Langmuir 1994,10, 3743-3748
3743
Acid-Base Equilibria of Merocyanine Air-Water Monolayers Robert A. Hall, Peter J. Thistlethwaite,* and Franz Grieser School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
Nobuo Kimizuka and Toyoki Kunitake Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan Received February 14, 1994. In Final Form: June 19, 1994@ The acid-base equilibrium of air-water monolayers of l-docosyl-4-(4-hydroxystyryl)pyridiniumbromide (HSP) on a range of subphases was investigated. On an aqueous subphase the observed PKa at halfionization was found to be considerablylower than that measured in mixed solvent systems. This difference is attributed in part to the enhanced stabilization of the unprotonated form of HSP in the monolayer environment by keto-enol tautomerization. The PKa of 11.0 measured for HSP monolayers on dextran sulfate solution was rationalized in terms of the acid group of HSP experiencing a net negative surface potential. Reflection spectra of the HSP monolayer on an anionic cyanine dye solution subphase indicated that only the protonated HSP speciesis complexed by the dye, givingsupport to the idea that the unprotonated form lacks a site of appreciable positive charge.
Introduction Merocyanines have aroused some interest for their potential as the active element in nonlinear optical devices. Several workers have measured second harmonic signals from transferred monolayers of a docosyl derivative of a merocyanine d ~ e . l -It~ was found that the dye is second harmonic active as the zwitterion, but the cation is second harmonic inactive.' This indicates that a fuller understanding of the acid-base equilibrium of the merocyanine monolayers is necessary to maximize their potential as the basis of nonlinear optical devices. The acid-base equilibrium of the methyl derivative of merocyanine has been studied in aqueous s ~ l u t i o n ~ with - ~ reported pK, values of 8.37-8.57. It was also noted that the unprotonated form exists as a resonance hybrid of the zwitterionic and quinoidal forms. It is widely recognized that the ionization at an interface may differ appreciably from that in aqueous s ~ l u t i o n . ~ For a prototropic group residing a t a charged interface, the observed pK, is given by8
where pK0 is the unknown intrinsic pK, a t the uncharged interface, e is the electronic charge, the surface potential, k Boltzmann's constant, and T the absolute temperature. The intrinsic pK0 is often approximated by the pK, measured in some model neutral interface. However,
* Author to whom correspondence should be addressed.
Abstract published inAdvance ACSAbstructs, August 15,1994. (1)Girling, I. R.;Kolinsky, P. V.; Cade, N. A.; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985,55,289. (2)Girling, I. R.; Cade, N. A,; Kolinsky, P. V.; Montgomery, C. M. Electron. Lett. 1986,21, 169. (3)Stroeve, P.; Srinivasan, M. P.; Higgins, B. G.; Kowel, S. T. Thin Solid Films 1987,146,209. (4)Davidson, S.J.;Jencks, W. P. J . Am. Chem. SOC. 1969,91,225. ( 5 ) Kuder, J. E.; Wychick, D. Chem. Phys. Lett. 1974,24,69. (6)Steiner, U.; Abdel-Kader, M. H.; Fischer, P.; Kramer, H. E. A. J . Am. Chem. SOC.1978,100,3190. (7)Venvey, E.J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elesvier: New York, 1948. (8)Drummond, C. J.;Grieser, F.; Healy, T. W. J . Phys. Chem. 1989, 92, 2604. @
some pH indicators are also solvatochromic, and this property can be used to estimate pK0.s2g The potential for some merocyanines to act as solvent property indicators has been known for some time.IO Solvatochromism of the merocyanines has been employed to determine solvent properties in micellar systemssJ1J2 and bilayer vesicle^.'^ This solvatochromism can enable a n estimate of pK0 in the monolayer system to be made. Acid-base equilibria of a range of surface-bound chromophores have been investigated in air-water and transferred monolayer systems by several workers.14-ls In some cases the Gouy-Chapman prediction for the variation of pK,b, or surface potential with surface charge density agrees well with experiment.16J7 However, some systems displayed other complicating factors, such as aggregation of the ionizing species.14J5J8 This paper presents surface pressure-area isotherms and absorption and reflection spectra of HSP monolayers spread on a variety of subphases. The acid-base equilibrium is monitored by using subphases of different pH, and the effect on the equilibrium of a complexing polymer in the subphase is also investigated. A water-soluble cyanine dye is used to obtain semiquantitative data on the degree of complexation.
Experimental Method The merocyanine dye l-docosyl-4-(4-hydroxystyryl)pyridinium bromide (HSP)was purchased from Aldrich Chemical Co. The chemical structures of the cationic and the unprotonated (9)Drummond, C. J.; Grieser, F.; Healy, T. W. Faraday Discuss. Chem. SOC.1986.81. 95. (10)Brooker, L. G: S.; Keyes, G. H.; Heseltine, D. W. J.Am. Chem. SOC.1961,73,5350. (11)Minch, M. J.;Sadiq Shah, S. J. Org. Chem. 1979,44,3252. (12)Donchi, K. F.; Robert, G. P.; Ternai, B.; Derrick, P. J. A u t . J . Chem. 1980,33,2199. (13)Dion, F.; Bolduc, F.; Gruda, I. J . ColZoidInterfaceSci.1986,106, 465. (14)Lovelock, B.; Grieser, F.; Healy, T. W. J . Phys. Chem. 1986,89, 501. (15)Lovelock, B.; Grieser, F.; Healy, T. W. Langmuir 1986,2,443. (16)Petrov, J. G.; Mobius, D. Langmuir 1990,6, 746. (17)Murray, B. S.;Godfiey, J. S.;Grieser, F.; Healy, T. W.; Lovelock, B.; Scales, P. J. Lungmuir 1991,7,3057. (18)Hall, R.A.;Hayes, D.; Thistlethwaite, P. J.;Grieser, F. Colloids Surf. 1991,56,339.
0 1994 American Chemical Society 0743-7463/94/2410-3743$04.50/0
Hall et al.
3744 Langmuir, Vol. 10,No. 10,1994 50 A
cation
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Figure 1. Chemical structures of the cationic, zwitterionic, and quinoidalforms of 1-docosyl-4-(4-hydroxysty1yl)pyridinium bromide (HSP).
Figure 3. Surface pressure-area isotherms of HSP on water subphases of different pH.
- - - .5.7
(4 H
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Figure 2. Chemical structures of (a) sodium dextran sulfate and (b)sodium 1,1'-disulfopropyl-(2-methyl)-2,2'-thiacarbocyanine.
resonance hybrid species are given in Figure 1. The polyanion sodium dextran sulfate(MW approximately 8000)and the watersoluble cyanine dye sodium l,l'-disulfopropyl-(2-methyl)-2,2'thiacarbocyaninewere obtained from Sigma Chemical Co. and Nippon Kankoh-Shikiso Kenkyusho Co., respectively,and their chemical structures are shown in Figure 2. The concentrations of the subphase additives were M per repeat unit for the polyanion and 5 x 10-6 M for the cyanine dye. Experiments were carried out on a poly(tetrafluoroethy1ene) trough. Absorption and reflection spectra were recorded on a Otsuka MCPD-1000diode array spectrometerusing a bifurcated fiber as described previously.lg The bifurcated fiber was placed 2-3 mm above the water surface,with the incident light normal to the interface. A mirror was placed at the bottom of the trough for absorption measurements, while for reflection experiments a black plate was used in place of the mirror. Each spectrum was an averageof 25 scans,with the total measurementtime per spectrumof approximately 2 s. Surfacepressure measurements were made using the Wilhelmy plate technique using a 0.5 cm wide piece offilter paper suspended from a pressure transducer.
Results and Discussion HSP on an Aqueous Subphase. Surface pressurearea isotherms of monolayers of HSP spread on subphases of different pH are shown in Figure 3. On a subphase of pH 3.0 the isotherm displays expanded, coexistence, and condensed regions. The well-defined plateau in the pH 3 isotherm might be taken to indicate a phase change or a change in orientation ofthe HSP head group that occurs when the surface pressure reaches 20 mN m-l. This (19)Kimizuka, N.; Kunitake, T. Chem. Lett. 1988,827.
300
350
400
450
500
550
600
Wavelength (nm)
Figure 4. Absorption spectra of HSP monolayers measured at 18 f 2 mN m-1 on water subphases of different pH.
question will be taken up again later. At pH 5.7 the expanded phase is similar to that for pH 3.0 but the coexistence region is reduced and there is no condensed phase. The isotherm recorded a t pH 11.0 shows no evidence of a condensed phase, even as part of a coexistence region. Figure 4 displays the absorption spectra measured a t a surface pressure of 18 f2 mNm-l for a range of subphase pH values. At low pH there is an absorption maximum a t 390 nm which is attributed to the protonated species. At higher pH values the peak a t 390 nm diminishes and a peak a t about 470 nm is observed, corresponding to the unprotonated species. There is difficulty in deriving the pKobs of the HSP monolayer on water as the imperfect quality ofthe spectra (as evidenced by the lack of a precise isosbestic point) limits the extent of the analysis. Nonetheless, an estimate ofthe pKobswhen the surface is 50%ionized provides some information of the position of the acid-base equilibrium of the system. An approximate value for the pKobswhen a = 0.5 is obtained by plotting the absorbances at 390 and 470 nm as a function of pH and noting the point of intersection of the two lines. For the HSP/water system, pKobsa t a = 0.5 is approximately 5.0. This pKobsvalue for half monolayer ionization of 5.0 is considerably smaller than the pKa in water for the methyl derivative of HSP of about 8.5.4-6 One might initially consider the difference between these values in terms of the effective dielectric and surface potential contributions to the shift in pKa in going from water to the interfacial
Langmuir, Vol. 10, No. 10, 1994 3745
Acid-Base Equilibria of Merocyanine region.2O The contribution of the effective dielectric to the p K a shift can be determined by using the solvatochromic properties of the unprotonated species of HSP, with the Am= at 18 mN m-l being 472 nm. Drummond et al.s have reported absorption wavelength maxima and pKa values for the propyl homologue of HSP in a range of 1,4-dioxane/water mixtures of known dielectric constant. These data can be used to deduce a n estimate of the effective dielectric constant and hence a value of pK0 (i.e., the pKaat a neutral interface) from the observed monolayer Am= values. With the teff corresponding to the surface pressure of approximately 18 mN m-l, the pK0 for the monolayer system represented by the spectra in Figure 4 can be determined. For = 52, a shift of the pKa from that in water is approximately 0.5, resulting in a predicted pK0 of about 9.0. Thus the pKa shift then attributable to the surface potential is of the order of 4 pH units, from the pK0 of 9.0 to the observed pKob,(a = 0.5) of 5.0. This PKa shift requires a surface potential of about 230 mV. However, this surface potential of 230 mV appears unreasonably high when compared with other monolayer and micelle systems of comparable charge densities and electrolyte concentrations for which surface potentials up to about 170 mV have been reported.21 Since it appears that the surface potential of 230 mV required to account for the difference between pK0 and p&, is unrealistically large, it is worthwhile to consider alternative explanations which may contribute to the low pKobsvalue of 5.0. A possible explanation for the low pKobs lies in the electronic structure of the deprotonated species and in the dielectric asymmetry ofthe air-water interface. The spectroscopic behavior of a n aminostyrylpyridinium in a lipid bilayer observed by Loew et a1.22was attributed to the different dielectric environments experienced by the pyridinium and aniline nitrogens of the oriented chromophore. That is, when the long axis of the chromophore is oriented toward the surface normal, the environment experienced by the pyridinium nitrogen is considerably different from that sensed by the aniline nitrogen. Such oriented monolayer systems have a pronounced dielectric asymmetry, and the notion of an “average” effective dielectric constant for the interfacial region is a n oversimplification. In the event that the HSP chromophores are oriented close to the surface normal, the pyridinium nitrogen and the hydroxyl groups experience environments of quite different polarity. With the OH group directed to the aqueous subphase, as ionization would require, the pyridinium nitrogen experiences a much less polar environment. In such a situation, conversion to the quinoidal tautomer on deprotonation would remove the instability of the cationic charge on the pyridinium nitrogen in the unfavorable nonpolar environment. This form of stabilization of the unprotonated species relative to the protonated species does not apply in the homogeneous, mixed solvent studies on which the estimate of 9.0 for the pKowas based. The above argument can be summarized by saying that deprotonation is facilitated by the stability of the quinoid product in the inhomogeneous interfacial environment. If the above argument is correct, the need to explain the discrepancy of 4 pH units between the assumed pK0 of 9.0 and observed pKobsof 5.0 purely in terms of surface potential is removed, and the requirement for an anomalously large surface potential disappears. It is most likely (20)Fernandez, M. S.;Fromherz, P. J.Phys. Chem. 1977,81,1755. (21)Grieser, F.; Drummond, C. J. J . Phys. Chem. 1988,92, 5580. (22)Loew, L. M.; Simpson, L.; Hassner, A.; Alexanian, V. J.Am. Chem. SOC.1979,101,5439.
that both some surface potential and the presence of the quinoidal tautomer contribute to the low pKobs of 5.0. Further evidence for the dominance of the quinoidal form over the zwitterion is presented in subsequent sections. The proposal that the low value of the observed pKobs is due to the unprotonated form existing a s the predominantly quinoidal resonance hybrid must be reconciled with the observed isotherms. The monolayer exists entirely as the protonated (cationic) species a t pH 3.0, and the isotherm has a n obvious condensed region. In the case of the purely unprotonated (net zero charge) film a t pH 11.0, there is no condensed region. This finding appears at first glance to be inconsistent with the literature. DaviesZ3suggested that the surface pressure could be conceived of as being the sum of two components, a n electrostatic term and a nonelectrostatic term, with the electrostatic term related to the free energy of formation of the electrical double layer. The Davies equation predicts that at a given area per molecule the surface pressure for a charged film will be greater than that for a comparable uncharged film. Although few systems produce quantitative agreement with the Davies equation, there is a general qualitative trend of ionized monolayers being “expanded” compared to neutral fiims.1*724-27
The distinction between isotherms of charged and uncharged monolayers predicted by the Davies equation is not present in the HSP system. Over the range of subphase pH values the expanded regions of the isotherms are almost identical, indicating that there is little difference in the interfacial charge of the protonated and unprotonated forms. Using the posulate that the unprotonated form is largely present a s the neutral quinoid tautomer leads to the requirement for the protonated form to exist as a net neutral monolayer. This leaves two aspects of the isotherm data to be resolved. First, how does the cationic protonated species produce a monolayer with approximately net zero charge to almost match the expanded region of the isotherm of the unprotonated film and, more significantly, form a condensed phase? Second, what prevents the neutral unprotonated monolayer from forming a condensed film? The ability of the protonated species to form a condensed film requires a net zero charge for the monolayer, and this can be explained by counterion binding. Davidson and Jencks have observed a small change in the absorption spectrum of a n aqueous solution of a merocyanine attributable to the binding of counter ion^.^ A later report has identified this as the binding of a n anion to the protonated (cationic) merocyanine in cationic micelles.8 Appreciable anion binding to the protonated species will reduce the electrostatic repulsion between the amphiphiles and facilitate the formation of a close-packed condensed film. The postulate of counterion binding implies accessibility of the N+ to counterions, i.e., that the N+ is oriented downward into the subphase, the reverse ofthe orientation just assumed in the deprotonated case. There is nothing intrinsically implausible about such a proposal. Molecular models show that a low cross sectional area conformation, in which the hydrocarbon tail folds back alongside the rodlike chromophore, is quite easily attained. A similar (23)Davies, J. T.Proc. R. SOC.London 1961,-4208,224. (24)Matijevic,E.;Pethica, B. A. Trans.Faraday Soc. 1958,54,1400. (25)Mingins, J.;Pethica, B. A. Trans.Faraday SOC.1963,59,1892. ( 2 6 )Patil, G.S.;Matthews,R. H.; Cornwell,D. G.J . Lipid Res. 1972, 13,574. (27)Bouloussa, 0.; Michel, J.; Dupeyrat, M. In Physical Chemistry of Transmembrane Ion Motions; Spach, G.,Ed.; Elsevier: Amsterdam, 1983.
Hall et al.
3746 Langmuir, Vol. 10,No. 10,1994 50 1
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"folded" conformation has been proposed for the closepacked monolayer of a diactylene phthalate.28 In the light of this possibility, the shape of the pH 3 isotherm might be seen as originating in a flat-to-"folded vertical" conformational transition initiated a t 20 mN m-l surface pressure. Molecular models confirm that the area per molecule values a t the start and end of the plateau are reasonable for such a postulate. One might expect that accompanying such a conformational change will be a change in the wavelength maximum of the absorption spectrum. Shown in Figure 5 are the wavelength maximum and the surface pressure of a n HSP monolayer at pH 3 as a function of area. It can be clearly seen from the figure that the changes in I,, closely follow the features observed in the isotherm. The inset in Figure 5 shows a schematic diagram of the proposed orientations of the HSP chromophore in the (a) condensed and (b) expanded regions of the isotherm. Further evidence for the pyridinium nitrogen being directed into the subphase in the low pH case will arise later. It is now necessary to address the reasons for this proposed conformational difference for the protonated and unprotonated forms and the failure of the neutral unprotonated species to form a condensed film. The proposal that the deprotonated species exists as the quinoid tautomer has two implications. First it allows the orientation necessary for deprotonation without the destabilization associated with placing the pyridinium charge in a nonpolar environment. Conversion to the quinoidal tautomer also removes the rodlike character of the chromophore and allows free rotation around the central C-C bond. This may be enough to prevent formation of the close-packed, folded vertical structure suggested earlier for the protonated form. HSP on a Dextran Sulfate Solution Subphase. It may be anticipated that the complexation of a polyanion to the cationic monolayer will have a considerable effect on the acid-base equilibrium of the monolayer as the nature of the driving force for complexation is electrostatic attraction. Furthermore, the nature of the resonance hybrid may be investigated because the different electron density distributions of the zwitterionic and the quinoidal forms of HSP. Figure 6 displays the surface pressure-area isotherms of HSP on dextran sulfate solution subphases with (28) Scoberg, D. J.; Furlong, D. N.; Drummond, C. J.; Grieser, F.; Davy, J.; Prager, R. H. Colloids Surf. 1991,58, 409.
0.4
0.2
Area per molecule (nm*)
Figure 5. Absorptionmaximum (circles)and surface pressure (solidline)as a functionof area per molecule ofa HSP monolayer on an aqueous subphase of pH 3. Inset shows the conformation ofthe HSP chromophore in the (a)condensed and (b) expanded monolayers.
0.6 0.8 Area per molecule (nm2)
1
1.2
Figure 6. Surfacepressure-area isothermsof HSP on dextran sulfate solution subphases of different pH. 0.0251
3
0.015. 0.01.
3
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'
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.
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Figure 7. Absorption (pH 5.9 and 11.2)and reflection (pH 13.0)spectra of HSP monolayers on sodium dextran sulfate solution subphases of different pH.
different pH. In a fashion similar to that observed for the HSP on pure water, the dextran sulfate complexed HSP shows both expanded and condensed phases a t the lower pH and only a n expanded phase a t high pH. More interestingly, these isotherms are evidence that the acidbase equilibrium for the complexed monolayer is shifted several pH units from that of the uncomplexed film. There is a n appreciable condensed region in the isotherm a t pH 11.2, suggesting that even at this high pH there is a significant amount of the cationic species present in the film. The similarity of the HSP isotherms on dextran sulfate solution a t pH 13.0 and pure water at pH 11.0 shows the minimal effect of dextran sulfate on the unprotonated film. The absence of complexation of the polyanion to the unprotonated film indicates that the unprotonated resonance hybrid has no distinct center of positive charge which is required for complexation. This supports the proposal of the unprotonated resonance hybrid existing primarily as the quinoidal form. The absorption spectra of HSP on sodium dextran sulfate solution with pH values of 5.9and 11.2 along with the reflection spectrum for pH 13.0, all at a surface pressure of 18f2 mNm-l, are shown in Figure 7. Several attempts were made to record the absorption spectrum of the HSP/dextran complexed film a t pH 13. Spectra recorded immediately after spreading were of reasonable quality. However, as the monolayer was compressed, the spectra deteriorated due to drifting of the baseline. In order to verify the composition of the film at high pH, the reflection spectrum of the complexed HSP film was
Langmuir, Vol. 10, No. 10,1994 3747
Acid-Base Equilibria of Merocyanine 0.025
300
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Wavelength (nm)
Figure8. Absorption spectra of an HSP monolayer at different surfacepressures on sodium dextran sulfate solution subphases. recorded. At a pH of 5.9 the film is entirely protonated and the unprotonated form only appears in significant quantities above 11. By pH 13 the film is completely unprotonated. From the spectra in Figure 7, pKobs(a = 0.5) for the dextran sulfate complexed monolayer is approximately 11.0. For HSP on a n aqueous subphase, the difference between the pK0 of 9.0 and the pKobsof 5.0 cannot plausibly be explained in terms of surface potential alone, as discussed above. Nevertheless, the approach of using the pKo obtained from mixed solvent data to calculate the surface potential from eq 1may be helpful for the dextran sulfate complexed system. With a procedure similar to that for HSP on water, the Am= of 490 nm for HSP on dextran sulfate produces a n E,R of 32 and hence a pK0 of 9.5. The measured pKobsof 11.0 for the complexed film with the pK0 of 9.5 leads to a n estimate of the surface potential of -90 mV. In view of the possible influence of keto-enol tautomerization on the pK0 for HSP on a n aqueous subphase, it is necessary to find further evidence for the net negative surface potential of the complexed film. Figure 8 shows the absorption spectra of a n HSP monolayer at various surface pressures on dextran sulfate solution at a pH of 11.2. At zero surface pressure, the unprotonated species is favored. However, a s the surface pressure increases, the protonated form becomes more favored and dominates above about 10 mN m-l. This behavior can be best explained if a net negative surface potential is assumed. Upon compression the magnitude of the surface charge density will increase as the area is reduced, by the same number of charges existing in a smaller area. Hence the surface potential will also become more negative, although the variation of the surface potential with surface charge density is not very marked at high charge densities. From eq 1, a greater negative surface potential will shift the pKa to a higher value; Le., the acid-base equilibrium moves toward the protonated species. The proposal of a net negative surface potential for the dextran sulfate complexed monolayer is dependent upon the validity of the pK0 of 9.5. It was suggested earlier that the pK0 for HSP on a n aqueous subphase was considerably lower due to the ability of the deprotonated species to adopt the quinoid structure. In the case of the dextran sulfate complexed film, this mechanism will be bypassed. Complexation by the polyanion can be expected to electrostatically stabilize the cationic pyridinium nitrogen, reducing the driving force for the formation of the deprotonated quinoidal form.
Even though there is some evidence for a net negative surface charge, the character of this charge requires discussion. As noted above, only the protonated species is capable of being complexed by the polymer as the unprotonated form has a n essentially quinoidal character. That is, the unprotonated form does not undergo complexation, and so its contribution to the net surface charge in zero. For the dextran complexed film a t pH 11.0 we have equal amounts of the protonated and unprotonated species. Thus to achieve a net negative surface charge, the overall stoichiometry of the complexation of the polymer to the cation must be greater than 1:l. Recently several studies have been undertaken to investigate adsorption of polyelectrolytes onto interf a c e ~ . ~These ~ - ~indicate ~ that the mean thickness of the adsorbed polymer layer to be of the order of 5-20 nm, considerably greater than the polymer chain dimensions normal to the chain axis. This indicates that the polymer has a n appreciable degree of looping, where only a portion of the polymer is in direct contact with the surface and the remaining segments form loops which extend some considerable distance from the surface. The monolayerpolyanion complex could conceivably also exhibit this looping. In such a case, each cationic chromophore will be complexed by a sulfate unit, and the adjacent uncomplexed sulfates present in the loops of the polymer will contribute to a negative potential. Thus, the apparent stoichiometry of the complexation may be greater than one sulfate unit to each HSP cation, hence a net negative interfacial charge. HSP on a Cyanine Dye Solution Subphase. Since the protonated form of HSP undergoes complexation and the unprotonated form does not, there is a variation in the degree of complexation by the polyanion as the monolayer is titrated. This change may be investigated by employing a complexing agent which can be observed spectroscopically. There are several reports in the literature of the attachment of water-soluble anionic cyanine dyes to charged monolayer^.^^,^^ These studies have shown from W-visible spectroscopic and electron diffraction observations that the cyanine dye attaches to cationic monolayers in a highly ordered array with two molecules per unit cell. As the reflection maxima of the cyanine dye are different from those ofHSP, the presence of complexed dye, its aggregation state, and the ionization state of the HSP monolayer can all be monitored by W-visible spectroscopy. The reflection spectra of the HSP/dye system at pH 6.9 a t various surface pressures are shown in Figure 9. First, the degree ofionization of the monolayer can be estimated by the HSP contribution to the reflection spectra, which should be evident a t 385-400 nm for the protonated form and 470-480 nm for the unprotonated form. There is a reflection peak a t 390 nm which increases in intensity as the surface pressure (and also surface concentration) increases. This is due to the cation, and since there is no observed reflectance peak a t 470-480 nm which would indicate the unprotonated form, the HSP exists entirely in the cationic form. Focusing on the dye contribution to the reflectance spectra (Le. 450-750 nm), the behavior observed here is almost identical to that of the dye complexed to conven(29)Kawaguchi, M.; Hayashi, K.; Takahashi, A. Colloids Surf. 1988, 31, 73.
(30)Trau, M.; Grieser, F.; Healy, T. W.; White, L. R.Langmuir 1992, 8,2349. (31)Trau, M.; Grieser, F.; Healy, T. W.; White, L. R. J.Chem. SOC. Faraday Trans., in press. (32)Kirstein, S.;Mohwald, H. Chem. Phys. Lett. 1992,189, 408. (33)Kirstein, S.:Mohwald, H.; Shimomura, M. Chem. Phys. Lett. 1989,154,303.
Hall et al.
3748 Langmuir, Vol. 10, No. 10, 1994 170 160
1
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I t
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Wavelength (nm)
Figure 9. Reflection spectra of an HSP monolayer at 3, 5, 7,
9, and 20 mN m-1 on a cyanine dye solution subphase with a pH of 6.9. The arrow indicates the direction of increasing pressure. 125
120
480 nm greater than that at 390 nm. That is, about half of the HSP molecules are unprotonated. This reduction in net positive charge generated by the surfactant minimizes the formation of the complex. At low pressure the cyanine dye monomer band of 565 nm is clearly evident. The monomer appears to be present also a t higher pressures, although it is partially hidden by the long wavelength tail of the unprotonated HSP band. Clearly there is no aggregate formation, indicating the absence of an ordered array of positive charge generated by the monolayer. A comparison of the reflection spectra at low surface pressure a t the two subphase pH values suggests that only the cationic species can form a complex with the anionic dye. At pH 6.9 the monolayer is cationic and the change in reflectance a t 566 nm due to the monomeric cyanine dye is about 10%. On a subphase with pH 11.0, about half of the monolayer is composed of cationic HSP, and the change in reflectance a t 565 nm is approximately 5%. The isotherms for HSP on cyanine dye subphases a t pH 6.9 and 11.0(not shown)display essentiallythe same phase behavior as the HSP/water and HSP/dextran sulfate systems. The complexed cationic film (pH 6.9) has expanded coexistence and condensed phases, while the condensed phase is reduced for the mixed protonatedunprotonated film (pH 11.0).
Conclusion 100 95 J 350
1 400
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Wavelength (nm)
Figure 10. Reflection spectra of an HSP monolayer at various
surface pressures on a cyanine dye subphase with a pH of 11.0.
tional cationic surfactant^.^^ That is, the spectra show a transition from the monomer to an aggregate as the surface pressure is increased. The monomeric form of the dye has two maxima a t 516 and 565 nm, and these are seen in the spectrum a t 3 mN m-l. With increasing surface pressure a complex aggregate structure is formed, which has two bands at 539 and 608 nm. The similarity between the spectra ofthe dye complexed to HSP and other cationic monolayers suggests that there is a homogeneous distribution of positive charge of the surfactant. It moreover indicates that the cyanine dye is able to bind strongly to the positive charges of the HSP film and supports the earlier conclusion that, for the protonated HSP film, the pyridinium nitrogens are directed downward into the subphase. Figure 10 shows the reflection spectra of the HSP/ cyanine dye system a t pH 11.0 over a range of surface pressures. In this case, the hydroxyl group is ionized to a considerable degree, with the reflectance intensity at
The acid-base equilibria ofHSP monolayers on a range of subphases were investigated. For HSP on an aqueous subphase, the pKa was appreciably lower than that measured in mixed solvent systems. This difference was in part rationalized by the enhanced stability in the monolayer environment of the unprotonated form, an essentially quinoidal resonance hybrid. The ability to switch to the quinoid form allows the chromophoreto adopt an orientation at the interface that is favorable to ionization of the OH group without the attendant destabilization associated with an unfavorably located pyridinium group. The complexation of an HSP monolayer by a polyanion confers improved film stability but also has a marked effect on the pKa. Dextran sulfate stabilizes the pyridinium nitrogen and a t the same time favors a head group orientation less favorable to deprotonation. This together with a “dielectric constant’’ contribution accounts for the much higher PKa when dextran sulfate is present. The dextran sulfate complexed film has an observed pKa of 11.0,indicating that the acid group of the HSP molecule experiencesa net negative surfacepotential. Complexation of the HSP monolayer by a water-soluble anionic cyanine dye showed that only the protonated species of HSP, and not the unprotonated, can be complexed. This work has revealed the complicated nature of both the ionization behavior and the resonance hybrid composition of HSP monolayers and suggests that further work is required if the merocyanine derivative is to be effectively employed in a nonlinear optical device.
