J. Phys. Chem. 1980, 84, 77-82
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Carbon-13 NMR Investigation into the Interaction of Sodium Sorbate and Sorbic Acid with Various Micelle Forming Surfactants Daniel L. Regel* and M. M. Habib Department of Chemistry, University of South Carolina, Columbia, South Carollna 29208 (Received March 1, 1979; Revised Manuscript Received July 23, 1979) Pubiicatlon costs asslsted by the Petroleum Research Fund
The locations of the 13CNMR resonances for sodium sorbate have been shown to change substantially in the presence of increasing concentrations of a cationic (dodecyltrimethylammonium bromide) and a nonionic (poly(oxyethy1ene) (23)-lauryl ether) micelle forming surfactant. The changes are small at low surfactant concentrations (but above the critical micelle concentration, cmc) and increase to a near maximum whlen the surfactant concentration is about one-half of the sorbate ion concentration. There is virtually no interaction of the sorbate ions with an anionic surfactant (sodium dodecyl sulfate). Also, sorbic acid interacts with reversed micelles formed in wet benzene by the nonionic surfactant. These results are rationalized in terms of changes in the hydrogen bonding of the sodium sorbate or sorbic acid molecules as an increasing percentage are solubilized within the micelles. Surface tension measurements of cmc values of the surfactants and of the surfactaints in the presence of sodium sorbate indicate that the chemical shift changes are not due to micellation of the sodium sorbate. The 13C chemical shift assignment of sodium sorbate and sorbic acid is also presented.
Introduction Proton NMR has been used to estimate critical micelle concentrations (cmc) by observing changes in chemical shifts' and irelaxation times2 of both surfactant and water protons. Information relating to structural feature^,^ molecular conformation^,^ and the interaction of micelles with carboxylic acids: aromatic compounds,6 fatty acids: and pyrene derivatives8with aqueous micellar solutions has also been obtained by using 'H NMR techniques. A general observation is, however, that the proton chemical shifts change only very slightly on micelle formation. Also, only a limited number of protons can be followed unambiguously because of the small range of chemical shifts generally observed in 'H NMR. To circumvent these problems, a number of investigations of fluorine labeled surfactants which used changes in 19Fchemical shifts have been carried out? Although experimentally very attractive, fluorine labeling may have an effect on the surfactant association when compared to hydrogen in the regular surfactants. More recently, 13CNMR has been used very successfully to study micelle solutions. For example, Cordes and co-workers1° have used 13Cspin-lattice relaxation time measurements to study the segmental motion of surfactant molecules in water and micelles. Chemical shift studies are also quite attractive because of the large range of chemical shift values generally observed for 13C NMR. While carrying out investigations aimed at delineating the effects of micellar solutions on the K,[Co(CN),H] catalyzed hydrogenation of conjugated dienes to monoenes, it was observed that the concentration of surfactant was an important variable in these reactions, even at concentrations of surfactant above the cmc. More specifically, it was found that when sodium sorbate (sodium salt of 2,4-hexadieinoic acid) is hydrogenated by K3[Co(CN)5H] in aqueous micellar solutions at concentrations close to or slightly higher than the cmc only very slight variations in the distribution of the expected products are observed when compared to the same reaction in water.'' However, when the concentration of surfactant is increased substantidly above the cmc, appreciable changes are observed in the hydrogenation reaction. Methyl sorbate solubilized by micelles i3hows even a larger effect. We have reported similar obseivations for simple conjugated olefins.12 These 0022-3654/80/2084-0077$0 1.OO/O
results prompted an investigation into the changes in the 13CNMR spectrum of sodium sorbate in aqueous micellar solutions containing various concentrations of cationic, nonionic, and anionic surfactants in order to determine the extent of the interaction of the sorbate ion with the micelles at various surfactant concentrations. Reported here are the changes in the 13C chemical shifts of individual carbon atoms of sodium sorbate as a function of increasing concentration of surfactant. Also, changes in the 13C chemical shifts of sorbic acid in reversed micelles of the neutral surfactant in benzene are reported. Carblon-13 assignments of sorbic acid and sodium sorbate are presented as well as surface tension measurements of aqueous solutions containing sodium sorbate and various concentrations of the surfactants.
Experimental Section The neutral surfactants poly(oxyethy1ene)(23)-lauryl ether (Brij 3 9 , C12H2&OCH2CH2),30H, 1-bromododecane, CH3(CH2)11Br(Aldrich), sodium dodecyl sulfate (SDS), acid, CH3(CH2)110S03Na,trans,trans-2,4-hexadienoic CH,CH=CHCH=CHCOOH (sorbic acid), malonic acid, CH2(COOH)2(Eastman Kodak), crotonaldehyde, CH3CH=CHCHO (Matheson Coleman and Bell), anid trimethylamine, N(CH3I3(Matheson Gas Products) were all purchased and used as received. Dodecyltrimethylammonium bromide (DTAB), CH&CH2)11N+(CH3)3Br-, was prepared from 1-bromododecaneand trimethylamine in ethanol according to the known procedure in the literature.I3 This material was recrystallized five timee from ethanol-acetone solution to yield white shiny crystals which melt at 242-243 "C (lit.14 241.5-243.5 "C). Preparation of trans,trans-Hexadienoic-2,4-d2 Acid. The deuterated acid perdeuteriomalonic acid, CD2(COOD)2,was readily prepared by simply dissolving malonic acid (10 g, 0.14 mL) in 3.0 g of D20 (0.15 mol) and leaving the solution to stand at room temperature for 40 min. The water was then evaporated at 40 "C in vacuo. Thiis procedure was repeated six times. The acid was then dried overnight under vacuum. A 13C NMR spectrum of this material showed partial deuteration of this acid (estimated at better than 40%). The desired acid was made by reacting this perdeuteriomalonic acid (8 g, 0.1 mol), crotonaldehyde (9 mL, 0.11 mol), and 13.0 mL of anhydrous 0 1980 American Chemical Society
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Figure 2. Protondecoupled natural-abundance 13C Fourier transform NMR spectrum of sorbic-Z,4-d2 acid in CDCI,.
pyridine by a procedure similar to that described by Kluge and Lillya.15 The final product weighed 2.4 g (21% yield) with a melting point of 131.5-133 "C (lit.15133-134 "C). Instrumentation and Methods. All 13CNMR spectra were obtained by using a Varian Associates CFT-20 spectrometer. Unless stated differently, all spectra were recorded with proton decoupling. In order to prepare samples to measure the change in the chemical shift of sodium sorbate vs. the amount of added surfactant, sodium sorbate (0.0603 g, 4.47 X loy4mol) and a weighed amount of the surfactant were added to an 8-mm NMR tube and 1.5 mL of H20 was added via a 2-mL syringe. Chemical shifts were determined vs. external ethylbenzene recorded immediately before and after each experiment. Repeated control experiments demonstrated that this technique yielded highly reproducible data. Chemical shift data are reported vs. MeSi (6,(C1 in benzene ring) = 144.2 ppm and 6,(CH3) = 15.7 ppm). To carry out similar experiments for sorbic acid under reversed micelle conditions, sorbic acid (0.0504 g, 4.5 X mol) and a weighed amount of Brij 35 were added to an 8-mm NMR tube containing 1.5 mL of benzene and 0.05 mL of distilled water. The surface tension measurements were obtained by means of Cenco-DuNouy tensiometer instrument (precision direct reading model). All measurements were made a t 26 f 1 "C. Each run was repeated three or four times.
Experiments were repeated for different Brij 35 and DTAB concentrations and then with each surfactant mixed with a 0.30 m concentration of sodium sorbate. Special care was taken with the Brij 35 measurements. Stock solutions were made and left stand overnight, and the system was left to equilibrate undisturbed for 15 min before the measurement of the surface tension was recorded. This procedure was to avoid aging effects mentioned for surface tension measurements for nonionic surfactants."
Results The assignment of the 13Cresonances of sorbic acid and sodium sorbate was accomplished with the three spectra shown in Figures 1-3. Figure 1shows simply the proton decoupled spectrum of sorbic acid for which the carbonyl (C,) and methyl (C,) carbons are readily assigned, Figure 2 shows the proton decoupled spectrum of sorbic-2,4-dz acid (ca. 50% deuterium enriched). The two more shielded resonances are clearly assigned to Czand C4because these resonances show less intensity (because approximately half these carbons now contain a deuterium) and the appearance of the slightly shifted triplets as expected for these carbons when deuterated (the deuteriums are not decoupled and, because the spin of deuterium is 1, each carbon becomes a triplet). The more shielded resonance of the Cz,C4 pair can be assigned to Cz by using Figure 3, the
The Journal of Physical Chemistry, Vol. 84,
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Flgure 3. Proton-coupled natural-abundance 13C Fourier transform NMR spectrum of sorbic acid in CDCl3
TABLE 1; I3C Assignmentsa for Sorbic Acid and Sodium Sorbate sorbic 172.90 118.07 147.21 129.55 140.63 18.55 acid sodium 173.64 1 2 1 . 8 2 139.54 127.08 135.31 15.19 sorbate'
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Values are reported in ppm from Me,Si. Chemical shifts were measured by using CDCl, as the solvent and internal standard, 6,(CDCl,) = 76.90 ppm. ' Chemical shifts were measured by using ethylbenzene as an external standard, 6,(CH,) = 15.70 ppm.
proton coupled spectra of sorbic acid. This carbon is coupled only to the hydrogen bonded directly to it, presumably due to the trans geometry of the diene system. The resonance assigned to C4 is quite complex as would be expected because it should be coupled to the methyl group hydrogens as well as the hydrogens on C3 and C5. For the other pair, C5 is assigned to the more shielded resonance as it is more complex with coupling arising from the methyl group hydrogens as well as the hydrogens on Cq.The C3 carbon would not be expected to couple to the methyl hydrogens and is thus less complex. The above assignments are also correct for sodium sorbate for the same reasons. See Table I for the exact chemical shift data for each. The 13C spectra were obtained for 3.0 X lo-' m sodium sorbate solutions and added amount of Brij 35 in the to 4.0 X lo-' m soluconcentration range from 2.0 X tions. Chemical shifts were measured vs. external ethylbenzene because no internal standard could be added to the solutions. Figure 4 (see Table I1 for complete datal6) is a plot of the change in chemical shift for each carbon (A.6) in ppm against the concentration of Brij 35. Negative values indicate more shielding of these carbons compared to the chemical shifts of sodium sorbate in water alone without added surfactant, while positive values indicate deshielding of these carbons. Although only small shifts are observed a t low concentrations (although above the cmc) of Brij 35, significant changes are observed at higher concentrations. These changes level off at high surfactant concentrations. The carbon atoms a t each end of the conjugated diene show the greatest change, with C2 shifting downfield and C5 shifting to higher field. The resonance for C3 also shows a moderate upfield shift while resonances for C4 and c6 show smaller downfield shifts. A complete set of data for the carbonyl carbon atom was not obtained. A similar set of measurements was carried out with the cationic micelle forming surfactant, dodecyltrimethyl-
Flgure 4. The change in the chemical shifts (A&, in ppm) of sodium sorbate carbons (0.3 msolutiins) as a function of Brij 35 concentration in aqueous micellar solutions.
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Figure 5. The change in the chemical shifts (A&, in ppm) of sodium sorbate carbons (0.3 rn solution) as a function of dodecyttriethylammonium bromide concentration in aqueous micellar solutions.
ammonium bromide (DTAB), in the concentration range of 1 X 10-3-9 X lo-' m solutions. Figure 5 (see Table 111 for complete datal6) is a plot of Ab against the concentration of DTAB. The trends in the chemical shifts for this surfactant are similar to those observed with Brij 35. However, the magnitude of the shifts is larger in this case. Again, both C2 and C5 show the largest changes. The carbonyl carbon was measured in these experiments and shows an appreciable shift to higher field. To test the interaction of sodium sorbate with an anionic surfactant, sodium dodecyl sulfate (SDS) was chosen and
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Figure 6. The change in the chemical shifts (As,, in ppm) of sorbic acid carbons (0.3 m solution) as a function of Brij 35 concentration in benzene reverse micelle solutions.
experiments were carried out by varying the solution concentration from 1 X lV3to 9 X 10-1 m. In this case, very small shifts are observed (see Table I V 9 . Figure 6 (see Table V for complete datal6) shows the change in chemical shifts for each carbon atom in sorbic acid upon changing from a benzene solution containing ca. 0.05 mL of H20 to solutions containing an increasing amount of added Brij 35, which causes reversed micelles to form. The direction of the change in chemical shifts is the same as that found for sodium sorbate in the aqueous micelle solutions. However, there are a few noticeable differences between the two systems. With reversed micelles, the magnitude of the changes in the chemical shifts of all carbon atoms is larger than that in the aqueous micelles. Moreover, the C3 resonance change is of a larger magnitude than that of C5. Unfortunately, the resonance of C4was masked by the benzene resonance and could not be measured. The carbonyl carbon shows the most drastic change in chemical shift. Note that the spectrum of sorbic acid in either wet or dry benzene is essentially the same (Table V16). The spectrum of sodium sorbate in the presence of the nonmicelle forming salt tetramethylammonium chloride was measured at concentrations up to 2.0 m (Table VI1% Very small changes were observed. In an ethanol-water or dioxane-water mixture, substantial shifts were observed, similar in direction and magnitude for those found at high concentrations of either the neutral or cationic surfactants (Table V P ) . The dioxane mixture showed the larger effects of the two solvents. Shown also in Table VII6 are data demonstrating that, if the surfactant concentration is held constant and the sorbate concentration increased, the A6 values decrease. Surface tension data for aqueous DTAB solutions over the concentration range 8.0 X 10-4-1.25 X 10-1m and for 3.0 X lo-' m solutions of sodium sorbate with DTAB to 6.0 X 10 m were concentration ranging from 5.0 X recorded (Table V I P ) . As shown in Figure 7, both sets of data yield similar results. The cmc concentration for m and is lowered to 5.0 X m when DTAB is 7.9 X sodium sorbate is also present. Analogous data were also recorded for Brij 35 (Table VIII16). In this case, 3.0 X 10-1 m sodium sorbate lowers the cmc from 5.4 X loT5to 1.6 X m. No inflection could be detected when surface tension measurements were carried out for neat sodium sorbate solutions in the concentration range 1 X 10-'-5 X 10-1 m. However, a continuous slight decrease in surface
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Figure 7. Surface tension (dynkm) of dodecyltrimethylammonium bromide aqueous solutions with (A)(0.3 m ) and without (0)sodlum sorbate.
tension was obtained by increasing sodium sorbate concentrations.
Discussion The 13C NMR spectrum of sodium sorbate has been clearly assigned from deuterium labeling experiments and the complexity of certain resonances in the proton coupled spectrum caused by long-range carbon-hydrogen coupling (see Table I). It is interesting to note that the ordering of the diene carbons follows the electron density trends and for calculated15 for trans,trans-2,4-heptadien-2-one all-S-trans-2,4,6-heptatrienal according to CND0/2 calculations.l* Thus C2is the most shielded resonance of the diene set and the calculations predict this carbon to be the one with the highest electron density. The remaining carbons follow this same trend. The dominant influence on the electron densities is the expected charge alternation along the carbon chain due to conjugation with the carbonyl group. As shown graphically in Figures 4 and 5,sodium sorbate strongly interacts with both the cationic and neutral surfactants. At low surfactant concentrations (but above the cmc) the effect, as measured by the change in the I3C chemical shifts of individual sodium sorbate carbon atom resonances (As), is relatively small but the interaction increases substantially at higher concentrations of surfactant. The relatively small interaction at low surfactant concentrations is expected because only a limited number of micelles are available for interaction with the sodium sorbate ions. As the concentration of surfactant increases, a larger percentage of the sorbate ions incorporate into the micelles. At higher concentrations of surfactant, almost all the sorbate ions are interacting with the micelles and further increases in the concentration of surfactant causes only small additional shifts of the sodium sorbate resonances. These chemical shift changes are not due to micellation of the sorbic acid molecules themselves. Surface tension measurements show that sodium sorbate does not form micelles in solutions as concentrated as 0.5 m. Also, if a mixture of two micelles was forming in the solution containing both sodium sorbate and surfactant, the cmc for this mixture would lie somewhere between the cmc's of the two comp~nents.'~ The surface tension data (shown in Figure 7 for DTAB) demonstrate clearly that this does not happen in these two systems. For both Brij 35 and DTAB, the addition of 0.30 m sodium sorbate simply
The Journal of Physical Chemlstry, Vol. 84, No. 1, 19(80 81
C-13 NMR of Sodium Sorbate and Sorbic Acid
lowers the cmc slightly as would be expected. These measurements also demonstrate that for all the A6 measurements rghown in Figures 4 and 5, the surfactant concentration is above the cmc for the system in question. The explanation(s) for the direction and magnitude of the 13Cchemical shifts observed for sodium sorbate as the surfactant concentration increases is difficult because of the complexity of the system. Obviously, the bonding and nonbonding interactions between the sorbate anion and water molecules are radically altered as these anions change from a polar purely aqueous environment to the environment of the micelles. Expected changes in NMR chemical shifts with changes in solvent have been discussed in broad terms.20 Generally speaking, in water, hydrogen bonding interactions will be the dominant solvent-solute interactions. Other factors that will affect chemical shifts are charge transfer, dipole-dipole and van der Waals interactions between solute and solvent molecules, the overall bulk susceptibility of the solution, the anisotropy in the magnetic susceptibility of the solvent molecules, and permanent or induced "reaction field" dipoles in neighboring solvent molecules which polarize the solute and hence alter its electronic environment. For the surfactant DTAB, the negatively charged carboxylate groups of the sorbate ions associate with the positively clharged ammonium head groups on the surface of the micelle. The hydrophobic carbon chain protrudes into the nonpolar micellar interior. In this environment the sorbate ions will not be as highly hydrated as when free in water solution only. This will reduce the hydrogen bonding of water molecules to the sorbate ions. The effect on the electron density of the carbonyl carbon as a result of changing the environment of ketones, esters, or acid molecules is well documented.20*21As seen most appropriately in the 13C NMR studies,2I downfield shifts of the carbonyl carbon (a decrease in electron density) are observed upon mixing a carbonyl containing compound with protic solvents. On the other hand, upfield shifts (an increase in electron density) are observed upon mixing with nonpolar solvents when compared to the neat compound. For sodium sorbate, the expected increase in electron density at the carbonyl carbon caused by a reduction of hydrogen bonding interactions when the ions move from an aqueous environment to the micellar environment will be transmitted down the conjugated system. In valence bond terms, resonance forms c, d, and e will become less 0
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important. 'Thus one would predict that the 13Cresonance of the diem carbons 3 and 5 would move upfield and carbons 2 and 4 downfield. This is the trend measured in these experiments. Support for this argument arises from the A6 values of sodium sorbate in mixed water-dioxane or water-ethanol solvents. In these mixed solvents, hydrogen bonding forces
will be weaker, analogous to the micellar environment. The changes in chemical shifts in mixed solvents are very compatible to those measured with surfactants. It is interesting to note that it takes about 11.7 and 17.0 m solutions of dioxane and ethanol, respectively, to produce comparable changes to those observed with ca. 0.10 m DTAB solution. The other expected factors that will affect the chemical shifts certainly must also be operative, but it is believed that these effects are not as important as changes in hydrogen bonding. Clearly the alkene carbons move from an aqueous to a less aqueous environment when sorbate ions are solubilized in the micelles. This will certainly cause conformational changes in the sorbate chain. This is probably why the methyl group at the end of the chain shows different chemical shift behavior in the micellar and the mixed solvents experiments. This also probably explains the difference in magnitude for A6 of some carbons, Le., C2and C5 in these different environments. Also, some changes are expected as a result of the ion pairing between the carboxylate group and the ammonium head groups. That this causes only small A6 is shown by experiments in which the 13Cspectrum of sodium sorbate was measured in the presence of high concentrations of N(CH3)&X Only very small shifts were observed. The A6 values of sodium sorbate as the concentration of the neutral surfactant Brij 35, C12H25(0CH2CH2P230H, is increased are generally the same as those observed in DTAB solutions; only the magnitude of the shifts is smaller. Once again in this case, moving the sorbate (anions from an aqueous environment to the less polar environment of the micelles should reduce hydrogen bonding interactions with the carboxylate groups. The chemical shift changes are smaller in this case because some hydirogen bonding will certainly be taking place between the sorbate anions and the alcohol head groups of the surfactant. However, this hydrogen bonding is weaker than that with water molecules. Although the sorbate ions interact quite strongly with neutral and cationic surfactants, there is virtually no interaction of this anion with the anionic surfactant SDS. The chemical shifts of the carbons in sodium sorbate change only slightly in the presence of high concentrations of this anionic surfactant. In this case, the sorbate ions are clearly not associated with or solubilized by the micellar phase. The effect of the sorbate ions on the micelles is also of interest. Unfortunately, chemical shift values of the surfactant carbon atoms can only be analyzed clearly at surfactant concentrations higher than 0.1 m solutions in these experiments. At this and higher concentrations, no changes in the 13C resonances of the surfactant molecules can be observed when mixed with 0.30 m sodium sorbate solutions. This is presumably due to the low sorbate/ micelle ratio. However, in experimentsin which the IITAB concentration is held constant at 0.20 m and the sodium sorbate concentration is raised to 0.90 and to 1.8 m, the two methylene carbons next to the ammonium head group broaden and the first one virtually disappears at the higher concentration. A similar but smaller effect is observed for the first methylene carbon next to the hydroxyl group in Brij 35. Clearly a high concentration of sodium sorbate in the micelles reduces the mobility of the head groups and the methylene carbon next to it,lo thus decreasing the relaxation time of these carbons giving the line broadening observed. This effect is not observed for the other carbons in the surfactant molecules and virtually no shifts of these carbons are observed. Thus the environment of the in1
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terior of these micelles change very slightly. It should be noted for these last sorbate experiments that increasing the concentration of sorbate gives lower absolute values of A6 for all of the sorbate carbon atoms when compared to data taken at lower concentrations of sodium sorbate at the same DTAB concentration, This again proves the sensitivity of this system to the micelle/sorbate ratio. Finally, the explanation for the observed chemical shifts of sorbic acid as the environment changes from wet benzene to solutions containing increasing amounts of Brij 35 is complicated because the structure of the reversed micelles which thus form has not been studied in as much detail as ‘‘normal” micelles. Presumably in benzene or wet benzene (the 13C shifts are the same), sorbic acid exists mainly as hydrogen bonded dimers. As increasing numbers of reversed micelles form, the acid dimers will be broken up because of interactions with the polar reversed micelles. This will weaken the hydrogen bonding of the acid molecules and the changes in chemical shifts shown in Figure 6 are thus expected to be the same as those observed for the sodium sorbate in normal micelles. This is similar to the accepted picture for explaining the spectral data of acetic acid in polar and nonpolar solvents. Infrared22and proton magnetic resonance23data suggest that acetic acid exists largely as dimers in both its pure liquid state and in solutions of nonpolar solvents. The dimer will dissociate when mixed with polar solvents to give a substantial upfield shift of the carbonyl carbon.21g Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this project. D. L. Reger thanks Professor M. F. Lappert and the University of Sussex for hospitality afforded during a sabbatical leave. Supplementary Material Available: Tables II-VI giving exact A6 values for sodium sorbate or sorbic acid as in-
dicated in the text and Tables VI1 and VI11 giving surface tension data (7 pages). Ordering information is available on any current masthead page. References and Notes J. Clifford and B. A. Pethica. Trans. Faradav SOC.. 60.1483 119641. (a) J. Clifford and B. A. Pethica, Trans. Farahy Soc., 61, 182 i1965k (b) J. Clifford, ibid., 61, 1276 (1965). D. C. Poland and H. A. Scherana, J. Phys. Chem., 89,2431 (1965). T. W. Johnson and I. M. Klotc, J. Phyi. Chem., 75, 4061 (1971j. C. A. Bunton and M. J. Minch, J. Phys. Chem., 78, 1490 (1974). E. J. Fendler, C. L. Day, and J. H. Fendler, J. Phys. Chem., 76, 1480
(1972). B. Lincoln, S. Frlberg, and S.Grasholt, CoiioU Polym. Sci., 252, 39
(1974). M. Grotzel, K. Kalyanasundaram, and J. K. Thomas, J. Am. Chem. Soc., 96,7869 (1974). N. Muller, J. H. Pellerin, and W. W. Chen, J. Phys. Chem., 76, 3012 (1972),and references therein. E. Williams, B. Sears, A. Allerhand, and E. H. Cordes, J. Am. Chem. Soc., 95, 4871 (1973). D. L. Reger and M. M. Habib, J . Mol. Catai., in press. D. L. Reger and M. M. Hablb, J. Mol. Catal., 4, 315 (1978). A. B. Scott and H. V. Tartar, J . Am. Chem. Soc., 65, 692 (1943). F. M. Menger and C. E. Pwtnoy, J. Am. Chem. Soc., 89,4698(1967). A. F. Kluge and C. P. Lillya, J. Org. Chem., 36, 1977 (1971). Available as supplementarymaterial. See paragraph at end of text regarding supplementary material. L. S. Wan and P. F. Lee, J . Pharm. Sci., 63, 136 (1974). D. J. Bertelll and T. G. Andrews, Jr., J. Am. Chem. Soc., 91,5280
(1969). H. Inoue and T. Nakagawa, J. Phys. Chem., 70, 1108 (1966). For reviews in this subject see: (a) A. D. Buckingham, T. Schaefer, and W. G. Schneider, J. Chem. Phys., 32, 1227 (1960); (b) N. Lumbroso, T. K. Wu, and B. P. Dailey, J. Phys. Chem., 67, 2469 (1963);(c) P. Laszb, Prog. N.M.R. Spectrosc., 3 (1967);(d) J. Homer, Appi. Spectrosc. Rev., 9, 1 (1975). (a) P. C. Lauterbur, Ann. N. Y . Acad. Scl., 70, 841 (1958);(b) G. E. Maciel and G. C . Ruben, J. Am. Chem. Soc., 85, 3903 (1963); (c) G. E. Maciel and D. D. Traficante, J. Phys. Chem.,69,1030 (1965); (d) W. H. de Jeu, Mol. Phys., 18, 31 (1970);(e) G. E. Maciel and R. V. James, J. Am. Chem. Soc., 86, 3893 (1964);(f) G. E. Maciel and J. J. Natterstad, J. Chem. Phys., 42, 2752,(1985);(9) G. E. Macle1and D. D. Traficante, J. Am. Chem. Soc., 88,220 (1966). L. J. Bellamy, R. F. Lake, and R. J. Pace, Spectrochlm. Acta, 19,
443 (1963).
N. Muller and P. I. Rose, J. Am. Chem. Soc., 85, 2173 (1963).
A Critical Comparison of Methods for Analysis of Linear Dichroism of Solutes in Stretched Polymers Erlk W. Thulstrup‘ and Josef Michl” Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12 (Received Aprll23, 1979) Publication costs assisted by the U.S. Public Health Service
The procedures for analysis of linear dichroism of solutes contained in stretched polymers developed by Eggers, Thulstrup, and Michl, by Tanizaki et al., by Popov, and those based on the Fraser-Beer description of the orientation of the polymer (Yogev et al., and NordBn, et al.) are cast into a common formalism and their scope and limitations are evaluated. Such an analysis is at variance with a number of conclusions published by authors who appear to be unaware of the consequences of the assumptions built into the evaluation procedures. Introduction In 1965-1970, a simple general procedure for quantitative analysis of linear dichroism of solutes contained in stretched polymers was described by Eggers, Thulstrup, and MichlZ4 (the TEM model). Unlike the previously published procedure of Fraser and Beer’ and the concurrently developed model of Tani~aki,”~ it was free of assumptions concerning the nature of the solute orientation distribution function. The TEM procedure has since been
applied to a large number of aromatic An expanded version of it is described in detail and documented on a large number of experimental data elsewhere.16,21 One of the advances built into the TEM model was the recognition that two rather than just one independent parameters are needed to describe adequately the orientation distribution for a symmetrical solute. These reflect the average alignmeht of the three molecular axes with the
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