Langmuir 1994,10, 1122-1128
1122
Deuterium NMR Studies of the Local Ordering and Dynamics of Sodium Dodecyl Sulfate at the Alumina/Water Interface Erik Soderlind Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden Received October 5,1993. I n Final Form: January 14,1994"
2HNMR spectra of sodium dodecyl sulfate (SDS),deuterium labeled at the a-position of the alkyl chain, adsorbed on alumina particles of three different average diameters are presented. It is shown that the spectra gradually broaden to eventually split into a powder-type pattern as the particle size increases. It is concluded that the most likely structure of the surface-bound SDS aggregates at the adsorption plateau is a bilayer conformation, partly or completely covering the alumina surface. 2H NMR band shape calculations imply that the a-carbon order parameter is smaller than found in most other SDS aggregates. Indications are also found that the diffusion coefficient for lateral diffusion over curved surfaces is reduced for adsorbed SDS as compared to micellar or liquid crystalline systems. Spin relaxation measurement data are interpreted by means of a simple motional model, and by using the order parameter obtained from the band shape calculations, the correlation time for the fast local motions is calculated. It is found that these motions are sufficiently fast to fall within the extreme narrowing regime, which is also indicated by the temperature dependence of the longitudinalrelaxation time (Tl). Nevertheless,they are significantly slower than in other SDS aggregates investigated. An apparent activation energy is calculated from an Arrhenius plot of the TI data, and the comparison with similar data of alkylsilanes chemisorbed on silica indicates that the motional processes behind the 21' relaxation are dominated by the alkyl chain transgauche isomerization. Introduction The state and dynamics of hydrocarbon chains in aggregated surfactant systems have been the topic of several nuclear magnetic resonance (NMR) studies over the years.lq In particular, 2Hand l3C spin relaxation measurements have proved useful in such investigations. It has been the general finding that several motionalmodes have to be invoked to account for the relaxation behavior. The so-called two-step model4for surfactant motions has generally been accepted as a good description. According to this model, the surfactants undergo two different types of motions: local anisotropic motions of the hydrocarbon segments superimposed by overall motions of the entire molecule, each being described by a motional correlation time. The state of the hydrocarbon chains is described by the order parameter,which is also directly obtainable from 2HNMR spectra of anisotropic phase^.^ For the surfactant in the present study, sodium dodecyl sulfate (SDS),order and dynamics have been studied to a relatively large extent by NMR techniques. In particular, by using selectively 2Hlabeled SDS, micelles and microemulsionshave been characterized with respect to ordering and molecular dynamics as well as micellar size and shape."1° Similarly, lyotropic nematic and hexagonal phases of SDS, water, and decanol have also been Abstract published in Advance ACS Abstracts, March 1,1994. (1) Chachaty, C. Mol. Eng. 1992,2,65. (2) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987,19, 183. (3) Lindman, B.; SBderman, 0.;Stilbs, P. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum Press: New York, 1989; Vol. 7, p 1. (4) Halle, B.; Wennerstrbm, H. J. Chem. Phys. 1981, 75, 1928. (5) Seelig, J. Q. Rev. Biophys. 1977, 10, 353. (6) SBderman,0.;Jonstrbmer, M.;van Stam, J. J. Chem. Soc., Faraday Tram. . .. 1993.89. 1759. (7) Ceglie,A.; Colafemmina,G.; Della Monica, M.; Olsson, U.; Jbnsson, B. Langmuir 1993,9, 1449. (8) Monduzzi, M.; Ceglie, A.; Lindman, B.; SMerman, 0. J. Colloid Interface Sci. 1990, 136, 113. (9) Ginley, M.; Henrikseon, U.; Li, P. J. Phys. Chem. 1990,94,4644. (10) Sbderman, 0.;Carlstrbm, G.; Olsson, U.; Wong, T. C. J. Chem. SOC.,Faraday Trans. 1 1988,84, 4415. ~
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0743-7463/94/2410-ll22$04.50/0
investigated.11J2 It was found in those studies that the lateral diffusion coefficient, the a-carbon order parameter, and the rate of the local chain motions were similar in all the investigated phases. SDS has also been well studied with regard to its ability to adsorb on solid surfaces from aqueous solution.lgn Alumina has been the most frequently used substrate,13-u but other solids, such as activated carbon,% surfacemodified and polystyrene have also been investigated. In the past, mainly bulk properties such as the adsorption isotherm and tpotential were studied, but recently an increasing number of studies concerned with molecular properties of the adsorbed surfactants have appeared. The aggregation number of adsorbed SDS aggregates and their internal micropolarity and microviscosity have been determined using electron spin resonance (ESR), luminescence, and Raman spectro(11) Quist, P.-0.; Halle, B.; Fur6, I. J. Chem. Phys. 1992, W,3876. (12) Quist, P.-0.; Halle, B.;Fur6, I. J. Chem. Phys. 1991, 95, 6945. (13) Hough, D. B.; Rendall, H. M. In Adsorptionfrom Solution at the
SolidlLiquid Interface;Pditt, G. D., Rochester, C. H., E ~ s .Academic ; Press: New York, 1983; p 247. (14) Somasundaran, P.; Kunjappu, J. T. Colloids Surf.1989,37,245. (15) Waterman, K. C.; Turro, N. J.; Chandar, P.; Somaeunaaran, P. J. Phys. Chem. 1986,90,6828. (16) Somasundaran, P.; Turro, N. J.; Chandar, P. Colloids Surf. 1986,
20. 145. (17) Chandar, P.; Somasundaran, P.; Waterman, K. C.; Turro, N. J. J.Phys. Chem. 1987,91,148. (18) Chandar, P.; Somasundaran, P.; Turro, N. J. J.Colloid Interface Sci. ... 1987. 117. 31. -(19) Somasundaran, P.; Kunjappu, J. T.; Kumar, C. V.; Turro, N. J.; Barton, J. K. Langmuir 1989,5, 215. (20) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500. (21) Sjbblom, J.;Blokhus, A. M.; Holiland,H. J.ColloidInterface Sci. 1990,136,584. (22) Sjcblom, J.; Blokhus, A. M.; Sun, W. M.; Friberg, 5. E. J.Colloid Interface Sci. 1990, 140, 481. (23) Blokhus, A. M.; Sjbblom, J. J. Colloid Interface Sci. 1991, 141, I
RQR
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(24) Blokhus, A. M. Colloid Polym. Sci. 1990,268, 679. (25) Ihara, Y. J. Appl. Polym. Sci. 1992, 44, 1837. (26) Leimbach, J.; Rupprecht, H. Colloid Polym. Sci. 1993, 271, 307. (27) Brown, W.; Zhao, J. Macromolecules 1993, 26, 2711.
0 1994 American Chemical Society
Local Ordering a n d Dynamics of Sodium Dodecyl Sulfate
Langmuir, Vol. 10, No. 4, 1994 1123
Table 1. Particle Characteristics of the Alumina Adsorbents, the Obtained Adsorption Levels, F, and Average Areas Occupied per Adsorbed SDS Molecule mean BET particle surface act?$Zn, areaper alumina sue (pm) area (m2/g) r (pmoVm2) molecules (A2)
again adjusted to 4, and the samples were rotated at least 1 h after the fiial adjustment. The suspensions so obtained were centrifuged, and excess supernatant was removed for analysis, leaving behind a dense sediment. The sediments were transferred into 10-mm NMR tubes, which were then sealed. The equilibrium surfactant concentration in the supernatant was determined by two-phase titration according to the method described in ref 37, using chloroform as the organic solvent and bromophenol blue as the indicator. The equilibrium concentrations of SDS were found to be above the critical micelle concentration in all samples. The adsorption levels, r,of all three samples are summarized in Table
A-1 (?Alios) A-2 (~AliOa) A-3 (a-AlaOs) a
0.01 0.3 0.4
103 13.4 6.8
6.4 6.1 6.2
26 27 27
The area per molecule is calculated 88 A = l/rNA.
s ~ o p y . ~ Nevertheless, ”~~ the structure and dynamics of the surfactant aggregates on the surface are still subject to some debate. At the adsorption plateau level, which is reached at a surfactant concentration in solution close to the critical micelle ~oncentration,l3-~ both bilayerlk19 and m ~ n o l a y e r ~coverages l - ~ ~ have been suggested. According to another model, the surfactants form small surface aggregates when adsorbed on solids even to saturation.28 Only very few attempts have been made to study the molecular organization and dynamics of surfactants on solid surfaces with NMR techniques. However, 2H NMR studies of zwitterionic phosphocholine-based surfactants adsorbed on polystyrene latex were recently presented.2H2 The spin relaxation behavior was investigated and the sensitivity to surface charge of the 2Hquadrupole splitting demonstrated. Furthermore, the aggregate structure of sodium 4-hexadecylbenzenesulfonate (SHBS) on alumina,33 and the structure and dynamics of a number of cationic surfactants on s i l i ~ a , 3has ~ ~been ~ ~ determined. This study concerns SDS adsorbed on alumina to the plateau adsorption level. In an attempt to bring further light on the adsorbate structure and molecular dynamics, we have investigated the 2H NMR spectra of SDS-dz,the spectral changes with increasing particle size, and the spin relaxation behavior. It is demonstrated that the adsorbed surfactant aggregates differ from other SDS aggregates in several aspects. These include the local motion dynamics and ordering, as well as the lateral diffusion rate over curved surfaces.
Experimental Section Materials. Sodium [l,l-2H21dodecyl sulfate (SDS-dZ) was synthesized from dodecanoicacid, obtained from Aldrich Chemie (Steinheim, Germany), according to the method outlined in ref 36. Three different adsorbents were used: y A l 2 0 3 (A-1)obtained from Johnson Matthey (Ward Hill, MA), a-Al203 (A-2) from Fluka Chemie (Buchs, Switzerland), and a-Al2Os (A-3) from Sumitomo Chemical Co. (Tokyo, Japan). The particle characteristics accordingto the manufacturers are summarized in Table 1. All adsorbents were dried for several days at 120 OC before use. 2H-depleted water, obtained from IC Chemikalien GmbH (Ismaning, Germany), was used in all NMR samples. Methods. The adsorption samples were prepared by weighing appropriate amounts of alumina and water into screw-capped centrifugation tubes to obtain approximately 10-15 w t % alumina suspensions. The alumina suspensions were thoroughly shaken both before and after the pH was adjusted to 4 by addition of 0.1 M HCl. The larger particles were also ultrasonicatsd in pulse mode, using 60-W output power. The surfactant was weighed into the tubes, and the samples were allowed to equilibrate at room temperature by slow rotation for several days. The pH was (28) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991,269, 506. (29) Macdonald, P. M.; Yue, Y.; Rydall, J. R. Langmuir 1992,8,164. (30) Yue, Y.; Rydall, J. R.; Macdonald, P. M. Langmuir 1992,8,390. (31) Kuebler, S. C.; Macdonald, P. M. Langmuir 1992,8, 397. (32) Macdonald, P. M.; Yue, Y. Langmuir 1993, 9, 1206. (33) Werlind, E.;Blum, F. D. J. Colloid Interface Sci. 1993,157,172. (34) Sbderlind, E.; Stilbs, P. J. Colloid Interface Sci. 1991,143,586. (35) Sbderlind, E.; Stilbs, P. Langmuir 1993, 9, 2024. (36) Berr, S. S.; Coleman, M. J.; Marriott Jones, R. R.; Johnson, J. S., Jr. J. Phys. Chem. 1986,90,6492.
1.
The 2HNMR spectra were recorded at 30.72 MHz on a Bruker MSL 200 spectrometer. Longitudinal relaxation times, TI,were measured by the standard inversion recovery pulse sequence, and in the case of transverse relaxation times, Tz,a Carr-PurcellMeiboom-Gill (CPMG) sequence was employed. The relaxation data were evaluated by fitting appropriate functions to the signal intensities. In order to minimize the influence of the narrow resonance present in most spectra, the intensities were taken to be the total resonance areas. The narrow component is alluded to surfactant dissolved in the aqueous phase and contributes only marginally (less than approximately 5 % ) to the total resonance area. The 90° pulse length was typically 10 ps, the recycle delays were 3-5T1, and the pulse spacing in the CPMG sequence was 30 ps. Unless stated otherwise, the NMR probe temperature was kept at 25 OC. Band Shape Simulations. In order to correctly account for molecular motions occurring on an intermediate time scale, the band shape simulations are based on the stochastic Liouville equation for the time evolution of the density operator.-’ The physical model for the calculations is similar to that used for NMR band shape calculations of liquid crystalline systems42 and the spin relaxation theory for surfactant aggregates.14 The quadrupolar interactions are averaged in several ‘steps” by different motional modes occurring on separate time scales.These motions include fast internal motions, such as tram-gauche isomerization, rotation around the molecular long axis, and reorientations in an anisotropic potential. The residual anisotropy produced by these motions is then averaged further by the lateral diffusion of the molecules over the curved surface in conjunction with particle overall tumbling. The parameter set of this model includes the followingparameters: Bo, the magnetic field;x,the quadrupole coupling constant; S,the order parameter; T., the joint correlation time for rotational tumbling and lateral diffusion; and R,, the residual l i e broadening due to fast internal motions. Several values of x have been suggested (167-185 kHz); here we have used the value x = 181kHz.a The magnetic field was fixed at Bo = 4.7 T, and the residual line broadening was 50 Hz. Only S and T~ were varied to get the best fit to the experimental band shapes. The band shape simulation program was kindly provided by Dr. E. Berggren, University of Bologna, Italy. It is written in Fortran, and was run on VAX/VMS workstations.
Theoretical Considerations 2H spin relaxation of selectively deuterium labeled surfactants is a well-established experimental technique to study surfactant dynamics in self-assembledsurfactant systems.l-l2 Having spin I = 1,2Hpossesses a quadrupole moment (eQ) which interacts with the local electric field gradient (efg) tensor. For aliphatic deuterons the efg tensor is approximately cylindricallysymmetric about the CAH bond vector (a = 0). The spin relaxation of 2H is entirely governed by the quadrupolar interactions, and (37) Barr, T.; Oliver,J.; Stubbings, W. V. J. Soc. Chem. Ind., London
1948. fi7. - -_, - . ,4K. ._ .
(38) Kubo, R. J. Math. Phys. (N.Y.) 1963,4, 174. (39) Freed, J. H.; Bruno, G. V.; Polnaszek, C. F. J. Phys. Chem. 1971, 75, 3385. (40) Lynden-Bell, R. M. Mol. Phys. 1971,22,837. (41) Baram, A.; Luz, 2.;Alexander, S. J. Chem. Phys. 1973,58,4558. (42) Berggren,E. PhD. Thesis,UniversityofStockholm,Sweden, 1991. (43) Saerman, 0. J. Magn. Reson. 1986,68,296.
1124 Langmuir, Vol. 10, No. 4,1994
SiSderlind
the longitudinal (R1)and transverse (Rz)relaxation rates are given by the following general expressions:4
R,=-=1 37r2
T, 40 X2(3J(O)+ 5 J b O ) + W(2wo)) (2) where J ( w ) is the reduced spectral density function of the relevant molecular motions, x the quadrupole coupling constant, and wo the Larmor frequency. For surfactants both aggregated in solution and adsorbed on solid surfaces, there are several modes of motions that contribute to the spin relaxation. It has been well established that there is a clear time scale separation of these motional modes into fast local motions (internal modes) and slow motions involving the entire surfactant m o l e ~ u l e . ~The - ~ fast ~ ~ ~local ~ motions are slightly anisotropic due to the preferred orientation of the molecule. However,the residual anisotropy is in most cases averaged to zero by the slow isotropic motions. For spherical aggregates the local motions are to good approximation cylindrically symmetric around the preferred orientation, given by the aggregate normal, and the residual anisotropy is in that case described by the local order parameter S, according to (3)
where 8 represents the angle between the CSH bond vector and the aggregate normal. The angular brackets indicate that this is a time average taken over a time appropriate to average the fast motions but not the slow motions. The most commonly used spectral density function to describe the molecular motions of aggregated surfactants is given by4 J(0)=
(1 - S2)Jf(W)
+ S2Js(0)
(4)
where Jf(w) and J,(w) represent the contributions from the fast (local) and slow (global) motions, respectively. If both motional modes are described by single-exponential correlation functions, then
Here q,, denotes the correlation times for the fast and slow motions, respectively. The fast motions normally fall in the extreme narrowing regime (TWO
