9480
J. Phys. Chem. C 2007, 111, 9480-9487
Potential-Dependent Adsorption of Amphoteric Rhodamine Dyes at the Oil/Water Interface as Studied by Potential-Modulated Fluorescence Spectroscopy Toshiyuki Osakai,*,‡ Hiroshi Yamada,‡ Hirohisa Nagatani,*,† and Takamasa Sagara† Department of Chemistry, Graduate School of Science, Kobe UniVersity, Nada, Kobe 657-8501, Japan, and Department of Applied Chemistry, Faculty of Engineering, Nagasaki UniVersity, Bunkyo, Nagasaki 852-8521, Japan ReceiVed: March 24, 2007; In Final Form: May 7, 2007
Ion transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), and Rhodamine 110 (R110), and a cationic rhodamine, Rhodamine 123 (R123), at a polarized 1,2-dichloroethane/ water (DCE/W) interface, were studied by means of cyclic voltammetry and potential-modulated fluorescence (PMF) spectroscopy. For all rhodamines, a well-defined voltammetric wave was obtained and the pH dependence of the reversible half-wave potential (i.e., midpoint potential) was investigated to prepare the ionic partition diagram. Theoretical considerations of the diagrams showed that the voltammetric waves obtained for the amphoteric rhodamines were not due to a simple transfer of the protonated form (R+) but due to the transfer of H+ facilitated by the amphoteric form (R() in DCE (and partly in W for R110): H+(W) + R((DCE or W) f R+(DCE). In PMF spectroscopy, the PMF signal due to the adsorption of R+ at the interface could be obtained for RB, R19, and R123, only when the Galvani potential difference across the interface W (∆W O φ) was lower than -0.14 V, suggesting a significant role of ∆O φ in the interfacial adsorption of the rhodamines. The PMF spectrum obtained for the rhodamines under these conditions suggested that the xanthene ring of the adsorbed species should be located in the DCE phase. Furthermore, the dependence of PMF on the angle of polarization of the excitation beam suggested that the longitudinal axis of the xanthene ring should tilt only by 20-25° with respect to the interface.
Introduction The elucidation of the adsorption behaviors of surface-active species at oil/water (O/W) interfaces is a fundamental issue for understanding heterogeneous charge transfer, phase-transfer catalysis, self-assembly of surfactants, and so forth.1,2 Particularly for charged species, it would be important to clarify the crucial role of the Galvani potential difference across the O/W W - φO) in the adsorption/desorption interface (∆W Oφ t φ processes. Thus far, electrochemical studies via interfacial tension,3-8 capacitance,8,9 and charge-transfer admittance10 have been carried out to investigate the adsorption behavior of phospholipids,4,6,7,9 surfactants,5 proteins,3,8 and dyes10 at polarized O/W interfaces. These classical methods mainly provide us with basic thermodynamic information about the adsorption reactions but not about the molecular-level structures of adsorption species at the O/W interfaces. Meanwhile, recent advances in spectroelectrochemical techniques have breathed fresh life into this research field, for example, surface secondharmonic generation (SHG)11-15 and potential-modulated spectroscopy.16-22 A quasi-elastic laser scattering method23,24 developed for interfacial tension measurements has also been used to study the adsorption of a water-soluble zinc porphyrin at the polarized O/W interface.15 One of the authors (H. N.) and co-workers have developed a potential-modulated fluorescence (PMF) spectroscopy.17-21 In this method, a sinusoidal potential modulation is superimposed * Corresponding authors. E-mail:
[email protected]; nagatani@ nagasaki-u.ac.jp. † Nagasaki University. ‡ Kobe University.
on the dc bias, generating a periodic perturbation of the interfacial concentration of fluorescent ion. The ac interfacial concentration manifests itself as a modulated fluorescence signal that can be measured by phase-sensitive detection. The amplitude and phase shift of the optical signal contains information on the nature of the charge-transfer and adsorption processes.18 The developed method has been applied successfully to study the ∆W O φ-dependent dynamics of charge-transfer and adsorption processes for metallo- and free-base porphyrins,17,18,20 1-pyrene sulfonate,19 and Sulforhodamine 101 (SR101).21 In the study of free-base porphyrins,20 we succeeded in obtaining the PMF spectra corresponding to the emission spectra of interfacial species, which gave information about their solvation structures at the O/W interface. Furthermore, PMF dependence on the polarization of the excitation beam allowed us to estimate the average molecular orientation of the adsorbed species.17 Thus, PMF spectroscopy has been revealed to be a promising tool for the analysis of charged species adsorbed at potentialcontrolled O/W interfaces. In this study, PMF spectroscopy was employed to study the charge transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), and Rhodamine 110 (R110), and, for comparison, a cationic rhodamine, Rhodamine 123 (R123), at the polarized 1,2-dichloroethane/ water (DCE/W) interface. The molecular structures of the rhodamine dyes are shown in Scheme 1. Rhodamine dyes are used widely in biological, analytical, and optical applications (e.g., as staining fluorescent dyes, chelating reagents, and laser dyes). Recently, certain attention has been drawn to amphoteric rhodamines, which make up the majority of commercial
10.1021/jp0723315 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007
Adsorption of Amphoteric Rhodamine Dyes SCHEME 1: Molecular Structures of the Rhodamine Dyesa
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9481 SCHEME 2: Schematic Representation of the Electrochemical Cella
a The pH of the W phase (III) was adjusted with appropriate buffers or HCl (for details, see the text).
For all of the rhodamines, the cationic forms (denoted by R+ in the text) are shown. a
rhodamines but whose adsorptivity at O/W interfaces has not been fully elucidated because of the complicated effects of positive and negative charges on the interfacial processes. Some papers have been devoted to the clarification of the adsorptivity and the excitation energy transfer dynamics of amphotetic rhodamines at nonpolarized O/W interfaces.25-27 Recently, the charge-transfer properties of amphoteric SR101 at a polarized O/W interface could be observed as the function of ∆W O φ, and the spectroelectrochemical responses indicated the presence of an adsorption process from the organic side of the interface.21 Although the ion transfer of an amphoteric rhodamine, RB, across polarized O/W interfaces was described briefly in a previous report,28 the more detailed analysis made in the present study clarified the reaction mechanism for the ion-transfer process. On the basis of a newly established reaction mechanism for rhodamine dyes, the results from PMF measurements were analyzed to show that ∆W O φ plays an important role in the adsorption processes at the interface. Furthermore, the solvation structure and molecular orientation of rhodamine dyes at the DCE/W interface were estimated from the PMF spectra. Experimental Section Reagents. Chloride salts of RB (g98%; Tokyo Chemical Industry), R110 (g99%; Fluka), and R123 (g95%; Aldrich) were used without further purification. Unless noted otherwise, the counteranion of R19 received as the perchlorate salt (∼95%; Aldrich) was ion-exchanged to the chloride ion through the extraction of the amphoteric form of R19 from a 50 mM LiOH aqueous solution to DCE followed by back-extraction into 2 mM HCl as the cationic form. DCE for HPLC (Wako Pure Chemical Industries) was used without further purification. The supporting electrolyte for the DCE phase, bis(triphenylphosphoranylidene)-ammonium tetrakis(chlorophenyl)borate (BTPPATClPB), was prepared by metathesis of bis(triphenylphosphoranylidene)-ammonium chloride (BTPPACl; Aldrich) and potassium tetrakis(chlorophenyl)borate (Dojindo Laboratories) in a 4:1 mixture of methanol and water, followed by recrystallization from acetone. All other reagents were of the highest grade available and used as received. Electrochemical Cell. A four-electrode spectroelectrochemical cell analogous to one reported previously17 was used in both cyclic voltammetry and PMF spectroscopy. A flat DCE/W interface (interfacial area 0.52 cm2) was formed at the tip of an aqueous solution-filled glass tube introduced downward into the DCE solution. Platinum wires were employed as counter electrodes in both aqueous and organic phases. Two Luggin
capillaries were provided for the reference electrodes in respective phases. The DCE/W interface was polarized by means of a Hokuto Denko HA1010mM1S potentiostat equipped with a positive feedback circuit for IR compensation.29 Unless noted otherwise, the electrochemical cell shown in Scheme 2 was used. Rhodamine dyes were dissolved in the W phase, the pH of which was adjusted with 5 mM NaH2PO4NaOH buffer for pH 7.0-6.0, 5 mM CH3COOH-CH3COOLi buffer for pH 5.5-3.6, and HCl for pH 2.4-3.0. The ∆W O φ of the test DCE/W interface was estimated by measuring the reversible half-wave potential (i.e., the midpoint potential in cyclic voltammetry) for the transfer of the tetramethylammonium ion, whose standard potential was reported to be 0.160 V.30 PMF Spectroscopy. PMF measurements were performed using an optical setup similar to the one reported previously.17 The excitation light source was a cw DPSS laser at 532 nm (Edmund Optics 90572-F; 5 mW) for RB and R19 or at 473 nm (Photop Suwtech DPBL-9010F; 10 mW) for R110 and R123, respectively. The DCE/W interface was illuminated under a total-internal reflection (TIR) condition from the organic phase. The critical angle for the DCE/W interface is 67.6°, and the angle of incidence to the interface was set at ca. 80°. The fluorescence was collected perpendicular to the interface by an optical fiber fitted to a photomultiplier tube (PMT) through a monochromator (Shimadzu SPG-120S). The ac modulated fluorescence signal was analyzed by a digital lock-in amplifier (NF LI 5640). Unless noted otherwise, the potential modulation was 10 mV and the potential sweep rate was 5 mV s-1. The dependence of the PMF on the angle of light polarization was studied by placing a linear polarizer for the unpolarized laser beam just before the spectroelectrochemical cell. Determination of Distribution Constants. The distribution constant for the amphoteric form (R() of RB, R19, or R110 at the DCE/W interface, KD ) [R(]O /[R(]W, was evaluated from the measurement of the distribution ratio of the rhodamine, D ) [R(]O /([R(]W + [R+]W) (R+: the cationic form), in the pH range of 3-7. These distribution experiments were performed by adding supporting electrolytes (5 mM BTPPATClPB and 10 mM LiCl) to the respective phases, in the same manner as the electrochemical measurements. The rhodamine concentration in the aqueous phase was determined by means of a Shimadzu UV-visible spectrophotometer (UV-2400PC). Using the pKa values of 3.2,31 3.15,32 and 3.2 (assumed) for RB, R19, and R110, respectively, the KD values were obtained as the averaged values over the pH range, which are shown in Table 1. It should be noted that in the above definition of KD, the existence of the lactone form is not taken into account, although the colorless lactone form may be produced to some extent from R( in the o TABLE 1: Values of log KD and ∆W O φR+ for the Rhodamines at the DCE/W Interface
rhodamines
log KD
o ∆W O φR+/V
RB R19 R110 R123
3.4 2.1 0.041
-0.37 -0.28 -0.01 -0.18
9482 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Osakai et al. r For this mechanism, ∆W O φ1/2 is given by
r W o ∆W O φ1/2 ) ∆O φR+ +
Figure 1. Cyclic voltammograms of the DCE/W interface, which were obtained by the addition of 0.1 mM RB, R19, R110, and R123 to the aqueous phase (pH 6.8). For R19, the perchlorate salt was added to the aqueous phase so that a wave due to the interfacial transfer of perchlorate ion appeared at around -0.15 V. For R123, 1.0 mM Span 20 was added to the DCE phase. The current increase in the voltammogram for each rhodamine corresponds to the increase in the scan rate: 10 f 20 f 50 f 100 f 200 mV s-1.
less-polar organic phase31 (i.e., [R(]O is considered here as the sum of the concentrations of the amphoteric and lactone forms). Results and Discussion Voltammetric Data. Figure 1 shows the cyclic voltammograms obtained when RB, R19, R110, and R123 were added to the aqueous phase. For a cation, anodic (i.e., positive) and cathodic (i.e., negative) currents correspond to its transfer from the aqueous to the organic phase and the reverse transfer, respectively. Well-developed waves were obtained for the transfer of RB, R19, R110, and R123 at around +0.05, +0.05, +0.25, and -0.18 V, respectively, at pH 6.8. However, abnormal current increase due to the electrochemical instability of the O/W interface, which should be originated by ionic adsorption,33,34 was observed at a lower pH of 3.0 for RB and R19 and in the whole pH range of 3.0-6.9 for R123 (data not shown). In such cases, the abnormal current was effectively suppressed by the addition of 1-2 mM sorbitan monolaurate (Span 20) to the organic phase.35 As seen in Figure 1, the voltammetric waves for the rhodamines showed essentially reversible characteristics, though they were possibly influenced by the interfacial adsorption of rhodamine species.17,20 The anodic and cathodic peak currents were proportional to the square root of the scan rate (V), suggesting that the charge-transfer process was diffusioncontrolled. The separation between the anodic and cathodic peak potentials was close to the theoretical value (ca. 60 mV) for the reversible transfer of a monovalent ion at the O/W interface, although it showed a tendency to increase with V, probably due to uncompensated IR drop. The midpoint potential between the anodic and cathodic peaks was dependent on pH but practically r independent of V. The reversible half-wave potential (∆W O φ1/2) was then evaluated from the midpoint potential and plotted against pH for the four different rhodamines. r As shown in Figure 2D, the ∆W O φ1/2 value for R123 was pHindependent, suggesting that the charge-transfer process can be described by a simple transfer of the monocation (R123+) across the O/W interface:
R123+(W) h R123+(O)
(1)
RT ln F
x
DRW+ DRO+
(2)
o where ∆W O φR+ denotes the standard potential for the transfer of O + R (here, R123+); DW j and Dj are the diffusion coefficients of + species j (here, R ) in the aqueous and organic phases, respectively; and R, T, and F have their usual meanings. Because DCE has a viscosity close to that of water, the DRW+/DRO+ ratio o was assumed to be unity. Therefore, the value of ∆W O φR+ could W r be determined as the constant value of ∆O φ1/2 () -0.18 V). r Regarding RB, R19, and R110, however, ∆W O φ1/2 shifted linearly with pH with a slope of ∼59 mV () 2.303RT/F) as shown in Figure 2A-C, suggesting the participation of protons in the charge-transfer processes. To elucidate the pH dependence r of ∆W O φ1/2 for the amphoteric rhodamines, we employed a generalized reaction scheme (Figure 3) for the partition of amphoteric rhodamines at the polarized O/W interface. In this scheme, it is assumed that the pH of the aqueous phase is adjusted with an appropriate buffer and that R( and its protonated form (R+) can be transferred across the O/W interface, but only R+ with a positive charge gives a faradaic current. Also, it is assumed that neither R+ nor R( forms any ion pair with supporting-electrolyte ions in both phases, though in practice some ion-pair formation would occur in the lesspolar DCE phase. Using a theoretical approach similar to those r described previously,36,37 ∆W O φ1/2 for this reaction scheme can
Figure 2. Ionic partition diagrams for (A) RB, (B) R19, (C) R110, r and (D) R123 at the DCE/W interface. (B), (2), (9), (1) the ∆W O φ1/2 values. The solid lines for A-C represent the regression curves that were obtained by using eq 7. The vertical dashed lines are drawn by eq 9 or pH ) pKa (for details, see the text). Note that these diagrams were prepared based on the voltammetric data in the presence of 5 mM BTPPATClPB in DCE and 10 mM LiCl in W.
Figure 3. Generalized reaction scheme for the partition of amphoteric rhodamines at the polarized O/W interface. diff. ) diffusion.
Adsorption of Amphoteric Rhodamine Dyes
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9483
be obtained as (see the Supporting Information) r W o ∆W O φ1/2 ) ∆O φR+ +
x
K1 D h W RT ln + O F 1 + K1 DR+
RT ln F
(
{
RT ln 1 + F
x
)
D h
W
) }
(
DRO+
1 K 1 + K1 D
(3)
where K1 is defined by
K1 )
[R+ ]W
(4)
[R(]W
and related to the acid dissociation constant, Ka, in the aqueous phase by
K1 )
10-pH Ka
(5)
And D h W in eq 3 represents the “effective” diffusion coefficient of the rhodamine in the aqueous phase, which is defined by
D hW )
DRW( + K1DRW+ 1 + K1
(6)
Because DRW( and DRW+ are probably almost equal, D h W can be W W approximated to be DR( or DR+. Also, considering the similar viscosities of DCE and water (and thus, D h W/DRO+ ≈ 1), we can W r obtain an approximate expression for ∆O φ1/2:
(
)
RT 1 + K1 + KD r W o ln ∆W O φ1/2 ) ∆O φR+ + F K1
(7)
Using this equation with eq 5, a regression analysis was r performed for the pH dependence of ∆W O φ1/2 for RB, R19, and R110 shown in Figure 2A-C. In this regression analysis, the above-mentioned values of Ka and KD were employed, and then o the value of ∆W O φR+ was used as a sole adjusting parameter. W o The ∆O φR+ values thus determined for the amphoteric rhodamines are also shown in Table 1. As seen in the table, the hydrophobicity of the R+ ions is reduced in the following order: RB > R19 > R123 > R110. Figure 2A-D represent the ionic partition diagrams for the rhodamines at the DCE/W interface, which show the pHpotential regions where the designated species are dominant. Judging from the higher KD values for RB and R19, these rhodamines would exist in the organic phase even at lower potentials, unless the pH is extremely low (note that in practice the rhodamines would exist as the dication at pH 0 to -131). In the higher pH and lower potential condition (i.e., right lower region of the diagram), the rhodamine exists as the amphoteric form in the organic phase, R((O).38 In the higher potential condition (i.e., upper region), the rhodamine should exist as the cationic form in the organic phase, R+(O). Accordingly, the r observed pH dependence of ∆W O φ1/2 for RB and R19 should not be described by the simple transfer of rhodamine species but rather in terms of the transfer of H+ facilitated by R((O):
H+(W) + R((O) h R+(O)
(8)
For RB and R19, the cationic form would exist in the aqueous phase only in the lower pH and lower potential region (left lower) indicated by R+(W) in Figure 2A and B. The pH value
Figure 4. Potential dependences of the PMF response for RB at (A) pH 6.8 and (B) pH 3.0. RB was initially added to the aqueous phase38 at the concentration of 1.0 × 10-5 M. The solid and dashed curves represent the real and imaginary components of the PMF signals, respectively. The excitation and detected emission wavelengths were 532 and 580 nm, respectively. The vertical dashed line indicates the ion-transfer potential.
at which [R+]W is equal to [R(]O (shown by vertical dashed lines) should be given by
pH ) pKa - log KD
(9)
Thus, the pH value representing the boundary line becomes much lower than pKa (∼3.2) owing to the large KD values (Table 1). The partition diagram for R110 (Figure 2C) is similar to those for RB and R19; however, in the right lower region, R110 exists as the amphoteric form not only in the organic phase but also in the aqueous phase because the KD value is close to unity. In this case, the boundary line between R+(W) and R((O) is given by pH ≈ pKa as coinciding with that between R+(W) and R((W). The partition diagram for the cationic rhodamine, R123, is simple as shown by Figure 2D. In the potential region higher r + than ∆W O φ1/2, R123 exists in the organic phase independent of pH, whereas in the lower potential region it exists in the aqueous phase. PMF Responses Depending on the Galvani Potential Difference. The PMF indicates the characteristic response for the transfer and adsorption of fluorescent species depending on 18 ∆W O φ and the frequency of a sinusoidal potential modulation. In the present study, the the PMF response was measured at the emission maximum wavelength of the rhodamine species in the aqueous phase, that is, 580, 548, 522, and 528 nm for RB, R19, R110, and R123, respectively. Figure 4 shows the representative PMF responses for RB at higher and lower pH values (pH 6.8 and pH 3.0). As seen in both panels, a large PMF signal was usually observed at around +0.25 V for all of the rhodamines. This PMF response will be discussed in the last part of this section. At pH values higher than 4.0, as shown in Figure 4A, a characteristic response for the transfer of a cationic fluorescent species, though small, was observed around the ion-transfer potential, that is, the midpoint r potential (∼∆W O φ1/2) in cyclic voltammetry, whose position is shown by a vertical dashed line. The real (∆Fre) and imaginary (∆Fim) components of the PMF response were observed mainly
9484 J. Phys. Chem. C, Vol. 111, No. 26, 2007
Osakai et al. where R is the overall transfer coefficient for the adsorption process, c is the concentration of the adsorption species just outside the electric double layer in the organic phase (note that W r c is a function of ∆W O φ and ∆O φ1/2), θ0 is the surface coverage at a given potential, ω is the angular frequency, Ca is a constant including optical properties of the adsorption species and the experimental configuration, and the subscripts 0 and 1 denote the dc and ac components. Considering the effect of the electric double layer, the adsorption (ka) and desorption (kd) rate constants are given by
[
ka,0 ) koa exp -
[
kd,0 ) kod exp
Figure 5. Frequency dependence of the PMF response for RB at pH 3.0. RB was initially added to the aqueous phase38 at the concentration of 1.0 × 10-4 M. The measurements were performed while setting ∆W O φ at -0.11 V. The potential modulation was 50 mV. The dashed line is the theoretical curve obtained by curve-fitting (see the Supporting Information).
as positive and negative values, respectively. This type of response is analogous to that for the interfacial transfer of a cationic fluorescent species.18 However, the above partition diagram (Figure 2A) shows that, in the pH range studied, RB does not exist in the aqueous phase but in the organic phase, where RB assists the interfacial transfer of H+ as shown in eq 8. The observed PMF response would be described by a comparative increase in the fluorescence of the rhodamine dye, which is brought about by the formation of the protonated form, R+(O), via the facilitated H+ transfer. It has been shown that R+(O) (with R ) RB and R19) exhibits a higher fluorescence intensity than R((O) at the chosen wavelength, that is, 580 nm for RB or 548 nm for R19 (see the Supporting Information, Figure S1). At pH 3.0, as shown in Figure 4B, a well-developed PMF response for RB was observed around -0.1 V that is lessnegative than the ion-transfer potential (shown by a dashed line). In contrast to the above “ion-transfer” response, ∆Fre and ∆Fim were observed mainly as negative and positive values, respectively. These features of PMF suggest the adsorption of a cationic fluorescent species (i.e., R+) from the organic-phase side of the interface.18 This has been supported by the frequency dependence of the PMF response shown in Figure 5. The PMF complex plane was measured as a distorted semicircle in the second quadrant, which is typically observed for the adsorption process of a cationic species from the organic phase. According to the previous theory,18 the ideal real (∆FOa,re) and imaginary (∆FOa,im) components of PMF for the adsorption are given by
∆FOa,re ) -Ca
[
]
∆wo φ1{ka,0c0R(1 - θ0) - kd,0(R - 1)θ0}(ka,0c0 + kd,0)
∆FOa,img ) Ca
(ka,0c0 + kd,0)2 + ω2
[
]
(10)
∆wo φ1{ka,0c0R(1 - θ0) - kd,0(R - 1)θ0}ω (ka,0c0 + kd,0)2 + ω2
(11)
RzF O W b ∆O φ RT
]
(1 - R)zF O W b ∆O φ RT
(12)
]
(13)
where bO∆W O φ is a portion of the Galvani potential difference employed for the adsorption process, z is the charge number of the species (z ) 1 for R+), and koa and kod are the adsorption and desorption rate constants at ∆W O φ ) 0 V. The observed PMF response, however, seemed to be influenced by the effect of the uncompensated IR drop in the higher frequency region (>7 Hz) and also by the contribution from the ion-transfer process at lower frequencies (5.0), the ion-transfer response could not be observed because of overlapping with the large PMF response that appeared at potentials higher than +0.1 V. On the other hand, the cationic rhodamine, R123, gave only an adsorption response except for the large response at higher potentials (see Figure 6B). The adsorption response was observed at the potential of ca. -0.1 V, which is somewhat higher than the ion-transfer potential (≈ -0.18 V). It was judged from the signal phase that the PMF response was mainly due to the adsorption of R123+ from the organic phase. In the pH-potential diagram in Figure 7, the region where adsorption responses were observed for the rhodamines is shown by shadow. As seen, only at lower potentials (
