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Langmuir 1997, 13, 6669-6682

6669

Adsorption and Aggregation of Semifluorinated Alkanes in Binary and Ternary Mixtures with Hydrocarbon and Fluorocarbon Solvents B. P. Binks, P. D. I. Fletcher,* S. N. Kotsev, and R. L. Thompson Surfactant Science Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received April 21, 1997. In Final Form: October 6, 1997X The phase behavior, adsorption, and aggregation of semifluorinated alkanes (SFAs) of general structure F(CF2)n(CH2)mH (abbreviated to FnHm) in hydrocarbon and fluorocarbon solvents has been investigated. The SFAs form a novel class of structurally primitive surfactants, and their behavior in mixtures with hydrocarbon and fluorocarbon solvents is compared with that of conventional surfactants in mixtures with oil and water. Miscibility studies of hydrocarbon and fluorocarbon solvents of different chain lengths show that the free energies of transfer of -CH2- from hydrocarbon to fluorocarbon phases and -CF2- from fluorocarbon to hydrocarbon are 1.1 and 1.4 kJ mol-1 respectively. These values are approximately onethird of the free energy of transfer of a -CH2- group from alkane to water. Hence, SFAs are expected to be only weakly amphiphilic when compared with conventional surfactants of similar chain lengths in oil + water systems. This expectation is confirmed by SFA solubility and aggregation measurements which show that SFAs aggregate weakly in both hydrocarbon and fluorocarbon solvents to give aggregates with aggregation numbers in the range 2-10. SFAs adsorb at the hydrocarbon-air and hydrocarbonfluorocarbon interfaces but not at the fluorocarbon-air surface. A limited number of two-phase ternary mixtures of SFA with hydrocarbon and fluorocarbon solvents were studied. No evidence for the formation of “middle phase” microemulsions was observed, again consistent with the relatively weak amphiphilicity of SFAs.

Introduction Hydrocarbon and fluorocarbon chains tend to demix. Thus, binary mixtures of alkanes with perfluoroalkanes are only partially miscible at temperatures lower than the upper critical solution temperature (UCST) for the liquid pair. Semifluorinated alkanes (SFAs) of general structure F(CF2)n(CH2)mH (abbreviated to FnHm) have the amphiphilic, diblock architecture required to operate as surfactants, either in binary mixtures with hydrocarbon or fluorocarbon solvents or in ternary mixtures with both hydrocarbon (H) and fluorocarbon (F) solvents. SFAs might be anticipated to show adsorption and aggregation behavior in H and F solvents and in their mixtures in a manner analogous to the behavior shown by “conventional” surfactants (i.e. amphiphiles with a polar plus nonpolar diblock architecture) in either water or oil or in oil-water mixtures. The motivation to study the surfactant properties of SFAs in apolar solvents arises because of the structural simplicity of the systems. In general, the complex patterns of behavior shown by ternary mixtures of a conventional surfactant + water + oil are due to the mutual interactions between the surfactant head and tail groups, oil and water. These interactions are complex and include electrostatic, dipolar, dispersion, solvation, and short range repulsive forces between all the different chemical moieties in the system, i.e., head with tail, head with oil and water, tail with oil and water, and oil with water together with the interactions between like species.1 In contrast, SFA systems with H and F solvents are uncharged and possess negligible dipoles, and thus the forces are likely to be only of relatively short range. Additionally, only H chain-H chain, F chain-F chain and H chain-F chain interactions * To whom correspondence should be addressed. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems. Formulation, solvency and physical properties; Surfactant Science Series 30; Marcel Dekker Inc.: New York, 1988.

S0743-7463(97)00408-3 CCC: $14.00

are involved. These considerations suggest that SFA systems are likely to be more amenable to realistic computer modeling using molecular dynamics simulation methods than are conventional surfactant systems with their complex, longer range interactions. In order to capture the adsorption and aggregation behavior of surfactants, molecular dynamics methods require large numbers of molecules to be simulated over long times. Despite advances in computational power, this requirement limits the method to greatly simplified interaction potentials of short range. Some examples of this approach are given in refs 2-6. SFA systems potentially offer an experimental realization of a “simple” surfactant system to bridge the gap between the complexity of conventional surfactants and this type of computer simulation. There is a limited amount of relevant literature concerning the physico-chemical behavior of solutions of SFAs in hydrocarbon or fluorocarbon solvents. At high concentrations and low temperatures, SFAs in alkane solvents precipitate from solution to yield two-phase mixtures of a solid phase plus a saturated solution. The two-phase mixtures commonly exhibit gel behavior, which is thought to arise as a result of the formation of a network of anisotropic crystals containing entrapped solvent. Phase behavior and microstructure for these systems are discussed in refs 7-14. Since the liquid-vapor surface tensions of fluorocarbons are generally lower than the (2) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os, N. M.; Schlijper, A. G. Nature 1990, 348, 624. (3) Smit, B.; Hilbers, P. A. J.; Esselink, K. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S.-H., et al., Kluwer: Amsterdam, 1992; p 519. (4) Karaborni, S.; van Os, N. M.; Esselink, K.; Hilbers, P. A. J. Langmuir 1993, 9, 1175. (5) Hariharan, A.; Harris, J. G. J. Chem. Phys. 1994, 101, 4156. (6) Klopfer, K. J.; Vanderlick, T. K. Colloids Surf. A 1995, 96, 171. (7) Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361. (8) Rabolt, J. F.; Russell, T. P.; Siemens, R.; Twieg, R. J.; Farmer, B. Polymer Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1986, 27, 223. (9) Hopken, J.; Pugh, C.; Richtering, W.; Moller, M. Makromol. Chem. 1988, 189, 911. (10) Hopken, J. Ph.D. Thesis, University of Twente, 1991. (11) Hopken, J.; Moller, M. Macromolecules 1992, 25, 2482.

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corresponding alkanes, SFA species are expected to adsorb at the hydrocarbon-air surface but not at the fluorocarbon-air surface. Gaines has measured the surface tensions of dodecane solutions of F12Hm (m ) 4, 8, 14 and 18) and FnH12 (n ) 8 and 10) at room temperature.15 The surface activity was found to increase with increasing H chain length of the SFA. Maximum surface pressures were only 0-4 mN/m, and no behavior indicative of micelle formation at concentrations less than the gel formation (solubility) limit was observed. The surface activity of a range of SFAs in solution in “vaseline oil” was investigated by Napoli et al.16 They observed maximum surface pressures of up to 10 mN/m at 20 °C and showed that the surface activity (judged by the initial slope of the tension vs concentration curve) increases with increasing F chain length and passes through a minimum with increasing H chain length. Effects of alkane solvent chain length and temperature on the adsorption of SFAs at the hydrocarbon-air surface have been reported.17 Increasing the alkane solvent chain length and decreasing the temperature led to increased adsorption. In strongly adsorbing systems such as F12H14 in pentadecane at 20 °C, the adsorbed monolayers were found to be very condensed with limiting areas per SFA molecule of 0.26 nm2, very close to the cross-sectional area of a vertical, fully extended -CF2- chain (0.28 nm2). Again, in this study, no evidence for micelle formation in the alkane solvents was observed at SFA concentrations lower than the solubility limit. Micelle-like aggregation of SFAs has been observed in only a few cases. Hopken et al.9 report a light scattering study of F12H10 in octane at 35 °C. A Zimm plot of the data over the concentration range 3-25 mM showed the presence of aggregates containing approximately 130 monomers. This observation appears to be questionable since subsequent surface tension and vapor pressure studies on related systems indicate no micelle formation in short chain alkanes below the solubility limit. For example, vapor pressure osmometry measurements showed that F12H14 behaves ideally in dodecane at 50 °C up to the solubility limit of approximately 2 mol %.17 Turberg and Brady18 investigated the aggregation of F8H16 in perfluorooctane at 40 °C. They detected a break point in the light scattering intensity and in the extent of solubilization of a dye at an SFA concentration of approximately 5 wt %. They estimated the aggregation number of the aggregates formed to be approximately 4-6. They also report that F8H12 does not aggregate in perfluorohexane at concentrations up to 10 wt %. Lo Nostro and Chen12 reported a more detailed study of F8H16 solutions (0-12 wt %) in perfluorooctane at 41 °C using viscosity and light and neutron scattering measurements. They concluded that this system forms small, spherical aggregates containing approximately 95 monomers at concentrations above the cmc of 4 wt %. In contrast to this relatively high aggregation number, Binks et al.19,20 report vapor pressure osmometry measurements for a range of SFA/solvent binary mixtures for which aggregation numbers in the range 2-10 are observed. Finally, (12) Lo Nostro, P.; Chen, S-H. J. Phys. Chem. 1993, 97, 6535. (13) Lo Nostro, P.; Ku, C. Y.; Chen, S.-H.; Lin, J.-S. J. Phys. Chem. 1995, 99, 10858. (14) Napoli, M. J. Fluorine Chemistry 1996, 79, 59. (15) Gaines, G. L., Jr. Langmuir 1991, 7, 3054. (16) Napoli, M.; Fraccaro, C.; Scipioni, A.; Alessi, P. J. Fluorine Chem. 1991, 51, 103. (17) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 1995, 11, 977. (18) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (19) Binks, B. P.; Fletcher, P. D. I.; Thompson, R. L. Ber. BunsenGes. Phys. Chem. 1996, 100, 232. (20) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. J. Molecular Liquids 1997, 72, 177.

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Fulton et al.21 report aggregation numbers of not more than 4 for F10H10 in super critical CO2. In this work, we report a systematic study of the surfactant properties of a range of FnHm SFAs in binary and ternary mixtures with linear alkanes (H(CH2)mH abbreviated to Hm) and linear perfluoroalkanes (F(CF2)nF abbreviated to Fn) of different chain lengths. The overall aim of the study was to map out the surfactant properties of the SFAs and compare the behavior with that of conventional surfactants in binary and ternary mixtures with water and oil. This paper is organized as follows. Following the Experimental Section, we report solubility data for alkanes in perfluoroalkanes (and vice versa) as functions of both the temperature and the chain lengths of the solvents and solutes. These data allow a comparison to be made between the antipathy of H and F chains with that between oil and water. Second, we report the temperature dependence of the solubility of SFAs in either Hm or Fn solvents. In the third section, aggregation of SFAs in Fn and Hm solvents is discussed. Fourth, the adsorption properties of SFAs at the solvent-air interface are described. In the fifth section, we describe the phase behavior of SFAs in ternary mixtures with Hm and Fn solvents and adsorption of the SFAs at the Hm-Fn liquidliquid interface. Finally, the main conclusions of the study are listed. Experimental Section The SFAs were synthesized according to the method of Rabolt et al.22 by Synprotec (Manchester, U.K.). Samples were analyzed using GC with mass spectroscopic detection and yielded a single chromatographic peak which was estimated to correspond to a minimum purity of approximately 98%. Samples were found to contain a minor dodecane-insoluble impurity which did not show up on the GC-MS. This impurity was removed by passing a pentane solution of the SFA over an alumina column and evaporating the solvent (yield approximately 90%). The apolar oils perfluoroheptane (Fluorochem, 95-97%), perfluorooctane (Fluorochem, >98%), perfluorononane (Fluorochem, 99%), perfluorodecalin (Fluorochem, 95%), isooctane (2,2,4-trimethylpentane, Lancaster, 99%), octane (Fluka, >99.5%), decane (Aldrich, 99+%), dodecane (Aldrich, 99+%), tetradecane (Aldrich, 99+%), hexadecane (Aldrich, 99+%), toluene (Sigma-Aldrich, 99.8%), and CCl4 (May & Baker, 99.8%) were passed over an alumina column prior to use to remove polar impurities. The solids perfluorotetradecane (Fluorochem, >97%) and eicosane (Sigma, 99%) were used as received. The extents of mutual miscibility of Fn + Hm solvent pairs were determined by preparing solvent mixtures of known composition in flame-sealed glass tubes. The temperature limits of the single-phase region for each mixture was determined by visual observation of the samples at different temperatures. Increasing or decreasing the temperature yielded consistent results within approximately 1 °C. SFA solubilities in Fn or Hm solvents were determined similarly by visual observation of the temperature at which opaque crystals of the SFA were observed to appear or disappear on decreasing or increasing the temperature. Vapor pressure osmometry (VPO) measurements were made using a Gonotec Osmomat 070 instrument. Each value was the mean of at least four separate determinations. VPO measurements were restricted to solvent systems showing a vapor pressure at the measurement temperature of greater than approximately 10 mmHg. Light scattering measurements were made using a Malvern PCS4700 instrument. Samples were filtered using Whatman Anatop inorganic membrane filters of pore diameter 0.02 µm. A software algorithm to reject light scattering data perturbed by the presence of residual “dust” (21) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241. (22) Rabolt J. F.; Russell T. P.; Twieg, R. J. Macromolecules 1984, 17, 2786.

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particles was implemented. Surface tensions were measured using the drop volume technique using Hamilton gastight syringes. Stainless steel tips were used for tension measurements at the liquid-vapor surfaces and Teflon tips were used for fluorocarbon-hydrocarbon interfaces. Full experimental details of the VPO, light scattering and tension measurements are given in ref 23. Densities were measured using a Paar DMA55 densimeter, and refractive index values were determined using an Abbe refractometer.

Results and Discussion Miscibility of Hydrocarbon and Fluorocarbon Solvents. The main driving force for the adsorption and aggregation of conventional surfactants in water arises from the antipathy between hydrocarbon chains and water (the hydrophobic effect). As for alkanes with water, the tendency of hydrocarbon and fluorocarbon solvents to demix can be quantified by measurement of their mutual miscibility. In general, linear-chain hydrocarbon and fluorocarbon solvents are miscible in all proportions above an upper critical solution temperature (UCST) and show partial mutual solubility at temperatures below the UCST. Values of the UCST have been determined for a range of binary hydrocarbon/fluorocarbon solvent pairs.24-29 It is found that the UCST increases, indicating a greater tendency to demix, as the chain length of the hydrocarbon solvent increases. Increasing the chain length of the fluorocarbon solvent is found to cause only a small increase in the UCST. When two species R and β are partially miscible, mixtures at equilibrium form two phases A and B, being rich in species R and species β, respectively. When the mutual miscibility is low, the difference in standard chemical potential of species R caused by transferring it from phase A to phase B at infinite dilution, ∆µotr, is

∆µotr ) -RT ln xR(B)

Figure 1. Variation of (a) solubility and (b) ∆µotr with temperature for H8 (diamonds), H10 (triangles), H12 (circles), and H16 (squares) into F9.

(1)

where xR(B) is the mole fraction solubility of species R in phase B. The quantity ∆µotr can be split into its entropic and enthalpic components as

∆µotr ) ∆Hotr - T∆Sotr

(2)

Thus, the enthalpic term ∆Hotr can be determined from the slope of a plot of ln(solubility) vs 1/T and the entropic term ∆Sotr is given by d∆µotr/dT. A series of solubility measurements as a function of temperature were made for solvent pairs of different chain lengths in order to obtain the standard partial molar free energies, enthalpies and entropies of transfer per -CH2of the alkane species and per -CF2- of the fluorinated species. A typical set of results is shown in Figure 1 for the solubility of Hm in F9. The solubility increases with increasing temperature and is higher for the short chain Hm species. The transfer process is endothermic and is associated with a positive entropy change. The variation of the standard partial molar free energies, enthalpies, and entropies of transfer with alkane chain length are shown in Figure 2. The free energy of transfer of Hm into F9 becomes increasingly unfavorable by approximately (23) Thompson, R. L. Ph.D. Thesis, University of Hull, U.K., 1997. (24) Hildebrand, J. H.; Cochrane, D. R. F. J. Am. Chem. Soc. 1949, 71, 22. (25) Scott, R. L. J. Phys. Chem. 1958, 62, 136. (26) Young, C. L. Trans. Faraday Soc. 1969, 65, 2639. (27) Hurle, R. L.; Toczylkin, L. S.; Young, C. L. J. Chem. Soc. Faraday Trans. 2 1977, 73, 618. (28) Hicks, C. P.; Hurle, R. L.; Toczylkin, L. S.; Young C. L. Aust. J. Chem. 1978, 31, 19. (29) Lo Nostro, P. Adv. Colloid Interface Sci. 1995, 56, 245

Figure 2. Derived plots of the standard chemical potential (a), partial molar enthalpy (b), and partial molar entropy (c) of transfer for Hn from Hn into F9 at 25 °C.

1.1 kJ mol-1 for each additional methylene group in the alkane solute at 25 °C. This value quantifies the antipathy between alkane and perfluoroalkane chains and is approximately one-third of the corresponding value for the transfer of alkane into water (the hydrophobic effect) which has a magnitude of 3.7 kJ mol-1.30 The increment of ∆µotr for transfer of Hm into F9 per methylene group comprises a positive enthalpic contribution (approximately 1.4 kJ (30) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley and Sons: New York, 1980.

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Table 1. Values of ∆µotr (kJ mol-1), ∆Hotr (kJ mol-1), and ∆Sotr (J mol-1 K-1) for Hydrocarbon and Fluorocarbon Liquidsa solute

solvent

∆µotr (25 °C) ((0.1)

∆Hotr ((2)

∆Sotr ((7)

∆µotr/dn ((0.1)

H8 H10 H12 H16

F9 F9 F9 F9

Hn in F9 5.9 8.1 10.5 14.9

17 20 23 28

37 41 42 45

1.1 1.1 1.1 1.1

F5 F6 F7 F8 F9

H12 H12 H12 H12 H12

Fn in H12 5.7 7.7 9.1 10.0 11.6

12 18 20 18 22

22 37 36 28 38

1.4 1.4 1.4 1.4 1.4

H10 H10 H10 H10 H10 H10

F5 F6 F7 F8 F9 PFD

H10 in Fn 8.0 7.6 7.8 7.7 8.1 6.9

17 21 28 22 20 21

31 45 68 46 41 48

0.0 0.0 0.0 0.0 0.0

F9 F9 F9 F9 F9 F9

H8 H10 H12 H16 toluene CCl4

F9 in Hn 9.2 10.9 11.6 12.5 15.0 10.7

20 22 22 25 31 29

36 37 38 42 53 62

1.0 0.6 0.3 0.0

a The right hand column shows the increment in ∆µo (kJ mol-1) tr at 25 °C per additional carbon in the solute or solvent species (n). PFD is perfluorodecalin.

mol-1) and a positive entropic contribution (approximately 1 J mol-1 K-1). The uncertainty in the enthalpic and entropic components is approximately (50%. Thermodynamic parameters for a range of solute and solvent species are summarized in Table 1. For all cases investigated, the solute transfer is endothermic and corresponds to a positive ∆Sotr. For the solubility of Fn in H12, the increment in ∆µotr per -CF2- group at 25 °C is 1.4 kJ mol-1, similar to that for Hm in F9 and less than that for Fn in water (estimated to be on the order of 1.7 times the value for Hm into water, i.e., approximately 6.5 kJ mol-1).31 The solubility of H10 in Fn shows little variation with the chain length of the fluorocarbon solvent. This observation is consistent with the fact that the UCST of alkanes with perfluoroalkanes is only weakly dependent on the fluorocarbon chain length. F9 solubility in Hm decreases with increasing alkane chain length, but the increment in ∆µotr per additional methylene group decreases with increasing hydrocarbon chain length. The main conclusions drawn from these solubility results are, first, that the antipathy between hydrocarbon and fluorocarbon chains (expressed per carbon atom) is approximately one-third of the hydrophobic effect. Second, the antipathy between H and F chains increases with H and F chain length of the solute species and also with the H chain length of the hydrocarbon solvents but is insensitive to the F chain length of the solvent species. Table 1 also includes data for the solubility of H10 in perfluorodecalin (PFD) and F9 in toluene and CCl4. The antipathy between H10 and PFD is similar to that for the linear chain perfluoroalkanes whereas that for F9 with the aromatic oil toluene is greater than that for hexadecane. The antipathy between F9 and CCl4 is similar to that for F9 with H10 at 20 °C but appears to decrease more rapidly with increasing temperature. (31) Ravey, J. C.; Stebe, M. J. Colloids Surf. A 1994, 84, 11.

Solubility of SFAs in Fn and Hm Solvents. The amphiphilic strength of a solute in a solvent is reflected by the extent to which the temperature dependence of the solubility deviates from that of a solute behaving ideally. For an ideal solute, the variation of solubility with temperature is given by

ln S ) (∆Hmelting/R)(1/Tm - 1/T)

for T > Tm (3)

where S is the solubility (in mole fraction units), ∆Hmelting is the enthalpy of melting of the solute, T is the absolute temperature, and Tm is the melting temperature. Solutes which show aggregation to form micelles deviate strongly from ideal behavior in that the solubilities are generally less than the values predicted by the ideal curve. Additionally, the solubility increases very sharply at the temperature (known as the Krafft temperature) at which the surfactant monomer solubility equals the critical micelle concentration (cmc).32 Thus solubility measurements can yield the critical aggregation concentration (cac) at the Krafft temperature (Tk). (In this paper the more general term cac is used in preference to cmc as the nature of the aggregates formed in SFA systems are not necessarily similar to micelles formed by conventional surfactants.) SFAs in apolar hydrocarbon or fluorocarbon solvents at concentrations in excess of their solubility do not show the usual precipitation of a solid phase. Generally, these systems form gels which are thought to consist of an intermeshed network of elongated crystals.7-14 Additionally, solid phases of SFAs may show a second phase transition in addition to the main melting transition. This premelting transition is thought to be associated with “melting” of the H chains of the SFAs11 and is seen for SFA species with high H:F chain length ratios. For such solutes, the equation describing the ideal solubility behavior is modified to include the enthalpy change associated with the premelting transition over the appropriate temperature range. Ideal solubility behavior was calculated based on enthalpy data taken from refs 10-12 with ∆Hmelting for F10H16 obtained by interpolation using values for F8H16 and F12H16. Figure 3 shows an example of the variation of solubility with temperature for F8H16 and F10H16 in various perfluorocarbon solvents. In all cases the experimental data deviate strongly from the ideal solubility curve. The SFA concentrations corresponding to the break points in the solubility curves (cac) are not greatly affected by the nature of the fluorocarbon solvent or by changing the length of the F chain of the SFA from 8 to 10. Solubility curves for the same SFA species in various hydrocarbon solvents are shown in Figure 4. For F8H16 in isooctane (where the driving force for aggregation of the SFA is expected to be weak) the solubility curve is relatively close to the ideal curve and shows little evidence of a Krafft-type discontinuity. Stronger surfactant-like behavior is obtained for F10H16 where the longer F chain of the SFA is expected to increase the driving force for aggregation. Overall, for all the SFA/solvent systems the discontinuity in the solubility curves is generally less sharp than for conventional surfactants in water,32 indicating that the SFA aggregation properties are somewhat weaker than for conventional surfactants. Aggregation of SFAs in Hm solvents is expected to lead to aggregates consisting of a “core” of F chains with a “shell” of H chains solvated by the hydrocarbon solvent. This type of aggregation is denoted F/H and is driven by (32) Laughlin, R. G. The Aqueous Phase Behaviour of Surfactants; Academic Press: London, 1994.

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∆µoagg ) RT ln(cac)

Figure 3. Temperature variation of the solubility of F8H16 (a) and F10H16 (b) in various perfluorocarbon solvents. Data for F8H16 in F8 was taken from ref 12. The solid lines show ideal behavior.

Figure 4. Temperature variation of the solubility of F8H16 (a) and F10H16 (b) in various hydrocarbon solvents. Data for F8H16 in iso-H8 was taken from ref 12. The solid lines show ideal behavior.

the tendency of the F chains of the SFA to minimize contact with the H solvent. The situation is reversed for SFA aggregation in Fn solvents for which H/F aggregates are expected. For uncharged solutes forming aggregates of large aggregation number, the change in standard partial molar free energy associated with the aggregation (∆µoagg) is given by the approximate equation

(4)

For H/F aggregate formation by an SFA species FnHm, the value of ∆µoagg is expected to be of a similar magnitude to the free energy of transfer of an alkane Hn from a fluorocarbon to a hydrocarbon solvent. The free energy change associated with F/H aggregate formation is similarly expected to be dominated by the free energy of transfer of the Fn species from hydrocarbon to fluorocarbon. Values of the cac and ∆µoagg estimated from solubility measurements for a range of SFA and solvent systems are summarized in Table 2. It is of interest to compare the increments in ∆µoagg with increasing chain lengths for SFA aggregation with those noted previously for solvent mutual solubility. The results in Table 2 show the following trends with chain lengths of the various species. (i) For FnH16 aggregation in H16, the cac falls from 0.12 to 0.05 as n is increased from 8 to 10. This is expected since the aggregation in this case (F/H type) is driven by the antipathy of F chain of the SFA for the H16 solvent and this antipathy increases with increasing n. The increment in -∆µoagg per additional -CF2- group is approximately 1.4 kJ mol-1, similar to the increment seen for the solubility of Fn species in H12 solvent at 25 °C (1.4 kJ mol-1). Because the temperatures and the H solvent chain lengths are different for the aggregation and solvent solubility data, the agreement in the exact magnitude of the increment is likely to be fortuitous. (ii) For F10Hm aggregation in F9 (H/F aggregates formed), the cac falls from 0.14 to 0.08 as m is increased from 10 to 16. The increment in -∆µoagg is approximately 0.3 kJ mol-1 per -CH2-, significantly smaller than the corresponding increment for the solubility of Hm in F9 at 25 °C (1.1 kJ mol-1). Since the effect of the different temperatures for the cac and solubility data is estimated to perturb the comparison by less than 0.1 kJ mol-1 per -CH2-, the smaller increment seen for aggregation suggests that the aggregation does not lead to complete separation of the Hm chain from the F9 solvent. (iii) The cac values for FnH16 in F9 do not vary with n (varied from 8 to 10). This independence is expected since the driving force for the H/F aggregate formation in this case should be controlled by the (constant) antipathy of the -H16 chain of the SFA with the F9 solvent. (iv) For F10Hm in H16 a similar independence to that noted in 3 above is expected. However, the cac decreases from 0.10 to 0.05 as m is increased from 10 to 16. In this case, it is speculated that a more favorable packing within the aggregates might be responsible for the reduction in cac. (v) For F8H16 in Fn (and PFD), the cac values are virtually independent of the chain length of the fluorocarbon solvent. This observation is consistent with the fact that ∆µotr for H10 into Fn is independent of n (Table 1). (vi) F8H16 shows a cac value of 0.12 in H10 and H16 but behaves close to ideally in isooctane (i-H8). This indicates that the antipathy of the F8 chain of the SFA for the solvent is low for the short chain i-H8 but is appreciable for the longer chain H solvents. This behavior is consistent with the observations in Table 1 for the solubility of F9 in Hm solvents. The comparison of SFA aggregation and solvent solubilities discussed above is rather crude for a number of reasons. First, the cac values estimated from SFA solubility measurements refer to the Krafft temperatures which necessarily change with the SFA species. Hence the variation of cac with SFA chain length includes a contribution from changing temperature which has a significant effect on the antipathy between H and F species.

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Table 2. Tm, Tk, and Cac Values (Where Present) Estimated from Solubility Data for Various Binary SFA plus Solvent Systems with Some Entries Repeated To Allow Easy Comparison SFA

solvent

Tm/°C

Tk/°C

cac/mole fraction

-∆µoagg/ kJ mol-1

F8H16 F10H16

F9 F9

FnH16 in F9 53 42 71.4 56

0.08 0.08

6.6 6.9

F8H16 F10H16

H16 H16

FnH16 in H16 53 30 71.4 48

0.12 0.05

5.3 8.0

F10H10 F10H16

F9 F9

F10Hn in F9 61 30 71.4 56

0.14 0.08

5.0 6.9

F10H10 F10H16

H16 H16

F10Hn in H16 61 41 71.4 48

0.10 0.05

6.0 8.0

F8H16 F8H16 F8H16 F8H16

F9 F8 F7 PFD

F8H16 in Fn 53 42 50.1 41 53 39.5 53 34

0.08 0.10 0.08 0.10

6.6 6.0 6.6 5.9

F10H16 F10H16

F9 PFD

F10H16 in F9 and PFD 71.4 56 0.08 71.4 47 0.06

6.9 7.5

F8H16 F8H16 F8H16

i-H8 H10 H16

F8H16 in Hn 50.1 53 26 53 30

0.12 0.12

5.3 5.3

F10H16 F10H16

H16 H20

F10H16 in Hn 71.4 48 71.4 54

0.05 0.06

8.0 7.6

Second, the values of -∆µoagg were derived from the measured cac values using eq 4 in which it was assumed that the aggregation numbers of the aggregates were large. As will be seen later, SFAs generally form only small aggregates and thus the values of -∆µoagg should be regarded as rather approximate. In spite of these uncertainties, it can be seen that the variation of cac values with chain lengths of the SFA and solvents broadly follow the trends expected on the basis of the solvent miscibilities. In addition to the SFA solubility measurements described above, a limited number of systems were examined using light scattering. In order to obtain the variation of scattered intensity for solutions containing aggregates, these measurements were made at temperatures above Tk. Figure 5 shows the static scattering (normalized with respect to the value measured for pure toluene) as a function of concentration for F8H16 and F10H16, each in F9 and H16 solvents. F8H16 solutions in H16 at 45 °C show an abrupt increase in scattering intensity at a mole fraction of 0.14, taken to be the cac. This is in reasonable agreement with the cac estimated from solubility measurements (0.12). The ratio of the slopes of intensity vs concentration above and below the cac was taken to be an approximate estimate of the weight average aggregation number of the aggregates (〈N〉W) and was estimated to be 250 ( 200 in this case. For F9 as solvent the curve shows a break at 0.05 mole fraction, again in agreement with the cac estimated from solubility data. The initial slope of the intensity plot above the cac yields an approximate value of 〈N〉W of 10 ( 8. The decrease in d(I/Itoluene)/dx at higher concentrations is probably a consequence of repulsive interactions between the aggregates as the volume fraction of aggregates reaches approximately 30% in this case. F10H16 in F9 shows a cac value in reasonable agreement with the solubility value whereas the curve for H16 as solvent shows apparently anomalous behavior. For this latter system, solubility data indicate a cac of

Figure 5. Variation of normalized light scattering intensity (I/Itoluene) for (a) F8H16 in F9 (filled circles) and H16 (open circles) at 45 °C and (b) F10H16 in F9 (filled circles) and H16 (open circles) at 64 °C. Table 3. Summary of Cac Values and Aggregation Numbers 〈N〉W Estimated from Light Scattering SFA

solvent

temp/°C

cac/mole frac

〈N〉W

F8H16 F8H16 F10H16 F10H16

F9 H16 F9 H16

45 45 64 64

0.05 ( 0.02 0.14 ( 0.01 0.06 ( 0.02 0.15 ( 0.02

10 ( 8 250 ( 200 6(4 4(3

0.05 whereas the scattering intensity curve shows no sign of a break at this concentration. The reasons for this are unclear, but one possibility is that it is due to the difference in temperature for the two measurements (64 °C for the scattering and 48 °C for the solubility). The values of cac and 〈N〉W are summarized in Table 3. Aggregation of SFAs in Fn and Hm Solvents. In addition to the very limited light scattering data for aggregation numbers, we have used vapor pressure osmometry (VPO) to investigate the aggregation behavior of FnHm in selected solvents. Since the VPO technique could only be used with solvents showing vapor pressures higher than approximately 10 mmHg at the measurement temperature, the results are limited to liquid fluorocarbon and short chain hydrocarbon solvents. As discussed by Corkill et al.,33 the signal measured in VPO is a colligative property which is determined by the mole fraction of solvent xs. In a system in which the (involatile) solute exists in both monomeric and associated states, a solute colligative mole fraction (xc) is defined by

xc ) x1 +

∑xn) 1 - xs

(5)

where x1 and xn are the mole fraction of the monomeric and nth associated species respectively and the summation extends over all aggregated species. The solute colligative mole fraction xc is obtained from the measured VPO signal by calibration of the VPO using measurements of VPO signal vs concentration of an ideal solute in the solvent (33) Corkill, J. M.; Goodman, J. F.; Walker, T.; Wyer, J. Proc. R. Soc. London, A 1969, 312, 243.

Semifluorinated Alkanes in Mixtures

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Figure 6. (a) Comparison of xc vs xt for F8H16 (filled squares) and F10H16 (unfilled squares) in F9 at 63.7 °C. (b) Monomer concentration x1 vs xt for the same systems. In each case the curved lines are polynomial fits to the data and the straight lines show ideal behavior.

of interest. Eicosane and perfluorotetradecane were used as ideal, “calibration” solutes for hydrocarbon and fluorocarbon solvents respectively. Linear calibration plots were obtained for each solvent and temperature investigated thereby confirming the ideal behavior of these systems. The total solute mole fraction xt is given by

xt ) x1 +

∑nxn

(6)

The mole fraction of monomer (x1) may be obtained from the measured dependence of xc on xt since a thermodynamic analysis33 yields

dxc/xt ) d(ln x1)

(7)

Integration of eq 7 using an experimentally determined relation between xc and xt (choosing as the lower limit for the integration a solution concentration sufficiently dilute such that aggregation is negligible, i.e., a concentration such that xc ) xt) yields the variation of x1 as a function of xc and hence of xt. Combination of these relations gives the number average aggregation number 〈N〉N of the aggregates (i.e. excluding the monomer concentration) as a function of xt according to

〈N〉N ) (xt - x1)/(xc - x1)

(8)

The aggregation behavior of F8H16 and F10H16 in F9 at 63.7 °C is compared in Figures 6 and 7. For F8H16 and F10H16 the cac (estimated from solubility) is 0.08 at their Krafft temperatures of 42 and 56 °C respectively. Figure 6a shows the variation of xc with xt for the two SFAs with the linear, ideal behavior for comparison. The plots for the SFAs fall below the ideal line indicating aggregation is occurring and were analyzed according to eq 7 to yield the derived plots of x1 vs xt shown in Figure 6b. The monomer concentrations deviate progressively from ideal behavior at concentrations below the apparent cac and appear to approach the cac asymptotically at high total

Figure 7. Number average aggregation number 〈N〉n vs xt for F8H16 (filled squares) and F10H16 (unfilled squares) in F9 at 63.7 °C.

concentrations. The SFA aggregation behavior resembles the weak aggregation seen for either short chain surfactants in water34 or for alkylpolyoxyethylene surfactants in water-free alkane solvents35 where the curves lack a very sharply defined cmc due to the small aggregation numbers. The derived values of the number average aggregation number 〈N〉n are shown in Figure 7 where it can be seen that 〈N〉n is approximately 3 for both SFA species and increases slightly with increasing SFA concentration. The uncertainty in 〈N〉n is estimated to be (0.5. For these systems, where H/F aggregates are formed, the aggregation behavior is not significantly affected by the length of the F chain of the SFA. Figure 8 shows the effect of varying the H chain length of the SFA on the aggregation in F9. As seen above, the formation of H/F aggregates is unaffected by the F chain length of the SFA, presumably due to the fact that the driving force for H/F aggregation is primarily determined by the antipathy between the H chain of the SFA and the F9 solvent. On this basis, decreasing the H chain length of the SFA is expected to decrease the aggregation and this can be seen in Figure 8. Whereas F10H16 aggregates relatively strongly, F10H10 behaves almost ideally up to the maximum concentration for which VPO measurements could be made. The aggregation of F8H16 in a range of perfluorinated solvents at 45 °C is compared in Figures 9 and 10. Considering the linear perfluoroalkanes first, the Krafft temperatures and cac values estimated from solubility are 42, 41, and 39.5 °C and 0.08, 0.10, and 0.08 for F9, F8, and F7 respectively. Examination of the plots of xc vs xt (Figure 9) for F9, F8, and F7 show that deviations from ideal behavior occur for the lowest concentrations tested for F9 and F7 whereas F8 behaves ideally up to a mole fraction of approximately 0.03. When analyzed according to eqs 5-8, these differences in the shapes of the xc vs xt plots result in quite large differences in aggregation numbers (Figure 10). F9 and F7 show aggregation numbers of 2-3 whereas values of 7-10 are found for F8. Since the (34) Clint, J. H.; Walker, T. J. Chem. Soc., Faraday Trans. 1 1975, 71, 946. (35) Jones, P.; Wyn-Jones, E.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 1, 1987, 83, 2735.

6676 Langmuir, Vol. 13, No. 25, 1997

Figure 8. Comparison of xc vs xt for F10H10 (filled circles) and F10H16 (unfilled squares) in F9 at 63.7 °C. The straight line shows ideal behavior.

Figure 9. Variation of xc with xt for F8H16 in F9 (filled circles), F8 (filled squares), F7 (unfilled triangles), and perfluorodecalin (unfilled circles). The straight line shows ideal behavior.

driving force for aggregation (H/F type) is not expected to be very sensitive to the chain length of the fluorocarbon solvent, the origin of the apparently anomalous behavior for F8 as solvent is unclear at present. It can be speculated that it may be due to favorable packing interactions associated with matching of the F chain length of the SFA and solvent or it may arise from an incompatibility of odd and even carbon number chains. The aggregation of F8H16 in perfluorodecalin is intermediate in behavior between that for F8 and the odd carbon number perfluoroalkanes and shows aggregation numbers of 5-6. The aggregation of F8H16 in F8 at temperatures just above the Krafft temperature has been examined previously by Turberg and Brady18 and Lo Nostro and Chen.12 On the basis of static light scattering measurements, Turberg and Brady suggest that the aggregation number is 4-6. Lo Nostro and Chen, using viscosity, small angle

Binks et al.

Figure 10. Variation of 〈N〉n with xt for F8H16 in F9 (filled circles), F8 (filled squares), F7 (unfilled triangles), and perfluorodecalin (unfilled circles).

neutron scattering, and dynamic light scattering techniques, conclude that spherical H/F aggregates of aggregation number approximately 95 are formed in this system. Our VPO data support the conclusion of Turberg and Brady. We also note that the light scattering data discussed earlier provides approximate estimates for the weight average aggregation numbers for F8H16 and F10H16 in F9 (at temperatures slightly above their respective Krafft temperatures) of 10 ( 8 and 6 ( 4 respectively. The variation of aggregation behavior with increasing temperature was examined for F8H16 in F9. The Krafft temperature for this system is 42 °C and the cac (from solubility) is 0.08. The plots of xc vs xt (Figure 11a) show that the deviation from ideal behavior at high SFA concentration becomes more pronounced as the temperature is decreased toward the Krafft temperature. This behavior is expected since the antipathy of hydrocarbon and fluorocarbon chains is decreased at higher temperatures. Below the Krafft temperature the deviations from ideality up to the solubility limit are relatively small. The derived plots of monomer concentration (Figure 11b) show that the monomer concentrations remain low at low temperatures but increase progressively with increasing temperature. The aggregation numbers are virtually temperature independent at 2-4 (Figure 12). Thus it can be seen that the main effect of the temperature dependence of the antipathy of the H and F chains on the patterns of aggregation is to change the monomer concentration with little effect on 〈N〉n. Finally, a limited number of VPO measurements were made for hydrocarbon solvents for which F/H aggregation is expected. Figure 13 shows plots of xc vs xt for F8H16 in isooctane at 40.2 °C and for F12H14 in H12 at 70 °C. In each case the systems are slightly above the estimated Krafft temperatures. It can be seen that F8H16 in isooctane deviates only slightly from ideal behavior up to the maximum concentration measurable on the VPO instrument. This is expected since the antipathy of the F chain of the SFA and the short chain hydrocarbon is expected to be weak and the solubility of F8H16 in isooctane deviates only slightly from ideality (Figure 4a). F12H14 in H12 shows

Semifluorinated Alkanes in Mixtures

Figure 11. (a) Variation of xc with xt for F8H16 in F9 at 40.2 (unfilled squares), 45.0 (filled triangles), 54.8 (filled circles), and 63.7 °C (filled squares). (b) Variation of x1 with xt for the same systems. The straight lines in each plot show ideal behavior.

Figure 12. Variation of 〈N〉n with xt for F8H16 in F9 at 45.0 (filled triangles), 54.8 (filled circles), and 63.7 °C (filled squares).

strong deviations from ideality, consistent with a stronger antipathy between the F12- chain of the SFA and the H12 solvent. The aggregation number for this system is independent of SFA concentration and is equal to 3. This behavior is similar to that seen for F10H16 in H16 at 64 °C using light scattering (Table 3) for which 〈N〉w is 4 ( 3. The aggregation number estimated using light scattering for F8H16 in H16 at 45 °C (〈N〉w ) 250 ( 200) appears anomalously high. Although it may possibly be due to favorable packing within the aggregates in this particular system, it would be preferable to obtain an independent confirmation of this result before speculating further. Unfortunately, the low vapor pressure of H16 prevented the use of VPO for this solvent. Overall, it is concluded that SFAs can form either F/H or H/F aggregates in either hydrocarbon or fluorocarbon solvents when the antipathy between the SFA chain forming the aggregate core and the solvent is sufficiently

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Figure 13. Variation of xc with xt for (a) F8H16 in isooctane at 40.2 °C and (b) F12H14 in H12 at 70 °C. The straight lines show ideal behavior.

high. With the exception of the (possibly anomalous) result for F12H14 in H16 at 45 °C, the aggregation numbers are generally less than 10. Adsorption of SFAs at the Liquid-Vapor Surface. We have previously reported surface tension measurements of the adsorption at the hydrocarbon-air surface for a range of SFA plus solvent systems at temperatures below the Krafft temperatures.17 Systems for which the antipathy between the F chain of the SFA and the solvent is weak (i.e. short F chain SFAs with short chain hydrocarbons) show weak adsorption with the minimum area per adsorbed SFA molecule being on the order of several nanometers2. Systems for which the antipathy is strong form highly condensed monolayers with a minimum area per adsorbed SFA of 0.26 nm2, close to the crosssectional area of a vertical, all-trans fluorocarbon chain. In all cases, the maximum surface pressure of the adsorbed films was in the range 0-10 mN m-1, typical of the difference in surface tension for hydrocarbon and perfluorocarbon liquid surfaces.36 The driving force for the adsorption of SFAs at the hydrocarbon surface is due to the lower surface energy of a fluorocarbon surface as compared with a hydrocarbon surface. SFA adsorption, and hence tension reduction, is not expected at fluorocarbon-air surfaces since adsorption would result in replacement of the low energy fluorocarbon surface with a higher energy surface of H chains of the SFA. This expectation was confirmed experimentally as follows. The surface tension of a 7.05 wt % solution of F8H16 in F8 (close to the solubility limit) was found to be 12.5 mN m-1, identical within experimental error to that of the pure F8 solvent (12.4 mN m-1). Previous measurements of adsorption of SFAs were for systems below the Krafft temperature.15,17 We report here the adsorption of F10H16 at the H16-air surface at 64 °C, above its Krafft temperature of 48 °C. Figure 14 shows the variation of surface pressure Π with the concentration of F10H16. The slope dΠ/dc decreases at an SFA concentration of 6 mol %, taken to be the cac. (It was not possible (36) Fletcher, P. D. I. In Specialist Surfactants; Robb, I. D., Ed.; Blackie Academic & Professional: London, 1997; Chapter 5.

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Binks et al.

Figure 14. Variation of surface pressure of F10H16 at the H16air surface at 64.0 °C. The vertical dashed line corresponds to the estimated cac.

to extend the data to higher SFA concentrations due to limited quantities of the SFA.) The cac estimated from the surface tension data is in reasonable agreement with the value estimated from solubility (5 mol % at the Krafft temperature of 48 °C). The surface pressure data was used to obtain the average area occupied per SFA molecule, A, using the appropriate form of the Gibbs adsorption equation:

A ) kT/(dΠ/d(ln(a)))

(9)

where a is the activity of the SFA solute. It was assumed that F10H16 behaves ideally in H16 for concentrations up to the cac, and thus a was equated with the concentration of SFA. Figure 15 shows the surface pressure-area isotherm derived from the tension data. It can be seen that the adsorption is relatively weak with the minimum value of A being approximately 2 nm2. This is in contrast to the highly condensed SFA monolayers observed at temperatures below the Krafft point.17 Figure 15 also includes theoretical Π-A isotherms calculated according to the ideal 2-D gas (eq 10) and Volmer (eq 11) surface equations of state.

ΠA ) kT

(10)

Π(A - Ao) ) kT

(11)

Both theoretical isotherms correspond to 2-D “gaslike” monolayers. The Volmer surface equation of state includes a correction term for a “hard-disk” excluded area of Ao per molecule. The value of Ao assumed was 0.30 nm2, which is close to the cross-sectional area of a vertical, all-trans fluorocarbon chain. It can be seen in Figure 15 that the Volmer equation provides a reasonable description of the high pressure region of the experimental isotherm, suggesting that the adsorbed SFA molecules adopt a configuration in the surface in which the F chains are close to vertical. The deviations at lower surface pressure (higher areas) may result from a change in orientation of the SFA molecules from perpendicular to parallel to the liquid surface.

Figure 15. Variation of surface pressure with area per adsorbed molecule of F10H16 at the H16-air surface at 64.0 °C. The solid lines correspond to the ideal (lower curve) and Volmer surface equations of state (with Ao ) 0.30 nm2) respectively.

Adsorption of SFAs at the Hydrocarbon-Fluorocarbon Interface. The adsorption of SFAs at the hydrocarbon-air surface is next compared with the equilibrium adsorption at the liquid-liquid interface formed in two-phase mixtures of hydrocarbon and fluorocarbon solvents. These experiments were performed as follows. Two-phase mixtures of the hydrocarbon and fluorocarbon solvents with SFA were allowed to equilibrate (after shaking) for 12 h in the sample chamber of the drop volume tensiometer. Interfacial tensions were measured by recording the volumes of drops of the more dense (fluorocarbon) phase suspended from a PTFE tip immersed in the less dense (hydrocarbon) phase. The choice of hydrocarbon and fluorocarbon solvents for the two phase mixtures was primarily dictated by the need for low mutual miscibility. For this reason solvents with the longest chain lengths corresponding to liquid mixtures at the measurement temperatures (F9 with either H16 or H20) were used. Figure 16 shows the variation of surface pressure with F8H16 concentration for F9-H16 phases at two temperatures above the Krafft temperatures for this SFA in either pure solvent. The adsorption is virtually independent of temperature, consistent with the temperature independence of the aggregation properties of F8H16 in F9 as seen in Figure 12. The temperature independence of the adsorption is in contrast to the strong temperature effects noted for F12H14 adsorption at the hydrocarbon-air surface at temperatures below the relevant Krafft temperature. We attempted to obtain the adsorbed amounts from the plots of Figure 16 using the Gibbs equation. However, the slopes of the plots, (which, for ideal solutions are proportional to the surface excess concentration) decrease with increasing concentration. Since adsorption is not expected to decrease with increasing concentration, this behavior is interpreted as being due to solute nonideality corresponding to a degree of progressive aggregation of the solute in one or both of the phases. The slope of the interfacial pressure plots at an F8H16 concentration of 2.5 mol % yields an apparent value of the area per adsorbed F8H16 molecule of approximately 2 nm2 at both temperatures (assuming ideal behavior at this concentration).

Semifluorinated Alkanes in Mixtures

Figure 16. Variation of interfacial pressure with F8H16 concentration (in the total system) for two-phase mixtures of F9 and H16 at 45.0 (circles) and 55.0 °C (crosses). The solid and dotted lines correspond to interfacial tensions of zero at 45.0 and 55.0 °C, respectively.

Figure 17. Interfacial pressure vs F10H16 concentration for the F9-H20 interface (squares) and the F9-H16 interface (circles) at 64.0 °C. The solid and dotted lines correspond to interfacial tensions of zero at the F9-H20 and the F9-H16 interfaces respectively.

Surface pressure-concentration plots for F10H16 at the F9-H16 and F9-H20 interfaces at 64.0 °C (above the Krafft temperatures) are compared in Figure 17. The effect of increasing the hydrocarbon solvent chain length is to increase the adsorption; i.e., the SFA concentration required to achieve a certain surface pressure is reduced. This is consistent with an increased antipathy between the F- chain of the SFA and the hydrocarbon solvent providing an increased driving force for adsorption. We can also compare the adsorption at the F9-H16 interfaces for F8H16 (Figure 16) and F10H16. (Since the adsorption of F8H16 is virtually temperature independent, the difference in the measurement temperatures for the two systems is ignored.) It can be seen that the concentration

Langmuir, Vol. 13, No. 25, 1997 6679

Figure 18. Interfacial pressure vs area per adsorbed molecule of F10H16 at 64.0 °C at the F9-H20 interface (filled squares), the F9-H16 interface (filled circles), and the H16-air surface (open circles). The solid line indicates the ideal 2-D gas surface equation of state. The dashed lines are guides for the eye only.

of F8H16 required to achieve a certain interfacial pressure is approximately 1.6 times greater than that for F10H16, indicating that the adsorption is promoted by increasing the F-chain length of the SFA. The data for Figure 17 were analyzed using the Gibbs equation to yield the variation of interfacial pressure Π with area per adsorbed F10H16 molecule, A, for SFA concentrations less than 2.6 mol %. It was assumed that the solute behaves ideally over this concentration range. Figure 18 compares the Π-A isotherms of F10H16 at the hydrocarbon-fluorocarbon interface with that for the same SFA at the H16-air interface at the same temperature. The monolayer at the liquid-liquid interface is slightly more expanded than at the hydrocarbon-air surface, consistent with a greater degree of solvent penetration in the former case. This conclusion is somewhat speculative since, because of possible nonideal behavior, the areas estimated using the Gibbs equation substituting concentrations for activities correspond to upper limits. The F10H16 concentrations corresponding to the points of inflection of the surface pressure plots in Figure 17 may be taken as estimates of the cac in these ternary mixtures. The cac values estimated in this way are approximately 3 and 6 mol % (with respect to the total system) for the H16 and H20 mixtures, respectively. As will be discussed in the following section, the SFA concentration required for aggregate formation in these two-phase systems depends on both the cac in the separate phases and the SFA distribution between the phases. SFA Distribution in Two-Phase Ternary Mixtures of SFA + Fn + Hm. The typical behavior of ternary mixtures of a conventional surfactant, water plus oil, is as follows (see, for example, ref 37). We consider the addition of surfactant to a two-phase system containing comparable volumes of water and oil. Below the critical surfactant concentration required for aggregate formation (cac), only surfactant monomers are present, and these may distribute between both phases. For ionic surfac(37) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3111.

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tants, the monomer distribution generally lies strongly in favor of the aqueous phase whereas the opposite is commonly the case for many nonionic surfactants. In general, as the overall surfactant concentration is increased, the monomer concentration in each phase increases with the concentration ratio being maintained equal to the distribution coefficient. As the monomer concentration increases, there is increased adsorption (and a resultant lowering of the tension) at the oil-water interface. This behavior is seen for SFAs adsorbing at the hydrocarbon-fluorocarbon interface in Figures 16 and 17. At an overall surfactant concentration such that the monomer concentration becomes equal to the cac in one of the phases, surfactant aggregates form in either the water, the oil, or a third phase and the oil-water tension remains virtually constant. Aggregation occurs in the phase in which the cac is reached first.38 For aggregation in the aqueous phase, the aggregates are oil-in-water microemulsion droplets which coexist with a phase of excess oil. Aggregation in the oil phase gives a waterin-oil microemulsion phase plus excess water. Aggregation to form a third phase gives a three-phase system comprising a microemulsion (commonly of bicontinuous structure) plus excess oil and water phases. At overall surfactant concentrations in excess of that required for aggregate formation, the monomer concentration present in both the oil and water phases remains virtually constant, and further addition of surfactant leads to a higher concentration of aggregates in the microemulsion phase. The behavior described above refers to strongly amphiphilic conventional surfactants (i.e. those with long chain lengths) in oil + water systems. Short chain length, weakly amphiphilic conventional surfactants generally show similar patterns of phase behavior except that the phase exhibits a decreased level of microstructure, i.e., weaker aggregation. One characteristic difference between weak and strong amphiphiles is that weak surfactants do not generally exhibit third phase (bicontinuous) microemulsion formation when the affinities of the surfactant for the oil and water phases are matched.39-41 We have investigated the extent to which the behavior of ternary mixtures of SFA + hydrocarbon + fluorocarbon exhibit the microemulsion phase behavior seen for conventional surfactant + oil + water systems. As for the adsorption results in the ternary mixtures, the fluorocarbon and hydrocarbons selected for this phase of the study were the long chain species F9 with either H16 or H20 (above its melting point), chosen to have the lowest possible mutual miscibility (i.e. the greatest mutual antipathy) between the liquid solvents. Three ternary SFA + Fx + Hy systems were investigated: F8H16 with F9 and H16 at 45 °C, F10H16 with F9 and H16 at 64 °C, and F10H16 with F9 and H20 at 64 °C. In each case, the temperature was slightly above the Krafft temperatures of both the SFA + solvent binary mixtures. A series of sealed tubes each containing the same weights of the Fx and Hy solvents (corresponding to approximately equal volumes of the two solvents) with different weights of SFA was prepared, vigorously shaken and allowed to reach equilibrium at the measurement temperature. The samples were equilibrated for a minimum of 24 h in each case. The densities of both the hydrocarbon-rich and fluorocarbon-rich phases and the refractive index of the hydrocarbon-rich phase were then determined for each (38) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (39) Kahlweit, M, Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 490. (40) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1994, 10, 1134. (41) Schubert, K.-V.; Strey, R.; Kline, S. R.; Kaler, E. W. J. Chem. Phys. 1994, 101, 5343.

Binks et al.

Figure 19. Variation of ∆F for the hydrocarbon and fluorocarbon phases (circles and squares respectively) with SFA concentration in the total system for (a) F8H16 with F9 and H16 at 45 °C, (b) F10H16 with F9 and H16 at 64 °C, and (c) F10H16 with F9 and H20 at 64 °C.

sample. (The refractometer used did not allow the measurement of the refractive index of the fluorocarbonrich phases which are less than 1.3.) The density difference (∆F) between the sample density (F) and the density of the same phase equilibrated with the second solvent in the absence of SFA (Fo) was then calculated (∆F ) F - Fo). The density difference ∆F provides a measure of the SFA concentration in each phase. The densities of H16, F9, and F8H16 are approximately 0.7, 1.7, and 1.2 g cm-3 respectively. Hence, ∆F is increasingly positive with increasing SFA concentration in the H16 phase and increasingly negative for the F9 phase. Figure 19 shows the variation of ∆F with SFA concentration in the total mixtures for the three systems where it can be seen that ∆F is positive for the hydrocarbon phase and negative for the fluorocarbon phase showing that the SFA partitions to a comparable extent between both phases. For the first two systems (F8H16 and F10H16 in F9 + H16) the slopes of the plots (d∆F/dc) decrease in magnitude with increasing SFA for the fluorocarbon phase but increase for the hydrocarbon phase at higher SFA concentrations. This behavior is consistent with the formation of F/H aggregates in the hydrocarbon phases at SFA concentrations above the cac for these systems since this type of aggregate should partition exclusively to the hydrocarbon phase. The final system (F10H16 in F9 + H20 at 64.0 °C) shows slightly increasing magnitudes of slopes for both the hydrocarbon and fluorocarbon phases. The variation of distribution behavior with SFA concentration is more clearly revealed in Figure 20a in which the ratio of density differences for the hydrocarbon and fluorocarbon phases (-∆F(H)/∆F(F)) are plotted vs SFA concentration. The intercepts of these plots yield the approximate distribution coefficient of the monomeric SFA

Semifluorinated Alkanes in Mixtures

Figure 20. Variation of (a) the ratio of ∆F for the hydrocarbon phase to that for the fluorocarbon phase (-∆F(H)/∆F(F)) and (b) the difference in ∆F (∆F(H) - ∆F(F)) with SFA concentration for F8H16 with F9 and H16 at 45 °C (filled squares), F10H16 with F9 and H16 at 64 °C (open circles) and F10H16 with F9 and H20 at 64 °C (filled triangles). The dashed lines in part a are guides for the eye and the solid line in part b indicates the behavior expected in the absence of solubilization by the SFA.

species whereas the slopes indicates the degree to which the distribution changes as a result of progressive aggregation at higher concentrations. Positive slopes indicate F/H aggregate formation in the hydrocarbon phase and negative slopes indicate H/F aggregation. For all three systems, the SFA monomers partition preferentially to the hydrocarbon phases, i.e. the intercepts are greater than 1. The order of the slopes suggests that H/F aggregation is increasingly favored in the series (F10H16 in F9 + H20) < (F10H16 in F9 + H16) < (F8H16 in F9 + H16). This sequence follows the trend of decreasing antipathy between the hydrocarbon solvent and the F chain of the SFA. The system F10H16 in F9 + H20 shows virtually equal affinity for both the hydrocarbon and fluorocarbon phases; i.e., the slope is approximately zero in this case. In this situation, the formation of middle phases (bicontinuous microemulsions) is generally observed for strongly amphiphilic conventional surfactants with oil plus water whereas weakly amphiphilic surfactants are too soluble to show third phase formation.39-41 No third phase formation is observed for the SFA system, suggesting that the amphilicity is insufficiently strong to drive the separation of a middle phase. The density results provide a limited amount of information concerning the extent to which the SFAs solubilize fluorocarbon solvent in hydrocarbon and vice versa. In order to estimate the extent of solubilization, we have compared the experimentally measured sum of the magnitudes of the density differences for both phases (|∆F(H)| + |∆F(F)| ) ∆F(H) - ∆F(F)) with theoretical values based on the density difference expected for SFA solutions with no solubilization. The SFAs were assumed to mix ideally (i.e., with no volume change on mixing) and to have a density of 1.2 g cm-3 (estimated from density measurements in binary systems). The experimental data for the three SFA systems (Figure 20b) lie approximately 20% above the line for no solubilization indicating that 5

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Figure 21. Variation of refractive index of the equilibrated hydrocarbon phase with SFA concentration for F8H16 with F9 and H16 at 45 °C (circles) and F10H16 with F9 and H16 at 64 °C (squares). Table 4. Values of Cacov and P for the Ternary Systemsa

system F8H16 + F9 + H16 F10H16 + F9 + H16 F10H16 + F9 + H20

cacov wt frac P 0.12 0.06 0.05

3.0 2.0 1.8

c(H)/mol c(F)/mol frac frac cac(H) cac(F) at cacov at cacov mol frac mol frac 0.10 0.04 0.08

0.04 0.02 0.04

0.13 0.05 0.06

0.07 0.07 0.08

a The SFA concentrations in the hydrocarbon and fluorocarbon phases at cacov (c(H) and c(F)) are compared with the cac values in the corresponding binary SFA + solvent mixtures (cac(H) and cac(F)).

molecules of SFA solubilizes approximately 1 molecule of either hydrocarbon or fluorocarbon solvent. This solubilization is significant but small relative to that seen for typical microemulsion forming, strongly-amphiphilic conventional surfactants which can commonly solubilize up to 100 molecules of water in a w/o microemulsion or 20 molecules of oil in an o/w microemulsion per surfactant molecule.42 Two-phase systems containing conventional surfactants with oil plus water generally exhibit a sharp break in surfactant distribution and other solution properties at the overall surfactant concentration required for aggregate formation.37,41 For the SFAs in ternary mixtures with Hx and Fy, the density plots of Figure 19 show gradual rather than discontinuous changes in slope indicating weak, progressive aggregation as discussed previously for binary SFA + solvent mixtures. The lack of any clear discontinuity in the density plots precluded the estimation of cac values from the density data. Cac values were estimated from the interfacial pressure plots (Figure 17) and from measurements of the refractive index of the equilibrated hydrocarbon phases in the ternary systems (Figure 21). Table 4 shows the values of the cac (expressed as weight fraction in the total mixtures), and it can be seen that the values decrease with increasing F chain length of the SFA consistent with an increased antipathy between the SFA and the phase in which aggregation occurs (hydrocarbon phase for these systems). (42) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989, 5, 1210.

6682 Langmuir, Vol. 13, No. 25, 1997

As discussed earlier, surfactant aggregation in twophase systems occurs in the phase for which the cac is reached first. Thus, the overall surfactant concentration required for aggregation (cacov) depends on the cac in the hydrocarbon phase (cac(H)), the cac in the fluorocarbon phase (cac(F)), the distribution coefficient of the SFA monomer, and the phase volumes. The ratio of the zero SFA concentration limiting values of density change with SFA concentration for the H and F phases (i.e., -{(d∆F(H)/dc)/(d∆F(H)/dc)}cf0, summarized in Table 4) provides an estimate of the SFA monomer partition coefficient P. P is equal to the ratio of volumes of SFA in the H and F phases which, for equal volumes of the two solvents as used in these experiments, is equal to the ratio of volume fractions of SFA in the two phases. The densities of the SFAs were taken to be 1.2 g cm-3 for these calculations. The values of P for the systems F8H16 with F9 and H16 at 45 °C, F10H16 with F9 and H16 at 64 °C, and F10H16 with F9 and H20 at 64 °C were found to be 3.0, 2.0, and 1.8 respectively. The values of P and cacov were used to estimate the SFA concentrations in each phase when the total SFA concentration in the two-phase mixtures is equal to the cacov. The SFA concentrations in each phase at cacov are compared with the cac values for the corresponding binary mixtures in Table 4. For the F8H16/F9/H16 and F10H16/ F9/H16 systems, it can be seen in Table 4 that the SFA concentration in the H phase at cacov is similar to the cac value estimated for the corresponding binary mixture of SFA and hydrocarbon solvent. The SFA concentrations in the fluorocarbon phases are significantly less than the cac values for the corresponding binary mixtures of SFA and fluorocarbon solvents. This is consistent with the conclusion (noted earlier) that SFA aggregation in these systems occurs primarily in the hydrocarbon phases. The values for the F10H16/F9/H20 system, for which SFA aggregation appears to occur equally in both the hydrocarbon and fluorocarbon phases, appears anomalous in that the concentrations suggest that aggregation is expected to occur in the hydrocarbon phase. It should be borne in mind that the comparison of values in Table 4 is rather crude since cac values in binary SFA/solvent mixtures are not expected to be identical with cac values for SFAs in a solvent saturated with a second (solubilized) species. Conclusions We have attempted to establish the extent to which the adsorption and aggregation properties of SFAs in binary and ternary mixtures with hydrocarbon and fluorocarbon solvents resembles that of conventional surfactants in mixtures with oil and water. The main conclusions are as follows.

Binks et al.

Miscibility studies of hydrocarbon and fluorocarbon solvents of different chain lengths show that the free energies of transfer of a -CH2- group into fluorocarbon and a -CF2- group into hydrocarbon are approximately 1.1 and 1.4 kJ mol-1 respectively. These values are approximately one-third of the free energy of transfer of a -CH2- from alkane to water, the main driving force for surfactant properties of conventional surfactants. Thus, SFAs are expected to be relatively weakly amphiphilic in hydrocarbon + fluorocarbon solvents when compared with conventional surfactants of similar chain lengths in oil + water. The solubilities of SFAs show an abrupt increase at a particular temperature, similar to the Krafft point of conventional surfactants in water. The Krafft point discontinuity is not as sharp as for conventional surfactants of similar chain lengths in water, indicating SFA aggregation is relatively weak. Trends in critical aggregation concentration with SFA and solvent chain lengths correlate with the strengths of the antipathy between the SFA and solvent. SFAs aggregate in either hydrocarbon or fluorocarbon solvents when the antipathy between the SFA and solvent is sufficiently strong. Aggregation numbers are generally in the range 2-10. As expected for systems forming aggregates of low aggregation number, the aggregation occurs progressively with increasing SFA concentration rather than at a sharply defined criticial concentration as seen for conventional surfactants in water. SFAs adsorb at the hydrocarbon-air and hydrocarbonfluorocarbon interfaces but not at the fluorocarbon-air surface. At temperatures below the Krafft temperature, the adsorbed films are commonly very condensed with limiting areas close to the values expected for vertical, all-trans fluorocarbon chains. At higher temperatures the adsorbed films are much more expanded. SFA distribution in two-phase mixtures with hydrocarbon and fluorocarbon solvents suggests that weak aggregation occurs predominantly in the hydrocarbonrich phase. “Third phase” microemulsion formation has not been observed. Acknowledgment. We are grateful to Dr. W. D. Cooper (Shell Additives International Ltd., Thornton, U.K.) for helpful discussions and to Shell Additives International Ltd., Thornton, the EPSRC (U.K.) and the EC TEMPUS Programme for financial support. We thank Mr. R. Clark of the University of Hull for making some of the solubility measurements of fluorocarbons in dodecane. LA970408I