Langmuir 2001, 17, 7873-7878
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Fluorinated Nonionic Surfactants Bearing Either CF3- or H-CF2- Terminal Groups: Adsorption at the Surface of Aqueous Solutions Julian Eastoe,* Alison Paul, Alex Rankin, and Ray Wat School of Chemistry, University of Bristol, Bristol, BS8 1TS U.K.
Jeff Penfold and John R. P. Webster ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON, OX11 0QX U.K. Received June 25, 2001. In Final Form: September 5, 2001 Four nonionic fluoro-surfactants have been synthesized and their surface adsorption and micellization properties investigated. The compounds were perfluoroalkyl triethyleneoxide methyl ethers X-(CF2)mCH2-O-(C2H4O)3-CH3, with X either H or F, and m either 4 or 6 (H4EO3, F4EO3, H6EO3, and F6EO3). A strong structure-activity relationship was observed, depending on the nature of the hydrophobic chain. Initial surface tension measurements, using long-time dynamic drop volume tensiometry (DVT), were consistent with trace hydrophobic impurities, which could be eliminated by vacuum distillation to yield surface chemically pure surfactants. Neutron reflectivity (NR) measurements were performed to determine surface excess as a function of bulk concentration, and there was good agreement with tensiometrically derived coverages. Switching the terminal group H-CF2- to CF3- reduced the cmc by a factor of 4, lowered the limiting molecular areas acmc by ∼ 10 Å2, and reduced the cmc surface tension γcmc by 9 mN m-1. Increasing the chain length also gave rise to significant changes. Therefore, surfactants of this kind are unusual in that physicochemical properties can be controlled over a wide range, but with only minor variations in chemical structure. Furthermore, it is shown that with high purity surfactants tensiometric measurements are able to distinguish variations in surface coverage which arise from such subtle structural changes.
Introduction With surfactants hydrophobic chain structure is a major factor controlling important physicochemical properties such surface excess Γ, surface tension γ, and cmc.1 A recent study with double-chain, partially fluorinated, anionic sulfosuccinates has identified significant effects on these parameters when a CF3- chain terminal group is switched for H-CF2-.2 Such large changes in surfactant properties, which are detailed below, cannot be so easily achieved with standard hydrocarbon amphiphiles. Therefore, such fluorocarbon surfactants are interesting systems, which display unusually sensitive structure-activity relationships. Here studies are reported of single chain nonionics, bearing similar fluorocarbon tails to the sulfosuccinates. The aims were to examine the generality of these reported effects, and compare nonionic behavior with anionics.2 Dynamic drop volume tensiometry (DVT), the duNouy ring (DNR) method, and neutron reflection (NR) have been applied to characterize adsorption layers at air-water (a-w) interfaces. These neutron reflection results appear to be the first for aqueous solutions of nonionic fluorosurfactants. These surfactants are analogues of n-alkyl triethyleneoxide methyl ethers (CiE3-CH3), with general formulas X-(CF2)m-CH2-O-(C2H4O)3-CH3. The terminal atom X is either H- or F-, and chain length m either 4 or 6; hence they are denoted H4EO3, F4EO3, H6EO3, and * To whom correspondence should be addressed. Tel.: UK + 117 9289180. fax: UK + 117 9250612. E-mail:
[email protected]. (1) Pitt, A. R.; Morley, S. D.; Burbidge, N. J.; Quickenden, E. L. Colloids Surf., A 1996, 114, 321. (2) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591.
F6EO3, respectively. The two F6 compounds were first reported by Selve et al.,3 but the F4 analogues are unique to this work. Although some surface tension data were described,3 it is shown here that a final vacuum distillation step is necessary to obtain surface chemically pure surfactants. The substitution of chain tip F by H in the ω-H analogue results in a permanent dipole moment, for example, in CF3CF2H, µ ) 1.54D.4 For ease of synthesis methoxy-capped surfactants were chosen in preference to related molecules with terminal -OH hydrophilic groups. This structural difference has little effect on dilute aqueous phase behavior.5 Previous detailed work describes sodium bis(1H,1H perfluoropentyl)-2-sulfosuccinate (di-CF4) and bis(1H,1H,5H octafluoropentyl)-2-sulfosuccinate (di-HCF4), which are fluorinated analogues of Aerosol-OT (AOT)2. In general, an ω-H surfactant is less effective, having a higher cmc (∼ x 10 i.e., x 5 per chain), a 10 mN m-1 higher limiting surface tension at the cmc γcmc, and a greater limiting molecular area acmc (∼10 Å2) than the equivalent perfluoromethyl-tipped compound. Despite these significant differences, NR partial structure factor analyses for ω-H and perfluoromethyl chain ends indicated similar layer structures, in terms of interfacial thicknesses and penetration of water into the films. This sensitive response of physicochemical parameters (γ, Γ, and cmc) to the chain tip group or atom is an important aspect of surfactant behavior which has received only little attention.1-3 (3) Selve, C.; Achilefu, S. J. Chem. Soc., Chem. Commun. 1990, 911. Selve, C.; Achilefu, S.; Stebe, M. J.; Ravey, J. C.; Delpeuch, J. J. Langmuir 1994, 10, 2131. (4) Buckley, G. S.; Rodgers, A. S. J. Phys. Chem. 1983, 87, 126. (5) Conroy, J. P.; Hall, C.; Leng, C. A.; Rendall, K.; Tiddy, G. J. T. Prog. Colloid Polym. Sci. 1990, 82, 253.
10.1021/la010958n CCC: $20.00 © 2001 American Chemical Society Published on Web 12/04/2001
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Therefore, with these anionic data as a foundation it is of interest to investigate nonionic analogues. Neutron reflection is a direct surface sensitive method, whereas interpretation of tensiometric γ-ln c curves relies on an adsorption equation. Therefore, NR is an important method for characterizing new surfactants, and validating any interpretation of tensiometric data. Fluorinated surfactants are convenient for NR work due to a high scattering length of fluorine (bF)5.650 fm cf. bD)6.674 fm), and this has the advantage that costly isotopic substitution is not necessary (or possible). Although there is a significant literature with hydrocarbon surfactants, which has recently been reviewed,6 only few NR studies with fluoro-surfactants have been published. This is surprising considering the various industrial applications of fluorocarbon surfactants.7 Previously, Thomas et al.,8 Ottewill et al.,9 and Eastoe et al.10 have studied various perfluorocarboxylates, and as mentioned fluoro-AOT analogues have also received attention.2 As found with the F-sulfosuccinates,2 a strong structure-function link is also observed here with nonionics: the switch terminal F- to H- causes similar shifts in cmc, γcmc, and acmc as documented above. These results highlight the importance of chain-tip chemistry on physicochemical properties of surfactants, and demonstrate such effects should be considered in surfactant molecular design. Experimental Section Synthesis. A similar method that of Selve et al.3 was employed, and an outline synthesis for H4EO3 is given below. Activation (Tosylation): CH3-(OC2H4)3-OH f CH3-(OC2H4)3-Ots. Triethyleneglycol monomethyl ether (55 g, 1.4 eq. Fluka, 97%) dissolved in THF (100 mL, Sigma, anhydrous) was mixed with aqueous sodium hydroxide (5 M, 100 mL, 2.0 eq., BDH Aristar). The mixture was cooled to 0 °C in an ice-bath. Tosyl chloride (59.8 g, 1.3 eq., Sigma) in THF (100 mL) was added dropwise with constant stirring, maintaining a temperature below 5 °C. Once addition was complete, the reaction mixture was stirred for a further 30 min at 0 °C before being poured over ice water (500 mL). This solution was divided into two, and each half extracted with dichloromethane (3 × 100 mL). The organic phase was washed with water (2 × 150 mL, BDH) and a saturated aqueous solution of sodium chloride (1 × 150 mL, BDH), then dried over magnesium sulfate (BDH). Rotary evaporation of the solvent gave colorless oil, which was used without further purification. (89.2 g, 89%) Condensation: CH3-(OC2H4)3-OTs f CH3-(OC2H4)3-OCH2(CF2)4H. Tosyl-triethyleneglycol monomethyl ether (30.0 g, 1.0 eq.), 1,4-dioxane (400 mL, BDH), and 1H,1H,5H-perfluoropentan-1-ol (26.2 g, 1.2 eq., Fluorochem, 98%) were mixed, and finely ground potassium hydroxide (21.2 g, 4.0 eq. Fluka, >85%) was added with stirring. The mixture was refluxed with vigorous stirring for 24 h. Solvent was removed on a rotary evaporator and the resulting slurry dissolved in a minimum amount of water (∼100 mL). The aqueous solution was divided into two portions, each of which was extracted with diethyl ether (3 × 100 mL). The organic phase was washed once with a small amount of water and the combined extracts dried over magnesium sulfate. Removal of the solvent gave crude surfactant, which was purified (6) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv Colloid Interface Sci. 2000, 84, 143. (7) Kissa, E. Fluorinated Surfactants-Synthesis, Properties aAnd Applications; Surfactant Science Series 50; Marcel Dekker: New York, 1994. (8) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 2446. (9) Downes, N.; Ottewill, G. A.; Ottewill, R. H. Colloids Surf., A 1995, 102, 203. and Simister, E. A.; Lee, E. M.; Lu, J. R.; Thomas, R. K.; Ottewill, R. H.; Rennie, A. R.; Penfold, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3033. (10) Downer, A.; Eastoe, J.; Pitt, A. R.; Penfold, J.; Heenan, R. K. Colloids Surfs., A 1999, 156, 33.
Eastoe et al. by column chromatography over silica, eluting with ethyl acetate (yield 32.9 g, 92%). For other surfactants the starting alcohols were H6EO3 (yield 82%), 1H,1H,7H-dodecafluoro-1-heptanol (Fluorochem, 98%); F4EO3 (yield 87%), 1H,1H-perfluoropentan1-ol (Fluorochem, 98%); and F6EO3 (yield 89%), 1H,1H-perfluoro1-heptanol (Fluorochem, 98%). Final yields were similar to those reported before.3 Purification. The crude surfactants were obtained as orange or brown viscous fluids that were sparingly water soluble, and did not foam well. Column chromatography (Si60 silica 40-63 µm, BDH) was used in the first instance resulting in yellowtinged oils. Initial tensiometry experiments, described below, were consistent with trace hydrophobic contamination. After a final vacuum distillation step colorless liquids were obtained, which gave single spots by TLC (Si60 F254, Merck), and tensiometry was characteristic of surface chemically pure surfactants. Results from elemental analysis and 1H, 19F, and 13C NMR (JEOL Lambda 300 MHz in CDCl3) spectroscopies were obtained and were found to be consistent with the desired products. Mass spectroscopy (Fisons Autospec electron impact and chemical ion impact ionization, samples 5 mg ml-1 in ethyl acetate) gave no peaks for higher/lower chain homologues, indicating the surfactants were effectively single MW compounds. Cleaning. All glassware was cleaned using Micro cleaning fluid (∼5% aqueous solution) by heating in an ultrasonic bath. Rinsing with tap water, acetone and deionized water was followed by oven drying. Syringes and capillaries for DVT were cleaned by repeatedly rinsing with deionized water, which was obtained from a RO100HP Purite water purification system. Tensiometry. Tensiometers (Kru¨ss K10, fitted with a duNouy ring, and Lauda TVT1 drop-volume) were calibrated using ethanol/water mixtures11 and cleanliness of syringes, rings, etc., was confirmed by checking γ for water between surfactant solutions. Aqueous solutions at concentrations from roughly 5 × cmc to 0.05 × cmc were prepared by mass, and appropriate corrections made for density. The temperature was 25 °C. The surface excess Γ and area per molecule as were obtained using the Gibbs isotherm.
Γ)-
dγ 1 nRT d ln c
(1)
The prefactor n is theoretically dependent upon the surfactant type and structure, as well as the presence of extra electrolyte.12,13 For various neutral hydrocarbon nonionic and zwitterionic surfactants, a combination of NR and tensiometry has been used to confirm the n-factor should be 1,6,14-17 which was used here. Neutron Reflection (NR). Experiments were performed using the SURF reflectometer at ISIS at the Rutherford Appleton Laboratories, Didcot, UK.18 The specular neutron reflectivity R(Q) was measured normal to the a-w interface. A grazing angle of 1.5° and incident wavelength range 0.5-6.5 Å gave an accessible Q range of 0.05-0.65 Å-1. The measurements were made at 25 °C. To remove trace ionic impurities2,10 D2O was twice distilled before use, and surfactant solutions prepared in null reflecting water (NRW, 8.0 mol % D2O in H2O). Therefore, the reflectivity was from adsorbed monolayer only. The reflectometer was calibrated using D2O, and a flat background (∼7 × 10-6, determined by extrapolation of the high Q data) subtracted from the curve. The nonionic surfactants studied here are only short chain molecules (maximum C14 with C6 fluorinated). Consequently, relatively weak scattering was observed which, coupled with the low concentrations (10-3 - 10-6 mol dm-3), (11) Strey, R.; Viisanen, Y.; Aratono, M.; Kratohvil, J. P.; Yin, Q.; Friberg, S. E. J. Phys. Chem. B 1999, 103, 9112. (12) Hall, D. G. Colloids Surf., A 1994, 90, 285. (13) Hall, D. G.; Pethica, B. A.; Shinoda, K. Bull. Chem. Soc. Jpn. 1975, 48, 324. (14) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. Langmuir 1992, 8, 1837. (15) Thomas, R. K.; Lu, J. R.; Lee, E. M.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352. (16) Hines, J. D.; Garrett, P. R.; Rennie, G. K.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 7121. (17) Hines, J. D.; Garrett, P. R.; Rennie, G. K.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 9215. (18) See: http://www.isis.rl.ac.uk/.
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Langmuir, Vol. 17, No. 25, 2001 7875 Table 1. Comparison of Data for Column-Cleaned Surfactantsa cmc/(mmol dm-3) H4EO3 F4EO3 H6EO3 F6EO3
b
c
0.34 0.10
4.80 2.50 0.31 0.10
γcmc/(mN m-1) b
c
36 18
35 24 35 24
Uncertainties on γcmc and acmc are ( 0.1 mN respectively. b Selve et al.3 c This work. a
acmc/(Å2) b
c 46 55 51
37 m-1
and ( 3 Å2,
Figure 1. Surface tension curve for columned H4EO3, measured by the duNouy method. meant that long data acquisition times were required (typically 2 h). Due to limitations on beamtime it was only possible to study three of the four surfactants by reflectivity. NR Data Analysis. Reflectivity profiles were fitted using a least-squares routine6,18 to determine layer thickness τ and scattering length density F. In the simplest case a sharp interface is assumed between the surfactant monolayer and the solution subphase, and the monolayer is considered to be of uniform thickness and F. The area per molecule and, hence, adsorbed amount were determined using eq 2.6
∑b
i
as )
i
Fτ
1
)
(2)
ΓNa
Σb represents the sum of isotopic scattering lengths in the molecule. The value of τ depends on the distribution function used to describe the scattering length density profile though surface normal. However, as described by Thomas et al.,6 the surface coverage obtained in eq 2 is model independent, as although various values for τ may give a satisfactory fit, the fitted value of F tends to compensate, such that the product τF remains constant. Under the kinematic approximation, if a Gaussian distribution of F (being of width σ) is assumed, eq 3 may be applied after subtraction of the background.
[ ] R(Q)Q2 2
≈ (ΓNA
∑b ) exp
(-Q2.σ2)
i
(3)
Figure 2. a: Equilibration time for a column-cleaned surfactant. H6EO3 at ∼2 × cmc. b: Equilibration time for a vacuum distilled surfactant. F4EO3 at ∼1.8 (2) and 0.7 (b) × cmc.
Tensiometry. Comparison of Methods and Surfactant Purity. With column-cleaned surfactants duNouy Ring (DNR) tensiometry was used to obtain initial values for cmc, γcmc, and acmc. An example γ-ln c plot is given in Figure 1, and the overall shape of the curve is consistent with pure surfactant. No minimum was observed and a sharp break point, characteristic of a well-defined cmc is evident. Furthermore, as shown by the data in Table 1
there was good agreement in surface parameters derived from these measurements and previous work.3 However, it was apparent that the solutions took a long time to reach equilibrium, even at and above the cmc’s. For a pure surfactant surface tension should decay to a steady equilibrium value, the time scale of this process depends principally on concentration. Around the cmc’s of these fluoro-nonionics adsorption should be complete within 1060s.20 Adsorption dynamics are often neglected in surface tension experiments, but it is essential to consider this in careful work, as eq 1 applies strictly to equilibrium systems. Drop Volume Tensiometry (DVT) allows long time scales (up to 15 min per drop) to be investigated, and the decay of γ with time may be followed to ensure that a true equilibrium value is reached. Therefore, dynamic studies by DVT were carried out for a more in-depth characterization (Figures 2 and 3). As shown in Figure 2 a, even above the cmc around 300 s were required to reach the equilibrium tension, well outside the normal dynamic region for this concentration. Such an effect could due to hydrophobic trace impurities, which since present at a much lower concentration than the surfactant, would adsorb at a slower rate. Also, the first determinations of cmc’s gave lower values of acmc for the H terminated
(19) Li, Z. X.; Lu, J. R.; Thomas, R. K. Langmuir 1997, 13, 3681.
(20) Eastoe. J.; Dalton, J. S. Adv. Colloid Interface Sci. 2000, 85, 103.
16π
i
At low Q the reflectivity is given by ΓNA∑ibi.6,19 A plot of ln[((R(Q)Q2)/16π2)] vs Q2 therefore yields Γ from the intercept. This approach was used to crosscheck the values obtained with the sharp interface approach. Equation 3 is valid at low Q, and hence only data in the range 0.05 < Q < 0.10 Å-1 were considered. To determine reliable measures of as the data were fitted in several different ways: (i) both τ and F were “floated” to obtain a minimum χ2 value and (ii) to ensure that a global (rather than local) minimum in χ2 was obtained, τ was fixed at values from 10 to 30 Å and F fitted. A minimum in χ2 was obtained at the same thickness as obtained in (i), and the fitted value F decreased with increasing τ to give a constant value for as (τF). (iii) Finally, low Q data were analyzed with eq 3.
Results
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Eastoe et al.
Figure 3. Surface tension data for purified nonionics. Lines are quadratic fits to the pre-cmc data. O: H4EO3. ]: H6EO3. b: F4EO3. [: F6EO3. Table 2. Parameters Derived from Surface Tension Data with Vacuum-Distilled Non-Ionic Surfactantsa
H4EO3 F4EO3 H6EO3 F6EO3 di-HCF4b di-CF4b
cmc/ (mmol dm-3)
γcmc/ (mN m-1)
acmc/ (Å2)
Γcmc/ (10-6 mol m-2)
8.0 2.0 0.3 0.08 16.0 1.57
33.0 24.4 33.2 24.6 26.8 17.7
55 43 48 37 65 56
3.01 3.89 3.47 4.37 2.55 2.96
a Typical uncertainties are (0.1 mN m-1 on γ 2 cmc, (2-4 Å on acmc, and (0.25 × 10-6 mol m-2 on Γ. b Data for anionic surfactants di-HCF4 (25 °C) and di-CF4 (30 °C) taken from ref 2.
compound H4EO3 (46 Å2) compared to F4EO3 (55 Å2), in contradiction to the trends observed previously with fluorosulfosuccinates.2 In light of these results the surfactants were further purified by vacuum distillation, and adsorption behavior reevaluated. Figure 2 b shows much faster equilibration times after this final purification, with stable tensions obtained after 10 s or so. Hence, this extra purification step appears to be an important treatment, which had previously been overlooked.3 Effect of Hydrophobic Chain Structure. Surface tension isotherms for the four clean surfactants are shown in Figure 3 and the values derived from these data are summarized in Table 2. (Neutron reflection experiments, described below, confirm these values derived from DVT data.) As can be seen, there is a strong structure-activity relationship. It is remarkable that small variations in molecular structure, changing the chain length by only two -CF2- units, or replacing a fluorine atom for hydrogen in the terminal methyl group, have such significant effects on adsorption and aggregation. The cmc for the CF6 compound is ∼ 25 lower than the equivalent CF4 analogue; each additional CF2 therefore causing a factor of ∼5 decrease in the cmc, consistent with previous studies.2,3 For typical hydrocarbon nonionics the factor would be ∼3, which reflects the relatively high surface activity of fluorocarbons. Furthermore, in terms of cmc depression, it is found that CF2 is roughly equivalent to 1.7 CH2 groups, in agreement with Selve et al.3 For the F4EO3 and F6EO3 increasing fluorocarbon chain length results in a slight contraction of the layer, in that the effective molecular area acmc decreases by ∼6 Å2. Although slightly counterintuitive, this trend has been observed previously (see 6 and references therein for various examples). Lukenheimer et al.21 observed acmc decreases with chain length for two series of di-alkyl phosphine oxides. This was first (21) Lukenheimer, K.; Haage, K.; Hirte, R. Langmuir 1999, 15, 1052.
attributed to a variation in headgroup hydration, but later to a change in conformation of the hydrocarbon chains.21 Eastoe et al.22 observed the same effects for several dichain anionics surfactant, and attributed the change in acmc to increasing chain-chain attraction. Variations in acmc are often small enough that results within a series are only just outside the experimental errors. Certainly it is essential that steps be taken to remove all surfaceactive impurities (as was the case in the examples quoted above). It is clear that a terminal H has a significant effect on adsorption. The added dipole increases surfactant hydrophilicity, resulting in a 4-fold increase in cmc for an equivalent chain carbon number. The surface energy of an H-CF2 group is higher than that of F-CF2- which, combined with the lower surface excess, results in a higher limiting surface tension by ∼9 mN m-1. This enhanced interaction increases acmc by ∼10 Å2 for the H-terminated compounds, compared to the fully fluorinated analogues. Comparison of Single-chain Nonionics and Doublechain Anionics. Table 2 contains also data for the doublechain anionics di-HCF4 and di-CF4,2 which bear the same hydrophobic chains as H4EO3 and F4EO3, respectively. Looking at the decrease in cmc’s per individual chain, on changing H-CF2- to CF3-, there is a factor of 4 for the nonionics and 5 (i.e., 10/2) for the corresponding anionics. At the respective cmc’s limiting tensions for both series decrease H-CF2- vs CF3- consistently by ∼ 9 mN m-1. The differences in molecular areas are between 11 Å2 (nonionics) and 9 Å2 (anionics), which are consistent given the uncertainties of 2-4 Å2. Therefore, the order and scale of changes in surfactant properties for the two series are comparable, highlighting the direct effect of hydrophobic chain structure on surface properties. However, doublechain surfactants have lower limiting tensions and this must be due to the greater surface chain density, which can be achieved owing to the two-tail structure. Compare F4EO3 with di-CF4; the effective areas per chain at the cmc’s are 43 and 23 Å2, respectively (both ( 3 Å2). For the anionic compound, this represents the physical crosssection area of a fluorocarbon chain.23 Therefore the anionic results in a highly efficient surface coverage, which is significantly reduced on switching to a single chain nonionic, with a larger headgroup (-EO3-OCH3). Hence, the surprisingly high surface activity of a compound like di-CF4 can be understood by comparing the results for the different surfactant analogues. Neutron Reflection. Typical R(Q) data are shown in Figure 4, for H6EO3 along with fits to the single layer optical matrix model. The steady decrease in R(Q) at low Q is consistent with a reduction in surface coverage on dilution, and this is reflected in the fitted values for F, as, and Γ which are given in Table 3. As a crosscheck on this analysis, the kinematic approximation expression (equation 3) was also used to obtain Γ. An example of this treatment is shown in Figure 5 for the H6EO3, and as can be seen from Table 3 good agreement was achieved between the two measures of adsorption. As described above, due to restricted beam-time it was only possible to carry out a complete study three surfactants, however, similar data as show in Figures 4 and 5 were obtained for the HF4EO3 and F4EO3. The layer appears to thin with dilution (Table 3), an observation common to many other surfactants.6 Compared with deuterated compounds, the plane of neutron contrast is less well defined with these fluorinated-ethylene oxides and so it is difficult to make (22) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733. (23) Mengyang, L.; Acero, A.; Huang, Z.; Rice, S. Nature 1994, 367, 151 and Arrington, C. H. ; Patterson, G. D. J. Phys. Chem. 1953, 57, 247.
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Langmuir, Vol. 17, No. 25, 2001 7877
Figure 4. Reflectivity profiles for H6EO3 at selected concentrations. Lines are fits to the single layer model. Concentrations in mmol dm-3. ]: 0.3. 4: 0.1. 0: 0.05 × 0.01. Table 3: Fitted and Derived Parameters for H6EO3 NR Dataa c/ (mmol dm -3)
τ/(Å)
10-10F/ (cm-2)
0.3 0.2 0.1 0.05 0.025 0.010
27.8 26.8 25.1 27.7 24.4 21.1
0.93 0.83 0.86 0.66 0.75 0.66
Γ/(10-6 mol m-2) as
/(Å2)
45.1 52.2 54.1 63.5 63.5 83.4
b
c
3.58 3.18 3.07 2.61 2.62 1.99
3.57 3.15 3.14 2.76 2.77 2.30
a The parameters, and typical uncertainties at the cmc, are layer thickness τ ((1 Å), layer scattering length density F, molecular area as ((2 Å2), and surface excess Γ ( 0.15 × 10-6 mol m-2. b Singlelayer fit. c Kinematic approximation, eq 3.
Figure 6. Adsorption Isotherms for (a) H4EO3, (b) F4EO3, (c) H6EO3. O, b, [: Tensiometry. 4: Neutron reflection. Table 4. Comparison of Surface Excess at the cmc Derived from Tensiometry and NRa Γcmc/(10-6 mol m-2)
acmc/(Å2)
Figure 5. Analysis of neutron reflection data from H6EO3 with the kinematic approximation. Lines are least squares fits to low Q data. Concentrations in mmol dm-3. ]: 0.3. 4: 0.1. 0: 0.05 × 0.01.
much of the absolute layer thicknesses. The tip-to-tail molecular length of H6EO3 can be estimated as ∼ 26 Å; therefore, the values for τ are at least consistent with a monolayer. Comparison of Results from NR and Tensiometry. Figure 6 gives adsorption isotherms derived from tensiometric (eq 1) and neutron reflection (eq 2) measurements for the compounds investigated by both methods. For comparison purposes, the concentration has been scaled to the respective cmc. Excellent agreement was obtained for H6EO3 and F4EO3, whereas H4EO3 there appears to be a systematic overestimation of ∼ 10%, which is just outside the uncertainties. The reason for this slight
H4EO3 F4EO3 H6EO3 F6EO3
DVT
NR
DVT
NR
55 43 48 37
50 44 46 -
3.01 3.89 3.47 4.37
3.29 3.78 3.58
a Uncertainty for tensiometry is ( 0.25 × 10-6 and ( 0.15 × 10-6 mol m-2 for neutrons.
difference with H4EO3 is difficult to ascertain without further experiments. However, the general agreement, and progression of trends with molecular structure validate the interpretation of tensiometry data discussed above. Furthermore, a similar close agreement between tensions and neutrons has been found for di-HCF4 and di-CF4,2 justifying the discussion comparing adsorption parameters for nonionics versus anionics given above. Table 4 gives (where available) molecular areas and surface excess derived by DVT and NR for the four compounds.
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Summary and Conclusions Surface chemical properties of nonionic fluoro-surfactants have been investigated: the compounds were X-(CF2)m-CH2-O-(C2H4O)3 -CH3, with X being either H- or F-, and m either 4 or 6 (H4EO3, F4EO3, H6EO3, and F6EO3, respectively). Although there have been reports of the C6 analogues,3 the shorter chain C4 surfactants are new. Where there was overlap,3 good agreement between the cmc values and limiting surface tensions γcmc were found (Table 1). However, extensive studies of long-time dynamic drop volume tensiometry (DVT), such as shown in Figure 2 a, indicate surface impurities (perhaps alcohol) are present in surfactants produced by the standard published procedure.3 Stable tensions were more rapidly achieved after an additional vacuum distillation purification step, consistent with the removal of hydrophobic contaminants. With these surface chemically pure surfactants a sensitive link was identified between surfactant chain chemical structure, surface activity, and micelle formation (Figure 3, Table 2). Neutron reflectivity (NR) measurements were possible owing to high neutron contrast against null reflecting water neutron (Figures 4 and 5, Tables 3 and 4). For three of these surfactants surface coverages determined by neutron reflectivity (NR) and DVT agreed well (Figure 6, Table 4), meaning that firm conclusions could be drawn concerning structure-activity relationships. Direct comparisons could be made with surface properties of the double-chain anionics, di-HCF4, and di-CF4,2 which have the same chain structures as H4EO3 and F4EO3, respectively. Switching the terminal group H-CF2- to CF3- causes almost identical changes in cmc’s (factor of 4-5 lower), limiting molecular areas (acmc down by ∼ 10 Å2), and tensions (9 mN m-1 decrease in γcmc) for both surfactant series (Table 2). Hence, regardless of headgroup type there is a strong structure-function relationship, which is owing to the differences in fluorocarbon structure only. For analogous hydrocarbon EO3 nonionics, with CH3- terminal hydrophobic groups, chain length affects strongly the cmc, whereas limiting tensions
Eastoe et al.
remain constant at ∼ 30 ( 2 mN m-1.24 The same is true for dichain anionics;22 chain length controls the cmc, but has little effect on γcmc. Therefore, such a high sensitivity of surface adsorption and aggregation as seen for these fluoro-surfactants is not really possible with normal hydrocarbons. Hence, it would seem that surfactants bearing these fluorocarbon chains are quite unique in that physicochemical properties can be controlled over a wide range, but with only minor variations in chemical structure. An aspect that deserves mention relates to agreement between two independent methods DVT and NR. With compounds of appropriate high purity tensiometric experiments do indeed have “molecular resolution”, in that it is possible to distinguish quite subtle variations in surface coverage which arise form minor changes in molecular structures. Recent work22 on a wide range of linear and branched hydrocarbon sulfosuccinates, anionics for which there are different surface purity issues to address, supports this statement. It would be interesting to see if such significant structure-function responses can be predicted and explained by Blanckschtein’s molecular thermodynamic model [e.g., ref 25], and hence provide a theoretical basis for the effects documented here. Acknowledgment. This work was funded by EPSRC as part of programs into surfactants for CO2 (GR/L05532) and dynamic surface tension (GR/M83780). A.P. and A.R. acknowledge the support of EPSRC in terms of studentships. We also thank CLRC (Rutherford Appleton Laboratory) for allocation of beam time at ISIS and contributions toward consumables and travel. Drs. Martin Murray and Ken MacNeil from the School of Chemistry are thanked for assistance with NMR and mass spectroscopies, respectively. LA010958N (24) Eastoe, J., Lodhi. A. Unpublished results. In ref 15 the limiting surface tension for C12E3 is given as ∼29 mN m-1. For example values for a range of CiEj′’s, see: van Os, N. M.; Haak, J. R.; Rupert, L. A. M. In Physio-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993; p 247. (25) Puvvada, S.; Blankschtein, D. J. Chem. Phys. 1990, 92, 3710.
