Langmuir 1990,6, 941-944 in the dense L1 phase. The surfactant has been added to the mixture in Figure 10, causing the formation of threephase Ll-L2-G equilibrium at constant temperature and pressure. The polar dye is now observed to be almost exclusively in the surfactant-rich L2 phase. In Figure 11, the pressure is increased isothermally to 1055 psia, where the L3 phase is formed. The three distinctly clear L1, L3, and gas phases are contrasted by the red-orange color of the L2 phase. The general conclusion that can be reached from these dye solubilization experiments is that no evidence has been found for the formation of reverse micelles in the L3 or G phases or for the formation of micelles in the L1 phase for either C4E1 or C8E3. Since nearly all of the surfactant in both these ternary mixtures in found in the L2 phase, where both dyes are also most evident, it is
941
obvious that the dyes partition due to some sort of surfactant-dye interaction. Clearly, further study is needed to determine the nature of this interaction.
Acknowledgment. We gratefully acknowledge financial support provided by the National Science Foundation (CPE 8351228) and the following industrial sponsors: Air Products and Chemical Co., E. I. du Pont de Nemours and Co., Dow Chemical Co. Foundation, Exxon Education Foundation, Imperial Chemical Industries, Merck and Co., and Proctor and Gamble Co. We are particularly indebted to Merck and Co. for a Pre-Doctoral Fellowship supporting J. M. Ritter. Registry No. C4E1,111-76-2; C4E3, 19327-38-9;Sudan 111, 85-86-9;COz, 124-38-9.
Articles 2HNuclear Magnetic Resonance Investigations of the Dynamics of Alkyl-Modified Silica in the Presence of Solvent, Surfactant, and Mesogenic Molecules M. E. Gangoda and R. K . Gilpin' Department of Chemistry, Kent State University, Kent, Ohio 44242 Received April 17, 1989.In Final Form: December 5, 1989 Specifically deuterated dodecyltrichlorosilane, (C13Si(CH2)loCD2CH3), was synthesized and reacted with porous chromatographicgrade silica to obtain a labeled alkyl-modified surface. Subsequently, 2H NMR quadrupole echo spectra were recorded, and spin-lattice relaxation times ( T1) were calculated for the material in the presence of organic and hydroorganic solvents, an aqueous surfactant solution, and the nematic phase of a thermotropic liquid crystal. Line width, shape, and T1 were sensitive to changes in the interfacial conditions. The spin-lattice relaxation rate (T1-l) was found to be linearly related to the square of the line width at its half-height (Av1p2). These results suggest that the motional averaging process for the C-2H bond is a combination of fast and intermediate molecular fluctuations similar to those observed in other restricted systems such as lipid bilayers and micellar systems.
Introduction Alkyl-modified surfaces are important stationary phases in chromatography as well as for bulk filler and additive applications. Generally, these materials are prepared by reacting either alkylchloro or alkylalkoxysilanes with silica. In the former case, particles with controlled size, geometry, and porosity often are used to produce packing used in high-performance liquid chromatography in the reversed-phase (RP) mode. The interfacial properties of RP surfaces have been investigated by numerous chromatographic and spectrometric methods. Infrared (IR),l-3 nuclear magnetic resonance (NMR),4-' and electron spin resonance (ESR)8.9
* Author to whom all correspondence should be addressed.
(1)Sander, L. C.; Callis, J. B. Anal. Chem. 1983,55, 1068. (2) Suffolk, B. R.; Gilpin, R. K. Anal. Chem. 1985,57, 596. (3) Suffolk, B. R.; Gilpin, R. K. Anal. Chem. Acta 1986, 181, 259. (4) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. SOC.1983, 105, 1848.
0743-7463/90/2406-0941$02.50/0
spectrometric techniques are some of the tools which have been utilized to examine various conformational and dynamic aspects of the interface. In the case of the NMR studies, 13C experiments have been used most often. Investigations have been carried out under both dry state and solvated conditions and have provided information respectively about the gassolid and liquid-solid interfaces. Although wide-line 2H NMR techniques have been used widely in characterizing macromolecules,10Jl mesogens,l2J3 molecular as(5) Gilpin, R. K.; Gangoda, M. E. J. Magn. Res. 1985, 64,408. (6) Gilpin, R. K.; Gangoda, M. E. Anal. Chem. 1984,56, 1470. (7) Gangoda, M. E.; Gilpin, R. K.; Fung, B. M. J. Magn. Res. 1987, 74, 134. (8) Gilpin, R. K.; Kasturi, A.; Gelerinter, E. Anal. Chem. 1987, 59, 1177. (9) Miller, C.; Dadoo, R.; Kooser, R. G.; Gorse, J. J. Chromatogr. 1988, 458, 255.
(IO) Meirovitch, E.; Samuleki, E. T.; Leed, A.; Scheraga, H. A.; Rananavare, S.; Nemethy, G.; Freed, J. H. J.Phys. Chem.. 1987,91,4840. 0 1990 American Chemical Society
942 Langmuir, Vol. 6, No. 5, 1990
Gangoda and Gilpin
semb1es,l4JSand membranes, its reported applications for studying modified surfaces are limited especially for silica.'e18 The theory that describes conformational and motional aspects of the bonded chains is similar to that applied to other semirigid systems.'+lS An aliphatic C-2H bond in a semirigid sample which can undergo a rapid motion about a definable axis produces a uniaxial powder pattern with a splitting A v l p
where Qo is the static quadrupole coupling constant (i.e., 165 kHz for an aliphatic C-2H bond). The order parameter, SCD,which is given by eq 2, is related to the angular fluctuations of the C-2H bond with respect to the definable axis of motional averaging
motion) to the relaxation process. In the latter case
T1,,-' = BSC;u: (4) where B is a constant, wo is the resonance frequency for a given magnetic field, and n is either -1/2, -1, or -2, depending on the model used to describe the motions.lg If the motions are noncollective and can be described by a single effective correlation time T18-'= BSC;~i2 (5) If the overall chain motions are collective (Le., their director fluctuations are correlated)
TIS-'= BSc;o,,-'
(6)
for two-dimensional systems such as for lipid bilayers and
(7) for three-dimensional systems such as liquid crystals. For a given magnetic field and in cases were Tlf-l is The time-dependent orientational fluctuations which affect constant, TI-' is linearly related to the square of the quathe C-2H bond are an ensemble average, ( ), over a period drupole splitting ( A v l p ) . In the present study, this relawhich is long compared to l/Qo, where is the instantationship has been used to examine the effect of interfaneous angular dependency between the C-2H bond and cial condition on the fast and intermediate motions for the axis of motional averaging. the immobilized alkyl chains. The line shape for an aliphatic C-2H bond can vary In a previous investigation,18 we have studied changes from a uniaxial power pattern in those cases where either in deuterium line shapes and spin-lattice relaxation times (1)intermediate motions exist with correlation times in as a function of position of labeling along the chain for the order of the inverse quadrupole coupling constant, alkyl-modified surfaces in the dry state. Broad Lorent(2) a unique axis of reorientation is absent, or (3) a comzian lines were observed and were ascribable to the presbination of both of these factors exists. In such instances, ence of both fast and intermediate motions. Similar line the line shapes are sensitive to changes in motion and widths for deuterons attached to carbons near the surproduce a series of dynamic line shapes which range from face and for deuterons attached to carbons at intermebroad Lorentzian to uniaxial powder patterns. diate positions indicated restricted overall chain motions. The sources of the averaging motions are single and Further, line widths were such that they were indicative multiple rotational isomerizations. In semirigid cases, such of an averaging process occurring via one-bond rotaas the gel phase of lipid bilayers, changes in line shape tional isomerization. Near the end of the chain, addias a function of the pulse delay used in quadrupole echo tional bond rotational isomerizations were found to coninversion recovery experiment and angular dependentribute to motional averaging. In these same studies,18 cies of T1 have been o b ~ e r v e d . ~Further, ~ , ~ ~ . the ~ ~depenobserved changes in spin-lattice relaxation times with dency of TI-' on line width has been utilized to investitemperature also were consistent with a model where at gate the superposition of differing motional regime which contribution to the relaxation p r o c e s ~ e s . ~For ~ * the ~ ~ * ~ ~least two different motional domains exist along the chains. In the present investigation, selectively deuterated dodecase of a fast and intermediate component and with no cyltrichlorosilane was synthesized and used to chemicross correlation between these differing motional regimes cally modify porous chromatographic grade silica. Subsequently, the chain motions of this material under dif(3) fering interfacial conditions have been studied by 2H NMR spectrometry. The surface has been examined in the dry where T l r l is the relaxation contributions from fast state as well as in the presence of solvent, surfactant, motions (rotational isomerization) and TIs-' is the relaxand mesogenic molecules. ation contribution from intermediate motion (overall chain Experimental Section (11)Spiess, H.W.Colloid Polym. Sci. 1983,261,193.
S,, = ((1/2)(3cos219- 1))
(2)
(12)Janik, B.; Samulski, E. T.; Toriumi, H. J. Phys. Chem. 1987,91, 1842. (13)Lifshitz, E.;Goldfarb, D.; Vega, S.; Luz, Zimmermann, H. J. Am. Chem. SOC. 1987,109,7280. (14)Brown, M. F.; Seelig, J.; Haberlen, U. J. Chem. Phys. 1979,70, 5045. (15)Siminovitch, D.J.; Ruocco, M. J.; Olejniczak, E. T.; Das Gupta, S.K.; Griffin, R. G. Biophys. J. 1988,54,373. (16)Boddenberg, B.; Grosse, R.; Breuninger, U. Surf. Sci. 1986,173, L655. (17)Kelusky, E.C.;Fyfe,C. A. J. Am. Chem. SOC.1986,108,1746. (18)Gangoda, M.;Gilpin, R. K.; Figueirinhas, J. J. Phys. Chem. 1989, 93,4815. (19)Jarrell, H.C.; Smith, C. P.; Jovall, P. A.; Mantsch, H. H.; Siminovitch, D. J. J. Chem. Phys. 1988,88,1260. (20) Siminovitch, D. J.; Ruocco, M. J.; Olejniczak, E. T.; Das Gupta, S. K.; Griffin, R. G. Chem. Phys. Lett. 1985,119,251. (21)Wittebort, R. J.;Oleiniczak, E. T.; Griffin, R. G. J. Chem. Phys. 1987,86,5411. (22)Brown, M. F. J. Chem. Phys. 1982, 77,1576. (23)Brown, M.F.; Davis, J. H. Chem. Phys. Lett. 1981,79,431.
Material. 11-Undecenoic acid, lithium aluminum deuteride, toluenesulfonyl chloride, methyllithium, cuprous iodide, and trichlorosilane were purchased from the Aldrich Chemical Company (Milwaukee, WI). LiChrosorb SI-60 silica and the liquid crystal NP-5 (eutectic mixture of C H ~ O C E H ~ N = N ( O ) C E H ~ ( C H ~ ) ~were C H ~obtained ) from EM Science (Cherry Hill, NJ). Sodium dodecanesulfonate (SDS) w a ~from the Eastman Kodak Company (Rochester, NY). All reagents were used without further purification. Synthesis. The 11-undecenoic acid was converted to its methyl ester with methanol in the presence of concentrated sulfuric acid. The methyl ester was reduced with LiAlD4 to yield the deuterated alcohol, CH~=CH(CH~)ECD~OH, which was reacted with toluenesulfonyl chloride. The resulting tosylate was treated with excess LiCu(CH&, prepared from CUI and LiCH3. The deuterated alkene that formed, CH2=CH(CH&CD2CHs, was hydrosilylated with HSiC13 in the presence of chloroplatinic acid to obtain the final product C13Si-
Langmuir, Vol. 6, No. 5, 1990 943
ZH NMR of Alkyl-Modified Silica
A
B
Figure 2. Partially relaxed 2H NMR spectra of the modified surface in an excess of the liquid crystal, NP-5. Delay between x and x / 2 pulse (in s): (A) 0.001, (B) 0.003, (C) 0.007, (D) 0.01, (E) 0.015, (F) 0.03, (G)0.05, (H) 0.01, (I) 0.15, (J) 0.2.
A I
I
20000
0
"
1
I
-20000
HZ
Figure 1. 2H NMR spectra of silica modified with C13Si(CH2)10CD&H3. Interfacial Conditions: (A) dry, (B) methanol, (C)hexane, (D) 6040 MeOH/H20, (E) SDS, (F) NP-5. Table I. Line Width at Half-Height (Av1/2) and Spin-Lattice Relaxation Time (TI)Data dry
methanol hexane 60:40 (MeOH/H20) SDS NP-5
10.3
0.09
4.4
0.16 0.17 0.11
3.0 9.8
15.0 20.3
0.05 0.03
( C H ~ ) ~ O C D ~ The C H alkylchlorosilane ~.~~ was purified by vacuum distillation and characterized by NMR and IR spectroscopy. The silica (average dp = 10 pm) was pretreated with water, dried, and reacted with the above silane under toluene reflux conditions to obtain the chemically modified surface.6 Subsequently, the material was dry packed into a 4.5 mm X 25 cm stainless steel column and rinsed by sequentially pumping through it 500 mL of methanol, water, and methanol. The material was extruded from the column by using methanol and dried at 100 "C overnight. Measurements. All NMR experiments were carried out at ambient temperature on a General Electric (Fremont, CA) Model GN-300 NMR spectrometer at a frequency of 46.1 MHz. Spectra were acquired by using a solid quadrupole echo pulse sequence. Spin-lattice relaxation times were measured via an inversion recovery method. Data were collected on the dry sample; on the sample solvated with either methanol, hexane, or a 6040 binary mixture of methanol-water; on the sample in contact with an excess of the liquid crystal NP-5 in the nematic phase; and on the sample saturated with sodium dodecanesulfonate. In the latter case, approximately 0.3 g of the alkyl-modified silica was equilibrated with a total of 200 mL of a 0.2 M aqueous solution of sodium dodecanesulfonate (i.e., this was carried out by successively equilibrating the sample with 20 10-mL portions of solution) and the excess water decanted off.
Results and Discussions Shown in Figure 1 are representative 2H NMR spectra for the C ~ & ( C H Z ) ~ ! D Z Cmodified H~ silica in the dry state and in contact with methanol, hexane, an aqueous surfactant solution of sodium dodecanesulfonate(SDS), and the nematic phase of the liquid crystal NP-5. Cor(24) Gangoda, M. E.;Gilpin, R. K. J. Labeled Compound Radiopharm. 1982, X I X , 283.
t 0
100
200
300
400
500
@vi/ d 2 Figure 3. Plot of 1/T1 vs Av1p2. responding values for the line widths at half-height (Avlp) and spin-lattice relaxation times (TI) are summarized in Table I. The narrow liquid-like resonance which is superimposed on and appears in the center of the spectra for the aqueous samples is due to natural abundant deuterium in the nondepleted water used. A broad resonance with an ll-kHz width at halfheight and a T1 of 0.09 s was observed for the dry surface (Figure 1A). Similar values for line width (9.9 kHz) and relaxation time (0.11 s) were obtained for the surface solvated by a 60:40 binary mixture of methanol and water (Figure 1D). The line shapes between these two conditions were indistinguishable. However, when the material was placed in either methanol (Figure 1B) or hexane (Figure IC), narrower and distinctly sharpened resonances were obtained. The widths at half-height were 4.4 kHz for methanol and 3.0 kHz for hexane, and the corresponding spin-lattice relaxation times were respectively 0.16 and 0.17 s. When the modified silica was placed in contact with either the surfactant, SDS (Figure lE),or the nematic phase of the liquid crystal, NP-5, the resonances significantly deviated from a Lorentzian shape. A broad flattened resonance with a T1 of 0.05 s and a Avlp of 15 kHz was observed in the presence of 0.2 M SDS, and a partially developed uniaxial powder pattern with a TI of 0.03 s and A v l p of 20 kHz was present from the NP-5 modified sample. In all cases, the line widths (Figure 1) were less the rigid limit value (124 kHz) due to partial motional averaging of the quadrupole interactions. Av1p as well as TI data depend on the interfacial conditions with the greatest motional averaging in methanol and hexane and the
Langmuir 1990, 6, 944-948
944
least in the liquid crystal. The overall trend was methanol and hexane > dry state and 60:40 methanol-water > SDS > NP-5. In addition, Figure 1 shows a sharpening of the lines (spectra B and C)with increasing motion compared to the partially developed uniaxial powder pattern for the liquid crystal modified sample (Figure 1F). These trends are similar to changes observed in other systems such as lipid bilayers, polymers, and liquid crystals, where the intermediate averaging motions are in the 10-5-10-7-s time ~ ~ a l e . ~ ~However, p ~ 5 - ~in~ most cases, dynamic line shapes have been produced by varying Sample temperature. In the current study, this has been carried out by varying the interfacial conditions. To further evaluate the motional averaging processes, partially relaxed spectra were recorded for each of the interfacial conditions and the corresponding spinlattice relaxation times calculated. Shown in Figure 2 are the resulting 2H NMR spectra of the alkyl-modified surface in contact with NP-5. The shapes of these spectra were sensitive to the delay time between ir and x / 2 , which is most easily seen near the null point. These data indicate a small angular dependency in T1 which is consistent with increased ordering due to the presence of liquid crystal molecules. Similar trends have been observed for lipids when they are in the gel phase.2OS2l The spectra for the other samples were nondefinitive. The spin-lattice relaxation times for the various contact conditions were linearly related to the square of A v l p as shown in Figure 3. The correlation coefficient, slope, and intercept were 0.995, 0.07, and 4.249, respectively. Similar linear dependencies have been reported for lipid bilayers19~22~23~2* and liquid crystals,29where averaging of the quadrupole interactions is due to a combination of (25) Blume, A.; Rice, D.M.; Wittebort, R. J.; Griffin, R. G. Biochemistry 1982,21, 6220. (26) Huang, T. H.;Skarjune, R. P.; Wittebort, R. J.; Griffin, R. G.; Oldfield. E. J . Am. Chem. SOC.1982.102.7377. (27) Meirovitch, E.; Freed, J. H.C h e k Phys. Lett. 1979,64, 311.
fast and intermediate molecular motions as described by eq 3-7. These results are also consistent with the observed changes in line shape as discussed above. Further, the magnitude of the intercept of the plot shown in Figure 3 (4.249) is in good agreement with the reported values for lipid bilayers and indicates the fast reorientations are due to bond rotational isomerizations.22 In the present study, the motions sensed by the deuterons at the 11th position are most disordered on the NMR time scale in methanol and hexane by an increased contribution from the collective motions. Likewise, in the case of the surfactant and liquid crystal modified Samples, higher ordering and slower collective motions were present. These results are similar to those observed for bilayers in the presence of long-chain alcohols and cholesterol esters.30
Conclusion The current investigation demonstrates the potential use of 2H NMR spectrometry to study the influence of solvents and secondary modifying reagents on the interfacial properties of chemically modified silica. We are especially interested in carrying out similar experiments to examine changes in motional/conformational processes in the presence of different surfactants and mesogens. In the latter case, it should be possible to examine and investigate interfacial ordering as a function of temperature. Acknowledgment. Support from DARPA-ONR Contract N0014-86-K-0772 is acknowledged. Registry No. SDS, 151-21-3;NP-5,9016-45-9 MeOH, 67-56-1; hexane, 110-54-3. (28) Mayer, C.; Muller, K.; Weisz, K.; Kothe, G . Lip. Cryst. 1989, 3, 797. (29) Ukleja, P.; Pirs, J.; Doane,J. W. Phys. Rev. 1976, A14, 414. (30) Yue, J.; Thewalt, L. J.; Cushley, R. J. Chem. Phys. Lip. 1988, 49, 205.
Solubilization of Methanol by Calcium Alkylarenesulfonates in Hydrocarbon Media Tze-Chi Jao* and Wendy S. Joyce Texaco Research Center, P.O. Box 509, Beacon, New York 12508 Received August 2, 1989. In Final Form: November 22, 1989 Solubilization of methanol by the calcium salts of four different sulfonate surfactants, dinonylnaphthalenesulfonate, a “synthetic” sulfonate, mixed petroleum/synthetic sulfonate, and petroleum sulfonate, in hydrocarbon media has been studied by gas chromatography (GC). It was found that the calcium synthetic sulfonate had a greater ability to solubilize methanol than calcium petroleum sulfonate. Methanol solubilization appears to depend on the carrier solubility parameter. The results of a complementary NMR study on the calcium mixed synthetic/petroleum sulfonate complement the GC findings, indicating that the solubility of methanol in the bulk carrier strongly influences the methanol solubilization by sulfonate. It was shown that the presence of CaC03 in the polar core of the (inverted) sulfonate micelle reduces the ability of sulfonate to solubilize methanol. Introduction
Oil-soluble metal salts of synthetic and petroleum sulfonic acids are widely used in engine lubrication. Because 0743-7463/90/2406-0944$02.50/0
of the broad application, a wide variety of these sulfonates are manufactured. Some are strictly neutral salts, while others may contain up to 40 mol of colloidal inorganic basic calcium per 1 mol of sulfonate. I t is gener0 1990 American Chemical Society
