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Charge-Transfer Interactions and the Polarity at the Surface of Micelles of Long-chain Pyridinium Iodides’
by Pasupati Mukerjee2 and Ashoka Ray Department of Physical Chemistry, Indian Association fw the Cultivation of Science, Jadavpur, Calcutta 38, India (Received October 88, 1966)
The absorption spectra of micelles of dodecylpyridinium iodides, characterized as charge transfer (CT) bands involving the interaction of pyridinium ions and iodide ions, provide an experimental probe for studying directly the innermost part of the electrical double layer or the Stern layer. These interionic CT bands are highly sensitive to the polarity of the environment. The micellar bands in water are found to be very different from the ion-pair bands in water. The difference is attributed to the reduced effective polarity a t the micelle surface. Comparison of ion-pair bands in various solvents to the micellar band leads to an estimated “effective” dielectric constant of 36 for the micelle surface in water. Small shifts in the micellar band position for different environments produced by mixed counterions, added KI, and some other additives, including a nonionic association colloid, have been observed. The “effective” dielectric constant has been compared to theoretical expectations and agrees with some rough estimates made using Booth’s theory of dielectric saturation or the effect of high concentrations in the Stern layer. The close similarity of the shape of the CT bands of micelles and ion pairs has been interpreted to show the essential homogeneity of the adsorption sites in the Stern layer.
Introduction Perhaps the least understood region of micelles of association colloidal electrolytes, and highly charged colloids in general, is the so-called “Stern” layer, or the innermost part of the double layer. The characteristic distance of interaction here is more likely to be the average separation of charges (5-10 A) rather than the Debye-Hiickel thickness of the diffuse double layer and thus be comparable to the dimensions of ions and solvent molecules even when the concentration of small ions in the intermicellar fluid is small. This makes all continuum approximations in theories extremely hazardous. Short-range forces, usually of diverse origin and generally intractable, add to the difficulties. On the experimental side, few approaches are available for investigating the Stern layer directly. The interesting optical absorptions characteristic of micelles of long-chain pyridinium iodidess-6 seem to be a very useful tool from this point of view. These micellar bands are very probably due to charge transfer (CT) interactions between alkylpyridinium and iodide ions.‘ The Journd of Phyeicd Chemistry
As these interactions must involve electronic orbitals, they must be of very short range. The study of the CT bands, therefore, provides an experimental probe for the innermost part of the double layer. The present paper is concerned with the interpretation of the position and shape of these bands for various micellar environments, as compared to bands due to ion pairs in various solvents. Experimental Section The materials used and the experimental procedures have been described previously.6 (1) Taken in part from the doctoral dissertation of A. Ray, Calcutta University, 1963. (2) Department of Chemistry, University of Southern California, Loa Angeles, Calif. 90007. Requests for reprints should be sent to this address. (3) W.D.Harkins, H. Krizek, and M. L. Corrin, J. Colloid Sci., 6, 576 (1951). (4) P. Mukerjee and A. Ray, J. Phya. Chem., 67, 190 (1963). (5) A. Ray and P. Mukerjee, ibid., 70, 2138 (1966).
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Results and Discussion Qualitative Nature of the Micellar Band. The wavelength of maximum absorption (Amax) of the CT bands for dodecylpyridinium iodide (DPI) micelles in water at 25" has been estimated by the band-matching technique described previously5 to be 286 mp. In contrast, methylpyridinium iodide (MePI), which does not form micelles, gives in water a CT band due to ion pairs whose maximum lies too far into the ultraviolet to be directly determined.6 Kosower and Skorce' have been able to estimate A,, for h!IePI by an indirect method, namely by extrapolation of a plot of ET vs. Z: where ET and Z are transition energies, corresponding to band maxima, in kcal/mole, of MePI and a related standard compound, 1-ethyl-4-carbomethoxypyridinium iodide, respectively. The value so obtained is 266 =t1 mp. This red shift of about 30 mp of CT bands for DPI micelles as compared to MePI ion pairs, both in aqueous media, is of central interest to this paper. Of the various alternative explanations possible, the possible effect of chain-length variation alone must be negligible since MePI, ethylpyridinium iodide, and DPI have CT bands in about the same position in chloroform.6 A second possibility arises from the consideration that the ion-pair bands are essentially due to 1: 1 interactions between pyridinium and iodide ions, whereas on the surface of i,he micelle, containing a large number of monomers and a large fraction of the counterions, the ions are in close proximity. Although CT interactions must be of extremely short range, some cooperative effect involving several ions cannot be ruled out a priori. This possibility was tested experimentally by absorbance measurements in mixed micelles. In the first experiment, the spectrum of a concentrated dodecyltrimethylammonium iodide (DTAI) solution well above the cmc was compared to a similar solution containing, in addition, 1 mole % of DPI. DTAI does not undergo any CT reaction, as is evident from the lower curve in Figure 1, where the longer wavelength absorbance is very small, considering the high concentration used, and is roughly proportional to It can, therefore, be ascribed entirely to scattering from micelles. On the other hand, the mixed micellar system containing 1 mole % of DPI has the characteristic micellar band, very similar in shape to the band obtained for DPI alone at 45", but somewhat shifted toward the red (Figure 1). This experiment was performed at 45" because of the low solubility of DTAI at lower temperatures. Results of a similar experiment with myristylpyridinium chloride (MyPC), to which small amounts of K I were added, are shown in
1.00-
50-
t ,x2 40
-
.os
-
'02
284
I
xh,
"
3 s
39
h
'
3ta
INY-
1 m
3
4w
'
Figure 1. Absorbance data a t 45": A, 9.59 X 10-2 M DTAI against water; 0, 9.59 X 10-2 M DTAI 9.19 X 10-4 M DPI against 9.59 X M DTAI; 0, micellar difference spectrum for D P I a t 45", after suitable vertical shift and a horizontal shift of 5 mH toward the red.
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Figure 2. Again, the micellar band is evident even when the CI-/I- ratio is 75. We have thus no indication of any cooperative phenomenon involving either the pyridinium or the iodide ion. The third and most likely possibility is that the band shift for micelles is due to a reduced polarity at the micelle surface. Kosowe9 has shown that one of the characteristics of the pyridinium CT bands is their strong dependence on the solvent polarity, presumably because of the charged character of the ground state. A decrease in polarity causes a red shift. of the band. A probable explanation is that the ground state is destabilized and thus brought closer to the essentially uncharged excited state. It is also generally recognized that at the surface of highly charged colloids, the polarity is reduced because of dielectric s a t u r a t i ~ n , ~ the high concentration of ions,l0 and the proximity of the hydrocarbon core in the case of a micelle. Qualitatively, therefore, the position of the micellar band may be attributed to a combination of these factors. (6) E. M. Kosower and P. E. Klinedinst, J . Am. Chem. Soc., 78, 3493 (1956). (7) E. M. Kosower and J. A. Skorcz, ibid., 82, 2195 (1960). (8) E.M.Kosower, ibid., 80, 3253 (1958). (9) F. Booth, J . Chem. Phys., 19, 391, 1327, 1615 (1951). (10) M. J. Sparnaay, Rec. Trau. Chim., 77, 872 (1958).
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IN
Figure 2. Absorbance data a t 25': X, 6.12 X 10-aM MyPC against water; 0, 3.58 X 10-2 M MyPC 4.41 X 10-8 M K I against 3.58 X 10-2 M MyPC; 0, 1.796 X M MyPC 2.23 X 10-8 M KI against 1.796 X 10-2 M MyPC; M KI against A, 6.12 X 10-3 M MyPC 7.59 X M MyPC; o-,3.98 X loe2M MyPC 6.12 x 5.13 X M K I against 3.98 x M MyPC; 6 ,micellar difference spectrum for DPI a t 25'.
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Comparison of the Micellar Band with Ion-Pair Bands. In order to make a more quantitative study of this effect, it was of interest to make a comparison of the micellar band position with the ion-pair bands in various solvent media of different dielectric constants. These positions have therefore been determined in solvent mixtures of methanol, ethanol, and ethylene glycol with water. The concentrations used were well below the estimated critical micelle concentrations in these media. Small amounts of sodium thiosulfate were added to all of these solvents to prevent any triiodide formation. It was found that the addition of the small amount of thiosulfate does not materially alter the shape or the position of the band because of the close similarity of the CT bands with thiosulfates and iodides. Figure 3 shows an example of the characteristic variation of the CT bands of the ion pairs of DPI in such solvent mixtures: the band always shifts toward the red with increasing concentration of the organic component. The shapes of the bands on the long wavelength side remain the same, however, and by matching these bands against The Journal of Phyeical C h a i a t r y
mn
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Figure 3. Absorbance data of DPI in methanol-water mixtures a t 25" : (1) 100% methanol, 4.29 X M DPI 3.0 X 10-4 M NazStOa; (2) 90% methanol, 4.40 X 10-8 M DPI 3.0 X 10-4 M Na&03; (3) 80% methanol, 9.87 X lo-' M DPI 3.3 X 10-4 M NazSzOa. Measurements were made against the solvent; percentage of alcohol by weight.
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the CT band in chloroform, as discussed previ~usly,~ the A,, values could be estimated. A plot of these A, values against the dielectric constants (D) of the solvent mixtures is shown in varies Figure 4. For each solvent combination, Am, smoothly with the dielectric constant D. The methanol-water and glycol-water curves overlap almost completely, and the ethanol-water curve lies close to them. The micellar A,, (286 mp) corresponds to dielectric constants of 36 f 2 of these solvent mixtures, which is, therefore, the effective D at the micelle surface. This is unlikely, however, to be a true estimate 01 the polarity a t the micelle surface, as discussed below, although it may be a close approximation. The important point is that even in an aqueous environment (of bulk D = 79) the CT band a t the micelle surface corresponds to a macroscopic dielectric constant of 36. This provides very strong evidence of the reality of the dielectric saturation and other such polarityreducing effects of highly charged surfaces. On the other hand, the reduction in D is not as excessive as is sometimes assumed. The Polarity at the Micelle Surface. The uncertainty
CT INTERACTIONS ON MICELLES OF LONG-CHAINPYRIDINIUM IODIDES
Figure 4. Plot of the dielectric constant of the solvent against Am,= for DPI a t 25’: 0, methanol-water mixtures; A, ethanol-water mixtures; 0, glycol-water mixtures.
of the above estimated “effective” D a t the micelle surface comes from two sources. First, the criticism can be made that in mixed solvents, fractionation of the solvent molecules may occur around an ion pair and, therefore, the bulk value of D is not representative of the microscopic value. However, in view of the close agreement of pure ethylene glycol (D = 37.7) with the other solvent mixtures, and the closeness of its Amax value to that of the micellar band, it seems that any preferential solvation effect is probably of little importance, a t least in the systems considered here. More important, to accept the value of 36 for the micelle surface, we must neglect any dielectric saturation effect for the ion pair, i.e., assume that the bulk D is identical with the microscopic D affecting the interactions of the ion pair. The neglect of this factor leads to an overestimate of D a t the micelle surface. The error, however, is probably small. A recent estimate of D in water around a monovalent ion with a radius of 2 A, comparable to the radius of 2.16 A for I-, shows that D increases rapidly between 2 and 3 A from the center of the ion, and the value of D a t 3.5 A, the minimum distance of separation between the charges in the pyridinium iodide ion pair, is about 72.l’ In the mixed solvents, with lower D,the saturation effect is expected to be even smaller. Environmental Efects on the Micellar Band Position. If the positions of the CT bands on the micelle surface are truly indicatJive of the effective polarity, and the value of the effective D at the micelle surface is caused by the factors mentioned before, it would be expected that alterations of the surface conditions should affect the Amax of the CT bands. Small variations in A,, were indeed found. Thus, the CT band for trace
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amounts of I- on MyPCl micelles, shown in Figure 2, is shifted slightly toward the ultraviolet, as the comparison with the micellar band of DPI in water shows. The estimated shift in A,, is 3 mp. Since the specific interactions of I- with pyridinium micelles is much stronger than that of C1-,12 the chloride micelles are expected to have a somewhat lower counterion concentration and field strength a t the micelle surface than iodide micelles. n!loreover, the concentration effect of C1- is less than that of iodide, the molar decrement in D of aqueous solutions being 11 and 15 for NaCl and NaI, respectively.la It is thus expected that the effective polarity on the chloride micelle is higher, in agreement with the blue shift of the micellar band. Similar conclusions have been reached previously in a different connection. Similarly, with increasing concentration of KI, as the micelle increases in size and charge density14 and there is a stronger specific adsorption of I- ions,12 the CT bands of DPI shift to higher wavelengths (Figure 5). The shift is small at 0.02 M KI, the estibeing 288 mp compared to 286 mp for DPI mated A,, micelles in water. At the higher concentration of K I , 0.1 M , the shift is more pronounced and the band actually crosses the bands of DPI in water and in 0.02 M KI. The estimated A,, here is 292 mp. The CT band for a trace of DPI in DTAI (Figure 1) is about 5 mp toward the red as compared to the DPI micellar band in water a t 45”, separately determined. Here again, the high concentration of DTAI used appears to be the main reason. Some experiments were also performed using various additives. I n the presence of 7.0 X M dodecylM DPI was amine, the band position of 5.75 X identical with that of DPI alone, but the intensity was about 55% higher than the value found for DPI alone, showing that micelle formation increased considerably, presumably because of induced micellization. l6 When 5% by weight of ethanol was added to 6.55 X M DPI, the band shifted slightly toward the blue by about 2 mp. The intensity was about 24% lower than that for DPI alone, showing that micellization is decreased by the addition of ethanol. When a nonionic association colloid, Lubrol-W, was added, the characteristic micellar bands appeared when DPI concentrations well (11) B. E. Conway, J. E. Desnoyers, and A. C. Smith, Phil. Trans. Roy. SOC.(London), A256, 389 (1964). (12) P. Mukerjee and A. Ray, J . Phys. Chem., 70, 2150 (1966). (13) (a,) J. B. Hasted, D. M. Ritson, and C. H. Collie, J. Chem. Phys., 16, 1 (1948); (b) P. Mukerjee and K. Banerjee, J. Phy8. Chem., 68,3567 (1964). (14) H. C. Parreira, Anais. Acad. Brasil. Cienc., 32, 207 (1960). (15) P. Mukerjee and K. J. Mysels, J. Am. Chem. Soc., 77, 2937 (1955).
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PASUPATI MUKERJEE AND ASHOKA RAY
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d d
260
I
I
I
I
I
280
300
320
340
360
31
X in m),
Figure 5. Difference micellar spectra of DPI at 25": 0,in water, 6.57 X lodaM DPI against 5.77 X M; 0,0.020 M KI, 3.066 X 10-3 M DPI against 2.40 X M; A, 0.100 M KI, 1.664 X 10-3 M DPI against 1.175 X M.
below the cmc were used, showing the formation of mixed micelles. On increasing the concentration of the nonionic detergent by a factor of 3, the intensity decreased by about 30%. This is probably due to the dilution of DPI, leading to lower charge densities on the micelles and thus lower binding of the counterions. Unexpectedly, however, the position of the band, as compared to DPI micelles in water, shifts toward the red by 4 and 6 mp in 0.44 and 1.36% Lubrol-W, suggesting that the CT interaction centers are in an environment of a somewhat lower effective polarity, compared to DPI micelles in water, even when the nonionic fraction predominates in the mixed micelle. Lubrol-W contains polyethylene oxides as the head groups. It appears that these bulky head groups, which are, nevertheless, mainly organic in character, containing ether linkages, produce a region of low effective polarity a t the micelle surface, a not unlikely result when it is remembered that ethyl ether has a dielectric constant as low as 4.3. Comparison with Theory. Any detailed theory of the effective polarity at the micelle surface must include at least the three factors mentioned above: namely, the field-strength effect on dielectric saturationlSthe concentration effect,lo*la and the proximity of the hydrocarbon core. The factors, however, are not independent. The third factor can be evaluated separately, assuming its independence from the others. l6 The calculated D depends critically on the assumed The Journal of Physical Chemistry
position of the charges in relation to the hydrocarbon sphere, i.e., whether the charges are considered to be just inside the surface of the sphere or just outside. In the former case, low effective D values are obtained, but in the latter case, using any reasonable geometry of charge separation and micelle radius, the effective D is only slightly lower than the medium value, not nearly enough to explain the observed value of 36. It seems to us that this latter assumption is physically more plausible and, therefore, the effect of the proximity of the hydrocarbon core is small. The macroscopic dielectric saturation effect can be calculated on the basis of an average field strength on the basis of a continuum model. The Gouy-Chapman surface potential calculated for DPI micelles in water is about 200 mv. Consideringthe specific adsorption of I- ions,12 it is expected that the electrokinetic potential for DPI micelles should be considerably lower than those of micelles of sodium lauryl ~ulfate.'~Thus, a fall of approximately 100 mv for the Stern layer is unlikely to be an overestimate. In view of the expected roughness of the micelle surface,17and the attendant snuggling of the counterions, the average separation between the surfaces describing the positive and negative charges on the micelle is certainly much less than the sum of the radii of I- and of the nitrogen charge center of pyridine (3.5 A). If we accept a reasonable value of 1.5 A for this separation,17 the field strength is about 7 X lo6 v/cm for which Booth's theoryg predicts a dielectric constant of 38. The excellent agreement with the experimental "effective" value is fortuitous, but it suggests that if the dielectric saturation approach is to be Booth's theory should be superior to the older theories of Debye and others, which, as treated by Conway, et a2.,l8 predict a value of only 16 for the above field strength. Reasonable agreement with the experimental value can also be obtained using an estimate based on the concentration effect alone. The molar dielectric decrements are 11 and 15 for NaCl and NaI,13 and about 17 for tetraethylammonium chloride, as recently determined.20 For the pyridinium iodide, the value should be in the range of 15-20. Assuming an average thickness of 4-5 A for the Stern layer, its concentration is (16) K. Lindenstr~m-Langand S. 0. Nielsen, "Electrophoresis," M. Bier, Ed., Academic Press Inc., New York, N. Y.,1959, Chapter
11. (17) D. Stigter and K. J. Mysels, J. Phve. Chem., 59, 45 (1955). (18) B. E. Conway, J. O'M. Bockris, and I. A. Ammar, Trans. Faraday SOC.,47, 756 (1951). (19) J. Lyklema and J. Th. G. Overbeek, J . Colloid Sci., 16, 501 (1961). (20) W.D. Kraeft and E. Gerdes, Z . Physik. Chem. (Leipaig), 2 2 8 , 331 (1965).
CT INTERACTIONS ON MICELLES OF LONG-CHAIN PYRIDINIUM IODIDES
calculated to be 2-3 M , which is enough to account for the value of D at the micelle surface. The fact that the two approaches are individually enough to explain the effective D, whereas the two factors should be additive, suggests the need for caution. It seems that the measured bulk dielectric constant of an ionic solution does not necessarily represent its value at the microscopic level, and the average field strengths calculated on the basis of a continuum model may be overestimated. The effect of high electrolyte concentrations on the positions of ion-pair bands should be of considerable interest, in this connection, to examine. The Homogeneity of the Stern Layer. An important conclusion can be drawn regarding the Stern layer from the matchability of the CT bands6 of ion pairs and of micelles. Although the CT interaction is probably confined6 to the “intimate” ion pairs and is not exhibited by all ion pairs, there is no evidence of any heterogeneity of these “intimate” ion pairs; ;.e., to the best of our knowledge, all “intimate” ion pairs
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involve the same interactions. Thus, the characteristic shape and width of the CT bands of ion pairs appear to correspond to a single class of absorbers. The same conclusion, therefore, follows for the micelle surface. Otherwise, if counterion adsorption sites of different energies or local environments with different effective polarities were present, it would be expected that the characteristic CT bands for the different kinds of sites would be differently placed along the wavelength axis, and the experimentally observed sum would be more diffuse and have a larger half-width than the ionpair bands. It appears, therefore, that at least as far as the CT interaction centers are concerned, the Stew layer is quite homogeneous. It thus seems fairly safe to treat Stern layers as such in theoretical formulations.
Acknowledgment. The support of PHS Research Grant GM 10961-01 from the Division of General Medical Services, Public Health Service, during the preparation of this manuscript is gratefully acknowledged.
Volume 70,Number 7 July 1966
