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ASHOKA RAYAND PASUPATI MUEERJEE
Some Aspects of Interionic Charge-Transfer Interactions of
Alkylpyridinium Ions in Ion Pairs and on Micelles1
by Ashoka Ray and Pasupati Mukerjee2 Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta $9, I n d h (Received October 88,1966)
The characteristic micellar absorption bands of long-chain alkylpyridinium (RPy +) iodides have been recently interpreted as charge-transfer (CT) bands. Similar absorption bands characteristic of interionic CT interactions are obtained on RPy +-type micelles in aqueous solution with Br-, s2032-,SOa2-, and N3- also, but not with C1-, S O P , NO3-, NO2-, CIOa-, BrOs-, IOa-, or formate. In the ion-pair form, in chloroform, RPy+ ions do show CT interactions with C1- but not with Clod-. The anions which show CT interactions with RPy+ ions also have absorptions characteristic of CT to the solvent. No evidence of any CT interactions with I- or Br- was found for tetraalkylammonium or trialkylphenylammonium ions in chloroform or a t the surface of micelles. It is suggested that the planar geometry of the nitrogen charge center in RPy+ ions facilitates CT interactions. The effect of alkyl chain length on the CT band position in chloroform is negligible. The CT bands with I- ions for both micelles and ion pairs in various solvent media have been found to have the same shape and half-width when plotted on a wavelength (A) scale. The bands can thus be matched by shifting positions along the X scale, and h for maximum absorption for partially hidden bands can be determined. The average extinction coefficients for ion pairs of two pyridinium iodides in alcoholic solvent media have been estimated. The values are much smaller than the extinction coefficients of ion pairs in chloroform, suggesting that in different media different fractions of ion pairs are in the “contact” or “intimate” class. This approach is expected to be of some use in the study of the equilibrium between “intimate” and “solvent-sharing” or “solventseparated” ion pairs.
Introduction The two following papers in this series report the results of our investigations on the characteristic spectra exhibited by micelles of long-chain pyridinium iodides in water, which have recently been interpreted as charge-transfer (CT) bands,8 and the relation of these bands to the properties of the innermost part of the electrical double layer, the so-called Stern layer. The present paper deals with some problems of general interest concerning interionic CT interactions, particularly in alkylpyridinium salts, both as ion pairs in vari DUS solvents and a t the surface of micelles. The unusual spectra of alkylpyridinium iodides have been noticed by Hantzsch.‘ Their interpretation has been possible only recently after the work of
Mulliken and others has clarified the essential features of the CT interactions in molecular c~mplexes.~ Kosower and his co-w~rkerse-~have used these ideas (1) Taken in part from the doctoral dissertation of A. Ray, Calcutta University, 1963. (2) Chemistry Department, University of Southern California, Los Angeles, Calif. 90007. Requests for reprints should be sent to this address. (3) P. Mukerjee and A. Ray, J . Phys. Chem., 67, 190 (1963). (4) A. Hantzsch, Ber., 44, 1783 (1911). (5) R. 9. Mulliken, J . Am. Chem. Soc., 74, 811 (1952); J. Phys. Chem., 56, 801 (1952). (6) (a) E. M. Kosower, J. Am. Chem. Soc., 7 7 , 3883 (1955); (b) E. M. Kosower, ibid., 78, 3493 (1956). (7) E.M.Kosower and J. C. Burbaoh, {bid., 78,5838 (1956). (8) E. M. Kosower, ibid., 80, 3253 (1958).
CHARGE-TRANSFER INTERACTIONS OF ALKYLPYRIDINIUM IONS
in their study of the spectra of alkylpyridinium iodides and have shown that most of their features can be understood in terms of interionic charge transfer. On the absorption of a photon, it is assumed that an electron from the iodide ion of the charged ground state of the alkylpyridinium ion pair is transferred to an orbital of the x-electron system of the pyridinium ion to produce a neutral excited state. The situation in this respect is the reverse of the charge transfer in molecular complexes for which the ground state is neutral. The interionic CT interactions thus exhibit some differences from CT interactions in molecular complexes.
Experimental Section Materials. Two samples of dodecylpyridinium iodide (DPI) were used. One was prepared by heating stoichiometric amounts of pyridine (E. Merck, freshly distilled) and pure dodecyl iodide (Columbia Chemicals) in an evacuated and sealed tube in an oil bath a t 80" for about 24 hr. The sample was recrystallized six times from a 1:6 mixture of absolute alcohol and absolute ether. The equivalent weight of the sample determined by ion-exchange analysis was 99.8% of theory. The critical micelle concentration (cmc) remained unchanged by two further recrystallizations. A second sample was prepared from dodecylpyridinium chloride supplied by Milton Industrial Chemicals, U. K. The chloride was washed with ether, recrystallized from dioxane, and precipitated twice from concentrated potassium iodide solutions, and finally re crystallized three times from water. The iodide was filtered in the cold, with as little passage of air as possible, washed with ice-cold water, and dried under vacuum. The two samples gave identical cmc's in O.OOO1 M NazSz03. The value of the cmc in water, 5.26 X 10-3 MIo is about 5% higher than that obtained previously by the same method" and considerably higher than a conductometric value reported.12 A light-scattering value of 5.60 X Mlg is probably unreliable because high turbidities were observed below the cmc, suggesting impurities. Dodecylpyridinium bromide (DPB) and myristylpyridinium chloride (MyPC) were gift samples from Diversey (U. K.) Ltd., and described as being of single chain length. The samples were slightly colored initially. They were treated with sugar charcoal in hot methanol for decolorization, followed by evaporation on a water bath and three recrystallizations from acetone. Dodecyltrimethylammonium bromide (DTAB) waB supplied by Milton Industrial Chemicals Ltd., and was used after recrystallization from acetone. The ion-exchange analysis waa 99.8% of theory.
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The corresponding iodide (DTAI) was prepared from DTAB by precipitation with KI in aqueous solution and was recrystallized twice from water. Myristylpyridinium perchlorate (MyPP) was prepared by precipitation from an aqueous solution of MyPC with excess KC104, filtration, repeated washing with KC104 solution and finally water, and drying under vacuum. Methylpyridinium iodide (MePI) and ethylpyridinium iodide (EtPI) were prepared by refluxing pyridine (E. Merck, freshly distilled) and methyl or ethyl iodide (Riedel de Haen, Hanover, Germany, also distilled), taken in stoichiometric amounts, in absolute alcohol for 6 hr. The cooled mixtures were seeded with some previously prepared crystals. The solid materials were washed with alcohol and dried under vacuum. Ion-exchange analysis agreed to within 0.1% of theory. Phenyltrimethylammonium iodide (PhTAI) was obtained from the British Drug Houses, Ltd. All inorganic salts used were of analytical reagent grade from E. Merck or the British Drug Houses, Ltd. The methanol and ethanol used were dried over calcium oxide and distilled before use. E. Merck's pure variety ethylene glycol was dried with anhydrous N&SO4 and distilled under vacuum just before use. The chloroform used was from E. Merck and contained a small amount of ethanol used as a preservative. Apparatus. A Hilger Uvispek spectrophotometer was used for absorbance measurements. Silica cells were used throughout. The cell chamber was thermostated, when necessary, to within 0.2". Solutions, in most cases, were prepared and diluted by weight.
Results and Discussion The E$ect of Alkyl Chain Length. For understanding the properties of the micelles of long-chain alkylpyridinium iodides in aqueous solution, it was necessary to investigate whether the long alkyl chains had any effect on the CT spectrum. The CT spectra of MePI, EtPI, and DPI in chloroform were found to be nearly identical in comparable concentrations, showing that the chain-length dependence of the position of the spectrum is very small as previously noted for methyl, ethyl, and isopropyl derivatives by K o s ~ w e r . ~ ~ (9) (a) E. M. Kosower, J. A. Skorcz, W. M. Sohwartz, Jr., and J. W. Patton, J . A m . Chem. Soc., 82, 2188 (1960); (b) E. M. Kosower and J. A. Skorcz, ibid., 82, 2195 (1960). (10) P. Mukerjee and A. Ray, J . Phys. Chem., 7 0 , 2150 (1966). (11) W.D.Harkins, H. Krizek, and M. L. Corrin, J . Colloid Sci., 6 , 576 (1951). (12) K. Meguro and T. Kondo, Nippon Kagaku Zasshi, 80, 818 (1959). (13) H.C. Parreira, Amis. Acad. Brasil. Cienc., 32, 207 (1950).
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Figure 1. Absorption spectra in chloroform a t 25': (1) DPI, 5.89 x 10-4 M ; (2) DPB, 8.5 x 10-4 M ; (3) MyPC, 6.7 X 1 0 - 4 M ; (4) PhTAI, 4.7 X M; (5) DTAI, 6.8 X l o d 4M ; (6) MyPP, 5.2 X M; (7) KI, saturated solution; (8) DTAB, 8.4 X M ; DTAC a t 6.0 X 10-4 M has negligible absorption above 250 mp.
The Efect of Concentration. On changing the concentration of DPI in chloroform from 3.2 X to 5.9 X lo-* mole/l., we observed a slight blue shift of about 1 mp of the band. Larger shifts have been observed by Kosower for more concentrated solutions of 1-ethyl-4-carbomethoxypyridinium iodide.8 No such shift was observed for alcoholic solutions. The blue shifts on changing the solvent medium are usually associated with increasing polarity.* The same explanation may be advanced here as the effective polarity of a highly nonpolar medium is expected to increase with the concentration of dissolved electrolyte.l4 Interionic Charge Transfer in Various Systems. Previous studies have been primarily concerned with iodide salts of alkylpyridinium ions. To investigate how general the CT interactions are, we have deter'mined the spectra of a large number of salts. Figure 1 shows some spectra in chloroform of several salts. All three of the alkylpyridinium halides undergo CT interactions, the bands appearing at higher wavelengths in the sequence I > Br- > C1-. The Iand Br- bands are easily recognizable. For the chloride, the comparison of MyPC and MyPP shows the presence of CT interaction, since the absorbance of The Journal of Physical C h a i a t r y
ASHOKA RAYAND PASUPATI MUKERJEE
C1- and c104-are negligible in this region. The perchlorate does not seem to undergo any CT interaction as the steep spectrum of MyPP seems to be due to the pyridinium ion. The quaternary ammonium and the quaternary anilinium iodides seem to show no CT interaction, their absorbances being due to that of I- alone.l5* The absorbances of DTAB also seems to be due to that of Br- alone. It seems that the pyridinium moiety is particularly suited for CT interactions, presumably because of its geometry which allows the anions to come close to the positive charge center on nitrogen, as compared to substituted anilinium and tetralkylammonium ions. However, in highly nonpolar solvents like Ccl4, even the tetraalkylammonium iodides can undergo CT interactions as recent work has shown.l5S16 For aqueous micellar systems, we have studied the interactions of I-, S2032-,S032-, N3-, Soh2-,c103-, BrOs-, IO3-, formate, Nos-, N02-, Br-, and C1with dodecyl- or myristylpyridinium ions. Positive evidence for CT interactions was obtained for I-, S2032--,S032-,and N3- (Figure 2) and for Br- (Figure 3). The nearly flat part of the spectrum obtained a t the higher wavelengths for Br- (Figure 3), C1-, and other salts were ascribed to the scattering from the micelles, in view of their low intensities and the approximate dependence on but there is sufficiently strong absorption below 340 mp for DPB (Figure 3) to show that the effects of CT interactions at the micelle surface are appreciable in this region, the absorptions of the individual ions being negligible. Indeed, the absorbance values can be used to determine the critical micelle concentration of DPB.1° No evidence of CT interactions was observed for Sod2-, c103-,Br03-, IO3-, formate, No3-, and NO2-. The pyridinium thiosulfate band for micelles is extremely similar to the pyridinium iodide band, as far as the position and the shape are concerned. The similarity was also found in the ion-pair form in methanol.l7 The extinction coefficient for the thiosulfate seems to be lower, however. Some unusual concentration dependence of absorption of tetramethylpyridinium iodide in ethanol obtained by Kosower in alcohol7 may have been due to a high concentration of LizSzOa used, whose effect was not recognized. As described in the following paper, there is no (14) H. L. Friedman, J. Phys. Chem., 66, 1595 (1962). (15) (a) T. R. Griffiths and M. C. R. Symons, Mol. Phys., 3, 90 (1960); (b) M. L. Blandamer, T. E. Gough, and M. C. R,Symons, Trans. Faraday SOC.,60, 488 (1964). (16) F. S. Larkin, ibid., 5 9 , 403 (1963). (17) P. Mukerjee and A. Ray, J. Phys. Chem., 70, 2144 (1966).
CHARGE-TRANSFER INTERACTIONS OF ALKYLPYRIDINIUM IONS
2141
h IN@-
Figure 2. Absorption spectra at 25" in water: V, DPI, micellar difference spectrum,96.57 X 10-3 M against 5.76 x 10-3 M ; 0,MYPC (6.25 x 10-3 M ) N ~ S * O ~ (6.92 X 10-8 M ) against water; 0,MyPC (3.13 x 10-8 M) NaN3 (1.28 X M) against water; A, MyPC (6.25 X M ) NsSOs (8.08 X M) against water.
+
+
+
evidence of CT interactions on the micelles of longchain trimethylammonium iodides. The above results establish the generality of CT interactions both in chloroform (as ion pairs) and on micelles (presumably at the surface) between alkylpyridinium ions and a variety of anions. The same anions, namely C1-, Br-, I-, SZO~~-, N3-, and SOa2-, have absorptions in the ultraviolet in solution which have been attributed to charge transfer to the solIt is to be expected that CT interactions with pyridinium ions will be related to those with solvents, the main difference being the localized character of the acceptor for the former case. CT interactions with RPyf ions may be useful for diagnosing the existence of charge-transfer-to-solvent bands of anions and vice versa. Comparison of C T Bands in Various Media and the Isolation of Amax. The CT bands of pyridinium iodides on micelle surfaces are clearly seen to the extent of only about one-half of the band, on the long wavelength side. The other part is in the region of the very intense absorption of the pyridinium ions and partly the iodide ions themselves. For intercomparison of the bands in various media it was desirable to estimate
Figure 3. Absorbance data on DPB at 25" in water. Concentration: (1) 0.5037 M, (2) 0.2519 M , (3) 0.1260 M , (4) 0.0630 M , (5) 0.0315 M , (6) 0.01575 M , (7) 0.00788 M.
the wavelengths of these band maxima for these partly visible bands. We attempted a gaussian analysis of the bands, but small deviations seem to occur from the strictly gaussian behavior so that, when the, , ,X for a wellresolved band (in chloroform) was estimated from the long-wavelength half of the band alone and compared to the actual Amax, the uncertainty was about 3 mF, which was unsatisfactory. Resolution of the partially visible band by correcting for the absorption of the individual ions did not prove satisfactory either. The isolation of A,,, was achieved on the basis of an interesting observation on the shapes of the bands. When log OD is plotted against the wavelength, as in Figure 4, the effect of a different concentration of the absorbing species is a vertical parallel displacement of the whole curve. It was found empirically that the long-wavelength parts of the CT bands in micellar systems, and for ion pairs in various solvent media including chloroform, were very similar in shape in (18) M. Smith and M. C. R. Symons, Tram. Faraday SOC.,54, 338 (1958). (19) R. Sperling and A. Treinin, J. Phya. Chem., 68, 897 (1964). (20) G. Stein and A. Treinin, TTUnS. Faraday Soc., 55, 1086, 1091 (1959). (21) I. Burak and A. Treinin, J . Chem. Phya., 39, 189 (1963).
Volume 70, Number 7 Jdy 1966
ASHOKARAYAND PASUPATI MUKERJEE
2142
7
Figure 4. Matching of spectral shapes of DPI in different media at 25’: 0, micellar difference spectrum in water; A, in CHCla; 0, in ethanol (1000/,); D, in ethanol-water (80:20 by weight); V, in methanol-water (9O:lO by weight).
such plots when data above 290 mp were used and could be superposed on one another by a horizontal shift of the spectrum, i e . , by a displacement along the wavelength coordinate, after a suitable vertical displacement to allow for differences in absorption intensities. This superposability, or the matching of band shapes, is satisfactory over a wide range of optical densities, about a factor of 10 from the band maximum. Figure 4 shows examples of this matching of the spectra. Absorbance data for five systems are shown, including chloroform. The peaked continuous curves shown are all the same, however, namely the chloroform band, plotted after suitable vertical and horizontal displacements. The absorbance data in the long-wavelength region are clearly well represented by a single curve. From such comparisons, Amax for a partially resolved band can be estimated from the Amax of the wellresolved band in chloroform. A stencil defining the shape of the long-wavelength half of the band in chloroform was found useful for such intercomparisons. The horizontal shift of the stencil which makes it match any band gives directly the difference between the Am, of the band and that of chloroform. The uncertainty of these estimates of Amax was about f1 mp. The Journal of Physical Chemistry
The above technique may be useful in extending the Z-value scale of the empirical measure of solvent polaritiesa to solvents of high polarity, in which the band maxima cannot be directly isolated. For the Z-value scale, Kosower, et used the Xlnax values of 1-ethyl-4-carbomethoxypyridinium iodide, a system which shows much more pronounced effect of CT than an alkylpyridinium iodide. The A,, values for this compound and those estimated by us for DPI show a fair linear correlation. The observed matchability of the CT bands means that the width of the band AX, the difference between Xmax and XlI2, the wavelength at which the absorption is half that at the maximum, is constant. Its value is 3940 mp for the alkylpyridinium iodides. In terms of frequencies (v), over limited variations in A,, the corresponding relation is vmax - VI/, = KY’max, where K is a constant. Briegleb and Czekallaz2obtained a linear relation between vmax - vl/, and vmax for intermolecular CT interactions of different donors and acceptors. The relationship observed here is probably characteristic of medium effects on CT interactions. The Extinction Coeficients of CT Bunds. It has been observed by Kosowel.8 that when the polarity of the medium increases, along with a blue shift of the CT band, the intensity at Xmax usually decreases when the concentration of the pyridinium iodides are kept constant. The most likely explanation, of course, is that the extent of ion pairing decreases. It is also likely, since CT interactions must involve short-range forces, that only a fraction of the ion pairs act as the absorber, the “contact” or “intimate” ion pairs, as compared to “solvent separated” or “solvent sharing” ion pair^,^^^^ and the fraction is dependent on the medium. Because of the intrinsic interest of this question, and for comparisons with apparent intensities for micelles, we estimated the molar extinction coefficients of the CT bands for some ion pairs. As the “intimate” ion pairs remain in equilibrium with the other types, they can be considered to be a constant fraction of all ion pairs, independent of concentration. The equilibria may be expressed as (Rp~+I-)intimateS (RPy+I-)so~ventsharing (RPy+I-)solvent separated S RPy+ -I- I-. If the total concentration of the salt is C and that of all ion pairs is CZ,and if the absorbance is due entirely to the ion pairs and not the free ions, then from the defi[& = (C nitions for the dissociation constant C,)z/C,] and the molar extinction coefficient E averaged _____
(22) G. Briegleb and J. Czekalla, Z.Physik. Chem. (Frankfurt), 24, 37 (1960).
CHARGE-TRANSFER INTERACTIONS OF ALKYLPYRIDINIUM IONS
301
75
1
1
I
80
85
90
81
4T/C Figure 5. Plot of a/C against --a/C: A, l-ethyl4carbomethoxypyridinium iodide in ethanol, a p b a n c e measurements at Am=; 0, DPI in 80% methanol-water, absorbance measurements at 290 mM. Arrows indicate 1 2 % error in absorbance values.
over all ion pairs ( E = a/Cs where a is the absorbance for 1-cm path length), we obtain the relations
and
A plot of a/C against a‘/’/C should give a straight line from the slope and intercept of which the e and K d values may be evaluated. For the approximate calculations involved here, the activity coefficients are considered to be unity.
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Figure 5 shows plots of this type for DPI in 80% methanol-water (by weight) and 1-ethyl-4-carbomethoxypyridinium iodide in ethanol. The data for the second system were taken from ref 8 . The estimated K d values, 4.1 X and 5.4 X lovs (mole/l.) for DPI and the carbomethoxy compound show the expected magnitude and trend for these solvent media. The corresponding e values are only 430 and 450, much lower than the values of 1140 and 12008 for chloroform. If the apparent oscillator strengths are estimated from a plot of a against Y, assuming symmetrical bands, the differences are reduced somewhat but still remain large. The most likely explanation seems to be that the fraction of the ion pairs exhibiting CT interactions, the “intimate” ion pairs, is much smaller in alcoholic solvents than in chloroform. This conclusion involves the assumption that the intrinsic oscillator strength of the CT band remains constant. It is expected that this approach will be of some use in investigating the equilibrium between “intimate” and other types of ion pairs, at least on a relative basis, as a function of the medium. The problem of estimating absolute fractions of “intimate” ion pairs would probably require some other approach. As discussed later,” when micellar band intensities are compared to those of chloroform, it appears that not all ion pairs, even in such a nonpolar medium as chloroform with a dielectric constant of 5, are “intimate.”15s The high intensities, -lo4, obtained for the tetraalkylammonium iodides in even less polar solvents such as carbon tetra~hloridel~~ seem to be in accord with this conclusion.
Acknowledgment. The writing of this paper was supported in part by PHS Research Grant GM 10961-01 from the division of General Medical Services, Public Health Service.
Volume 70,Number 7 July 1966
