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Langmuir 1999, 15, 4392-4396
Neutron Reflectivity Studies of the Surface Excess of Gemini Surfactants at the Air-Water Interface Z. X. Li, C. C. Dong, and R. K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K. Received November 3, 1998. In Final Form: March 30, 1999 Neutron reflection has been used to determine the surface coverage of a series of cationic gemini surfactants of the general formula [CmH2m+1N(CH3)2-(CH2)s-N(CH3)2CmH2m+1]Br2 designated Cn-Cm-Cn, where n and m denote the number of carbon atoms in the chains of the free alkyl chain and the spacer, respectively. The compounds studied were C12-C3-C12, C12-C4-C12, C12-C6-C12, C12-C12-C12 and C12-xylyl-C12, where the spacer is the xylene group, -CH2-φ-CH2, in the last of these compounds. By the use of partially deuterated forms of the surfactants in null reflecting water, neutron reflection was used to measure directly values of the area per molecule at the critical micelle concentration (cmc) of 66, 82, 95, 140, and 97 ( 3 Å2, respectively. Comparison with the results from surface tension measurements shows that the appropriate prefactor P in the Gibbs equation, dγ ) -PRTΓ d ln c, is approximately 2 for all except the compound with the xylyl spacer, for which P is about 3. A tentative explanation of the unexpected value of 2 is that the dicationic surfactant ions are all in the form of a 1:1 complex with the Br- ions in the bulk solution. Some support for this comes from the behavior of the C12-C6-C12 compound, for which P changes to 3 at concentrations much lower than the cmc, consistent with dissociation of the complex.
Introduction Gemini surfactants essentially consist of two surfactants attached by a spacer group close to the two head groups. They have the technological advantages of lower critical micelle concentrations (cmc), greater surface tension lowering at the air-water interface, and better wetting properties than achievable with monomeric surfactant species.1-12 Although more difficult to prepare than their single-chain counterparts, they have generated much interest, and this is summarized for the cationic gemini surfactants by Zana.4 The most widely studied series of gemini surfactants is the set of dicationic quaternary ammonium compounds
[CmH2m+1N(CH3)2-(CH2)s-N(CH3)2CmH2m+1]Br2 (1) which we designate Cn-Cm-Cn, where n and m denote the number of carbon atoms in the chains of the free alkyl chain and the spacer, respectively. At the air-water interface, the adsorption behavior per molecule is mainly determined by the strong hydrophobicity of the two end alkyl groups, but as the hydrophobic spacer length increases, it starts to exert an important effect on the adsorption, and it is found from surface tension measurements that the area per molecule passes through a maximum at m ) 10-12 for a given alkyl chain length,10 (1) Deinega, Y. F.; Ullberg, Z. R.; Marochko, L. G.; Rudi, V. P.; Denisenko, V. P. Kolloidn. Zh. 1974, 36, 1974. (2) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (3) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (4) Zana, R. Curr. Opin. Colloid Sci. 1996, 1, 566. (5) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (6) Zana, R. Langmuir 1996, 12, 1208. (7) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (8) Hirata, H.; Hatton, N.; Ishida, M.; Okabayashi, H.; Frusaka, M.; Zana, R. J. Phys. Chem. 1995, 99, 17778. (9) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (10) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1996, 12, 1149. (11) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (12) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261.
although the physical reasons for this are not yet understood. However, the areas per molecule obtained by applying the Gibbs isotherm to the surface tension results are unexpectedly large. The correct prefactor in the Gibbs equation, i.e., P in
dγ ) -PRTΓ d ln c
(2)
is 3 for a divalent surfactant counterbalanced by two monovalent counterions. However, because the derived areas per molecule have been unacceptably large, it has been widely assumed that it is more appropriate to use a value of 2 for P. No clear physical reason has been given as to why this should be the case, although Alami et al. suggested that binding of one Br- counterion would have such a consequence.7 Unlike surface tension, neutron reflection measures the surface excess of the surfactant ion directly and without assumptions.13 In all cases where there seems initially to be a conflict between neutron reflection results and those from the Gibbs equation, this conflict has been resolved by more careful assessment of exactly what ions are present in the solution.14-16 In situations that are straightforward to analyze, such as the nonionic series of surfactants C12Em, neutronmeasured surface excesses agree well with those from surface tension and the Gibbs equation. In any case, even if there is an unresolvable disagreement between the two types of measurement, the neutron measurement is a direct measure of the surface excess and is the appropriate quantity to use in any discussion of maxima in the areas occupied by gemini surfactants as a function of length of spacer group. In this paper, we present neutron reflection measurements on the dicationic compounds C12-C3-C12, C12-C4-C12, C12-C6-C12, C12-C12-C12, and C12-xylylC12. In the last of these compounds, the spacer is the xylene (13) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1992, 96, 1373. (14) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 2446. (15) Li, Z. X.; Lu, J. R.; Thomas, R. K. Langmuir 1997, 13, 3681. (16) Hines, J. D.; Thomas, R. K.; Garrett, P. R.; Rennie, G. K.; Penfold, J. J. Phys. Chem. B, in press.
10.1021/la981551u CCC: $18.00 © 1999 American Chemical Society Published on Web 05/21/1999
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Table 1. Parameters of Dicationic Gemini Surfactants at the cmca
dC12-dC3-dC12 dC12-dC4-dC12 dC12-dC6-dC12 dC12-dC12-dC12 dC12-xylyl-dC12 (40 °C) a
cmc mM
γcmc mN m-1
1.1 (0.91) 1.2 (1.17) 1.1 (1.12) 0.28 (0.28) 1.2 (1.0b)
36.8 (35.0) 39.9 (39.8) 42.5 (42.5) 42.8 (41.5) 40.0
An Å2 As (2) Å2 66 82 95 140 97
65 (70) 76 (77) 95 (95) 143 (150) 60
Numbers in brackets are from Alomi et al.10
b
As (3) Å2 98 (105) 114 (116) 143 (143) 224 (226) 90
Done at 50 °C.3
group, -CH2-φ-CH2, where φ represents the benzene ring and was chosen because the xylyl group is a rigid spacer and may cause different kinds of this gemini to behave differently at the surface.3 Experimental Section In neutron reflection, the surface excess of a surfactant is most easily determined by measurement of the reflectivity from a solution of the deuterated (or partially deuterated) surfactant in null reflecting water. This is water of composition 0.088 mol % of D2O in H2O which has a scattering length density the same as that of air. The neutrons do not respond to the boundary between this water and air, and the signal is then entirely from the layer of deuterated surfactant ion at the interface. Such experiments require the synthesis of the deuterated surfactants. The deuterated gemini surfactants were prepared using the method of Menger and Littau.3 In this method, the temperature is kept below about 40 °C, which we believe to be a preferable procedure to methods that use higher temperatures because of the possibility of decomposition of the quaternary ammonium salt. A large excess of dodecyldimethylamine was reacted with the appropriate dibromoalkane in warm acetone until precipitation was complete. Chain-deuterated dodecyldimethylamine was prepared by direct reaction of deuterated bromododecane with dimethylamine in methanol. Bromododecane was prepared by reduction of dodecanoic acid with LiAlD4 to give dodecanol followed by bromination of the dodecanol. Fully deuterated dibromohexane and dibromododecane were prepared in the same way as bromododecane but starting with the appropriate diacid. Dibromobutane was prepared by bromination of deuterated tetrahydrofuran. Deuterated dibromopropane was obtained from Merck, Sharp, and Dohme. The only compound not prepared with a deuterated spacer was the one with the xylene spacer. All compounds were purified by not less than three recrystallizations from a mixture of ethanol and ethyl acetate, the ratio of the two varying according to the compound. The fully protonated surfactants were prepared in exactly the same way. The purity of the compounds was assessed from surface tension measurements and comparison with known values of the critical micelle concentrations,6 and the comparisons are given in Table 1. No minima were observed, and the good agreement between the cmc’s of the deuterated and protonated species shows both that any isotope effects are relatively small and that the purity of these compounds was high. The only discrepancy between the data for our materials and those previously published is for the C12-C3-C12 compound, for which our cmc and limiting surface tension are both slightly higher than those in Zana.6 The surface tension measurements were performed on a Kruss K10 tensiometer using a Pt/Ir ring. Reflectivity measurements were made on the SURF neutron reflectometer at the Rutherford Appleton Laboratory, Didcot, U.K.17 using procedures described previously.18 The measurements were made at 298 K except for C12-xylyl-C12, which was measured at 313 K. (17) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. W. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E. J.; Roser, S. J.; McLure, I. A.; Hillman, R. A.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc. Faraday Trans. 1997, 93, 3899. (18) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381.
Determination of Coverage We have given a detailed comparison of the determination of surface coverage by neutron reflection and by surface tension measurements earlier,14,15,19 and here we only give an abbreviated description of what is measured by the two methods. In the neutron reflection experiment, an adsorbed monolayer of deuterated surfactant forms a layer of scattering length density different from that of air or the underlying null reflecting water (NRW). The scattering length densities of the air and the NRW are exactly the same, and apart from a low level incoherent background, the signal is therefore entirely from the adsorbed layer of surfactant. The scattering length density F is given by
F ) bn
(3)
where b is the total scattering length of the surfactant molecule and n is its number density. Because b is known from the isotopic composition of the surfactant, a determination of F of the surfactant layer leads directly to its number density and hence the number of moles per unit area in the layer Γm. The scattering length density of the surfactant layer is most conveniently determined by fitting the model of a uniform monolayer to the reflectivity using standard formulae for the reflected intensity.20 An approximate formula for the reflectivity for the uniform layer is 2 κ2R 2 sin (κτ/2) = (Γ N b) m a 16π2 (κτ/2)2
(4)
where R is the reflectivity, Na is Avogadro’s number, τ is the thickness of the layer, and κ is the momentum transfer defined by
κ)
4π sin θ λ
(5)
where θ is the incident glancing angle and λ is the wavelength of the neutrons. Although other models of the surfactant layer might be more realistic, we have shown that the derived coverage is not sensitive to the choice of model. Different models give rise to different thickness parameters and different scattering length density distributions, but these two quantities compensate each other so that the resultant value for the coverage is always the same. The surfactant ion excess may consist of two parts, surfactant adsorbed in a monolayer, Γm, and a negative excess in the region below the monolayer, Γd, so that the overall excess of the surfactant ion is21,22
Γ ) Γ m + Γd
(6)
The effect on the reflectivity of any surfactant ion desorbed from the submonolayer region of the interface is negligible, and therefore, the coverage measured in a neutron experiment is Γm. Because it is essentially Γ that is determined by the application of the Gibbs equation to surface tension data, the two experiments may give different results if Γd is significant. In the absence of any added electrolyte, Hall et al. have shown that the Gibbs (19) Hines, J. D.; Garrett, P. R.; Rennie, A. R.; Thomas, R. K.; Penfold, J. J. Phys. Chem. B 1997, 101, 7121. (20) Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1970. (21) Hall, D.G. Colloid Surf. A 1994, 90, 285. (22) Hall, D. G.; Pethica, B. A.; Shinoda, K. Bull. Chem. Soc. Jpn. 1975, 48, 324.
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Li et al.
Figure 1. Variation of surface tension with concentration for C12-C6-C12.
equation for a divalent ionic surfactant with monovalent counterions becomes
-1 dγ R R ) Γm 3 1 - + 1 - c1β RT d ln c1 2 2
[(
) (
) ]
(7)
where R is the degree of dissociation and β accounts for the variation of activity coefficient with concentration and can be put to unity at the low concentrations being used for the present systems. The left-hand side of the equation can be equated to the Gibbs surface excess, ΓG. Thus
[
Γ G ) Γm 3 -
3R R + 1 - c1β = Γm(3 - R) 2 2
(
) ]
(8)
where the approximate expression, holding when the activity coefficients are unity, identifies the prefactor in the Gibbs equation with 3 - R. Although this equation might suggest that any discrepancy between the Gibbs surface excess ΓG and the neutron coverage Γm can be resolved by a nonzero value for R, we have shown elsewhere that, at the low concentrations characteristic of surfactant solutions, it is physically unrealistic to envisage a situation where R is significantly different from 0.14 Thus, surface tension and neutron reflection should always give the same coverage unless the assumptions that lead to the construction of a particular Gibbs isotherm are incorrect. This means either that other ions, usually impurities, are playing a role or that the state of the surfactant and ions in the bulk phase is not as has been assumed. Results and Discussion The surface tension as a function of concentration is shown for C12-C6-C12 at 298 K in Figure 1. A fit of the curve near the cmc to a quadratic equation and subsequent application of the Gibbs equation (eq 8 with R ) 0) gives the result that the limiting area per molecule at the cmc would be 143 Å2/molecule (using the correct Gibbs prefactor of 3), in excellent agreement with the results of Zana et al.6 The variation of this area with concentration will be discussed after we have presented the neutron results. However, we note that it becomes very difficult to measure the surface tension below about cmc/10. Neutron reflection profiles at different bulk concentrations are shown for four of the gemini compounds in Figures 2 and 3. The incoherent background has been subtracted in these plots and would have a level equivalent to a reflectivity of about 6 × 10-6, which is 2 or 3 orders of magnitude below the true reflected signal at the lowest κ values measured. The continuous lines in these figures represent the best fit of a uniform layer model to the data
and yield a thickness and scattering length density of the layer, from which the coverage can be extracted using eq 4. The surface excesses obtained from neutron reflection, Γm, at the respective cmc’s are plotted as a function of number of carbons in the spacer group in Figure 4. Also plotted are the values derived from our own surface tension data using a Gibbs isotherm prefactor of either 2 or 3. For all of the simple alkyl spacers, the prefactor 2 agrees well with the direct result from neutron reflection, but for the xylyl group, a better value of the prefactor is 3. In Figure 4, we have assigned the xylyl group a length of 6.5 carbons because its geometrical length is equivalent to about (CH2)6.5. We emphasize that, whatever the reasons for the discrepancy between neutron and surface tension data, the neutron measurement is the direct measurement and should be used in any discussion of how the area per molecule varies with size of the spacer group. The neutron reflection results for the gemini compounds with alkyl spacers show that the Gibbs prefactor at the cmc is about 2. This is the value appropriate to a univalent surfactant ion and a univalent counterion. Thus, it would be explained if the divalent surfactant cation was entirely in the form of a 1:1 complex with a bromide ion in the bulk solution because the situation would then become the same as for a monovalent surfactant ion with a monovalent negative counterion. Note that the concentration terms in the Gibbs equation refer to the species in the bulk; they have nothing to do with any binding at the interface itself. It seems probable that an anion might be tightly bound by the bidentate surfactant ion and it may be a key feature that the dication is sufficiently flexible to bring both charged groups into play. The dication would then be acting like a chelating agent. The stability of such a complex would diminish with decreasing concentration, by the law of mass action. Thus, provided that a lowenough concentration could be reached to ensure that the complex ion dissociated, the prefactor in the Gibbs isotherm would be expected to change toward the value of 3 as the concentration is lowered. This happens for the C12-C6-C12 compound, as shown in Figure 5, where the neutron surface coverage is compared with that from the surface tension using either 2 or 3 as the prefactor. The system changes from 2 at high concentration to 3 at low concentration. This is consistent with the formation of a moderately stable 1:1 complex between a gemini cation and one Br- ion in the bulk which starts to dissociate as the concentration drops below the cmc. On the basis of the unreasonably large limiting areas per molecule derived from the Gibbs isotherm with P equal to 3, Alami et al. assumed that P was 2 and made the suggestion that the binding of a Br- counterion was responsible.7 Our results give the actual surface areas occupied by a selection of gemini molecules at the cmc and confirm that, in practice, similar results can be obtained in many cases by taking P equal to 2. However, the conclusion about the formation of a complex between gemini dication and a Br- ion must be regarded as tentative. Indirect support comes from the fact that the compound with the xylyl spacer has a Gibbs prefactor of 3, which indicates that it does not form any complex. This can be rationalized in two ways. It is reasonable to suppose that for chelation to be effective requires both charged groups on the dication to participate, and the steric effect of the rigid xylyl group may make this impossible. Alternatively, delocalization of the charges onto the xylyl group might weaken the local field sufficiently that the complex is not formed. Either effect might prevent formation of the complex. The main argument against
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Langmuir, Vol. 15, No. 13, 1999 4395
Figure 2. Neutron reflectivity at different concentrations of (a) dC12-dC6-dC12, (b) dC12-xylyl-dC12. The concentrations are (b) cmc, (O) cmc/3, (+) cmc/10, (9) cmc/30, and (×) cmc/50.
Figure 3. Neutron reflectivity at different concentrations of (a) dC12-dC3-dC12 and (b) dC12-dC12-dC12. The concentrations are (b) cmc, (O) cmc/3, (+) cmc/10, (9) cmc/30, and (×) cmc/100.
Figure 4. Variation of area per molecule at the cmc (O) neutron reflection and surface tension using Gibbs prefactors of (+) 2 and (b) 3. The results for the xylyl spacer are boxed, and the size of the spacer has been set equal to 6.5 carbons.
the interpretation in terms of a complex is the fact that it is difficult to measure with sufficient reliability surface tension values as the coverage decreases. There are relatively few measurements of the surface tension of cationic surfactants at concentrations below the cmc in the literature, and although no reasons have been set down, except possibly in Simister et al.,13 this is generally held to be because such measurements are unreliable. In keeping with this, our surface tension measurements at low concentrations on the other three gemini surfactants,
Figure 5. Comparison of the surface coverage of dC12-dC6dC12 determined by neutron reflection (O) and surface tension with Gibbs prefactors of 2 (+) and 3 (b).
C12-C3-C12, C12-C4-C12, and C12-C12-C12 do not give results that are consistent with either the Gibbs prefactor remaining at 2 throughout the concentration range or the systematic change shown in Figure 5. The surface tension results for these compounds give coverages that fall off faster than those observed by neutron reflection. In anionic compounds, discrepancies between neutron reflection and surface tension have all been rationalized in terms of impurity divalent ions such as Ca2+ and Mg2+. It is possible that the discrepancies in the cationic geminis also have their origin in impurity ions. The difficulty is
4396 Langmuir, Vol. 15, No. 13, 1999
that there are no impurity ions that would obviously have such an effect. Two possible candidates are OHand HCO3 . These are present in very low concentration, but if they interacted strongly with a gemini cation, they could affect its surface tension behavior. However, the effect would have to be extraordinarily strong to explain the observed behavior, and there is no obvious reason for such a strong ion selective interaction. In summary, neutron reflection shows exactly how the actual area per molecule of this series of gemini surfactants at their cmc’s varies with length of spacer. They show a steady increase up to and including the compound with the C12 spacer and are therefore consistent with conclusions from earlier surface tension measurements, except
Li et al.
that they are based on much firmer evidence. The one compound with a rigid spacer, the xylyl compound, has an area per molecule similar to that for an alkyl spacer of the same length, i.e. a saturated straight 6.5 carbon chain. This conclusion could not have been made from the surface tension results because of the different prefactor in the Gibbs isotherm. It is possible that the compounds with alkyl chain spacers interact sufficiently strongly with Br- to form complexes, but further evidence would be needed to confirm this. Acknowledgment. We thank the EPSRC for support for this work. LA981551U
