Thermal Expansion of Surface-Frozen Monolayers of Semifluorinated Alkanes E. Sloutskin, † H. Kraack, † B. Ocko, ‡ J. Ellmann, § M. Mo ¨ller, § P. Lo Nostro, | and M. Deutsch* ,† Physics Department, Bar Ilan University, Ramat Gan 52900, Israel, Physics Department, Brookhaven National Laboratory, Upton, New York 11973, Organische Chemie III, Universita ¨ t Ulm, D-89069 Ulm, Germany, and Dipartimento di Chimica e CSGI, Universita ´ di Firenze, I-50019 Sesto Fiorentino, Italy Received October 14, 2001. In Final Form: November 26, 2001 The linear coefficient of thermal expansion of a quasi-2D surface-frozen crystalline layer is measured, using surface X-ray diffraction, for three different semifluorinated alkane diblocks. The values obtained are in good agreement with those of surface-frozen monolayers of fully protonated alkanes. An unexpected strong dependence on the protonated block’s length is found and discussed. I. Introduction Surface freezing (SF), the formation of an ordered monolayer on the surface of a melt a few degrees above the bulk solidification temperature, was discovered to occur in several families of chain molecules, 1-3 e.g., alkanes, 4 alcohols, 5,6 alkenes, 7 etc. While for normal- alkanes the existing theoretical model provides a reason- able quantitative description of the effect, 8-11 significant deviations between theory and experiment exist in some derivative molecules. 12 Semifluorinated alkanes (SFA), F(CF 2 ) m (CH 2 ) n H (ab- breviated as F m H n ), are molecules that, as normal alkanes, possess only pure van der Waals (vdW) interactions. However, the molecule’s diblock structure and the mutual phobicity of the fluorinated and hydrogenated parts 13,14 break the inversion symmetry of these molecules. This symmetry breaking is further enhanced by the different structure of the two blocks, a planar zigzag for the H block and a helical one for the F block, and their different rigidities and cross-sectional areas, both significantly larger for the F block. SFAs allow varying the relative strength of the interactions by using different molecular lengths, m + n, and block size ratios, n/m. These molecules were shown 12,15 to exhibit SF over a range of block lengths n and m. While the temperature range of existence of the surface frozen phase in normal alkanes is smaller than ∼3 °C, some of the SFAs show significantly broader ranges of surface freezing, 16 thus permitting the measurement of the thermal expansion of the quasi-2D surface-frozen crystal over a significant temperature range. Two different phases were reported 12,15 to exist for the SFA: the low n/m molecules exhibit a reversible first-order surface transition to an hexagonally packed monolayer, while the high n/m ones form an in-plane disordered layer, which melts by a second-order-like continuous transition. Herein we are concerned solely with the low n/m species. The surface-frozen layer consists, in this case, of a monolayer of surface-normal SFA molecules, the F blocks of which point into the vapor, while its H blocks extend loosely into the bulk. We note in passing that a more elaborate structure where the SF layer is a bilayer with a 20-30% coverage in the lower monolayer was found to be consistent as well with experiment. 12,15 In both models the surface- normal-oriented F blocks reside at the free surface of the sample. Since the average cross-sectional area of the F block is ∼28 Å 2 , 17,18 as compared to only ∼19 Å 2 for a normal-alkyl chain, 19 the close-packed ordering observed in the SF layer implies that the order is dominated by the larger F-block’s cross-sectional area. Thus one would expect that the F blocks dominate the structural properties of the SF monolayer and, in particular, the thermal expansion. Moreover, the cross section of the very rigid, helical F block is independent of the F block length. Thus, no variations are expected in the expansion coefficient upon varying n or m in these materials. Contrary to these expectations, the present study demonstrates a pro- nounced dependence of the structure and possibly also the thermal expansion, on the length n of the H block and on n/m. Possible reasons for this dependence are presented and discussed. II. Experiment A. Samples. The samples (F8H8,F10H8, and F10H6) were synthesized by reacting F(CF2)mI with CH2d(CH2)n-1H and reducing the product with tributyl tin hydride to remove the iodine. 20,21 The purity of the samples was >96% and >99% for F10H6 and for F10H8, respectively, and unknown, but similar, for F8H8. B. Measurements. The surface structure was studied using surface-specific X-ray techniques, which have been described * Corresponding author. E-mail: deutsch@mail.biu.ac.il. † Bar Ilan University. ‡ Brookhaven National Laboratory. § Universita ¨t Ulm. | Universita ´ di Firenze. (1) Wu, X. Z.; et al. Science 1993, 261, 1018. (2) Wu, X. Z.; et al. Phys. Rev. Lett. 1993, 70, 958. (3) Earnshaw, J. C.; et al. Phys. Rev. A 1992, 46, R4494. (4) Ocko, B. M.; et al. Phys. Rev. E 1997, 55, 3164. (5) Gang, O.; et al. Phys. Rev. E 1998, 58, 6086. (6) Deutsch, M.; et al. Europhys. Lett. 1995, 30, 283. (7) Gang, H.; et al. J. Phys. Chem. B 1998, 102, 2754. (8) Tkachenko, A. V.; Rabin, Y. Phys. Rev. E 1997, 55, 778. (9) Tkachenko, A. V.; Rabin, Y. Phys. Rev. Lett. 1996, 76, 2527. (10) Tkachenko, A. V.; Rabin, Y. Phys. Rev. Lett. 1997, 79, 532. (11) Sirota, E. B.; et al. Phys. Rev. Lett. 1997, 79, 531. (12) Gang, O. Ph.D. thesis, Bar-Ilan University, 1999 (unpublished). (13) Christensen, J. J.; et al. Handbook of Heats of Mixing; Wiley: New York, 1982. (14) Dunlap, R. D.; et al. J. Am. Chem. Soc. 1959, 81, 2927. (15) Gang, O.; et al. Europhys. Lett. 2000, 49, 761. (16) Sloutskin, E.; et al. To be published. (17) Lo Nostro, P. Adv. Colloid Interface Sci. 1995, 56, 245. (18) Bunn, C. W.; et al. Nature 1954, 174, 549. (19) Small, D. M. The Physical Chemistry of Lipids; Plenum: New York, 1986. 1963 Langmuir 2002, 18, 1963-1967 10.1021/la0156309 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/16/2002