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Langmuir 2007, 23, 10140-10149

Equilibrium Surface Adsorption Behavior in Complex Anionic/ Nonionic Surfactant Mixtures J. Penfold,*,† R. K. Thomas,‡ C. C. Dong,‡ I. Tucker,§ K. Metcalfe,§ S. Golding,§ and I. Grillo| STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford, UnileVer Research and DeVelopment Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral, and Institute Laue LangeVin, 6 Rue Jules Horowitz, F-38042 Grenoble, Cedex 09, France ReceiVed April 19, 2007. In Final Form: July 2, 2007 Neutron reflectivity (NR) and small angle neutron scattering (SANS) have been used to investigate the equilibrium surface adsorption behavior and the solution microstructure of mixtures of the anionic surfactant sodium 6-dodecyl benzene-4 sulfonate (SDBS) with the nonionic surfactants monododecyl octaethylene glycol (C12EO8) and monododecyl triiscosaethylene glycol (C12EO23). In the SDBS/C12EO8 and SDBS/C12EO23 solutions, small globular mixed micelles are formed. However, the addition of Ca2+ ions to SDBS/C12EO8 results in a transition to a vesicle phase or a mixed vesicle/micellar phase for SDBS rich compositions. In contrast, this transition hardly exists for the SDBS/C12EO23 mixture, and occurs only in a narrow composition region which is rich in SDBS. The adsorption of the SDBS/C12EO8 mixture at the air-solution interface is in the form of a mixed monolayer, with a composition variation that is not consistent with ideal mixing. In water and in the presence of NaCl, the nonideality can be broadly accounted for by regular solution theory (RST). At solution compositions rich in SDBS, the addition of Ca2+ ions results in the formation of multilayer structures at the interface. The composition range over which multilayer formation exists depends upon the Ca2+ concentration added. In comparison, the addition of a simple monovalent electrolyte, NaCl, at the same ionic strength does not have the same impact upon the adsorption, and the surface structure remains as a monolayer. Correspondingly, in solution, the mixed surfactant aggregates remain as relatively small globular micelles. In the presence of Ca2+ counterions, the variation in surface composition with solution composition is not well described by RST over the entire composition range. Furthermore, the mixing behavior is not strongly correlated with variations in the solution microstructure, as observed in other related systems.

Introduction The widespread use of surfactants in the many applications of a range of home and personal care products usually involves mixtures.1,2 This is because they provide synergistic enhancements of many aspects of performance and behavior and greater flexibility in processing and formulation, and because many of the commercial surfactants are inherently mixtures due to the distribution of carbon chain lengths found in natural- and oleochemical raw material feedstocks. Recent developments in modern experimental techniques3,4 and in new theoretical treatments5,6 have considerably improved our understanding of surfactant mixing in micelles and at interfaces. In spite of this, there are many aspects and many systems that are either poorly understood or relatively unexplored, especially in situations where strong interactions occur and significant departures from ideality arise. Hence, the study of surfactant mixing remains a rich and interesting area of investigation. †

STFC, Rutherford Appleton Laboratory. Oxford University. § Unilever Research and Development Laboratory. | Institute Laue Langevin. ‡

(1) Scamehorn, J. F. In Mixed surfactant systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1992. (2) Rosen, M. In Phenomena in mixed surfactant systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1988. (3) Penfold, J.; Tucker, I.; Thomas, R. K.; Staples, E.; Schuermann, R. J. Phys. Chem. B 2005, 109, 10760. (4) Lu, J. R.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2000, 84, 143. (5) Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1992, 96, 5567. (6) Holland, P. M., Rubingh, D. N., Eds. Cationic surfactants; Surfactant Science Series; Marcel Dekker: New York, 1990; Vol. 37.

Linear alkyl benzene sulfonates, SDBS, are a particularly important class of anionic surfactants. They are one of the most commonly used commercial surfactants, and they usually exist as a mixture of alkyl chain homologues with a range of headgroup positional isomers. In applications, they are often used in combination with a range of different ethoxylated nonionic surfactants. In spite of their importance and widespread use, there is relatively little published on the fundamental properties of such surfactants and their mixtures with other surfactants; some notable exceptions are the recent paper by Ma et al.7 on the different SDBS isomers and the paper by Verma and Kumar8 on SDBS/nonionic surfactant mixtures. Although the role of the electrolyte in modifying surfactant adsorption and self-assembly and its effect on surfactant mixing have been extensively studied,3 the emphasis has been primarily on simple monovalent counterions. An exception to this is the study of the effect of the aromatic counterions, such as benzoate, salicylate, and tosylate,9 where the impact on self-assembly can be particularly striking. No less striking is the impact of multivalent counterions, as recently demonstrated by Alargova et al.10 Here, the strong binding triggers dramatic micellar morphology changes, but the corresponding impact on the surface adsorption behavior is not well established. In the context of detergency based applications, hard water (and hence Ca2+ and Mg2+ counterions) is known to have a profound effect upon performance. (7) Ma, J. G.; Boyd, B. J.; Drummond, C. J. Langmuir 2006, 22, 846. (8) Verma, S.; Kumar, V. V. J. Colloid Interface Sci. 1998, 207, 1. (9) Penfold, J.; Tucker, I.; Staples, E.; Thomas, R. K. Langmuir 2004, 20, 8054. (10) Alargova, R. G.; Petkov, J. T.; Petsev, D. N. J. Colloid Interface Sci. 2003, 261, 1.

10.1021/la701151m CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007

Surface and Solution BehaVior of SDBS Mixtures

Langmuir, Vol. 23, No. 20, 2007 10141

In this paper, we focus on the surface and solution behavior of mixtures of SDBS and the nonionic surfactants C12EO8 and C12EO23 and on the impact of NaCl and CaCl2 on that behavior. Predominantly, small angle neutron scattering (SANS) is used to determine the nature of the self-assembly in solution. Neutron reflectometry (NR) is used to determine the adsorption behavior at the air-solution interface, and some complementary surface tension measurements were also made to augment the scattering data. Experimental Details 1. Neutron Reflectivity. The specular neutron reflectivity measurements were made on the SURF reflectometer11 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, U.K. The measurements were made using a single detector at a fixed angle, θ, of 1.5° using neutron wavelengths, λ, in the range 0.5-6.8 Å to provide a wave vector transfer, Q, in the range of 0.048-0.5 Å-1, and using what are now well-established experimental procedures. The basis of a neutron reflectivity experiment is that the variation in specular reflection with Q (the wave vector transfer normal to the surface, defined as Q ) (4π/λ) sin θ, where λ is the neutron wavelength and θ is the grazing angle of incidence) is simply related to the composition or density profile in a direction normal to the interface. In the kinematic or Born approximation, it is just related to the square of the Fourier transform of the scattering length density profile, F(z)12 R(Q) )

|∫

16π2 Q2

|

F(z) e-iQz dz 2

(1)

where F(z) ) Σini(z)bi, ni(z) is the number density of the ith nucleus, and bi is its scattering length. The scattering length density or neutron refractive index profile (where the neutron refractive index is defined as n ) 1 - λ2F(z)/2π at the interface) can be manipulated using hydrogen (H)/deuterium (D) isotopic substitution (where H and D have vastly different scattering powers for neutrons). This has now been extensively used for the study of surfactant adsorption (for the determination of adsorbed amounts and surface structure) in a wide range of surfactants, surfactant mixtures, and polymer/surfactant mixtures,12 and the principles of its application are briefly described here. For a deuterated surfactant in null reflecting water, nrw (92 vol % H2O/8 vol % D2O has a scattering length of zero, the same as air), the reflectivity arises only from the adsorbed layer at the interface. This reflected signal can be analyzed in terms of the adsorbed amount at the interface and the thickness of the adsorbed layer. The most direct procedure for determining the surface concentration of surfactant is to assume that it is in the form of a single layer of homogeneous composition. The measured reflectivity can then be fitted by comparing it with a profile calculated using the optical matrix method for this simple structural model.13 The parameters obtained for such a model fit are the scattering length density, F, and the thickness, τ, of the layer. For a binary mixture, as studied here, the area per molecule of each component is calculated from F)

∑b /A τ + ∑b /A τ 1

1

2

2

(2)

where bi and Ai are the scattering lengths and area per molecule, respectively, of each component in the binary mixture. Making three different reflectivity measurements, with both surfactants deuterium (11) 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, A. R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Simister, E. A.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899. (12) Lu, J. R.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2000, 84, 143. (13) Penfold, J. In Neutron, X-ray, and light scattering; Lindner, P., Zemb, T., Eds.; Elsevier: New York, 1991.

labeled and with either of the two surfactants deuterium labeled, provides a self-consistent estimate of the surface composition, and a minimum requirement is two measurements with each component deuterium labeled in turn. For the neutron reflectivity data reported here, there are regions of surfactant concentration where the adsorbed layer is well described as a thin monolayer of uniform composition (and hence density) and regions where the surface structure is more complex. In these cases, the simplest model consistent with the data is used to describe the surface structure. Where a pronounced interference fringe is observed, two or three layers are mostly sufficient to describe the data. In cases where a pronounced Bragg peak is observed, this is indicative of more extensive multilayer formation at the interface. Here, a different approach is required to evaluate the surface structure. In the kinematic approximation, and following the approach of Tidswell et al.14 and Sinha et al.,15 the specular reflectivity for such a multilayer at the interface can be written as R(Q) )

|∑(F - F

16π2

N

Q4

i)0

i

i+1)

exp(-iQdi) exp(-Q2σi2/2)

|

2

(3)

where Fi is the scattering length density of the ith layer, i ) 0 represents the subphase, di is the distance of the interface between the ith and i+1th layers from the subphase, di ) ∑ili, li is the thickness of the ith layer, σi is the roughness between the ith and i+1th layers, F(N + 1) is the upper bulk phase (air), and N is the number of layers (N/2 is the number of bilayers). The definition of the bilayer structure with increasing depth is modified by an exponential decay constant, such that F(i) ) FN - ∆F exp(-di/η)

(4)

and η is a damping coefficient. The contribution from the surface monolayer in equilibrium with the multilayer structure is included in the form 16π2 (2F)2 sin(Qd/2)2 Q4

R(Q)

(5)

where d and F are the thickness and scattering length density of the monolayer, respectively, and the scattering length density of the subphase is assumed to be zero (nrw). Equations 3 and 5 are added together to provide the total reflectivity. 2. Small Angle Neutron Scattering. The SANS measurements were made on the LOQ diffractometer16 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory and on the D11 diffractometer 17 at the Institute Laue Langevin, Grenoble, France. The measurements on LOQ were made using the white beam timeof-flight method in the scattering vector, Q, range of 0.008-0.25 Å-1. The measurements on D11 were made using a wavelength of 6 Å (∆λ\λ ∼ 10%) and three different detector/collimation distance combinations (1.1/8, 5/8, and 16.5/16.5 m) to cover the Q range of ∼0.003-0.25 Å-1. The samples were contained in Starna 1 mm path length quartz spectrophotometer cells and maintained at a temperature of 25 °C. The data were corrected for background scattering, detector response, and the spectral distribution of the incident neutron beam, and were converted to an absolute scattering cross section (I(Q) in cm-1) using standard procedures.18,19 (14) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. ReV. B 1990, 41, 1111. (15) Sinha, S. K.; Sanyal, M. K.; Satija, S. K.; Majkrzak, C. F.; Neumann, D. A.; Homma, H.; Szpala, S.; Gibaud, H.; Morkov, H. Physica B 1994, 198, 72. (16) Heenan, R. K.; King, S. M.; Penfold, J. J. Appl. Crystallogr. 1997, 30, 1140. (17) Neutron beam facilities at the high flux reactor available for users, ILL, Grenoble, France, 1994. (18) Heenan, R. K.; King, S. M.; Osborn, R.; Stanley, H. B. RAL Internal Report, RAL-89-128, 1989. (19) Ghosh, R. E.; Egelhaaf, S. U.; Rennie, A. R. ILL Internal Report, ILL 98GH14T, 1998.

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Penfold et al.

In the micellar phase, the micelle structure is determined by analyzing the scattering data using a standard and well-established model for globular micelles.20 For a solution of globular polydisperse interacting particles (micelles), the scattered intensity can be written in the “decoupling approximation” 20 as I(Q) ) n[S(Q)|〈F(Q)〉Q|2 + 〈|F(Q)|2〉Q - |〈F(Q)〉Q|2]

(6)

where the averages denoted by 〈Q〉 are averages over particle size and orientation, n is the micelle number density, S(Q) is the structure factor, and F(Q) is the form factor. The micelle structure (form factor) is modeled using a standard “core and shell” model.20 The model comprises an inner core made up of the alkyl chains only and constrained to space fill a volume limited by a radius, R1, and the fully extended chain length of the surfactant, lc. For larger aggregation numbers, ν, volumes greater than that defined by R1 (as is found in this study) are accommodated by a prolate elliptical distortion with dimensions R1 and eR1 (where e is the elliptical ratio). The outer shell, of dimensions R2 and eR2, contains headgroups and the corresponding hydration water. Representative hydration values for the ethylene oxide (EO) headgroup, the cation, and the bound counterions are included as fixed values,20 and the modeling is not particularly sensitive to variations in hydration. The packing constraints include the measured or assumed micelle composition (see later discussion). From the known molecular volumes and neutron scattering lengths, the scattering length density (F) for the core, shell, and solvent can be estimated.20 For the mixed systems, the two surfactant components in the binary mixture are accommodated by assuming ideal mixing, which has been shown to be consistent with previous observations for concentrations well in excess of the mixed cmc.21 The interparticle interactions are included using the rescaled mean spherical approximation (RMSA) calculated for a repulsive screened Coulombic potential22,23 defined by the surface charge, z, the micelle number density, n, the micelle diameter, and the DebyeHuckel inverse screening length, κ.22 The model parameters refined are then ν, z, and e, and an acceptable model fit requires the shape of the scattering to be reproduced and the absolute value of the scattered intensity to be predicted to within (10%. The scale factor, f (the ratio of the measured intensity to model calculated value), shows the variation in the absolute scaling of the data fits. For mixtures containing components with a more complex geometry than the single alkyl chain, the simple constraints described above are not sufficient, and this was previously observed for the dialkyl chain cationic/nonionic surfactant mixtures.24,25 To accommodate disruption to the simple packing arguments, an additional model parameter, ext, is included. This allows a modification to the constraint that the inner dimension of the micelle, R1, is limited by the extended alkyl chain length, such that it can be greater or smaller than that value. It can be considered as a packing parameter that allows the partial molar volume in the core to change as a result of those packing constraints. Although much of the scattering data is in the micellar phase, in the presence of CaCl2, there are mixed phase (micellar/lamellar) and lamellar regions in the phase behavior. This data has not been analyzed quantitatively, but the form of the data has been used quantitatively to identify the form of the microstructure and distinguish between pure one component and mixed phase regions. 3. Materials and Measurements. SDBS, sodium 6-dodecyl benzene-4 sulfonate, that is, the 6-phenyl isomer (see Figure 1), was synthesized in two forms, with and without the dodecyl alkyl chain deuterium label (d-SDBS and h-SDBS, respectively). (20) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1032. (21) Staples, E.; Penfold, J.; Thompson, L.; Tucker, I.; Hines, J.; Thomas, R. K.; Lu, J. R. Langmuir 1995, 11, 2479. (22) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (23) Hayter, J. B.; Hansen, J. P. Mol. Phys. 1982, 42, 651. (24) Penfold, J.; Staples, E.; Ugazio, S.; Tucker, I.; Soubiran, L.; Hubbard, J.; Noro, M.; O’Malley, B.; Ferrante, A.; Ford, G.; Buron, H. J. Phys. Chem. B 2005, 109, 18107. (25) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2004, 20, 1269.

Figure 1. Representation of the molecular structure of the SDBS isomer, sodium 6-dodecyl benzene-4 sulfonate. The synthesis was performed in four steps. The two hydrocarbons were made using standard reactions: (i) Friedel-Craft reaction of pentanoyl chloride and bezene, (ii) addition of the Grignard reagent of n-bromoheptane to the product from (i), and (iii) simultaneous elimination and reduction of the product from (ii) using perchloric acid as the catalyst. The resulting hydrocarbons (protonated and perdeuterated) were purified by fractional distillation before sulfonation and purification following the method of Gray et al.26 Finally, the alkylbenzene was sulfonated by reaction with SO3. The purity of the intermediates was assessed by NMR and gas chromatography mass spectrometry (GCMS), but the final purity of SDBS was verified from surface tension and neutron reflectivity measurements. Surface tension data (measured on a Kruss K10T maximum pull digital tensiometer with a du Nouy ring) gave a cmc for SDBS of ∼2.0 mM, broadly consistent with other reported literature values,7,27 and the data showed no pronounced minimum that would be a clear signature of impurities. C12EO8 was obtained from Nikkol and used without further purification, and C12EO23 was obtained from Sigma and used as supplied. All the solutions for the SANS measurements were made in D2O, which was obtained from Fluorochem. The solutions for the neutron reflectivity measurements were made in nrw, and high purity water (Elga Ultrpure) was used with the D2O. Analar grade NaCl and CaCl2 were used for the solutions in electrolyte. All glassware and sample cells were cleaned using alkaline detergent (Decon 90), followed by copious washing in high purity water. On D11, the SANS measurements were made in 2 mM CaCl2 for surfactant concentrations of 2, 10, 15, and 25 mM for the SDBS/ C12EO mixture in D2O, in the composition range 100/0-30/70. On LOQ, the SANS measurements were made for SDBS/C12EO8 and SDBS/C12EO23 mixtures in D2O, 6 mM NaCl, and 2 mM CaCl2 at surfactant concentrations of 5, 10, and 25 mM and for compositions from 95/5 to 5/95. The neutron reflectivity measurements were made at the air-solution interface for 2 mM SDBS/C12EO8 mixtures in nrw, 6 mM NaCl, and 2 mM CaCl2 in the composition range 95/ 5-0/100, and for the isotopic combinations d-SDBS/h-C12EO8/nrw and h-SDBS/d-C12EO8/nrw. Further measurements were made for the isotopic combination of d-SDBS/h-C12EO8 in nrw at a surfactant concentration of 2 mM and for the composition range of 100/080/20. These measurements were made as a function of CaCl2 concentration for 0, 0.1, 0.25, 0.5, 1.0, and 2.0 mM CaCl2. Some additional limited measurements were made at higher electrolyte concentrations (100/0 and 85/15 d-SDBS/h-C12EO8 in 5 and 10 mM CaCl2 and 100:0 and 80:20 d-SDBS/h-C12EO8 in 15 and 30 mM NaCl).

Results and Discussion 1. Solution Microstructure. The solution microstructure of the SDBS/C12EO8 and SDBS/C12EO23 mixtures has been determined using data from SANS measurements (measured on both the LOQ and D11 diffractometers). Measurements were made for both mixtures at 5, 10, and 25 mM for compositions from 100/0 to 0/100 in D2O and in 6 mM NaCl (measured on the LOQ diffractometer) and at concentrations of 2, 10, 15, and 25 mM in 2 mM CaCl2 (measured on the D11 diffractometer). Some additional measurements in CaCl2 were also made on the LOQ diffractometer, but these were made predominantly in the micellar region (C12EO8 rich solution compositions). In D2O and in 6 mM NaCl, the data over the whole composition range are well described as arising from relatively small globular (26) Gray, F. W.; Gerecht, J. F.; Krems, I. J. J. Org. Chem. 1955, 20, 511. (27) Farquhar, K. D.; Misran, M.; Robinson, B. H.; Steytler, D. C.; Morini, P.; Garrett, P. R.; Holzwarth, J. F. J. Phys.: Condens. Matter 1996, 8, 9397.

Surface and Solution BehaVior of SDBS Mixtures

Figure 2. Scattered intensity, I(Q) (cm-1), as a function of wave vector transfer, Q (Å-1), for 25 mM SDBS/C12EO8 mixtures in D2O for (+) 100/0, (O) 95/5, (4) 80/20, (2) 50/50, (3) 20/80, (0) 5/95, and (b) 0/100 compositions. Solid lines are model fits to the data as described in the text and for the parameters in Table 1.

interacting mixed micelles. Typical SANS data for the SDBS/ C12EO8 mixture in D2O and in 6 mM NaCl are shown in Figure 2 and in Figure 1 in the Supporting Information. Similar data were obtained for the SDBS/C12EO23 mixture, and the key model parameters are included in Table 2 in the Supporting Information. In D2O, the data in Figure 1 for 100% C12EO8 is typical of that previously reported for relatively small globular nonionic surfactant micelles,3 and for 100% SDBS the scattering is strongly dominated by the intermicellar structure factor, S(Q), as observed in most charged surfactant micellar structures.3,20 As the composition changes from 100% SDBS to 100% C12EO8, the changes in the scattering reflect the reduction of the charge on the micelle and hence the contribution from S(Q). The changes in the scattering also reflect a modest increase in the micelle aggregation number or size from pure SDBS to pure C12EO8 micelles. The data for SDBS/C12EO8 in 6 mM NaCl are rather similar (see Figure 1 in the Supporting Information for more details) except that the addition of NaCl partially suppresses (or screens) the repulsive interactions between the micelles, and here the contribution from S(Q) is less pronounced. The data have been analyzed using the well-established model for small globular interacting micelles, as described in the earlier section on SANS, and the key model parameters are summarized in Table 1 and in Tables 1 and 2 in the Supporting Information. The variation in the aggregation number with solution composition for SDBS/C12EO8 and SDBS/C12EO23 is plotted in Figure 3 for solutions in D2O and in 6 mM NaCl. For the SDBS rich solutions, the aggregation number (∼30) is relatively small (in the range 20-40, depending upon the surfactant concentration), and it increases linearly as the mole fraction of the nonionic surfactant in solution increases. This increase is less pronounced for the SDBS/C12EO23 mixture than for the SDBS/C12EO8 mixture, reflecting the greater intrinsic curvature associated with C12EO23 compared to C12EO8. Similar, but less pronounced, trends were previously reported for SDS/ C12EO6 and SDS/C12EO8 mixtures;3 however, in that case, this was because the aggregation number for SDS is intrinsically larger than that for SDBS. The addition of NaCl results in only a modest increase in the micellar aggregation, and this is consistent with previous observations.3,20 Notably, its impact upon the nonionic rich compositions is minimal, and the most significant impact is observed for the SDBS rich compositions. This is similar to what was observed for the SDS/nonionic mixtures.3 The

Langmuir, Vol. 23, No. 20, 2007 10143

aggregation number reported here for C12EO8 is consistent with previously reported values.3 Caponetti et al.28 have compared the structure of linear SDBS with the same branched SDBS isomer as studied here using SANS. Although their model was constrained in a different way, the resulting aggregation numbers were comparable to those reported here for pure SDBS. They quoted an aggregation number of ∼45 for 70 mM SDBS, compared with a value of ∼40 measured at at 25 mM. Over the entire composition range for both SDBS/C12EO8 and SDBS/C12EO23, the core-shell model of globular interacting micelles, as described earlier, provides a good description of the scattering data. The molecular dimensions and packing constraints are consistent with an elliptical shape, where the elliptical ratio varies from ∼1.3 to 2.4, dependent upon the concentration and composition. The additional model parameter, ext, which allows for a disruption to the simple packing contraints for single alkyl chain surfactants, is included, as discussed earlier. Here, it varies from ∼1.0 to 1.4 from nonionic to SDBS rich micelles, and this range of values is similar to that reported for other related systems.24,25 However, its origin here is somewhat different, as there is uncertainty as to whether the phenyl ring of SDBS should be accommodated in the inner hydrocarbon core or located in the hydrophilic outer shell of the micelle. For the parameters in Table 1 and in Tables 1 and 2 in the Supporting Information, it was assumed to be in the outer shell. Alternatively, it can be located in the inner core of the micelle, and the lc value of SDBS is then increased by ∼3 Å to account for the phenyl ring. In this case, the parameter ext remains as ∼1.0 ( 0.05 over the entire composition range. However, importantly equivalent model fits were obtained, and the key model parameters (ν, z, R1, R2, and e) are within error, similar to those in Table 1 and in the Supporting Information. Hence, the variation in the parameter ext quoted above can be considered in this case as just reflecting the contribution from the phenyl ring. Goon et al.29 have used 1H NMR to identify aggregation induced conformational changes in several of the SDBS isomers and in commercial SDBS. Their results indicate that the phenyl ring is partially hydrated. That is, the solvent boundary is between the ortho- and metaprotons on the phenyl ring, and this is consistent with the interpretation of the SANS data discussed above. Their data also imply that the two alkyl chains experience dissimilar environments on micellization, consistent with one of the chains being closer to the micelle palisade layer. The modeling of the SANS data is not sufficiently sensitive to such detail, and so it can neither confirm nor refute such refinements. The Israelachvili, Mitchell, and Ninham packing parameter,30 imn () V/Al, where V is the alkyl chain molecular volume, l is the extended chain length, and A is the area per molecule), based on geometrical packing arguments has been highly effective in predicting self-assembly behavior in surfactant systems. The preferred morphology is spherical for imn < 1/3, elongated for 1/ < imn < 1/ , and planar for 1/ < imn < 1. Using typical 3 2 2 values for C12EO8 (V ) 327 Å3, l ) 16.7 Å, and A ) 60 Å2 (ref 3)), the imn value is ∼0.32, whereas for SDBS (V ) 327 Å3, l ) 8.9 Å, and A ) 65 Å2 (ref 7)) the imn value is ∼0.56 (note that, following the arguments in the previous paragraph, it does not matter if the phenyl ring is included in the hydrophobic core in this calculation, as V and l change by compensating amounts). For C12EO8, the value of 0.32 is at the spherical/elongated (28) Caponetti, E.; Triolo, R.; Ho, P. C.; Johnson, J. S.; Magid, L. J.; Butler, P.; Payne, K. A. J. Colloid Interface Sci. 1987, 116 (1), 200. (29) Goon, P.; Das, S.; Clemett, C. J.; Tiddy, G. J. T.; Kumar, V. V. Langmuir 1997, 13, 5577. (30) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans 2 1976, 72, 1525.

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Table 1. Model Parameters for SDBS/C12EO8 Mixed Micellesa solution composition (mol % SDBS)

n

z ((2)

R1 ( 1 (Å)

R2 ( 1 (Å)

e ((0.1)

ext ((0.1)

f

1.0 0.95 0.8 0.5 0.2 0.05 0.0

39 ( 3 41 47 63 86 ( 5 104 105

14 13 15 15 13 5 6

12 12 13 14 16 17 17

16.1 16.6 18.0 20.7 24.1 26.8 25.8

1.6 1.6 1.6 1.6 1.6 1.7 1.8

1.4 1.4 1.3 1.1 1.1 1.0 1.0

0.96 0.94 0.83 0.80 0.78 0.78 0.96

a 25 mM in D20. ν ) aggregation number; z ) surface charge; R1) inner shell radius; R2 ) outer shell radius; e ) elliptical ratio (Rlong/Rshort); ext ) R1/lc, where lc is the extended chain length of the surfactant; and f ) scale factor, i.e., ratio of the measured intensity to the model calculated value.

Figure 3. Variation in micelle aggregation number with solution composition for (a) SDBS/C12EO8 and (b) SDBS/C12EO23 in D2O and in 6 mM NaCl, as annotated in the figure.

boundary and this is broadly consistent with the observed micelle structure and aggregation number for the pure C12EO8 micelles. In contrast, the corresponding value for SDBS of 0.56 is at the boundary between elongated and planar structures. This is not consistent with the observed scattering data, which is consistent with relatively small globular micelles with an elliptical ratio of ∼2 and a relatively modest aggregation number. The value of A ∼ 65 Å2 is taken from surface tension7 and neutron reflectivity measurements at the air-solution interface. This implies that the packing within the micelle must be different from that at a planar interface, as normally a value of A from such measurements would provide a consistent estimate for the micelles. Hence, to be consistent with the scattering data and for the imn packing

Figure 4. Scattered intensity, I(Q) (cm-1), as a function of wave vector transfer, Q (Å-1), for SDBS/C12EO8/2 mM CaCl2 mixtures at (a) 25 mM and (b) 10 mM for solution compositions of (O) 50/50, (b) 60/40, (4) 70/30, (2) 80/20, (0) 90/10, (9) 95/5, and (+) 100/0. Each curve is shifted by ×2 for clarity.

parameter to be in the range 1/3-1/2, this implies that the effective area per molecule, A, must be in the range 73-110 Å2, that is, at least 12% greater. This could be a direct manifestation of the conformational changes reported by Goon et al.,29 resulting in a larger area per molecule in the micelle. In 2 mM CaCl2, the SANS data for SDBS/C12EO8 measured at 2, 10, 15, and 25 mM show a rather different behavior compared to that in D2O or in 6 mM NaCl. This is illustrated in Figure 4, which shows the SANS data for concentrations of 10 and 25 mM for the SDBS/C12EO8 mixtures at compositions of 50/50, 60/40, 70/30, 80/20, 90/10, 95/5, and 100% SDBS. At 25 mM (Figure 4a), the scattering for solution compositions of 70/30 and richer in C12EO8 is consistent with interacting globular micelles, broadly similar to those observed in D2O and

Surface and Solution BehaVior of SDBS Mixtures

in 6 mM NaCl. A concentration of 2 mM CaCl2 is the same equivalent ionic strength as 6 mM NaCl, and for the C12EO8 rich solutions it is not sufficient to promote any significant micellar growth. For solutions richer in SDBS than 70/30, the form of the scattering changes markedly, and there is a significant increase in the scattered intensity for Q values < 0.01 Å-1. The scattering in this region of Q has a Q-2 dependence, and it is attributable to a more planar geometry, and probably vesicles. Hence, between compositions of 100/0 and 80/20, there is a coexistence of vesicles (liposomes), Lv, and globular micelles, Lv/L1. At 10 mM (Figure 4b), the SANS data show some of the features seen at the higher concentration, but the evolution of the scattering with solution composition is different. At a composition of 50/50, the microstructure is micellar, L1, and between 60/40 and 80/20 it is in the form of coexisting Lv/L1. For solution compositions greater than 80/20, the solution microstructure is predominantly liposomal, Lv. At a concentration of 2 mM, the scattering is from micelles for compositions of 40/60 and richer in nonionic mixtures. At a composition of 80/20, the scattering has a Q-2 dependence and the microstructure is liposomal. For solutions richer in SDBS than 80/20, the scattering has a Q-4 dependence and a pronounced Bragg peak at higher Q values (∼0.2 Å-1, corresponding to a layer spacing of ∼32 Å); and in this region, the microstructure is in the form of large multilamellar aggregates or lamellar fragments. Ockelford et al.31 reported the existence of two coexisting lamellar phases for sodium 5-dodecyl benzene-4 sulfonate at relatively high surfactant concentrations (30-65 wt %), but coexisting lamellar phases are not observed here at the much lower concentrations studied. Richards et al.32 reported a “d spacing” in concentrated SDBS dispersions of 32.5 Å at low temperatures, and a range of coexisting lamellar components with larger “d spacings” (up to 45 Å) at high temperatures. For the SDBS/C12EO23 mixtures, SANS measurements in 2 mM CaCl2 were only made on the LOQ diffractometer. Although, in the micellar region, the Q range is sufficient to obtain a good description of the micelles, the limited Q range to lower Q enables only an approximate assignment of the microstructure for the SDBS rich compositions. However, what is evident from this more limited data is that the micellar phase persists to much richer SDBS compositions, such that the transition from L1 to more planar structures (probably Lv) occurs only for solution compositions in the region of 80/20-100/0. A summary of the quantitative analyses of the micellar regions for SDBS/C12EO8 and SDBS/C12EO23 in the presence of CaCl2 are summarized in Table 3 in the Supporting Information. The evolution of the microstructure for the SDBS/C12EO8 and SDBS/C12EO23 mixtures with solution composition and concentration in the presence of 2 mM CaCl2 are summarized as approximate phase diagrams in Figure 5. The addition of CaCl2 to the SDBS/nonionic mixtures results in a more complex phase behavior, with the transition from a micellar to a lamellar/vesicle phase at relatively low surfactant concentrations. For SDBS alone7,32,33 and for SDBS/C12EO6 mixtures,34 the phase behavior (in the absence of electrolyte) undergoes a transition from L1 to L1/LR and to LR at higher surfactant concentrations, typically >10 wt %, significantly higher than the observation here in the presence of CaCl2. The phase behavior reported here for both SDBS/C12EO8 and SDBS/C12EO23 has broad similarities with the trends observed in the dialkyl chain cationic/nonionic surfactant mixtures24,25 and in a range (31) Ockelford, J.; Timimi, B. A.; Narayan, K. S.; Tiddy, G. J. T. J. Phys. Chem. 1993, 97, 6767. (32) Richards, C.; Tiddy, G. J. T.; Casey, S. Langmuir 2007, 23, 467. (33) Brinkmann, U.; Neumann, E.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1998, 94, 1281.

Langmuir, Vol. 23, No. 20, 2007 10145

Figure 5. Phase diagram for (a) SDBS/C12EO8 and (b) SDBS/ C12EO23 in 2 mM CaCl2. The symbol (+) indicates points where SANS measurements have been made.

of phospholipid/nonionic mixtures.35 The results here are also broadly similar to those of Richards et al.34 for SDBS/C12EO6, although their studies focused on the temperature and concentration dependence of the phase behavior for only a 50/50 mixture. Brinkmann et al.33 and Farquhar et al.27 reported the formation of SDBS vesicles at higher electrolyte (50 mM NaCl) concentrations but at relatively low surfactant concentrations and the transition to mixed micelles with the addition of SDS. Lin and Fu36 reported spontaneous vesicle formation in SDBS solutions with the addition of Ca2+/Mg2+ counterions. For the SDBS/C12EO23 mixture, the micellar phase persists to solutions much richer in SDBS, and this can be attributed to the associated higher curvature of C12EO23 arising from its larger headgroup. Alargova et al.10 have shown that the strong binding of multivalent counterions can promote micellar growth and a transition to more planar structures in anionic and anionic/nonionic surfactant mixtures. In particular (and of relevance to this work), they showed that the impact of Al3+ ions on alkyl oxyethylene sulfate surfactants is different in the presence of C12EO4 and C12EO10. The effects are markedly reduced in the presence of C12EO10 compared to C12EO4, and this is attributed to the steric hindrance and the greater hydration of the larger EO10 group, preventing trimer formation. It would seem most likely that this type of effect is the origin of the difference between the behaviors of SDBS/C12EO8 and SDBS/C12EO23. (34) Richards, C.; Tiddy, G. J. T.; Casey, S. Colloids Surf., A 2006, 288, 103. (35) Almgren, M. Aust. J. Chem. 2003, 56, 458. (36) Lin, Z.; Fu, Y. C. J. Colloid Interface Sci. 1996, 184, 325.

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2. Surface Adsorption. Neutron reflectivity measurements were made for the SDBS/C12EO8 mixture in nrw, in 6 mM NaCl, and in 2 mM CaCl2 for the isotopic combinations d-SDBS/hC12EO8 and h-SDBS/d-C12EO8 at 2 mM surfactant for the compositions 95/5, 90/10, 80/20, 60/40, 40/60, and 20/80 and for the pure SDBS and C12EO8 components. In these measurements, the hydrogenous component is effectively matched to the solvent (nrw) and only the deuterated component contributes to the reflectivity. Apart from the SDBS/C12EO8 mixtures rich in SDBS (80 mol % and greater) in 2 mM CaCl2, all the other reflectivity data are consistent with a simple mixed monolayer at the interface. Some typical data for d-SDBS/h-C12EO8 in nrw for solution compositions from 95/5 to 20/80 are shown in Figure 2 in the Supporting Information. In the Q range for that data, the mean level of the reflectivity is related to the amount of surfactant at the interface and the slope to the thickness of the adsorbed layer. Hence, it is evident that, as the solution composition varies from 95/5 to 20/80, the amount of SDBS at the interface decreases but the thickness of the adsorbed layer, within error, is broadly constant. The data are analyzed as a single layer of uniform composition and distribution, as discussed in the Experimental Details section, to provide a layer thickness, d, and a scattering length density, F. Using eq 2 and in combination with the data for the alternative “contrast”, h-SDBS/d-C12EO8, the amount adsorbed of each component at the interface can be obtained, and the surface composition can be estimated. The key model parameters, adsorbed amounts, and compositions are summarized in Tables 4 and 5 in the Supporting Information. However, the variation in the surface composition with solution composition is shown in Figure 6a, and the variation in the total adsorbed amount is shown in Figure 6b. In nrw and in 6 mM NaCl, the variation in the total adsorption with solution composition is relatively small, and the addition of 6 mM NaCl has little impact on the adsorbed amount or its variation with composition. In 2 mM CaCl2, this is not the case, and although the impact of CaCl2 is negligible for the C12EO8 rich compositions, the total adsorption does start to increase significantly for solutions progressively richer in SDBS. From previous studies,3 the area per molecule for C12EO8 is ∼62 Å2, and it is consistent with the trends in the data measured here for an increasing mole fraction of C12EO8. The adsorption isotherm was measured for SDBS, and it is shown in Figure 3 in the Supporting Information. The limiting area per molecule is ∼57 ( 5 Å2, compared with a value of ∼67 Å2, obtained by surface tension.7 For solution compositions of 80/20 and those even richer in SDBS, the addition of CaCl2 results in a more complex adsorption pattern and adsorbed layer structure. This region was investigated as a function of CaCl2 concentration in more detail, and it is discussed later in this section. With the exception of that data, in Figure 6a, the variation in the surface composition with solution composition shows a marked departure from ideal mixing, which is progressively larger in 6 mM NaCl and 2 mM CaCl2, compared to the data in nrw. Regular solution theory6 is extensively used to treat nonideal mixing in mixed surfactant systems, where the departure from ideal mixing is encapsulated in a single interaction parameter, β. Hence, the cmc of the mixture, c*, can be expressed in terms of the cmc of the individual components, such that

(1 - R) R 1 ) + c* f1c1 f2c2

(7)

where c1 and c2 are the cmc’s of the two components, R is the

Figure 6. (a) Surface composition (mole fraction of C12EO8) as a function of solution composition (mole fraction of C12EO8) for 2 mM SDBS/C12EO8 in (blue) nrw, (red) 6 mM NaCl, and (green) 2 mM CaCl2. The lines are RST calculations for βs values of (long dashed line) -4, (short dashed line) -7, and (dotted line) -10 using the parameters described in the text. (b) Total surfactant adsorption (×10-10 mol cm-2) as a function of solution composition (mole fraction SDBS) for 2 mM SDBS/C12EO8 in (black) nrw, (red) 6 mM NaCl, and (blue) 2 mM CaCl2.

mole fraction of component 1 in solution, the activity coefficients are f1 ) exp β(1 - x)2 and f2 ) exp βx2, and x is the mole fraction of component 1 in micelles. Extensions of this approach provide access to the micelle and monomer concentrations, and, of particular relevance here, using the approach of Holland, it can be applied to the surface compositions, such that

π)

fixi RT ln + πimax Ai fsixsi

(8)

where π is the total surface pressure, Ai is the area per molecule of component i at the interface, πimax is the surface pressure of component i above the cmc, xi and xsi are the micelle and surface mole fractions of component i, and fi and fsi are the corresponding activity coefficients as defined above. Following the approach of Holland described briefly above, RST was used to estimate the variation in the surface composition, and this is shown also in Figure 6a. Calculations were made using the SDBS and C12EO8 cmc’s (2 × 10-3 and 9 × 10-5 M, respectively) and assuming mixed cmc’s of ∼10-4, 7 × 10-5, and 4 × 10-5 to give a surface

Surface and Solution BehaVior of SDBS Mixtures

Langmuir, Vol. 23, No. 20, 2007 10147

Figure 8. Neutron reflectivity as a function of wave vector transfer, Q (Å-1), for 2 mM d-SDBS/h-C12EO8 in 2 mM CaCl2 and nrw. Solid lines are model fits as described in the text and for the parameters in Table 3.

Figure 7. (a) Neutron reflectivity as a function of wave vector transfer, Q (Å-1), for 2 mM d-SDBS/h-C12EO8 in 1 mM CaCl2 for (black) 100/0, (red) 95/5, (green) 90/10, (blue) 85/15, and (magenta) 80/20 compositions. (b) Surface phase diagram for SDBS/C12EO8. The symbol (+) indicates points where the neutron reflectivity measurements have been made.

interaction parameter, βs, of ∼ -4, -7, and -10, respectively. The RST calculations for βs in the range -7 to -10 are broadly consistent with the variation in surface composition, measured by neutron reflectivity. Previous surface tension measurements37 on SDBS/C12EO7 and SDBS/C12EO30 mixtures provided a micelle interaction parameter, βm, of ∼ -7 to -8, and this is consistent with the βs value estimated from the surface adsorption data. Related data on the surface composition for the DHDAB/C12EO6 mixtures25 also show a marked departure from ideal mixing at the interface. For that mixture, the nonideality was not compatible with RST, and it was attributed to the change in monomer concentration and composition in solution associated with a change in the microstructure from the micellar to mixed micellar/lamellar (and ultimately lamellar) phase. Here, the predictions of RST for a relatively large negative interaction parameter correspond well to the data in nrw. The departure from the predictions of RST is increasingly larger in 6 mM NaCl and in 2 mM CaCl2. It is especially poor in 2 mM CaCl2 for solution compositions relatively rich in the nonionic surfactant C12EO8. However, this corresponds to the micellar region of the phase behavior, and there are no significant changes in the measurable micelle size or structure. There are few studies on the adsorption of SDBS/nonionic mixtures at interfaces reported in the literature, and so the scope for comparison with other work (37) Thorely, D. Private communication.

is relatively limited. Verma and Kumar8 reported measurements on oil-water interfacial tensions for SDBS/C12EO3 (C12EO7) mixtures in the context of oily soil removal, but no interpretation in terms of adsorption was presented. In the SDBS/C12EO8 composition range from 80/20 to 100/0 and in the presence of CaCl2, the surface structure is more complex and evolves from a simple monolayer to a bilayer/trilayer and ultimately to a multilayer at the interface. The reflectivity data (for the isotopic combination d-SDBS/h-C12EO8) in Figure 7a show this evolution in surface structure with the variation in solution composition from 80/20 to 100/0 in the presence of 1 mM CaCl2. For compositions of 80/20 and 85/15, the data are consistent with a simple monolayer, whereas for a composition of 90/10 a well defined interference fringe is formed. At solution compositions of 95/5 and 100% SDBS, a “Bragg” peak, associated with multilayer formation at the interface, is evident in the data. The evolution in the surface structure is summarized in Figure 7b, where the occurrence of surface monolayers, a single bilayer, or multiple bilayers (multilayers) is shown as a function of solution composition (mol % SDBS) and CaCl2 concentration (mM). This is, in essence, a surface phase diagram. Comparing this surface phase behavior (Figure 7b) with the solution phase behavior (Figure 5a) shows a rather close correspondence. The region where surface monolayer formation persists correlates with the solution micellar region, L1, whereas the region of multilayer formation at the interface coincides with the LR and LR/L1 regions in solution. A detailed quantitative analysis of the neutron reflectivity data has been made for SDBS rich SDBS/C12EO8 compositions in CaCl2. The data in the presence of a well defined “Bragg” peak have been analyzed either as a single monolayer, a single bilayer, or a single trilayer, as described earlier. The data showing well defined “Bragg” peaks have been analyzed assuming a multiple bilayer (multilayer) structure (as described by eqs 3-5). The key model parameters from the analysis of this data are summarized in Tables 2 and 3, and typical model fits are shown in Figure 8. A limited range of measurements were made at higher CaCl2 concentrations (for 100% SDBS and for SDBS/C12EO8 compositions of 85/15 and 80/20, measurements were also made in 5 and 10 mM CaCl2). At a composition of 80/20, consistent with the data at lower CaCl2 concentrations (e2 mM), only monolayer adsorption was observed, and the adsorbed layer thickness and

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Table 2. Key Model Parameters from Analyses of SDBS/C12EO8 Surface Adsorption in CaCl2 in the Composition Range 80/20-100/0 for a Single Monolayer, Bilayer, or Trilayer solution composition (mol % SDBS)

CaCl2 concentration (mM)

0.8 0.85 0.90 0.95 1.0

d ( 1 (Å)

F ( 0.1 (×10-6 Å-2)

dF (×10-5 Å-1)

A (Å2)

Γ ( 0.1 (×10-10 mol cm-2)

0.0

19 19 18 17 18

2.4 2.3 2.8 3.1 3.6

4.6 4.4 5.0 5.3 6.5

75 ( 3 78 68 65 54

2.2 2.1 2.4 2.6 3.1

0.8 0.85 0.9 0.95

0.1

19 18 20 16

3.6 2.9 3.0 2.7

5.8 5.6 5.4 5.0

59 61 64 68

2.8 2.7 2.6 2.4

0.8 0.85 0.9 0.95

0.25

20 18 17 19 29

2.7 3.1 3.2 3.8 0.9

5.4 5.4 5.6 7.3 2.6

63 63 61 47 133

2.6 2.6 2.7 4.8

0.8 0.85 0.9

0.5

19 18 17 27

2.8 3.0 3.6 0.5

5.2 5.6 6.0 1.2

65 62 57 283

2.6 2.7 3.5

0.8 0.85 0.8

1.0

19 18 22

2.8 3.2 2.6

5.4 5.7 5.5

64 60 62

2.6 2.8 2.7

2.0

Table 3. Key Model Parameters from Multilayer Analyses of SDBS/C12EO8 Surface Adsorption in CaCl2 in the Composition Range 80/20-100/0 multilayer parameters solution composition (mol % SDBS)

CaCl2 concentration (mM)

1.0 1.0 0.05 1.0 0.95 0.9 1.0 0.95 0.9

0.25 0.5 1.0 2.0

initial monolayer

d1 ( 0.5 (Å)

d2 ( 0.5 (Å)

F1 ( 0.2 (×10-6 Å-2)

F2 ( 0.2 (×10-6 Å-2)

N

η (Å)

d(2 (Å)

F ( 0.2 (×10-6 Å-2)

17 16 16 16 16 17 16 16 16.5

17 16 16 16 16 17 16 16 16.5

1.2 4.6 2.9 4.4 2.9 0.07 4.8 4.4 3.9

0.1 2.2 0.1 1.8 0.1 0.01 1.8 1.4 1.7

4 20 3 30 20 3 30 30 20

1000 2000 200 3000 2000 300 3000 3000 2000

45 45 45 45 45 45 45 45 45

2.8 1.5 2.1 1.7 2.3 2.7 1.5 1.5 1.1

adsorbed amounts were similar to those observed at 2 mM CaCl2. Similarly, for 100% SDBS and for the SDBS/C12EO8 composition of 85/15, the data exhibited a pronounced “Bragg” peak and were essentially identical to those measured at 2 mM CaCl2. Penfold et al.25 have previously reported the transition from monolayer to multilayer adsorption at the air-water interface for the dialkyl chain cationic surfactant DHDAB, which occurs with the addition of electrolyte (0.1 M KBr). At the relatively low level of NaCl added here (6 mM), no such salt induced effects are observed for SDBS or SDBS/C12EO8 mixtures. For 100% SDBS and an 80/20 SDBS/C12EO8 mixture, some measurements were also made at higher NaCl concentrations (15 and 30 mM). Only relatively modest increases in the adsorption were observed, and the adsorbed layer remained as a monolayer. Following the earlier discussions about the solution phase behavior in the presence of CaCl2, these results also highlight the importance of complex formation and the strong binding of multivalent counterions to the surface structure. It is that strong binding that is reducing the effective area per molecule such that multilayer formation at the interface is induced. The presence of C12EO8 increasingly disrupts that complexation due to the steric hindrance and associated hydration of the EO8 headgroups.

At compositions of 80/20 and richer in C12EO8, complexation is sufficiently disrupted that multilayer formation is increasingly difficult. 3. General Discussion. For the SDBS/nonionic surfactant mixtures in the presence of CaCl2, there is a transition in the solution microstructure from micellar, L1, to lamellar fragments/ liposomes, LR (Lv), at SDBS rich compositions. The composition at which this transition occurs depends strongly upon the nature of the nonionic surfactant. Here, we have contrasted the impact of C12EO8 and C12EO23, where the greater curvature associated with C12EO23 results in the transition being shifted to more SDBS rich compositions compared to what is observed for C12EO8. This contrasts markedly with what is observed in water and in NaCl. For the concentrations studied here, only relatively small globular micelles are observed over the entire composition range, and the addition of NaCl results in only modest micellar growth. Broadly similar phase behavior has been observed in other systems in the absence of electrolyte, notably in the dialkyl chain cationic/ nonionic surfactant mixtures24,25 and in a range of phospholipid/ surfactant mixtures.35 A similar or parallel behavior is observed at the air-water interface. In water and in NaCl, mixed monolayer adsorption is observed. The variation in the surface composition with solution composition is nonideal, and it is broadly consistent with regular

Surface and Solution BehaVior of SDBS Mixtures

solution theory. At SDBS rich compositions for the SDBS/C12EO8 mixture, the addition of CaCl2 results in a transition from a monolayer to a bilayer/trilayer structure at the interface and ultimately to multilayer formation. As the solution becomes richer in C12EO8, the onset of this transition from a monolayer to a more complex surface structure occurs at higher CaCl2 concentrations, and it was not observed for solution compositions of 80/20 and richer in C12EO8 in the SDBS/C12EO8 mixture. In the presence of CaCl2, the variation in the surface composition with solution composition was not well described by RST, and it showed a marked departure from the predictions of RST. This is not attributable to associated changes in the solution microstructure as was found for DHDAB/C12EO6.25 Multilayer formation at the interface has been observed in an increasingly wide range of other systems, such as lung surfactants38 and dialkyl chain surfactants39-41 (both in electrolyte and at higher solution concentrations), and in polyelectrolyte/ surfactant mixtures42 which showed a particularly strong surface interaction. In this study, it is the strong interaction or complexation of the divalent counterion which is driving the multilayer formation at relatively low surfactant (2 mM) and electrolyte (e2 mM) concentrations. This is not observed with simple monovalent electrolytes, where much higher concentrations are needed to induce such effects.25 Lamellar formation in solution and multilayer structures at the interface are generally more associated with much higher solution concentrations (approximately a few weight %), even for dialkyl chain surfactants, but their formation at such relatively low concentrations (2 mM) is unusual. The measurements made so far have been with one specific SDBS isomer (sodium 6-dodecyl benzene-4 sulfonate) and two different nonionic surfactants, C12EO8 and C12EO23, both of which have relatively high intrinsic curvatures. In commercial systems, SDBS is a mixture of different isomers, and studying the role of those different isomers in modifying this behavior will be important. Furthermore, it is evident that changing from C12EO23 to C12EO8 has a significant impact upon the solution properties and decreasing the EO chain length from EO23 to EO8 promotes a more planar structure for the SDBS rich compositions. However, C12EO8 is still a relatively high curvature surfactant (38) Follows, D.; Thomas, R. K. In preparation. (39) Penfold, J.; Sivia, D. S.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir 2004, 20, 2265. (40) McGillivray, D. J.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Sivia, D. S. Langmuir 2003, 19, 7719. (41) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Weller, A.; Penfold, J.; Webster, J. R. P.; Sivia, D. S.; Rennie, A. R. Langmuir 2001, 17, 5850. (42) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061.

Langmuir, Vol. 23, No. 20, 2007 10149

(on its own, it forms relatively small globular micelles in dilute solution). From these results, it is expected that multilayer formation at the interface can be further optimized by reducing the EO chain length using a nonionic cosurfactant with a more planar geometry and by reducing the alkyl chain length of the cosurfactant to more closely match that of SDBS. Furthermore, with such manipulation, the surface and solution properties can be tuned as a function of CaCl2 concentration.

Summary The mixing behavior of SDBS/nonionic (C12EO8 or C12EO23) surfactant mixtures in solution and at the air-water interface has been explored. In water and in NaCl, relatively small globular mixed micelles are observed in solution at modestly low surfactant concentrations. At the interface, under similar conditions, mixed monolayers are adsorbed, and the nonideal variation in surface composition with solution composition can be broadly described in terms of regular solution theory. In the presence of CaCl2 and for SDBS rich solution compositions, there is a transition from micelles, L1, to LR/Lv in solution and from a monolayer to multilayer formation at the interface. The solution conditions for the complementary surface and solution structural variations are strongly correlated. These surface and solution transitions are sensitive to the type of nonionic cosurfactant and the CaCl2 concentration. The transition is shifted to the SDBS richer solution compositions for the nonionic surfactant with the greater intrinsic curvature (C12EO23) and for lower CaCl2 concentrations. The variation in the surface composition with solution composition departs more markedly from the predictions of RST in CaCl2 than in water or NaCl. Unlike the surface structural changes observed for the SDBS rich compositions, the variations in the surface composition are not strongly correlated with any clear changes in the solution behavior. Acknowledgment. The support of the ISIS and ILL facilities is gratefully acknowledged. In addition, the authors wish to thank David Thorley for contributions to the surface tension measurements and Professor D. W. Thornthwaite for helpful discussions about SDBS synthesis. Supporting Information Available: Scattered intensity plot for 25 mM SDBS/C12EO8 mixtures, neutron reflectivity plot for 2 mM d-SDBS/h-C12EO8 in nrw, adsorption isotherm for SDBS, model paramters for SDBS/C12EO8 and SDBS/C12EO23 mixed micelles, and model parameters for neutron reflectivity data for SDBS/C12EO8. This material is available free of charge via the Internet at http://pubs.acs.org. LA701151M