1996
Langmuir 1997, 13, 1996-2000
Microspectroscopic Study on Dye Association at a Single Laser-Trapped Water Droplet/Oil Interface Hiroshi Yao, Hiroshi Ikeda, and Noboru Kitamura* Division of Chemistry, Graduate School of Science, Hokkaido University, Kita-ku, Sapporo 060, Japan Received October 4, 1996. In Final Form: January 27, 1997X Scanning laser manipulation and microspectroscopic techniques were applied to analyze a molecular association of malachite green (MG) in single water droplets dispersed in dioctyl phthalate. Droplet size effects on MG dimer formation were observed in micrometer dimension; dimer formation was facilitated in the smaller water droplets. Concentration effects of MG and an added electrolyte on dimer formation were also studied in detail, and the results were discussed in terms of Langmuir-type adsorption of the MG dimer at the water droplet/oil interface.
Introduction Investigations of the properties and chemical reactions at liquid/liquid interfaces have attracted scientific interest in connection with phase transfer catalysis, ion extraction, or ion transport through bilipid membranes.1 In most cases, the studies along the line mentioned above have been conducted for macroscopic flat liquid/liquid or emulsion systems. In emulsion systems, particularly, the properties and chemical reactions at a droplet interface are expected to be dependent on the size and interfacial structure (i.e., tension, adsorption, and so forth) of the droplet. Furthermore, characteristic behavior of a spherical droplet, different from that of a flat liquid/liquid interface, may also be expected. Therefore, single-droplet measurements are quite necessary to elucidate liquid/ liquid interfacial phenomena. Recently, we developed a new methodology, capable of simultaneous manipulation and chemical analyses of micrometer-sized droplets by combining laser trapping with a microspectroscopy/ microelectrochemistry technique.2-4 Laser trapping by a focused laser beam can be applied to microparticles whose refractive indices (np) are higher than that of the surrounding medium (ns): np > ns. This is the case for oil droplets or polymer particles (np > 1.33) in water (ns ) 1.33). Actually, we reported direct measurements of electron or mass transfer processes across a microdroplet/water interface as well as of ion diffusion in single polymer microparticles on the basis of the laser-trapping-microspectroscopy technique. When np < ns, as in the case of a water droplet (np ) 1.33) in a common organic solvent, contrarily, the droplet receives repulsive force, so that a water droplet cannot be trapped by irradiating a focused laser beam. In order to trap a water droplet in a medium where ns > np, therefore, a laser-scanning method has been developed, by which a focused laser beam is repetitively scanned around the droplet to produce a “photon cage”. The water droplet experiences the repulsive force from all the directions in the cage, so that the droplet is trapped three-dimensionally at the center of the photon cage. Recently, we succeeded in photometric analyses of a solute in single water droplets X
Abstract published in Advance ACS Abstracts, March 1, 1997.
(1) Benjamin, I. Acc. Chem. Res. 1995, 28, 233. (2) Masuhara, H., DeSchryver, F. C., Kitamura, N., Tamai, N., Eds. Microchemistry; Spectroscopy and Chemistry in Small Domains; NorthHolland: Amsterdam, 1994. (3) Nakatani, K.; Misawa, H.; Sasaki, K.; Kitamura, N.; Masuhara, H. J. Phys. Chem. 1993, 97, 1701. (4) Kim, H.-B.; Hayashi, M.; Nakatani, K.; Kitamura, N.; Sasaki, K.; Hotta, J.; Masuhara, H. Anal. Chem. 1996, 68, 409.
S0743-7463(96)00966-3 CCC: $14.00
dispersed in an organic medium on the basis of a scanning manipulation-microspectroscopy technique. Direct measurements of individual micrometer-sized water droplets are very important to elucidate molecular mechanisms of reactions in water-in-oil emulsions. Also, the studies are an indispensable basis for understanding liquid/liquid extraction mechanisms. As a model system, therefore, we studied molecular association of dye molecules (malachite green: MG) in single water droplets dispersed in an oil and demonstrated previously that dimer formation of MG was facilitated in micrometer-sized droplets compared to that in aqueous thin films. It was suggested that the water droplet/oil interface played an essential role in MG dimer formation. However, detailed experiments could not be done, owing to an instability in manipulating water droplets and also to photobleaching of MG. Fortunately, such problems were overcome by a choice of an oil phase. In this study, therefore, we examined concentration effects of both MG and added KCl on MG dimer formation in individual water droplets in detail. On the basis of the present results, micrometer size effects on dimer formation were discussed in terms of adsorption of the MG dimer on a water/oil interface. Experimental Section Apparatus. An experimental setup of a scanning laser manipulation-microspectroscopy system has been reported elsewhere.5 Briefly, a fundamental beam from a CW Nd3+:YAG laser (Spectron; SL902T) was introduced to an optical microscope (Nikon; Optiphoto 2) and focused to a ∼1 µm spot through an oil immersion objective lens. For manipulation of a water droplet in an organic solvent (np < ns), the focused beam was scanned circularly around the droplet to produce a “photon cage”, as mentioned before. Beam scanning was performed by computercontrolled galvano mirrors (GSI; G325DT, Marubun; TI-325 controller, NEC; PC9801VX computer). A probe Xe light beam (Hamamatsu Photonics, L2273) was introduced to the microscope and used to irradiate (∼2 µm in beam diameter) the center of a trapped droplet to obtain an absorption spectrum. The light intensity of the Xe beam through a droplet (I) was analyzed by a polychromator (Jobin-Ybon, HR-320)-multichannel photodetector (Princeton Instruments, IRY-512G SMA) system. The incident light intensity of the Xe beam (I0) was determined under the same optical conditions without a droplet. For obtaining one spectrum, I or I0 data were accumulated for 40 shots with an exposure time per shot of ∼2.4 ms. Under the present conditions, photodegradation of MG during spectrum measurements was negligible (see also below), so that a spectrum with a good S/N ratio was obtained even for single water droplets. The spec(5) Yao, H.; Inoue, Y.; Ikeda, H.; Nakatani, K.; Kim, H.-B.; Kitamura, N. J. Phys. Chem. 1996, 100, 1494.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 7, 1997 1997
Figure 1. Absorption spectra of MG (10 mM) in water thin films (∼30 µm thickness) at [KCl] ) 0, 100, and 400 mM. troscopic response of the present setup was checked by measuring single Ru(bpy)32+ (bpy: 2,2′-bipyridine)/water droplets with various diameters, whose spectra were the same as that recorded by a conventional spectrometer. Conventional absorption spectroscopy for solution films was conducted with a Shimadzu UV365 spectrometer. Chemicals. Malachite green oxalate (MG; GR grade) and potassium chloride (KCl; GR grade) were purchased from Kanto Chemicals and Wako Pure Chemical Industries, respectively, and used as received. Pure water was obtained by an Aquarius GSR-200 (Advantec Co. Ltd.). Dioctyl phthalate (DOP: phthalic acid di(2-ethylhexyl) ester; GR grade, ns ) 1.486) as an oil was used without further purification. In our previous study, di-nbutyl phthalate (DBP) was used as an oil phase. However, we used DOP instead of DBP, since DOP was more preferable for trapping of single water droplets as well as for the photostability of MG. Water-in-oil emulsions were prepared by dispersing a DOP-saturated aqueous dye solution into a water-saturated DOP with a volume ratio (water/DOP) of 0.05. The pH of the aqueous solution was adjusted to ∼4.5 with HCl to prevent photobleaching of MG. Absorption spectroscopy was performed for freshly prepared emulsions, which were sealed between two glass plates. The MG solution films for bulk measurements were obtained by setting the solution between two slide glasses with thicknesses of ∼30 µm.
Results and Discussion Effects of Added KCl on Association of MG in Water. Figure 1 shows absorption spectra of MG (initial concentration (C0) ) 10 mM) in water (thin films) at various KCl concentrations. The peaks at 615 and 580 nm have been assigned as the monomer and dimer bands of MG, respectively.5 The absorbance ratio of the dimer to the monomer increased with an increase of the KCl concentration. As reported previously, MG dimer formation is expressed as in eq 1,
2MG+ S (MG+)2
(1)
so that the relevant association constant (K) of MG can be given as
K)
CD CM2
)
x 2C0(1 - x)2
(2)
where CD and CM represent the concentrations of the dimer and the monomer, respectively. x is the mole fraction of the dimer to C0. In the absence of KCl, the K value has been determined to be 135 M-1.5 Also, the molar absorption coefficients of the monomer (M) and the dimer (D) at 615 and 580 nm, respectively, are known as follows: M615 ) 8.0 × 104, D615 ∼ 0, D580 ) 5.2 × 104, and M580 ) 3.6 × 104 M-1 cm-1.5 The K values at various [KCl] were
Figure 2. Ionic strength effects on MG dimer formation.
thus estimated from the relevant absorption spectra. Figure 2 shows the relationship between log K and the square root of the ion strength of the solution ((I)1/2), which demonstrates the increase in K with I1/2. When the ionic atmosphere produced by added KCl affects the association equilibrium of a cationic MG, the plot is expected to be fitted by eq 3,
log K ) R -
AβxI 1 + BaxI
(3)
where A and B are constants at 0.51 and 0.33, respectively, for a 1:1 electrolyte in water at 25 °C.6 The a value is an ion size parameter defined by Debye-Hu¨ckel theory and is 3.8 for an aqueous KCl solution ([KCl] < 1 M).7 The observed data were well simulated with R ) 2.03 and β ) -2.47, as shown by the solid curve in Figure 2. Ionic strength effects on rates of ion-ion reactions are well known.7 As an example, Taube et al. reported that the ionic strength effects on the equilibrium constant for ligand substitution reactions of Co3+ complexes with ionic ligands could be well explained by eq 3.8 Therefore, the results in Figure 2 indicate that MG dimer formation is facilitated by Coulomb screening through positive charge neutralization of MG by Cl anions. Estimation of the MG Dimer Geometry. The dimer band (580 nm) showed a hypsochromic shift compared to the monomer band (615 nm). The structural requirements for MG dimer formation are not well-known, so that we evaluated the geometrical structure of the dimer on the basis of a simple theoretical model. Since MG itself possesses a planar structure, the molecules are assumed to be aligned parallel to each other.9,10 In such a case, an exciton model can be applied. According to the model, namely, the electronic transition of an aggregate appears at a shorter wavelength (higher energy) than that of the monomer, when the angle θ between the center-to-center line of the chromophores and the transition moment of a monomer is larger than the magic angle, 54.7°. Actually, it has been reported that the absorption spectrum of a methylene blue dimer can be well understood by this model.11 On the basis of the exciton model, a shift energy between the dimer and monomer bands (∆ν) is given as in eq 4, (6) Christian, G. D. Analytical Chemistry, 4th ed.; John Wiley & Sons: New York, 1986. (7) Harned, H. S.; Owen, B. B. The Physical Chemistry of Electrolytic Solutions, 3rd ed.; Reinhold Publishing Corp.: New York, 1964. (8) Posey, F. A.; Taube, H. J. Am. Chem. Soc. 1956, 78, 15. (9) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (10) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (11) Bergmann, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 2169.
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Figure 4. Droplet diameter dependencies of the dimer/ monomer concentration ratio of MG in single water droplets at several MG concentrations. The solid curve represents the simulation curve of eq 6.
Figure 3. Absorption spectra of single MG ((a) 10 mM and (b) 5 mM)/water droplets in DOP. The droplet was laser trapped by circular scanning of the focused 1064 nm laser beam with the power and the repetition rate ∼4 W and ∼30 Hz, respectively.
( )( )
∆ν ) 4
N - 1 m2 (1 - 3 cos2 θ) N r3
(4)
where N and m are the aggregation number and the transition moment of MG, respectively, and r represents the center-to-center distance between two molecules. In the present case, N equals 2 and m2 can be calculated to be 9.2 × 10-35 esu2 cm2. Assuming r to be 4 Å,12 θ should be 56.1° to explain the observed shift energy of 1.95 × 10-13 erg. Since the θ value is larger than the magic angle, the hypsochromic shift of the MG dimer can be reasonably explained within the exciton model. Therefore, we suppose that the MG dimer is produced by association of two molecules in a parallel fashion (r ∼ 4 Å) with θ being ∼56°. Micrometer Size Effects on MG Dimer Formation. Concentration Effects of MG. Absorption microspectroscopy on single MG/water droplets at various MG concentrations (C0 ) 5∼10 mM) has been conducted. Figure 3 shows absorption spectra of individual MG/water droplets dispersed in DOP (10 and 5 mM for parts a and b, respectively). Figure 3 demonstrates that the increase in the MG concentration brings about an increase of the absorbance ratio of the dimer to the monomer. Furthermore, the absorbance ratio increased when the droplet diameter was decreased from ∼35 to ∼16 µm irrespective of the MG concentration. Previously, we performed analogous experiments on single MG/water droplets dispersed in di-n-butyl phthalate (DBP) and demonstrated the MG concentration and droplet size effects on MG dimer formation. However, the experiments on the MG concentration effects were conducted at a fixed droplet diameter, while the droplet (12) Rossi, U. D.; Daehne, S.; Reisfeld, R. Chem. Phys. Lett. 1996, 251, 259.
size effects on the spectrum were studied at a fixed MG concentration (10 mM). In DBP, photodegradation of MG took place efficiently, so that detailed spectroscopic measurements varying both the MG concentration and the droplet diameter were very difficult. In the present study, since the experimental conditions were improved by the use of DOP as an oil (see also Experimental Section), we carried out the concentration and droplet size dependencies of the spectrum in detail. For quantitative discussion, the concentration ratio of the dimer to the monomer (CD/CM) in the single droplet was estimated on the basis of the absorption spectrum and eq 2. By varying C0 from 5 to 10 mM, x changed from 0.43 to 0.55. The CD/CM value in each MG/water droplet was then calculated with the known parameters of D580 and M615. The droplet diameter (d) dependencies of CD/ CM at several MG concentrations are summarized in Figure 4. The CD/CM value increased with decreasing d at any MG concentration. Furthermore, it is worth pointing out that, at a given MG concentration, the CD/CM value for the water droplet (d < 35 µm) is always larger than that observed in a water thin film (thickness ) ∼30 µm and CD/CM ) 0.382 (5 mM) ∼ 0.608 (10 mM)). The results demonstrate that the spherical structure of the droplet is very favorable for MG dimer formation and that the water droplet/oil interface plays a role in MG dimer formation. The micrometer size effects on CD/CM indicate that the efficiency of MG dimer formation is dependent on the surface area/volume ratio of the droplet. We assume that the association equilibrium of MG in the water droplet is the same as that in the bulk phase and that the characteristic behavior in the droplet observed in Figure 4 originates from the MG dimer formation at the water/ oil interface. In order to test such an idea, we introduce a parameter, s, which is the mole number of the monomer to form the dimer at the interface (per unit surface area; mol‚cm-2). Dimer formation at the interface renders a decrease in the monomer concentration in the droplet. Therefore, the decreased amount of MG (mole number) in the droplet by dimer formation can be expressed as 4π(d/2)2‚s. Dividing by the volume of the droplet (4/3‚π(d/ 2)3), we obtained the monomer concentration of MG (CM) as in eq 5a,
CM ) C0(1 - x) -
6s d
(5a)
Since MG dimer is supposed to exist preferentially at the interface, the surface dimer concentration in the droplet
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Langmuir, Vol. 13, No. 7, 1997 1999
Figure 5. Absorption spectra of single MG (10 mM)-KCl (100 mM)/water droplets in DOP. The laser-trapping conditions were the same as those in Figure 3. Table 1. Concentration Effects of MG and KCl on MG Dimer Formation ionic strength
[MG]/mM
[KCl]/mM
s/10-10 mol‚cm-2
0.0075 0.009 0.0105 0.015 0.065 0.1075 0.115 0.415
5 6 7 10 10 5 10 10
0 0 0 0 50 100 100 400
1.40 1.55 1.75 1.80 1.65 1.60 1.75 1.80
(CDS) is given as 1/2‚4π(d/2)2‚s/{4π(d/2)2‚δ} ) 1/2‚s/δ, where δ (δ , d) is the thickness of the surface layer for MG dimer formation. Taking the optical path length for absorption spectroscopy on the droplet (2δ) into account, the dimer concentration (CD) obtained from the absorption spectrum is expressed as 1/2‚C0x + CDS‚2δ/d, where the first and second terms represent the dimer concentrations in the droplet interior and in the surface layer, respectively. Using the relation CDS ) s/(2δ), we obtain
CD )
1 2s C x+ 2 0 d
(
)
(5b)
Thus we can derive eq 6,
CD/CM )
[
2C0 - 8s/d
(C0x + 2s/d)
]
-1
-2
(6)
Equation 6 predicts the d dependence of CD/CM and should explain the observed data in Figure 4 at given C0. The simulation of the data based on eq 6 is shown by the solid curve in Figure 4. The droplet size dependence of CD/CM was well reproduced by eq 6 with the parameter s, and the s values thus obtained at various MG concentrations are listed in Table 1. With an increase of the MG concentration from 5 to 10 mM, s increased from 1.40 × 10-10 to 1.80 × 10-10 mol‚cm-2, demonstrating that the increase in [MG] brings about preferential dimer formation at the droplet interface. For example, the s value of 1.80 × 10-10 mol‚cm-2 implies that one MG dimer sits on the water/oil interfacial area of 1.8 nm2. The mole number of MG located at the interface is calculated to be 5.4% of that dissolved in a single droplet with d ) 20 µm. KCl Effects on MG Dimer Formation. Analogous to the results in Figure 1, remarkable KCl effects on MG dimer formation were confirmed for individual MG/water microdroplets (10 mM) in DOP, as typical absorption spectra at [KCl] ) 100 mM were shown in Figure 5. With a decrease in the droplet diameter, an increase of CD/CM was also observed. Similar micrometer size effects were observed at several KCl concentrations, and the results
Figure 6. Droplet diameter dependencies of the dimer/ monomer concentration ratio of MG (10 m) in single water droplets at [KCl] ) 0, 100, and 400 mM. The solid curves represents the simulation curves of eq 6.
are summarized in Figure 6 as the d dependence of CD/CM ([MG] ) 10 mM). Although the data at [KCl] ) 400 mM seem to be somewhat scattered, the results were simulated on the basis of eq 6. The s parameter estimated for each KCl concentration is included in Table 1, together with that for MG (5 mM)-KCl (100 mM)/water droplets. Table 1 demonstrates that, at [MG] ) 5 mM, the s value increased with an increase in KCl, while it scarcely changed with a variation of [KCl] at [MG] ) 10 mM. This will be ascribed to saturation of the s value, since the concentration of MG is very high, as discussed again later. Adsorption Equilibrium of MG Dimer at the Water Droplet/Oil Interface. MG dimer formation in the water droplets was shown to be dependent on both the droplet diameter and the added KCl concentration. At given [MG], furthermore, the CD/CM value in the droplet is larger than that in a water thin film. These results suggest surface adsorption of MG on the water droplet/oil boundary. We analyzed the results in Table 1 on the basis of a Langmuirtype adsorption mechanism.13 We assume adsorption sites of the dimer at the interface. At an equilibrium, rates of adsorption (ka) and desorption (kd) of the MG dimer are equal, so that we obtain eq 7,
kdφσ ) ka(1 - φ)CD0
(7)
where φ and σ are the fraction of the surface occupied by the MG dimer and the amount of the MG dimer at saturated adsorption, respectively. CD0 represents the dimer concentration in the absence of adsorption, which is assumed to be equal to that in a water thin film at given [MG]. Here the parameter s, representing the mole number of MG at the surface area, can be written as 2σ‚φ. Introducing an equilibrium constant KL ()ka/kd), the mole number of the dimer adsorbed on the surface can be expressed as in eq 8,
σKLCD0 s ) 2 σ + KLCD0
(8)
We thus obtain eq 9,
2 1 1 1 + ) ‚ s KL CD0 σ
(9)
by which the KL and σ values can be evaluated through the slope and intercept values of a 1/s versus 1/CD0 plot, respectively. (13) Watarai, H. Talanta 1985, 32, 817.
2000 Langmuir, Vol. 13, No. 7, 1997
Figure 7. Relationship between 1/s and 1/CD0. The data were compiled from those in Figures 4 and 6.
As shown in Figure 7, actually, we obtained a fairly good linear relationship between 1/s and 1/CD0, indicating that the assumption of Langmuir-type adsorption was adequate for the present analyses. The slope and the intercept values of the plot gave KL ) 2.6 × 10-4 cm and σ ) 1.0 × 10-10 mol‚cm-2 (i.e., s ) 2.0 × 10-10 mol‚cm-2 under the saturated condition). The amount of saturated adsorption, σ, indicates that one MG dimer occupies 1.6 nm2 of surface area of the droplet by adsorption. The s value at saturated adsorption (s ) 2.0 × 10-10 mol‚cm-2) is very close to those observed at [MG] ) 10 mM (Table 1). Therefore, the absence of the KCl concentration effect on s at [MG] ) 10 mM, described in the preceding section, will be explained by saturated adsorption of the MG dimer on the interface. According to the molecular model of the MG dimer, a cross section of the MG dimer is estimated to be about 1.5-1.7 nm2, so that the MG dimer would adsorb horizontally on the water/DOP interface. An observation of the droplet size effect in the d ) 15∼30 µm region is very unique, since the size of the droplet is very large compared to that of MG. However, analogous micrometer size effects have been observed for electron
Yao et al.
and mass transfer processes across single oil droplet/water interfaces.14,15 Changes in the fluorescence lifetime of a dye have also been reported for micro-oil-droplets.16,17 Thus, it is true that spherical micro liquid/liquid interfaces show a characteristic nature, not expected simply from the conventional microemulsion regime. In the present system, MG acts as a surfactant. As reported previously, namely, minute and stable water droplets in dibutyl phthalate can be prepared in the presence of MG. Without MG, on the other hand, preparation of water-in-oil emulsions was very difficult and well-dispersed emulsions could be obtained by adding poly(vinyl alcohol) as an additive. These results support preferential location of MG at the water droplet/oil boundary. The MG dimer is supposed to be more hydrophobic than the monomer, so that adsorption of MG on the interface will be facilitated by the dimer formation. If this is important, the dimer formation kinetics will be different in the surface layer of the droplet, which might be one of the other possible reasons for the present size effect. Further systematic investigations are required to reveal the micrometer size effects on molecular interactions, and work along that line is now in progress in this laboratory. Acknowledgment. This work was supported partly by the Grant-in-Aid for Scientific Research on PriorityArea-Research “Photoreaction Dynamics” from the Ministry of Education, Science Sports and Culture of Japan (08218201) and the Toray Science Foundation. LA960966H (14) Nakatani, K.; Chikama, K.; Kim, H.-B.; Kitamura, N. Chem. Phys. Lett. 1995, 237, 133. (15) Nakatani, K.; Uchida, T.; Kitamura, N.; Masuhara, H. J. Electroanal. Chem. 1994, 375, 383. (16) Pandey, K. K.; Hirayama, S. J. Photochem. Photobiol. A: Chem. 1996, 99, 165. (17) Barnes, M. D.; Kung, C.-Y.; Whitten, W. B.; Ramsey, J. M. Phys. Rev. Lett. 1996, 76, 3931.
