Langmuir 2006, 22, 8403-8408
8403
Study on the Surface Density of Surface-Active Substances through Total-Reflection X-ray Absorption Fine Structure Measurement Kaoru Kashimoto,*,† Youichi Takata,† Takashi Matsuda,† Norihiro Ikeda,‡ Hiroki Matsubara,† Takanori Takiue,† Makoto Aratono,*,† Hajime Tanida,§ and Iwao Watanabe# Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu UniVersity, Fukuoka 812-8581, Japan, Faculty of Human EnVironmental Science, Fukuoka Women’s UniVersity, Fukuoka 813-8529, Japan, Experimental Facilities DiVision, Japan Synchrotron Radiation Research Institute, Hyogo 679-5198, Japan, and Department of Chemistry, Graduate School of Science, Osaka Prefecture UniVersity, Sakai, Osaka 590-0035, Japan ReceiVed March 24, 2006. In Final Form: May 9, 2006
The total-reflection X-ray absorption fine structure (XAFS) method previously employed for the adsorption of dodecyltrimethylammonium bromide (DTAB) at the air/water interface was applied to that in the presence of NaBr. The surface concentration of the bromide ions Γ XB of DTAB and NaBr was evaluated by using the Br K-edge absorption jump values of the total-reflection XAFS spectra and was compared to the corresponding value Γ HB estimated from the dependence of surface tension on the bulk concentrations of DTAB m1 and NaBr m2. The Γ XB values trace almost perfectly the Γ HB versus m1 curve up to a concentration near the critical micelle concentration (cmc) and deviate gradually above the concentration. This behavior is basically similar to that of the single DTAB system and ensures that the XAFS method is also applicable to the DTAB system, even in the presence of NaBr. In addition, this method was extended to the single nonionic amphiphile with covalently bonded bromine, and the surface concentrations of 6-bromo-1-hexanol (BrC6OH), Γ X1 and Γ H1 , were evaluated and compared with each other. It was found that the Γ X1 value almost perfectly traces the Γ H1 versus m1 curve, even at high surface concentrations. The excellent coincidence confirmed that the total-reflection XAFS method can be applied to the nonionic amphiphile system as well as a cationic surfactant with or without an added salt system. Finally, the difference between the Γ XB and Γ HB values observed in the DTAB with and without an added salt system is briefly described.
Introduction Surface-active substances are generally used for controlling the property of the liquid surfaces because they are adsorbed at the air/water interface, form the soluble monolayers in which the hydrophobic chains protrude to the air phase, and thus alter the nature of the surface dramatically.1,2 Their surface concentrations are very important for understanding the character of the liquid surfaces and have usually been evaluated thermodynamically by applying the adsorption equation.3-5 Recently, spectroscopic techniques such as ellipsometry,6 sumfrequency spectroscopy,7,8 neutron reflection (NR),9,10 X-ray reflection (XR),11,12 and X-ray absorption fine structure (XAFS) under total reflection conditions13-15 have been developed to investigate adsorbed films at air/liquid interfaces and have been * Corresponding author. E-mail:
[email protected]; telephone number: +81-92-642-2580; fax number: +81-92-642-2577 (K.K.). E-mail:
[email protected]; telephone number: +81-92642-2577; fax number: +81-92-642-2577 (M.A.). † Kyushu University. ‡ Fukuoka Women’s University. § Japan Synchrotron Radiation Research Institute. # Osaka Prefecture University. (1) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (2) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990. (3) Defay, R.; Prigogine, I. Surface Tension and Adsorption; Longmans: London, 1966. (4) Chemical Society of Japan. Colloid Chemistry; Tokyo Kagaku Dohjin: Tokyo, 1995. (5) Motomura, K. J. Colloid Interface Sci. 1978, 64, 348. (6) Goates, S. R.; Schofield, D. A.; Bain, C. D. Langmuir 1999, 15, 1400.
successfully applied to examine the structure of soft liquid interfaces as well as that of solid surfaces. Among them, NR, XR, and XAFS are also utilized for the determination of surface concentrations. Therefore, the evolution of such a spectroscopic study on an adsorbed film coupled with a thermodynamic approach will provide new insight on the structure and property of the adsorbed film. In our previous study on the application of the total-reflection XAFS method developed by Watanabe et al.13 to the aqueous dodecyltrimethylammonium bromide (DTAB) solution surface, the Br K-edge absorption jump J value was evaluated at various bulk concentrations of DTAB m1, and its m1 dependency was compared with the surface concentration of bromide ions (Br-) ΓHB obtained from the surface tension measurement.15 The following results were obtained: the J value can trace the ΓHB versus m1 curve, and thus it is proportional to ΓHB , and the shape of the (7) Bell, G. R.; Li, Z. X.; Bain, C. D.; Fischer, P.; Duffy, D. C. J. Phys. Chem. B 1998, 102, 9461. (8) Casson, B. D.; Bain, C. D. J. Phys. Chem. B 1999, 103, 4678. (9) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (10) Thomas, R. K.; Penfold, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 23. (11) Als-Nielsen, J.; Jacquemain, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251. (12) Schlossman, M.; Pershan, P. In X-ray and Neutron Scattering from Liquid Surfaces; Langevin, D., Ed.; Marcel Dekker: New York, 1991; Chapter 18. (13) Watanabe, I.; Tanida, H.; Kawauchi, S.; Harada, M.; Nomura, M. ReV. Sci. Instrum. 1997, 68, 3307. (14) Watanabe, I.; Tanida, H. Anal. Sci. 1995, 11, 525. (15) Takiue, T.; Kawagoe, Y.; Muroi, S.; Murakami, R.; Ikeda, N.; Aratono, M.; Tanida, H.; Sakane, H.; Harada, M.; Watanabe, I. Langmuir 2003, 19, 10803.
10.1021/la0526784 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/26/2006
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J versus m1 plot is that of Frumkin’s type,16 which can predict a phase transition in the adsorbed film.17 In addition, we realized that the surface concentration estimated by the J value, Γ XB , near the critical micelle concentration (cmc) is about 10-20% larger than the corresponding Γ HB value, although the reason such a difference appears remains unknown. Furthermore, we have demonstrated from total-reflection XAFS measurements that the generally accepted criterion of the ideal mixing of binary ionic surfactant mixtures in adsorbed films is wrong, and, instead, the criterion that we have derived from the thermodynamics of interfaces, which is totally different from the generally accepted one, is correct.18 These studies display the usefulness of the total-reflection XAFS method in fluid surface systems, and, as far as we are aware, only our group has carried out the study of adsorbed film through this method. The main purpose of the present study is to determine whether the total-reflection XAFS method is applicable for estimating the Γ XB values to the DTAB system, even in the presence of inorganic salt (NaBr), and to the nonionic system of 6-bromo1-hexanol (BrC6OH), of which bromine is covalently bonded to the hydrophobic chain. Then we briefly examine whether these results provide any information on the origin of the difference between Γ XB and Γ HB observed in the DTAB system. Experimental Section Materials. DTAB purchased from Tokyo Chemical Industry Co., Ltd. was purified by being recrystallized three times with a ethanol/ acetone mixture with a 1/5 volume ratio. Sodium bromide (NaBr) purchased from Wako Pure Chemical Industries, Ltd. (99.9%) was used without further purification. The purity of DTAB was checked by observing no minimum on the surface tension versus molality curve in the vicinity of the cmc. BrC6OH purchased from Tokyo Chemical Industry Co., Ltd. was purified by being distilled two times at around 105 °C and 4-6 mmHg. The purity of BrC6OH was confirmed by gas-liquid chromatography and elemental analysis. Water was distilled three times: the second and third stages were carried out from alkaline permanganate solution. Surface Tension Measurements. The surface tension γ of the aqueous solution was measured as a function of molalities under atmospheric pressure by the drop volume technique.19 The apparatus is illustrated in Figure 1, and the principal procedure is as follows. A drop with about 95% of its final volume was formed on the capillary tip of a glass syringe by pushing a plunger with the motor-driven micrometer (OPT MIKE-E, Sigma Koki Co., Ltd.) operated automatically by its controller (OPT MIKE-E CONTROLLER OMEC2BF, Sigma Koki Co., Ltd.) through the RS232C interface. After the thermal and adsorption equilibrium was established, the drop was grown again very slowly by pushing the plunger, and it eventually fell off the capillary tip. The falling was detected by a photoelectric sensor (FU-12, Keyence Corporation) located outside the glass measurement cell. The electricity of the sensor was provided by switching the power supply (MS-H50, Keyence Corporation). The optimum light intensity for detecting the falling drop was tuned by an amplifier unit (FS-V11, Keyence Corporation), and its electric signal from the sensor was transmitted to a personal computer. The consecutive operation was wholly automated by the computer system. The surface tension γ value is related to the volume of the drop V by the equation γ ) (V∆Fg/r)F
(1)
(16) Frumkin, A. Z. Phys. Chem. 1925, 116, 466. (17) Aratono, M.; Uryu, S.; Hayami, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1984, 98, 33. (18) Aratono, M.; Kashimoto, K.; Matsuda, T.; Muroi, S.; Takata, Y.; Ikeda, N.; Takiue, T.; Tanida, H.; Watanabe, I. Langmuir 2005, 21, 7398. (19) Matsuki, H.; Kaneshina, S.; Yamashita, Y.; Motomura, K. Langmuir 1994, 10, 4394.
Figure 1. Schematic diagram of the surface tension apparatus: (a) motor-driven micrometer; (b) plunger; (c) spring; (d) glass cell; (e) syringe; (f) sample solution; (g) drop; (h) red light-emitting diode; (i) photoelectric sensor; (j) sensor cable. where ∆F is the difference in density between the air and aqueous solution, g is the local acceleration of gravity, r is the capillary radius, and F is the correction factor, which was taken from the table of Lando and Oakley as a function of r/V1/3.20 Since the bulk concentration of DTAB is very low, the density of pure water was employed in the absence of NaBr, and that of aqueous NaBr solution was used in the presence of NaBr.21 The temperature was kept constant at 298.15 ( 0.05 K by immersing the measurement cell in a thermostated water bath (NTT-2200, EYELA). The experimental error of γ was less than (0.05 mN m - 1. Total-Reflection XAFS Measurements. The total-reflection XAFS experiment was performed by making use of the synchrotron X-ray at BL-7C22 of the Photon Factory of the National Laboratory for High Energy Physics (Tsukuba, Japan). Since the full description is given in our previous papers,13,15 we briefly mention the setup. The X-ray beam monochromatized by a double-crystal monochromator Si(111) is tilted downward by a mirror to make the incident angle 1 mrad and then strikes the solution surface under the total reflection condition. The incident beam intensity I0 was detected by a gas ionization chamber in front of the measurement cell. The Auger electron signal I was detected by the total-conversion heliumion yield method, and thus the XAFS spectrum was obtained by plotting I/I0 against the X-ray energy. The Br K-edge absorption jump was observed at around 13470 eV, and its value J was determined as the distance between the two best-fitted linear lines in the lower and higher energy regions around the absorption edge energy. Furthermore, the corrections of the J values for the deviation of the incidence angle from 1 mrad and the background from the bulk solution were carried out in the same manner described previously.15 The J value is correlated with the concentration profile of Bror BrC6OH, C(z), and the intensity of evanescent wave, P(z), is (20) Lando, L. J.; Oakley, T. H. J. Colloid Interface Sci. 1967, 25, 526. (21) Chemical Society of Japan. Kagaku Benran, 5th ed.; Maruzen: Tokyo, 2004. (22) Nomura, M.; Koyama, A.; Sakurai, M. KEK Report 91-1; National Laboratory for High Energy Physics: Tsukuba, Japan, 1991.
Total-Reflection XAFS Surface Density Measurements
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Figure 2. Surface tension versus DTAB molality curves at constant NaBr molalities: m2/mmol kg-1 ) (1) 0, (2) 5.00, (3) 10.0, (4) 20.0, (5) 50.0, (6) 1.00 × 102, (7) 2.00 × 102, (8) 5.00 × 102, and (9) 1.00 × 103. given as a function of the position z normal to the surface from the top of the surface at z ) 0 by J ) kS
∫
∞
0
C(z)P(z)dz ) kSP(0)
∫
∞
0
C(z) exp(-z/λ)dz
(2)
where k is a proportional constant, S is the footprint area of X-rays on the air/solution interface, P(0) is the intensity of the incident X-ray, and λ is the penetration depth, estimated to be 7 nm in the present study.15 The J value is assumed to be roughly attributable to the concentration of counterions Br- (DTAB-NaBr system) or that of the surface-active molecules (BrC6OH system) existing in the surface region as J = kSP(0)Γ
Figure 3. Surface tension versus NaBr molality curves at constant DTAB molalities: m1/mmol kg-1 ) (1) 0.05, (2) 0.1, (3) 0.2, (4) 0.4, (5) 0.75, (6) 1, (7) 1.5, (8) 2, (9) 3, (10) 5, (11) 8, and (12) 12.
by assuming that DTAB and NaBr are 1:1 strong electrolytes. Here, mR and γR are the molality and activity coefficient, respectively, of ion R (R ) D (dodecyltrimethylammonium ion), N (sodium ion), and B (bromide ion)) in the bulk solution, and ΓHR is the surface concentration of ion R, which is defined with respect to the two dividing planes, making the excess numbers of moles of air and water zero simultaneously.5,23 Using the electroneutrality conditions for the bulk solution mD ) m1, mN ) m2, mB ) m1 + m2, and that for the surface
ΓHD + ΓHN ) ΓHB eq 4 is rewritten as
[(
(3)
where Γ is the surface concentration of Br- or BrC6OH molecules per unit area. The proportional constant k is determined by using the Γ value evaluated from the surface tension measurement for a standard solution.
ΓHN
[(
( (
) )
∂ ln γDmD ∂ ln γNmN ∂ ln γBmB dγ ) -RT ΓHD + ΓHN + ΓHB dm1 ∂m1 ∂m1 ∂m1 -RT
ΓHD
∂ ln γDmD ∂ ln γNmN ∂ lnγBmB + ΓHN + ΓHB dm2 ∂m2 ∂m2 ∂m2 (4)
-RT ΓHD
)
∂ ln γDγB 1 1 + + + m1 m1 + m2 ∂m1
dγ ) -RT ΓHD
Results and Discussions Cationic Surfactant with Inorganic Salt (DTAB-NaBr) System. The surface tension γ of the DTAB-NaBr system was measured as a function of the bulk concentrations of DTAB m1 and NaBr m2 at 298.15 K under atmospheric pressure. The results are shown as the γ versus m1 curves at fixed m2 in Figure 2. The γ value decreases with increasing m1 at the lower concentrations, while it is almost constant at the higher concentrations, and thus all the curves have a distinct break point at the cmc. The shape changes gradually with increasing m2 from curve to curve. By reading the γ values at a given m1 from Figure 2 and plotting them against m2, we obtain the γ versus m2 curves shown in Figure 3. The γ value decreases with increasing m2, and the shape of the curve changes regularly with m1. Here, we briefly mention the thermodynamic relations for the calculation of surface concentrations employed in this work. For the air/aqueous solution of DTAB-NaBr, the total differential of γ is expressed as a function of m1 and m2 at a constant temperature T and pressure p as
(5)
(
)]
∂ ln γNγB 1 + dm1 m 1 + m2 ∂m1
)
∂ ln γDγB 1 + + m1 + m2 ∂m2
ΓHN
(
)]
∂ ln γNγB 1 1 + + dm2 m2 m1 + m2 ∂m2
(6)
γR is approximately calculated by the Debye-Hu¨ckel equation
log γR ) -
AzR2xI
(7)
1 + BaRxI
where A and B are the Debye-Hu¨ckel electrostatic terms, I is the ionic strength of the solution (I ) m1 + m2), and aR is the mean ionic diameter.24,25 For example, the activity coefficient is calculated as 0.8 when I is 100 mmol kg-1. The partial differential of log γR with respect to mi is written as
(
)
∂ log γR ∂mi
T,p,mj*i
)-
A
, i ) 1,2
2(1 + BaRxI)2xI
(8)
(23) Motomura, K.; Aratono, M. Langmuir 1987, 3, 304. (24) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; WileyInterscience: New York, 1989. (25) Boucher, E. A.; Grinchuk, T. M.; Zettlemoyer, A. C. J. Am. Oil Chem. Soc. 1968, 45, 49.
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Figure 4. Surface concentration of Br ion evaluated from surface tension measurement versus DTAB molality curves at constant NaBr molalities: m2/mmol kg-1 ) (1) 0, (2) 10.0, (3) 20.0, (4) 50.0, (5) 1.00 × 102, (6) 2.00 × 102, and (7) 5.00 × 102; (b) Γ H,c B .
and then
(
)
∂ ln γRγβ ∂ ln mi
)-
[
2.303Ami
1
2xI
(1 + BaRxI)2
T,p,mj*i
]
1
(1 + BaβxI)2
+
≡ fi(R,β) (9)
Figure 5. Surface concentration of Br ion evaluated from surface tension and total-reflection XAFS measurements versus DTAB X molality curves at constant NaBr molalities: (s) Γ HB ; (b) Γ H,c B ; ΓB -1 2 at m2/mmol kg ) (O) 0, (3) 10.0, (]) 50.0, (0) 1.00 × 10 , and (4) 2.00 × 102.
and
[
]
∂ ln(γR/γβ) ∂ ln mi
)-
T,p,mj*i
[
2.303Ami
1
2xI
(1 + BaRxI) 1
]
(1 + BaβxI)2
Table 1. Differences between the Plateau Values of Surface Concentrations, ΓH,c B (from the Surface Tension) and X,p Γ B (from the Total-Reflection XAFS), at Different NaBr Bulk Concentrations m2
2
≡ gi(R,β) (10)
are obtained. Making use of eqs 5, 6, 9, and 10, Γ HB is evaluated by the dependence of γ on m1 and m2 as
ΓHB ) -
[
( )
(1 - g2(D,N))m1 ∂γ 1 + RT 2 + f1(D,B) + f2(N,B) ∂m1
( )]
(1 + g1(D,N))m2 ∂γ 2 + f1(D,B) + f2(N,B) ∂m2
(11)
In the absence of added salt, putting m2 ) 0 in eq 11 and assuming that the aqueous solution is ideally dilute, we have the equation
ΓHB ) -(m1/2RT)(∂γ/∂m1)T,p
(12)
We evaluated ΓHB by applying eqs 11 and 12 to the γ versus m1 and m2 curves in Figures 2 and 3. The results are shown in Figure 4. The ΓHB values increase with increasing m1 and reach H,c the values at the cmc ΓH,c B depicted by the closed circle. The ΓB values increase gradually with increasing m2 because the electrostatic repulsion between the headgroups of surfactant ions are reduced because of the shielding of positive charges by an ionic atmosphere of Br-. The surface concentration of Br- from the total-reflection XAFS measurement Γ XB is evaluated by multiplying a factor to the J value. The factor is determined for the Γ XB value of the standard solution to coincide with the corresponding ΓHB
NaBr molality/mmol kg-1
H,c -2 Γ X,p B - Γ B /µmol m
0 10 50 100 200
0.48 0.37 0.27 0.21 0.15
obtained by the surface tension measurement. The Γ XB values are plotted against m1 at various m2 values together with the Γ HB versus m1 curves in Figure 5. It is seen that the Γ XB values trace the ΓHB versus m1 curves almost perfectly up to a concentration near the cmc at all m2 values. However, we note that, although X ΓHB practically reaches ΓH,c B in the vicinity of the cmc, the Γ B value still increases around the cmc and finally reaches another plateau value Γ X,p B at higher concentrations above the cmc. This behavior is essentially the same as that observed for the DTAB system. Therefore it is concluded that the present XAFS method is also applicable to the system in the presence of inorganic salt. X,p Here it should be noted that the difference between ΓH,c B and Γ B tends to decrease with increasing m2, as listed in Table 1. Single Nonionic Amphiphile (BrC6OH) System. The surface tension γ of its aqueous solution was measured as a function of the molality m1 up to about 14 mmol kg-1, corresponding to the solubility limit of BrC6OH at 298.15 K under atmospheric pressure. The results are shown in Figure 6: the γ value decreases with increasing m1, and the curve has an inflection point at a low concentration (m1 ) 2-3 mmol kg-1). Applying the equation
ΓH1 ) - (m1/RT)(∂γ/∂m1)T,p
(13)
Total-Reflection XAFS Surface Density Measurements
Figure 6. Surface tension versus molality curve of the BrC6OH aqueous solution.
Figure 7. Surface concentration versus molality curve of the BrC6OH aqueous solution.
Figure 8. XAFS spectra for the BrC6OH aqueous solutions at given BrC6OH molalities: m1/mmol kg-1 ) (1) 2.00, (2) 4.00, (3) 6.00, (4) 8.00, (5) 10.0, (6) 12.0, and (7) 14.0.
to the γ versus m1 curve, the surface concentration of BrC6OH ΓH1 is evaluated and plotted against m1 in Figure 7: the ΓH1 value increases with increasing m1 and the shape is sigmoidal with the inflection point. The typical XAFS spectra at several bulk concentrations are drawn in Figure 8, in which the I/I0 values at the lowest energy of all the spectra were moved to the same position to visualize clearly the concentration dependency of the spectra above the absorption edge. The concentration dependence of the I/I0 jump value was very similar to that observed in the DTAB system, and
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Figure 9. Surface concentration evaluated from surface tension and total-reflection XAFS measurements versus molality curves for the BrC6OH aqueous solution: (s) Γ H1 ; (O) Γ X1 .
the surface concentration Γ X1 was evaluated from the XAFS spectra by the same procedure employed in the DTAB system.15 The Γ X1 values are plotted against m1 together with the Γ H1 versus m1 curve in Figure 9. It is said that ΓX1 and Γ H1 coincide with each other within their experimental error and that the γ versus m1 curve at the dilute region does not suggest the Langmuir isotherm26 but the Frumkin one. It should be noted that Γ X1 almost perfectly traces the ΓH1 versus m1 curve, even in a high surface concentration region up to the solubility limit. This indicates that the total-reflection XAFS method is also effective for the nonionic surface-active substance system similarly to the cationic surfactant system. In this way, the applicability of the XAFS method to the two cases, that is, the ionic surfactant with added salt and nonionic amphiphile with covalently bonded bromine, was established. Consulting the results given above, let us briefly describe the X,p 15 difference between ΓH,c B and Γ B . In our previous paper, it was suggested that a structure change from an almost flat arrangement of the surfactant headgroup to a staggered one claimed by Penfold et al.9 may increase (i) the surface concentrations of both the surfactant ion and the counter Br-, or (ii) that of only Br- due to a change in Br- distribution in the electrical double layer with keeping the surface concentration of the surfactant ion constant. If case (i) is true, it is concluded that the ΓHB values given here were not correctly estimated at concentrations very near to the cmc, because nonideality corrections due to any possible cases such as ion pair and/or premicelle formation (small aggregate) were not taken into account, although the activity correction by the Debye-Hu¨ckel approximation was properly performed. As far as the authors are aware, however, there is no report on how to properly measure such nonideality corrections very near to the cmc of surfactants. If the difference between Γ XB and ΓHB is responsible for the Br- distribution in the interfacial region, the difference ought to disappear at very large concentrations of added salt because ionic surfactants practically behave as if they were nonionic. The experimental finding given in Table 1 undoubtedly fulfills this condition. However, this situation can happen in both (i) and (ii) and thus does not necessarily guarantee case (ii). Thus, the change in surface concentration near and above the cmc is still controversial.27,28 In this respect, a method that can (26) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (27) Lu, J. R.; Simister, E. A.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1993, 97, 13907.
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directly determine the number of surfactant hydrophobic groups in the interface is desired. Infrared reflection absorption spectroscopy has progressively become an available technique for this,29 and we are making progress in this direction. (28) Tajima, K.; Muramatsu, M.; Sasaki, T. Bull. Chem. Soc. Jpn. 1970, 43, 1991. (29) Wen, X.; Lauterbach, J.; Franses, E. I. Langmuir 2000, 16, 6987.
Kashimoto et al.
Acknowledgment. The present study has been examined under the approval of the Photon Factory Advisory Committee (Proposal Nos. 2002G106 and 2004G109). This work was supported partly by the Grant-in Aid for Scientific Research (B) of the Japan Society for the Promotion of Science (No. 16350075). LA0526784
