Unusual Surface Structure in Layers of Cationic Gemini Surfactants

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Langmuir 2002, 18, 6614-6622

Unusual Surface Structure in Layers of Cationic Gemini Surfactants Adsorbed at the Air/Water Interface: A Neutron Reflection Study Z. X. Li, C. C. Dong, J. B. Wang, and R. K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, United Kingdom

J. Penfold ISIS, CCLRC, Chilton, Didcot, Oxon., OX11 0QX, United Kingdom Received April 1, 2002. In Final Form: May 23, 2002 We have used neutron reflection measurements to determine the structure of monolayers of the cationic gemini surfactants R,ω-bis(N-alkyl dimethylammonium) alkane bromides (CnCmCn) adsorbed at the air/ water interface at the critical micelle concentration and at 1/10 of this concentration. The compounds studied all had hydrophobic chains of 12 carbons (n ) 12), and the spacers were m ) 3, 4, 6, 12, and xylyl. Partial deuteration was used to distinguish the spacer and chain groups. In all of the compounds except that with m ) 12, the spacer remains close to the headgroups and partially immersed in water. The C12 spacer, however, is part of the strongly hydrophobic region of the layer. The most surprising feature of the layers is that a low-concentration single layer of surfactant forms below the main layer at an approximate distance of 15-20 Å from the center of the headgroup distribution. The area per molecule in this secondary layer varies between about 800 and 2500 Å2 and does not seem to show any particular concentration dependence. More detailed fitting leads to the tentative conclusion that the molecules in the secondary layer are oriented with their charged groups pointing away from the aqueous subphase and toward the charged groups in the main monolayer. It is possible that this sublayer is a manifestation of the tendency of the geminis to undergo premicellar aggregation that has been proposed independently by other researchers.

Introduction Gemini surfactants essentially consist of two surfactants attached by a spacer close to the two headgroups. As surfactants, they have the advantages of lower critical micelle concentrations (cmc’s) and better wetting properties than those achievable with monomeric surfactant species.1-12 Although more difficult to prepare than their single-chain counterparts, they have generated much interest (a recent summary is given by Zana4). The most widely studied series of gemini surfactants is the set of dicationic quaternary ammonium compounds Rω-bis(N-alkyl dimethylammonium) alkane halides, which we designate CnCmCn, 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 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 (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.

surface tension measurements that the area per molecule passes through a maximum at m ) 10-12 for a given alkyl chain length.10 This result has been accounted for by Diamant & Andelman in terms of a competition between the elasticity of the spacer and the hydrophobic attraction between the chains.13,14 In its ability to explore the interior structure of a surfactant monolayer, neutron reflection should be able to offer some insight into the validity of the structural assumptions made by Diamant & Andelman and hence contribute to a better understanding of these interesting materials. Experimental Section 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 of the Rω-bis(N-dodecyl dimethylammonium) alkane dibromide 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.15 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.15 Deuterated dibromopropane was obtained (13) Diamant, H.; Andelman, D. Langmuir 1994, 10, 2910. (14) Diamant, H.; Andelman, D. Langmuir 1995, 11, 3605. (15) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Orgainic Chemistry, 5th ed.; Longman: Essex, U.K., 1989.

10.1021/la020302+ CCC: $22.00 © 2002 American Chemical Society Published on Web 07/20/2002

Structure in Layers of Cationic Gemini Surfactants

Langmuir, Vol. 18, No. 17, 2002 6615

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 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. All the glassware and poly(tetrafluoroethylene) troughs were cleaned by soaking them in alkaline detergent overnight and then rinsing several times with ultrapure water (Elgastat UHQ, Elga, U.K.). The surface tension of the aqueous copolymer solutions was determined on a Kru¨ss K10T digital tensiometer using the du Nou¨y ring method with a Pt/Ir ring. Before each measurement, the ring was rinsed with pure water and flamed to remove contaminants. The temperature was maintained to within 0.2 K. The neutron reflection measurements were carried out on the reflectometers CRISP and SURF at Rutherford Appleton Laboratory (Didcot, U.K.). The instruments and the procedure for making the measurements have been fully described elsewhere.16,17

Results The main purpose of the neutron reflection measurements was to determine the structure of the layers. We have already used neutron reflectometry to determine the surface excesses of these geminis at the air/water interface.18 Measurement of the reflectivity of the fully deuterated surfactants in null reflecting water (NRW) gives the coverage and the distribution of the surfactant molecule as a whole along the direction normal to the interface. The distribution of water at the interface and its position relative to the surfactant layer can be obtained by combining the results from the three combinations dCndCmdCn/NRW, hCnhCmhCn/D2O, and dCndCmdCn/D2O, as has been described elsewhere.19 More detailed information about the internal structure of the layer may come from the modeling of these three reflectivities but is more reliably obtained by additional measurements on combinations such as dCnhCmdCn/NRW, hCndCmhCn /NRW, dCnhCmdCn/D2O, and dCnhCmdCn/D2O. We therefore made measurements at all these seven contrasts and did this at two surface concentrations, one at the cmc and one at the cmc/10. However, in several cases the signal from the hCndCmhCn/NRW contrast was too low with respect to the background to be useful. The set of six profiles is shown for C12C6C12 at its cmc in Figures 1 and 2. The fitting of this set of data, shown as continuous lines, is discussed below. Some qualitative conclusions can be drawn about the layer structure by comparing the reflectivity profiles with that of D2O on its own, shown as a dashed line. Two structural features tend to depress the reflectivity strongly relative to clean D2O. The first is a layer of deuterated hydrocarbon fragments above the water, which has the maximum effect when the scattering length density of the layer is about half that of D2O. The equivalent (16) Penfold, J.; Ward, R. C.; Williams, W. G. J. Phys. E 1987, 20, 1411. (17) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. 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.; 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) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (19) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.

Figure 1. Neutron reflectivities of C12C6C12 at three isotopic compositions at the cmc: (a) dC12dC6dC12 in NRW (b) and hC12dC6hC12 in D2O (O); (b) dC12dC6dC12 in D2O (b). The continuous lines are calculated for the best fitting model consisting of only a simple monolayer.

protonated hydrocarbon layer would have no effect on the reflectivity. The second is penetration into the water by protonated groups. This has the effect of lowering the scattering length density of the topmost layer of water and again will be a maximum when the scattering length density of this layer is half that of D2O. If the groups penetrating the water are deuterated, their scattering length density will be about the same as that of D2O and then the layer will not affect the reflectivity. These effects can be seen in Figures 1 and 2. Thus, the dCndCmdCn layer depresses the reflectivity from that of D2O by a large amount because of the upper layer of deuterated chains. Interestingly, however, the hCnhCmhCn depresses the reflectivity by only a relatively small amount, indicating that the protonated heads and spacer do not penetrate the water very strongly. However, that there is some penetration of the water by the spacer is indicated by the fact that the hCnhCmhCn depresses the D2O reflectivity more than the hCndCmhCn. To analyze the data quantitatively, we use a recently developed method which uses numerical Fourier transformation to calculate the reflectivity from realistically shaped distributions of the different fragments of the layer. In the present case, a key reason for adopting this approach is that we could not find a division into uniform layers that would adequately fit the data. However, the general advantage is that the Fourier transform method allows the use of more realistic fragment distributions and greater flexibility in testing unusual distributions, a feature that turns out to be very important for the geminis. We divide the layer into surfactant chains, C, heads, H, spacer, X, and water. The distributions of the first three are likely to be well described by Gaussians of the form

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Langmuir, Vol. 18, No. 17, 2002

Li et al. Table 1. Scattering Characteristics of the Fragments of the Cationic Gemini Surfactants fragment

volume/Å3

104 × scattering length/Å

C12H25 (C12D25) N(CH3)2 C3H6 (C3D6) C4H8 (C4D8) C6H12 (C6D12) C12H12 (C12D24) xylyl

350 100 75 100 150 300 150

-1.37 (23.46) 0.07 -0.25 (5.71) -0.33 (7.61) -0.50 (11.42) -1.00 (22.84) 2.33

numerical Fourier transformation to obtain a single partial structure factor for each component. The partial structure factors can then be combined with each other and the scattering lengths of each fragment to give the kinematic reflectivity using

RKIN )

Figure 2. Neutron reflectivities of C12C6C12 at three isotopic compositions at the cmc: (a) dC12hC6dC12 in NRW (b) and hC12hC6hC12 in D2O (O); (b) dC12hC6dC12 in D2O (b). The continuous lines are calculated for the best fitting model consisting of only a simple monolayer.

fX(z) )

( )

2 4z2 exp - 2 σX σXAxπ

(1)

where A is the area per fragment, σX is the width parameter (full width at 1/e of the height), and z is the distance along the surface normal. The description of the surfactant layer on its own is completed with the distances between the centers of the distributions of chains, heads, and spacer, δCH and δCX. The water distribution is taken to be one that fills up any space left available by the surfactant up to a certain level in the surface, that is,

fW(z) )

1 - (VCfC(z) + VHfH(z) + VXfX(z)) VW

zGξ

where Vi are the appropriate molecular volumes (see Table 1), fi(z) are the distributions, and ξ is a cutoff distance above which there is no water. To remove the unphysically sharp cutoff in the water distribution, we assume that beyond the cutoff the water decays as a half-Gaussian with a width parameter σW. Thus the remainder of the water distribution is

fW(z) )

1 - (VCfC(ξ) + VHfH(ξ) + VXfX(ξ)) × VW

(

exp -

)

4(z - ξ)2 σ2

z < ξ (2)

Although all the distributions except for the water can be broken down into components that can be Fourier transformed analytically, it is more convenient to keep the distributions for each species intact and to use

16π2 [ κ2

2bibj Re(F ˆ iF ˆ j)] ∑i bi2|Fˆ i|2 + ∑ iEj

(3)

where the F ˆ i are the various Fourier transforms and the bi are the scattering lengths of the fragments. Finally, this is converted to the true reflectivity using the Crowley correction,20 which has been given elsewhere and which has been shown to be almost exact for this type of system except very close to the critical angle, a region that we shall not utilize in the fitting. The Fourier transforms were done numerically, and truncation errors were avoided by Fourier transforming the differential distributions, that is, using

RKIN )

16π2 [ κ4

2bibj Re(F ˆ ′iF ˆ ′j)] ∑i bi2| Fˆ ′i|2 + ∑ iEj

(4)

ˆ ′i are now where we now have κ4 instead of κ2 and where F the Fourier transforms of the differentials f′(z) of the distributions. The number of independent parameters needed to fit a particular set of data depends on what constraints are applied. For the present systems, we found that the chain distributions are generally quite narrow and the value of σC is not much larger than would be expected for the roughness of the layer, as defined by the width of the headgroups. Therefore, instead of using independent values for the four different σi we use a single value. This reduces the number of fitting parameters to six; the area per molecule A, the width σ, the three separations δCH, δCX, and δCW, and the cutoff distance ξ. A further complication for the gemini surfactants was that when the coverage could be determined directly, for example, for the fully deuterated and chain deuterated samples, there were small differences in the values obtained. This could be a genuine isotope effect, or it could be the cumulative errors in sample purification, solution preparation, alignment, and measurement. An indication that the cause might be a genuine isotope effect is that in three out of four cases the protonated compound was found to be slightly less surface active than the deuterated one. We therefore took the precaution of fitting the data in two sets. In the first, we made a simultaneous fit to dCndCmdCn/NRW, dCndCmdCn/D2O, and hCndCmhCn/D2O, and in the second to dCnhCmdCn/NRW, dCnhCmdCn/D2O, and hCnhCmhCn in D2O. However, this was probably an unnecessary precaution because the isotope effect is most strongly manifested in the fits to the two samples in NRW because it causes a small difference in the surface coverage (20) Crowley, T. L. Physica A 1993, 195, 354.


Figure 3. Partial structure factor/interference function for C12C6C12 at six isotopic compositions at the cmc: (a) dC12dC6dC12 in NRW (b, upper), hC12dC6hC12 in D2O (O), and dC12dC6dC12 in D2O (b, lower); (b) dC12hC6dC12 in NRW (b, upper), hC12hC6hC12 in D2O (O), and dC12hC6dC12 in D2O (b, lower). The continuous lines are calculated for the best fitting model consisting of only a simple monolayer.

and it is these two profiles that are the most sensitive to coverage. The other four reflectivity profiles are more sensitive to the structure of the layer than to the coverage, and since this varies only slowly with coverage, it would have been easy to fit them simultaneously using the average coverage from the two NRW profiles. The fits to the C12C6C12 data are shown as continuous lines in Figures 1 and 2. While these fits are adequate, there are some deviations which do not show up well on the logarithmic scale of Figures 1 and 2 but are revealed more clearly when the κ-4 decay of the reflectivity is removed and the data are plotted on a linear scale. For the runs in NRW, this is done by simple multiplication by κ4 which gives the partial structure factor of the deuterium-labeled fragment. The best way of extracting just the interference function from reflectivity profiles in D2O is to subtract the calculated profile for perfectly smooth D2O from the observed profile and then to multiply by κ.4 In the special case where the surfactant is entirely null scattering (approximately satisfied by the fully protonated form), this procedure gives the partial structure factor for D2O. In general, the function obtained is a combination of partial structure factors, and we therefore refer to it as the interference function. We show the experimental results for C12C6C12 plotted in this form in Figure 3. The fits shown in Figures 1-3 are not the final fits, but they are the best fits for a model based on a single surfactant layer at the surface using the division into Gaussian sublayers described in the previous paragraph. While the fits shown in Figures 1 and 2 are adequate, the second group of data (isotopic species with a protonated spacer) are not quite as well fitted when one examines the

Langmuir, Vol. 18, No. 17, 2002 6617

Figure 4. Partial structure factor/interference function for C12C12C12 at seven isotopic compositions at the cmc: (a) dC12dC12dC12 in NRW (b, upper), hC12dC12hC12 in D2O (O), dC12dC12dC12 in D2O (b, lower), and hC12dC12hC12 in NRW (4); (b) dC12hC12dC12 in NRW (b, upper), hC12hC12hC12 in D2O (O), and dC12hC12dC12 in D2O (b, lower). The continuous lines are calculated for the best fitting model consisting of only a simple monolayer.

linear fits (Figure 3b). There appears to be a slight dip in the data for NRW at about κ ) 0.12 Å-1, and the data for hC12hC6hC12 and dC12hC6dC12 in D2O show significant deviations at similar values of κ. This feature is seen in all the compounds to a greater or lesser extent. The effect is weakest for the C3 spacer, that is, the C12C3C12 profiles are quite well fitted by the model above, but is very large for the C12 and xylene spacers. Figure 4 shows these large deviations for the C12C12C12 compound. There is clearly some structural feature of the real layer that has not been included in the model, and the Fourier component in the interference function indicates that the length scale of this structural feature is significantly larger than the dimensions of a single surfactant layer. A first obvious possibility is that the general disorder observed in these surfactant layers may cause a small proportion of alkyl chains to point downward into the subphase. Increasing the thickness of the layer would introduce a larger scale Fourier component. However, introduction of such a feature into the model of the layer with physically reasonable distances does not give Fourier components corresponding to a large enough length scale. Thus, although some chains may be pointing downward, they are not the cause of the unusual profiles shown in Figure 4. It turns out that only the introduction of a small fraction of surfactant molecules at a distance of 15-20 Å below the main monolayer can account for the observations. Figure 5 shows the considerable improvement in the fits for C12C12C12 when such a layer is included. The actual contributions of the sublayer to the various interference functions are shown in Figure 6 where the calculated

6618


Figure 5. Partial structure factor/interference function for C12C12C12 at seven isotopic compositions at the cmc: (a) dC12dC12dC12 in NRW (b, upper), hC12dC12hC12 in D2O (O), dC12dC12dC12 in D2O (b, lower), and hC12dC12hC12 in NRW (4); (b) dC12hC12dC12 in NRW (b, upper), hC12hC12hC12 in D2O (O), and dC12hC12dC12 in D2O (b, lower). The continuous lines are calculated for the best fitting model consisting of a monolayer and a sublayer.

interference functions for five of the contrasts have been divided into those from a simple monolayer and those from the sublayer. Since such a feature has not been observed in any system below the cmc before, the evidence for its existence needs to be established with maximum certainty, and we therefore show the fits to the data for two of the other geminis, C12C4C12 in Figure 7 and two concentrations of C12xC12 in Figure 8. For both molecules at the cmc, the effects identified in Figures 4-6 can be seen clearly. The effect is also very pronounced for the C12xC12 compound at 1/10 of the cmc. What is really surprising about the C12xC12 is that the concentration of molecules in the sublayer is approximately constant between the cmc and cmc/10. The fits of the two-layer model to the data for all the geminis are given in Tables 2-6, which include, in addition to the parameters for the main monolayer that have already been identified, the area per molecule in the sublayer, ASM, and the distance from the headgroup in the main monolayer. The additional amount of surfactant in the submonolayer adds a noticeable amount to the overall surface excess as is shown by the mean area per molecule at the interface, ATOT, although this is not so large as to invalidate the conclusions about the surface excess presented in the previous paper. For the fits given in the tables, we have assumed that the fragments of each surfactant molecule in the sublayer have the same width as they do in the main monolayer and that heads, chains, and spacers are centered at the same distance, δSM, from the surface. There is some difficulty in how best to characterize the water distribution because it does not

Li et al.

Figure 6. Contributions to the partial structure factor/ interference function from the main monolayer and the sublayer. In all cases, the more rapidly varying function is from the sublayer. In (a), the area per molecule in the sublayer is approximately 2500 Å2, and in (b) it is about 700 Å2. (a) dC12dC12dC12 in NRW (continuous line) and hC12dC12hC12 in D2O (dotted line); (b) dC12hC12dC12 in NRW (continuous line), hC12hC12hC12 in D2O (dotted line), and dC12hC12dC12 in D2O (dashed line).

have a well-defined analytical form. In the tables, we give δWH as the distance between the center of the headgroups and the point where the water density is half its bulk value and, in brackets, we give the cutoff point defined by ξ as defined above. The volumes and scattering lengths used for the various fragments are given in Table 1. Although the model can be refined to improve some of the fits, it already provides the key features needed for satisfactory fits to all the data. The values of ASM fall in the range 800-2700 Å2. They are smallest (highest adsorption) for the C4, C12, and xylene spacers, and they are also generally smaller for the isotopes with the protonated spacer. This may be because the reflectivity from species with the protonated spacer is more sensitive to the effect, but the differences are mostly greater than the experimental error. The very interesting result is that the total area per molecule in the sublayer does not seem to increase systematically on changing from the cmc to cmc/10. It might be thought that the neutron reflection experiment could not detect such low concentrations. The sensitivity of neutron reflection to molecules forming a typical surfactant monolayer is certainly such that it would not be able to detect adsorption at these low levels. However, in circumstances where there is already a monolayer and/or where the composite thickness of the layer increases above about 30 Å the sensitivity increases dramatically, as is shown by the contribution of the sublayer to the interference function in Figure 6. One other feature of interest for the structures in the tables is the position of the spacer group in the main monolayer.


Figure 7. Partial structure factor/interference function for C12C4C12 at seven isotopic compositions at the cmc: (a) dC12dC4dC12 in NRW (b, upper), hC12dC4hC12 in D2O (O), dC12dC4dC12 in D2O (b, lower), and hC12dC4hC12 in NRW (4); (b) dC12hC4dC12 in NRW (b, upper), hC12hC4hC12 in D2O (O), and dC12hC4dC12 in D2O (b, lower). The continuous lines are calculated for the best fitting model consisting of a monolayer and a sublayer.

For all except the C12 spacer, the spacer lies close to the headgroups. However, the much larger hydrophobicity of the C12 spacer and its additional flexibility allow it to move into the hydrophobic region of the layer. In doing so, it evidently causes the chain layer to become slightly thinner. Discussion The marked difference in the positions of the spacers in the interface between the small-spacer geminis and the C12C12C12 is shown most clearly in the average distributions for C12C4C12 and C12C12C12 in Figure 9. The variation of the spacer/water overlap with spacer is made more quantitative in Table 7, where a number of properties of the monolayers are collected together. When the total spacer/water overlap is converted to an overlap per CH2, it results in a value of about 0.1 for all the spacers except C12, for which it is only 0.025. At first glance, this ought to result in a lower surface tension for the C12C12C12 compound. However, its limiting surface tension is in fact the highest in the series and higher even than for the single-chain C12TAB. This must be a result of the much lower volume fraction of hydrophobic units covering the surface (first row of Table 7), which in turn results from the much higher area per chain. Even if the sum of chains and spacer is taken, the volume fraction of hydrophobe is still lowest for the C12C12C12 compound (second row of Table 7). Another interesting comparison is that of the C12C6C12 gemini and single-chain C12TAB, and this is made both in Table 7 and in Figure 10. The comparison is interesting because the area per chain is almost identical for these two compounds. It is clear that the presence of the spacer makes the environment of the headgroup in

Langmuir, Vol. 18, No. 17, 2002 6619

Figure 8. Partial structure factor/interference function for C12xC12 at three isotopic compositions at (a) the cmc and (b) cmc/10: dC12xdC12 in NRW (b, upper), hC12xhC12 in D2O (O), and dC12xdC12 in D2O (b, lower). The continuous lines are calculated for the best fitting model consisting of a monolayer and a sublayer. In (a), the areas per molecule are 97 Å2 in the main monolayer and 900 Å2 in the sublayer, and in (b) they are 140 and 900 Å2, respectively.

the gemini more hydrophobic (Figure 10b). If the volume occupied by the spacer were replaced with water, the overlap between the resultant water distribution and the headgroup would be very similar to that for the singlechain surfactant (dashed-dotted line in Figure 10b). The other noticeable difference between the two surfactants is that the chains in the gemini are somewhat closer to the headgroups, and this is what would be expected from the more hydrophobic environment of the latter. Although Figure 10 does show distinct differences between the two surfactant monolayers, the parameters averaged over the whole layer and tabulated in Table 7 are within error the same. It is only when the contribution from the chain/ water overlap from the subsurface layer is removed (in brackets in Table 7) that the more hydrophobic surface of the gemini is revealed in the values of Table 7. This should lead to a lower surface tension for the C12C6C12 gemini in comparison with C12TAB. That it does not must be because any favorable contribution of the hydrophobic layer to the free energy of the surface is opposed by the immersion of the spacer in the water, that is, the hydrophobic repulsion between spacer and water may be an important structural feature of the layer. Diamant & Andelman have proposed a model that accounts for the variation of surface excess of the geminis with spacer length, and it is interesting to examine their assumptions about the structure in light of the present results.13,14 The crucial part of the model is accounting for the behavior of the spacer. Diamant & Andelman first determined the elasticity of the spacer by using the

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Li et al.

Table 2. Best Fitted Parameters for the C12C3C12 Surfactant (cmc ) 1.1 mM) concn/M cmc cmc/10

isotope

σ/Å

δCH/Å

ddd dhd ddd dhd

14 ( 1 14 ( 1 13 ( 1 13 ( 1

-6 ( 1 -6 ( 1 -5.5 ( 1 -5.5 ( 1

δSH/Å

δWH/Å

AM/Å2

ASM/Å2

ATOT/Å2

δSM/Å

3(1 3(1 1.5 ( 1 1.5 ( 1

67 ( 3 75 ( 5 95 ( 5 105 ( 5

850 ( 50% 1750 1900 2100

62 72 90 100

20 ( 5 20 20 20

Table 3. Best Fitted Parameters for the C12C4C12 Surfactant (cmc ) 1.2 MM) concn/M cmc cmc/10

isotope

σ/Å

δCH/Å

δSH/Å

δWH/Å

A/Å2

ASM/Å2

ATOT/Å2

δSM/Å

ddd dhd ddd dhd

14 ( 1 13.5 ( 1 11 ( 1 11 ( 1

-5 ( 1 -5 ( 1 -3.5 ( 1 -3.5 ( 1

-1 ( 3 0(3

2.5 ( 1(-3) 2.5 ( 1(-3) 2 ( 1(-2) 2 ( 1(-2)

85 ( 2 83 ( 3 124 ( 3 121 ( 3

1700 ( 50% 900 900 1000

81 76 109 108

20 ( 5 20 20 20

Table 4. Best Fitted Parameters for the C12C6C12 Surfactant (cmc ) 1.1 mM) concn/M cmc cmc/10

isotope

σ/Å

δCH/Å

δSH/Å

δWH/Å

A/Å2

ASM/Å2

ATOT/Å2

δSM/Å

ddd dhd ddd dhd

14.5 ( 1 14 ( 1 12 ( 1 14 ( 1

-5 ( 1 -5 ( 1 -4.5 ( 1 -4 ( 1

2(2 2(2 -2

3 ( 1(-2) 3 ( 1(-1) -(0)

92 ( 3 104 128

1850 ( 50% 2100 1300

88 99 117

20 ( 5 20 20

Table 5. Best Fitted Parameters for the C12XC12 Surfactant at 313 K (cmc ) 1.2 mM) concn/M

σ/Å

δCH/Å

δSH/Å

δWH/Å

A/Å2

ASM/Å2

ATOT/Å2

δSM/Å

cmc cmc/10

14 ( 1 13 ( 1

-5.5 ( 2 -5 ( 2

0(3 0(3

3 ( 2(0) 3.5 ( 2(-1)

97 ( 5 140 ( 5

900 ( 50% 950

88 122

20 ( 5 20

Table 6. Best Fitted Parameters for the C12C12C12 Surfactant (cmc ) 0.28 MM) concn/M

isotope

σ/Å

δCH/Å

δSH/Å

δWH/Å

A/Å2

ASM/Å2

ATOT/Å2

δSM/Å

cmc

ddd dhd

13 ( 1 12.5 ( 1

-5 ( 1 -5 ( 1

-6 ( 3 -6 ( 3

0.5 ( 1(-2) 0.5 ( 1(-2)

134 ( 3 134 ( 3

2700 ( 50% 2400

128 127

20 ( 5 20

rotational isomeric state model to calculate the spacer chain conformations. They took into account three factors. These are the constraint imposed on the ends of the spacer by the two anchoring headgroups, the preference for the spacer to loop away from the aqueous side of the interface, and the flexibility of the spacer. Diamant & Andelman made separate calculations for different models of the spacer. The simplest was to use the rotational isomeric state model to calculate the number of accessible configurations in the hydrophobic half space. At the next level, additional nonbonded repulsions along the spacer chain were included, and in the third model a hydrophobic repulsion was introduced between the water and the hydrophobic CH2. Two of these models account well for the maximum in the area per molecule at saturation that has been observed by means of surface tension measurements (see Figure 11). The third model, which includes the hydrophobic repulsion term, is more consistent with the area per molecule approaching a plateau rather than a maximum, although there is a very weak maximum. As we have pointed out in a previous paper, the direct measurement of surface area from neutron reflection makes it clear that the application of the Gibbs equation to surface tension data for the geminis is risky and we would not regard the maximum as necessarily established, given this uncertainty. Although we cannot make a direct comparison of our experimental values for the area per molecule with those of Diamant and Andelman, it is interesting to plot scaled values on their graph, and this is done in Figure 11. The neutron data are distinctly more consistent with the third of their models, the one including a hydrophobic repulsion term. We believe that it is possible to rationalize this as follows. In a typical texbook representation of a surfactant at the surface of water, the water cuts off very sharply and the approximation of calculating the accessible conformations in the hydrophobic half space would be appropriate. In the real system, not only is the water distribution more blurred but there

Figure 9. Mean distributions of the different fragments in the layers at the cmc of (a) C12C4C12 and (b) C12C12C12: chains (continuous line), heads (dotted line), spacer and water (dashed lines).

is also disorder in the position of the spacer, that is, its two anchors (the charged headgroups) are themselves moving. There is therefore significant overlap of the spacer


Langmuir, Vol. 18, No. 17, 2002 6621

Table 7. Comparison of Overlaps and Related Parameters for the Gemini Surfactants at Their Critical Micelle Concentrations spacer

C3

C4

C6

C12

X

C12TAB

total chain total chain + spacer chain-water overlap/Å2 spacer-water overlap area per chain (monolayer)/Å2 surface tension/mN m-1

10.6 11.8 1.0(0.55) 0.3 36 37

9.0 10.4 1.4(0.9) 0.4 42 40

7.6 9.3 1.3(0.9) 0.7(0.6) 49 42

5.9 8.6 1.4(0.8) 0.3 67 43

8.1 10.0 1.4(0.65) 0.8(0.65) 48 40

7.3

Figure 10. Mean distributions of the different fragments in the layers at the cmc of (a) C12TAB and (b) C12C6C12: chains (continuous line), heads (dotted line), spacer and water (dashed lines). The C12TAB data are from ref 22.

Figure 11. The variation of area per molecule with spacer length calculated for three models by Diamant & Andelman (ref 13) with the observed areas plotted as points.

and the water. Table 7 gives the values of the overlap of water and spacer per CH2 group calculated by integrating the product of the water and spacer distributions. In all cases except for the C12 spacer, this is significant and the hydrophobic repulsion used by Diamant & Andelman in

1.4 48 41

their third model, which takes the form of a repulsive interaction for every CH2 falling within 2 Å of the air/ water interface, would therefore seem to be a necessary inclusion. We suggest that this explains the better agreement of the variation of coverage with spacer length given by their third model. The most dramatic result from the neutron reflection experiment is the presence of the sublayer. Although related layers have been observed above the cmc,21 this is the first observation of one below the cmc, and it is remarkable that it persists even at 1/10 of the cmc. The strength of the feature this contributes to the reflectivity and the consistency of its appearance indicate that this result is unlikely to be an artifact. However, we cannot exclude the possibility that the observed surface is in a state of metastable rather than true equilibrium. Although the measurements were typically made on a surface of 2 or 3 h of age, it is a surface where there is free exchange of water with the enclosed vapor phase. If such a situation can lead to the creation of some sort of metastable structure at the interface, it would also be a structure that would be found under most other measurement circumstances. However, we have re-examined a large fraction of the extensive data we have accumulated on other surfactants and found no trace of comparable behavior. There are two features of the main monolayer of the geminis that make it very different from that of a singlechain surfactant. The first has already been identified, and that is that the lower side of the layer is substantially more hydrophobic because of the presence of the spacer. It is also possible that because of the competition between spacer and chain to occupy the hydrophobic region, there will be a small fraction of chains pointing down into the subphase. This will further enhance the hydrophobicity of the underside of the main monolayer to a point where it may be favorable for a small number of molecules to adsorb. However, there is another possible reason for a sublayer. Several authors have suggested that there is preaggregation of the geminis at concentrations below the cmc, for example, refs 3, 23-30. We have shown that the prefactor in the Gibbs equation for the surface tension indicates extensive association in the solution below the cmc, which we attributed to the formation of ion pairs between the doubly charged gemini and the singly charged (21) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 13907. (22) Lyttle, D. J.; Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 1001. (23) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148. (24) Rosen, M. J.; Mathias, J. H.; Davenport, L. Langmuir 1999, 15, 7340. (25) Frindi, M.; Michels, B.; Levy, H.; Zana, R. Langmuir 1994, 10, 1140. (26) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. B 1998, 102, 6152. (27) Hattori, N.; Horata, H.; Okabayashi, H.; O’Connor, C. J. Colloid Polym. Sci. 1999, 277, 361. (28) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149. (29) Rosen, M. J.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885. (30) Zana, R. J. Colloid Interface Sci. 2001, 246, 182.

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counterion, but it could also be consistent with aggregation of the gemini ions themselves. Zana has also found evidence for ion association in some of the compounds.30 In completely unrelated experiments, it has also been shown by neutron reflection and soap film balance studies that above the cmc micelles of the single-chain cationic surfactant C14TAB aggregate below the surface.21 If preaggregates exist for the geminis, it is possible that they will also adsorb at the monolayer surface and create an effect comparable to what we observe here, although the phenomenon that we observe here is of a much smaller spacing than that for C14TAB. We have examined to what extent we can determine whether there is any preferential orientation of the molecules in the sublayer. In the fits given in the diagrams and tables above, we assumed that the heads, spacers, and chains were all distributed randomly about the same depth in the layer. For the C12C12C12 gemini, we tried various orientations of the

Li et al.

spacer and the chains. Orientations where the spacer is closer to the main monolayer give distinctly better fits to the data than those where the chains are closer, and this would then suggest that the origin of the effect is some sort of electrostatic interaction. The alternative of having the spacer pointing toward the subphase gives the wrong phase for the long-range Fourier component in the reflectivity. Although these observations would seem to be fairly conclusive, the uncertainties in the assumptions about the overall model make the interpretation at such a level of detail risky, and this orientation must therefore be regarded as tentative. Acknowledgment. We thank the Engineering and Physical Sciences Research Council for supporting this work. LA020302+

Unusual Surface Structure in Layers of Cationic Gemini Surfactants

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