Langmuir 1997, 13, 3681-3685
3681
Neutron Reflectivity Studies of the Adsorption of Aerosol-OT at the Air/Water Interface: The Surface Excess Z. X. Li, J. R. Lu, and R. K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K. Received August 26, 1996. In Final Form: April 29, 1997X Neutron reflection and surface tension measurements have been used to show that the surface properties of sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT or AOT) at concentrations below the critical micelle concentration (cmc) are dominated by divalent ion impurities. Previous determinations of the area per molecule at the cmc using surface tension measurements and the Gibbs equation have given values of about 100 Å2 or higher. Neutron reflection gives a lower value of 78 ( 3 Å2. A similar value can be obtained from surface tension measurements only in the presence of species which remove divalent ions from the solution. This suggests that the lower value is much closer to the correct value and that surface tension measurements on AOT will usually give misleading values of the coverage.
Introduction The anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol-OT or AOT) has been very widely used in both research and technological applications; see, for example, ref 1 . In common with most other surfactants, little is known about its structure at interfaces because of a general lack of experimental techniques capable of probing wet interfaces. We have started using the relatively new technique of neutron reflection to study the structural characteristics of AOT layers at the liquid/ air2 and solid/liquid3 interfaces. In particular we wish to identify whether there are any special structural characteristics of AOT layers that can be identified, such as, for example, the roughness of the layer, the degree of immersion of the surfactant in the water, and the conformation of the two branched chains, and we have already shown for other surfactants that all of these features are accessible to neutron reflection (see ref 4). During the course of work on AOT we have found large discrepancies between the surface excesses determined by neutrons and surface tension, although we have used material of at least as high purity as used by other researchers. The discrepancy is most easily described in terms of the value of the prefactor necessary to reconcile the coverages determined from application of the Gibbs equation to surface tension data and from neutron reflection. For an ionic surfactant the correct factor is 2, but for AOT and other anionic surfactants the prefactor usually has to take a value somewhat less than 2 for the two sets of measurements to agree. Analysis of such a discrepancy for the anionic surfactants formed from perfluorooctanoic acid showed that it was entirely a result of trace contamination by divalent ion impurities.5 The anomaly is not easily revealed by surface tension measurements alone (although see ref 6 for early work on this subject) but most clearly manifests itself when the surface coverage is measured independently as, for example, by radiotracer measurements or by neutron reflection. That
there may be problems in the determination of the surface coverage of AOT at its critical micelle concentration (cmc) was indicated in the first radiotracer measurement of surface excess, where the radiotracer excess was found to be significantly larger than that from surface tension.7 This was interpreted in terms of hydrogen ions replacing sodium ions in the layer. The problem with this explanation is that the acid is a strong one and it is unlikely that sodium ions would be displaced by hydrogen ions. Although several surface tension measurements have been done on AOT, hardly any authors analyze their data to give a surface excess; see, for example, ref 8. Even in the absence of direct structural information, conclusions are often drawn about the structure of monolayers from the coverage, for example, how loosely packed the layer is and therefore how disordered the chains are. It is clearly important at the start of any study of AOT to have accurate values of its cross sectional area at an interface. Since this has proved far from trivial to establish, we devote this paper to the problem of the coverage determination of AOT at the air/water interface. Determination of Coverage: Theory We have given a detailed comparison of the determination of surface coverage by neutron reflection and by surface tension measurements in ref 5, 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 surfactant forms a layer of different refractive index from that of the underlying solution and the neutron reflectivity is sensitive to the presence of the layer. The neutron reflective index is close to unity, and it is convenient to discuss reflection in terms of the scattering length density, related to the refractive index by
η2 ) 1 -
Abstract published in Advance ACS Abstracts, June 15, 1997.
(1) Porter, M. R. Handbook of Surfactants, 2nd ed.; Blackie: London, 1994. (2) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. Prog. Colloid Polym. Sci. 1995, 98, 243. (3) Fragneto, G.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1996, 178, 531. (4) Lu, J. R.; Smallwood, J. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1995, 99, 8233. (5) An, S. W.; Lu, J. R.; Thomas, R. K.; Penfold, J. Langmuir 1996, 12, 2446. (6) Pethica, B. A. Trans. Faraday Soc. 1954, 50, 413.
S0743-7463(96)00847-5 CCC: $14.00
(1)
where η is the refractive index, λ is the wavelength, and F is the scattering length density given by
F(z) ) X
λ2 F π
∑b n (z) j j
(2)
j
where bj is the scattering length and nj is the number density of atomic species j. For surfactant solutions the difference between the scattering of the isotopes of hydrogen, for which bH ) -3.74 × 10-5 Å and bD ) 6.67 × 10-5 Å, is important for aqueous (7) Salley, D. J.; Weith, A. J., Jr.; Argyle, A. A.; Dixon, J. K. Proc. R. Soc. (London) 1950, A203, 42. (8) Williams, E. F.; Woodberry, N. T.; Dixon, J. K. J. Colloid Interface Sci. 1957, 12, 452.
© 1997 American Chemical Society
3682 Langmuir, Vol. 13, No. 14, 1997
Li et al.
surfactant solutions because it is possible to choose the isotopic composition of water so that its scattering length density is the same as that of air, i.e. zero. There is no reflection at all from the air/water interface for water of this composition (0.088 mole fraction of D2O), which we refer to as null-reflecting water (NRW). For a dilute solution of deuterated surfactant in NRW reflection is only from the surfactant monolayer formed at the surface; i.e., the technique is surface specific. A typical signal from a deuterated surfactant monolayer at its cmc is 1000 times the residual background signal, and there are no ambiguities in the calibration of the signal or in the determination of the background level. The surface coverage is most conveniently determined by fitting the model of a uniform monolayer to the reflectivity using standard formulas for the reflected intensity.9 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
(3)
where b is the total scattering length of the surfactant molecule, Γm is the surface coverage, Na is Avogadro’s number, τ is the thickness of the layer, and κ is the momentum transfer defined by
4π sin θ κ) λ
(4)
where θ is the incident glancing angle and λ is the wavelength of the neutrons. Another model that is often used for the layer is the Gaussian distribution
( )
F ) F0 exp -
4z2 σ2
(5)
where σ is the full width at 1/e of the maximum and the reflectivity for this model is
κ2R = (ΓmNab)2 exp(-κ2σ2) 16π2
(6)
Although the thickness of the layer is clearly sensitive to the choice of model for the distribution of surfactant, the derived coverage is not at all model dependent. This is because as the product of momentum transfer and thickness tends to zero, the reduced reflectivity (the quantity on the left-hand sides of eqs 3 and 5) tends to (ΓmNab)2, which depends only on the coverage. Since this region of the reflectivity profile is where the signal is strongest, the coverage is determined very accurately and independently of any assumptions about the model. This point is discussed in more detail in ref 11. There are two possible sources of difficulty in determining the coverage from neutron reflection. The first is that if the layer breaks up into islands and the dimensions of these are comparable with the coherence length for the experiment, then the reflectivity will have a different dependence on the coverage from that of a uniform layer.10 The number density of surfactant n is the average number density over the whole surface, and the reflectivity is proportional to n2. If the adsorbed layer breaks up into islands which cover only a fraction θ of the surface, the experiment will observe the average reflectivity, which will be proportional to θ(n/θ)2. In interpreting the reflection from islands in terms of a uniform layer, the coverage would be overestimated by a factor (1/xθ). The changeover between the two reflection regimes should occur when the dimensions of the islands are of the order of microns, depending on the resolution of the experiment, and it should be identifiable because it should be associated with enhanced off-specular scattering. This is not expected to be a problem for surfactant monolayers. The second is that damping effects, associated with uncorrelated roughness of the upper and lower surfaces of the surfactant layer, might (9) Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1970. (10) Roser, S. J.; Richardson, R. M. Langmuir 1991, 7, 1458. (11) Lu, J. R.; Lee, E. M.; Thomas, R. K. Acta Crystallogr. 1996, A52, 11.
reduce the signal below what is expected,12 but this also is unlikely to be a problem for the surfactant ion itself, although it might give rise to an inaccurate determination of the surface coverage of a counterion. In practice, the counterion usually has such a relatively small scattering length that its contribution is, in any case, negligible. 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 is13,14
(7)
Γ ) Γm + Γd
The effect on the reflectivity of any surfactant ion desorbed from the sub-monolayer region of the interface is negligible, and therefore the coverage measured in a neutron experiment is Γm. Since it is 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. The Gibbs equation for an ionic surfactant in the presence of coion is13,14
-dγ ) Γ1 dµ1 + Γ2 dµ2 + Γ3 dµ3
(8)
where the subscripts 1, 2, and 3 refer to surfactant, counterions, and co-ions, respectively, γ is the surface tension, Γi are the surface excesses defined with respect to Γsolvent ) 0, and the µi are the chemical potentials. When the electroneutrality conditions are included in eq 7, we obtain
-dγ ) Γ1(d ln c1f1 + d ln c2f2) + Γ3(d ln c3f3 + d ln c2f2) (9) RT where fi are the mean activity coefficients of the electrically neutral surfactant and supporting electrolyte. Further manipulation gives the equation most typically used,
() [
-1 ∂γ RT ∂c1
)
( ) ( )]
Γ1 Γ1 + Γ3 ∂ ln f1 + + Γ1 c1 c1 + c3 ∂c1
c3
+ Γ3
∂ ln f3 ∂c1
c3
c3
(10)
A further equation that is useful when discussing the effects of added electrolyte is
() [
-1 ∂γ RT ∂c2
)
( ) ( )]
Γ3 Γ1 + Γ3 ∂ ln f1 + + Γ1 c3 c1 + c3 ∂c3
c1
+ Γ3
c1
∂ ln f3 ∂c3
c1
(11)
Because the monolayer repels co-ions, we can expect Γ3 to be negative. There will be a negative contribution, Γd, to Γ1 because surfactant ions will be repelled from the region just below the monolayer because of the charge on the surfactant layer. Hall has defined a surface degree of dissociation, R, which is
Γd + Γ3 Γm
R ) -2
(12)
where Γm is as defined in eq 6. If the activities of surfactant anion and any added anion are taken to be the same and we also assume that
Γd Γ3 ) c1 c3
(13)
we obtain the following equation (12) Pershan, P. S. J. Phys.: Condens. Matter 1994, 23A, 37. (13) Hall, D. G. Colloids Surf., A 1994, 90, 285. (14) Hall, D. G.; Pethica, B. A.; Shinoda, K. Bull. Chem. Soc. Jpn. 1975, 48, 324.
Adsorption of Aerosol-OT at the Air/Water Interface
[
( )
( )(
Langmuir, Vol. 13, No. 14, 1997 3683
)]
Rc1 c1 R -1 ∂γ c3 ) Γm 1 + c1β + 1RT ∂ ln c1 2 c1 + c3 2(c1 + c3) (14) where
β)
( ) ∂ ln fi ∂cj
(15)
ci
When there are no co-ions, only eq 13 applies and it reduces to
R -1 dγ ) Γm 2 - R + 1 - c1β RT d ln c1 2
[
(
) ]
(16)
Figure 1. Surface tension of a purified sample of commercial NaAOT, the same in the presence of EDTA, the same in the presence of cationic exchange resin (Na+ form), and Ca(AOT)2.
The right-hand side is the Gibbs surface excess, Γ. Thus
[
(
Γ ) Γm 2 - R + 1 -
) ]
R c β = Γm(2 - R) 2 1
(17)
where the approximate expression, holding when the activity coeffients are unity, identifies the prefactor in the Gibbs equation with (2 - R). At the low concentrations characteristic of surfactant solutions it is difficult to envisage a situation where R is significantly different from zero5 and surface tension and neutron reflection should then give the same coverage.
Experimental Details Protonated AOT ((C8H17OOC)2C2H3SO3Na (abbreviated to h-AOT)) was obtained from Sigma and purified by liquid/liquid extraction as described below. The deuteriated surfactant (C8D17OOC)2C2H3SO3Na (abbreviated to d-AOT) was synthesized as follows. Ethylhexanoic acid was synthesized by successive reactions of butyl and ethyl bromide with diethyl malonate using standard procedures.15 After decarboxylation of the product LiAlD4 was used to reduce the acid to ethylhexanol. After purification on a silica gel column using an ether/hexane solvent, the ethylhexanol was reacted with fumaric acid to give the diester of the acid, from which residual ethylhexanol and monoester were removed on a silica gel column using ether/hexane solvent. The diester of fumaric acid was reacted with an aqueous solution of sodium bisulfite to give the sulfosuccinate AOT.16 The residual salt was removed by successive extraction, evaporation, and filtration using methanol. The purification of AOT has been assessed by Williams et al. We have followed their procedure but with one useful modification. The product obtained after extraction with methanol is dissolved in a methanol/water mixture and a liquid/liquid extraction with hexane performed for 24 h, to remove any unreacted diester, and monoester or ethylhexanol produced by hydrolysis in the addition step. The modification, which we found improved the extraction, was to hold the AOT solution at 0 °C during the liquid/liquid extraction, which reduces the rate of hydrolysis of the ester groups in the AOT. Initially the purity was assessed from the surface tension as a function of concentration. No minimum was observed and the cmc was found to be 2.5 × 10-3 M, in good agreement with other measurements.17 No differences were observed between the two AOT isotopes when H2O was the solvent. The surface tension measurements were performed on a Kruss K10 tensiometer using a Pt/Ir ring. Reflectivity measurements were made on the CRISP neutron reflectometer at the Rutherford Appleton Laboratory, Didcot, U.K., using procedures described previously,18 and all measurements were made at 298 K. (15) Furniss, B.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Practical Organic Chemistry, 5th ed.; Longman: Harlow, U.K., 1989. (16) Jaeger, A. O. U.S. Patent 2028091, 1936. (17) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NSRDS-NBS 36, (U.S. Feb. 1971); National Bureau of Standards: Washington, DC. (18) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381.
Figure 2. Surface coverages derived from the Gibbs equation (eq 10) using R ) 0 for the three samples of NaAOT whose surface tensions are shown in Figure 1, and those derived from the neutron reflectivity profiles shown in Figure 3.
Results The surface tension as a function of concentration is shown for a purified sample of Sigma AOT in Figure 1. The cmc is at the generally accepted concentration, and a fit of the curve with a quadratic and subsequent application of the Gibbs equation (eq 16 with R ) 0) gives the result that the limiting area per molecule at the cmc is 100 Å2 per molecule, in reasonable agreement with comparable published data, for example in ref 8. The variation of this area with concentration is shown in Figure 2. However, a problem that is immediately apparent from the surface tension curve shown in Figure 1 is that, for this sample, although the coverage is rapidly tending to zero (see Figure 2), the surface tension is tending toward a limiting value that is considerably less than the expected value of 72 mN m-1. Neutron reflection profiles of the deuterated AOT in null-reflecting water (NRW) at various concentrations are shown in Figure 3. The method of analysis of such profiles has been described above and should give the coverage of AOT in the monolayer (Γm) with an accuracy of better than 5%.19 The surface area as a function of concentration is compared in Figure 2 with that determined via the Gibbs equation with R ) 0. It is immediately clear that the difference in the results from the two techniques is large even at the cmc, where the area per molecule from the neutron results is 78 Å2. The neutron result is more in line with the value of 66 Å2 of Fontell et al.20 obtained from an X-ray diffraction study on the lamellar phase of AOT, where there can be no ambiguity of interpretation. The neutron and Gibbs surface excesses can be made to agree by including a significant value for Γd in eq 6 (19) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1992, 96, 1373. (20) Fontell, K. J. Colloid Interface Sci. 1973, 44, 318.
3684 Langmuir, Vol. 13, No. 14, 1997
Li et al.
Figure 5. Surface coverages for Ca(AOT)2 determined using surface tension and the Gibbs equation and neutron reflection. Figure 3. Neutron reflectivity profiles of deuteriated NaAOT at various concentrations in null-reflecting water. The concentrations used were cmc (2.5 × 10-3 M), cmc/30, cmc/150, and cmc/300 with the reflected intensity diminishing with concentration. The continuous lines are fitted to the model of a uniform monolayer of thickness 18 Å and areas per molecule 78, 90, 118, and 137 Å2 per AOT unit.
Figure 4. Effects of different electrolytes on the surface tension of a 10-4 M solution of NaAOT. The concentration is the concentration of added electrolyte. The dashed line is the surface tension/log c plot of NaAOT on its own, and the vertical dashed line marks the starting concentration of NaAOT. The concentration c used on the horizontal axis is the concentration of added electrolyte.
above or a nonzero value for R. Within the framework of the surface tension measurements the test of whether the prefactor in the Gibbs equation should be less than 2 is to study the variation of the surface tension with added electrolyte. However, the interpretation of such data may not be as definite as has sometimes been suggested, and the problem has been discussed clearly by Hall et al.13,14 We have done several such measurements, and the effects of different electrolytes on the surface tension of a 10-4 M solution of NaAOT are shown in Figure 4. Equation 10 can be modified to account for the addition of 1:1 electrolyte to give
(
)
-1 ∂γ c = Γ1(1 - R) RT ∂ ln c2 1
(18)
where we have omitted the activity coefficient terms. If R is zero, as it should be,5 then the slope gives the coverage of the counterion. If the surface of NaAOT at 10-4 M consisted of pure AOT, then eq 17, over the range of added NaCl from about 10-6 to 10-3, would have a slope of Γ1, which should be approximately half of the slope of the dashed line for the surfactant on its own. Although Figure 4 is not the correct plot to demonstrate this because c signifies added electrolyte rather than c2, it is easy to see by inspection that the addition of up to 10-3 M NaCl to
the solution has almost no effect on the surface tension whereas the curve for the surfactant on its own is varying sharply at 10-4 M. This indicates that the amount of Na+ in the layer is substantially less than that of AOT ion. When the added electrolyte has no ion in common with the surfactant, as for HCl, there should be no change in surface tension unless the cation is present in the surfactant layer. Figure 4 shows that there is no H+ in the layer above a pH of about 5. This is an important conclusion because earlier discrepancies between radiotracer and surface tension measurements on di-n-octylsulfosuccinate were attributed to undissociated acid. This is clearly not the case as long as the pH does not drop below 5. The form of eq 17 changes for the divalent electrolyte, but the conclusions are sufficiently straightforward that it is unnecessary to give a quantitative argument. Thus the addition of MgCl2 has no effect on the surface tension initially but from about 3 × 10-6 M upward the surface tension drops linearly. This indicates that there is no Mg2+ in the NaAOT layer until [Mg2+] reaches 3 × 10-6 M but that above this concentration the surface consists of pure Mg(AOT)2. Likewise, addition of CaCl2 converts the NaAOT layer into a pure Ca(AOT)2 layer even when its concentration is only 10-6 M. The suspicion that divalent ions play an important role in determining the surface properties of AOT came in part from an attempt to purify AOT on a silica column, when the final product gave a surface tension curve quite different from that of NaAOT with both a lower cmc and a lower limiting surface tension, in part from an earlier observation of the presence of magnesium in commercial AOT,21 in part from the effects of added electrolyte described above and from results on other anionic surfactants.5,22 We have used two further procedures to demonstrate that divalent ions make an important contribution: to remove divalent ions from the NaAOT solutions, on the one hand, and to make measurements on Ca(AOT)2, as a representative M(AOT)2, on the other. The surface tension curve for pure Ca(AOT)2 is shown in Figure 1. The cmc is approximately an order of magnitude lower than that for NaAOT, and the surface tension in the region just below the cmc is about 20 mN m-1 lower. It is noteworthy that the surface tension of the supposedly pure NaAOT approaches the Ca(AOT) curve quite closely at very low concentrations, and it might even be concluded that the two curves become identical at low concentrations. The surface coverage using the Gibbs equation for Ca(AOT)2 is compared with that measured by neutrons in Figure 5, and they are in good agreement, indicating that there are no unexpected artifacts in either measurement on Ca(AOT)2. The surface (21) Parrott, D. Ph.D. Thesis, University of Hull, 1989. (22) Cross, A. W.; Jayson, J. J. J. Colloid Interface Sci. 1994, 162, 45.
Adsorption of Aerosol-OT at the Air/Water Interface
coverage per AOT fragment is also slightly lower than that for NaAOT, the limiting area being 68 Å2 in comparison with 78 Å2 for NaAOT. The difference between the two forms of AOT is less than that between the neutron and surface tension measurements of the surface excess of NaAOT. There are two possible ways of removing divalent ions, by the addition of EDTA, which is a powerful sequestering agent for Ca2+, or by using an ion exchange resin. Surface tension plots are shown in Figure 1 with and without added EDTA and with ion exchange resin present in the solution. As we have discussed elsewhere, the addition of EDTA will cause a small increase in the ionic strength, which, if anything, will lower the surface tension. However, its effect is to increase the surface tension considerably at all concentrations except at the cmc. The effect on the slope of the surface tension curve is even more dramatic, and the resulting calculation of the area per molecule is now seen (Figure 2) to be in good agreement with the neutron surface coverage in the region of the cmc. This is even more so for the surface tension measurement in the presence of ion exchange resin, except that there is now an uncertainty about the bulk concentration of AOT, since the ion exchange resin may absorb some AOT. Hence the concentration of AOT may be less than the apparent value. Discussion We have discussed in full how ion contamination of anionic surfactants is most clearly manifested by comparing surface coverages determined by surface tension and the Gibbs equation on the one hand with those determined by a direct method such as neutron reflection or radiotracers on the other hand.5 Our earlier reflection results on the perfluorooctanoates were done over a wider range of conditions than we have used here but it is clear that AOT follows a similar pattern to that for the fluorooctanoates, the only difference being that the problem is substantially more serious for AOT. Cross and Jayson22 have shown that a similar problem exists for SDS by using radiotracers to monitor the calcium ion impurity directly. We emphasize that it is only possible to identify such impurities by surface tension measurements alone if the effect of suspected impurities on the surface tension is systematically tested, and it may still be difficult to obtain quantitative information. We cannot quantify the level of impurity in NaAOT. The surface tension behavior on addition of CaCl2 suggests that the NaAOT layer at 10-4 M becomes a pure Ca(AOT)2 layer when the Ca2+/Na+ ratio is about 0.01. However, the monitoring of the surface tension depression as a function of added electrolyte becomes an inaccurate procedure at these low concentrations, and it is possible that there are still significant surface effects at much lower concentrations. The source of the impurity is also not easily established. AOT ion is clearly an efficient scavenger of divalent ions, and it may be impossible to purify NaAOT to a level where the true surface tension/
Langmuir, Vol. 13, No. 14, 1997 3685
concentration variation can ever be obtained. It is also likely that, even were it prepared with the required purity, it binds these ions so strongly that it will scavenge them from any glassware used in the course of the measurement. Although the surface behavior of dilute NaAOT solutions is dominated by ionic impurities, this may be peculiar to dilute solutions. The many experiments that have been done using AOT as surfactant have mainly been done at concentrations above the cmc. However, even at the cmc the slope of the plot in Figure 4 where Na+ is in considerable excess indicates that the monolayer is not a pure NaAOT layer until an Na+ concentration of 10-2 M, and it may be safer to operate at concentrations greater than about 3(cmc), where it is likely that the less soluble divalent compounds will be solubilized. The situation is further complicated by the easy hydrolysis of AOT in water. This leads to the mono-AOT and free ethylhexanol. Neither of these are expected to affect the surface properties at low concentrations, although it has been suggested that they do affect the phase diagram at higher concentrations.23 We are confident that these two impurities were completely eliminated from our samples, although we found that AOT solutions do deteriorate with time. The discrepancy between the coverages determined from the Gibbs equation and neutron reflection is similar to that observed using radiotracers on bis(n-octyl)sulfosuccinate by Dixon et al.,7 although they attributed the discrepancy to the presence of undissociated acid at the surface. Given that the acid is a strong one, this was always an unlikely explanation and, not unexpectedly, we have found that there is no effect of acid at pH values above about 5. The value we obtain for the area per AOT fragment at the surface is 78 Å2, which can be compared with a value of 66 Å2 in the lamellar phase,20 68 Å2 for calcium AOT, and 80 Å2 at the hydrophobic solid/liquid interface.7 The higher ionic strength in the lamellar phase leads to screening of the head group repulsion, and hence one would expect a smaller value of the area per head group in the lamellar phase. Similarly, the tighter binding of the AOT fragments to a divalent ion would also lead to a smaller area per AOT unit in the divalent salts. Our value of 78 Å2 is therefore much more in line with expectations than the varying values of 100 Å2 and upward obtained from surface tension measurements. The main purpose of the project is, however, to determine the structure of the AOT layer. We have already made a preliminary study at the cmc2 and will give a fuller description in another paper. Acknowledgment. We thank the EPSRC for support for this work. Z.X.L. thanks the British Council for a grant. LA9608472 (23) Sager, W.; Strey, R.; Kuhnle, W.; Kahlweit, M. Prog. Colloid Polym. Sci. 1994, 97, 141. (24) Fragneto, G.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1996, 178, 531.
