2034
Langmuir 2006, 22, 2034-2038
Fluorosurfactants at Structural Extremes: Adsorption and Aggregation Julian Eastoe,* Sarah E. Rogers, Laura J. Martin, and Alison Paul School of Chemistry, UniVersity of Bristol, Bristol, BS8 1TS, U.K.
Fre´de´ric Guittard and Elisabeth Guittard Laboratoire de Chimie des Mate´ riaux Organiques et Me´ talliques (CMOM), UniVersite´ de Nice Spohia-Antiplois, Parc Valrose, 06108 Nice - Cedex 2, France
Richard K. Heenan and John R. P. Webster ISIS-CCLRC, Rutherford Appleton Laboratory, Chilton, Oxon, OX11 0OX, U.K. ReceiVed October 30, 2005. In Final Form: January 10, 2006 Fluorosurfactants with several structural modifications have been synthesized, and the air/water interface and bulk aggregation properties investigated. The compounds were fluorinated ethylene oxide (EO) nonionics where the number and position of the hydrophilic group(s) has been radically altered to generate linear, bolaform, and Y-shaped analogues. A noticeable structure-interfacial packing relationship was observed via both tensiometric measurements and neutron reflection studies: the limiting molecular areas, acmc, and surface excesses, Γcmc, are strongly dependent on the number and position of the EO headgroups. Differing bulk aqueous properties were also observed. Small-angle neutron scattering shows an evolution of micelle structure from cylindrical to disklike aggregates on changing from Y-shaped to bolaform molecular structure.
Introduction Various structural modifications have been explored in the search for new and improved surfactants. These have included double-chained amphiphiles such as AOT,1 double-head, doublechain cationic geminis,2 unusual hetero-geminis, bearing two different chains and two different heads,3 and bolaform compounds with two headgroups at each end of a central hydrophobic block.4 An interesting area, which has received less attention, is the case of double-headed compounds comprising one hydrophobe and two headgroups.5-8 This arrangement can give rise to unusual interfacial packing and bulk aqueous properties. In this paper, three related, but structurally different, surfactants have been synthesized and characterized. Molecular structures are shown in Figure 1: a linear surfactant (L) comprising of one fluorinated hydrophobic group and one nonionic headgroup; a bolaform (B), which may also be considered as mini, monodisperse fluorinated block copolymer EO-CF-EO; and a Y-shaped (Y) surfactant where now both headgroups are at the same end of the hydrophobic group. Note that B is not truly * To whom correspondence should be addressed. Tel: + 117 9289180. Fax: + 117 9250612. E-mail:
[email protected]. (1) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. (2) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (3) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 113, 1451; 1993, 113, 10083. (4) Shin, J. Y.; Abbott, N. L. Langmuir 1999, 15, 4404. (5) Guittard, F.; de Givenchy, E. T.; Szonyi, F.; Cambon, A. Tetrahedron Lett. 1995, 36, 7863. (6) Guittard, F.; de Givenchy, E. T.; Cambon, A. J. Colloid Interface Sci. 1996, 177, 101. (7) (a) Mureau, N.; Trabelsi, H.; Guittard, F.; Geribaldi, S. J. Colloid Interface Sci. 2000, 229, 440. (b) Guittard, F.; Geribaldi, S. J. Fluor. Chem. 2001, 107, 363. (8) (a) Clarey, L.; Gadras, C.; Greiner, J.; Rolland, J.-P.; Santaella, C.; Vierling, P.; Gulik, A. Chem. Phys. Lipids 1999, 99, 125. (b) Xu, B.; Li, L.; Yekta, A.; Masoumi, Z., Kanagalingam, S.; Winnik, M. A.; Zhang, K.; MacDonald, P. M. Langmuir 1997, 13, 2447. (c) Kausch, C. M.; Kim, Y.; Russell, V. M.; Medsker, R. E.; Thomas, R. R. Langmuir 2003, 19, 7182.
Figure 1. 3D structures of the three surfactants: the linear (L), bolaform (B) and Y-shaped surfactant (Y).
analogous to L, and this is due to the lack of availability of an appropriate starting material to synthesize the direct homologues. It has been shown in previous publications how hydrophobic chain structure9 and terminating group chemistry10 greatly influence important physicochemical properties such as surface excess, Γ, surface tension, γ, and cmc. Earlier work has been carried out on perfluoroalkyl triethyleneoxide methyl ethers X-(CF2)m-CH2-O-(C2H4O)3-CH3 with X being either H or F and m ) 4 or 6, and this has shown that altering the chain length by 2 -CF2- groups or replacing a F atom for H in the terminal group can have dramatic effects on adsorption and aggregation.9 It was shown that each additional CF2 group caused a decrease in the cmc by a factor of ∼5 (compared to ∼3 for a typical HC nonionic), and in terms of cmc depression it was shown that a CF2 group is equivalent to 1.7 CH2 groups. This demonstrates the high surface activity of fluorocarbon (FC) material. The increased hydrophobicity of a surfactant with a terminal H produced a 4-fold increase in cmc compared to the (9) Eastoe, J.; Paul, A.; Rankin, A.; Wat, R.; Penfold J.; Webster, J. R. P. Langmuir 2001, 17, 7873. (10) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591.
10.1021/la0529153 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006
Fluorosurfactants at Structural Extremes
equivalent F-tipped surfactant. The H-CF2-capped surfactants also had higher limiting surface tensions (∼9 mN m-1) and larger acmc (∼10 Å2). Similar effects have been reported for dichained surfactants also.10 For hydrocarbon surfactants there is no thermodynamic tendency to adsorb from common H-based organic solvents, whereas fluorocarbon surfactants will adsorb out of both aqueous and normal hydrocarbon media. This is due to the antipathy between fluorocarbon and hydrocarbon material.11-14 This means that fluorosurfactants find numerous industrial applications in nonaqueous solvents.15 The purpose of this work is to examine how such dramatic structural variations, as depicted in Figure 1, affect packing in adsorbed monolayers and aggregated micelles in water. This is a necessary first stage in developing efficient FC surfactants based on these structures as effective amphiphiles for applications in novel solvents, including supercritical CO2. Dynamic drop volume tensiometry (DVT) and neutron reflection (NR) have been employed to characterize the adsorbed layers at the air/water (a/w) interface and small-angle neutron scattering (SANS) to study the bulk properties. Neutron reflectometry provides a direct measurement of surface excess, whereas the interpretation of tensiometric data is dependent on an adsorption isotherm equation. Therefore, NR is a valuable method for characterizing new surfactants and validating interpretations of tensiometric studies. NR is also relatively convenient for the study of fluorinated surfactants due to a high scattering length of fluorine (bF ) 5.650 fm, cf. bD ) 6.674 fm), meaning that the fluorinated layers can be effectively contrasted against a mixed H2O/D2O subphase (air contrast matched water, ACMW, 8.0 mol% D2O in H2O). SANS was employed to study the bulk aggregation properties of these surfactants in D2O. As found in other studies (e.g., refs 9, 10) small changes in surfactant molecular structure have significant effects on physicochemical properties. The findings described here show how altering the number and position of the headgroup(s) can greatly alter the surface energies and packing in adsorbed a/w monolayers and the size and shape of micellar aggregates. This work highlights the profound effects of molecular architecture on surface and bulk properties, which are important considerations for rational design of efficient amphiphiles for speciality applications, such as in nonaqueous solvents. Experimental Section Chemicals. The linear and Y-shaped surfactants were all synthesized according to previous routes.9,16 For the bolaform compound, detail on the synthesis is given in the Supporting Information. All samples for the tensiometric measurements were made in deionized water that was obtained from a RO 100HP Purite water purification system. SANS samples were made up in D2O (Goss, 99.9% D). Cleaning. All glassware was cleaned using Micro cleaning fluid (∼5% aqueous solution) by heating in an ultrasonic bath. Rinsing with hot tap water, acetone, and deionized water was followed by oven drying. The syringe and capillary for the DVT were cleaned by repeated washings with deionized water. (11) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley and Sons: New York, 1980. (12) Binks, B. P.; Fletcher, P. D. I.; Kotsev, S. N.; Thompson, R. L. Langmuir 1997, 13, 6669. (13) Kabalnov, A. S.; Shchukin, E. D. AdV. Colloid Interface Sci. 1992, 38, 69. (14) Kabalnov, A. S.; Makarov, K. N.; Shcherbakova, O. V. J. Fluorine Chem. 1990, 50, 271. (15) Kissa, E. Fluorinated Surfactant Synthesis, Properties and Applications; Surfactant Science Series 50; Marcel Dekker: New York, 1994. (16) Kratzat, K.; Guittard, F.; Taffin de Givenchy, E.; Cambon, A. Langmuir 1996, 12, 6346.
Langmuir, Vol. 22, No. 5, 2006 2035 Tensiometry. The tensiometer (Lauda TVT1, drop-volume) was calibrated using ethanol/water mixtures,17 and the cleanliness of the syringe was confirmed by measuring γ for water between surfactant solutions. Aqueous solutions at concentrations ranging from approximately 10 × cmc to 0.05 × cmc were prepared by mass, and the appropriate corrections for density were made. To ensure determination of true equilibrium surface tensions, experimental protocols detailed elsewhere9 were employed. Measurements were made at 25 °C. The surface excess, Γ, and area per molecule, as, were calculated using the Gibbs isotherm. Γ)-
1 dγ nRT d ln c
(1)
The pre-factor, n, is dependent upon the surfactant structure and type and the presence of extra electrolyte.18,19 A combination of tensiometric and NR measurements have been used to confirm that the pre-factor for nonionic surfactants is unity,20-24 which was used here. Neutron Reflection (NR). Experiments were performed using the SURF reflectometer at ISIS at the Rutherford Appleton Laboratories, Didcot, UK.25 The specular neutron reflection, R(Q), was measured normal to the a/w interface. The Q range of 0.050.65 Å-1 was achieved by employing a grazing angle of 1.5° and an incident wavelength range of 0.5-6.5 Å. Measurements were made at 25 °C. Trace ionic impurities10,26 were removed by distilling D2O before use. To ensure the reflectivity signal was only from the adsorbed monolayer, surfactant solutions were prepared in air contrast matched water (ACMW, 8.0 mol% D2O in H2O). Calibration of the reflectometer was performed using D2O, and a flat background (∼2 × 10-6, determined by extrapolation of the high Q data) subtracted from the curve. Due to the low concentrations of surfactant used (10-3-10-6 mol dm-3), long count times were required (1-2 h). NR Data Analysis. Reflectivity profiles were fit using a leastsquares routine20,25 to give the layer thickness, τ, and scattering length density, F. A sharp interface was assumed between the surfactant monolayer and the solution subphase; the monolayer is considered to be of uniform thickness and scattering length density. The area per molecule and adsorbed amount were determined using eq 2.20
∑b
i
as )
i
Fτ
)
1 ΓNa
(2)
∑b is the sum of isotopic scattering lengths in the molecule, which is calculated on the basis of the chemical isotopic structure. Small-Angle Neutron Scattering (SANS). Experiments were carried out on the time-of-flight LOQ diffractometer at ISIS. The incident wavelength range of 2.2-10.0 Å25 gave rise to a Q range of 0.009-0.249 Å-1. Absolute intensities for I(Q) (cm-1) were determined within 5% by measuring the scattering from a partially deuterated polymer standard. Standard procedures for data treatment were employed.25 Measurements were made at 25 °C. All samples were made up in D2O and run in 2 mm quartz cells. SANS was not (17) Strey, R.; Viisanen, Y.; Aratono, M.; Kratohvil, J. P.; Yin, Q.; Friberg, S. E. J. Phys. Chem. B 1999, 103, 9112. (18) Hall, D. G. Colloids Surf., A 1994, 90, 285. (19) Hall, D. G.; Pethica, B. A.; Shinoda, K. Bull. Chem. Soc. Jpn. 1975, 48, 324. (20) Lu, J. R.; Thomas, R. K.; Penfold, J. J. AdV. Colloid Interface Sci. 2000, 84, 143. (21) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. Langmuir 1992, 8, 1837. (22) Thomas, R. K.; Lu, J. R.; Lee, E. M.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352. (23) Hines, J. D.; Garrett, P. R.; Rennie, G. K.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 7121. (24) Hines, J. D.; Garrett, P. R.; Rennie, G. K.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 9215. (25) See: http://www.isis.rl.ac.uk/. (26) Downer, A.; Eastoe, J.; Pitt, A. R.; Penfold, J.; Heenan, R. K. Colloids Surf., A 1999, 156, 33.
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Eastoe et al.
Figure 2. Surface tension curves for the purified nonionics. Lines are quadratic fits for the pre-cmc data: b, L; 9, B; 2, Y. Table 1. Values Derived from Surface Tension Dataa L B Y a
cmc (mmol dm-3)
γcmc (mN m-1)
2.00 0.12 0.38
24.4 38.7 23.7
Typical uncertainties on γcmc are (0.1 mN m-1.
Table 2. Comparison of Surface Excesses at the cmc Derived from Tensiometry and NR τ (Å) L B Y
acmc (Å2)
Γcmc (10-8 mol m2)
NR
DVT
NR
DVT
NR
27.0 35.0 20.0
43.0 25.0 55.0
44.0 24.0 52.0
3.9 6.5 3.1
3.8 6.9 3.2
possible on the L surfactant due to lack of solubility in D2O at sufficient multiples of the cmc to yield measurable systems (typical requirement is micellar volume fraction > 0.01). SANS Analysis. Detailed information for the scattering from cylinder- and disk-shaped particles and the application of the FISH analysis program can be found in the Supporting Information.27 The model used for both the B and the Y surfactant was core shell rod, and the ratio of length to radius was altered depending on whether the best fit form factor was a cylinder or a disk. The Guinier approximation was also employed to estimate the dimensions and shape of the different micelles.28
Results and Discussion Tensiometry. Surface tension isotherms for the three surfactants are shown in Figure 2, and the values obtained from these data are displayed in Tables 1 and 2. (Neutron reflection experiments described below are consistent with these derived values). Altering the position and number of surfactant headgroup(s) had significant effects on both the adsorption and aggregation properties. The γcmc values of both the L and Y surfactants were typical of nonionic fluorinated surfactants in water (∼20 mN m-1).9,29-36 However, interestingly, the limiting surface tension (27) Heenan, R. K. FISH Data Analysis Program; Rutherford Appleton Laboratory; Report RAL-89-129; CCLRC: Didcot U.K., 1989. (28) Zemb, T.; Lindner, P. Neutron, X-ray and Light Scattering: Introduction to an InVestigation Tool for Colloidal and Polymeric Systems; Elsevier Science, New York, 1991. (29) Selve, C.; Achilefu, S.; Ste´be´, M. J.; Ravey, J. C.; Delpuech, J.-J. Langmuir 1994, 10, 2131. (30) Selve, C.; Achilefu, S. J. Chem. Soc., Chem. Commun. 1990, 15, 911. (31) Ravey, J.-C.; Ste´be´, M.-J. Colloids Surf., A 1994, 84, 11. (32) Ravey, J.-C.; Gherbi, A.; Ste´be´, M.-J. Prog. Colloid Polymer Sci. 1988, 76, 234. (33) Matos, L.; Ravey, J.-C.; Serratrice, G. J Colloid Interface Sci. 1989, 128, 341.
Figure 3. Reflectivity profiles: O, L; 0, B; 4, Y.
of bolaform B was in the range of a typical hydrocarbon bolaform (30-40 mN m-1).9,37 This result agrees with previous literature in that a bolaform structure is much less efficient at reducing tension than a corresponding conventional linear surfactant.38,39 The cmc of B is ∼21 lower than that of L and ∼ 3 lower than that of Y, showing a strong dependence of the cmc on the number of headgroups rather than the actual molecular architecture. The difference depends on the hydrophile-lipophile balance of the headgroup and can be very large, the more so the more hydrophilic the group. The terminal groups on the hydrophilic moiety on Y are hydroxyl groups rather than the methoxyl groups present on B and L. It has been demonstrated that the terminal atom on the hydrophilic moiety does not significantly affect the surface activity of surfactants or polymers for the linear form.29,34,40 However, the methoxylated version of Y has been synthesized and characterized, and a cmc of 0.16 mmol dm-3 and γcmc of 20.7 mN m-1 were found.6 This cmc value is approximately half that of the hydroxyl-capped Y-shaped surfactant, though the two γcmc values are comparable. This indicates that the surface activity is affected by the nature of the hydrophilic terminal group. Neutron Reflection. Typical R(Q) data are shown in Figure 3 along with fits to the single layer model. As expected, the layer of surfactant at the a/w interface thins on dilution for all three surfactants studied. The surface coverage of all three compounds also decreases on dilution. The B, Y, and L surfactants all formed a monolayer at the a/w interface, as demonstrated previously by both hydrocarbon and other fluorinated nonionic surfactants.9,10,41-43 In all cases, the conformation at this interface seems to be extended molecules: the fitted layer thickness is approximately the respective molecular length. Hence, it was found that the layer thickness increases as the surfactant length increases. The values of acmc and τ obtained for B indicate that it has a configuration similar to that shown in Figure 4(i). This is inconsistent with an inverted U-shaped (34) Selve, C.; Ravey, J.-C.; Ste´be´, M.-J.; El Moudjahid, C.; Moumni, E. M.; Delpuech, J.-J. Tetrahedron 1991, 47, 411. (35) Fletcher, P. D. I. Fluorinated and Semi-Fluorinated Surfactants. In Specialist Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional: London, 1997. (36) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (37) Craine, L.; Greenblatt, J.; Woodson, S.; Hortelano, E.; Raban, M. J. Am. Chem. Soc. 1983, 105, 7252. (38) Menger, F. M.; Wrenn, S. J. Phys. Chem. 1974, 78, 1387. (39) Zana, R. Bolaform and Dimeric (Gemini) Surfactants. In Specialist Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional: London, 1997. (40) Kelarakis, A.; Havredaki, V.; Booth, C.; Nace, V. M. Macromolecules 2002, 35, 5591. (41) Lu, J. R.; Simister, E. A.; Lee, E. M.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Langmuir 1992, 8, 1837. (42) Lu, J. R.; Lee, E. M.; Thomas, R. K.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352. (43) Lu, J. R.; Li, Z. X.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2408.
Fluorosurfactants at Structural Extremes
Langmuir, Vol. 22, No. 5, 2006 2037
Figure 4. Possible orientations of bolaform surfactants at the a/w interface.
Figure 6. Adsorption isotherms. Tensiometry: b, L; 9, B; 2, Y. NR: O, L; 0, B; 4, Y. NB: values for L are multiplied by 0.5. Figure 5. Possible orientation of the Y surfactant at the a/w interface.
configuration (Figure 4(ii)), such as proposed elsewhere for hydrocarbon38,44 and partially fluorinated bolaforms.8c Significantly, this work differs from previous related literature8,38,44 in two important ways: (a) here the direct structural technique of neutron reflectivity is employed, rather than tensiometry alone, which is an indirect method and (b) the central hydrophobic section of the bolaform B is fully fluorinated, rather than hydrocarbon38,44 or merely partially fluorinated.8c Hence, (a) the layer thickness can be accurately determined, to infer the interfacial molecular orientation, and (b) the fully fluorinated B is much less flexible,45-49 reducing the likelihood of U-shaped interfacial configurations. As far as can be ascertained, to date, there are no published NR studies of related hydrocarbon bolaforms with which to compare the findings presented here. The molecular area, acmc, was too small and the layer thickness, τ, too large to accommodate for flat orientation of B, as depicted in Figure 4(iii). This layer configuration, and therefore low value of acmc, is a reasonable explanation as to why the surface excess, Γcmc, is so much larger for the B surfactant compared to that of Y and L. The Y surfactant, with an acmc over double that of B, and τ of approximately one surfactant molecule length, may have the conformation shown in Figure 5 at the a/w interface. The values of acmc and τ found for L were consistent with a monolayer.9 Comparison of Results from NR and Tensiometry. Figure 6 shows adsorption isotherms derived from both tensiometric (eq 1) and NR (eq 2) measurements. For comparative purposes, the concentrations have been scaled to the respective cmc and the values for L have been multiplied by 0.5. Good agreement was obtained for all three surfactants, and this validates the interpretation of tensiometric data discussed above. Table 2 gives the acmc and Γcmc derived by DVT and NR and τ derived from NR for all three surfactants. SANS from Micellar Solutions. Figure 7 shows the SANS profiles of B (a) and Y (b) with the appropriate Guinier Plots displayed as insets. (SANS was not possible on L as explained previously). The decrease in I(Q) seen in the two sets of data is consistent with a decrease in the number of scattering centers on dilution. (44) Mao, G.; Tsao, Y.-H.; Tirrell, M.; Davis, H. T. Langmuir 1994, 10, 4174. (45) Matsuoka, K.; Moroi, Y. Curr. Opin. Colloid Interface Sci. 2003, 8, 227. (46) Riess, J. G. Tetrahedron 2002, 58, 4113. (47) Barton, S. W.; Goudot, A.; Bouloussa, O.; Rondelez, F.; Lin, B.; Novak, F.; Acero, A.; Rice, S. A. J. Chem. Phys. 1992, 96, 1343. (48) Shin, S.; Collazo, N.; Rice, S. A. J. Chem. Phys. 1993, 98, 3469. (49) Pierre Krafft, M.; Goldmann, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 243.
Figure 7. SANS profiles for (a) B and (b) Y with Guinier plots as insets: b, 10; O, 7.5; 1, 5; 3, 2.5 wt%.
From the data obtained and the analysis performed (see Supporting Information), it was clear that B formed disk structures and Y formed cylindrical micelles. This agreed with the packing parameters, Pc, calculated for the two surfactants. Pc is calculated using eq 3 (for full details, see Supporting Information):
Pc )
V a0lc
For B, the packing parameter was calculated as:
Pc ) and for Y:
449 ) 1.19 24.5 × 15.4
(3)
2038 Langmuir, Vol. 22, No. 5, 2006
Pc )
352 ) 0.49 53.5 × 13.3
Eastoe et al. Table 3. Best Fit Parameters to SANS Data for the Y Surfactant rcyl (Å)
The volumes were calculated from the molecular fragments of the surfactants.10 Tables 3 and 4 show the comparison in parameters found from Guinier analysis28 and the models used (details found in the Supporting Information). Good agreement was found in all cases. These derived parameters also confirm the aggregate structures described earlier. For both the B and Y surfactant, the SANS and NR data were consistent with one another. In the case of B, the thickness of the disks was fixed in analysis to the approximate length of one surfactant molecule. This indicates that large islands of surfactant aggregates are found in the bulk and that structures of these aggregates are similar to that found at the a/w interface by neutron reflection. For Y, the radius of the cylindrical micelles was similar to the thickness of the monolayer found at the a/w interface, and both are comparable to the length of one surfactant molecule.
Summary and Conclusions Adsorption and aggregation properties of three novel and structurally modified nonionic fluorinated surfactants have been investigated to reveal how dramatic structural variations affect surfactant properties and identify efficient molecular architectures. These compounds were the linear surfactant (L) comprising of one fluorinated hydrophobic group and one nonionic headgroup; the bolaform (B) which may also be considered as a unique mini, monodisperse fluorinated block copolymer EO-CF-EO; and the Y-shaped (Y) surfactant where now both headgroups are at the same end of the hydrophobic group. A strong link between the number and position of the headgroup(s) and the interfacial and bulk aqueous properties has been identified (Figures 2, 3, and 7; Tables 1, 3, and 4). NR and SANS measurements have been possible due to the neutron contrast between the fluorinated surfactants and air contrast matched water (NR experiments) and D2O (SANS experiments), and in all cases, the results from the two techniques have been in good agreement. For all three compounds, the surface coverages and excesses determined by NR and DVT studies agreed well (Figure 6, Table 2) as did the parameters found from Guinier analysis and model fitting for SANS studies (Figure 7, Tables 3 and 4). Therefore, strong structural-interfacial packing and structural-aggregation relationships are observed for such fluorosurfactants with such widely different molecular architectures. Interestingly, for the bolaform B, surface coverages determined by both NR and tensiometry are consistent with densely packed monolayer of rigid, extended molecules (Figure 4(i)). This contrasts with the bent U-shaped conformation (Figure 4(ii)) that has been proposed,
lcyl (Å)
[Y] (mol dm-3)
Guinier
model
Guinier
model
0.14 0.10 0.07 0.03
20.1 20.4 19.7 19.8
18.8 18.9 19.3 19.2
91 107 128 161
93 116 153 179
Table 4. Best Fit Parameters to SANS Data for the B Surfactanta rdisk/Å
tdisk/Å
[B] (mol dm-3)
Guinier
model
Guinier
modelb
0.13 0.10 0.06 0.03
183 183 205 205
246 272 261 265
35.5 33.0 35.2 36.2
35.0 35.0 35.0 35.0
a Uncertainties in absolute intensity is (5% and dimensions rcyl and tdisk (2 Å, Lcyl and rdisk (20 Å. b Parameter was fixed in the analysis.
on the basis of tensiometric studies, with more flexible hydrocarbon38,44 and partially fluorinated bolaform molecules.8c Hence, NR studies with standard hydrocarbon bolaform surfactants should be performed to shed more light onto these unusual interfaces. Since surface tensions of typical hydrocarbon-based solvents (e.g., ketones ∼25 mN m-1) are so much lower than water (∼72 mN m-1), normal hydrocarbon amphiphiles generally do not display any significant adsorption in such nonaqueous solvents. However, owing to strong antipathy between hydrocarbons and fluorocarbons, surfactants bearing certain fluorinated moieties can adsorb and aggregate in solvents. In favorable cases, these tailored fluoro-amphiphiles can display classic surfactant behavior, in both water and nonaqueous solvents. The results described here provide new insight into the molecular design requirements to achieve highly efficient surfactants for speciality applications in nonaqueous media. Acknowledgment. S.R. thanks the University of Bristol for a studentship. L.M. thanks Kodak and EPSRC for a jointly funded studentship. A.P. was supported by a Faraday IMPACT fellowship. We also acknowledge CCLRC for allocation of beam time at ISIS and grants toward consumables and travel. The Omnova Foundation is thanked for financial support. Supporting Information Available: Details of the synthesis of B surfactant and SANS data treatment and analysis. This material is available free of charge via the Internet at http://pubs.acs.org. LA0529153
