J. Phys. Chem. B 2000, 104, 9873-9877
9873
Acid-Base Equilibrium Constants at the Water Surface and Distribution Coefficients between the Surface and the Bulk as Studied by the Laser Two-Photon Ionization Technique Miki Sato,† Takeshi Kaieda,‡ Kohshin Ohmukai,‡ Hirofumi Kawazumi,‡ Akira Harata,† and Teiichiro Ogawa*,† Department of Molecular and Material Sciences, Kyushu UniVersity, Kasuga-shi, Fukuoka 816-8580, Japan, and Department of Industrial Chemistry, Faculty of Engineering, Kinki UniVersity in Kyushu, Iizuka, Fukuoka 820-8555, Japan ReceiVed: May 2, 2000; In Final Form: August 9, 2000
A new method has been proposed to determine the equilibrium between the bulk and the surface by directly measuring surface concentrations using laser two-photon ionization. This method has been applied to pyrenebutyric acid. The surface concentrations depended on the pH of the solution and were analyzed on the basis of two equilibrium constants and two distribution coefficients. Most pyrenebutyric acid stays on the surface at pH ) 2.2. The equilibrium constant, pKa, of pyrenebutyric acid on the water surface was determined to be 7.85 ( 0.13, and this value is shifted to higher value than that in the bulk (4.76). The distribution coefficient of the neutral pyrenebutyric acid was determined as (5.9 ( 2.8) × 10-2 m, and that of pyrenebutyric anion as (4.82 ( 0.1) × 10-5 m. The ratio of the distribution coefficient of the neutral pyrenebutyric acid to that of pyrenebutyric anion was determined to be (1.2 ( 0.6) × 103. These findings indicate that the equilibrium shifts toward the neutral form on the water surface. Laser two-photon ionization was found to be a sensitive and powerful technique to analyze equilibrium on the surface and that between the surface and the bulk.
Introduction The surface of water is an important subject not only in basic science but also as an environmental issue because the contamination of the surface of river, lake, and ocean water has a greater environmental impact, in many cases, than contamination of the deep bulk. Water surface has, however, only recently been investigated on a microscopic level using new surfacespecific laser techniques. The most popular technique has been second harmonic generation,1-8 which is sensitive to the first layer of the surface. Another important technique is two-photon ionization.9-12 Both methods have shown the water surface to be less polar than the bulk.3,6,7,12 A molecule can be ionized in a stepwise two-photon process upon irradiation by an intense laser. Measuring the photocurrent induced by the electrons and ions that are produced, two-photon ionization is a very sensitive technique for determining the concentration of photoabsorbing molecules in solutions13-18 and on surfaces.9-12,19,20 This method determines the surface concentration of a solute on the water surface as well as the second harmonic generation method does, although the depth resolution is slightly different. It should be possible to analyze the acidbase equilibrium on the surface and the distribution equilibrium between the surface and the bulk when the surface concentration is known. Although the acid-base equilibrium3,7 and the adsorption4 of neutral and charged molecules on the water surface have been investigated based on the second harmonic generation method, no investigation has been carried out on the distribution coefficient of a solute between the surface and the bulk using either laser technique. * Author to whom correspondence should be addressed. † Kyushu University. ‡ Kinki University.
The solubility of an ionic species depends on the pH of the solution.21 We have shown that the two-photon ionization signal depends on solubility and that the technique is especially sensitive with poorly soluble molecules.10,11 The solubility of polycyclic aromatic hydrocarbons can even be determined on the basis of the two-photon ionization.22 In the present study, we have determined the amount of pyrenebutyric acid on the water surface by varying the pH of the solution. We have also obtained the equilibrium constant on the surface and the distribution coefficient of the neutral species and the ionized species between the surface and the bulk of water. We believe this is the first investigation for the determination of not only the equilibrium constant but also the distribution coefficients based on a laser technique. Experimental Section The experimental apparatus was described previously;10,11 only brief descriptions are given here. The laser beam of a nitrogen laser (Molectron UV24, 337 nm, 9 mJ, 10 ns width) was focused softly by a quartz lens on the solution surface at an incident angle of 85°, as shown in Figure 1. A stainless steel vessel (0.600 mL, surface area 2.56 cm2) was filled with the sample solution. A disk electrode was placed 8 mm above the solution surface and was biased positively at 1.5 kV. The photocurrent signal was fed to a current amplifier (Keithley 427, time resolution 10 µs) and a digital storagescope (Iwatsu DS6411). The photoionization signal was obtained as a photoionization charge by integrating the photocurrent over its time profile. The signal was accumulated for 32-128 pulses of the laser, and an average of three to 10 sets of such data was taken as an observed result. Averaging and soft focus are essential to obtaining quantitative data because the molecule would produce
10.1021/jp001646g CCC: $19.00 © 2000 American Chemical Society Published on Web 09/30/2000
9874 J. Phys. Chem. B, Vol. 104, No. 42, 2000
Sato et al.
Figure 1. Experimental apparatus.
domains on the water surface and the surface concentration may fluctuate at a high time resolution and over a small region of the surface.23-25 Pyrenebutyric acid and pyrenehexadecanoic acid were obtained from Molecular Probes and used as received. The water was purified with a Milli-Q (Millipore) system. The pH of the solution was adjusted using a buffer solution prepared with H3PO4 and NaOH. The pH was monitored with a pH meter (Horiba M-8E). The ionic strength of the bulk was kept constant at 0.5 mol/L by the addition of KCl. Pyrenebutyric acid and pyrenehexadecanoic acid were dissolved in benzene. The solution was spread on the buffer solution in the ionization vessel, and the benzene was evaporated. In the case of pyrenebutyric acid, the signal showed a steady-state value within 10 min at pH ) 12 and within 2 min at pH ) 2.8.21 All measurements were done at a steady-state condition at a room temperature (25 ( 2 °C).
Figure 2. Dependence of the photoionization signal on laser pulse energy. Pyrenebutyric acid (0.60 nmol in the vessel, pH 4 in the bulk solution).
Results and Discussion Characteristics of the Photoionization Signal. The photoionization current can be observed only when the disk electrode is positively biased.9 Its time profile consists of two components: a fast one due to electrons ejected from the surface and a slow one due to oxygen anions.10 The escape depth of electrons from water has not been measured, but it can be estimated to be about 1 nm from the escape depth26 of electrons from D2O ice. Thus, a surface layer of water about 1 nm thick should be probed by the laser photoionization method. Therefore, this technique can probe more deeply than can the second harmonic generation method, which allows probing of only the first layer of the surface. The photoionization signal of pyrenebutyric acid (0.600 nmol in the vessel, pH 4 in the bulk solution), and pyrenehexadecanoic acid (0.020 nmol in the vessel, pH 4 in the bulk solution) is quadratically proportional to the laser pulse energy for 10200 and 30-500 µJ/pulse, as shown in Figure 2 for pyrenebutyric acid. All of the experiments were carried out within this range. The photoionization signal was proportional to the amount of solute spread on the water surface up to 0.70 nmol in the cell,9,10 and it tended to saturate at larger amounts, as shown in Figure 3 for pyrenebutyric acid. This saturation is due to neither electrical nor optical problems but to the saturation of the solute on the water surface because the signal can be extended linearly well above 5 pC, as indicated in Figure 2. All of the following experiments were carried out with the
Figure 3. Dependence of the photoionization signal on concentration. Pyrenebutyric acid (pH 4 in the bulk solution); laser pulse energy 100 µJ/pulse.
photoionization signal proportional to the amount of sample and with the surface concentration far below the monolayer coverage. The adsorption to and desorption from the surface were in equilibrium, and there was no serious aggregation of molecules on the water surface. The ionization efficiency of a neutral form may be different from that of the corresponding ion. To clarify this difference, the photoionization signal of pyrenehexadecanoic acid (0.020 nmol in the vessel) was measured from pH 2.2 to pH 11, as shown in Figure 4. The surface concentration of pyrenehexadecanoic acid was calculated as 7.8 × 10-8 mol/m2 based on its molecular area27 of 0.34 nm2 because this molecule is insoluble; 7.8 × 10-8 mol/m2 corresponds to 0.016 of the monolayer coverage. There was no signal change within experimental uncertainty at any pH region even though the molecules remained as neutral species on the water surface in the low-pH region and as ionized species in the high-pH region. Because this molecule has a long hydrocarbon chain and stays on the water surface even in an anion form, the sum of the concentration of the neutral and ionic forms should be constant at any pH on the surface. This finding indicates that the neutral and the anion forms have approximately the same ionization efficiency. It also indicates that ionization efficiency depends
Acid-Base Equilibrium Constants at the H2O Surface
J. Phys. Chem. B, Vol. 104, No. 42, 2000 9875 A-) and the distribution coefficients between the two phases can be expressed as follows:28
Figure 4. Dependence of the photoionization signal of pyrenehexadecanoic acid (7.8 × 10-8 mol/m2, coverage 0.016) on pH. Laser pulse energy: 100 µJ/pulse.
TABLE 1: The Observed Photoionization Signal and the Calculated Amount of the Neutral and Ionized Species of Pyrenebutyric Acid on the Surface and in the Bulk of Water pH bulk
observed signal (pC)
2.2 3 4 5 6 7 8 9 10 11
2.16 2.17 2.00 1.89 1.39 0.202 0.154 0.106 0.0433 0.0473
amount on the surface (nmol) neutral anion total 0.577 0.577 0.573 0.541 0.345 0.074 0.008 0.001 0 0
0 0 0 0.001 0.005 0.011 0.012 0.012 0.012 0.012
0.577 0.577 0.573 0.541 0.349 0.085 0.020 0.013 0.012 0.012
amount in the bulk (nmol) neutral anion total 0.023 0.023 0.023 0.022 0.014 0.003 0 0 0 0
0 0 0.004 0.037 0.237 0.512 0.580 0.587 0.588 0.588
0.023 0.023 0.027 0.059 0.251 0.515 0.580 0.587 0.588 0.588
The calculated amounts are given in absolute values for an easier comparison; 1.000 nmol on the surface is equal to 3.906 × 10-6 mol/ m2 and 1.000 nmol in the bulk is equal to 1.667 × 10-3 mol/m3. The total amount in the cell was 0.600 nmol.
mainly on the electronic property of the aromatic ring and has little to do with the hydrocarbon chain. Thus, we can conclude that the two-photon ionization signal of an aromatic molecule that has a hydrocarbon chain with a COOH group at the end gives the sum of the surface concentration of the neutral form and that of the anion form of the molecule. This is also the case for pyrenebutyric acid as justified by its nearly identical fluorescence excitation spectrum at pH 2-11. Surface Concentration and pKa. Dependences of the photoionization signal on the pH of the solution are shown in Table 1 for pyrenebutyric acid. The photoionization signal of a partly soluble weak acid molecule tends to increase with decreasing pH because the ionized species are more soluble. A signal change indicates a proportional change in the amount of neutral species and the total surface concentration. The pH dependence of the surface concentration should be directly related to dissociation constants (Ka) on the surface and in the solution bulk, as well as with the distribution coefficients of the neutral species (KDHA) and the anionic species (KDA) of the molecule between the surface and the bulk. If the surface can be regarded as a phase, we can carry over a wellknown set of equations for the extraction of a dissociative species from one phase to another. The acid-base equilibrium constants of pyrenebutyric acid (existing as two forms: HA and
HA ) H+ + A-
(1)
Kas ) [A-]s[H3O+]s/[HA]s
(2)
Kab ) [A-]b[H3O+]b/[HA]b
(3)
KDHA ) [HA]s/[HA]b
(4)
KDA ) [A-]s/[A-]b
(5)
where subscript s denotes the properties at the surface and subscript b denotes those in the bulk. The observed photoionization signal is proportional to [A-]s + [HA]s. Both neutral and ionic forms of pyrenebutyric acid can, in principle, stay both in the bulk and on the surface of water. It has, however, been found21 that the neutral species of pyrenebutyric acid stays mostly on the surface and that the ionic species (pyrenebutyric anion) prefers to stay in the bulk of the water because the substituent is small and the ionic species is, therefore, soluble in water. Thus, a sharp increase/decrease in the photoionization signal indicates sharp increase/decrease in the concentration of the neutral species on the water surface and should be related to an acid-base equilibrium at the surface and in the bulk. It would be expected that the signals of pyrenebutyric acid at around pH ) 2-3 approximately represent the concentration of the neutral species on the surface and that the small signals of pyrenebutyric acid at pH ) 10-11 represent the concentration of the ionic species on the water surface. The pH value may differ between the surface and the bulk. The difference should be calculated in order to determine the dissociation constant. The surface pH (pHs) on a charged surface is related to its bulk pH (pHb):3
pHs ) pHb + eΦ/2.3RT
(6)
where the surface potential Φ can be determined using the Gouy Chapman Model of electrical double layers.3 For negatively charged ionized species such as pyrenebutyric acid, Φ is negative and pHs is smaller than pHb since a negatively charged interface will attract hydrogen ions, effectively decreasing pHs with respect to the bulk. The number density of the ionized species in the present case is, however, small as described later, and thus the difference in pH values between the surface and the bulk was found to be negligible. Because the dissociation constant of pyrenebutyric acid in the water was unknown, its value was estimated by taking the value of a similar compound: γ-phenylbutyric acid, pKa ) 4.76 (Kab ) 1.738 × 10-5).29 This estimation can be justified by the fact that the dissociation constant depends little on any group to which -(CH2)3COOH is attached; for example, pKa of butyric acid is 4.81,29 which is nearly identical to that of phenylbutyric acid. To solve eqs 1-5, one additional parameter was necessary. We used the amount of pyrenebutyric acid on the surface at pH ) 2.2 as a parameter; the concentration on the surface was defined as mol/m2. The best fit with the experimental results was obtained when the amount of pyrenebutyric acid on the surface was 2.25 × 10-6 mol/m2 (0.577 nmol out of 0.600 nmol). The calculated results are also shown with the experimental results in Table 1 and are summarized as follows:
9876 J. Phys. Chem. B, Vol. 104, No. 42, 2000
Figure 5. Dependence of the photoionization signal of pyrenebutyric acid (0.60 nmol) on pH. Laser pulse energy: 100µJ/pulse. The observed results are shown as [ with an error bar. The calculated results are shown as a solid line for the best fit (0.225 × 10-6 mol/m2 at pH 2.2), and as a long dashed line (0.221 × 10-6 mol/m2 at pH 2.2) and a short dashed line (0.229 × 10-6 mol/m2 at pH 2.2) for two reference values.
The amount of pyrenebutyric acid (HA + A-) on the water surface at pH ) 2.2:
(2.25 ( 0.04) × 10-6 mol/m2 Kas ) (1.42 ( 0.57) × 10-8 (pKas ) 7.85 ( 0.13) KDHA ) (5.9 ( 2.8) × 10-2 m, KDA ) (4.82 ( 0.01) × 10-5 m KDHA/KDA ) (1.2 ( 0.6) × 103 Pyrenebutyric acid remained as the neutral species at pH ) 2.2. The amount of pyrenebutyric acid on the surface at pH ) 11 was found to be about 0.047 × 10-14 mol/m2 (0.012 nmol on the surface), and it remained as the anion at pH ) 11. This number of ions induces a pH difference below 0.08 based on eq 6. Thus, we can conclude that the effect of the difference in the pH values between the surface and the bulk was smaller than the experimental uncertainty and could be considered negligible. The calculated and experimental results are compared in Figure 5, in which not only the best-fit value (2.25 × 10-6 mol/m2 at pH ) 2.2: 0.577 nmol on the surface) but also the values obtained at 2.21 × 10-6 mol/m2 (0.567 nmol on the surface at pH ) 2.2) and at 2.25 × 10-6 mol/m2 (0.587 nmol on the surface at pH ) 2.2) are shown for comparison. The decrease in the signal at pH ) 4-7 is due to the decrease in the neutral species. These findings indicate that the neutral form of pyrenebutyric acid stays mostly on the surface and that the ionized form stays mostly in the bulk. The findings agree with the results21 of time dependence of the surface concentration. Characteristics of the Present Method. Two laser techniques can be used to determine small surface concentration: second harmonic generation (SHG) and two-photon ionization (TPI). TPI is, however, more sensitive than SHG; the sensitivity of SHG30 has been reported to be about 5.0 × 10-8 mol/m2 and that of TPI19 about 0.25 × 10-8 mol/m2. Then, the lowest surface concentration measured in the present study was below the detection limit of a typical SHG technique. Thus, TPI is a more favorable method than SHG for the analysis of equilibrium
Sato et al. involving the surface, because the equilibrium analysis should be carried out much below the monolayer coverage in order to remove mutual interactions of solutes. An additional advantage of the high sensitivity is that we could ignore pH changes. The equilibrium on the surface was measured using SHG for p-hexadecyl aniline3 and Eosin B.7 However, the former molecule is insoluble in bulk water, stays only on the surface, and is easy to analyze. The present molecule, pyrenebutyric acid, as Eosin B, can stay both on the surface and in the bulk, and the analysis is more complicated. However, this allows us to determine not only the equilibrium constant but also the distribution coefficient of the solute molecule. The adsorption of alkyl phenols, anilines, and their respective ions to the surface was investigated using SHG,4 but pH dependence was not measured. Thus, we have shown that both the equilibrium constant and the distribution coefficient can be obtained by sensitively measuring the surface concentration at various pH values. The present investigation has shown that the equilibrium constant (pKas) on the surface is shifted to higher value than that in the bulk (pKab). This indicate that the surface is more favorable for a neutral species than the bulk and that the polarity of the surface is smaller than that in the bulk. This conclusion agrees with the previous results obtained by the two-photon ionization11 and by SHG.3,6,7 We have clarified that the TPI technique is useful for measuring the distribution of ions and neutrals between the bulk and the surface of water and also for determining the dissociation constant at the surface and the distribution coefficient between the surface and the bulk. Schechter et al.22 analyzed a relation between the photocurrent and the solubility of hardly soluble molecules and clarified a distinctive advantage of the TPI technique. Because this technique probes the water surface at a slightly different depth resolution than does SHG, the comparison of results using the two techniques will enhance the understanding of equilibrium phenomena involving the water surface. Acknowledgment. The present work was partially supported by Grant-in-Aids for Research (No. 07228253, 08555211) from the Ministry of Education, Science and Culture. References and Notes (1) Shen, Y. R. Annu. ReV. Phys. Chem. 1989, 40, 327. (2) Eisenthal, K. B. Annu. ReV. Phys. Chem. 1992, 43, 627. (3) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Chem. Phys. Lett. 1990, 171, 558. (4) Castro, A.; Bhattacharyya, K.; Eisenthal, K. B. J. Chem. Phys. 1991, 95, 1310. (5) Eisenthal, K. B. J. Phys. Chem. 1996, 100, 12997. (6) Wang, H.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. 1997, 101, 713. (7) Tamburello-Luca, A. A.; Hebert, Ph.; Antoine, R.; Brevet, P. F.; Girault, H. H. Langmuir 1997, 13, 4428. (8) Tsukanova, V.; Harata, A.; Ogawa, T. Langmuir 2000, 16, 1167. (9) Masuda, K.; Inoue, T.; Yasuda, T.; Nakashima, K.; Ogawa, T. Anal. Sci. 1993, 9, 297. (10) Inoue, T.; Masuda, K.; Nakashima, K.; Ogawa, T. Anal. Chem. 1994, 66, 1012. (11) Chen, H.; Inoue, T.; Ogawa, T. Anal. Chem. 1994, 66, 4150. (12) Ogawa, T.; Chen, H.: Inoue, T.; Nakashima, K. Chem. Phys. Lett. 1994, 229, 328. (13) Voigtman, E.; Jurgensen, A.; Winefordner, J. D. Anal. Chem. 1981, 53, 1921; (14) Voigtman, E.; Winefordner, J. D. Anal. Chem. 1982, 54, 1834. (15) Yamada, S.; Kano, K.; Ogawa, T. Bunseki Kagaku 1982, 31, E247; (16) Yamada, S.; Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983, 55, 1914. (17) Yamada, S.; Ogawa, T. Prog. Anal. Spectrosc. 1986, 9, 429.
Acid-Base Equilibrium Constants at the H2O Surface (18) Ogawa, T.; Kise, K.; Yasuda, T.; Kawazumi, H.; Yamada, S. Anal. Chem. 1992, 64, 1217. (19) Ogawa, T.; Yasuda, T.; Kawazumi, H. Anal. Chem. 1992, 64, 2615. (20) Gridin, V. V.; Korol, A.; Bulatov, V.; Schechter, I. Anal. Chem. 1996, 68, 3359. (21) Kawazumi, H.; Kaidea, T.; Ohmukai, K.; Sato, M.; Inoue, T.; Ogawa, T. Anal. Sci. 1997, 13 suppl., 49. (22) Gridin, V. V.; Litani-Barzilai, I.; Kadosh, M.; Schechter, I. Anal. Chem. 1998, 70, 2685. (23) Matsuzawa, Y.; Seki, T.; Ichimura, K. Thin Solid Films 1997, 301, 162.
J. Phys. Chem. B, Vol. 104, No. 42, 2000 9877 (24) Dutta, A. K.; Salesse, C. Langmuir 1997, 13, 5401. (25) Li, Y.-Q.; Slyadnev, M. N.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 3035. (26) Jo, S. K.; White, J. M. J. Chem. Phys. 1991, 94, 5761. (27) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Lett. 1986, 124, 326. (28) Christian, G. D. Analytical Chemistry; John Wiley: New York, 1994; Chapter 16, pp 484-487. (29) CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Astle, M. J., Eds.; CRC: Boca Raton, FL,1982; pp D168-172. (30) Inoue, T.; Moriguchi, M.; Ogawa, T. Anal. Sci. 1995, 11, 671.
