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Ordered Structures of Dichain Cationic Surfactants at Interfaces† Duncan J. McGillivray and Robert K. Thomas* Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
Adrian R. Rennie Department of Chemistry, King’s College London, Strand, London WC2R 2LS, U.K.
Jeffrey Penfold and Devinder S. Sivia ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K. Received October 24, 2002. In Final Form: January 17, 2003 We have observed stable layered structures adsorbed at the silicon-water and air-water interfaces from N,N-didodecyl-N,N-dimethylammonium bromide surfactant (DDAB) and the corresponding diundecyl compound (DUDAB). The structures were studied by neutron reflection at the wet hydrophilic silicon interface, and at the air-water interface, at concentrations of 0.2-2% w/w. The structures formed are found to have a high sensitivity to temperature, with the repeat spacing decreasing approximately reversibly by about 50% with an increase of temperature of about 40 °C. Associated with the specular scattering there is pronounced off-specular scattering, indicative of a degree of lateral structure or dynamic fluctuation. The observations are consistent with two to four correlated layers which could arise from adsorbed monodisperse unilamellar vesicles, a lamellar phase, or a structured bicontinuous phase. Studies of the bulk solutions with X-ray scattering and cryo transmission electron microscopy show only coexisting unilamellar vesicles and lamellar phase.
Introduction When the packing parameter (pp), defined by Israelachvili et al. as pp ) v/al,1 approaches 1 (i.e., surfactant shape approaches cylindrical, with v ) volume of the surfactant, a ) area of the headgroup, and l ) length of the surfactant), the geometric rationale originally proposed by Tanford2 and developed by Israelachvili1 indicates that surfactants will adopt lamellar type aggregates in their aqueous solutions, rather than the spherical and elongated micellar forms which are observed for surfactants of lower pp. Lamellar aggregates may have a variety of morphologies, which include lamellar sheets, bicontinuous sponge phases, and lamellar vesicles. Vesicles are of particular interest in a number of fields, in particular in biochemical fields where stable vesicles are being investigated for their usefulness in drug delivery3 or as biological membrane models.4 Vesicles are well-known in phospholipid systems,5,6 where they are well characterized, but much recent research has focused on alternate surfactant systems which also produce these structures. Thus systems involving single-chain nonionic surfactants7,8 and mixtures * To whom all correspondence should be addressed. † Part of the Langmuir special issue dedicated to neutron reflectometry. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568. (2) Tanford, C. J. Phys. Chem. 1972, 76, 3020-3024. (3) Miller, A. D. Angew. Chem., Int. Ed. 1998, 37, 1769-1785. (4) Carmona-Ribeiro, A. M. Chem. Soc. Rev. 2001, 30, 241-247. (5) Bucher, P.; Fischer, A.; Luisi, P. L.; Oberholzer, T.; Walde, P. Langmuir 1998, 14, 2712-2721. (6) Rapuano, R.; Carmona-Ribeiro, A. M. J. Colloid Interface Sci. 2000, 226, 299-307. (7) Ma, G.; Barlow, D. J.; Lawrence, M. J.; Heenan, R. K.; Timmins, P. J. Phys. Chem. B 2000, 104, 9081-9085.
of cationic and anionic surfactants9-12 as well as copolymer surfactant species13,14 have all been shown to form vesicular solutions under appropriate conditions. In particular, pure solutions of dialkyldimethylammonium bromide (DAB) surfactants have been shown to form bulk vesicles in relatively dilute solutions.15-18 There is some debate about the long-term stability of vesicles formed from pure surfactants in bulk solution and about whether they form spontaneously or require an energy input (such as shaking, heating, or sonication).12,15 Nevertheless different solutions have produced both monodisperse and polydisperse vesicles, and these have been unilamellar and/or multilamellar. Such structures show at least longterm metastability and may be thermodynamically stable. The experiments reported here investigate a region of the surfactant/water phase diagram for didodecyl dimethylammonium bromide (DDAB) and diundecyl dimethylammonium bromide (DUDAB) in which lamellar structures are expected to be present; for DDAB unilamellar and multilamellar vesicles have been shown to (8) Le, T. D.; Olsson, U.; Mortensen, K. Phys. Chem. Chem. Phys. 2001, 3, 1310-1316. (9) Bergstrom, M.; Pedersen, J. S. Langmuir 1998, 14, 3754-3761. (10) Bergstrom, M.; Pedersen, J. S.; Schurtenberger, P.; Egelhaaf, S. U. J. Phys. Chem. B 1999, 103, 9888-9897. (11) Horbaschek, K.; Hoffmann, H.; Hao, J. C. J. Phys. Chem. B 2000, 104, 2781-2784. (12) Marques, E. F. Langmuir 2000, 16, 4798-4807. (13) Discher, B. M.; Won, Y. Y.; Ege, D. S.; Lee, J. C. M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143-1146. (14) Harris, J. K.; Rose, G. D.; Bruening, M. L. Langmuir 2002, 18, 5337-5342. (15) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352-1360. (16) Caboi, F.; Monduzzi, M. Langmuir 1996, 12, 3548-3556. (17) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. D.; Lindman, B. J. Phys. Chem. B 1999, 103, 8353-8363. (18) Haas, S.; Hoffmann, H.; Thunig, C.; Hoinkis, E. Colloid Polym. Sci. 1999, 277, 856-867.
10.1021/la026745v CCC: $25.00 © 2003 American Chemical Society Published on Web 03/19/2003
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be present in solutions of about 5% w/w.18 We present here observations from neutron reflectometry, which show a layered structure at the interface at concentrations in the range 0.2-2% w/w. The structure is ordered and stable and can be manipulated through the variation of the temperature of the solution. This structure also gives evidence of a degree of ordering in a plane parallel to the surface which gives rise to pronounced off-specular scattering. To complement the study of the interfacial structure, we have also examined the structure of aggregates in bulk solution using cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. Experimental Details Neutron reflectometry measurements were performed predominantly on the SURF reflectometer at the ISIS spallation neutron source, Rutherford Appleton Laboratory, Didcot U.K. This is a time-of-flight reflectometer, described in detail elsewhere,19 which is equipped with both linear and multidetectors. This enables measurements of both the specular and off-specular components of the reflected beam, with typical resolutions of ∆Q/Q e 5% (Q is the momentum transfer defined by Q ) 4π sin θ/λ, where 2θ is the angle through which neutrons of wavelength λ are scattered). This spectrometer has a horizontal geometry, making it suitable for solid-liquid and air-liquid interfaces. N,N-Didodecyl-N,N-dimethylammonium bromide, DDAB, was used as received from Aldrich (purity >98%). N,N-DiundecylN,N-dimethylammonium bromide was prepared by direct reaction of 1-bromoundecane and dimethylamine and was purified by recrystallization from acetone. Measurements were performed at concentrations between 0.2% (w/w with solvent) and 2% w/w. Solutions of the surfactant were sonicated when first made up to assist the dissolution of the surfactant, and after dissolution solutions were visually turbid. No phase separation was observed over a period of several months at room temperature for these solutions in D2O. For measurements made at the hydrophilic silica-water interface, a pure silicon crystal, cut to give a (111) surface, was lapped and polished in-house (3 µm diamond polish on a copper lapping plate, followed by polishing with 1 µm diamond polish and colloidal alumina on a polishing cloth). The surface was cleaned through sonication in ethanol for 20 min, followed by copious rinses with UHQ water. The crystal was then cleaned by immersion in a solution of concentrated H2SO4/H2O2/water (by volume in the ratio 4:1:5) at 80 °C for 15 min. The hydrophilic oxide layer that naturally forms on the Si(111) surface was characterized through neutron reflection in clean solutions of D2O and a mixture of H2O and D2O contrast-matched to the scattering length density (SLD) of the pure silicon crystal (2.07 × 10-6 Å-2), which we refer to as CMSi. A typical oxide layer determined by this characterization is a layer 12-15 Å thick, with an SLD of 3.4 × 10-6 Å-2 and an interfacial roughness of 5 Å. This surface layer was taken into consideration in modeling all reflectivity profiles from the silicon/silica/water interface. Temperature control during experiments was achieved by use of a water bath, stable to (4 °C, which circulated fluid around the aluminum housing of the block. The system temperatures measured were between 25 and 70 °C. Measurements were made at both 0.35° and 0.8° incident angles, giving an effective QZ range of between 0.01 and 0.08 Å-1, where QZ is the component of the momentum transfer normal to the interface. Typical measurements took around 1 h. To obtain measurements at different angles for air-water measurements, a neutron supermirror was used to deviate the neutron beam from its fixed 1.5° natural angle to the horizontal free liquid surface. The supermirror also had the effect of acting as a discriminator against short-wavelength neutrons, which meant that measurements were needed at 0.35°, 0.5°, and 0.8° (19) 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, 3899-3917.
McGillivray et al. to cover the same QZ range. Air-water measurements were made in two solvents, D2O and CM4 (which has an SLD of 4.0 × 10-6 Å-2, chosen because the step in the SLD from air to CM4 is approximately the same as the step between silicon and D2O). The range of temperature for air-water measurements is limited by problems of evaporation, one of which is changes in surface height, which is sufficient to affect alignment at the lowest angle of measurement. Hence, the air/water interface was studied only between 25 and 45 °C. The main source of background scattering in these measurements is the sample-dependent isotropic background caused by incoherent scattering from the sample, which is of the order of 10-6. The reflectivity from silicon/D2O drops to less than this background at a typical momentum transfer value g0.2 Å-1, but the background is not significant in the region of specular scattering with which we are most concerned, namely, QZ less than about 0.08 Å-1. Data modeling was performed using optical matrix methods, in which the interface region is modeled as a series of homogeneous layers. The layers are each characterized by thickness (τ/Å), SLD (F/Å-2), with an interfacial roughness between adjacent layers (σ/Å-1). The exact reflectivity can be calculated from such a model, which is then compared with the data, and the model is then refined using least-squares minimization routines. For a more detailed discussion of the modeling of neutron reflectometry data in general, refer to Li et al.20 In the modeling performed here the roughness was not varied freely but was fixed at appropriate values for different interfaces. Small-angle X-ray scattering was performed at beamline ID02 at the ESRF, Grenoble.21 Measurements were made in 1 mm capillary tubes in a housing heated by a circulating water bath, and at 1 and 10 m camera distances. Cryo-TEM measurements were performed on the instrument at the Centre for Chemistry and Chemical Engineering at Lund University, Sweden, on a 100 nm graphite fiber mesh, made negatively charged. DDAB solutions (2% w/w) were equilibrated to room temperature (ca. 20 °C) before being flash frozen in near freezing ethane and thereafter handled in liquid nitrogen. For more details on the cryo-TEM technique, see ref 22 for a good review.
Specular Scattering Figure 1 shows the specular reflection profile for a 2% w/w solution of DDAB at 65 °C, in D2O, both as an ordinary reflectivity profile and as (reflectivity × Q4) in which the differences from ordinary Fresnel scattering from a smooth interface are highlighted. The pattern shows a series of regularly spaced peaks, indicative of a repeating layer structure present at the interface. In specular reflection the change in momentum transfer is entirely perpendicular to the surface, and hence the specular profile gives information only about the density profile of the interface along the surface normal direction. In any finite structure containing a repeating motif, subsidiary peaks occur between the main diffraction features. The number of such peaks is n - 1 where n is the number of repeating units. Since one such subsidiary peak appears in the profile of Figure 1, we conclude that the structure can be approximately described as consisting of two more or less equally spaced layers at a separation of about 900 Å. This is borne out by fitting the data, for which we have used two approaches, the results of which are compared in Figure 2. Figure 2a shows the best fit using the maximum entropy method23 for which the structure (Figure 2b) shows two more or less equally well defined layers with a weaker (20) Li, Z. X.; Thirtle, P. N.; Weller, A.; Thomas, R. K.; Penfold, J.; Webster, J. R. P.; Rennie, A. R. Physica B 1998, 248, 171-183. (21) Technical information on the instrument is available through the Internet at http://www.esrf.fr/exp_facilities/ID2/handbook/ handbook.html. (22) Almgren, M.; Edwards, K.; Karlsson, G. Colloid Surf., A 2000, 174, 3-21. (23) Sivia, D. S.; Webster, J. R. P. Physica B 1998, 248, 327-337.
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Figure 1. Typical reflectivity profiles, here from a 2% w/w solution of DDAB in D2O at 65 °C: (a) the reflectivity (on a log scale) against the momentum transfer; (b) the reflectivity × Q4 against the momentum transfer (on a linear scale).
Figure 2. Alternative fits to the specular reflectivity profiles from 2% w/w solution of DDAB in D2O at 65 °C: (a) the maximum entropy method for which the derived SLD profile is shown in (b); (c) the fit of the simpler structure shown in (d) using the optical matrix method. The parameters of the optical matrix fit are given in Table 1. Table 1. Optical Matrix Fit Parameters for 2% w/w Solution of DDAB in D2O at 65 °C, Fitted as in Figure 2d layer SLD/Å-2 thickness/Å roughness/Å solvent/%
SiO2 silicon layer layer 1 layer 2 layer 3 layer 4 2.07 n/a 5.0 n/a
3.4 12.0 5.0 0
-0.2 690 40 100
-0.2 170 30 90
-0.2 630 40 100
-0.2 165 30 93
third one and traces of a fourth. In this fit the layers clearly become less well defined further from the surface. This fitting procedure is the least biased way of fitting such a set of data. The second fit (Figure 2c) uses the optical matrix method with just two layers and also gives a reasonable fit, although not as good as the more extended structure. The parameters of the optical matrix fit are given in Table 1. Note that the comparison of data on a (reflectivity × Q4) scale, as in Figure 2a, which eliminates the Fresnel component of the reflectivity, is a much more severe test than the comparison using reflectivity on a log scale, as in Figure 2c. The excellent maximum entropy fit indicates that we are observing a two to four layer structure adsorbed at the interface. We defer the discussion
of the detailed structure to the later discussion but note that the layers are both much broader than would be expected for simple bilayer lamellae and contain a high fraction of water. A possible explanation of such a structure is that the mean structure is a continuous, or nearly continuous, bilayer but that fluctuations of static disorder broaden it substantially. We discuss this further after considering other aspects of the system. Further evidence that the multilayer is adsorbed at the surface and that we are not merely observing small-angle scattering from the bulk structure is (i) that the spacing is different from its value in the bulk (see below), (ii) that there is off-specular scattering associated with the diffraction peaks (see below), and (iii) that there is a marked curvature in the off-specular scattering, which is caused by the difference in refractive index across the surface (see below). Temperature Variations The spacing of the layer structure shows a marked temperature dependence, as shown in Figure 3, with the layer spacing generally decreasing with increasing temperature. The structure has been found at all temperatures
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Figure 3. Specular reflectivity profiles from a 1% solution of DDAB in D2O heated from 25 to 70 °C and then recooled to 25 °C. The solid circles are measurements made on the heating cycle, while the hollow triangles are measurements made on the cooling cycle.
measured for both DDAB and DUDAB, although becoming weaker at lower temperature. We have not reached an upper limiting temperature. The variation of spacing of the adsorbed layer structure is similar to the behavior exhibited by the lamellar phase of a 2% solution of NaAOT at the silicon-water interface, where the spacing changes from 270 Å at 5 °C to 165 Å at 35 °C.24 The spacings given in Table 2 are, however, much larger than those obtained for NaAOT. Figure 3 also gives some indication of the degree of reversibility of the 1% DDAB system. The spacing of the layers seems to be completely reversible on heating (24) 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, 10800-10806.
Table 2. Layer Repeat Distances (in Å) with Temperaturea concentration
25 °C 30 °C 40 °C 47 °C 55 °C 65 °C 70 °C
DUDAB 0.2% 1520b 1350 1.0% 970 970 DDAB 0.2% n/a 1.0% 1600 1400 2.0%
1120 970 n/a 1140 1520
960 920
850 720
740 560
950
850 1000
700 850
580
a Temperature/concentration combinations marked with an “n/a” were measured but showed no or only very weak lamellar structure. Blank cells are combinations that have not been measured. b Weak.
and subsequent cooling except at the lowest temperature of 25 °C, but the intensity of the peaks drops in the cooling
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cycle, particularly that of the secondary peaks. At 25 °C the layer structure is largely lost in the cooling cycle but, intriguingly, small remnants are found at the spacing associated with the secondary peaks in the original structure. For DDAB, the temperature behavior of the surface adsorbed layer is significantly different than the layer structure found in the bulk solution (described below). The spacings are different and there are quite large differences in the spacings in the heating and cooling cycles. Off-Specular Scattering Figure 4 shows multidetector measurements for the 1% w/w DUDAB system in D2O and CMSi over a range of temperature. The plots show the scattering angle (equivalent to vertical position on the detector) against the wavelength of the incident neutron. The third dimension, intensity of signal, is given by color (note the nonlinear scale). The measurements show a strong specular ridge across the center of the pattern, where the angle of reflection is equal to the angle of incidence (here 0.35°). The plots of specular reflectivity shown (e.g., Figure 3) map the intensity along this ridge. The position of the critical edge, where total reflection occurs at the siliconwater interface, is clearly marked on the pattern by the drop-off in the intensity of the specular ridge and the increase in the off-specular background. Neutrons with a wavelength above the critical wavelength at this angle do not interact with the subphase, and hence the incoherent scattering from the solution, which gives rise to most of the isotropic off-specular background, does not play a role. It is also possible to see the effect of the beam stop used to protect against the transmitted beam and the small portion of the transmitted beam that misses the beam stop, at the bottom of the pattern. The most striking feature in the scattering patterns of Figure 4 is the relatively strong and structured off-specular scattering. Off-specular scattering arises when there is a change in the momentum transfer of the neutrons in a direction parallel to the surface. It normally accounts only for a very small proportion of the scattered beam, primarily from interfacial roughness, which for smooth surfaces is less than the isotropic background except in the Yoneda wing near total reflection. Pronounced and structured offspecular scattering has been observed in the past from highly ordered systems, such as diffraction gratings.25,26 In the present system the bands of off-specular scattering occur at a constant vertical momentum transfer, QZ, and over a range of QX values. The slight curvature of the bands in the D2O profiles is due to refraction effects and is not apparent for CMSi subphase measurements where there is no SLD difference between the silicon and the subphase. The off-specular scattering bands occur only at QZ values corresponding to the diffraction peaks in the specular scattering pattern and not with any of the intermediate secondary peaks. The intensity of the offspecular scattering is significantly less than that of the specular ridge but is clearly greater than the background. Along an off-specular scattering band there are no features within the resolution of the measurement, only a gradual decay away from the maximum at the specular ridge. The coincidence of the maximum intensity of the off-specular scattering with the specular ridge, the correspondence of (25) Richardson, R. M.; Webster, J. R. P.; Zarbakhsh, A. J. Appl. Crystallogr. 1997, 30, 943-947. (26) Ott, F.; Menelle, A.; Fermon, C.; Humbert, P. Physica B 2000, 283, 418-421.
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the strong scattering with constant QZ values, and the effects of refractive index are each inconsistent with the possibility that the off-specular scattering might be smallangle scattering from the bulk solution. The important feature of off-specular scattering that is different from bulk small angle scattering is that it is centered on the reflected beam rather than on the direct beam. The off-specular scattering closely resembles that observed for NaAOT. Following the analysis of the offspecular reflection of NaAOT presented by Li et al.,27 it must arise from some sort of variation in the structure parallel to the surface which is not so large that it destroys the correlation between the layers. This variation could be static or dynamic. Static structures that could produce both the observed specular reflection pattern and the strong off-specular bands are an adsorbed stack of approximately monodisperse vesicles or an adsorbed slice of sponge phase. Each would give a repeat distance along the surface normal and would also show enough variation in the lateral direction to give rise to strong off-specular scattering. Such static ordering might have a sufficiently well defined lateral repeat distance that it would generate a constructive interference peak in the off-specular scattering. The repeat distance would be approximately comparable to the vertical repeat distance (at most 1500 Å) and any interference peak would then correspond to a horizontal momentum transfer, QX, that would be, unfortunately, beyond the range of our measurements. The alternative to a static variation in the plane of the surface is the dynamical fluctuations caused by thermal undulations of individual layers in the adsorbed structure. The fluctuations of individual layers only need to be partially correlated to produce the observed effects, the off-specular scattering showing a more rapid decay in intensity away from the specular ridge the poorer the correlation. The interesting observation that the offspecular scattering is only apparent from the diffraction peaks could result either from the lack of correlation in fluctuations between the silica surface and the outermost layer or because there is no diffraction intensity to boost this scattering, as there is for the individual layers. In the case of NaAOT, we attributed the off-specular scattering to dynamical fluctuations of the individual lamellae in an adsorbed fragment of lamellar phase. In the DDAB and DUDAB systems, the structures observed in the bulk solution phase make such an attribution more speculative. Air-Water Interface Qualitatively the same general behavior was observed at the air-water interface, although the definition of both the diffraction peaks and the off-specular structure was not as good (see Figure 5). At the silica/water interface the effective surface is generated by a bilayer of DDAB adsorbed directly on the silica. The outer part of this layer, presented to the aqueous phase, is then a layer of DDAB headgroups. At the air/water interface there will be strong adsorption of a single monolayer and the surface presented to the underlying aqueous phase will also be the headgroups of the DDAB. The only differences are that the headgroup layer is likely to be more complete in the case of the air/water interface and that capillary waves in the free liquid surface may make the surface locally more rough than the silica-water interface. Another difference is that the silicon-water interface has a static roughness, while the air-water surface is roughened by thermal (27) 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, 5858-5864.
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Figure 4. Multidetector measurements for DUDAB, at 1.0% w/w in D2O (left) and CMSi (right). The temperatures for the three sets of measurements are 25, 55, and 65 °C from top to bottom. The specular ridge is marked in each case by a horizontal line, and on the first measurement the position of the critical edge and beamstop are also marked. Note there is no critical edge for the CMSi measurements.
fluctuations. However, the general similarity of the two surfaces makes it not surprising that there is qualitative similarity in the reflectivity, while the differing types and scales of roughness may be the reason for the apparent reduction in the order of the adsorbed structure.
Structure of the Bulk Solution Haas et al.18 have shown that in the region of 5% w/w the bulk solutions of the dialkyldimethylammonium DABs contain unilamellar and multilamellar vesicles, but the
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Figure 5. DDAB 1.0% w/w in CM4 (a mixture of H2O and D2O with a SLD of 4 × 10-6 Å-2) at 45 °C, measured at the air-water interface. The specular reflectivity (right) is a cross section taken from the multidetector measurement on the left. Note the much weaker off-specular scattering. Table 3. Lamellar Spacing (in Å) for a 1% w/w Solution of DDAB, Taken from Small-Angle X-ray Scattering from the Bulk Solution, Heated from 25 to 65 °C and Then Cooled Back to 40 °Ca heating cooling
45 °C
50 °C
55 °C
60 °C
65 °C
900 690
890 650
730 610
620 600
570
a Below 45 °C there were no peaks strong enough to assign a spacing.
Figure 6. X-ray small angle scattering from a 1% w/w solution of DDAB as the solution is heated to 65 °C. The data have been off-set from the 45 °C data for clarity.
authors gave no information about the internal spacing in the multilamellar vesicles. The presence of only unilamellar vesicles in the bulk solution would give some support to the adsorbed layer consisting of a stack of three or four unilamellar vesicles, especially if the unilamellar vesicles in bulk solution were monodisperse, as would be required in the adsorbed layer. Taking the adsorbed layer to be a layer of multilamellar vesicles does not seem to be a viable option because interference effects from the internal structure of the vesicle would be expected to be very prominent. However, an adsorbed layer consisting of a single layer of monodisperse multilamellar vesicles with an internal spacing corresponding to the observed diffraction peaks could explain the experimental observations if the vesicle were flattened sufficiently to generate the diffraction features of individual lamellae. The X-ray small-angle scattering from DDAB at different temperatures and on heating and cooling is shown in Figure 6. Diffraction peaks corresponding to spacings comparable with those observed for the surface layer are observed. However, the spacings are significantly smaller, the structures giving rise to the diffraction are apparently less well ordered than those at the surface, and the response to heating and cooling is not as reversible. These three observations, especially the first and last, confirm that the structure observed in neutron reflection does not arise from bulk small-angle scattering. We found it
impossible to model the small angle scattering in any simple way, but the values for the spacings associated with the diffraction peaks are easily extracted and are given in Table 3. The best we can conclude is that the small-angle scattering could arise from fragments of lamellar phase or from multilamellar vesicles. Two representative cryo-TEM images are shown for 2% w/w DDAB in Figure 7. Unilamellar vesicles are observed together with regions of lamellar phase. This confirms that the lamellar phase is not space-filling. In the lamellar regions of the images, adjacent bilayers are often connected and there is an alternation in the intensity of the interlayer space that suggests that the phase consists of layers of extended flattened vesicles (microtubules?). The widths of the flattened vesicles are mostly, though not always, similar to the separation between adjacent vesicles, which would have the result that any scattering experiment would effectively see a lamellar phase. These and intermediate features are observed in a large number of images. The results of cryo-TEM and small-angle X-ray scattering indicate that at these lower concentrations unilamellar vesicles and lamellar phase coexist, but there do not appear to be any multilamellar vesicles. Discussion and Conclusions The firm conclusions from the neutron reflection data are that, at the solid/liquid interface, there is an adsorbed layer of total thickness varying between about 2000 Å at the highest temperatures to about 5000 Å at the lowest temperatures and that this layer consists of between two and four repeating subunits. There is some partially correlated lateral structure in the subunits, which gives rise to the off-specular scattering, and the general level of scattering length density is consistent with the layer containing a low volume fraction of surfactant, although this is much higher than the mean volume fraction of
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Figure 7. Cryo-TEM of 2% w/w DDAB solutions frozen from ca. 20 °C on a graphite fiber mesh. The box in (a) shows the approximate position of the image in (b). The arrowhead in (b) points to the closure of the lamellar phase to give a microtubule, while the arrow points to a unilamellar vesicle coexisting with the closed lamellar phase. Scale bars are marked on the images.
surfactant in the bulk solution. The cryo-TEM indicates that the local surfactant structure is a bilayer. When taken in conjunction with the fits to the neutron reflection, this suggests that the individual bilayers in the adsorbed layer are strongly roughened. Similar behavior is observed at the air/water interface. Although the structure that we observe has the appearance of a sequence of a limited number of lamellae, this observation would be consistent with any structure in which the sequence of lamellae is the main feature contributing to the scattering. Since the lamellae themselves are not perfectly flat, there would seem to be three possible structures, (i) a simple sequence of lamellae, (ii) a stack of monodisperse unilamellar vesicles, or (iii) a few repeat units of a bicontinuous phase. The distinction between the latter two structures and the simple lamellar structure would be that the presence of the “walls” connecting the lamellae in the vertical direction will contribute to the scattering length density of the intervening medium. However, the effect would be smaller than the experimental error and hence the three structures cannot be distinguished using the reflectivity alone. The results from cryo-TEM and small-angle X-ray scattering, particularly from the former, indicate that the adsorbed layer structure derives either from unilamellar vesicles or from lamellar phase. Although this seems to exclude the possibility of a bicontinuous structure, the adsorption of a layer of vesicles would surely lead to rearrangements at the contacts between vesicles that would start to resemble a bicontinuous structure. Thus the structure of the bulk solution does not necessarily exclude any of the three structures. A multilayered lamellar structure at the interface has also been suggested for surfactants such as AOT at the silicon/water and air/ water interfaces,24,27 with lamellar spacings of 100-400 Å, or a mixture of an anionic and nonionic surfactant, C12E5 with SDS, which has a spacing of around 400 Å.28 In these two systems the spacing is much smaller and interpretation in terms of a lamellar phase is more plausible, but just as for the DDAB and DUDAB systems, the evidence is circumstantial. (28) Salamat, G.; de Vries, R.; Kaler, E. W.; Satija, S.; Sung, L. P. Langmuir 2000, 16, 102-107.
It is not clear what the driving force is for adsorption or why the adsorbed layer should have a finite and relatively small thickness (in comparison with the layer spacing). Electrostatic forces would be expected to play a role in the adsorption, but it should be noted that the interaction of the layer is not with the bare, negatively charged silica surface. The surface is covered by a bilayer of the cationic surfactant. Thus whatever adsorbs, vesicles or lamellar phase, is adsorbing on to a surface made up of the headgroups of previously adsorbed molecules. Given the low ionic strength of the solutions, repulsive electrostatic interactions between the bilayer-covered surface and subsequently adsorbed species are expected to be strong. There will also be strong repulsive undulation forces. The undulation forces are expected to increase with temperature, unless changes in hydration have an opposite effect by changing the elasticity of the layer. It is less clear how the electrostatic forces will change with temperature. On one hand, the increased dissociation will increase the charge on the bilayers, which will increase the repulsion. On the other hand, the increase in free surfactant ions will also lead to greater screening of the electrostatic repulsion. Given that the layer spacing decreases with increasing temperature, it would seem that the latter is the controlling factor. However, the effect is difficult to estimate because the ionic strength of these solutions is uncertain. We are currently exploring the effects of electrolyte on the surface behavior in order to elucidate the factors determining the spacing and the overall adsorbed layer thickness. Acknowledgment. We thank Dr. J. R. P. Webster and Dr. S. A. Holt at the Rutherford-Appleton Laboratory, United Kingdom, Dr. S. Finet at the ESRF, France, and Andrew Jackson and Hanna Vacklin, of the University of Oxford, for experimental assistance. We also thank Dr. Gunnel Karlsson at the University of Lund, Sweden, for measuring the cryo-TEM images. D.J.M. gratefully acknowledges the support, through a scholarship, of the Rhodes Trust, Oxford. LA026745V
