Langmuir 1993,9, 1678-1683
1678
Chain Conformation of Ionic Surfactants Adsorbed on Solid Surfaces from 13C NMR Chemical Shifts Erik Siiderlind and Peter Stilbs’ Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm, Sweden Received January 21,1993. In Final Form: April 2,1993
It is demonstrated that 13CNMR spectroscopy on adsorbed surfactant m o l d a at the liquidisolid interface is possible with present-day instrumentation,at a reasonable signallnoiseratio. lSCNMR spectra were recorded for four cationic surfactants, dodecylpyridinium chloride (DPCL) and three alkyltrimethylammonium bromides (CloTAB, C12TAB, and CleTAB), and two anionic surfactants, sodium dodecyl sulfate (SDS) and sodium odylbenzenesulfonate (SOBS),adsorbed on charged solid surfaces. Chemical shifts were compared with the corresponding data for the dissolved and the micellar state. Downfield shifts were observed for m a t carbone, particularly the centzal carbons in the hydrocarbon chains, on adsorption from solution, indicating the formation of surfactant aggregatea on the solid surface. Shift changes ale0 demonstrate that the trans conformer population of each carbon,with the exception of the f i t carbon of the chain, was increased upon aggregation. For the carbon bound to the headgroup, an upfield shift was observed for all surfactants, except DPCL. Comparisonswith the shiftsof corresponding micelles indicate that for most Surfactants, the surface aggregate does resemble the micellar state from a chain conformational point of view.
Introduction
use to characterize the nature of nonionic Surfactants on similar solid surfaces.lOJ1 Methodsbaaed on NMR spectroscopy have proven very feasible for studying dynamics and structure of other surfactant systems, such as micellar systems, liquid crystals, and micro emu lei on^.'"^^ Particularly valuable were 2H, ‘Y!, or 14Nspin relaxation measurements,which provide information about the molecular dynamics and organization. A number of 13Cshieldingstudies have also been where chemical shift changes upon aggregation were interpreted in terms of conformational changes of the hydrocarbon chains. In the field of amphiphiles on solid surfaces, only a few NMR studies have been report.ed,23-2ehowever. Those studies have exclusively utilized 2HNMR, in attempts to access the molecular dynamics and chemical exchange rates between bound and free states. In the present work, we apply 13C
The adsorption of surfactants onto solid surfaces has been the subject of many investigations. Current interest is focused on fundamental aspects,needed for an understanding of processes involved in a greatvariety of technical An increasing number of recent studies have been concerned with microscopic physicochemical properties, such as molecular organization and dynamics of the adsorbedsurfactantmolecules. A number of models of surfactant aggregation on solid surfaces have been proposed, but there is still some debate regarding the structure and also the nomenclature of those aggregates. The term ‘solloid”, including all types of surfactant or polymer surface bound aggregates, was recently suggested in order to generalize the terminology? ESR,Raman, and luminescence spectroscopy were used to study the micropolarity and microviscosityof sodium dodecylsulfate (9) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rannie, solloids on alumina- and cationic surfactantson s i l i ~ a . ~ ~A.~ R. Langmuir 1992,8, 1204. (10) C u m m h , P. G.; Staples, E.; Penfold, J. J.Phys. Chem. 1991,96, The structure of cationicand nonionic surfactants on silica 5902. has also been studied by means of the neutron reflection (11) Cummins, P. G.;Staples, E.;Penfold, J. J.Phys. Chem. 1990,94, techniq~e.~J’ Small angle neutron scattering also found 3740.
* To whom correspondence should be addressed.
(1) (a) Clunie, J. S.; Ingram,B. T. In Adsorption from Solution at the SolidlLiquid Interface; Pditt, G.D., Rochester, C. H., Ede.; Academic Press: New York, 1983; p 105. (b) Hough, D. B.; Randall, H. M. In Adsorption from Solution at the SolidlLiquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983; p 247. (2) (a) Ingram,B. T.; Ottewill, R. H. In Cationic Surfactants;Physical Chemistry; Rubmgh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 87. (b) herstemu, D. W.; Herrera-Urbina, R. In Cationic Surfactants; Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 407. (c)Laughlin, R. G. In Cationic Surfactants; Physical Chemistry; Rubingh, D. N.,Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 449. (d) Rubingh, D. N. In Cationic Surfactants; Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 469. (3) Somasundaran, P.; Kunjappu, J. T. Colloids Surf. 1989,37, 246. (4) Chandar, P.;Somasundaran, P.; Waterman, K. C.; Turro, N. J. J. Phys. Chem. 1987,91,148. ( 5 ) Chandar, P.; Somasundaran, P.; Turro, N. J. J. Colloid Interface Sci. 1987, 117, 31. (6) Esumi, K.; Nagahama, T.; Meguro, K. Colloids Surf. 1991,57,149. (7) Esumi, K.; Sugimura, A.; Yamada, T.; Meguro, K. Colloids Surf.
1992,62, 249. (8) Rannie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990,6, 1031.
0743-7463/93/2409-1678$o4.oo/0
(12) Chachaty, C. h o g . Nucl. Mugn. Reson. Spectrosc. 1987,19,183. (13) Lmdman, B.; SMerman, 0.; Stilbs,P. In Surfactants in Solution; Mittal, K.L., Ed.; Plenum Press: New York, 1989; Vol. 7, p 1. (14) Khan, A. InNuclear Magnetic Resonance; Webb, G.A., Ed.; The Royal Society of Chemistry: London, 1991; Vol. 20, p 497. (15) Persson, B.-0.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (16) Rueenholm,J. B.;Drakenberg, T.;Lindman, B. J.Colloid Interface Sei. 1978, 63, 538. (17) Maeda, H.; Ozeki, S.; Ikeda, S.; Okabayashi, H.; Matauehita, K. J. Colloid Interface Sci. 1980, 76, 532. (18) Ohbayashi, H.; Yoehida, T.; Matauehita, K.; Terada, Y. Chem. Scr. 1982. 20., 117. - .. . _ _ , -. - - .. (19) Kamo, 0.; Mataushita, K.; Terada, Y.; Yoehida, T.; Okabayashi, H. Chem. Scr. 1984,23, 189. (20) Corno, C.; Platone, E.;Ghelli, S.Colloid Polym. Sci. 1984,262, MI. .
(21) W , T.; Karlstriim, G.; Lindman, B. J.Phys. Chem. 1987,91, 4030. (22) SWerman, 0.; Guering, P. Colloid Polym. Sci. 1987,265, 76. (23) Sbderlind, E.; Stilbs, P. J. Colloid Interface Sci. 1991,143,586, (24) Werlind, E.;Blum, F. D. J.Colloid Interface Sci. 1993,157,172. (26) Sbderlind, E.;Stilbs, P. Submitted for publication. (26) Macdonald, P. M.; Yue Y.; Rydall, J. R. Langmuir 1992,8,164. (27) Yue, Y.; Rydall, J. R.; Macdonald, P. M. Langmuir 1992,8,390. (28) Kuebler, S. C.; Macdonald, P. M. Langmuir 1992, 8, 397. (29) Bayerl, T. M.; Bloom, M. Biophys. J. 1990,58, 357.
Q 1993 American Chemical Society
A 13C Shielding Study of Adsorbed Surfactants
Langmuir, Vol. 9, No. 7, 1993 1679
of 200 ma/g. The alumina was y-AlzOa, obtained from Johnson Matthey, Ward Hd,MA. The Nrbased surface area was determined to be 103 m2/g. The adsorbents were dried under vacuum at 100 O C for severalhours to remove excesswater. DPCL was obtained from Aldrich-Chemie,CieTAB from Merck, Clr TAB from Sigma,CloTAB from Eaatman Kodak, and SDS from FlukaChemie. SOBSwasakindgiftfromProfeseorB. Lindman, Lund, and was originally obtained from Profeasor Bothorel's group, Bordeaux. All Surfactants were purified by meam of recrystallization before use. Heavy water was used throughout all experiments. Theadsorptionsampleewereprepared by weighing appropriate amountsof adsorbentinto screw-capped centrifugationtuba. A (1) known amount of water was added and pH was adjusted to 4-6 ,6 = Pase+ ( 1 - PaMf (SDS and SOBS on alumina) or 10 (cationica on silica) by the where pa is the fraction of molecules that is aggregated. addition of HC1or KOH solutions. Finally, the surfactantswere In suspensions of solid particles in surfactant solutions, weighed intothe tubes,which were equilibrated by slow rotation the situation is similar. In this w e , the molecules exist for several days at ambient temperature. Special precautions in surface-bound aggregates or dissolved in solution. The were made to keep the temperature at or above 26 "C for the dissolved state may contain both micelles and free CTAB sample to avoid surfactant precipitation. Linear SOBS molecules. Provided the chemical exchange between the has a Kraffttemperature above26 O C , but it ie sufficiently soluble at room temperatureto obtain plateau adsorptionon alumina.g* adsorbed aggregatesand the solution is fast, the observed After equilibration, the suspensionswere centrifugedand excess chemical shift will be given by an expression equivalent supernatant was removed for analysis of the surfactant content. to eq 1. However, the 2H NMR spectra of centrifuged The concentrations of &TAB, &TAB, and SDS were detersamples of a number of cationic surfactants adsorbed on mined by two-phasetitration accordingto the method described silica or polystyrene particles consisted of a superposition ref 36, usingchloroformasthe organicsolventand bromophenol . ~ ~ ~ ~ ~in of a narrow resonance upon a broad r e s o n a n ~ e The blue asthe indicator. The CloTABconcentrationwas not poeeible narrow resonance was claimed to be dissolved surfactant to determine accurately by this method. However, lacchemical molecules,implyingthat the chemical exchange is slow on shifts of the supernatantshowed that only free molecules were the NMR time scale. Furthermore, the spectra were present. The DPCL and SOBS concentrationswere determined entirely dominated by the resonances of adsorbed molwith a W spectrophotometer (Shimadzu,W-110-2) at X = 286 nm. From the concentration in solution and the overall sample ecules. For these reasons, no corrections of the observed composition, the adsorption level (r)was calculated. shifta of adsorbed cationic surfactants are required. The '42 NhfR measurements were performed at 100.6 MHz Preliminary 2H NMR studies of sodium dodecyl sulfate on a Bruker AM 400spectrometer using field/frequencylock on adsorbed on alumina also indicated that anionic surfacthe DzOin the samples. The sample temperature was kept at tants behave similarly. 26 "C, except for the micellar SOBS sample which was kept at In thie work, we have chosen to study the adsorption of elevated temperatures. The NMR spectra of the adsorption ionic surfactants onto charged oxide surfaces. Four sampleswere recorded with a sweep width of 10 OOO Hz and 32K different cationicsurfactants,dodecylpyridinium chloride points, resulting in an optimal resolution of 0.008 ppm. The (DPCL), decyltrimethylammonium bromide (CloTAB), number of recorded transients varied with sample, but typically 40 OOO scam were performed. The chemical shifts are referred dodecyltzimethylammoniumbromide (CI~TAB), and hexato TMS,whichwasintroducedintothe samplwin asmallcapillary decyltrimethylammonium bromide (CISTAB) were adtube. The use of an external reference may provide a complisorbed on silica particles. Adsorption data for the CloTAB cation, since the volume magnetic susceptibility of the sample system has not been previously reported, but there are and the reference may be different. In the present work, where adsorption isotherms available for C12TAB and changes in chemical shifts between two states are monitored, on silica.g0 The adsorption isotherm of DPCL on porous susceptibility corrections only influence the absolute values of silica particles has also been described.6 The anionic the shift changes,but not the profile over the hydrocarbon chain surfactants studied were sodium dodecyl sulfate (SDS) as the correction ie equal for all carbons. Therefore, differences and sodium octylbenzenesulfonate (SOBS),adsorbed on in susceptibilities in sample and reference were not accounted for. '9c signal aeeignmentsof micellized SDSl- and C,,TAB%are alumina. The adsorption of SDS on alumina has been the available, whereas the spectrum of micellized SOBS and DPCL subject of many s t u d i e ~ . ~The * ~adsorption ~ * ~ ~ of SOBS was assignedfrom TIrelaxation data, assumingthat TIincreaeee on the same surface is less studied, but isotherms are continuouslyalong the chain,toward the terminal methyl group. Hence, all surfactant/solid systems are well The spectra of adsorbed surfactantswere assigned according to studied and characterized. A common feature of all these the assignment of micellized surfactants. adsorption isotherms is that a t concentrations in solution around the critical micelle concentration the adsorption Results levels off to a plateau. The aim of this study was to The surfactant adsorption levels, I' in mmoUg,of DPCL, investigate the conformation changes in the chains when CloTAB, C12TAB, and &TAB on silica, and SDS and the surfactants are adsorbed to a degree correspondingto SOBS on alumina, are shown in Table I. The mean area the plateau value. occupied per molecule, A (A2), is also shown. A wm ,'l as if all molecules were calculated from the adsorption, Experimental Section evenly spread out over the solid surface, usingthe relation The silica, Cab-0-Sil grade MS, was obtained from Fluka Chemie AG, Buchs. According to the manufacturer's specifiA = llNAF cations, the particles are nonporous with a specific surface area where N Arepresents Avogadro's constant. Obviously,the (30)Bijeterhh, B. H.J. Colloid Interface Sci. 1974,47,186. calculated A may deviate from the true occupied area per (31)Sjablom, J.; Blokhua, A. M.; Sun,W. M.; Friberg, S. E. J. Colloid molecule, since the structuring of the adsorbed layer has Interface Sci. 1990,140,481.
shifta to show that the hydrocarbon chains of a number of surfactants do undergo conformational changes when aggregated on solid surfaces. The shifta are also compared with the shifts of free and micellized molecules. In micellar solutions, the surfactant molecules exist in two states: "free", as monomers, or %ggregated" in micelles. Due to fast chemical exchange between the two states on the pertinent NMR time scale, the observed chemical shift, becomes a population weighed average of the shift of free molecules,6f, and the shift of aggregated molecules, 6,
(32) Blokhua, A. M.; Sjablom, J. J. Colloid Interface Sci. 1991,141, 395. (33)Denoyel, R.;Rouquerol,F.;Rouquerol,J. ColloiL Surf. 1989,37, 295. (34)Denoyel,R.;Rouquerol,J. J. Colloid Interface Sci. 1991,143,555.
(35)Barr,T.;Oliver, J.; Stubbinge, W. V. J. SOC.Chem. I d . , London 1948,67,45. (36) william^, E.; Sears, B.; Allerhand,A.; Cordea, E. H.J. Am. Chem. SOC.1973,96,4871.
SBderlind and Stilbs
1680 Langmuir, Vol. 9,No.7,1993 Table I. Adsorption Densities, I' in mmol/g, of the NMR Sampler, and the Corresponding Averse Amas Occupied per Adsorbed Molecule, A in At 0.64 >0.34 0.95 0.93 0.41 0.51
DPCL/silica CioTAB/silica C1zTAB/~ili~a CleTAB/dh
SDS/alumina SOBS/alumina
52
