more usual unit cell parameters a, b, and γ but are free from the ambiguity of the latter and allow more ready comparison between different packing symmetries common in monolayers. Only the lowest pressure set shows molecular tilt. In the other cases, the distortion of the unit cell is in the direction found in the common orthorhombic crystalline packing of aliphatic chains, the one with herringbone alternation of the chain zigzag-plane orientation. This is also the distortion direction found by Shih et al. in the LSI subphase of the LS region.14 There is only one data set corresponding to a surface pressure below the tilt transition. The two intensity maxima are both out of the plane of the water surface and with values of Qz essentially in the ratio 1:2. This observation confirms the previous reports of NNN tilt at this surface condition.15,16 (14) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. Phys. Rev. A 1992, 45, 5734 (15) Durbin, M. K.; Shih, M. C.; Malik, A.; Zschack, P.; Dutta, P. Colloids Surf. A 1995, 102, 173. (16) Shih, M. C.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 101, 9132.
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Figure 4. Best-fit curves of the profile along the principal Bragg rod with largest intensity integral. In the doubleLorentzian fit for the L2* case, d (degenerate) labels the individual Lorentzian with twice the integrated intensity of the n (nondegenerate) Lorentzian. Table 2. Lattice Parameters Corresponding to the Best-Fit Parameters of Table 1a data set T/°C π/mN m-1 Α/nm2
Figure 3. Single- and double-Lorentzian best-fit approximations to the integrated profiles. d (degenerate) labels the individual Lorentzian with twice the integrated intensity of the n (nondegenerate) Lorentzian in the double-Lorentzian fits.
In both the LURE and KEK data sets corresponding to pressures above the tilt transition there is just one intensity maximum, but the corrected peak is clearly asymmetric, with significant wings. The data quality is good enough to conclude that there are at least two Lorentzian peaks, but there is no clear evidence for a greater number of peaks. The relative positions and intensities of the peaks are similar to those seen in the LSI subphase of the LS region. On the assumption that there are two peaks, one is narrow and the other at least 3 times broader. It should be noted that the observation of two peaks in the powder pattern is not necessarily inconsistent with the hexagonal macroscopic symmetry of the phase. In liquid crystals it is well-documented that correlations of the herringbone order parameter which decay exponentially with separation can occur in the hexagonal smectic BH phase at temperatures near the transition to the smectic E phase.17 Just as in the present case, there are (17) Levelut, A. M. J. Phys. Fr. 1976, 37, 51.
03 59 57 17 56 55 16
17.5 20 20 20 20 20 20
40 20 27 30 32 35 40
LURE 0.2021 0.2008 0.1990 0.1975 0.1976 0.1987 0.1997
8 18 22 34 40
25 25 25 25 25
8 18 22 34 40
KEK 0.2017 0.1983 0.1972 0.1969 0.1968
ξ
ω
θ/°
0.006 93 0.016 40 0.008 38 0.002 72 0.007 20 0.009 25 0.009 98
NN NN NN NNN NN NN NN
0 0 0 0 0 0 0
0.000 53 0.003 17 0.004 65 0.006 31 0.002 44
NN NN NN NN NN
11.6 0 0 0 0
β NNN -
a Area per molecule, A; distortion magnitude, ξ, distortion direction, ω; tilt magnitude, θ; tilt direction, β.
two powder-pattern peaks, yet the hexagonal symmetry of the phase is apparent when the incident X-ray beam is normal to the smectic planes. On the basis of the X-ray data reported here, we confirm the existence of a tilted phase, L2*, and a nontilted highpressure phase, LS. If the high-pressure LS to LS′ transition found by Lawrie and Barnes11 in isotherm studies exists, it appears to be a first-order transition between phases of identical symmetry. However, the present technique does not allow unequivocal confirmation
GID of Octadecanol Monolayers
of its existence. The phase observed at high pressure in the present studies shows a distortion azimuth different from that of the X-phase reported by Dutta.14 The unusual diffraction peak profile observed from octadecanol monolayers reported here at high pressures has also been observed in more recent measurements.18 (18) Brezesinski, G.; Kaganer, V. M.; Mo¨hwald, H.; Howes, P. B. J. Chem. Phys. 1998, 109, 2006.
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Acknowledgment. We gratefully acknowledge the use of facilities at the Photon Factory and at LURE. We would like to thank Drs V. M. Kaganer and G. Brezesinski for stimulating discussions. One of us (I.R.P.) thanks the Rothschild Foundation, and another of us (R.S.) thanks the German Science Foundation (DFG) for financial support. LA980163P
