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H NMR Investigation of the Structure and Dynamics of the Nonionic Surfactant C12E5 Confined in Controlled Pore Glass Ying Qiao,† Monika Scho¨nhoff,*,† and Gerhard H. Findenegg‡ Max-Planck-Institute of Colloids and Interfaces, D-14424 Potsdam/Golm, Germany, and Stranski-Laboratorium fu¨ r Physikalische und Theoretische Chemie, Technische Universita¨ t Berlin, D-10623, Berlin, Germany Received March 18, 2003. In Final Form: May 13, 2003 The structure and dynamics of the nonionic surfactant C12E5 adsorbed from aqueous solution to the inner pore walls of two types of CPG porous silica glass (pore diameters 35 nm (CPG-240) and 13 nm (CPG-75)) have been investigated with 2H NMR. Broad Lorentzian shape peaks, instead of a Pake pattern, were observed, which imply the averaging of the anisotropic quadrupole interaction by isotropic motions inside the samples. The spin-spin relaxation rate R2 is 3 kHz, corresponding to motions of a correlation time of 3.8 µs, in both porous materials. Information about the aggregate curvature is derived from a relative order parameter, Srel, for three positions of the 2H label along the alkyl chain. Srel values in a range from 1.0 to 1.1 are found for all label positions and both glass samples, which suggests a flat bilayer structure of the surface aggregates. Lateral diffusion of surfactants along the pore wall is discussed as the dominant motional mode determining the relaxation rates. Diffusion coefficients along the pore walls are estimated to 1 × 10-11 m2/s and 7 × 10-13 m2/s for CPG-240 and CPG-75, respectively, based on a simple model. Both values are below the corresponding values in free surfactant layers, viz., D ) 3 × 10-11 m2/s. These facts suggest that the lateral diffusion of the surfactant on the pore wall of CPG-240 is decreased due to the interaction with the silica surface, and the much stronger decrease on the pore wall of CPG-75 is mainly due to the confined geometry as the pore size is approaching the thickness of the surfactant bilayer. The proposed distinction between the effects of surface interactions and of geometrical confinement on the diffusional motion of the surfactant is supported by a comparison of surfactant adsorbed layers on concave and convex surfaces.
Introduction Surfactants in aqueous solutions self-organize into various aggregates depending on surfactant concentration, temperature, pH, and salinity. The effect of geometrical confinement in narrow pores may further influence the surfactant aggregation, when the width of the pore is approaching the size of the surfactant aggregates. This confinement effect is the subject of the present study. The adsorption of ionic surfactants from aqueous solution onto a solid surface is mainly due to the electrostatic attachment of the headgroups to surface charge sites, supported by a minimization of the contact area between the hydrophobic tails and water. The adsorption of nonionic surfactants onto solid surfaces involves more complex interactions. For surfactants of the poly(oxyethylene) alkyl ether type (abbreviated as CnEm) on hydrophilic silica surfaces, hydrogen bonding of the ether groups to the silanol groups was suggested as the dominant binding mechanism.1,2 However, since the ether groups of the surfactant as well as the surface silanol groups are strongly hydrated in water, this attachment of the surfactant headgroups to the surface is a weak interaction, leading to a low surface concentration at a surfactant concentration well below the critical micelle * To whom correspondence may be addressed. E-mail:
[email protected]. Phone: +49-331-5679256. Fax: +49-331-5679202. † Max-Planck-Institute of Colloids and Interfaces. ‡ Technische Universita ¨ t Berlin. (1) Denoyel, R.; Rouquerol, J. J. Colloid Interface Sci. 1991, 143, 555-572. (2) Lindheimer, M.; Keh, E.; Zaini, S.; Partyka, S. J. Colloid Interface Sci. 1990, 138, 83-91.
concentration (cmc). Above a distinct concentration (≈0.60.9 cmc),3 surface aggregation is then occurring, and it is believed that the isolated surfactant molecules adsorbed at low concentrations represent nucleation sites (anchor molecules) for the surface aggregates, which may be small micelles or extended bilayers. The relative length of the surfactant headgroup and the alkyl chain, which is controlling the spontaneous curvature of surfactant aggregates in solution, will also control the size and shape of an aggregate: Short oxyethylene (EO) headgroups favor the formation of large bilayer-like aggregates, while long EO headgroups favor discrete micellar-like aggregates. For the intermediate range, the surface phase is sensitive to the surfactant concentration.4,5 On the other hand, surfaces with a low density of adsorbing sites favor the formation of micellar-like surface aggregates, while surfaces of a high concentration of adsorption sites will favor bilayer-like aggregates.1 For CnEm surfactants the adsorption furthermore increases when increasing temperature, since the headgroup hydration is temperature dependent. Little attention has been paid so far to the effects of confinement on the adsorption and surface aggregation of surfactant. Confinement in one dimension of space corresponds to the geometry of a liquid between two parallel surfaces, i.e., in a slit pore. Interesting details about surfactant adsorption in this geometry were obtained by force measurements between two solid surfaces using atomic force microscopy (AFM). In a recent study (3) Tiberg, F.; Jonsson, B.; Tang, J.; Lindman, B. Langmuir 1994, 10, 2294-2300. (4) Levitz, P.; Van Damme, H. J. Phys. Chem. 1986, 90, 1302-1310. (5) Grant, L. M.; Tiberg, F.; Ducker, W. A. J. Phys. Chem. B 1998, 102, 4288-4294.
10.1021/la034471l CCC: $25.00 © 2003 American Chemical Society Published on Web 06/18/2003
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of the adsorption of cationic surfactants between a silica particle and a silica sheet in aqueous medium,6 a complex dependence of the adsorption on the surface separation and concentration was observed: At concentrations below the cmc (0.01 cmc < c < cmc) desorption of the surfactant occurs as the surfaces approach each other, but at a higher concentration (4 cmc) first a large increase and then a large decrease in adsorption was noted. The authors attributed these changes in adsorption to the overlap of electrical double layers.6 Confinement in two dimensions is expected in cylindrical pores and related convex pore geometries that are commonly found in porous silica and other oxide materials. One may speculate that in these cases the surface aggregation of surfactants depends not only on the pore width but also on the mean curvature of the pore walls. Whereas for cylindrical pores the mean curvature C is only a function of the pore radius r0 (i.e., C ) 1/2r0), in materials with interconnected pores a more complex situation arises. For example, for bicontinuous pore morphologies the mean curvature of the pore wall is expected to vary continuously from positive to negative values as the porosity P increases from values 0.5, irrespective of the mean pore width. For this reason, it is desirable to study surfactant adsorption in mesoporous solids of different pore morphology. The controlled pore glass (CPG) is a mesoporous silica material obtained by acid leaching of a phase-separated Na2O-B2O3-SiO2 glass, by which the sodium borate phase (NaBO2) is removed, leaving behind a silicon oxide skeleton with a highly interconnected pore system. The resulting structure is reminiscent of those occurring in spinodal decomposition,7 although Haller pointed out that a nucleation and growth mechanism can equally lead to such morphologies.8-10 The porosity of the CPG materials is determined by the chosen composition ratio of Na2OB2O3 to SiO2, and the mean pore size can be controlled by an annealing process, i.e., reheating for variable periods of time after the initial temperature quench. CPG materials of relatively narrow pore size distribution and a wide range of controllable pore sizes (diameter 7.5-500 nm) are available as column packings in chromatography.9 CPG materials are well suited for studying confinement effects on the adsorption of surfactants in pores with hydrophilic pore walls, as the typical width of the surfactant surface aggregates (ca. 4 nm) is of the order of half of the pore width of CPG material of smallest mean pore diameter (ca. 13 nm). In a recent study of the adsorption of the surfactant C8E4 in a series of CPG materials, it was found that the plateau value of the adsorption isotherm decreases strongly as the pore width decreases below ca. 25 nm, i.e., as the pores become less accessible for surfactant adsorption.11 Here, we are reporting on the influence of confinement in pores of different size on the structures and dynamics of nonionic surfactant C12E5. NMR Methods. Dynamic NMR methods can monitor molecular motion and are thus sensitive to confinement effects. Although for a long time NMR techniques, due to their intrinsic low sensitivity, were applicable only for (6) Subramanian, V.; Ducker, W. J. Phys. Chem. B 2001, 105, 13891402. (7) Cahn, J. W. J. Chem. Phys. 1965, 42, 93. (8) Haller, W. J. Chem. Phys. 1965, 42, 686. (9) Haller, W. In Solid-Phase Biochemistry, Analytical and Synthetic Aspects; Chemical Analysis Series 66; Scouten, W. H., Ed.; John Wiley & Sons: New York, 1983. (10) Haller, W.; Macedo, P. B. Phys. Chem. Glasses 1968, 9, 153-&. (11) Dabiri, A. R.; Dietsch, O.; Findenegg, G. H. Publication in preparation.
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studies of fairly concentrated bulk samples, applications of NMR methods to organic adsorption layers at surfaces are currently evolving. Several kinds of adsorption layers, consisting of polymers, surfactants or lipids, have already been studied, as summarized in recent reviews.12,13 Surfactant adsorption onto nonporous particles has been studied by 2H NMR both for ionic14,15 and nonionic16 surfactants. For the investigation of molecular arrangement and dynamics in porous materials, where the surfaces are far more difficult to access by conventional methods as compared to planar surfaces, NMR methods are particularly promising. Because NMR is sensitive to the intramolecular structure, it is complementary to the long-range structure information provided by diffraction techniques. Studies of the structure and dynamics of ionic surfactants in porous materials using 13C NMR exemplify this point. In these studies, Wang et al.17,18 reported conformational changes in the headgroup region due to the interaction between the charged headgroup and a charged silica surface, as well as a decreased mobility of the methylene group adjacent to the headgroup. In addition to these surfactant studies, 2H solid-state NMR has been employed to investigate a range of different types of guest molecules trapped in pores, such as small organic molecules,19,20 liquid crystals,21,22 or polymers.23,24 In the present paper, 2H NMR is employed to investigate the local order and dynamics of the surfactant C12E5 in two CPG glass materials of different mean pore size. This paper is the first report of a series of studies in which the influence of pore size and shape on the adsorption of nonionic surfactants will be investigated by comparing results for several different mesoporous materials. Experimental Section Porous Glass. The substrates used in this work are two types of controlled pore glass (CPG) with nominal mean pore size 75 and 240 Å. These porous glasses were purchased from Fluka. In the following section, these two glasses are referred to as CPG240 and CPG-75. The surface and geometrical properties of the two materials were characterized by gas adsorption and by transmission electron microscopy (TEM) imaging. The nitrogen adsorption isotherm at T ) 77 K was obtained with a Micromeritics Tristar 3000 automated gas adsorption analyzer. The samples were initially degassed in a Micromeritics VacPrep061 degasser overnight at 150 °C under 100 µTorr pressure. The adsorptiondesorption isotherms are shown in Figure 1, where the square and diamond symbols stand for data obtained using CPG-240 and CPG-75, respectively. The two CPG materials show a (12) Blum, F. D. Annu. Rep. NMR Spectrosc. 1994, 28, 277. (13) Scho¨nhoff, M. In Novel Methods to Study Interfacial Layers; Mo¨bius, D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; pp 285-336. (14) So¨derlind, E.; Stilbs, P. Langmuir 1993, 9, 2024-2034. (15) Nagashima, K.; Blum, F. D. J. Colloid Interface Sci. 1999, 214, 8-15. (16) Scho¨nhoff, M.; So¨derman, O.; Li, Z. X.; Thomas, R. K. Langmuir 2000, 16, 3971-3976. (17) Wang, L. Q.; Liu, J.; Exarhos, G. J.; Bunker, B. C. Langmuir 1996, 12, 2663-2669. (18) Wang, L. Q.; Exarhos, G. J.; Liu, J. Adv. Mater. 1999, 11, 13311341. (19) Gedat, E.; Schreiber, A.; Albrecht, J.; Emmler, T.; Shenderovich, I.; Findenegg, G. H.; Limbach, H. H.; Buntkowsky, G. J. Phys. Chem. B 2002, 106, 1977-1984. (20) Shantz, D. F.; Lobo, R. F. J. Phys. Chem. B 1998, 102, 23392349. (21) Vilfan, M.; Apih, T.; Gregorovic, A.; Zalar, B.; Lahajnar, G.; Zumer, S.; Hinze, G.; Bohmer, R.; Althoff, G. Magn. Reson. Imaging 2001, 19, 433-438. (22) Iannacchione, G. S.; Crawford, G. P.; Qian, S.; Doane, J. W.; Finotello, D. Phys. Rev. E 1996, 53, 2402-2411. (23) Fischer, E.; Kimmich, R.; Beginn, U.; Mo¨ller, M.; Fatkullin, N. Phys. Rev. E 1999, 59, 4079-4084. (24) Mirau, P. A.; Heffner, S. A. Macromolecules 1999, 32, 49124916.
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Figure 1. Nitrogen adsorption-desorption isotherms at 77 K. Table 1. BET Specific Surface Area as and Mean Pore Diameter d of the CPG Materials Determined by Nitrogen Adsorption (77 K) d/nm as/m2 g-1 BJH(des) BJH(ads) DFT vP/cm3 g-1 P/% CPG-240 CPG-75
84 174
30.3 9.6
35.2 13.1
34.3 12.7
0.66 0.59
59 56
pronounced adsorption/desorption hysteresis, which is typical for the upper mesopore range. Analysis of the adsorption isotherm yields a Brunauer-Emmett-Teller (BET) specific surface area25 as of 84 m2/g for CPG-240 and 174 m2/g for CPG-75. The specific pore volume vp as determined from the adsorbed amount at relative pressure p/p0 ) 0.99 was 0.66 cm3/g for CPG-240 and 0.59 cm3/g for CPG-75. These values correspond to porosities P ) Fsvp/(1 + Fsvp) of 59% and 56% for CPG-240 and CPG-75, respectively, using a skeleton density Fs ) 2.2 g/cm3. The pore diameter d was evaluated from the adsorption and desorption branch of the isotherms by the Barrett-Joyner-Halenda (BJH) formalism26 and by density function theory (DFT).27,28 Results are listed in Table 1. The values of the pore diameter differ significantly from those given by Fluka. Our DFT values will be used in the discussion of the present paper. TEM. Pore size and shapes were further confirmed by imaging using transmission electron microscopy (TEM). TEM images were obtained on a Zeiss EM 912 Omega transmission electron microscope at an acceleration voltage of 120 kV. Samples were ground and taken up in pure ethanol. One droplet of the suspension was applied to a 400 mesh carbon-coated copper grid and left to dry in air. Details are shown in Figure 2 for CPG-240 and CPG-75, respectively. These images demonstrate that the shape of the pores is similar in both materials, but the mean pore size is clearly different. Adsorption Samples. The surfactant dodecyl penta(ethylene oxide) (C12E5) was employed in three different forms with a selective deuterium label in the R, β, or γ position of the alkyl chain, respectively. The synthesis of this chemical had been described previously.16 All samples are prepared in deuteriumdepleted water, deuterium content e1% of the natural abundance, purchased from Aldrich Chem. Co. All samples for NMR measurements were prepared by weight, employing two different methods: Samples denoted as mixingtype were prepared by mixing known amounts of surfactant, porous glass, and water, followed by grinding to distribute the surfactant uniformly into the pores. The ratio of surfactant to glass surface was 6 µmol/m2 for CPG-240 and 2 µmol/m2 for CPG75, which roughly corresponded to full surface coverage. These (25) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309-319. (26) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380. (27) Webb, P. A.; Orr, C. Analytical Methods in Fine Particle Technology; Micrometrics Instrument Corp.: Norcross, GA, 1997. (28) Ravikovitch, P. I.; Haller, G. L.; Neimark, A. V. Adv. Colloid Interface Sci. 1998, 77, 203-226.
values were taken from a study of adsorption isotherms of a similar surfactant, C8E4, in these glasses.11 In addition, the plateau value of the adsorption isotherm of C12E5 in CPG-75 was determined, which gave a value of 2.4 µmol/m2. Hence, in the mixing-type samples at least 80% of the full coverage was reached. The dense mixture was then transferred into a short NMR glass tube, about 30 mm in length and 5 mm in diameter. The particles were allowed to settle by gravity in the tubes and equilibrated over several weeks, then supernatant was removed and the samples were carefully flame-sealed. Samples denoted as washing type were prepared by placing porous glass and aqueous solution with an excess amount of surfactant, corresponding to 10 µmol/m2, into 1.5 mL Eppendorf tubes; 10 µmol/m2 is clearly exceeding the full coverage of C12E5 on planar silica surfaces (5.7 µmol/m2)3 in order to ensure that the pore surface is fully covered. The sample tube was sonicated for 10 min and shaken for 3 min with a MSI minishaker at the speed around 2000/min. Then the sample was equilibrated under gentle shaking on a VORTEX shaker for 2 weeks. Before the measurement, the sample was washed three times. Each washing cycle consisted of centrifugation and subsequent removal the supernatant, a clear solution. Water was added, and the above process was repeated from the shaking step on. After three washing cycles, the final sediment was transferred into a short 30 × 5 mm NMR tube, which was carefully flame-sealed. NMR Measurements. The NMR measurements were performed on a Bruker DMX 400 spectrometer, which operated at a magnetic field strength of 9.4 T. All spectra were recorded under static conditions with a solid-echo pulse sequence [d190°x-d2-90°y-d3-acquire]n, employing a broadband solid probe head with a 5 mm coil. The relaxation delay d1 was set to 200 ms, which is more than five times T1 (T1 ≈ 20 ms16). The echo delay d2 is set to 40 µs, if not mentioned otherwise. The 90° pulse length is about 3.0 µs, employing a low Q insert. All NMR measurements were performed at room temperature (23 °C), which is well below the cloud point of aqueous solutions of C12E5 (31 °C). Lorentzian fits of the spectra were performed using the program Origin, where the line width was extracted.
Results and Discussions 1. Evaluation of Relaxation Rates. Line Shape. The most pronounced feature of the spectra for both glass samples and all 2H label positions of the surfactant is that they exhibit no Pake pattern but a Lorentzian line shape. This is shown in Figure 3. The line shape indicates isotropic molecular motion in the sample, which averages the anisotropic quadrupolar interaction. In particular, the very large line width, on the order of kilohertz, may indicate possible contributions other than homogeneous broadening. The fact that the peaks are well represented by a single Lorentzian suggests a homogeneous broadening mechanism and the existence of one dynamic fraction only. However, this is not unambiguous proof of homogeneous broadening, since the spectrum might contain contributions from several components, leading by coincidence to a Lorentzian shape. Contributions to the Line Width. Additional contributions to the line width might arise from (a) residual quadrupolar coupling or (b) local magnetic field inhomogeneity due to susceptibility gradients at the surface. A number of experiments were performed to analyze the origin of the line width. A first possible contribution, residual quadrupolar coupling, would broaden the line shape in the case of molecular motions being not fast enough to lead to complete averaging of the anisotropic interactions. This implies that, in contrast to liquid spins, the solid spins are subject to a quadrupolar coupling. Then, a liquid type experiment, performed by a simple acquisition following a 90° pulse, would not refocus these coupled spins, and lead to a different shape of the spectrum. Figure 4 compares the spectra of a given sample (R-labeled CPG-
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Figure 2. TEM images of CPG-240 and CPG-75. The scale bar is 100 nm.
Figure 4. 2H NMR spectra of a washing-type sample of R-deuterated C12E5 confined in CPG-240. Spectrum a was obtained by a 90° pulse and spectrum b by a solid-echo pulse sequence. The solid lines are fits with Lorentzian functions.
Figure 3. Two sets of 2H NMR spectra of mixing-type samples, together with Lorentzian fit functions (solid lines).
240 washing-type sample) obtained with a liquid type acquisition and a solid echo, respectively. The spectra are indistinguishable both in shape and width, implying that the solid echo does not lead to the detection of possible additional solid spins. This result is considered as evidence that there are no solid spins present in the porous sample. A second possible contribution, local magnetic field inhomogeneity, would lead to an inhomogeneous broadening of the resonance due to the distribution of the local B0 field strength, since B0 is distorted locally due to gradients of the magnetic susceptibility in a heterogeneous
material. Such effects are well-known in porous materials. To investigate whether the large line width of the spectra arises from homogeneous or inhomogeneous broadening, a solid relaxation experiment was performed on one sample (CPG-75 mixing-type sample). The delay time dependencies of the peak width and peak intensity are given in Figure 5, in dependence on the total delay time t ) d2 + d3. The data point at t ) 0 represents the value obtained by a 90° pulse sequence. The peak width did not show a dependence on the total delay time. Figure 5b is based on the same experiments, and the echo decay of the intensity is well described by a monoexponential decay (solid line in Figure 5b), resulting in a relaxation rate R2 ) 2.6 kHz. From the peak widths of Figure 5a, R2 is calculated to about 3.0 kHz. Therefore, any inhomogeneous contributions to the line width are smaller than 0.4 kHz, which is considered negligible compared to the large homogeneous line width. Therefore, in this paper R2 data will be evaluated from the homogeneous line width and interpreted in terms of the motional dynamics of the surfactants. The broadening mechanism of the Lorentzian line is attributed to slow isotropic motion, and hence the line width of each peak may be taken as a reliable indicator of R2. Thus, R2 can be calculated directly from the line width w using R2 ) πw. This conclusion is applicable to samples prepared both with mixing and with washing methods and both glasses reported in this paper. For further studies, a variation of
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Figure 7. Dependence of R2 on label positions for C12E5 in CPG-75. The horizontal axis represents the deuterated position, R, β, and γ being denoted by 1, 2, and 3, respectively. The solid lines were drawn as a guide to the eyes.
Figure 5. Peak width (a) and peak intensity (b) as a function of delay time t. The solid line in (a) is a guide to the eye, while the solid line in (b) is a linear fit.
Figure 6. Dependence of R2 on label positions for C12E5 in CPG-240. The horizontal axis represents the deuterated position, R, β, and γ being denoted by 1, 2, and 3, respectively. The solid lines were drawn as a guide to the eye.
the temperature could be of interest, since at lower temperatures the dynamics would be decreased and a Pake pattern might be observed, giving information on the anisotropy. Interpretation of R2 Values. Figures 6 and 7 show the relaxation rate R2 for the three label positions of C12E5 in CPG-240 and CPG-75, respectively. It can be seen that R2 at the three label positions is almost equal. A second
feature is that the mixing-type and washing-type sample, which correspond to full coverage, agree well within error. Comparison of Preparation Methods. An important prerequisite for NMR experiments on adsorbed layers in pores is an accurate knowledge of the surface coverage, which can influence the surfactant dynamics significantly. For example, if nonadsorbed excess surfactant is present in the pores, free surfactant can contribute to the relaxation rate of the adsorbed species by fast molecular exchange. On the other hand, if the surface is not saturated, the surfactant dynamics might be faster or slower than that for the fully saturated surface. Both situations would complicate the interpretation of the NMR signal. For this reason, samples for CPG-240 were prepared according to the two different procedures. As explained in the Experimental Section, in washing-type samples, excess free surfactant is removed by the washing process, but the washing may cause desorption of the surfactant, so that the surface may not be fully covered by surfactant. On the other hand, for the mixing-type samples the amount of surfactant is well-defined, but complications may arise from the uncertainty of the specific surface area of the substrate and the plateau value of the adsorption isotherm which may cause either excess surfactant or the pore wall being not fully covered. Contribution of Free Surfactant to R2. Here, we discuss the effect of free surfactant on the NMR signal. To test the expectation that excess free surfactant will dramatically influence R2, a sample of CPG-240 was mixed with 12 µmol/m2 of R-labeled surfactant, i.e., the double amount of that required to fully cover the pore surface. The R2 for this sample, which is also shown as the triangle data point in Figure 6, is indeed much lower than R2 for the sample without excess surfactant. The excess amount of surfactant in the pores exists in the form of a solution of ca. 6 wt %. A bulk solution of this surfactant solution of 6 wt % would result in R2 about 1 kHz,16 while the surface aggregates result in R2 ) 3 kHz. The fact that we observed only one NMR peak suggests that the surfactant molecules exchange rapidly between the surface aggregate and free micelles or monomers. Such exchange processes were previously shown to be controlled by the free surfactant diffusion, and their time scale is dependent on the surfactant concentration.29 Though for C12E5 in a dilute system the exchange time scale is on the order of
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10 ms, faster processes can be expected for more concentrated systems. In fast exchange, a surface bilayer coexisting with 6 wt % free micelle solution would result in an average relaxation rate R2 ) 0.5R2,surface + 0.5R2,solution ≈ 2 kHz. This rough estimate agrees remarkably well with the experimental value (triangle data point in Figure 6), indicating that the assumption of fast exchange is reasonable in the case of CPG-240 containing a substantial amount of free surfactant. Note that a rapid exchange process requires the presence of excess surfactant inside the pore space, while excess surfactant outside the porous grains cannot exchange fast enough with molecules in the interior of the grains. Clearly, the good agreement of the data of the mixing type and washing type suggests that a surface concentration of 6 µmol/m2 is indeed the true full coverage for CPG240, and the R2 data are not influenced by fast exchange with potential excess surfactant. This also implies that in CPG-240 pores there is indeed no reduction of adsorption by confinement, as the plateau value of the adsorption agrees with that on a flat silica surface (5.7 ≈ 6 µmol/m2).3 This agrees with results for another surfactant, C8E4, where in CPG-240 the same coverage as on the planar surface was found.30 For CPG-75, the mixing sample was prepared with a surfactant amount corresponding to 2.0 µmol/m2, i.e., somewhat less than the plateau value of the adsorption isotherm (2.4 µmol/m2). Following the argument outlined above, a sample with excess surfactant (4.1 µmol/m2) was prepared. Assuming that 2.4 µmol/m2 surfactant was adsorbed and the remaining surfactant forms a solution in the pores or outside the grains, this corresponds to a concentration about 7 wt % of free surfactant, resulting in R2,solution ) 1 kHz.16 Accordingly, with R2,surface ) 3 kHz at full surface coverage, the relaxation rate is expected to be R2 ) 0.6R2,surface + 0.4R2,solution ≈ 2.3 kHz. This rough estimate is somewhat larger than the experimental data value of 1.6 kHz in Figure 7. 2. Relative Order Parameter Srel. An often employed model to interpret NMR relaxation data from surfactant aggregates is the “two-step model”31 in which the fast and slow motions are separated into two time scales. The fast motions, such as the motion of the C-D bond in the labeled C12E5 molecules, determine the spin-lattice relaxation rate R1. The slow motions, such as the rotation of a whole surfactant aggregate, are described by the slow motion correlation time τs and determine the spin-spin relaxation rate R2. Under the assumption of slow motions, ωτs . 1, a simplified expression for the relaxation rate difference ∆R is obtained
∆R ) R2 - R1 =
9π2 2 2 χ S τs 20
(1)
and with the condition R1 , R2, we have R2 ≈ ∆R. Here, χ is the quadrupole coupling constant and S is the anisotropic order parameter, defined as S ) (1/2)(3〈cos2 θ〉 - 1). Thus, changes in R2 indicate changes in the slow motion correlation time τs or changes in the order parameter S. We now discuss the dependence of R2 on the position of the label. Figures 6 and 7 show that for a given pore size and sample preparation, the R2 data are similar for (29) Scho¨nhoff, M.; So¨derman, O. J. Phys. Chem. B 1997, 101, 82378242. (30) Kiraly, Z.; Bo¨rner, R. H. K.; Findenegg, G. H. Langmuir 1997, 13, 3308-3315. (31) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860-6864.
Figure 8. Order parameter profiles of surface aggregates of C12E5, expressed by the relative order parameter as a function of 2H label position. The deuterated position, R, β, and γ is denoted by 1, 2, and 3, respectively. Lines represent literature data for volume phases taken from ref 16.
different label positions. Apparently, the R2 values are dominated by τs. However, on comparison of samples of identical composition, but different label position, the same τs is given, since the slow motion correlation time τs results from the motion of a whole surfactant molecule or a whole surfactant aggregate, instead of the motion of a single C-D bond. Thus, the slight change of R2 from R to β and to γ position reflects the change of the order parameter S (the packing efficiency) with the label position. The fact that R2 values do not differ much suggests that S does not differ much with the labeled positions. The order parameter S of the C-D bond is a measure of the anisotropy of the system. In the special case of our surfactant-pore system, the Pake pattern is totally reduced to a Lorenztian peak by isotropic motion. Therefore, it is not possible to calculate the absolute value of S directly from the line shape. For such a case, a relative order parameter Srel had previously been introduced,16 which is defined in relation to the R-position as a reference position
Srel )
( )
R2,x Sx ≈ SR R2,R
1/2
(2)
where x is the R, β, or γ position and where it is assumed that τs is the same for different label positions. It is of interest to compare this relative order parameter Srel vs label position for C12E5 in different environments. In previous work, a comparison of Srel values of two liquid crystalline phases and micelle solutions of various concentrations was given.16 An increase of Srel with increasing distance from the headgroup was a general feature for all aggregates. It was shown that the slope of Srel is directly linked to the curvature of the aggregate: an aggregate with a higher positive curvature resulted in a Srel profile with a steeper increase. This was attributed to a change of the cross-sectional area of the hydrocarbon chain as a function of the distance from the headgroup, which increases with increasing curvature and results in an increase of the order parameter with increasing distance from the headgroup.16 Figure 8 shows an application of this concept to C12E5 surface aggregates in the pore space of CPG silicas. Values of Srel are given as a function of the label position, with data for different volume aggregates given as lines for comparison. It is seen that all data for C12E5 in porous glass agree with the lamellar phase, while most of the data points are clearly below the line corresponding to
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Figure 9. Model of slow motions of surface aggregates inside the CPG pores. The shaded area is the silica skeleton and the empty areas are the pore space filled with surfactant aqueous solution. τex and τD represent the correlation times of exchange and diffusion, repectively.
the hexagonal phase. Thus we conclude that the aggregate structure has an average curvature similar to that of the lamellar phase. Therefore, the present results indicate a flat bilayer or a disrupted bilayer type structure of the surface aggregates. Small spherical surface micelles or cylindrical structures are not consistent with the results presented in Figure 8. 3. The Slow Motion. The slow motion correlation time τs may be estimated using eq 1 and assuming an absolute order parameter of the aggregate similar to that in the lamellar phase (SR ) 0.08).16 The quadrupole coupling constant is χ ) 167 kHz. A slow motion correlation time of τs ≈ 3.8 µs is then obtained for C12E5 in CPG-240 and CPG-75. The interpretation of the slow motion correlation time is model dependent and thus also depends on details of the pore geometry. We discuss here exchange with free surfactant molecules and diffusion along the surface as possible motional modes determining the slow motion correlation time. Exchange. The exchange of surfactant molecules in the bilayer with free excess surfactant molecules is a motional mode possibly causing isotropic averaging. Desorbed surfactant molecules can be incorporated into the bilayer at a random position and thus with a random orientation, so that on the time scale of exchange, τex, isotropic averaging is achieved; see the processes depicted in Figure 9. Surface Diffusion. For the case of CPG materials, the most plausible motional mode is the lateral diffusion of surfactant molecules within a closed layer adsorbed on the pore surface. If this is the fastest isotropic motional mode determining τs, then R2 is governed by the decay of the orientational autocorrelation of a surfactant molecule. The simplest model of the inner pore shape of the CPG is to consider the pore as spherical, where the lateral diffusion coefficient is given by
D ) r2/6τs
(3)
From Table 1, the mean pore radius is r0 ≈ 17.2 nm for CPG-240 and r0 ≈ 6.4 nm for CPG-75. D is estimated from τs = 3.8 µs and r ≈ r0 - δ, where δ ≈ 2.4 nm is half the thickness of a bilayer on bare silica (∼4.8 nm),32 which is subtracted from r0 to obtain an average radius of the bilayer aggregate (see Figure 9). Inserting these values into eq 3 yields DCPG240 ≈ 10-11m2/s and DCPG75 ≈ 0.07 × 10-11 m2/s. For comparison, in a bicontinuous volume phase a lateral diffusion coefficient of the free layer of C12E5 separating the two phases was determined to be Dfree ≈ 3 × 10-11 m2/s.33 Thus our estimate for DCPG240 agrees with Dfree within an order of magnitude. The finding that
the estimated value of DCPG240 is a factor of 3 lower than that for a free surfactant layer is plausible in view of the fact that the interaction with the pore wall is expected to cause a reduction of the diffusion constant. Hence for CPG240 the slow motion time scale is consistent with the interpretation of the isotropic dynamics in terms of diffusion along the pore surface. The diffusion constant in CPG-75, DCPG75 ≈ 0.07 × 10-11 m2/s, is lower than that for DCPG240 by more than an order of magnitude. Since the two CPG materials differ essentially only by the mean pore width, we have to conclude that this pronounced difference between DCPG240 and DCPG75 is caused by the geometrical confinement of the surfactant bilayer in CPG-75. If for simplicity DCPG240 is taken as the diffusion coefficient on flat silica surfaces, independent of the pore size, then in CPG-75 the lateral diffusion is reduced at least by a factor of 10 with respect to this flat geometry. This reduction may be caused by two facts: First, the surface concentration of C12E5 in CPG-75 amounts to 2.4 µmol/m2, which is only ca. 40% of the full coverage of 5.7 µmol/m2 on a flat silica surface. This means that the pore walls of CPG-75 are not completely covered even at maximum coverage. Hence a substantial part of the inner surface area is not accessible for the surfactant and the surface aggregate exists as separate islands, probably in the form of bilayer patches, as suggested in Figure 9. Diffusion is clearly hindered at the edges of the surface aggregates. The second fact is that the surface aggregates of the surfactant in CPG-75 experience a strongly curved surface. Locally this will cause a more dense or less dense packing of the alky chain or the headgroups, respectively, as compared to flat bilayers, possibly resulting in a lower diffusion coefficient. Despite these clear implications of confinement, however, the overall curvature of the surface aggregates in CPG-75, as deduced from the Srel profiles, is not affected. This is probably due to an averaging of positive and negative local curvature on a bicontinuous surface. Exchange processes between aggregates via surfactants dissolved in the volume phase could additionally contribute to the isotropic averaging dynamics, although exchange would then cause a substantial decrease of τs. Since the dynamics, as discussed above, are rather slow as compared to free layers, it seems unlikely that a contribution of exchange is relevant. Furthermore, since the excess surfactant concentration is substantially controlling the exchange dynamics,29,34 and the excess concentration of (32) Thirtle, P. N.; Li, Z. X.; Thomas, R. K.; Rennie, A. R.; Satija, S. K.; Sung, L. P. Langmuir 1997, 13, 5451-5458. (33) Olsson, U.; Wu¨rz, U.; Strey, R. J. Phys. Chem. 1993, 97, 45354539.
2H
NMR of C12E5 in Porous Glass
Langmuir, Vol. 19, No. 15, 2003 6167
Table 2. Slow Motion Correlation Time τs and Diffusion Coefficient D Obtained under the Assumption of Diffusion in a Closed Bilayer on a Spherical Surfacea 3D bicontinuous phase CPG-240 colloidal silica CPG-75
τs (µs)
D (10-11 m2/s)
3.8 2.2 3.8
3 1 0.5 0.07
a Data of bicontinuous phase and colloidal silica were taken from literature.31,16
surfactant is rather low (on the order of the cmc) in the present samples, the contribution of exchange to τs can probably be neglected. Above, we have considered only some major aspects of the dynamics of the surfactant in the pore space. It is expected that the system will exhibit further more complex dynamic modes than can be reasonably deduced from the current data. For instance, in a comprehensive analysis of the diffusion mechanisms it will be necessary to consider the different situation in the contact layer and inner layer of the surfactant bilayer in narrow pores. It is interesting to compare the present data for C12E5 in mesoporous glass substrates with R2 data of the same surfactant adsorbed on nonporous colloidal silica particles of 12 nm diameter, where R2 ∼ 1.8 kHz.16 This comparison is particularly revealing as the size of the silica particles is similar to the mean pore size of CPG-75 (13 nm), but the curvature of the particle surface is convex while the hydraulic radius of the pores in CPG-75 suggests a concave curvature of the pore wall. On convex particles, the relaxation rate was attributed to diffusion along the particle surface as the dominating dynamic mode, and this motion was slightly slower than expected from diffusion in free layers.16 An estimate of D assuming isotropic averaging to be controlled by lateral diffusion, results in Dcolloid ≈ 0.5 × 10-11 m2/s. All estimates of D are summarized in Table 2. The fact that Dcolloid is more similar to DCPG240 and much larger than DCPG75, despite the particle radius being very similar to the internal radius of CPG75 pores, supports our conclusion that the surface interaction is the dominant mechanism in decreasing D, leading to a slow lateral diffusion on convex particles and on CPG-240 surfaces. The spatial confinement and the surface curvature do not seem to have a major influence here. However, in CPG-75, when strong spatial confinement is effective, as the average pore diameter of 13 nm is only slightly larger than the thickness of two bilayers (≈10 nm), see also Figure 9, the confinement is leading to a major reduction of the diffusion motion along the surface. In conclusion, the surface interaction and spatial confinement are two distinct mechanisms, which could be separated in our experiments. In the discussion above, only one single-averaged D is considered for the two monolayers forming a bilayer aggregate. One expects that the surface interaction will cause a decrease of D in the contact layer, while D of the free layer of the bilayer is expected to be essentially unaffected by the silica surface. Furthermore, since the mean radii of the contact layer and the free layer are strongly different in pores or on particles of molecular dimensions, different diffusion dynamics may be effective in the two layers. The fact that the experiments exhibit only one Lorentzian peak may be explained in two different ways: (1) The surfactant molecules are undergoing fast exchange between these two layers, resulting in only one D and one slow motion correlation time. (2) There are (34) Scho¨nhoff, M.; So¨derman, O. Magn. Reson. Imaging 1998, 16, 683-685.
indeed two peaks induced by two distinct diffusion constants under slow exchange, but the difference of their widths is within the limits of experimental error and thus the two spectra cannot be resolved. Assuming that two lines could be separated if their width differs by at least 20%, it follows that with the assumption of slow exchange the diffusion coefficients of the contact and the free layer should not differ by more than 20%. In view of the discussion above, such a small difference seems unreasonable, and thus we conclude that fast exchange between the contact layer at the surface and the free layer occurs. Finally, it should be noted that diffusion even on a much shorter length scale may contribute to R2. If molecules diffuse along a surface of substantial roughness, then the diffusion can lead to orientational averaging already on the length scale of this roughness. If the adsorbed surfactant molecules can assume a certain range of orientations relative to the local surface normal, one expects a broad distribution of orientations on a rough surface, but some orientations will still not occur due to the limitation of allowed orientations for each molecule relative to the surface normal. For this reason the anisotropy would not be totally averaged. We believe that this scenario does not apply to the case of self-assembled surfactant films at the pore walls of CPG, although this motional mode was found to play a key role for the diffusion of smaller molecules, such as benzene.19 In other words, the surfactant may not see the roughness that a benzene molecule experiences, in particular, since surfactant aggregates are dominated by the strong lateral interactions rather than the surface interactions of the individual molecules. Conclusions The structure and dynamics of the nonionic surfactant C12E5 adsorbed from aqueous solution to the inner pore walls of two types of CPG porous silica glass (pore diameters 35 nm/CPG-240 and 13 nm/CPG-75) has been investigated with 2H NMR. Major conclusions of this study are as follows: (1) Broad Lorentzian shape peaks, instead of a Pake pattern, were observed in all experiments, which imply the averaging of the anisotropic quadrupole interaction by isotropic motions inside the sample. (2) The spinspin relaxation rate R2 is 3 kHz for both pore sizes, which corresponds to motions with a correlation time of 3.8 µs. (3) The relative order parameter Srel of the aggregates in all samples has values between 1.0 and 1.1, which suggests a flat bilayer structure of the surface aggregate. (4) Surfactant molecules undergo fast exchange between the monolayers within the bilayer aggregate. (5) The average lateral diffusion coefficient on the CPG-240 and CPG-75 pore walls are estimated to 10-11 and 7 × 10-13 m2/s, respectively, based on a simple model. These values are lower than the lateral diffusion coefficient within layers of a three-dimensional bicontinuous phase (3 × 10-11 m2/ s). These facts suggest that the lateral diffusion of the surfactant on the pore wall of CPG-240 is slowed due to the interaction with the silica surface. In comparison, the lateral diffusion of the surfactants on the pore wall of CPG-75 is slowed mainly due to the confined geometry, when the pore size is approaching the thickness of the surfactant aggregate bilayer. Acknowledgment. The authors wish to thank H. Mo¨hwald and M. Antonietti for helpful discussions, Z. X. Li and R. K. Thomas for providing the deuterated surfactant, and B. H. Han for performing the gas adsorption measurements and taking the TEM images presented in this paper. LA034471L
