Langmuir 1994,10,4195-4202
4195
2HNMR Study of Hydrocarbons Adsorbed by Ion-Exchange Resins: The Effect of Surfactants C.T.Yim* Department of Chemistry, Dawson College, 3040 Sherbrooke Street West, Westmount, Quebec, Canada H3Z 1A4
G. R. Brown Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received April 8, 1994. In Final Form: August 9, 199P The interactions of two nonpolar molecules, deuterated cyclohexane and benzene, with two strong anionexchange resins have been examined using deuterium NMR spectroscopy, The observed spectra consisting of overlapping narrow and broad peaks, which can be attributed to free and adsorbed hydrocarbon molecules, respectively, show strong dependence on the nature of the exchange resin. The effect of a cationic surfactant, sodium cholate, on the observed spectra suggests that the bound surfactant molecules are capable of “solubilizing”hydrocarbons into the resin phase. Longitudinal relaxationtimes were measured as a function of temperature at two different magnetic field strengths. The results reveal the presence of fast and slow molecular motions similar to those observed in other restricted systems such as micellar solutionsand lipid bilayers. The data were further analyzed in terms of the two-stepmodel, involving fast slightly anisotropic motions superimposed upon slow isotropic motions.
Introduction The retention behavior of many amphiphilic ions and neutral solutes on ion-exchange resins indicates the importance ofhydrophobic interactions between the solute and stationary phases.’ On the other hand, surfactant adsorption at the solid/liquid interface also offers a convenient means of altering the surface properties ofthe solid, particularly its hydrophobicity or hydrophilicity, and this method has been applied successfully in ion-pair chromatography to separate charged species on lipophilic stationary phases.’ It is of interest, therefore, to study the interactions between nonpolar solutes and ionexchange resins. Here we report the results of an investigation of the interactions of two hydrocarbon probe molecules, deuterated cyclohexane and benzene, with two different anion-exchange resins. The effect of the surfactant, sodium cholate, was also examined with the additional aim of investigating the possible formation of micelle-likeaggregates by the bound cholate ions. Several studies of the molecular organization and dynamics of adsorbed surfactants have been reported recently.2-8The results point to the formation of surface aggregates and, depending on the nature of surfactants and adsorbents, a number of structures such as bilayers,2-6 micelles, or mixed surface have been suggested. Previous
* To whom correspondence should be addressed.
* Abstract published inAdvance ACSAbstracts, October 1,1994.
(1)See, for example: Haddad, P. R.; Jackson, P. E. Ion Chromatography; Elsevier Science Publisher; Amsterdam, 1990; Chapters 6 and 7. (2) Soderlind,E.; Blum,F. D. J . Colloid Interface Sci. 1993,157,172. (3) Esumi, K.; Sugimura, A.; Yamada, T.;Meguro, K. Colloids Surf. 1992, 62, 249. (4)Esumi, K.; Watanabe, N.; Meguro, K. Langmuir 1991, 7, 1775. (5) Rennie,A. R.; Lee, E. M.; Simister,E. A.;Thomas, R. K. Langmuir 1990,6,1031. (6)Ruths, M.; Sj6blom, J.; Blokhus, A. M. J . Colloid Interface Sci. 1991, 145, 108. (7) Denoyel, R.; Rouquerol, F.;Rouquerol, J. J . Colloids Surf. 1989, 37, 295. (8) Denoyel, R.; Rouquerol, J. J . Colloid Interfme Sci. 1991, 143, 565.
work in this laboratory also demonstrated the importance of hydrophobic interactions in the binding of bile salts by ion-exchange resins,ll and aggregate formation within the beads has been proposed.12 NMR spectroscopy has been employed widely in studies of heterogeneous systems. Relevant information has been deduced from line splitting, chemical shielding anisotropy, relaxation rates, and self-diffision measurements. Among the commonly accessible nuclei, the deuteron has been particularly useful for probing the molecular dynamics and organization in anisotropic systems, such as micellar solutions,13lipid bilayers,14 liquid c r y ~ t a l s , l ~ -and ’ ~ interfacial regions.18 Interesting 2HN M R studies have been conducted on the dynamics of surface groups of alkylmodified silica in the “dry” state, and in the presence of wetting solvents, surfactants, and mesogenic molecule~.~ Recently, ~ - ~ ~ the line splitting and spin relaxation behavior have been determined for deuterated surfactant molecules adsorbed on nonporous silica gel,22,23 on porous alumina,2 and on polystyrene In some systems the spectra of adsorbed surfactant molecules showed well(9) Somasundaran, P.; Fuerstenau, D. W. J . Phys. Chem. 1966, 70, 90. (10)Dick, S. G.;Fuerstenau,D. W.; Healy,T.W. J . ColloidInterface Sci. 1971,37, 595. (11) Zhu, X. X.; Brown, G. R. Can. J.App1. Spectrosc. 1990,35,121. (12) LBonard, D.;Clas, S.-D.;Brown, G. R. Reactive Polym. 1993,20, 131. (13) Chachaty,C.Prog.Nucl. Magn. Reson. Spectrosc. 1987,19,183. (14) Seelig, J.; Macdonald, P. M. ACC.Chem. Res. 1987,20, 221. (15) Spiess, H. W. Adv. Polym. Sci. 1986, 66, 23. (16)Muller, K.; Meier, P.; Kothe, G. Prog. Nucl. Magn. Reson. Spectrosc. 1986, 17, 211. (17)Yim, C. T.;Gilson, D. F. R.; Budgell, D. R.; Gray, D. G. Liq. Cryst. 1989,14, 1445. (18) Grandjean, J. Annu. Rep. NMR Spectrosc. 1992,24, 181. (19) Gangoda, M. E.; Gilpin, R. K.; Figueirinhas,J. J . Phys. Chem. 1989,93,4815. (20) Gangoda, M. E.; Gilpin, R. K. Langmuir 1990, 6, 941. (21) Zeigler, R. C.;Maciel, G. E. J . Am. Chem. SOC.1991,113,6349. (22)Sbderlind, E.; Stilbs, P. J . Colloid Interface Sci. 1991, 143, 586. (23) Sbderlind, E.; Stilbs, P.Langmuir 1993, 9, 2024. (24) Macdonald,P.M.; Yue, Y.;Rydall, J. R. Langmuir 1992,8,164. (25) Kuebler, S. C.; Macdonald, P. M. Langmuir 1992, 8, 397.
0743-746319412410-4195$04.50/00 1994 American Chemical Society
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Yim and Brown
Table 1. Line Width and TIValues for Adsorbed C&p resin PAAl2 YO
(MHz)
76.73
temp ("C) 7 23 29 39 49
46.98
20 32 44 56
NaC addition no Yes no yes no Yes no yes no Yes no yes no Yes no yes no Yes
line width (Hz) 2010 3680 1300 1810 690 1140 424 620 210 331 1150 1920 740 1100 334 530 124 211
Ti (9) 0.064 (0.006) 0.055 (0.006)b 0.080 (0.005) 0.064 (0.005) 0.084 (0.005) 0.068 (0.005) 0.090 (0.005) 0.073 (0.005) 0.109 (0.005) 0.080 (0.004) 0.050 (0.004) 0.042 (0.004) 0.057 (0.004) 0.047 (0.004) 0.065 (0.004) 0.052 (0.004) 0.087 (0.005) 0.059 (0.003)
cholestyramine line width (Hz)
Ti (9)
2140
0.057 (0.006)
1340
0.069 (0.004)
800
0.070 (0.006)
540
0.076 (0.005)
231
0.081 (0.005)
1200
0.042 (0.004)
750
0.043 (0.004)
383
0.053 (0.004)
203
0.060 (0.003)
Estimated error for the line width is &lo%,and the number in parentheses is the estimated uncertainty in additional uncertainty due to the long pulse length used in the measurement.
defined powder patterns, while in others only broad, featureless peaks were observed. Deuterium NMR, dominated by quadrupolar interactions, is almost exclusively governed by the motions and orientation of the deuteron-containing bond (e.g., C-D bond) relative to the applied magnetic field. A C-D bond undergoing a rapid reorientation motion about a definite axis produces a uniaxial powder pattern with a splitting Av given by
[ll where (e2q&/h)is the deuterium quadrupole coupling constant, usually taken to be 170 and 185 kHz for the aliphatic and aromatic C-D bonds, respectively. The order parameter, S, is given by
[21 where 8 is the angle between the C-D bond and the unique axis of motional average, and the angular brackets indicate an average over all intramolecular and reorientation motions. In writing eq 1 and 2, we have neglected the small non-zero asymmetry parameter of the electric field gradient. In this work we examine the hydrocarbon-resin interactions by monitoring the 2HNMR spectra of deuterated hydrocarbons. In an attempt to quantify the motions of the adsorbed probe molecules, their spin relaxation behavior was studied as a function of temperature at two different magnetic field strengths.
Experimental Section Two strong anion-exchange resins, cholestyramine (Dowex 1x2-100, -[CH~-CH(C~H~-CHZ-N+(CH~)~C~-)-],, 2% crosslinked, 50-100 mesh, 3.3 meq/g) and a polyacrylamide-based resin (-[CH~-CH(CO-NH-(CH~)I~-N+(CH~~C~-)-L, 4%crosslinked, 20-40 mesh, 3.4 meq/g), designated as PAA12, were used in this study. The cholestyramine (Dowex) was obtained from Aldrich Co., and it was further treated using procedures described in ref 26. The polyacrylamide-based resin was synthesized in this laboratory.2' Both resins were kept under vacuum at room temperature for 24 h prior to use. The sodium cholate (NaC) was purchased from Sigma Chemical Co. and used without (26)Zhu, X. X.; Brown, G. R.; St-Pierre,L. E. J. Pharm. Sci. 1992, 81, 65.
(27) (a)Wu, G. Ph.D. Thesis, McGill University, Montreal, 1990. (b) Wu, G.; Brown, G . R.Reactive Polym. 1991, 14, 49.
7'1.
It may contain
W h e r purification. The zHNMR spectra were acquired at 45.98 and 76.73 MHz using Varian XL-300 and Unity 500 spectrometers, respectively. The spin-lattice relaxation times were measured using the inversion-recovery pulse sequence. All 21' values were evaluated using a nonlinear three-parameter least squares fitting procedurez8available on the Varian instruments. The n/2 pulse lengths were 16 and 55 ps on the XL-300 and Unity 500 spectrometers, respectively. The low pulse power available on the latter spectrometer compelledthe use of long pulse lengths. This may cause minor excitation profile problems and introduce additional uncertainty in the 2'1 values of the three broad signals observed at the lowest temperature (7 "C) and with line widths greater than 3.0 kHz (see Tables 1 and 2).28*29 "he samples were prepared by weighing appropriate amounts of resin, C6D12 or C6D6, and NaC (if required) into 1O-mm N M R tubes. Sufficient 0.050 M NaCl solution, prepared with deuterium-depleted water, was added to each sample to allow the resin to become fully swollen at equilibrium. The sample tubes were sealed, vigorously shaken, and kept in a 40-50 "C bath overnight, and then left at room temperature for at least 1 week before the spectra were recorded. There was no noticeablechange in the observed spectra during the course of the investigation (6 months). The total concentration of hydrocarbon (C6D12or C6D6) in each sample, expressed in terms of unit resin mass, was about 0.20 mmol per gram of resin. For samples containing sodium cholate its concentration was about 0.86 mmol per gram ofresin, and on the basis of previouslydetermined adsorption is0therms,2~ it is expected that more than 96%ofthe cholate ion was adsorbed by the resins. The cholate ion concentration in the isotropic phase was below 4 mM, the cmc value of sodium cholate in 0.050 M NaC1.30
Results and Discussion Figure 1shows spectra of samples containing CsDlz and PAAl2 resin (a and b) and of samples containing C6D12 and cholestyramine resin (c and d), all obtained at 29 "C with a spectrometer operating at 76.73 MHz. The effect of bound cholate ions can be seen clearly by comparing Figure l a with lb, and ICwith Id. Except for Figure IC, each spectrum consists of two narrow lines superimposed on a broad featureless peak. The narrow line at lower field can be attributed to the residual deuterium nuclei in water and that at higher field to C6D12, both in the isotropic phase. The broad peak, attributed to the adsorbed cyclohexane, indicates that the quadrupolar interactions may not be completely averaged out as a result (28) Sass, M.; Ziessow, D. J. Magn. Reson. 1977,25, 263. (29) Martin, M. L.; Martin, G. J.; Delpuech, J. J. Practical NMR Spectroscopy; Heyden & Son Ltd.: London, 1980; Chapter 7. (30)Carey, M. C.; Small, D. M. Arch. Intern. Med. 1972, 130, 506.
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Langmuir, Vol. 10, No. 11, 1994 4197
Table 2. Line Width and TIValues for Adsorbed c&"
resin PAAl2 YO (MHz)
temp ("C)
NaC addition
76.73
7
no
23 29 39 49 46.98
59 19 32 35 44 56
Yes no Yes no Yes no Yes
no
Yes Yes no Yes no Yes no Yes no Yes no Yes
line width (Hz) 1580 3700 650 1650 418 930 234 520 136 202
cholestyramine
Ti (8) 0.044 (0.004) 0.061 (0.007)b 0.050 (0.004) 0.049 (0.004) 0.055 (0.004) 0.051 (0.004) 0.061 (0.004) 0.053 (0.003) 0.069 (0.003) 0.054 (0.003)
690 1530 390 870
0.031 (0.003) 0.031 (0.003) 0.034 (0.003) 0.032 (0.003)
198 338 92 154
0.044 (0.004) 0.035 (0.003) 0.052 (0.004) 0.039 (0.004)
line width (Hz) 860 3360 314 1400 160 850 94 471 51 226 96 271 1340 162 790 155 750 104 346 38 166
Ti (8) 0.050 (0.005) 0.066 (0.006)b 0.055 (0.005) 0.056 (0.005) 0.064 (0.006) 0.057 (0.004) 0.063 (0.005) 0.055 (0.003) 0.077 (0.005) 0.056 (0.003) 0.057 (0.004) 0.034 (0.003) 0.036 (0.003) 0.031 (0.004) 0.032 (0.004) 0.041 (0.003) 0.036 (0.003) 0.048 (0.003) 0.035 (0.003) 0.059 (0.005) 0.037 (0.003)
a Estimated error for the line width is &lo%, and the number in parentheses is the estimated uncertainty in Ti. It may contain additional uncertainty due to the long pulse length used in the measurement.
Figure 1. 2H NMR spectra of CsDlz-resin samples at 29 "C, YO = 76.73 MHz: (a)CeD12-PAA12 in 0.05 M NaC1; (b)C6D12PAA12-NaCin 0.05 M NaC1; ( c )C&z-cholestyramine in 0.05 M NaCl; (d) C6D12-cholestyramine-NaC in 0.05 M NaC1. of reduced motional freedom. Similar spectra consisting of narrow peaks superimposed on a broad component were observed for the C~Dc-resinsystems. For all the spectra recorded a t room temperature or higher, the broad peak appears to have a Lorentzian line shape. However, several spectra recorded at the lowest temperature (7 "C) exhibit somewhat higher intensity in the wings. Here we did not observe a uniaxial powder pattern with reduced quadrupole splitting which, as discussed in the Introduction, is expected for a C-D bond undergoing rapid anisotropic reorientational motions about a unique axis. Similar featureless peaks have been reported for polymers,3l alkylmodified s i l i ~ a and , ~ ~other , ~ ~systems.23,32-34They have been explained in terms of(a)the presence of slow motions, isotropic or otherwise, with correlation times in the order (31)Meirovich, E.; Samulski, E. T.; Leed, A.; Scheraga, H. A.; Rananavare, S.; Nemethy, G.; Freed, J. H. J. Phys. Chem. 1987,91, AAAn ----.
(32) Lifshitz, E.; Goldfarb, D.; Vega, S.; Luz, Z.; Zimmermann, H. J.
J.Am. Chem. SOC.1987,109,7280.
(33)Meirovitch, E.; Freed, J. H. Chem. Phys. Lett. 1979,64, 311. (34) Davidson, D. W.; Garg, S. IC;Ripmeester, J. A. J.Mugn. Reson. 1978,31, 399.
of the inverse quadrupole splitting, or (b) the absence of a unique axis of reorientation due to the heterogeneity of the surface. In our case both factors may have affected the observed line shapes. The broad peaks narrow rapidly as the temperature increases. The line widths at halfheight, AYYZ,at several temperatures are listed in Tables 1and 2 for adsorbed C6D12 and CsD6, respectively. Within experimental error, the line widths are independent of the spectrometer frequency. The spectrum in Figure ICexhibits two narrow lines superimposed on a barely noticeable broad component, indicating that the cholestyramine adsorbs only very small amounts of C6D12. In this system most of the cyclohexane remained as a separate phase on the surface ofthe solution and was outside the detecting volume of the NMR coil. In this spectrum the total signal intensity for the adsorbed and nonadsorbed cyclohexane is, therefore, much less than that observed for the other systems studied. Due to the extremely low intensities of the broad peak, reliable measurements on its line width and relaxation time could not be obtained. The data in Tables 1and 2 show that in the absence of bound cholate the two resins behave quite differently toward the nonpolar probe molecules. The difference was observed either in the amount adsorbed (C6D12, as described above) or in the width of the broad component (C6D6). The larger line width observed with the Cd&PAAl2 system indicates less motional freedom, including the orientational freedom for adsorbed C6D6 molecules. We attribute this to the presence of the dodecyl groups in the resin side chain, which provide a hydrophobic milieu for the nonpolar molecules, and hence an environment with greater motional restriction. As shown in Figure Id, upon the addition of sodium cholate to the cholestyramine-C6Dl2 system, a broad peak appears with a n intensity comparable to those observed in other systems, clearly indicating the ability ofthe bound surfactant molecules to "solubilize" cyclohexane into the resin phase. For the other systems investigated in this study, there was no noticeable increase in the intensity of the broad peak upon the addition of sodium cholate. However, a substantial increase in the line width of the broad signal was observed in all cases. Measurements of the longitudinal relaxation time (2'1)
Yim and Brown
4198 Langmuir, Vol. 10,No. 11, 1994
k s 7oom
’Looms
loo ms 70ms
Q 50ms ms 30
Figure 2. Stack plots of the 2H NMR spectra of sample C6D12- cholestyramine -NaC at 32 “C (YO = 46.98 MHz), showing the modulation of the spectral intensity as a function of delay time z in the inversion recovery pulse sequence. The plots reveal that the two narrow resonances have much longer TIthan the broad signal. were performed at two different magnetic field strengths, and as shown by the typical stack plots in Figure 2, the two narrow lines have much longer T I values than the broad signal. The T Ivalues of the broad signal are listed with the line width data in Tables 1and 2. In addition, for a Lorentzian line, the transverse relaxation time, T2, can be obtained from the line width at half height Av112: TZ= (nAv~2)-l.To justify the use of this equation, and to verify the assumption that the broad signals are essentially Lorentzian, direct measurements of T2 at 32 “C were performed using the standard CPMG pulse sequence. For all of the systems studied, reasonable agreement was observed between two sets of T2 values obtained from CPMG measurements and from the line width data. From deuterons, the longitudinal and transverse relaxation rates, (T1-l)and (Tz-l),are dominated by a single mechanism of intramolecular origin, the quadrupolar interaction. In isotropic phases, the relaxation rates are given by
and
where wo is the Larmor frequency. The spectral density functions, J(w),are Fourier transforms of a time correlation function characterizing random molecular motions. If the time correlation can be described by single exponential functions, then
J(w)=
1
+
TC
where tcis the correlation time for the motion under consideration. Strictly speaking, eqs 3 and 4 are valid only for isotropic phases. Thus, their use implies that the
quadrupolar interactions have been completely averaged out. Although this assumption may not be exactly correct for these systems, any residual coupling, if present, is obviously quite small. Therefore, at least as a first attempt, the use of these equations for the interpretation of the relaxation data seems to be justified. The data in Tables 1and 2 show the following features: (a) The transverse relaxation time, Tz,is much shorter than TI. For C6D12 in the PAAl2 resin at room temperature, T I= 30OTz. (b)The T Ivalues show a fieldfrequency dependence with significantly lower values of T Iobserved at the lower field. ( c ) Neither the addition of sodium cholate nor the variation of temperature has a strong effect on the observed values of TI. In the presence of sodium cholate, the T I values of C6D6 show little variation with temperature. The last point is in striking contrast with the strong dependencies shown by the line widths of the same signal. More detailed calculations show that these features cannot be rationalized on the basis of slower isotropic molecular orientations with a distribution of reorientation rates because of the heterogeneity of the systems. Due to the J ( 0 )term in eq 4, the line width or T2 is dominated by relatively slow molecular processes, whereas the longitudinal relaxation time is determined mainly by relatively rapid reorientation motions. In terms of conventional theory of nuclear relaxation for the bulk phases, these features can be attributed to the presence of several motional processes with rather different correlation times. In further analysis ofrelaxation data, we w i l l first consider different types of motions which could contribute to spin relaxation in the systems and make an order of magnitude estimation of their individual correlation times. For the simple “probe”molecules, such as c6D6 or C6D12, the fast motion can only be molecular reorientation or rotation. Available data for solubilized hydrocarbons in micellar solutions36and for organic liquids in porous silica (35) Wasylishen, R. E.; Kwak, J. C. T.; Gao, Z.; Verpoorte, E.; MacDonald,J. B.; Dickson, R. M. Can. J. Chem. 1991,69,822.
Hydrocarbons Adsorbed by Ion-Exchange Resins
Langmuir, Vol. 10, No. 11, 1994 4199
glasses36suggest that the rotational correlation times for the deuterated probes are probably in the range of 5-50 ps. Thus, they are 5-50 times slower than in the bulk liquid but are still in the fast motion regime with orc > 1 and or, = 1) the spectral densities for a twodimensional liquid are much higher than for a threedimensional liquid, and that leads to shorter relaxation times especially Tz. Since the two-dimensional diffusion affects mainly the intermolecular contribution to the total relaxation rates, proton relaxation times with their strong intermolecular contribution are expected to be most affected by the restricted geometry. The restricted diffusion model gave a satisfactory account of proton relaxation behavior in heterogeneous systems, in which intermolecular couplings are deemed to be predominant. Recently, Jonas and co-workers have measured 2H relaxation times of several deuterated molecular liquids in porous silica glasses with narrow pore size distribution.36z52s53 In comparison with TI, much smaller values of Tz and TIQwere observed for these liquids. For example, (48) Avogadro, A.; Villa, M. J. Chem. Phys. 1977, 66, 2359. (49) Forb, J. P.; Winterhalter, M.; McConnel, H. M. J. Chem. Phys. 1984,80,1059. (50) Tabony, J.; Korb, J. P. Mol. Phys. 1986, 56, 1281. (51) Korb, J. P.; Xu,S.; Jonas, J. J. Chem. Phys. 1993, 98, 2411. (52) Lui, G.; Li, Y.; Jonas, J. J. Chem. Phys. 1991, 95,6892.
4202 Langmuir, Vol. 10, No. 11, 1994
they reported values of 160 and 23.0 ms for TI and T2, respectively, for liquid C6D12 in porous glass with a 3.0 nm pore size.52 Furthermore, for liquids confined in pores with a diameter of 3 nm or less, they observed a logarithmic frequency dependence of relaxation rates (TI and TI@),a distinctive character of two-dimensionalliquids as shown by the restricted diffusion model of Avogadro and Villa. Jonas and co-workers attributed their findings to the twodimensionalbehavior of liquids confined in smaller pores, but no detailed analysis was given concerning the effect of geometrical confinement on reorientational dynamics. More experimental and theoretical work is required in order to assess the contribution of the confined geometry to our relaxation data. It seems unlikely that the large difference in our longitudinal and transverse relaxation rates (TI 3ooT2) is mainly caused by geometrical confinement. Nevertheless, it may contribute to this difference and to the shortening of longitudinal relaxation time observed at lower fielafrequency, leading to an overestimation of S values. Even with a moderate contribution from the confined geometry effects, our relaxation data would still lead to relatively large order parameters. The large order parameters may indicate that these nonpolar molecules sense a more hydrophobic and spacially confined environment in our systems. We have assumed that the quadrupolar interactions have been completely averaged out by the slow motions. A unique axis of reorientation is also assumed for the adsorbed molecules so that a single order parameter can be used to define the amount of anisotropy in the fast motion. In the above we have employed the two-step model in an attempt to quantify the molecular motions of adsorbed (53) Lui, G.; Mackowiak,Y.; Li,Y.; Jonas, J. Chen. Phys. 1990,149, 165.
Yim and Brown
molecules in terms of correlation times and order parameters. It was further argued that, due to the complexity of the systems and the basic assumption that only two motions are involved, the simple model yielded unreasonably high order parameters. However, we are of the opinion that this approach has furnished a useful, although somewhat oversimplified, picture which allows qualitative discussions concerning the environment and dynamics of the adsorbed molecules.
Conclusions We have demonstrated that the two resins PAAl2 and cholestyraminebehave quite differently toward nonpolar hydrocarbons. The difference can be attributed to the presence of dodecyl groups on resin PAA12, which provide an amiable environment for the nonpolar molecules. We have also shown that the addition of sodium cholate enhances the resin-hydrocarbon interaction. The 2H relaxation measurements reveal a great difference between the longitudinal and transverse relaxation times, TI >> T2, and also a significant frequencylfield dependence of TI. These features indicate the presence of slow motions which contribute to the relaxation of deuterated hydrocarbons. Further analysis of the relaxation data yielded large order parameters, suggesting that there exists a strong hydrophobic interaction in these systems. The results also support the view that bound cholate ions participate in the formation of micelle-like surface aggregates in the resin phase.
Acknowledgment. We thank Drs. Kishore Pate1 and G. WU for synthesizing the polyacrylamide-based resin. Financial support in the form of operating grants from the Quebec Government (Fonds F C m ) is gratefully acknowledged.
