Langmuir 2000, 16, 4511-4518
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Adsorption of Ionic Surfactants at the Air-Solution Interface Julian Eastoe,* Sandrine Nave, Adrian Downer, Alison Paul, Alex Rankin, and Kevin Tribe School of Chemistry, University of Bristol, Bristol, BS8 1TS UK
Jeff Penfold ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON, OX11 0QX UK Received December 2, 1999. In Final Form: January 31, 2000 Neutron reflection (NR) and surface tension methods were compared for accessing the equilibrium adsorption isotherm of various ionic surfactants at the air-water interface. Four custom-synthesized anionics were investigated in detail: sodium dihexyl sulfosuccinate (di-C6SS), bis(1H,1H-perfluoro-npentyl)sodium sulfosuccinate (di-CF4), bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4), and bis(1H,1H,5H-octafluoropentyl)-2-sulfoglutaconate (di-HCF4GLU). Commercial n-alkyltrimethylammonium bromide (Cn-TAB) cationic surfactants with C12, C14, and C16 chain lengths were also studied. The experiments examined the validity of the Gibbs equation, and the prefactor 2, for these seven compounds. Effects of contaminants, trace levels of polyvalent metal ions, and hydrophobic impurities were assessed for the anionics. When added at low levels, tetrasodium ethylenediaminetetraacetate (EDTA) was effective for eliminating effects of metallic impurities, and foam fractionation was used to remove hydrophobic contaminants. The effects of these treatments on the apparent surface excess and procedures for obtaining agreement between the neutronic and tensiometric isotherms are described. Finally, it was confirmed that the Gibbs prefactor of 2 applies for all these 1:1 ionic surfactants.
Introduction The exact form of the adsorption isotherm for 1:1 ionic surfactants is a fundamental issue in surfactant science that is still a matter of debate (e.g., refs 1-25). Proper knowledge of the surface coverage is important for * E-mail:
[email protected]. (1) Shinoda, K.; Nakayama, H. J. Coll. Sci. 1963, 18, 705. (2) Tajima, K.; Muramatsu, M.; Sasaki, T. Bull. Chem. Soc. Jpn. 1970, 43, 1991. (3) Tajima, K.. Bull. Chem. Soc. Jpn. 1970, 43, 3063. (4) Tajima, K.. Bull. Chem. Soc. Jpn. 1971, 44, 1767. (5) Cross, A. W.; Jayson, G. G. J. Colloid Interface Sci. 1994, 162, 45. (6) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 2446. (7) Li, Z. X.; Lu, J. R.; Thomas, R. K. Langmuir 1997, 13, 3681. (8) Lu, J. R.; Li, Z. X.; Thomas, R. K.; Staples, E. J.; Tucker, I.; Penfold, J. Langmuir 1992, 8, 1837. (9) Thomas, R. K.; Lu, J. R.; Lee, E. M.; Penfold, J.; Flitsch, S. L. Langmuir 1993, 9, 1352. (10) Hines, J. D.; Garrett, P. R.; Rennie, G. K.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 7121. (11) Hines, J. D.; Garrett, P. R.; Rennie, G. K., Thomas, R. K., Penfold, J. J. Phys. Chem. B 1997, 101, 9215. (12) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (13) Bae, S.; Haage, K.; Wantke, K.; Motschmann, H. J. Phys. Chem. B 1999, 103, 1045. (14) Downer, A. D.; Eastoe, J.; Pitt, A. R.; Penfold, J.; Heenan, R. K. Colloids Surf. A 1999, 156, 33. (15) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591. (16) Williams, E. F.; Woodberry, N. T.; Dixon, J. K. J. Colloid Interface Sci. 1957, 12, 452 (17) Elworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 21, 331. (18) Mysels, K. J. Langmuir 1986, 2, 423. (19) Mysels, K. J.; Florence, A. J. Colloid Interface Sci. 1973, 43, 577. (20) Lukenheimer, K. J. Colloid Interface Sci. 1989, 131, 580. (21) Lukenheimer, K.; Haage, K.; Hirte, R. Langmuir 1999, 15, 1052. (22) Strey, R.; Viisanen, Y.; Aratono, M.; Kratohvil, J. P.; Yin, Q.; Friberg, S. E. J. Phys. Chem. B 1999, 103, 9112. (23) Hall, D. G. Colloids Surf. A 1994, 90, 285. (24) Hall, D. G.; Pethica, B. A.; Shinoda, K. Bull. Chem. Soc. Jpn. 1975, 48, 324.
explaining many surfactant-related phenomena, especially dynamic surface tension, in which an accurate measure of the surface excess is essential for understanding the adsorption mechanism. This article describes careful studies of the equilibrium adsorption behavior of four different custom-synthesized and highly purified anionics, which are shown in Figure 1. The sodium dihexyl sulfosuccinate (di-C6SS) is a structural relative of AerosolOT [AOT, bis(2-ethylhexyl)sodium sulfosuccinate].16 The next two, bis(1H,1H-perfluoro-n-pentyl)sodium sulfosuccinate (di-CF4) and bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4), are fluorinated AOT analogues.15 With bis(1H,1H,5H-octafluoropentyl)-2-sulfoglutaconate (di-HCF4GLU) there is an extra -CH2spacer in the headgroup region compared with the succinate di-HCF4. In addition, some commercial nalkyltrimethylammonium bromide (Cn-TAB) cationic surfactants also have been investigated to compare them with recent studies of related gemini and pyridinium surfactants.12,13 The equilibrium studies described here will form the foundation for investigations of dynamic surface tensions, which will be reported at a later date. The adsorption isotherm can be obtained indirectly by analyzing tensiometric data with the Gibbs equation, where the terms have their usual meanings.
Γ)-
dγ 1 mRT d ln a
(1)
The importance of using activity, a, rather than concentration or mole fraction, has been demonstrated recently for simple mixtures of water and alcohols.22 This finding is confirmed here for dilute ionic surfactants, for (25) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, 96, 1383.
10.1021/la991564n CCC: $19.00 © 2000 American Chemical Society Published on Web 03/31/2000
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scattering length density, F. These values are related to the area per molecule A via.
A)
Figure 1. Anionic surfactants investigated in this work.
which the critical micelle concentrations (cmcs) range from 1 to 10 mmol dm-3, and where activity coefficients can be calculated using the Debye-Hu¨ckel limiting law. The prefactor m theoretically is dependent on the surfactant type and structure, as well as the presence of extra electrolyte in the aqueous phase.23,24 For various nonionic and zwitterionic surfactants the expected value of 1 for the m-factor has been confirmed.8-11 With a 1:1 ionic surfactant, in the absence of extra electrolyte, the thermodynamic treatment requires m ) 2, implying an equimolar ratio of surfactant anion and countercation in the interface. The motivation of this work was to test this thoroughly for various ionics, and evaluate the criteria for “surface chemical purity”. A complementary method for measuring Γ is needed to verify the Gibbs equation. Early experiments used radiotracer (RT) measurements with tritiated surfactants2-5; more recently neutron reflection (NR)6-12,14,15 and surface second-harmonic generation (SHG) spectroscopy have been used.13 Although it is difficult to assess the accuracy of the other two methods, with NR the surface excess around the cmc can be measured to about 5%.25 With NR measurements, using null reflecting water (NRW, 8 mol % D2O), it is possible to determine the absolute amount of surfactant ion in the monolayer (e.g., refs 6-12, 14, 15). The measured reflectivity curve can be modeled in terms of a single, uniform layer to fit for the thickness, τ, and
∑bi ) Fτ
1 ΓNa
(2)
The sum of nuclear scattering lengths over the surfactant molecule, ∑bi, is a known factor, and Na is the Avogadro number. NR is a direct method, which essentially “counts” molecules in the film, whereas tensiometry is indirect, and interpretation of the γ - a curve always involves assumptions in terms of an adsorption equation. Tensiometric and neutronic adsorptions are compared for seven different ionic surfactants in this article. Unlike NR, it is still unclear exactly what is being measured in tensiometric experiments. For certain systems, the neutronic and tensiometric isotherms can only be reconciled by invoking a net charge on the monolayer (e.g., refs 6, 7). This interpretation requires that there is a depletion layer of unrealistically large dimensions (>1000 Å) below the surface. Hence, there must be another reason for these differences in apparent coverages, and impurities are the prime suspects. Surface-active impurities are expected to introduce discrepancies between any two independent methods. The RT measurements of Tajima et al.2 on sodium dodecyl sulfate, in the absence of added electrolyte, were consistent with m ) 2. However, more recent work by Cross and Jayson5 highlighted possible problems associated with trace inorganic contaminants. For anionics the most important contaminants are polyvalent metal ions Mn+, and their effects have been described in studies of AOT7 and in several other anionics.5-7,14,15 When used at the correct level, the tetrasodium salt of ethylenediaminetetraacetic acid (EDTA) is an effective chelating agent, which effectively replaces Mn+ with Na+.6,7,14,15 Even at ppm levels any Mn+ may give rise to a significant lowering of tension in the pre-cmc range.7,14 In general, at trace levels of Mn+, there is no dramatic effect on the form of the γ - ln a curve, for example the manifestation of a minimum or shoulder around the cmc. Therefore, inorganic contaminants are more difficult to detect than organic impurities, which can be removed effectively by separation procedures based on foam fractionation.15-21 In NR studies of Cs+, Na+, and H+ perfluorooctanoate6 and Na+ AOT,7 the prefactor 2 agreed with NR data only in the presence of EDTA. Downer et al.14 performed detailed work with sodium perfluorononanoate (C8F17COO- Na+) and 9-Hperfluorononanoate (H-C8F16COO-Na+). Background levels of Ca2+, the principal contaminant, were determined by atomic absorption spectroscopy; the ratio surfactant: Ca2+ was typically 10 000:1.14 Based on these results, careful studies of the effects of various Mn+ ions on precmc tensions were performed to determine the optimum level of EDTA. Comparison of the tensiometric and neutronic Γ - c curves indicated m ≈ 1.7. Next D2O, which generally is “used as received” for the NR experiments, was assessed for Ca2+ content, which indicates that this also must be considered. For example, a surfactant at its cmc of 1 mmol dm-3, made up in NRW typically would result in a surfactant:Ca2+ ratio of 333:1. Once this effect had been taken into account, repeat NR experiments with higher EDTA ratios were then consistent with m ) 2. In terms of cationics, Bae et al.13 recently have studied 1-dodecyl-4-dimethylaminopyridinium bromide by tensiometry and SHG, and the two methods indicated m ) 1.28 for this surfactant. An elaborate procedure was used
Ionic Surfactants Adsorbed at Air-Water Interface
to calibrate the SHG signal by studying the response for an insoluble monolayer of a related C20 chain compound as a function of surface pressure. Recently, the interesting case of gemini surfactants of the type [CmH2m+1 - N (CH3)2-(CH2)s - N - (CH3)2-CmH2m+1] Br2, has been examined using neutrons.12 These are 2:1 electrolytes for which the Gibbs factor is expected to be 3, and with an aromatic xylyl spacer group this was confirmed. However, for the more common systems with aliphatic spacer groups s ) 3, 4, 6, and 12, m tended to 2. The observations with cationics12,13 can be rationalized by invoking formation of a (partial) surfactant-counterion complex at the interface. This suggests that chemical specificity may play a role, and cationics must be assessed on a case-by-case basis, which is a point also borne out here for anionics. In this article adsorption isotherms for the four doublechain anionics shown in Figure 1 are shown to be consistent with m ) 2. However, it is demonstrated that to achieve this agreement extreme care must be taken to eliminate various sources of contamination. A more limited study of Cn-TABs, with n ) 12, 14, and 16, is also presented. The results for these systems at the cmc also indicate an m-factor of 2. Experimental Section a. Materials. Deuterated Sodium Dihexyl Sulfosuccinate (Ddi-C6SS). Deuterated n-hexanol (n-hexyl-d13 alcohol, CDN Isotope, 98.5%D), maleic anhydride (Avocado, 99%), and toluene4-sulfonic acid monohydrate (Aldrich, 98.5%+) were reacted in toluene, with use of a Dean and Stark trap to remove water.26 The mixture was refluxed with stirring for approximately 14 h. When the volume of water evolved from the reaction was comparable with the theoretical maximum, the mixture was cooled to 70 °C and washed with hot water to remove unreacted maleic anhydride and sulfonic acid. The crude diester was obtained by rotary evaporation and then purified by vacuum distillation. The pure diester was sulfonated in a 1:1 mixture of ethanol:water, which was refluxed with an excess amount of sodium metabisulfite (Avocado 97%). Note this reagent, which is the sole source of Na+, also contains trace levels of other cations. Thin layer chromatography (TLC) was used to check for disappearance of the high-running diester spot. The other diesters (see below) were also sulfonated in this way. The crude surfactant was obtained by decantation and rotary evaporation. Bis(1H,1H-perfluoro-n-pentyl)sodium Sulfosuccinate (di-CF4). Fumaryl chloride (Avocado, 95%) was added dropwise to a stirred solution of 1H,1H-perfluoro-n-pentanol (Fluorochem, 97%) and N,N-dimethylaniline (Aldrich, 99%) in dry tetrahydrofuran (THF). The reaction mixture was then refluxed until TLC indicated completion. After rotary evaporation, the residue was dissolved in diethyl ether and washed with 10% aqueous HCl, followed by a saturated aqueous sodium hydrogen carbonate solution. The ethereal layer was dried over magnesium sulfate and filtered, and the solvent was removed to yield the crude diester,15 which was then purified by vacuum distillation. The bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4) and the proteated sodium dihexyl sulfosuccinate (Hdi-C6SS), were also obtained in this way, using 1H,1H,5Hoctafluoro-1-pentanol (Fluorochem, 97%) or H-n-hexanol (Aldrich, 98%) as the starting alcohol. Bis(1H,1H,5H octafluoropentyl)-2-sulfoglutaconate (diHCF4GLU). 1H,1H,5H-Octafluoropentan-1-ol, glutaconic acid (Fluka puriss >97%) and toluene-4-sulfonic acid monohydrate were mixed with toluene, and the Dean and Stark method was used to obtain the diester, which was purified as described above. Purification of Anionic Surfactants. To obtain “surface chemically pure” surfactants four separate stages were necessary. 1. Soxhlet extraction. Soxhlet extraction removed inorganic material left over from the sulfonation step. The Soxhlet apparatus was protected from atmospheric moisture, the extrac(26) Yoshino, N.; Komine, N.; Suzuki, J.-I.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262.
Langmuir, Vol. 16, No. 10, 2000 4513 tion was performed for 24-48 h, using dried ethyl acetate and dried surfactants, and then the solvent was removed by rotary evaporation. 2. Washing. Washing removed residual inorganic material. The product from stage 1 was dissolved in the minimum quantity of dried, distilled methanol and centrifuged at 25 °C, 10 000 rpm for 45 min. The supernatant was decanted and any residual solids were discarded. The solvent was removed by rotary evaporation. 3. Recrystallization. The surfactants were recrystallized twice, or three times, from ethanol (di-CF4), ethanol/chloroform (diHCF4), methanol (D-di-C6SS, H-di-C6SS),16 or toluene (diHCF4GLU), and then dried under vacuum at 45-70 °C for 48 h. (All solvents were Analar grade). 4. Foam fractionation. Foam fractionation was the final purification step to remove surface-active impurities. Aqueous solutions of the surfactants were made up at just below the cmc.18 Nitrogen gas was passed through a calcium sulfate drying setup and a carbon filter then bubbled through aqueous solutions of HCl and NaOH, followed by several flasks of pure water before finally reaching the surfactant solution. It was necessary to control the gas flow and vacuum suction rates so that the foam had sufficient time to drain before being removed. The pure surfactants were recovered by rotary evaporation of water and dried as before. The final products were dried in a vacuum oven (60 °C, 5 mbar) for a least 30 h, and then stored in sealed vials in a desiccating cabinet over refreshed phosphorus pentoxide. All the diester intermediates and purified surfactants were investigated with a range of analytical methods, as appropriate. 1H, 13C, and 19F NMR (JEOL Lambda 300), mass spectroscopy (Fisons Autospec), and elemental analyses (H, C, F, and S) confirmed the desired products at >99% purity. Cationic Surfactants. n-Dodecyltrimethylammonium bromide (C12-TAB, 97%), n-tetradecyltrimethylammonium bromide (C14TAB, 98%), and n-hexadecyltrimethylammonium bromide (C16TAB, 98%) were obtained from Lancaster. The surfactants were recrystallized once from a 1:1 ethanol:acetone mixture, dried in a vacuum oven, then stored over refreshed P2O5 before use. Other Materials. All solvents used during purification were dried (MgSO4 or Na wire) and twice distilled before use. Water was from Millipore Milli-Q Plus or R0100HP Purite purification system. D2O (Fluorochem, 99% D-atom) used in the neutron reflection experiments was distilled twice to remove any inorganic contaminants.14 b. Methods. Tensiometry. Glassware was cleaned, soaked in Micro critical cleaning solution, rinsed repeatedly with water, soaked in 50% nitric acid solution, rinsed well, and finally checked for cleanliness over a steam bath. All solutions were prepared by mass, and the concentrations were corrected for density (Paar digital density meter DMA 35). EDTA (Sigma, 99.5% tetrasodium salt hydrate) was included in the aqueous subphase, and the appropriate level was carefully determined, as described below. The measurements were performed using a drop-volume instrument (Lauda TVT1) operating in dynamic mode. This ensures that a true equilibrium tension is achieved, and it is also a very sensitive method for detecting trace hydrophobic impurities. Selected systems were also investigated with the du Nouy and Wilhelmy methods (Kru¨ss K10). Typically 10 to 20 repeat measurements were made at each concentration, and reproducibility was checked periodically with fresh solutions. Thermostating to ( 0.1 °C was achieved with a Grant LTD6G circulating water bath. Experiments were conducted at 25 °C, except for di-CF4 and di-HCF4GLU (30 °C). Neutron Reflection. NR measurements were performed on the CRISP and SURF reflectometers at ISIS, Rutherford Appleton Laboratories, Didcot, UK, using the standard setup for free liquid surfaces.27,28 The specular NR R(Q) was measured normal to the interface as a function of the momentum transfer Q ) (4π sin θ)/λ. The angle of incidence (θ) was 1.5°, and the incident wavelength range was 0.5-6.5 Å. This results in an accessible Q range of 0.05-0.65 Å-1. Measurements were performed at selected concentrations using NRW (8.0 mol % D2O in H2O). The (27) see http://www.isis.rl.ac.uk/ (28) Penfold, J.; Thomas, R. K. J. Phys. Condens. Matter 1990, 2, 1369.
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Figure 2. Surface tension behavior of proteated sodium dihexyl sulfosuccinate H-di-C6SS solutions. The polynomial lines fitted to the pre-cmc data were used to calculate surface excesses. Table 1. Parameters Derived from Drop Volume Tension Measurements on H-di-C6SS at Different Stages of Purification H-di-C6SS no EDTA-no foam fractionation EDTA-no foam fractionation EDTA-foam fractionation
cmc γcmc Acmc (mmol dm-3) (mN m-1) (( 3/Å2) 12.2 12.5 12.7
29.15 29.06 30.09
72 60 56
instruments were calibrated with D2O in the usual way, and a flat background of ∼5 × 10-6 (determined by extrapolation of the data to high Q) was subtracted. This approach is generally considered valid when there is no small-angle scattering from the bulk, which is a reasonable assumption at the low surfactant concentrations involved here. Full accounts of the theory can be found elsewhere (e.g., ref 28); however, under these conditions the R(Q) curve can be modeled in terms of a single, uniform layer using the optical matrix method.29 The R(Q) decays were fitted by a least-squares program27 to yield the layer thickness τ and scattering length density F, and hence the area per molecule or surface excess via eq 2.
Results and Discussion Anionic Surfactants. 1. Sodium Dihexyl Sulfosuccinate di-C6SS. Studies of di-C6SS will be discussed first, because this will establish the procedures for investigating the other anionic compounds. a. Surface Tensions. Dynamic drop-volume tensiometry (DVT) benefits these studies because dynamic effects can be eliminated, since these are generally more difficult to pick up using a du Nouy or Wilhelmy plate method. The γ - ln a plots, before and after various purification treatments, are shown in Figure 2. Polynomials were fitted to the pre-cmc curves to generate local gradients, then the Gibbs equation was used with m ) 2 to give the area per molecule at the cmc. These results are given in Table 1. The cmc was determined to be (12.5 ( 0.3) mmol dm-3, and the three samples gave identical values within experimental error. This result compares well with reported data.16 All the γ - ln a curves show clean breaks at the cmc, with no minima or shoulders. However, both the limiting surface tensions γcmc and the pre-cmc portions of the curves differ significantly, with a radical change in the slope resulting in large discrepancies between the effective areas per molecule Acmc. As discussed below, this depends crucially on the purification treatment. These molecular areas drop from (72 ( 3) Å2 down to (56 ( 3) Å2 for the “raw” and “pure” surfactant, respectively. The difference is well outside the experi(29) Lekner, J. Theory of Reflection; Martinus Nijhoff: Dordrecht, 1987.
Figure 3. Effect of EDTA on surface tension of H-di-C6SS solutions at various concentrations below the cmc.
mental errors and points to the presence of impurities. Williams et al.16 studied dialkyl sodium sulfosuccinates and described purification procedures to obtain a clean break at the cmc. The different recrystallization and foaming processes covered were effective at removing traces of residual alcohol and other hydrophobic impurities. For di-C6SS they determined the limiting area per molecule to be 74 Å2, which matches the value for our so-called nonpurified di-C6SS (Table 1). Hence, there are still purity issues to resolve, and the first of these is the effect of trace polyvalent cationic species Mn+. i. Inorganic Contaminants. Polyvalent cations will adsorb preferentially to the sodium ions, and this can lower the surface tension in the pre-cmc region, an effect that is clearly demonstrated in Figure 2. A different curve is obtained if EDTA is added to the solution. The “best” level of EDTA was determined by measuring γ at a fixed surfactant concentration, but varying the amount of EDTA in the range 10-9 to 10-2 mol dm-3, as shown in Figure 3. These measurements were performed at approximately 1/24, 1/12, and 1/6 × cmc. In all three cases, a plateau of constant γ was reached between a range of EDTA from 5 × 10-5 to 1 × 10-4 mol dm-3. This region is presumably where all divalent species are completely complexed by EDTA, and thus are effectively removed from the interface. Above a concentration of 1 × 10-4 mol dm-3, a decrease in γ is observed, consistent with the onset of a “swamping electrolyte” condition. Sodium salts used in the sulfonation step are an obvious source of ions such as Ba2+, Ca2+, and Mg2+. As noted before,14 the ppm levels present in these reagents would be sufficient to cause the effects seen here. If the surfactant itself is the source of contamination, to be effective over the whole isotherm, a constant surfactantto-EDTA ratio must be used. By inspection of Figure 3, a ratio surfactant:EDTA of 100:1 was chosen, and this typically increases γ by 4 to 5 mN m-1 from the raw surfactant alone. An extensive range of anionic surfactants has been investigated in our laboratory; because of the small variability in purity of inorganic reagents, this level must be determined for each different surfactant, and even for different batches of the same compound. In this study, both the H- and D-di-C6SS required a ratio of 100: 1. Comparison of this with previous values,14,15 and those below for the fluorocarbons, shows that the optimum amount of EDTA must be determined separately for each different surfactant sample. ii. Hydrophobic Impurities. There may be hydrophobic adulterants, which produce a similar, but less pronounced, effect by introducing a slight time dependence of the surface tension. These may be alcohols and also longchain surfactant homologues. The criterion of a highly
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Table 2. Parameters Derived from Tensiometry and Neutron Reflectivity Measurements with D-di-C6SS in the Presence of EDTA D-di-C6SS DVT-no foam fractionation DVT-foam fractionation NR-foam fractionation
cmc γcmc (mmol dm-3) (mN m-1) Acmc (Å2) 12.4 12.9
28.60 30.02
60 ( 3 57 ( 3 59 ( 2
pure surfactant can also be considered in terms of a weak time dependence of γ. Mysels et al.17-19 first recognized the impact of trace hydrophobic impurities and discussed the slow tension decay during relatively long adsorption times. By “long” times we refer to times greater than the diffusion time for adsorption, above which complete adsorption should have occurred, that is, typically more than 30 s for most practical concentrations. This phenomenon can be investigated using the DVT, because temporal changes can be followed up to 15 min by using long-drop formation times. As an example, for di-C6SS at 0.8 × cmc, a long time decay was observed; there was a decrease of 0.4-0.5 mN m-1 between 35 and 950 s. This could be caused by hydrophobic trace impurities, which would adsorb at a much slower rate because they are present at a much lower concentration than the surfactant. Procedures based on foam fractionation17-21 were used as the final purification step to counteract this. Measurements of the dynamic surface tension, γ(t), were used to monitor the purification. After 10 h foaming, the decay had been reduced to approximately 0.2 mN m-1, between 35 and 950 s. After 72 h of foam fractionation the decay had disappeared completely, and a stable γ value was reached in less than 60 s, which remained constant up to 15 min. Both the H- and D-di-C6SS were foam fractionated for 3 days and the γ - ln a curves were reexamined. Tables 1 and 2 show that the γcmc values before purification differ significantly; 29.1 and 28.6 ( 0.1 mN m-1 for the H- and D-surfactants, respectively, which suggests they are contaminated to slightly different levels. However, after the foam fractionation was completed, the two values agreed within 0.1 mN m-1, and both compounds gave essentially identical γ - ln a curves. As shown in Figure 2, the isotherms before and after foam fractionation (both with EDTA) are very similar, but the largest discrepancies are seen for the EDTA-free systems. Figure 4a shows the adsorption isotherms of H-di-C6SS derived from the surface tension measurements at the different stages of the purification. The surface excess was calculated using the Gibbs equation, in terms of activity, and m ) 2. Clearly, addition of EDTA has a marked effect on the adsorption, whereas foam fractionation leads to a much smaller change. The effective areas, Acmc, before the foaming process were determined to be 60 ( 3 Å2 for both the H- and D-di-C6SS, respectively, compared with 56 and 57 ( 3 Å2 after complete purification (Tables 1 and 2). Considering the experimental error on the tensiometric measurements, this latter change is minor compared with the effect of EDTA, which decreased the Acmc by 12 ( 3 Å2. b. Neutron Reflection. Measurements were made in the range cmc/40 to 2 × cmc, in the presence of EDTA. Although the raw data are not shown here, analyses of the reflectivity curves in terms of a single uniform layer indicated a progressive thickening: at cmc/40 it was 11.4 ( 1.5 Å increasing to 17.5 ( 1.5 Å at the cmc. For the same concentrations, the molecular area decreased from 214 ( 8 to 59 ( 2 Å2. At twice the cmc, the area per molecule remained essentially the same as at the cmc, as expected for a pure surfactant; this is a key indication of purity. If
Figure 4. Adsorption isotherms obtained under various conditions of purity for (a) H-di-C6SS and (b) D-di-C6SS. Part c shows a comparison between measured surface tensions and the behavior predicted by integrating the neutron data (• in b above).
surface-active (hydrophobic) impurities were present, they would adsorb strongly below the cmc, but above the cmc, impurities would be dissolved in micelles, thereby altering the surface composition and the apparent adsorbed amount. Figure 4b shows the surface excess for D-di-C6SS determined by NR and tensiometry. Identical sets of foamed-cleaned samples were studied by both techniques and the Γ - c isotherms agree well. Remember that the surface excess can be measured to an accuracy of about 5% with NR.25 So the coverages close to the cmc obtained for the nonfoamed samples by DVT also agree reasonably well with the neutron data from the foam-fractionated system, at least within experimental error. A consequence of this agreement is that a NR isotherm, plotted as Γ ln a, can be integrated to yield the γ - ln a curve via eq 3 (e.g., ref 11),
∫lnlnaa
∆γ ) -mRT
2
1
Γd ln a
(3)
The constant of integration is taken to be the surface tension of pure water. Figure 4c shows the comparison, and the agreement is excellent between 30 and 52 mN
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Figure 5. Comparison of neutronic and tensiometric adsorption isotherms of D-di-C6SS with different values of the Gibbs prefactor m. Table 3. Parameters Derived from Tensiometry and Neutron Reflectivity Measurements with Fluorocarbon Anionics in the Presence of EDTA cmc γcmc tension neutrons (mmol dm-3) (mN m-1) (Acmc ( 3/Å2) (Acmc ( 2/Å2) di-CF4 di-HCF4 di-HCF4GLU
1.57 16.0 11.2
17.7 26.8 25.4
56.0 65.0 66.0
62.7 65.8 65.9
m-1. At higher tensions the deviation is a reflection of the uncertainties in determining Γ at relatively low coverage. The level of agreement indicated in Figure 4c is similar to that found for single-chain zwitterionic betaines.10,11 Figure 5 clearly demonstrates that the NR and tensiometric data are consistent with a prefactor of between 2.0 and 1.95. A more accurate assessment of m is outside the resolution of the two different methods. Nevertheless, to obtain this agreement it is essential that the surfactant is “surface chemically pure”, and extreme care must be taken to achieve this state. Therefore, for the di-C6SS both the anions and cations adsorb similarly, and the assumption of 1:1 ion adsorption, which is implicit in the Gibbs factor of 2, has been confirmed for this compound. In the next section, this result is tested for three fluorocarbon anionics, which are closely related to di-C6SS, and three common cationic surfactants. 2. Fluorinated Sulfosuccinates, di-CF4 and di-HCF4, and the Glutaconate di-HCF4GLU. Comparison between tensiometry and NR. An advantage of using fluorosurfactants for NR experiments is the inherently high neutron scattering length of F, which is similar to that of deuterium. For di-CF4 and di-HCF4, the surface tension behavior and the detailed film structures derived from partial structure factor NR measurements are described elsewhere.15 The Γ - c curves are included here for completeness, and also to draw comparisons with the related compound di-HCF4GLU. Screening of the fluorosurfactants for cationic impurities, as described above, indicated that the appropriate EDTA levels depend on the surfactant. For the di-CF4, di-HCF4, and di-HCF4GLU the surfactant:EDTA levels were 25:1, 60:1, and 40:1, respectively; therefore, the DVT and NR experiments were all performed under these conditions. The cmcs and limiting surface tensions are given in Table 3, as well as the effective molecular areas at the cmc (m ) 2 was assumed). For di-HCF4 and di-HCF4GLU, there is good agreement in the Acmc determined by both methods. However, for di-CF4 the difference is just outside the uncertainties, and this is discussed below. The adsorption isotherms for all three compounds are given in Figure 6, and error bars are indicated for some example points.
Figure 6. Adsorption isotherms of fluorinated surfactants obtained by neutron reflectivity and tensiometry measurements: (a) di-CF4, (b) di-HCF4, and (c) di-HCF4GLU.
For di-HCF4GLU it was possible to perform some reproducibility checks, which were done on freshly made samples, and also to repeat measurements of the same sample. These experiments demonstrate the 5% accuracy of the NR method.6-12,25 For all the fluoro-surfactants, the surface coverages measured above the cmc were essentially identical with coverages measured at the cmc, consistent with high-purity systems. An example of this is shown in Figure 6c for di-HCF4GLU, where the highest concentrations were 2 × cmc. Returning to di-CF4, as shown in Table 3, the Acmc from NR is slightly larger. Given the precautions taken, such a discrepancy is difficult to explain, but a cation contamination problem seems unlikely. Furthermore, there is no evidence for trace hydrophobic impurities: no minimum at the cmc; no long-time decay of γ below the cmc, at least outside of the dynamic region >30 s. However, when the entire tensiometric and neutronic isotherms are
Ionic Surfactants Adsorbed at Air-Water Interface
Figure 7. Values of area per molecule at the cmc Acmc for Cn-TAB cationic surfactants obtained by tensiometry (this work), and neutron reflection from refs 30 and 31.
compared in Figure 6a, this looks like only a minor problem. The discrepancies seen for di-CF4 may well indicate the true resolution of the two methods, that is, a worst case scenario. Nevertheless, for all the fluorosurfactants the surface tension behavior was faithfully reproduced, up to about 60 mN m-1, when the neutron data were treated using eq 3. Taking all four anionic surfactants together, and comparing Figures 5 and 6, indicates the behavior is again consistent with a Gibbs factor of 2. Cationic Surfactants. In terms of impurities it has been suggested above that synthetic intermediates may be the main culprits. To test this idea, commercially available cationic Cn-TAB surfactants, with C12, C14, and C16 chains, have also been studied. The chemical route to these is more straightforward than for the anionics: typically trimethylamine is coupled to a longchain alkyl bromide in an organic solvent. Also, in contrast with anionics there is no need to introduce inorganic reagents in the synthesis, and because unreacted material can be readily separated, surfactant homologues will be the main impurities for cationics. Comparison between Tensiometry and NR. One problem, which hampers the study of cationics by tensiometry, is the possibility of specific adsorption on the oxide layer of du Nouy ring (Pt/Ir) or the Wilhelmy plate (Pt). This can affect the wetting properties at the contact line between solid and solution phases. As discussed elsewhere,25 because of such experimental difficulty, neutron and surface tension measurements may seem to be at odds, especially via eq 3. Despite this, the agreement was excellent, between our tensiometric results (drop volume, ring and plate methods) and published neutron data for C-14 and C-16TAB.25,30 Because of the nature of the available reported data,25,30,31 it is more appropriate to compare values of Acmc, which are shown in Figure 7. Values from all three tensiometric methods agreed within the absolute uncertainties of (3 Å2, and so average values are plotted. Hence, these common cationics are also consistent with a Gibbs factor 2. It is not possible to put a more precise value on m, because this is outside the resolution of both NR and tensiometry. Conclusions Studies of seven different 1:1 ionic surfactants by neutron reflectivity and tensiometry demonstrate the validity the factor of 2 Gibbs equation. For four anionics (30) Lu, J. R.; Hromadova, M.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1994, 98, 11519. (31) Lyttle, D. J.; Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 1001.
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the effect of impurities has been addressed in detail. Most important is the issue of cation contamination, which is an inevitable and unavoidable consequence of the synthetic route. With sodium surfactants the Mn+ ions can be effectively removed from the interface, and the Na+ level replenished, by adding Na4EDTA. The optimum amount of EDTA must be determined experimentally for each batch of surfactant, so that there is sufficient to chelate the unwanted metals, but its presence does not exert any other (noticeable) influence on the surface tension. Hydrophobic impurities seem to present less of a problem for the custom-made surfactants studied here; however, these can be separated with a rather time-consuming foam fractionation process. For the Cn-TABS the molecular areas at the cmc Acmc also indicate an m-factor of 2. As expected, the purification procedures appear to be less stringent than for the anionics, and the only treatment was recrystallization. This conclusion is very different from that of Bae et al.,13 who found m ) 1.28 with 1-dodecyl-4-dimethylaminopyridinium bromide using surface SHG spectroscopy. Based on the theoretical work of Hall et al.,23,24 this value would imply a depletion layer below the surface of unrealistic dimensions, compared with the Debye length in this concentration range. However, neutron studies of the gemini cationics12 indicated that the expected Gibbs factor of 3 was only observed with an xylyl spacer separating the two cationic centers. With aliphatic spacers the value was closer to 2. These results suggest unusually specific chemical effects with cationics, and this certainly deserves more attention. Neutron reflection has been used to investigate counterion binding to the surface film by using isotopically labeled counterions,32-34 for example, the tetramethylammonium ion (TMA+). Studies of insoluble monolayers of C22-sulfate with TMA+ counterions indicated a lower than expected binding.32 Similar conclusions were drawn for TMA+ dodecyl sulfate,33 the neutrons detected only about 80% of the ions. This observation is consistent with a Gibbs factor of less than 2. However, a more likely explanation is that the remaining ions are located in a weak diffuse layer, which is difficult to detect by NR. In this counterion-labeling experiment, problems caused by the inherent film roughness and the relatively weak NR signal may conspire, putting an absolute determination out of reach.32,33 The findings show that adsorption isotherms for 1:1 ionic surfactants can be determined to a sufficient accuracy using surface tension measurements. However, all sources of contamination must be eliminated and dynamic surface tension effects must be taken into account. For anionics the purification methods described here serve as a helpful guide. This is especially important for surfactants with cmcs in the submillimolar range where equilibration times may be of the order of seconds - minutes. These results are of particular importance for understanding dynamic surface tensions (DSTs) of ionic surfactants, because in the diffusion-controlled model DST depends on Γ2, and therefore m-2!35 Hence, the findings are expected to be important for analyzing ongoing DST work using maximum bubble pressure,36 ellipsometry and (32) Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1997, 13, 2133. (33) Su, T. J.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1997, 101, 937. (34) Bell, G. R.; Bain, C. D.; Li, Z. X.; Thomas, R. K.; Duffy, D. C.; Penfold, J. J. Am. Chem. Soc. 1997, 119, 10227. (35) Dukhin, S. S.; Kretzschmar, G.; Miller, R. Dynamics of Adsorption at Liquid Interfaces; Elsevier: Amsterdam, 1995. (36) Eastoe, J.; Dalton, J. S.; Rogueda, Ph. G. A.; Crooks, E. R.; Pitt, A. R.; Simister, E. A. J. Colloid Interface Sci. 1997, 188, 423.
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surface light scattering,37 as well as direct measurements of the dynamic adsorbed amount by neutron reflectometry using an overflowing cylinder.38 (37) Manning-Benson, S.; Bain, C. D.; Darton, R. C.; Sharpe, D.; Eastoe, J.; Reynolds, P. Langmuir 1997, 13, 5808. (38) Manning-Benson, S.; Parker, S. R. W.; Bain, C. D.; Penfold, J. Langmuir 1998, 14, 990.
Eastoe et al.
Acknowledgment. S.N. thanks the University of Bristol for a Ph.D. scholarship. A.P., A.M.D., and A.R. acknowledge the support of EPSRC in terms of studentships. Kodak is thanked for providing a CASE award to A.M.D. We also acknowledge CLRC for allocation of beam time at ISIS and grants toward consumables and travel. LA991564N
