Langmuir 1991, 7, 3054-3056
3054
Surface Activity of Semifluorinated Alkanes: F(CF2)m(CH2)nH George L. Gaines, Jr. Department of Chemistry, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Received March 4, 1991. I n Final Form: April 8, 1991 The surface tensions of dilute dodecane solutionsof several semifluorinated alkanes have been measured at room temperature. Decrements of up to 4 mN/m have been observed. While other workers have obtained evidence of micelle formation by these compounds in other solvents, constancy of surface tension with increasing concentration, denoting micellization, has not been observed in the present work. This may result from the onset of gel formation, which occurs at very low concentrations in these systems. It has also been found that some of these compounds form fairly stable monolayers when spread at the air-water interface, and LangmuirBlodgett multilayer deposition has been demonstrated.
Introduction Perfluorinated alkanes and normal (hydrocarbon) alkanes are practically immiscible.' The fluorocarbons have some of the lowest surface energies known-for example, perfluoro-n-hexane has a surface tension at 25 "C of 11.4 mN/m.2 Depending on molecular weight, only some poly(dimethylsiloxanes) appear to possess lower surface energies than fluorocarbon^.^ The low surface energy, as manifested by low wettability, low friction, and poor adhesion, of poly(tetrafluoroethy1ene)makes this polymer practically valuable. Because of these facts, considerable effort has been devoted to examining the surface activity and properties of fluorinated compounds. Beginning in the early 1950's Zisman and his colleagues a t the Naval Research Laboratory studied the wettability of fluorinated polymer surfaces, the adsorption of fluorinated surfactants on solids, and the properties of insoluble monolayers of fluorinated compounds? More recently, the highly nonidea1 interactions between fluorocarbons and hydrocarbons have been studied by Mukerjee et al.5 Another interesting recent report concerns fluorinated silicone polymem6 Several years ago, Mahler at Du Pont and Twieg, Rabolt, and co-workers at IBM reported preparation of a series of compounds with the structure F(CF2),(CH2),H.7i8 These compounds have been called "(perfluoroalkyl) alk a n e ~ ~or' "semifluorinated alkanesne8 A convenient shorthand notation refers to them as "F,H,". They are in essence low molecular weight block copolymers (the largest compound so far reported as m + n = 32) of normal perfluorocarbons and hydrocarbons. Extensive spectroscopic and structural studies have been reported on these comp0unds,8*~ and an interesting gel phase is observed when some of them are dissolved in alkanes.1° (1)Le Grand, D.G.; Gaines, G. L., Jr. J.Colloid Interface Sci. 1975, 50,272. (2)Stiles, V. E.;Cady, G. H. J. Am. Chem. SOC.1952,74,3771. (3)Le Grand, D. G.; Gaines, G. L., Jr. J. Colloid Interface Sci. 1969, 31,162. (4)A review of this work has been given: Jarvis, N. L.; Zisman, W. A. In Encyclopedia of Chemical Technology; Wiley: New York, 1966;Vol. 9,p 707. (5)Mukerjee, P. J. Am. Oil Chem. SOC.1982,59, 573. (6)Kobayashi, H.;Owen, M. J. Macromolecules 1990,23,4929. (7)Mahler, W. Unpublished results. (8) Rabolt, J. F.; Russell, T. P.; Twieg, R. J. Macromolecules 1984,17, 2786. (9)Russell, T. P.;Rabolt, J. F.; Twieg, R. J.; Siemens, R. L.; Farmer, B. L. Macromolecules 1986,19, 1135.
In addition, a liquid-crystal phase has been observed in FloHlo." Mahler7 also had observed that, because of the presence of the unsymmetrical -CF2-CH2- linkage, these compounds are more polar than either fluorocarbons or hydrocarbons; measured dielectric constants ranged up to 6.5, and most of them (especially the hydrocarbon-rich species) are substantially more soluble in polar solvents like methanol than the fluorocarbon or hydrocarbon of similar chain length. Micelle formation in solution has also been ~ b s e r v e d , and ~ ~ Jfoaminess ~ suggests substantial surface activity.12 In the present work, we have provided further evidence of surface activity in two additional, direct ways: surface tension reduction in hydrocarbon solutions and formation of spread monolayers at the air-water interface.
Experimental Section All of thesemifluorinatedalkane samples studied were reported
to have purities (by vapor-phase chromatography) of >99%, except for F12HI8,which was 98.5 % . Solutions were prepared in n-dodecane (Philips99+ % ), which had been purified by passage through silica and alumina columns. The neat dodecane had a surface tension of 25.0 mN/m, in good agreement with the literature value.14 Initial concentrations were selected to be just below the concentrationwhere gel phase was formed, and then sequential dilutions were made until the measured surface tension was indistinguishable from that of the solvent. Surface tension measurementswere made by the drop volume technique, using a micrometer driven syringe. In a few cases, confirmatory measurements were made by the ring method. In the case of a few of the more concentrated solutions, the densities were measured; for the very dilute solutions, they were assumed to have the densityof dodecane. Standardcorrectionswere applied. Monolayers were spread on water (triply distilled) by using hexane as spreading solvent. II-A curves were recorded in our automatic film balance.15 All measurements were made at room temperature,24 1"C.
Results and Discussion Surface Tensions of the Neat Liquids. The surface of a block copolymer containing units of differing surface (10)Rabolt, J. F.;Russell, T. P.; Siemens, R.; Twieg, R. J.; Farmer, B. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1986,27(l),273. (11)Mahler, W.; Guillon, D.: Skou1ios.A. Mol. Cryst. Liq. - Cryst.,Lett. Sect. 1985,2, 111. (12)Houken, J.:Puph, C.;Richterina, W.: Moller. M.Makromol. Chem. 1988,189,911. (13)Turberg, M. P.; Brady, J. E. J. Am. Chem. SOC.1988,110,7797. (14)Jasper, J. J.; Kerr, E. R.; Gregorich, F. J . Am. Chem. SOC.1953, 75,5252. (15)Gaines, G. L.,Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966;pp 66-73.
0743-7463/91/2407-3054$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 12, 1991 3055
Surface Activity of Semifluorinated Alkanes 25
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Figure 1. Surface tension values at 25 O C obtained by Mahler m + n = 6; (0)m + n = 12. (ring tensiometer, ref 7): (0) SURFACE TENSION DECREMENT, mN/m
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Figure 2. Surfacetension reduction in n-dodecane solutionsby F12H,compounds: (v)m = 4; (A)n = 8; (0) n = 14;(0)n = 18.
Solid points denote samples in which a small amountof gel phase was present.
A'IMOLECULE
energy is generally enriched in the component of lower surface energy,16and the surface tension is closer to that of the corresponding homopolymer. The surface tensions measured by Mahler7 for the semifluorinated alkanes are shown in Figure 1, and it is apparent that they do exhibit this trend, especially for the longer chains. It has been found in the case of ethylene oxide-propylene oxide block copolymers that the surface tension becomes progressively lower (closerto that of poly(propy1eneoxide))as the chains are lengthened;17 clearly, the same effect is observed in the present case. It is also noteworthy that, in the series of dodecanes, the surface tension is practically constant when more than half the chain is fluorinated. Surface Tensions of Dodecane Solutions. As had been observed before,1° many of these compounds form a gel phase with n-dodecane when dissolved by heating and then allowed to cool to room temperature. Of these compounds studied in this work (FsH12, F10H12, F12H4, F12H8, F12H14, and F12H18), only F8Hl2 formed a clear solution a t room temperature at a concentration greater than 0.1 M. For F10H12, the gel dissolved below 0.05 M, while all of the others had to be diluted to 0.01 M or below to produce clear solutions. ~~
(16) Gaines, G. L., Jr. Macromolecules 1981, 14, 208. (17)Rastogi, A. K.; St. Pierre, L. E. J. Colloid Interface Sci. 1969,31,
168.
Figure 4. Initial rapid compression curves for semifluorinated alkanes spread at the air-water interface: (A) F12H8; (B)F10H12; (C)FizHis.
The measured surface tensions of the dodecane solutions are shown in Figures 2 and 3, plotted as the decrement in surface tension from that of the pure solvent. The longer the total chain length, the greater the surface tension depression, and the more highly fluorinated compounds generally are more effective, and at lower concentrations. However, in the case of the two F,H12 compounds (Figure 3), the longer fluorocarbon segment does not produce a greater depression of surface tension. In no case examined here does the surface tension stop decreasing, indicating the onset of micellization. As noted, micelles have been observed in solutions of F12H10 in n-octane;12J8the difference from the systems examined here may result from the lower concentration onset of the gel phase associated with the longer chain length of the solvent.lo Monolayers. When dissolved in hexane and applied to a water surface, several of the compounds spread to produce films that were apparently monolayers. The surface pressure-area isotherms of F12H8, F10H12, and (18)Pugh, C.; Hopken, J.; Moller, M. Polym. Prepr. (Am. Chem. SOC., Diu. Polym. Chem.) 1988,29, (l), 460.
Gaines
3056 Langmuir, Vol. 7, No. 12, 1991
F12H18 are shown in Figure 4. These curves were obtained with continuous rapid (10-20 A2/molecule/min) compression, and the curves were reproducible to within f0.5 A2/molecule at these compression rates. Some hysteresis was observed in a compression-expansion cycle, and if compression was stopped, small decreases of surface pressure were observed. F12Hl8 appeared to give the most stable film-for example, in one experiment it was compressed to 12.3 mN/m, and the pressure decayed to 11.9 mN/m in 3.5 min. In contrast, when a film of Fl2H8 was compressed to 4.2 mN/m, the pressure decayed to 2.5 mN/m in 1.5min. These kinetic effects were not explored in detail, and it is not known whether they resulted from some rearrangement, film collapse, or some other loss process such as evaporation. The observed areas per molecule are similar to those observed by Bernett and Zismanl9 for partially fluorinated fatty acids on water; for example, they obtained areas of 33-38 A2 per molecule for films of F(CF2)7(CHz)l&OOH. Clearly, there is not room enough in our films for both the hydrocarbon and fluorocarbon chains to extend up from the surface in a folded configuration; such an orientation might be expected from the presence of the polar center at the CF2-CH2 junction. The most probable configuration in our films has the perfluoroalkyl segment extending up from the surface, with the hydrocarbon segment immersed. While it is at first surprising that a chain as (19) Bernett, M. K.; Zisman,W. A. J. Phys. Chem. 1963,67, 1534.
long as (CH2)ls can be immersed in the water, it is true that alkanes as long as n-octadecane do have small, but measurable, water solubilities.20 Because of the larger cross-section of the perfluorinated chain, there is less energy cost for it to be out of the water, with the hydrocarbon immersed. Although only a few experiments have been performed, it is definitely possible to build up films of F12H18 as Langmuir-Blodgett multilayers. For example, a monolayer on water was compressed to 12.8 mN/m, and a piece of uniformly oxidized silicon that exhibited a blue interference color was dipped in and out four times. Goad Y-type deposition was observed, and the color in the dipped area had shifted to green. Small drops of water were applied to the dipped and undipped areas, and the film-covered area was clearly more hydrophobic (contact angles estimated roughly as -80' and -45').
Acknowledgment. I am grateful to Dr. Walter Mahler for providing his unpublished data and for permission to cite it. Dr. John F. Rabolt kindly supplied the samples of the semifluorinated alkanes. Some of the measurements reported here were carried out at the General Electric Research and Development Center, for which support is acknowledged. Registry No. F8H12,106873-67-0;F10H12,93454-71-8;FlzHd, 89109-69-3;F12H~,90499-31-3;F12H14,93454-73-0;F12H18,9345475-2; dodecane, 112-40-3. (20) McAuliffe, C. Science 1969, 163, 478.
