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Surface Ordering in Dilute Dihexadecyl Dimethyl Ammonium Bromide Solutions at the Air-Water Interface J. Penfold,*,† D. S. Sivia,† E. Staples,† I. Tucker,‡ and R. K. Thomas§ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, United Kingdom, Unilever Research and Development, Port Sunlight, Quarry Road East, Bebington, Wirral, United Kingdom, and Physical and Theoretical Chemistry, Oxford University, South Parks Road, Oxford, United Kingdom Received August 6, 2003. In Final Form: December 11, 2003 At elevated temperatures and in dilute solution, we have observed lamellar surface ordering at the air-water interface of dihexadecyl dimethylammonium bromide, DHDAB, in the presence of electrolyte. With increasing temperature, the onset in ordering is observed between 35 and 40 °C. At 40 °C, there is an abrupt change in the lamellar spacing, from ∼33 to ∼40 Å. Furthermore, in the presence of the cosurfactant benzyl alcohol, the ordering occurs at a lower temperature, between 20 and 25 °C. The change in lamellar spacing with temperature is attributed to a surface-induced transition, similar to the Lβ to LR phase transition observed in bulk lamellar dispersions.
Introduction Self-assembly and ordering at interfaces offer the potential to manipulate surface properties, which extends the opportunities and capabilities afforded by simple Gibbs monolayers; and has implications for the control of wetting, for stabilization, for the production of functionalized surfaces, and in lubrication. Surface or interfacially induced ordering and the adsorption of an ordered lyotropic mesophase have now been observed in a variety of different circumstances at the air-water, liquid-solid, and liquid-liquid interfaces. X-ray and neutron reflectivity have been extensively used to characterize and investigate such surface-induced ordering. At low concentrations, surfactant adsorption at the airwater interface is characterized by a simple Gibbs monolayer,1 whereas at the liquid-solid interface the adsorption is cooperative and is in the form of discreet aggregates for concentrations above the critical micellar concentration, cmc.2,3 At the air-water interface, aggregate adsorption at lower concentrations is generally not observed in simple low molecular weight surfactants, but sub-cmc adsorption of micellar-like aggregates is observed in a range of polymeric surfactants.4,5 A more recent example of this is the surface-induced ordering of triblock copolymer micelles that has been reported by Gerstenberg et al.6 A recently reported exception to this general observation regarding small-molecule surfactants is the unusual adsorption observed in gemini surfactants,7 †
ISIS Facility, Rutherford Appleton Laboratory. Unilever Research and Development, Port Sunlight. § Physical and Theoretical Chemistry, Oxford University. ‡
(1) Lu, J. R.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143. (2) Lee, E. M.; Thomas, R. K.; Cummins, P. G.; Staples, E.; Penfold, J.; Rennie, A. R. Chem. Phys. Lett. 1989, 162, 196. (3) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219. (4) An, S. W.; Su, T. J.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, J. P.; Penfold, J. J. Phys. Chem. B 1998, 102, 387. (5) Dewhurst, P. F.; Lovell, M. R.; Jones, J. L.; Richards, R. W.; Webster, J. R. P. Macromolecules 1998, 31, 7851. (6) Gerstenberger, M. C.; Pedersen, J. S.; Smith, G. S. Phys. Rev. E 1998, 58 (6), 8028. (7) Li, Z. X.; Dong, C. C.; Wang, J. B.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 6614.
where at low surfactant concentrations (∼cmc) an adsorbed sublayer beneath the surface monolayer is observed and is thought to be associated with premicellar aggregation. Surface-induced ordering and more complex adsorption than a single monolayer have been observed in surfactant systems at interfaces, but generally at much higher surfactant concentrations. At these higher surfactant concentrations, Lee et al.8,9 reported micellar ordering adjacent to the surfactant monolayer at the air-water interface for the cationic surfactant tetradecyl trimethylammonium bromide, C14TAB. Smit et al.10,11 subsequently demonstrated such surface micellar ordering in their pioneering mesoscale computer simulations. Hamilton et al.12,13 reported highly ordered shear-induced structures at the liquid-solid interface for the viscoelastic surfactant mixture of cetyl trimethylammonium, 3,5-dichlorobenzoate and bromide, where a near surface hexagonal structure of aligned rods was observed. Multilayered lamellar structures have been reported at the air-solution and liquid-solid interfaces for AerosolOT, AOT, solutions14-16 and for the anionic/nonionic mixtures of sodium dodecyl sulfate, SDS, and pentaethylene monododecyl ether, C12E5.17 Of particularly direct relevance to this paper is the observation of stable layered structures at the silicon-water and air-water interfaces (8) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (9) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 13907. (10) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; Van Os, N. M.; Schlijper, A. G. Nature 1990, 5848, 624. (11) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; Van Os, N. M.; Schlijper, A. G. J. Phys. Chem. 1991, 95, 6361. (12) Hamilton, W. A.; Butler, P. D.; Baker, S. M.; Smith, G. S.; Hayter, J. B.; Magid, L. J.; Pynn, R. Phys. Rev. Lett. 1994, 72, 2219. (13) Butler, P. D.; Hamilton, W. A.; Magid, L. J.; Hayter, J. B.; Slawecki, T. M.; Hammouda, B. Faraday Discuss. 1996, 104. (14) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. Faraday Discuss. 1996, 104, 117. (15) Li, Z. X.; Weller, A.; Thomas, R. K.; Rennie, A. R.; Webster, J. R. P.; Penfold, J.; Heenan, R. K.; Cubitt, R. J. Phys. Chem. B 1999, 103, 10800. (16) 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, 5858. (17) Salamat, G.; de Vries, R.; Kaler, E. W.; Satija, S.; Sung, L. P. Langmuir 2000, 16, 102.
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for N,N-didodecyl-N,N-dimethylammonium bromide (DDAB) and the corresponding diundecyl compound (DUDAB).18 The structures observed are consistent with an adsorbed lamellar phase or the adsorption of unilamellar vesicles. In general, these surface-induced layered structures are observed for solution concentrations in the range 0.2-5 wt % (>10 mM) and have lamellar spacings ranging from 100 to 1500 Å (depending on the particular system and the specific surfactant and solution conditions). The observations reported in this paper arise from our interest in the adsorption behavior of these dichain cationic surfactants and of their mixture with nonionic surfactants;19 and their role in manipulating surface properties, such as wetting and lubrication, in applications such as detergency and fabric conditioning. We report here the observation of ordered layered lamellar structures at the air-water interface for dihexadecyl dimethylammonium bromide (DHDAB) at low surfactant concentrations and in the presence of electrolyte. Although the system is related to those discussed in the previous paragraphs, the measurements are made in a different concentration regime and the observed structures arise from a different adsorption mechanism. In particular, we report the formation of ordered structures at the interface at surfactant concentrations between 10 and 100 times lower than is normally observed. Experimental Details The neutron reflection measurements were made on the SURF reflectometer20 at the ISIS pulsed neutron source. The measurements were made using the white beam time-of-flight method at an angle of incidence, θ, of 1.5° and using the wavelength, λ, range 0.5-6.7 Å to cover the q range of 0.048-0.35 Å-1 (the scattering vector, q, is defined as q ) 4π sin θ/λ). The specular reflectivity measurements were made using a single detector, and the off-specular measurements using a two-dimensional multidetector. The details of the measurement protocol and data normalization are described elsewhere.21 The alkyl chain deuterium labeled DHDAB, d-DHDAB, was synthesized at Oxford, as described previously.18 High-purity water (Elga Ultrapure) was used throughout, and the D2O was obtained from Aldrich. The poly(tetrafluoroethylene) (PTFE) troughs, used for the reflectivity measurements, and all glassware were cleaned in alkaline detergent (DECON 90), followed by copious washing in ultrapure water. The samples were prepared by dilution into the appropriate solvent and heated to 70 °C for 20 min to achieve a homogeneous dispersion. The visual appearance is of a solution with a faint hue, no evidence of undissolved surfactant, and no precipitation within 12 h. Measurements were made at the air-solution interface for a surfactant concentration of 3 × 10-4 M d-DHDAB in null reflecting water, nrw (a 92 vol % H2O/8 vol % D2O mixture, with a scattering length density or refractive index the same as that of air). Measurements were made in the temperature range 2550 °C (thermostated to (0.5 °C). Measurements were made in pure water, in 0.01, 0.05, and 0.1 M KBr, and for the addition of 1 g/L benzyl alcohol in 0.1 M KBr. To encapsulate the kinetics of the adsorption and the evolution of the surface structure, relatively short measurement times were used (∼15 min). The relatively broad q range covered simultaneously, using the white beam time-of-flight method, is well matched to that requirement. Measurements over an extended q range when the surface was at equilibrium (but not reported here) do not substantially improve the interpretation of the data. The specular reflectivity is related to the Fourier transform of the scattering length density (or refractive index) distribution (18) McGillivray, D. J.; Thomas, R. K.; Rennie, A. R.; Penfold, J.; Sivia, D. S. Langmuir 2003, 19, 7719. (19) Penfold, J.; Staples, E.; Tucker, I.; Thomas, R. K. Langmuir, submitted. (20) Penfold, J.; et al. J. Chem. Soc., Faraday Trans. 1999, 93, 3899. (21) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 581.
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Figure 1. Specular reflectivity for 3 × 10-4 M d-DHDAB/nrw at 25 °C for t0 (b), t0 + 40 min (2), and t0 + 180 min (9). The solid lines are model fits to the reflectivity data for a single thin layer of uniform density (as described in the text, and for the parameters in Table 1). in a direction (z) normal to the surface or interface, such that
R(q) )
16π2 | q2
∫F(z) exp -iqz dz|
2
(1)
where F(z) is the scattering length density distribution, F(z) ) ∑iNi(z)bi, and Ni and bi are the number density and scattering length of species i. q (or qz) is the scattering vector normal to the plane of the surface, as defined earlier in this section. For a deuterium labeled surfactant in nrw, it has been shown1 that the reflectivity arises only from the adsorbed surface layer. In the simplest case of a single monolayer, this can be analyzed as a layer of uniform composition to estimate the adsorbed amount,
F ) b/τA
(2)
where b is the scattering length of the adsorbed molecule, A is the area/molecule, and F and τ are the scattering length density and thickness of the adsorbed layer obtained from a model fit to the data. For more complex surface structures, the variation of reflectivity with q can be analyzed (usually through modeling) to yield a surface structure. For the layered structures presented here, this has been achieved using a maximum entropy reconstruction.22
Results and Discussion In the absence of electrolyte, the reflectivity measurements for 3 × 10-4 M d-DHDAB/nrw are consistent with a simple monolayer with a mean thickness in the range 18-24 Å, see Figure 1 and Table 1. Figure 1 shows the reflectivity at 25 °C at three different times (as indicated in Table 1). In this q range, it has been shown1 for a wide range of surfactant systems that the reflectivity can be reliably analyzed as described in the experimental details as a single layer of uniform composition to obtain a thickness and area/molecule. In broad terms, in this q range the slope of the reflectivity is related to the thickness of the layer, and the mean level to the adsorbed amount; and this is illustrated in Figure 1. The results for the measurements of the surfactant in the absence of electrolyte are summarized in Table 1. With increasing temperature from 25 to 50 °C, there is within error no variation in the area/molecule (or adsorbed amount). The initial measurements at 25 °C (for a freshly (22) Geoghegan, M.; Jones, R. A. L.; Sivia, D. S.; Penfold, J.; Clough, A. S. Phys. Rev. E 1996, 63, 825.
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Table 1. Thickness and Area/Molecule of the Adsorbed Layer for 3 × 10-4 M d-DHDAB/nrw as a Function of Time and Temperature temp, °C
time, min
layer thickness, Å
area/molecule, Å2
25 25 25 35 40 45 50
t0a t0 + 40 t0 + 180
18.0 ( 1 20.0 23.0 24.0 24.0 24.0 25.0
116 ( 4 78 ( 2 62 61 64 66 67
a t is the initial measurement after the sample is loaded into the 0 trough. At the higher temperatures (35 °C and higher), measurements were made at intervals over a period of 60 min per sample, and no further variation with time was observed.
loaded sample) showed a variation in area/molecule, from ∼120 to 64 Å2 with increasing time. This time dependence of the adsorption has been previously reported19 as part of a study on DHDAB/nonionic surfactant mixtures, where it was shown that the time and concentration dependence of the adsorption was consistent with a Langmuir type isotherm. The characteristic time for adsorption was reported as >100 min, the long time scale being a consequence of the low cmc (∼5 × 10-6 M19) and hence low monomer concentration. At the higher temperatures, the adsorption was followed in time, but apart from the initial time dependence at 25 °C no further variation with time was observed. A similar behavior was observed in the presence of 0.01 and 0.05 M KBr and for the addition of 12 g/L benzyl alcohol, where at 45 °C the area/molecule was 63 Å2 in 0.01 M KBr, ∼58 Å2 in 0.05 M KBr, and ∼64 Å2 for the addition of benzyl alcohol. For all the measurements where the area/molecule was in the range 50-64 Å2, the thickness of the adsorbed layer was ∼24 ( 1 Å. The specular reflectivity measurements in electrolyte, 0.1 M KBr, show a rather different pattern of behavior. In the presence of 0.1 M KBr, the area/molecule is smaller at the lower temperatures, ∼54 Å2 (compared to 64 Å2 in the absence of electrolyte), but increases with increasing temperature (64 Å2 at 32 °C). For temperatures greater than 32 °C, the data are no longer consistent with a simple monolayer (see Figure 2). Between 35 and 40 °C, the reflectivity changes markedly and a pronounced interference peak, indicative of a layered structure at the interface, appears and grows in intensity with time and increasing temperature. The peak initially appears at q ∼ 0.19 Å-1 (lamellar spacing ∼ 33 Å). At the slightly higher temperature, a second peak appears at a lower q value, ∼0.16 Å-1 (larger lamellar spacing ∼ 39 Å), and with increasing time the intensity of the lower q peak grows at the expense of the peak at higher q. The growth of the peak at 0.16 Å-1 is accompanied by the appearance of a second-order peak at a q ∼ 0.32 Å-1. Some increase in the intensity of the Bragg peak is observed in the temperature range 45-55 °C, and although the occurrence and position in q of the feature are reproducible some variation in the actual temperatures and evolution with temperature is encountered. The width of the Bragg peaks observed is dominated by the instrumental resolution, ∆q/q, which is ∼5%. The absence of secondary interference peaks adjacent to the Bragg peaks is consistent with an extended layered structure at the interface. The relative intensities of the first and second-order Bragg peaks provide a measure of the diffuse nature of the interfaces between the layers. The qualitative observations are confirmed by a quantitative analysis of the reflectivity data measured at 45 °C using the Bayesian reconstruction method.21 This is
Figure 2. Specular reflectivity for 3 × 10-4 M d-DHDAB/nrw/ 0.1 M KBr (a) at 25 and 35 °C for different times ((yellow) at 25 °C; (green, blue, and red) at 35 °C with increasing time at 20 min intervals) and (b) at 40 °C (green, blue, and red) for increasing times, at 20 min intervals.
consistent with a highly ordered multilayered structure, extending ∼1500 Å from the interface into the bulk solution (see Figure 3), with a layer spacing of ∼40 Å and diffuse interfaces. In broad terms, the definition of the layering decreases with increasing distance from the surface, as indicated by the variation in the amplitude of the oscillations in the scattering length density. These values imply the possibility of lateral inhomogeneities, an increasingly incomplete lateral coverage of the lamellar layers, and some variation in the distribution of orientations of the lamellae. That is, the layers become increasingly less well defined and more disordered. Similar patterns were observed for AOT14-16 and for DDAB and DUDAB,18 but with a much larger lamellar layer spacing. There was some discussion about the unusually long length scale of the layering, and it was tentatively ascribed to the adsorption of liposomes. For AOT,14-16 it was clearly associated with lamellar ordering. This distinction is consistent with the bulk phase behavior observed in these systems. The bulk solution microstructure for DHDAB19,22-26 at low surfactant concentrations is consistent with the coexistence of micelles and lamellae. (23) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1357. (24) Soubiran, L. Unpublished results. (25) Hass, S.; Hoffmann, H.; Thuinig, C.; Hoinkis, E. Colloid Polym. Sci. 1999, 277, 856. (26) Brady, J. E.; Evans, D. F.; Warr, G. C.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853.
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Figure 4. Scattering intensity for 3 × 10-4 M d-DHDAB/nrw/ 0.1 M KBr at 40 °C, showing the specular and off-specular scattering as a function of the scattering vectors qz (orthogonal to the plane of the sample) and qx (in the plane of the sample).
Figure 3. Analysis of specular reflectivity for 3 × 10-4 M d-DHDAB/nrw/0.1 M KBr at 40 °C: (a) specular reflectivity vs scattering vector, q; (b) specular reflectivity × q4 as a function of scattering vector, q; and (c) scattering length density distribution from Bayesian reconstruction analysis of the reflectivity data (see text for description). The solid line in panels a and b is the model fit to the data for the scattering length density distribution in panel c.
A marked feature of the AOT data14-16 was the appearance of pronounced off-specular scattering, that was attributed to conformal roughness in the lamellar stacks. In view of this and the evident imperfections discussed above, measurements were also made at 40 °C for 3 × 10-4 M d-DHDAB/nrw/0.1 M KBr, using the multidetector (see Figure 4), to establish the amount of diffuse scattering and to confirm that the features in the specular reflectivity arise from the surface region. The measurements using the multidetector provide a map of the scattering in the qz-qx plane (where qx is the scattering vector in the plane of the sample and is defined as qx ) 2π/λ(cos θ - cos θ1), θ is the angle of incidence as defined in the definition of qz, θ1 is the scattering angle, and for specular reflectivity θ ) θ1). The in-plane q range (qx) and the corresponding length scale probed are much
longer (∼microns compared to 10-1000 Å in the z direction). Here the off-specular scattering provides information about inhomogeneities or structure in the plane but on length scales of ∼microns. The specular ridge and the Bragg peak (first order) are clearly visible. There is little evidence of scattering emanating from the horizon (which would be indicative of bulk scattering) or scattering around the specular ridge (which would be indicative of pronounced in-plane inhomogeneities). This confirms that the structure that is observed in the specular reflectivity is definitely a surface structure and that there is no pronounced in-plane distribution on the length scale of the measurements (∼microns). This is in marked contrast to what is observed for AOT14-16 and for DDAB (C12) and DUDAB (C10),18 and this implies a more rigid surface structure. In the specular reflectivity, the same broad pattern of behavior is observed for samples loaded initially at 55 °C and cooled to 35 °C. The Bragg peak q ∼ 0.16 Å-1 appears between the temperatures of 42 and 35 °C. The peak is not so pronounced as that formed on heating. This is not necessarily surprising as the kinetics associated with the heating and cooling cycles may be rather different. In the absence of electrolyte, the surface adsorption behaves in an entirely expected manner, in that at these low concentrations the adsorption is consistent with a simple monolayer. Consistent with other observations,19 the low cmc and hence monomer concentration in solution result in very slow adsorption kinetics. In the presence of electrolyte, 0.1 M KBr, the surface adsorption behavior, the observation of surface ordering at such low surfactant concentrations, is unusual and unexpected. Furthermore, the evolution of the surface structure with temperature is unusual and is not consistent with this being simply attributed to an electrolyte-induced precipitation. This would normally be expected to be most pronounced the lower the temperature, but here the surface structuring occurs at a temperature 10-15 °C above room temperature. Furthermore, during the time scale of these
Surface Ordering in Dilute DHDAB Solutions
measurements no bulk precipitation is observed. The abrupt variation in the lamellar spacing at 35-40 °C is tentatively attributed to a surface transition, a transition analgous to the bulk LR/Lβ phase transition, from a rigid interdigitated bilayer to a fluidlike bilayer, commonly observed in bulk lamellar dispersions.22 Hass et al.25 have determined the phase diagram for DHDAB, and Brady et al.26 report a LR/Lβ phase transition temperature of ∼40 °C. Cosurfactants, such as benzyl alcohol, are known to lower the LR/Lβ phase transition temperature.24 We have tested this, and the addition of 1 g/L benzyl alcohol reduces the temperature at which the Bragg peak at 0.16 Å-1 occurs in the reflectivity to a temperature less than 30 °C. This was observed during cooling from 55 °C, and consistent with the earlier measurements, the Bragg peak was not so pronounced as that observed during heating. The similarity between the transition temperature for the surface Bragg spacing change from 33 to 39 Å and the bulk LR/Lβ phase transition temperature and its reduction to lower temperatures with the addition of a cosurfactant are consistent with the transition being a surface LR/Lβ phase transition. The role of electrolyte on the surface ordering observed here at low surfactant concentrations is different to that encountered in related systems where ordering was observed at much higher surfactant concentrations.14-16 For AOT, the addition of even low electrolyte concentrations (0.1 mM) suppressed the pronounced surface ordering that was observed in the absence of electrolyte. In contrast, small-angle neutron scattering (SANS) measurements showed that for AOT the addition of electrolyte had little or no effect on the bulk lamellar structure. This was, in that case, attributed to the modification of curvature in favor of spherulite formation, which would not adsorb so strongly at the interface as the lamellar phase. Arguments about relative curvature are not so relevant here as the bulk phase will be a dilute lamellar dispersion, with a lesser tendency to form spherulites. At this lower surfactant concentration, it is postulated here that the role of the electrolyte is in reducing the effective
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area/molecule to the level at which the surface charge density is sufficient to induce surface ordering. The area/ molecule in 0.1 M KBr is ∼54 Å2, compared to 64 Å2 in the absence of electrolyte. The area/alkyl chain (27 Å2) in 0.1 M KBr is now comparable to the area/chain in lamellar phase bilayers, and the surface can now provide an effective template for lamellar ordering from that surface layer. Similar well-defined “Bragg” scattering has been observed with neutron and X-ray reflectivity in the surfactant-templated growth of silicate films at interfaces,27 induced by the action of tetraethoxysilane. Here a long induction period is followed by the formation of a layered structure at the interface and the subsequent formation of a multilayered hexagonal structure at the interface. Although the resulting profiles have some similarity, the mechanism of formation is somewhat different. Summary In the presence of electrolyte, surface-induced lamellar ordering is observed at low surfactant concentrations for the dialkyl chain cationic surfactant DHDAB. This occurs at a temperature well above room temperature and is hence not associated with electrolyte-induced precipitation. The abrupt change in the bilayer spacing observed with increasing temperature is attributed to a surfaceinduced phase transition, similar to the bulk LR/Lβ phase transition, and is shown to be manipulated with the addition of the cosurfactant benzyl alcohol. The important factor inducing the surface structure is associated with reducing the area/alkyl chain to a value comparable to that found in lamellar bilayers. Acknowledgment. The authors acknowledge the contribution of Jenny Tucker in the production of the Table of Contents graphic. LA035432C (27) Brown, A. S.; Holt, S. A.; Reynolds, P. A.; Penfold, J.; White, J. W. Langmuir 1998, 14, 5532.
