Langmuir 2000, 16, 3971-3976
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Internal Dynamics and Order Parameters in Surfactant Aggregates: A 2H NMR Study of Adsorption Layers and Bulk Phases Monika Scho¨nhoff*,† Max Planck Institute of Colloids and Interfaces, 14424 Potsdam/Golm, Germany
Olle So¨derman Physical Chemistry 1, Chemical Center, University of Lund, 22100 Lund, Sweden
Zhi Xin Li and Robert K. Thomas Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K. Received October 5, 1999. In Final Form: January 12, 2000
Nonionic surfactant adsorption layers are investigated by applying 2H NMR to selectively deuterated dodecyl penta(ethylene oxide), C12E5, adsorbed on colloidal silica. Results for surface aggregates are compared with different types of bulk aggregates such as micelles, which show isotropically averaged Lorentzian line shapes, and with liquid crystalline phases, which show a Pake pattern spectrum. Surface aggregate spectra are isotropically averaged, with relaxation rates R2 of several kilohertz, indicating a slow motion correlation time of the order of microseconds. Possible mechanisms of isotropic averaging of the quadrupolar interaction in surface aggregates and the implications for the structural arrangement are discussed. To compare results from solid and liquid spectra, we introduce the concept of a relative order parameter profile Srel of the C-2H bond. Adsorption layers can thus be directly compared with micelles, and with lamellar and hexagonal phases with respect to their internal mobility. An increase of Srel with distance of the 2H label position from the headgroup is found for all bulk aggregate types, with the slope depending on aggregate curvature. The result is interpreted in terms of packing constraints and chain interaction. Comparison of the results for surface aggregates with these profiles gives an indication of their curvature and structure. Srel profiles of adsorption samples do not depend on water content or surface coverage. All results, that is, the structural implications from Srel profiles as well as time scales of surfactant motional modes, are consistent with the formation of large anisotropic surface aggregates, which can be described as a disrupted bilayer. The motions causing isotropic averaging probably are lateral diffusion in combination with surfactant exchange processes.
Introduction The properties of amphiphilic molecules at interfaces are in many cases governed by the collective behavior of the amphiphiles and can thus be undestood in terms of surface aggregates. Surface aggregates exhibit a large variety of shapes and structures, such as monolayers, bilayers, adsorbed spherical micelles or hemimicelles, or large anisotropic rod- or disc-shaped micelles. The structures depend on the nature of the surface and the solvent, and might differ significantly from the corresponding equilibrium aggregates in solution, because they are influenced by the interaction with the interface. Surfactant orientation and packing at the interface furthermore depend on the spontaneous curvature of the surfactant. Volume and interface techniques have been applied to study solution and surface aggregates, respectively. In this study, we are applying 2H NMR, which so far has predominantly yielded results for liquid crystalline phases, to different volume phases and particularly to surface aggregates on colloidal silica to directly compare the latter * Author for correspondence. E-mail:
[email protected] † Previous address: Physical Chemistry 1, Chemical Center, University of Lund, 22100 Lund, Sweden.
with bulk aggregates with respect to the internal dynamics of the surfactant. Nonionic alkylpoly(ethylene oxide) CnEm surfactant aggregates in solution show curvatures that strongly depend on the length of the headgroup and the chain, n and m, respectively. At hydrophilic surfaces, for example, layer thickness data were interpreted by a transition from a closed bilayer to small aggregates with increasing headgroup length,1,2 following the same trend in aggregate curvature as in bulk micelles. C12E5 is an interesting surfactant, because its spontaneous curvature in bulk is intermediate, leading to the formation of long rodlike micelles in the L1 phase, the size of which strongly depends on concentration. Adsorption layers of C12E5 on hydrophilic silica have so far been characterized by reflection methods on flat substrates, such as neutron reflectivity,1 ellipsometry,3 and small-angle neutron scattering of C12E5 adsorbed on colloidal silica.4 Data obtained by these (1) Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451. (2) Desbene, P. L.; Portet, F.; Treiner, C. J. Colloid Interface Sci. 1997, 190, 350. (3) Tiberg, F.; Jo¨nsson, B.; Tang, J.-a.; Lindman, B. Langmuir 1994, 10, 2294. (4) Cummins, P. G.; Penfold, J.; Staples, E. J. Phys. Chem. 1992, 96, 8092.
10.1021/la991317j CCC: $19.00 © 2000 American Chemical Society Published on Web 03/14/2000
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techniques generally have to be analyzed in the frame of a closed layer model; usually a water component in a bilayer accounts for deviations from the bilayer, for example, for the water content of a layer consisting of small surface micelles. Literature data for C12E5 are consistent with the picture of either a disrupted bilayer or micellar structures at the interface. Atomic force microscopy measurements as the only model-free approach could reveal the lateral nanoscale structure for a series of CnEm surfactants, but did not lead to reproducible results for C12E5.5 Applying 2H NMR, we can here access the bond order parameter, S, associated with the amplitude of the local molecular motions, to study the internal dynamics of surface aggregates, which will also lead to conclusions about the aggregate structure. 2H quadrupolar splittings and 2H relaxation on selectively deuterated C12E5 contain information about S and about the time scales of surfactant motional modes. The results for surface aggregates are compared with different bulk structures as reference systems. Materials and Methods Dodecyl penta(ethylene oxide), C12E5, is synthesized in three different forms with a selective deuterium label in the R, β, or γ position of the alkyl chain, respectively. This is achieved by the reaction of penta(ethylene glycol) with the suitably labeled dodecanol, which is prepared by (a) reduction of d-dodecanoic acid with LiAlD4; (b) exchange of the R hydrogens in dodecanoic acid with D2O/NaOD, followed by reduction with LiAlH4; and (c) by reduction of octanoic acid with LiAlD4, followed by conversion to octylbromide and reaction of the Grignard reagent with ethylene oxide. The degree of deuteration in the respective positions was determined from 1H spectra and amounts to 93% for R-, 16% for β-, and 43% for γ-deuterated surfactant. All samples are prepared in deuterium-depleted water (2H e 0.0001%; Fluka). For adsorption samples, colloidal silica CabO-Sil (Fluka) with a specific surface area of 200 m2/g is used as obtained. In samples with high surfactant concentration, such as the liquid crystalline phases, the deuterated surfactant is diluted with protonated C12E5 obtained from Nikko (Japan). Adsorption samples on silica are prepared by mixing deuterated surfactant and Cab-O-Sil in dilute aqueous solution at a weight ratio corresponding to full surface coverage, which is 45 Å2/ molecule, as determined from ellipsometry measurements.3 After equilibration under gentle shaking for several days the samples are filled into NMR tubes and centrifuged directly before measurement to achieve a higher concentration for optimized sensitivity. Spectra are taken on the centrifugation pellet containing Cab-O-Sil, after careful removal of the upper phase. All experiments are performed on a Bruker DMX 200 spectrometer. 2H liquids spectra are taken with a 90-acq sequence, and solids spectra are obtained by applying a quadrupolar echo sequence using a 2H solids probe head. Relaxation rates R1 are determined in an inversion recovery experiment using 30 wt % C12E5 micellar solutions.
Results and Discussion 1. Liquid Crystalline Phases. With increasing surfactant concentration, aqueous mixtures of C12E5 form a hexagonal and a lamellar phase, which are investigated here at 50% and 69 wt % surfactant, respectively. The spectra show a Pake pattern, resulting from the anisotropy of the quadrupolar interaction. The quadrupolar splitting ∆Q, determined from the frequency difference of the main maxima in such spectra, is mainly determined by the order parameter S of the C-2H bond, which is a measure of the residual anisotropy. S is defined by the angle ϑ between the C-2H bond and the local aggregate surface: (5) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288.
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S)
1 (3〈cos2 ϑ〉 - 1) 2
(1)
It is
∆Q )
3 Sχ 4n
(2)
where χ is the quadrupolar coupling constant, and n is a geometric factor. For the samples investigated here, ∆Q is evaluated from Pake pattern spectra, and order parameters are calculated from eq 2 using n ) 1 for the lamellar phase. In the hexagonal phase, fast diffusion in the angular direction in hexagonal rods causes a partial averaging of the quadrupolar interaction and a reduction of ∆Q by a factor of 2; therefore it is n ) 2. The order parameters are significantly larger in the hexagonal phase (see Figure 1), indicating a more constrained hydrocarbon chain packing. This can be interpreted in terms of the geometry, because the ethylene oxide headgroup is strongly hydrated, occupying a large cross-sectional area per molecule, so that the alkyl chain packing is more efficient at larger curvatures. In both phases, the order parameters are increasing toward the center of the hydrophobic region, indicating a higher order on the segmental length scale with increasing distance from the headgroup. It is concluded that order is driven by the aggregation of the chains, whereas the separation of the headgroup and chain regions plays a minor role. This agrees with the fact that for C12E5 the hydrophobicity of the headgroup and the alkyl chain does not differ as much as in ionic surfactants, where the typical behavior is a decrease of S with distance from the interface in a lamellar phase6 as well as in a hexagonal phase.7 The smaller contrast in hydrophobicity in the nonionic surfactant causes a comparatively rough and disordered headgroup/chain interface. These arguments also explain the small absolute values of S we find here, as opposed to order parameters of up to S ≈ 0.2 in ionic surfactants.6 In agreement with our interpretation of a disordered interface, in a study of a phospholipid lamellar phase a significant decrease of the lipid order parameter was observed on addition of C12E8. Furthermore the decrease of S was most pronounced on carbon atoms close to the lipid headgroup.8 Order parameters of a similar nonionic surfactant, perdeuterated C12E4 in a lamellar phase (at 60 wt %), were obtained by Ward et al.9 Values of the quadrupolar splittings in the R, β, and γ positions are slightly higher than for C12E5, which can be attributed to the smaller headgroup, which induces a lower degree of disorder. Although in that work the splittings could not be uniquely assigned in the region close to the headgroup, the results clearly show a maximum of S around the fourth carbon atom, in agreement with our results for C12E5, and a decrease of S at larger distance from the headgroup. 2. Micellar Solutions. Micellar solutions are investigated as a function of the surfactant concentration. Here, the anisotropy of the quadrupolar interaction is averaged by fast rotational tumbling and intramicellar diffusion. The resulting spectral shape is Lorentzian with a width (6) Davis, J. H. Biochim. Biophys. Acta 1983, 737, 117. (7) Henriksson, U.; O ¨ dberg, L.; Eriksson, J. C. Mol. Cryst. Liq. Cryst. 1975, 30, 73. (8) Thurmond, R. L.; Otten, D.; Brown, M. F.; Beyer, K. J. Phys. Chem. 1994, 98, 972. (9) Ward, A. J. I.; Ku, H.; Phillippi, M. A.; Marie, C. Mol. Cryst. Liq. Cryst. 1988, 154, 55.
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Figure 1. Order parameters in the lamellar and hexagonal phases, obtained from quadrupolar splittings. Positions 1, 2, 3 indicate deuteration on the R, β, and γ carbon of the alkyl chain, respectively. Solid lines are guides for the eye only, as in all following figures. Table 1. Longitudinal Relaxation Rates of C12E5 in Concentrated Micellar Solution (30% wt Surfactant), Determined in Inversion Recovery Experiment at 30.7 MHz R-C12E5 β-C12E5 γ-C12E5
T1 (ms)
R1 (Hz)
23.4 21.3 21.2
42.7 46.9 46.1
given by the natural line width, and therefore determined by relaxation. An often-used model for the interpretation of NMR relaxation data from surfactant aggregates is based on a separation of the molecular motions into two wellseparated time regimes. This model has been termed the two-step model:10 Local anisotropic motions, such as bond rotations, are “fast motions” determining the longitudinal relaxation rate R1. Isotropic motions, which average the residual anisotropy, such as aggregate rotation, occur on a much slower time scale, described by the slow motion correlation time τS. The transverse relaxation rate R2 is dominated by these “slow motions”. Under the assumption of ωτS . 1, a simplified expression for the relaxation rate difference ∆R ) R2 - R1 as a function of the correlation time is obtained:
∆R ) R2 - R1 )
9π2 2 2 χ S τS 20
(3)
Measurements of R1 ) 1/T1 are performed in concentrated micellar solutions. Table 1 gives the results, which show that the fast motions dominating R1 are almost independent of label position. Because these fast local motions can be assumed to be independent of aggregate type, and because furthermore it is R1 , R2 (compare with Figure 2), it follows that
R1 ∝ τSS2
(4)
From a Lorentzian fit of the 2H spectra of micellar solutions the line width is determined, from which R2 is (10) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860.
Figure 2. Relaxation rates R2 of bulk micelles at different concentrations of d-C12E5 in H2O.
calculated. The increase of R2 with concentration shown in Figure 2 mainly reflects an increase of τS: previous investigations had shown that C12E5 forms long rodlike micelles11,12 where the micellar size is increasing with surfactant concentration, already from concentrations just above the critical micelle concentration.13 Micellar growth causes an increase of the correlation time for tumbling motions, which is monitored by the relaxation rates. Furthermore, for each concentration, R2 is increasing with label position, revealing an increasing order parameter, similarly to the anisotropic phases. Because R2 depends on both τS and S, it is not possible to calculate absolute values of S from the relaxation rates. We are therefore introducing relative order parameters, defined by the relative value of S with respect to SR of the same system, so that Srel ) S/SR. Relative order parameters are calculated from eq 4 as Srel ) Sx/SR ) (R2,x/R2,R)1/2, where x is R, β, or γ. This equality is making use of the fact that τS is the same in samples with identical composition but different label position, and thus does not need to be known. Figure 3 shows a comparison of Srel values of micelles and anisotropic phases. For all aggregate types, an increase with label position is observed; thus the above arguments concerning the headgroup/chain interface in the LC phases generally apply. The increase of Srel is most pronounced in smaller micelles and is directly correlated to the curvature of the aggregate, with the smallest increase obtained for the lamellar phase. These results can be explained in terms of the aggregation of the alkyl chain: for C12E5, due to strong hydration of the ethylene oxide, the cross-sectional area of the headgroup is much larger than that of the alkyl chain. This hinders efficient packing of the chains in the lamellar phase, leading to small absolute order parameters (see Figure 1) due to chain disorder. As the aggregate curvature increases, the cross-sectional area starts to vary along the chain, and the mobility of a CH2 segment decreases with distance from the headgroup. This effect causes a larger absolute order parameter, as well as a larger increase of Srel for the hexagonal phase as compared with the lamellar phase. This trend is obviously continuing as the curvature (11) Nilsson, P. G.; Wennerstro¨m, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377. (12) Brown, W.; Pu, Z.; Rymden, R. J. Phys. Chem. 1988, 92, 6086. (13) Scho¨nhoff, M.; So¨derman, O. J. Phys. Chem. B 1997, 101, 8237.
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Figure 3. Relative order parameter profiles of bulk micelles and anisotropic phases determined from R2 and quadrupolar splittings, respectively.
further increases with decreasing concentration in the micellar phase. The internal order in the bulk aggregates of C12E5 can thus be understood in terms of the packing in the respective aggregate. An interesting issue is the fact that the slope of Srel monotonically increases with curvature, which will facilitate an understanding of the aggregate structures at the interface. 3. Surface Aggregates. 2H resonances of surfactant adsorbed to colloidal silica show a Lorentzian line shape for all preparation conditions and label positions investigated. No residual quadrupolar interaction is observed; consequently the existence of an isotropic motional mode present in surface aggregates has to be concluded. The dynamics of this isotropic mode are fast enough to average the quadrupolar interaction; thus it is τS < 150 µs. The shape of the resonances is Lorentzian, indicating that the line width is determined by homogeneous broadening and thus by the relaxation rate R2. R2 can thus be derived from the line width, resulting in values on the order of several kilohertz, which points at the existence of a very slow motional mode. The possible nature of the motions being responsible for isotropic averaging, but still slow enough to lead to large relaxation rates, will be discussed further below. Relaxation rates of surface aggregates depend on label position, surface coverage, and preparation history. In Figure 4, R2 values of R-deuterated C12E5 on silica are shown as a function of the centrifugation speed, which was applied before measurement. The data show a continuous increase due to either an increase of τS or S, until reaching a plateau. The effect of increasing centrifugation speed is to change the density of the colloidal particle sample by squeezing out water. Because the variation of R2 with decreasing water content is rather small and continuous, we assume that the aggregate structure remains intact during centrifugation. In contrast to this, in a previous communication14 we have reported a dramatic effect of repeated cycles of silica centrifugation and redispersion on R2, which was attributed to structural changes of the surface aggregates (data shown for comparison in Figure 4). After centrifugation treatment only, however, we believe the R2 variation to reflect (14) Scho¨nhoff, M.; So¨derman, O.; Li, Z. X.; Thomas, R. K. Bull. Magn. Reson. 1999, 20, 25.
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Figure 4. Relaxation rates of surface aggregates of R-d-C12E5 as a function of centrifugation treatment for two samples of different coverage. Filled symbols: 45 Å2/molecule; open symbols: 60 Å2/molecule. The largest R2 values (triangles) result from samples that were treated by repeated centrifugation and redispersion cycles.
Figure 5. Relaxation rates of surface aggregates as a function of 2H label position at different surface coverages.
changes of the dynamics represented by the correlation time τs, and not a structural transformation, so that the aggregate structure remains intact during centrifugation, which is also supported by the results below. To obtain reproducible results, samples are thus treated by centrifugation only, performed directly before measurement. A speed of 4000 rpm, corresponding to 2952g (g: earth gravitation), where R2 values reach a plateau, is chosen as standard treatment; results are averaged over at least two samples. The water content of the adsorption samples is determined by weight after centrifugation and removal of the upper phase: at 45 Å2 per molecule the weight ratio of surfactant to water is 12%, corresponding to a micellar L1 phase. In Figure 5 relaxation rates of surface aggregates are given as a function of label position. A comparison can be made with the L1 phase at the same surfactant/water ratio: from the data in Figure 2 an interpolation of R2 in dependence of concentration shows that at 12 wt %
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Figure 6. Relative order parameter profiles of surface aggregates as a function of 2H label position at different surface coverages.
surfactant, R2 of free micelles is smaller compared with R2 in adsorption samples. The reason can be either an increase of the motional correlation time due to immobilization of aggregates at the surface, or significant structural differences. Relative order parameters (Figure 6) of surface aggregates increase with label position, as in all bulk structures investigated. By comparison of the Srel increase with the data in Figure 3, conclusions on the curvature of surface aggregates can be drawn: the increase of the Srel data of surface aggregates is in the range of that of the lamellar or hexagonal phase, and distinctly lower than for small micelles, indicating surface structures in the curvature range between long rodlike aggregates and bilayers. Because of the large errors in Figure 6 a more precise interpretation is not possible. Generally, because of the normalization, the Srel data of the β and γ positions contain the error of the R2 measurement of the R position as well, which leads to large errors in the case of surface aggregates where the spectra contain a lot of noise. In samples with a lower surface coverage of 60 Å2 per molecule, R2 values are larger after identical treatment, whereas the order parameter profile is similar within the error range, probably caused by similar surface structures with a small curvature but a longer motional correlation time than at 45 Å2 per molecule. At lower centrifugation rates the relative order parameter profiles are the same within error range (data not shown); the internal aggregate structure and the curvature therefore do not depend significantly on water content. Only after redispersion and centrifugation cycles is the Srel increase larger; probably the large surface structures are destroyed by heavy mechanical treatment and smaller aggregates are formed. Using the concept of Srel profiles, it thus becomes feasible to obtain structural information from relaxation data. Extracting information on molecular motions from R2 data, however, is not straightforward, because the relaxation rates again depend on both order parameter and correlation time. According to the above discussion about the aggregate structures at the interface we can assume that the absolute order parameter SR will be of the same order as in a hexagonal or lamellar phase, and obtain an estimate of the motional correlation time in surface aggregates. Then, R2 data for centrifuged adsorption samples result
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in a correlation time in the range of τs ≈ 1.1 µs (according to eq 3 with SR taken from the hexagonal phase) to τs ≈ 2.2 µs (SR from the lamellar phase). We will now discuss different motions of surfactants that could occur in surface aggregate samples and would determine τs. These motions have to be isotropic to average the local anisotropy, as discussed above. Possible motions are: (I) intramicellar diffusion in case of small spherical surface micelles, (II) diffusion along the silica surface in case of closed layer aggregates, (III) rotation of the particle, or (IV) other mechanisms such as breaking and re-formation of aggregates or surfactant exchange between aggregates or between the surface and the aqueous region. For type I motions, an estimate of the expected correlation times τs in small spherical micelles results in nanoseconds and does not account for dynamics on the microsecond scale. Furthermore, as discussed above, the Srel profiles are not consistent with the formation of small spherical micelles at the surface. If large rodlike micelles are formed, diffusional averaging would be slower, but because large micelles have a nonspherical shape, the anisotropy of the quadrupolar interaction will only be reduced and an additional isotropic mode would be necessary for complete averaging. In the case of closed bilayers, diffusion around a spherical particle (type II) will completely average ∆Q. The diffusional correlation time is given by
τS )
R2 6D
(5)
where D is the lateral diffusion coefficient, defined at the headgroup/chain interface, and R ) Rparticle + Rlayer is the radius at that interface. With an average radius of Rparticle ) 6 nm for the silica, and with a lateral diffusion coefficient of D ) 3 × 10-11 m2/s (determined in a bicontinuous cubic phase15), τS for diffusion along the particle surface is estimated as 0.3 µs and 0.45 µs for the inner and outer layers, respectively, the different values resulting from different radii at the inner and outer bilayer parts. These correlation times are faster than the τS value estimated from R2, but of the same order of magnitude. Considering that diffusion might be slowed down in a disrupted bilayer, this scenario remains a possible one, and lateral diffusion might contribute to τS. Rotation of the particle itself, however, does not have to be considered as averaging mode, because the specific silica in use consists of interconnected particles, and is furthermore densely packed, preventing rotation. For motions of type IV it is very difficult to estimate a correlation time. Time scales of collective micellar reorganization processes are not known, and time scales of surfactant exchange have only been measured in dilute solutions, where the exchange time between free C12E5 and a colloidal surface is on the order of 12 ms,13 but decreases strongly with increasing concentration, because it is governed by diffusion onto the surface. Correlation times of type IV motions will therefore strongly depend on local surfactant concentration, and may very well be as fast as microsecond, possibly accounting for isotropic averaging. The picture arising from this discussion is therefore a surface structure that clearly is neither a closed bilayer nor small spherical surface micelles, but an intermediate structure ranging between either a disrupted bilayer that might be still continuous, but with a large number of defects slowing down lateral diffusion by a factor of at least four, or a discontinuous structure of bilayer patches, (15) Olsson, U.; Wu¨rz, U.; Strey, R. J. Phys. Chem. 1993, 97, 4535.
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for example large disc-shaped micelles. The motions determining R2 then contain a contribution from lateral diffusion, which is slower than the above estimated τS for a closed bilayer, and an additional contribution from exchange processes, which can lead to complete isotropic averaging even in laterally limited structures. At incomplete surface coverage (60 Å2/molecule) the relaxation rates are larger, and the Srel profile is the same within error range, so that the aggregate structure does not change significantly with coverage and the R2 difference is most probably due to a difference in τS. Slower motions at less surface coverage could reflect a larger number of defects in the distorted bilayer or a larger distance between neighboring surface aggregates, which slows down exchange processes. We thus obtain the same structural picture from the discussion of the molecular motions as from the Srel profiles, which is furthermore consistent with the literature data from reflection techniques. Conclusions Relaxation rates of selectively deuterated C12E5 in various kinds of aggregates were deduced from the line widths. Introducing the concept of relative order parameter profiles Srel, structures of aggregates in various bulk phases with very different motional time scales can be compared with each other. Order parameters in all bulk and surface aggregates increase with label position, indicating order to be induced by the hydrophobic aggregation, whereas the hydrophobic/hydrophilic interface is comparatively disordered. The slope of Srel is found to
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reflect the aggregate curvature in bulk aggregates of known shape, and the comparison to Srel in surface aggregates reveals the formation of large aggregates at the surface. The surface aggregate shape does not change significantly as a function of surface coverage or water content in the sample. Additional information is obtained by the R2 relaxation rate values and a discussion of possible motional modes in the system. The quadrupolar interaction is averaged by a slow isotropic motional mode with a time constant in the microsecond range. A discussion of the nature of this motional mode again indicates that the shape of the surface aggregates is large disc-shaped micelles or disrupted bilayers, with isotropic averaging most probably achieved by a combination of lateral diffusion and surfactant exchange processes. Thus the structural information gained from the dynamic NMR data leads to the same structural picture as obtained from reflection techniques.1,3,4 Acknowledgment. We thank Fredrik Tiberg for helpful discussions. M.S. was supported by a grant, partly from the European Commission TMR program and partly from the Deutsche Forschungsgemeinschaft (Scho 636/ 2-1). Further financial support was obtained from the Swedish Natural Science Foundation (NFR). The spectrometer was sponsored by the Swedish Council for Planning and Coordination of Research. LA991317J
