J. Phys. Chem. 1995, 99, 4113-4123
4113
Structure and Composition of Dodecane Layers Spread on Aqueous Solutions of Dodecyland Hexadecyltrimethylammonium Bromides Studied by Neutron Reflection J. R. Lu and R. K. Thomas* Physical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, England
B. P. Binks and P. D. I. Fletcher School of Chemistry, University of Hull, Hull, HU6 7RX, England
J. Penfold Rutherford-Appleton Laboratory, Chilton, Didcot, Oxon OX11 ORA., England Received: June 7, 1994; In Final Form: October 24, 1994@
Neutron reflection measurements have been used to study the composition and structure of mixed dodecanesurfactant monolayers at the aidwater surface. The surfactants were dodecyltrimethylammoniumbromide (C12TAB) and the corresponding C16TAB. Partially isotopically labeled C16TABs were used to localize the position of the oil in the layer with greater precision. The surfactant concentrations were chosen to be at M (C12TAB) and 9.1 x M (C16TAB). The area their critical micelle concentrations (cmc) of 14 x per molecule for both surfactants increased on the addition of oil from 48 to 54 f 3 A2 for C12TAB and 43 to 52 f 3 A2 for C16TAB. In each case the surfactant layer was found to be slightly thicker in the presence of dodecane than in its absence, although the increase in area would normally cause the thickness to decrease. The area per molecule of the dodecane was found to be 76 and 67 f 7 A2 on C12TAB and c16TAB, respectively, to be compared with earlier surface tension estimates of 78 and 56 A2. On CnTAB the thickness of the dodecane layer is slightly greater (18.5 f 2 A) than its fully extended chain length of dodecane (18 A), but on C16TAB it is significantly greater (20.5 f 2 A). The separations of the centers of the surfactant and dodecane distributions at the corresponding cmc’s were found to be 6.5 and 8.5 f 1 A. Labeling of the C6 fragments at either end of the C16TAB made it possible to locate the oil distribution more accurately. The center of the outer c6 (hexyl) group (i.e., the air side) of the surfactant tail was found to coincide exactly with the center of the oil distribution, whereas the inner C6 (hexamethylene) group was centered 8.5 f 1 8, from the center of the oil distribution.
Introduction When small quantities of long-chain liquid alkanes are placed on the surface of an aqueous surfactant solution, they often spread initially but then retract to form lenses of small contact angle. The equilibrium situation then consists of macroscopic oil lenses coexisting with a mixed monolayer of surfactant and penetrating or solubilized alkane molecules. Using surface tensiometry, the mixed monolayer composition has been quantified for a range of surfactant and alkane chain lengths as a function of the surfactant surface excess.1*2The extent of oil penetration into surfactant monolayers at the oiYwater interface is believed to be an important factor in determining the spontaneous curvature and bending elasticity of monolayers at the oiYwater interfa~e.~ The detailed nature of such mixed monolayers cannot be elucidated by surface tensiometry but requires a technique capable of determining the interfacial structure directly, such as neutron reflection. In a previous study we have used surface tensiometry and neutron reflection to investigate the composition and structure of surface films at the air-water interface formed in systems containing n-dodecane and the cationic surfactant tetradecyltrimethylammonium bromide ( C I ~ T A B ) .We ~ found partial penetration of the surfactant chains by the oil and were able to quantify the relative positions of oil and surfactant in the direction normal to the interface. From the original surface @
Abstract published in Advance ACS Abstracts, March 1, 1995.
0022-365419512099-4113$09.00/0
tension work it was shown that there was a pronounced dependence of monolayer composition on the chain length of both surfactant and oil. There is therefore some interest in extending the neutron reflection measurements to the other chain length surfactants in the series. Here we study the same oil, dodecane, on both the longer chain surfactant hexadecyltrimethylammonium bromide and the shorter chain dodecyltrimethylammonium bromide. However, the main purpose of the present set of experiments is to examine the relative positions of surfactant and oil at much higher effective resolution. This has been done by using more selective isotopic labeling of the alkyl chain of the surfactant. Experimental Details The various isotopes of C16TAB and C12TAB were made and purified as described in refs 4 and 5. Dodecane was obtained from Fluka (protonated) and Merck, Sharp, and Dohme (deuterated). Polar impurities were removed by passing over a column of alumina. Water was purified by ion exchange (Elgastat). The procedures for determining the reflectivity profiles have been fully described elsewhere.6 The measurements were made on the reflectrometer CRISP at ISIS, England. The instrument was calibrated using the reflectivity profile of pure D20, and a flat background determined at high momentum transfer was subtracted before processing the data. The solutions were contained in Teflon troughs mounted in a sealed thermostated 0 1995 American Chemical Society
4114 J. Phys. Chem., Vol. 99, No. 12, 1995
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container. At the start of each experiment possible surface active impurities were removed by foaming the solution in situ and removing the foam. A small quantity of dodecane was added onto the edges of the solution surface in the trough. More dodecane was introduced into a recess surrounding the trough in order to saturate the vapor. The neutron reflectivity is affected by the presence of lenses of oil on the solution, but, provided that the lenses cover less than about 10% of the surface, the effect should be small. Nevertheless, this undoubtedly led to some lack of reproducibility in the measurements, which, in turn, led to a larger error than we normally expect for surface coverages determined by reflection. The errors arising from this problem have been discussed fully in refs 4 and 7. All the measurements were made at 301 K. Results
(A) Surface Coverage. Neutron reflection measurements were made at the critical micelle concentration (cmc), 9.1 x M for C16TAB and 1.4 x lo-’ M for C12TAB. In both cases the isotopic compositions studied were selected in order to answer the simple question of where the oil (dodecane) is in relation to the entire surfactant molecule. For this the isotopic compositions studied were fully deuterated C16TAB, denoted dC16dTAB, and fully protonated dodecane, denoted by hO, in null reflecting water (nrw)and D20, hCl&TAB, and dO in nrw and D20, dC16dTAB and dO in nrw,and hC16hTAB and hO in D20. The same set of isotopic compositions were also studied for C12TAB. We also examined the structure of the C16TAB/ dodecane mixture at higher resolution by more selective isotopic substitution. For these experiments we determined the reflectivities of dC .c
u 10: aJ
-1
G aJ
a
-
10‘ -2
10‘:
-3
10-6
0.05
0.10
0.15
0.20
0.25
C 0
K/kl Figure 6. Reflectivity profiles of mixed partially deuterated C16TAB/ dodecane layers in D20 at 9.1 x M and 301 K. The isotopic compositions are (a) dCs“O”Cl&TAB/hO (0)and “O”ClodC&TAB/ hO (+) and (b) dC{‘O”Cl&TABldO (0)and “O”ClodC&TAB/dO (+). The fitted lines are exact reflectivities calculated using a two-layer model with the parameters given in Table 5 .
I
0.05
0.W
0.15
0.20
0.25
t
I
K 1A.l Figure 5. Partial structure factors of mixed ClzTAB/dodecane layers in D20 at 1.4 x M and 301 K for (a) h$), (b) /$I, and (c) h:’. The continuous lines are (a) fits of eq 15 using Gaussian distributions for oil and surfactant and (b and c) using eq 14 with Gaussian distributions for oil and surfactant and a tanh distribution for the water. The parameters used were d,, = 6.5 A, d,, = 4.5 A, and do, = 12.0 A.
7 instead of eq 3. The lower signal from the deuterated c6 groups means that there is a slightly greater uncertainty in the determination of the Gaussian widths of the c6 distributions given in Table 4. Both the end hexyl and end hexamethylene groups have the same width of 14 f 2 8,. Once again, however, the cross terms in the partial structure factors are remarkably sensitive to the 6 values. Figure 7 demonstrates that hclo and hczo are completely different, and, since h c ~ c ~ hand o o hczczhoo are identical, the difference results only from differences in the cos ~d term in eq 15, i.e. a difference between &lo and dczO. From the best fit to this data &lo is found to be quite large, at 8.5 ic 1 A, whereas dczOis very close to 0. This shows that the center of the oil distribution almost coincides with the center
of the hexyl group at the outer end of the surfactant chain. A similar sensitivity to the 6 values is found for the partial structure factors connecting the c6 fragments to the solvent, shown in Figure 8. Again, since the geometric means of the two self partial structure factors contributing to eq 14 are identical, the differences only depend on sin ~ d However, . now it is the c l fragment that is closest to the solvent, and Bels is found to be 5.5 f 1 A, whereas 6c2s is 15.0 f 1 8,. An important final observation on the partially labeled chain experiments is that they give two determinations of hb‘,‘ which are completely independent of the determination of this partial structure factor from the more simply labeled compounds described above. We compare the three measurements of hf) in Figure 9. In each case we have fitted the h!) calculated using the same value of do, of 15.5 A. Figure 9c is well filled by this value of 6 but Figure 9b apparently less well. However, in this second case small changes in 6 would not improve the fit significantly because the discrepancies in the fit must result from small systematic errors in the value of hb‘,)itself. That this must be so is because an increase of 6 from 15.5 8, would fit the data below 0.10 better than shown in Figure 9, but there would then be larger discrepancies above 0.10 The opposite would occur if 6 were decreased from 15.5 8,. That the three independent determinations of do, are the same within experimental error goes a long way to confirming the accuracy of the partial structure method of analyzing reflectivity data.
4120 J. Phys. Chem., Vol. 99, No. 12, 1995
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TABLE 5: Structural Parameters Used in the Two-Layer Model Fit to the Reflectivity Data for Partially Deuterated ClaTABu surfactant oil substrate tJA (12) tdA(fl) A,/A2 (f7%) AJAz (110%) fc (fO. 1) hCi2 nnv 14.0* 10.0 54* 66 0.05 dC$‘O”Cl&TAB 20.0* 10.0 54* 67* 0.05 dCi2 nnv dCc,“O”Cl&TAB 11.0* 13.0 54* 60 0.10 hCi2 Dz0 dC$‘O”Ci&TAB dCi2 Dz0 19.5* 10.0 53* 67* 0.10 dCc,“O”Cl&TAB 6.0 hCiz nnv 10.0 54* 67 0.60 “O”CiodC&TAB dCiz nnv 17.0* 10.0* 50* 60* 0.60 “O”ClodC&TAB Dz0
hCiz
“O”ClodC6hTAB
7.0
10.5*
54*
65
0.60
dCiz D20 20.0* 9.5* 54* 72* 0.60 “O”CiodC&TAB a The optimum fits to the whole set of data were found with molecular areas of the surfactant and oil of 52 and 66 Az, respectively. The asterisks denote the parameters that dominate the fitting.
-
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-3.5-
-3.0
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3.51 (b’ 3.0 2.5
2.0 1.5 1.0
t
0.5 0.0
0.05
0.10
0.15
020
0.25 0
KIP Figure 7. Partial structure factors of mixed Cl6TABldodecane layers at 9.1 x M and 301 K: (a) h6ib; (b) h$b. The continuous lines are fits of eq 15 using Gaussian distributions for oil and cl and c2 fragments with &lo = 8.5 8, and dcz0 = 0.5 8,.
Discussion The method of analyzing the reflection data using partial structure factors gives the separations between the centers of the distributions of the different components without any assumptions other than that the distributions are even or odd. It is not easy to assess just how well the distributions conform to this constraint. However, there are several factors that indicate that the deviations will not affect the determination of the 6 values more than the errors quoted. Firstly, even quite large deviations do not significantly affect the derived value of 6, provided that the fitting is weighted toward the low K data.gq10 Secondly, the contribution of roughness (at least some of which is from capillary waves) to the width of the profile of a given fragment apparently becomes comparable to the intrinsic thickness for fragments less than about eight carbon atoms in
-701
1
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1
0.10
1
0.15
K
Ik’
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0.25
030
Figure 8. Partial structure factors of mixed ClJAB/dodecane layers at 9.1 x M and 301 K (a) h$’,; (b) h$i. The continuous lines are fits of eq 14 using a tanh distribution for the water and Gaussian distributions for c l and c2 fragments with dCls= 5.5 8, and 6c2s= 15.0 8,. length,” and roughness is likely to lead to a Gaussian distribution of the fragment across the interface. Finally, the number of values of 6 has been overdetermined, and the self-consistency of the different values is excellent, as can be seen from Table 4, which would not happen if any one distribution deviated too far from the ideal. A comparison of the structural parameters of the mixed layer for different surfactant chain lengths is given in Table 6 , which also includes the corresponding parameters for C14TAB. The C14TAB parameters are not directly comparable because they were made on the chain-deuterated surfactant, and, for example, the value of 6,,in the paper on C14TAB refers to the separation of chain and oil rather than whole surfactant and oil. The difference between the two is not large and may be estimated from differences between the two relevant distances determined
J. Phys. Chem., Vol. 99, No. 12, 1995 4121
Dodecyl- and Hexadecyltrimethylaonium Bromide
TABLE 6: Structural Parameters of the OWSurfactant Layer for Different Surfactant Chain Lengths (All Dimensions in Angstrom@ aa go acl ac2 6, 6ao 60s &lo 6cls 6c20 6c2s
5
(b) 16.0 (c) 16.0
ClzTAEVdodecane 4.5 6.5 12.0 4.5 4.5
5.5 6.0 5.5
(a) 18 20 (b) 17.0
C14TATUdodecane 6.5* 6* 13.0 5.0
5.5 5.0
(a) 17.5 18.5
C16TAB/dodecane (a) 20.0 20.5 14 (b) 18.0 (c) 17.5
14 7.0 14 14 7.5 12.5 12.5 6.5
8.5 15.5 8.5
5.5 5.5 4.5
0.5
15.0 6.0 12.0 6.0 10.5 5.5
“(a) Oil present; (b) surfactant at c.m.c. with no 0il;8.12-13 (c) interpolated values for surfactant without oil at the coverage in the presence of oil. The asterisks denote values estimated from the chain and overall surfactant dimensions because the original measurement was done only on chain-labeled surfactant.
-4.01
1
0.05
0.10 0.15
020
0.2s
(
K/k’ Figure 9. Partial structure factors of mixed C~TABldodecanelayers at 9.1 x M and 301 K: (a) hb‘,’ from the reflectivities of the fully deuterated and fully protonated C16TAB samples; (b) hb‘,’ from the reflectivities using the partially deuterated dC{‘O”Cl&TAB; and (c) hb‘,‘ from the reflectivities using the partially deuterated “O”c~1dc6hTAB. The continuous lines are fits of eq 14 using a Gaussian distribution for the oil and a tanh distribution for the water with a,, = 15.5 8, in all cases. for the pure surfactant layers. Thus, although the values given for C14TAB in Table 6 are apparently different from the original reference,“ this does not involve a change of interpretation. Also included in Table 6 are the corresponding structural parameters for the surfactant only at its cmc.8,12,13Although the bulk concentrations of surfactants in the current experiments are at the cmc’s, it might be more appropriate to make the comparison at the same surface coverage of surfactants. While these measurements have not been done for either C16TAB or C12TAB, the variation of the structure has been studied as a function of surface concentration, and it is therefore possible to estimate interpolated values which are labeled as (c) in Table 6 . The distributions of the various fragments in the mixed surfactant-oil layer as determined directly from the experiment are shown for both surfactants in Figure 10. Distances are expressed relative to the position of the peak for the surfactant. These distributions are in terms of the number density, which
0
Distance normal to wrface/A Figure 10. Number density distributions for (a) C12TABldodecane and (b) Cl,jTAB/dodecane. The solid lines represent the distributions of surfactants, dashed lines the oil, and dotted lines the water. is what is determined in the experiment. However, estimated volumes of the fragments can be combined with the number density profiles to give the volume fraction profiles shown in Figure 11. The latter gives a more direct picture of the fraction of each type of material across the interface and also gives a test of the consistency of the set of distributions in that the total volume fraction should not rise above unity. Given that the shape of the distribution of each fragment has been assumed, both sets of distributions satisfy this criterion. We have argued elsewhere that the assumption that the water follows a tanh distribution is particularly suspect, and it is almost certain that deviations from this distribution cause the sliget unevenness in the overall volume fraction in the 10 or so A below z = 0. Also included for comparison in Figure 11 are the distributions
4122 J. Phys. Chem., Vol. 99, No. 12, 1995
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0.6-
0.20.4
0.0'
s e
z U rc
Distance normal to surface/A Figure 11. Volume fraction distributions for (a) ClzTAB/dodecane, (b) Cl6TAB/dodecane, and (c) C16TAB at an area per molecule of 52 A2.
of c16TAB at an area per molecule of about 52 A2 estimated as described in the previous paragraph. Examination of the parameters given in Table 6 and Figure 1lb,c shows that the surfactant chain distribution becomes more extended in the presence of oil, whether the initial reference point is taken at the cmc or approximately the same area per surfactant. Particularly striking is that the inner C6H12 group is hardly affected by the presence of the oil, but the outer Ca13 moves outward by 4.5 A from its position in the surfactant layer at the same area per molecule of surfactant. The distance between the centers of the two c6 fragments dong the fully extended c16 chain is 13.4 The actual values in the layer are such that this distance is about 6 8, at an area per molecule of 52 A2,I2which indicates that the chain is strongly tilted away from the surface normal. It is not possible from the data presently available to determine whether there is a uniform tilt angle, which would be about 60" away from the surface normal, or a distribution of tilt angles. However, the change from 6.0 to 9.5 8, in the presence of oil shows that the dominant effect of the oil is to cause the surfactant chains to adopt a much more upright conformation. In terms of a uniform tilt angle this is a change from 63" to 45".
The distributions of Figure 11 show that the addition of oil creates an oil-like layer on the surface, which does not exist for the surfactant alone. Thus, in the absence of oil, the hydrocarbon volume fraction is above about 0.6 only for 1 or 2 A after the water fraction has dropped to about 0.2. In the presence of oil there is a layer about 10 8, where the total hydrocarbon fraction maintains a volume fraction above about 0.6 and the water is below about 0.2. This prompts the question as to what extent the present system mimics the behavior of a surfactant at the oil/water interface. While this cannot be answered, it does seem probable that the only further change that could occur when the oil film is replaced by a bulk oil phase is that the surfactant molecule tilts slightly further toward the surface normal; that is, that the position of the lower part of the surfactant is not affected by the presence of the oil monolayer suggests that this part of the layer will not change significantly in the presence of a bulk oil phase. These measurements confirm the earlier conclusion for c14TAB that mixed monolayers are formed when dodecane lenses are added to the surface of aqueous solutions of both C12TAB and C16TAB. As stated in the previous paper,14 this is an important result because surface tension measurements alone
J. Phys. Chem., Vol. 99, No. 12, 1995 4123
Dodecyl- and Hexadecyltrimethylaonium Bromide cannot distinguish between mixed monolayer formation and the presence of a multilayer oil film coexisting in equilibrium with lenses of oil. A further conclusion is that the oil must be distributed evenly over the surface at least on the scale of the in-plane coherence length of the neutrons (about 5000 A at the resolution used). Although this has not been explicitly demonstrated in the text, it follows from two observations. The most important is the similarity between the coverages obtained from neutron reflection and from surface tension measurements. If the oil were separated into islands greater than the in-plane coherence length for neutrons, the reflectivity would be the average of the reflectivity from oil-covered and oil-free regions. This is different from the reflectivity from the same amount of oil distributed evenly over the surface because the reflectivity depends on the square of the scattering length density of the layer. In the analysis of the reflectivity data in the present paper the layer has been assumed to be uniform. If it were not uniform, our analysis would have overestimated the coverage relative to the surface tension measurement. In fact, in the case of C12TAB there is excellent agreement between the two, and in the case of Cl6TAB the discrepancy is in the opposite direction and can only be explained by experimental rather than interpretative error. The second reason for believing that the oil is uniformly distributed is similar in that it is impossible to fit the reflectivity data for the set of different isotopic species to a model where the reflectivity is an average of two differently reflecting parts of the surface.
Acknowledgment. We thank the Science and Engineering Council for their support of the project. References and Notes (1) Aveyard, R.; B a s , B. P.; Cooper, P.; Fletcher, P. D. I. Adv. CoZloid Interface Sci. 1990,33, 59. (2) Aveyard, R.; Cooper, P.; Fletcher, P. D. I. J. Chem. SOC., Faraday Trans. 1990,86, 3623. (3) Binks, B. P.; Kellay, H.; Meunier, J. Europhys. Lett. 1991,16, 53. (4) Lu, J. R.; Thomas, R. K.; Aveyard R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Sokolowski, A.; Penfold, J. J . Phys. Chem. 1992,96,
10971. (5) Simister, E. A,; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.: Coomr. P.: Fletcher. P. D. I.: Lu. J. R.: Sokolowski. A. J . Phvs. Chem. 1992,96, 1383. (6) Lee. E. M.; Thomas, R. K.;Penfold, J.: Ward, R. C. J. Phvs. Chem. 1989,93, 381. (7) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993,97, 6024. (8) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J . Phys. Chem. 1992,96, 1373. (9) Lu, J. R.; Simister, E. A.; Lee, E. M.; Thomas, R. K.; Rennie, A. R.; Penfold, J. Lanamuir 1992,8, 1837. (10) Simister, ETA.; Lee, E. M.; Thomas, R. K.; Penfold, J. Macromol. Rep. 1992,A29, 155. (11) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Condens. Matter 1994,6 , 1. (12) Lu, J. R.; Hromadova, M.; Simister, E. A.; Thomas, R. K. J . Phys. Chem. 1994,98, 11519. (13) Lu, J. R.; Lyttle, D.; Hromadova, M.; Penfold, J. Lungmuir, in press. (14) Tanfold, C. J. J. Phys. Chem. 1972,76, 3020. (15) Sears, V. F. Neutron News 1992,3, 26. JF'941392X
