Analysis of the Aggregation of an Anionic Porphyrin with a Cationic

An external reflection (ER) spectrometric device was developed to directly measure adsorbates at the supercritical carbon dioxide (SC–CO2)–water i...
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Analysis of the Aggregation of an Anionic Porphyrin with a Cationic Surfactant at the Supercritical Carbon Dioxide−Water Interface Using UV−Visible External Reflection Spectrometry Akira Ohashi,*,† Akihiro Yamagata,‡ and Haeng-Boo Kim† †

College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, Japan 310-8512 Graduate School of Science and Engineering, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, Japan 310-8512

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ABSTRACT: An external reflection (ER) spectrometric device was developed to directly measure adsorbates at the supercritical carbon dioxide (SC−CO2)−water interface. The aggregation of diprotonated species of 5,10,15,20-tetraphenyl-21H,23Hporphinetetrasulfonic acid (H4tpps2−) at the positively charged SC−CO2−water interface, prepared by adsorption by the cetyltrimethylammonium ion (CTA+), was studied using this device. Orientations of the H4tpps2− monomers and J-aggregates at the SC−CO2−water interface were assessed using s- and p-polarized external reflection (ER) spectra. It appeared that the porphyrin plane of the H4tpps2− monomer was nearly parallel to the SC−CO2−water interface, and that the long axis of the rodlike H4tpps2− J-aggregate was also nearly parallel to the interface. Dependence of the ER spectra on CTA+ concentration and CO2 pressure were investigated, and the interfacial CTA+ concentration was found to cause changes in the interfacial H4tpps2− species present. Increasing the CO2 pressure changed the interfacial species from the H4tpps2− monomer to the H4tpps2− Jaggregate because the interfacial CTA+ concentration increased as the pressure increased. This suggests that the interfacial chemical species can be changed by controlling the pressure and temperature of the SC−CO2. This is the first report of direct measurements of the chemical species at the SC−CO2−water interface, as far as we know. upercritical fluids have properties that are somewhere between those of gases and liquids. Manipulating the temperature and pressure of a supercritical fluid allows its physical properties such as density, dielectric constant, diffusion coefficient, and refractive index to be changed.1 The solubility of a solute in a supercritical fluid can also be changed by manipulating the temperature and pressure, and this is one of the most notable features of supercritical fluids. Supercritical carbon dioxide (SC−CO2) has been widely used as an alternative to conventional organic solvents with many applications such as extraction, chromatography, purification, synthesis, cleaning, etc., because it is nontoxic, cheap, environmentally acceptable, and has a relatively low critical point.1,2 It is thought that SC−CO2 is increasingly applied as a solvent in various fields. Liquid−liquid interfaces also have specific properties, including amphiphilicity, being two-dimensional, low capacities, and the ability to confine adsorbates in specific orientations. Chemical reactions at liquid−liquid interfaces have received a great deal of attention in a number of research areas, including

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solvent extraction, organic synthesis, and molecular recognition in biological membranes.3−5 In particular, formation of aggregates is one of the most specific reactions that occurs at an interface because the concentration of a monomer reactant at the interface can be increased by adsorption to a much higher concentration than can be achieved in the bulk phase. Several methods for making measurements at the liquid−liquid interface have been developed so that chemical reactions at the interface can be studied.6,7 As with chemical reactions at the liquid−liquid interface, those at the SC−CO2−water interface attract the interest of many research areas such as extraction, synthesis, and microemulsions in the SC−CO2−water two-phase system. Moreover, the SC−CO2−water interface has properties of both SC−CO2 and a liquid−liquid interface. The solubility of a solute in SC−CO2 can be changed by making relatively small Received: April 29, 2014 Accepted: August 13, 2014 Published: August 13, 2014 9518

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changes to the pressure and temperature; therefore, the adsorptivity of the solute at the SC−CO2−water interface can be changed by changing the pressure and temperature. This property allows the chemical species formed at the SC−CO2− water interface to be changed by changing the pressure and temperature of the SC−CO2. It is important to identify the chemical species at the SC−CO2−water interface in order to reveal the reaction mechanism in the SC−CO2−water twophase system and the unique property of the SC−CO2−water interface. Directly measuring the SC−CO2−water interface is an effective way of identifying the interfacial species. However, SC−CO2−water interfaces have been studied using indirect measurement methods such as an interfacial tension determination,8−11 which cannot be used to identify the chemical species formed at the interface. So far, there have been no reports of direct spectroscopic measurements at the SC−CO2− water interface. Therefore, the development of a method for direct spectroscopic measurements at the SC−CO2−water interface is desired so that interfacial chemical species can be identified. Porphyrins are most commonly used when measuring interfacial reactions by spectrometry because of their high absorptivities.12−14 A water-soluble anionic porphyrin, diprotonated 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps2−), has been observed to form aggregates with a coexisting cationic surfactant, cetyltrimethylammonium ion (CTA+), at the liquid−liquid interface.15,16 Moriya and coworkers demonstrated that UV−visible external reflection (ER) spectrometry was useful to measure Gibbs monolayers of porphyrin at a toluene−sulfuric acid interface.17,18 ER spectrometry involves irradiating the interface with incident light at an oblique angle from the lower refractive index medium to the higher one. If the ER technique was used in the SC−CO2−water system, we would selectively observe the spectrum of the interfacial adsorbate. In this study, we developed an ER spectrometric device to directly measure the adsorbate at the SC−CO2−water interface. The interfacial aggregation of H4tpps2− with CTA+ was observed using this device. ER spectroscopic measurements showed that changing the CO2 pressure caused different chemical species to be formed at the SC−CO2−water interface.

Figure 1. Schematic of the apparatus used to measure external reflection (ER) spectra from the supercritical CO2−water interface.

polarization prism is placed between the pressure-tight optical cell and optical fiber on the irradiation side. Temperature of the pressure-tight optical cell is monitored using a digital temperature indicator (Fenwal), and pressure inside the cell is monitored using a digital pressure gauge (Toyo Sokki). Measurement of External Reflection Spectra at the SC−CO2−Water Interface. An aqueous solution (5.7 mL) containing H2tpps4− and CTA+ was added to the pressure-tight optical cell using a syringe pump. CO2 was then introduced into the cell using a CO2 delivery pump, and the cell pressure was controlled using a back-pressure regulator. Temperature inside the cell was maintained at 318 K using a water jacket and a thermostat-controlled water circulator. The system was left to equilibrate for 7 h, light was irradiated obliquely onto the interface from the SC−CO2 medium side, and the ER spectrum was measured. The refractive indices of CO2 and SC−CO2 (1.02−1.12, respectively) are smaller than that of water (1.34).19 The incidence angle was fixed at 70°. The ER absorbance, AER, was defined as log(R0/R), where R0 and R are the reflectivities in the absence and presence of the absorbate, respectively. The absorption spectrum of the aqueous phase was measured simultaneously. H2tpps4− concentration was maintained at 2.0 μM, and CTAC concentration was varied from 0 to 9 μM. Pressure was varied from 1 to 10 MPa, and CO2 pressure and temperature were maintained within 0.2 MPa and 0.1 °C, respectively, of the desired value.

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EXPERIMENTAL SECTION Chemicals. Cetyltrimethylammonium chloride (CTAC) was purchased from Tokyo Kasei (Japan). 5,10,15,20Tetraphenyl-21H,23H-porphinetetrasulfonic acid disulfuric acid tetrahydrate (H6tpps·2H2SO4·4H2O) was purchased from Dojindo (Japan). Each stock solution was prepared in pure water. Liquid CO2 (99.99%) was purchased from Taiyo Nissan Co. Ltd. Water was double distilled and purified further using a Milli-Q system (Millipore). Apparatus. Figure 1 is a schematic of the ER spectroscopic system developed in this study. The system has a CO2 delivery pump (SCF-Get; Jasco), a syringe pump (KD210; Muromachi), a pressure tight optical cell (Jasco), two UV−vis spectrophotometers (USB3000; Ocean Optics), an arc lamp light source (APEX SOURCE ARC 150W XE OF; Newport), and a back-pressure regulator (880-80; Jasco). The pressuretight optical cell is cylindrical and made of stainless steel with a 10 cm3 inner volume, 15 mm path length, a pair of quartz windows (22 mm diameter, 15 mm thick), and a water jacket to control the temperature. Optical fibers and a focus lens are used to irradiate the sample and collect the reflected light. A

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RESULTS AND DISCUSSION Interfacial Aggregation Behavior of H4tpps2− with CTA+. CO2 dissolves in the aqueous solution in a SC−CO2− water system, causing the pH of the unbuffered solution to decrease to ∼3 because of the carbonic acid−carbonate equilibrium.20 H4tpps2− was, therefore, always retained in the aqueous phase because its pKa is ∼4.9.21 Figure 2a,b shows the typical time dependence of s- and p-polarized ER spectra, respectively, at the SC−CO2−water interface measured using the apparatus developed in this study. A negative band appeared at 417 nm in the s-polarized spectrum in the early stages of the study (0.5 h), and then two negative bands appeared at approximately 424 and 490 nm. However, a positive band was observed at 417 nm in the p-polarized ER spectrum in the early stages of the study, and then a negative band at 424 nm and a positive band at 490 nm were observed. These types of spectral changes were also observed when 9519

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been assigned to the characteristic linear oscillator polarized transition in the short axis. Therefore, it is convenient to imagine that the long axis of the rod-like H4tpps2− aggregate was almost parallel to the SC−CO 2 −water interface. Equilibration of the interfacial reaction was achieved within 12 h in all cases; therefore, ER spectra were measured after 12 h in all the subsequent experiments. Effect of CTA+ Concentration on H4tpps2− Aggregation. CTA+-concentration-dependent ER spectra were measured at a constant pressure (10 MPa). Figure 3 shows the s-

Figure 3. Changes in the s-polarized external reflection (ER) spectra of interfacial 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps2−) when cetyltrimethylammonium chloride (CTAC) was added. [H2tpps4−] = 2.0 μM; pressure = 10 MPa; temperature = 318 K.

polarized ER spectra of H4tpps2− at the SC−CO2−water interface at various CTA+ concentrations. Figure 4 shows the

Figure 2. (a) s-Polarized and (b) p-polarized external reflection (ER) spectral changes when 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps2−) and cetyltrimethylammonium (CTA+) aggregates were formed at the supercritical CO2−water interface. [H2tpps4−] = 2.0 μM; [CTAC] = 4.0 μM; pressure = 10 MPa; temperature = 318 K.

H4tpps2− J-aggregates formed at a toluene−water interface.16 The observed ER spectral changes showed, therefore, that the H4tpps2− monomer formed J-aggregates at the SC−CO2−water interface. We assigned the interfacial species peak at 417 nm to the H4tpps2− monomer and the peaks at 424 and 490 nm to the H4tpps2− J-aggregate.16 The feasibility of positive and negative bands appearing in the s- and p-polarized ER spectra was shown by Hansen’s approximate formulas.22 According to this formula, the ER absorbance, As‑ER, in the s-polarization measurements is always negative. In the p-polarization measurements, the electric transition moment that is parallel to the interface gives the negative p-polarized ER absorbance, Ap‑ER, at smaller angles of incidence below the Brewster’s angle, θB, and the positive Ap‑ER above θB. On the other hand, the electric transition moment that is perpendicular to the interface gives the positive Ap‑ER below θB and the negative Ap‑ER above θB.22,23 The incidence angle was much higher than the Brewster’s angle (about 50°) in our system; therefore, the positive band observed in the p-polarized ER spectrum was assigned to the chromophores with moments parallel to the interface, while the negative band was assigned to the chromophores with moments perpendicular to the interface. Therefore, we postulate that the porphyrin plane of the H4tpps2− monomer was almost parallel to the SC−CO2−water interface. However, according to literature,24 the absorption band at 490 nm should have been assigned to the characteristic linear oscillator polarized transition in the long axis of the rodlike H4tpps2− aggregate, and the band at 424 nm should have

Figure 4. s-Polarized external reflection (ER) absorbances (as an absolute amount) at 490 and 422 nm plotted against cetyltrimethylammonium (CTA+) concentration. Experimental conditions were the same as those for Figure 3.

ER absorbances (as absolute amounts) at 490 and 422 nm at different CTA+ concentrations. The ER spectrum of H4tpps2− at the SC−CO2−water interface was not observed when CTA+ was not present, suggesting that H4tpps2− was hardly adsorbed at the SC−CO2−water interface without a cationic surfactant. The H4tpps2− monomer band at 417 nm appeared when 1.2 μM CTA+ was present, suggesting that H4tpps2− monomer− CTA+ pairs were formed at the SC−CO2−water interface. ER spectra with a band at 490 nm were observed when CTA+ concentration was between 2.4 and 5.6 μM. The interfacial H4tpps2− concentration increased as CTA+ concentration increased, causing the H4tpps2− J-aggregate with CTA+ to be formed at the SC−CO2−water interface. The ER absorbance (as an absolute amount) at 490 nm reached a maximum when CTA+ concentration was 3.5 μM, and H4tpps2− absorbance in the aqueous phase was hardly observed (data not shown). The ER absorbance (as an absolute amount) at 490 nm decreased as CTA+ concentration increased above 3.5 μM, and the ER absorbance at 422 nm increased as CTA+ concentration 9520

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increased. The band at 490 nm disappeared when CTA+ concentration was relatively high (e.g., 7.0 μM), and a new band at 422 nm appeared instead. The 422 nm band was assigned to the H4tpps2− H-aggregate species because the peak wavelength was different from that of the H4tpps2− monomer (417 nm). However, further study, including measurements of the interfacial CTA+ concentration, will be required to clarify the aggregation mechanism at the SC−CO2−water interface. Effect of CO2 Pressure on H4tpps2− Aggregation. Pressure-dependent ER spectra were measured at a constant CTA+ concentration (4 μM) to determine the effect the CO2 pressure on H4tpps2− aggregation at the SC−CO2−water interface. Figure 5 shows the s-polarized ER spectra at the

interface. Information on the orientations of H 4 tpps 2− monomers and J-aggregates relative to the interface was obtained from s- and p-polarized ER spectra. The H4tpps2− monomer adsorbed to the SC−CO2−water interface when the pressure was low, but the H4tpps2− J-aggregate formed when the pressure was high. This was explained by the changing interfacial CTA+ concentration with the change in CO2 pressure. We demonstrated that changing the CO2 pressure caused different chemical species to be formed at the SC− CO2−water interface: that is to say the chemical species formed at the SC−CO2−water interface was able to be changed by changing the density of the SC−CO2 phase. This specific property of the SC−CO2−water interface could be revealed by identifying the chemical species by ER spectrometry. The method we developed will be applicable for the measurement of the interfacial reactions in the SC−CO2−water two-phase system and will allow us to obtain a great deal of information on the chemical species present at SC−CO2−water interfaces. The knowledge obtained by this method will promote a great deal of understanding on the reaction mechanism in the SC− CO2−water two-phase system and will encourage further developments in investigations in areas such as supercritical CO2 extraction and water in SC−CO2 microemulsions.

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Figure 5. s-Polarized external reflection (ER) spectra of interfacial 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonic acid (H4tpps2−) at different pressures. [H2tpps4−] = 2.0 μM; [CTAC] = 4.0 μM; temperature = 318 K.

AUTHOR INFORMATION

Corresponding Author

*A. Ohashi. Tel: +81-29-2288704. Fax: +81-29-2288403. Email: [email protected]. Notes

The authors declare no competing financial interest.

pressures that were tested. We observed that the ER spectra changed as the pressure increased. One negative band at 417 nm was observed in the lower pressure range. This absorption band was attributed to the H4tpps2− monomer being adsorbed at the SC−CO2−water interface, as mentioned above. Two negative bands at 424 and 490 nm appeared as the CO2 pressure increased, and these were attributed to the H4tpps2− Jaggregate with the formation of CTA+. In general, the adsorptivity of an ionic surfactant is higher at an oil−water interface than at an air−water interface. Moreover, it is reported that the concentration of a nonionic surfactant at the SC− CO2−water increases with an increase in the CO2 pressure.25 Therefore, we presumed that the adsorptivity of the surfactant at the SC−CO2−water interface would increase as the CO2 pressure increased, i.e., the interfacial CTA+ concentration should be higher at high pressures than at low pressures. This would result in a higher interfacial H4tpps2− concentration at high pressure than at low pressure, causing the H4tpps2− Jaggregate with CTA+ to be formed at the SC−CO2−water interface at high pressures. Our results imply that changing the interfacial chemical species by changing the pressure in the SC−CO2−water system is possible.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 26410143. The authors thank Enago (www.enago.jp) for the English language review.

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REFERENCES

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CONCLUSIONS We developed an ER spectrometric device for directly measuring adsorbates at the SC−CO2−water interface. We were successful in the measurement of ER spectra of the H4tpps2− monomer and aggregate with CTA+ at the SC−CO2− water interface using this device. This is the first report, as far as we know, of direct measurements of chemical species at a SC− CO2−water interface. We studied the aggregation of an anionic diprotonated porphyrin, H4tpps2−, prepared by the adsorption of cationic CTA+, at the positively charged SC−CO2−water 9521

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(15) Fujiwara, K.; Wada, S.; Monjushiro, H.; Watarai, H. Langmuir 2006, 22, 2482. (16) Moriya, Y.; Hasegawa, T.; Okada, T.; Ogawa, N.; Kawai, E.; Abe, K.; Ogasawara, M.; Kato, S.; Nakata, S. Anal. Chem. 2006, 78, 7850. (17) Moriya, Y.; Ogawa, N.; Kumabe, T.; Watarai, H. Chem. Lett. 1998, 221. (18) Moriya, Y.; Amano, R.; Sato, T.; Nakata, S.; Ogawa, N. Chem. Lett. 2000, 556. (19) Sun, Y.; Shekunov, B. Y.; York, P. Chem. Eng. Commun. 2003, 190, 1. (20) Toews, K. L.; Shroll, R. M.; Wai, C. M.; Smart, N. G. Anal. Chem. 1995, 67, 4040. (21) Maiti, N. C.; Ravikanth, M.; Mazumadar, S.; Periasamy, N. J. Phys. Chem. 1995, 99, 17912. (22) Hansen, W. N. Symp. Faraday Soc. 1970, 4, 27. (23) Hasegawa, T.; Umemura, J.; Takenaka, T. J. Phys. Chem. 1993, 97, 9009. (24) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993, 99, 4128. (25) Chen, X.; Adkins, S. S.; Nguyen, O. P.; Sanders, A. W.; Johnston, K. P. J. Supercrit. Fluid 2010, 55, 712.

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