2742
Langmuir 1995,11, 2742-2744
Thermal Expansion Coefficient of a Water-Supported Perfluoro-n-eicosaneMonolayer Zhengqing Huang,? Mark L. Schlossman,t,SAnibal A. Acero,? Zhongjian Zhang,S Ning Lei,? and Stuart A. Rice*,? Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637,and Department of Physics, University of Illinois, Chicago, Illinois 60607 Received March 13, 1995. In Final Form: May 18, 1995@ We report the results of grazing incidence X-ray diffraction studies, as a function of temperature, of monolayers of perfluoro-n-eicosane. The experiments were undertaken to test the suggestion that there is a phase transition in this monolayer between 20 and 25 "C, in analogy with the transition which occurs in Teflon at about the same temperature. The issue is of importance with respect to determining whether descriptionof the properties of a monolayer requires considerationof only the obvious quasi two dimensional character of the monolayer or requires inclusion of its three dimensional character. Our results do not support the existence of the suggested transition.
I. Introduction We recently reported1the results of a grazing incidence X-ray diffraction (GIXD) study of a monolayer of perfluoron-eicosane (F(CF&F) supported at the air-water interface. This monolayer is unusual in that the constituent molecules do not have conventional head groups which embed in the water surface, although the terminal CF bond is polar. The inspiration for the experiment was the combination of the observation that the crystal structure of perfluoro-n-eicosane is lamellar2with the observation that the chain-chain interactions in perfluoroalkanes are stronger than in the equivalent hydrocarbons. It then seemed plausible that strong lateral fluorocarbonfluorocarbon interactions coupled with weak van der Waals interactions between a layer of fluorocarbon molecules and the water surface could stabilize a Langmuir monolayer of perfluoroeicosane. Indeed, that was found to be the case. The GMD data reported in that paper revealed that the perfluoroeicosane monolayer is very well ordered in the temperature range studied, namely, 4-25 "C. Over this temperature range the chain molecules are close packed in a hexagonal lattice, with the long axes of the chains oriented perpendicular to the water surface. The GIXD data, which were taken a t 4, 20, and 25 "C, also showed that the chain-chain spacing decreased by about 0.35%when the temperature was changed from 25 to 4 "C, with a greater rate of change between 25 and 20 "C (from 5.677 to 5.$64 A) than between 20 and 4 "C (from 5.664 to 5.658 A). The latter observation led to the suggestion that there might be a phase transition between 20 and 25 "C in the perfluoro-n-eicosane monolayer, since there is known to be a structural transition in that temperature range in T e f l ~ n .Verifying ~ this suggestion is important since, if it is correct, it will be necessary to recognize that some of the properties of monolayers are characteristic of three dimensional behavior even though other properties are better described as characteristic of two dimensional behavior. t The University of
Chicago.
* University of Illinois. @
Abstract published in Advance ACS Abstracts, July 1, 1995.
. (1)Li, M.; Acero, A. A.; Huang, Z.; Rice, S. A. Nature 1994,367,151.
(2) Schwickert, H.; Strobl, G.;Kimmig, M. J . Chem.Phys. 1991,95, 2800. (3)Bunn, C. W.; Howells, E. R. Nature 1964,174, 549.
A
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Figure 1. Schematic diagram of the experimental setup at beam line X19C, NSLS: (A)toroidal focusingmirror; (B)Si(111) monochromator; (C and E) scintillation photon monitors; (D and G ) slits; (F)Langmuir trough; (H) Soller slits and photon
detector (position-sensitive linear detector or scintillation detector).
Our previous work, because of limitations in available synchrotron time, was limited to the study of the monolayer a t only three temperatures, and the number of measurements made at each temperature was different. Indeed, although we were able to carry out many GMD scans at 4 and 25 "C, at 20 "C we were able to carry out only two GMD scans. Consequently, the uncertainty associated with the difference between the inferred temperature dependences of the monolayer lattice constant in the two intervals 4-20 and 20-25 "C is substantial. More detailed measurements using a finer temperature mesh were needed to unambiguously locate the temperature range in which the transition occurs, if it does occur. To this end, we have undertaken a series of measurements of the temperature dependence of the structure of the perfluoro-n-eicosane monolayer using grazing incidence X-ray diffraction.
11. Experimental Details The GIXD measurementsreported in this paper were carried out using a newly commissioned X-ray surface-scattering spectrometer on beamline X19C at the National Synchrotron Light Source (NSLS)at Brookhaven National Laboratory (Upton,NY). Details of the characteristicsof this spectrometerwill be discussed in a separate report;4 a brief description is given here for convenience. A schematic of the experimental setupis displayed in Figure 1. X-rays from the storage ring are focused horizontally at the sampleposition with a cylindrical mirror. Vertical focusing is achieved by bending the long axis of the mirror. We have consistently obtained a focmed beam size of -1 mm x 0.4 mm (horizontal x vertical). The focused white beam, which travels downward at an angle of approximately 9 mrad from the horizontal, is monochromatized with a Si(111)crystal situated (4) Schlossman,M. L.; et al. Manuscript in preparation.
0743-7463/95/2411-2742$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 7, 1995 2743
A Water-Supported Perfluoro-n-eicosane Monolayer at the center of a Huber three-circlegoniometer. The (111)surface of the crystal is normally placed vertically, but the crystal can be rotated along an axis collinear with the beam path so that the monochromatized beam can be steered downward onto a liquid sample with an angle of incidence in the range of 0-0.18 rad. This large angular range was incorporated in the design to facilitate X-ray reflectivity measurements. For the grazing incidence diffraction measurements reported in this paper the angle of incidence was set at 2.2mrad. The whole monochromator assembly (including the Huber three-circle goniometer) is enclosed in a helium-filled stainless steel tank. A monochromatized X-ray beam is brought out of the tank through a Kapton window. The experiments reported here were all carried out using 8.05 keV (1.548)X-rays. After leaving the tank, the X-ray beam goes through a set of four-jaw slits. Photon intensity monitors were placed in front of and behind the slits. The vertical slit opening was set at 0.1 mm in this experiment. The X-ray beam is then incident on the monolayer sample inside a Langmuir trough assembly, a detailed description of which has been given elsewhere.5 Briefly, the Langmuir trough consits of a milled piece of solid Teflon and associated equipment for surface area and temperature control and for temperature and surface pressure measurements. The system is enclosed in a temperature controlled aluminum housing with a 180" Kapton window to permit entry of incident X-rays and exit of scattered X-rays. The temperature of the substrate water was stabilized to within h0.05 "C of the desired temperature. The accuracy of the temperature measurements was estimated to be better than 0.3 "C. The Wilhelmy method was used to measure the surface pressure with a precision of h0.05 dydcm. Monolayers were prepared by depositing a small volume of a dilute solution of perfluoro-n-eicosane in a 4:lmixture of 1,1,2trichlorotrifluoroethance and perfluoropentane (all from PCR Incorporated, Gainsville, FL) onto the water subphase and allowing the solvent to evaporate. GIXD measurements as a function of temperature were carried out for three monolayers. Two of the monolayer samples were prepared at low temperatures (2and 4 "C; monolayers A and B, respectively) and allowed to warm up, and one was prepared at 25 "C (monolayer C) and then cooled slowly as the X-ray measurements were made. Occasionally the direction of change in temperature was reversed so as to repeat the measurements at a previously used temperature. Typically it took 1-2 h to change the temperature by k2-4 "C and stabilize at the final temperature. After temperature and surface pressure stability were achieved, an in-plane GIXD scan was taken. One or two more scans would then be taken to provide an estimate of the uncertainty in the determination of the diffraction peak position at a given temperature. The diffracted X-ray intensity was monitored using a scintillation detector or a position-sensitive linear detector placed behind Soller slits. The combined in-plane angular resolution from the Soller slits and the X-rays is 0.016 A-l (fwhm) at 1.2 A-1. Each in-plane data point consisted of all photons collected over the range 0 < QL < 0.06A-l, where Qz is the momentum transfer perpendicular to the water surface. As already mentioned, our previous study established that the perfluoro-neicosane molecules are oriented perpendicular to the interface. From time t o time during the course of the current experiment we also took Bragg rod scans of the primary diffraction peak; the results obtained were consistent with the results of our earlier work. 111. Results
We have shown previously that the GIXD peak position in ordered monolayers of perfluoro-n-eicosane are independent of surface coverage. Consequently, all the d a t a reported in this paper were taken at surface densities in the small range from 45 to 50 AVmolecule. Figure 2 shows typical GIXD scans obtained in this experiment. A Gaussian line shape a n d a linear background were used to least-squares fit each diffraction peak. Figure 3a (5) Acero, A. A.; Li, M.; Lin, B.; Rice, S. A.; Goldman, M.; Azouz,I.
B.;Goudot, A,; Rondelez, F. J. Chem. Phys. 1993, 99,7214.
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Figure 2. Grazing incidence X-ray diffraction scans of a perfluoro-n-eicosane monolayer at 26.4 "C: (a)(1,O)diffraction peak; (b) (1,l)diffraction peak. Qxy is the in-plane momentum transfer. The dots with error bars denote experimental data, and the line is the least-squares fit to the data using a Gaussian line shape and linear background.
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Figure 3. Diffraction peak center Qw as a function of water temperature at which X-ray measurements were made: (a) (1,O)data; (b) (1,l)data. Data are shown for three newly prepared monolayers, A, B, and C. The least-squares linear fit (solid line) to the data set A is also shown to help to guide the eye. displays, for each monolayer, t h e dependence of t h e (0,l) diffraction peak center taken from the fit as a function of the water temperature. A few inferences can be drawn immediately from t h e d a t a displayed.
Huang et al.
2744 Langmuir, Vol. 11, No. 7, 1995
First, the temperature dependences of the (0,l) diffraction peaks of the three monolayers show very similar behavior. There is a small uniform difference (-0.0015 A-l) between the peak positions for monolayer A and monolayers B and C. We note that data from monolayer A were obtained in one trip to the NSLS, while those from monolayers B and C were obtained in another trip. Because our beam line was shared with other groups that employed white X-rays, it was necessary to realign some optical elements a t the beginning of each trip. The small difference in peak positions between data set A and data sets B and C, in effect, gives us an estimate of the reproducibility of this alignment procedure. If data set A were shifted downward by about 0.0015 A-l in Figure 3a, it would match the other two data sets very well. The agreement among the three data sets suggests that the monolayers were in thermally equilibrated states when the measurements were taken. Second,the data displayed do not support the suggestion that there is a phase transition between 20 and 25 "C that is associated with a change in the chain-chain spacing. From the scatter ofthe fitted values at a given temperature we estimate that the uncertainty in the determination of the diffraction peak ppsition is about 0.0005 A-1 but could be as high as 0.0015 A-l. Given the range and density of points shown in Figure 3a, the effect of this uncertainty on the inferred temperature dependence is negligibly small with respect to determining if a phase transition occurs in the temperature range studied. Figure 3b displays the temperature dependences of the (1,l)diffraction peak positions for the same three monolayers. The data are a lot noisier and therefore do not clearly show the above mentioned displacement between monolayer A and monolayers B and C. However, they do have nearly the same temperature dependence as do the (1,Oj data. The Qzyvalues of the ( 1 , l j peaks are 4 3 times those of the (1,O) peaks, which strongly suggests that the perfluoroeicosane molecules are packed in a hexagonal lattice. In Figure 4 we show the temperature dependent chainchain separation in the perfluoro-n-eicosane monolayer calculated using the data from Figure 3. A linear function was used to fit each of the six data sets. The average thermal expansion coefficient (of the chain-chain separation) from the fit is (7,l f .8) x for the (1,O) data and (8.0 f .8) x 10-4APC for the (1,l) data. These values are close to that estimated from the 4 and 25 "C data obtained in our earlier measurements. We have not been able to locate any reports of the thermal expansion of long-chain fluorinated alkanes. Studies of normal alkanes,6 however, reveal that the typical bulk expansion coefficient for the tightly packed low temperature orteorhombic phase of these molecules is around 0.006 (A3/ -CH2- j/"C, which corresponds to approximately 6 x H2I"C expansion in the intermolecular spacing (neglecting the temperature variation of the intramolecular C-C bond length). In comparison, the higher temperature loosely packed hexagonal phase of the alkanes has a much greater expansion coefficient, of the order of 5 x k C . Recently Bommarito et al. have reported the results of a study of phase transitions in a water-supported monolayer of behenic acid.7 They observed that, at a surface ( 6 ) Small, D. M. The Physical Chemistry ofLipids; Plenum: New York, 1986; Chapter 2. (7) Bommarito, G. M.; Schlossman, M. L.; Foster, W.; Pershan, P. S. Manuscript in preparation.
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Figure 4. Chain-chain spacing (latticespacing) as a function ofwater temperature for the three monolayers: (a)(1,O) data; (b) (1,l)data. The spacing is calculated from the Qxy values of Figure 3 based on hexagonal packing of the molecules. The least-squareslinearfit (solidline) to the data set Ais also shown to help to guide the eye. pressure of 11-12 dydcm and in the temperature range 3.3-8.3 "C, where the molecules pack in a centeredrectangular lattice, the monotayer has a thermaloexpansion coefficient of about 0.06 A2PC or 6.7 x APC for the chain-chain spacing. This is a rather surprising result because the packing in this monolayer is quite similar to the packing of molecules within an individual layer in the bulk orthorhombic phase of alkanes. The result suggests that either the addition of a polar head group or the transition from a three dimensional bulk structure to a two dimensional monolayer system, or the combination ofboth, is responsible for the large difference in the thermal expansion coefficients of these structures. Some preliminary results from our recent study of a monolayer of perfluorohexadecanoic acid (C15F31COOH)show, however, that its expansion coefficient is very similar to that reported in this paper for perfluoro-n-eicosane. Perhaps, because the diameter of the fluorocarbon chain is larger than the head group, the chain-chain interaction is not much affectedby the head group. In contrast, the diameter of the hydrocarbon chain is a little smaller than that of the head group. The observed large thermal expansion' might, then, reflect the response of the separation of the head groups, which strongly interact with the water surface, to the change in temperature.
Acknowledgment. This work was supported by a grant from the National Science Foundation. X-ray experiments were performed at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy's Office of Basic Energy Sciences. LA950 197M
