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Evacuation of Counteranions from Langmuir Monolayers of Double-Tailed Quaternary Ammonium Ions into Subphase at High Surface Pressures as Studied by ...
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J. Phys. Chem. C 2009, 113, 12476–12482

Evacuation of Counteranions from Langmuir Monolayers of Double-Tailed Quaternary Ammonium Ions into Subphase at High Surface Pressures as Studied by Total Reflection X-ray Absorption Spectrometry Makoto Harada* and Tetsuo Okada Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan ReceiVed: January 31, 2009; ReVised Manuscript ReceiVed: May 22, 2009

Total reflection X-ray absorption fine structure (TR-XAFS) has been utilized for the detailed analyses of the surface aggregation of double-tailed quaternary ammonium ion bromides. TR-XAFS at the Br K-edge gives us information on the population of Br- just below the subphase surface as well as its local structure. The spectra indicate that Br- is almost completely hydrated, even when it is attracted by the compressed monolayer. In addition, the signal intensities obtained for the monolayers of the ammonium amphiphiles with different hydrocarbon chain lengths suggest that the correction for the attenuation of X-rays due to the absorption by the hydrocarbon layer is necessary to obtain a reliable population of Br- on the subphase surface. The correction of the attenuation reveals that the population of Br- is lower than that predicted on the basis of the electrostatic attraction, and that this deviation becomes larger as the monolayer is compressed. These findings can be explained by the submergence of Br- into the subphase when the amphiphiles are substantially in contact with each other upon the compression of the monolayer. This consideration is also supported by the pressure-molecular area (π-A) isotherms. Introduction Interfaces have received increasing attention not only because of scientific interest but also from technological perspectives. Physical properties change at the surface of a liquid or solid phase in more drastic ways than at interfaces between condensed phases. Molecular motions on the solid surface are so severely restricted that clear physical views are, in general, depicted by the application of appropriate physical probes. In contrast, solution surfaces are so flexible as to be subject to macroscopic deformations by a small external force. Also, such powerful physical probes such as soft X-ray, vacuum UV, and electron are often inapplicable to solution surfaces because measurements should usually be done in vacuum. Nevertheless, solution surfaces have attractive features that solid surfaces do not possess: (1) Molecular aggregates can easily be formed on the solution surface. (2) These aggregates can be transferred on the solid surface as Langmuir-Brodgett films. (3) A subphase may dissolve various substances, which are expected to modify the properties of a solution surface. (4) Probes such as an electrode can be immersed in a subphase to study and vary the properties of solution surfaces.1-6 Surfaces are often modified by a self-assembled monolayer (SAM). Thiols and silane compounds have been extensively used to protect the solid surface and to give a particular functionality. However, it is a disadvantage of the SAM on the solid surface that the precise control of the molecular density is difficult and, in addition, the monolayer cannot be completely removed once formed. In contrast, the modification of solution surfaces can be realized with a Gibbs or Langmuir monolayer, which provides more convenient ways than the SAM on the solid surface. We can control the molecular density in the monolayer on the solution surface by adjusting the concentration * To whom correspondence should be addressed. E-mail: hmakoto@ chem.titech.ac.jp. Telephone and Fax: +81-3-5734-2233.

of amphiphiles in a subphase for a Gibbs monolayer and by varying the surface molecular area for a Langmuir monolayer. In both cases, the surface tension (pressure) measurements provide useful thermodynamic information. In addition, the adjustments of subphase pH and the addition of appropriate ions are efficient ways to modify the nature of the monolayer on the solution surface. Because such options are not available for the SAM formed on the solid surface, these versatilities of fundamental and practical use are characteristic of liquid surfaces. As stated above, development of appropriate methods is important not only to characterize surface monolayers but also for their particular applications. A Brewster angle microscope,1,2 nonlinear optical measurements such as sum frequency generation and second harmonic generation,6-8 and other modern spectrometric methods9-11 have been exploited to elucidate the interfacial phenomena occurring at interfaces, including solution surfaces. A surface potential measurement with an ionizing electrode or a vibrating electrode is also a powerful means for probing changes in surface potential due to the variation of the polarity of the molecules involved in a monolayer.12-15 The surface potential change is caused not only by the dissociation of ionizable groups in the molecules involved in a surface monolayer but also by their molecular orientation.16 The surface potential changes by the compression of a monolayer should be carefully interpreted because various effects possibly influence the surface potential. X-ray is also an effective probe to solution surfaces and has been used in various ways. One of the methods most widely applied to the solution surface is X-ray reflectivity, which allows structural analyses of surface monolayers.17-22 This method evaluates the electron distribution in a monolayer in the direction perpendicular to the subphase surface and eventually gives the distribution of atoms (and in turn molecules) in the vicinity of the surface. We have employed total reflection X-ray absorption fine structure (TR-XAFS) for

10.1021/jp900912z CCC: $40.75  2009 American Chemical Society Published on Web 06/23/2009

TR-XAFS Studies of Double-Tailed Ammonium Ions

Figure 1. Schematic representation of a total reflection total conversion helium yield XAFS measurement apparatus.

studying surface monolayers and have revealed that this approach is efficient not only for the analyses of the local structures of ions attracted by surface monolayers but also for the estimation of their population near the solution surface.23-31 XAFS itself has very high elemental selectivity and provides structural information for noncrystalline samples.32-34 A total reflection scheme adds another advantage to XAFS, i.e., interfacial selectivity. The total reflection of X-rays introduced at a grazing angle of incidence induces an evanescent wave at a solution surface, of which the penetration depth is typically some nanometers.29,35-38 Therefore, we can discuss the ionic distribution just below the solution surface on the basis of the signal intensities of the TR-XAFS spectra. In the present paper, we characterize the surface monolayers of dialkyldimethylammonium bromide (diCnABr, n ) 12, 16, and 18, where n is the number of carbon atoms in a hydrocarbon tail) spread on a KBr subphase by a cooperative approach of TR-XAFS and surface pressure-area (π-A) isotherm measurements. Although a π-A isotherm measurement is commonly used to characterize surface monolayers from a macroscopic point of view, their interpretations are not straightforward in many cases. The cooperative use of TR-XAFS, which gives the information of the distribution of counterions (Br- in this particular case), promises a great advancement in understanding surface monolayers and reveals a novel aspect involved in the well-known types of Langmuir monolayers. Experimental Section TR-XAFS Measurements at Controlled Surface Areas (or Pressures). Figure 1 illustrates the TR-XAFS apparatus. Although the details of the instrument were described in our previous papers,25-28 various modifications have been done on the prototype apparatus. A sample solution was put in a PTFE trough (210 mm × 210 mm × 2 mm) glued on an aluminum heat sink, in which thermostatted water was circulated to keep the temperature at 298 ( 0.5 K. The area of the solution surface was controlled by moving two PTFE barriers (the maximum distance between the PTFE barriers was 190 mm and the minimum interval was 50 mm). The height of the solution surface, which was liable to be lowered by water evaporation, was monitored by a laser focus displacement system. The trough was set on a stepping motor-driven Z-stage, which maintained the right height of the surface to allow irradiation of the incident X-ray around the center of the solution surface. The entire system, including the trough, the stage, etc., was installed on a vibration-free device. The atmosphere above the solution was completely replaced by helium to allow for total conversion He+-yield detection. The incidence angle was set to 0.79 mrad, which was smaller than the critical incidence angle on an aqueous solution surface at a Br K-edge (1.68 mrad). The details

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Figure 2. π-A isotherms of (a) diC12ABr, (b) diC16ABr, and (c) diC18ABr surface monolayers onto a 10 mM KBr subphase at 298 K.

of baseline corrections and edge jump determinations were described in our previous papers.23-31 A 30 µL aliquot of a 1 mM diCnABr solution prepared in chloroform was spread on the surface of a subphase. XAFS measurements were started 20 min after the monolayer spreading. The initial surface area per molecule was A ) 180 Å2. The monolayer was compressed up to A ) 50 Å2 at the compression rate of about 22 Å2 min-1. The compression was ceased every 9 Å2, and then a XAFS spectrum was measured. A TR-XAFS measurement at a given surface area was completed within 1 min. The measurements were performed at the beamline BL7C of Photon Factory, High Energy Accelerator Research Organization in Tsukuba, Japan. Surface Pressures. The π-A isotherms were measured with a USI System Langmuir trough (100 mm × 300 mm × 5 mm) Model FSD-220 (Fukuoka, Japan) with a paper Wilhelmy plate at 298 K. The π-A isotherm measurements were started 20 min after spreading 25 µL of a 1.0 mM diCnABr solution on an aqueous KBr subphase. Unless otherwise stated, the concentration of KBr in the subphase was 10 mM. A diCnABr surface monolayer was compressed by the speed of 15 cm2 min-1 (A ) 9.96 Å2 min-1). Reagent. All of the diCnABr was purchased from TCI (Tokyo, Japan) and used as received. The purity was not less than 95%. KBr (Wako, Japan) of the analytical grade was dissolved in ultrapure water with 18.2 M Ω cm resistivity and used as a subphase. Results Surface Pressure. Figure 2 shows the π-A isotherms for diCnABr monolayers on the 10 mM KBr aqueous subphase at 298 K. The surface pressure starts to rise at A ≈ 130, 120, and 110 Å2 for the diC12ABr, diC16ABr, and diC18ABr monolayer, respectively, corresponding to the transfer from gaseous to liquid-expanded films. The diC12ABr and diC16ABr molecules form liquid-type films at the surface of a 10 mM KBr subphase in the entire range of A, whereas the transition from a liquidexpanded to a liquid-condensed film is seen for a diC18ABr monolayer at A ) 75 Å2. The surface pressure increases by compressing the films up to A ) 60 Å2. The monolayers of diC12ABr, diC16ABr, and diC18ABr collapse when they are compressed to A ≈ 55, 52, and 50 Å2, respectively. These molecular areas correspond to the minimum surface area occupied by the headgroup of a diCnABr molecule, which was calculated to be 50 Å2, according to the molecular model depicted in Figure 3. It was reported that diC12ABr spread on pure water is gradually dissolved in the subphase when the monolayer is compressed.39 However, the dissolution into a

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Harada and Okada

Figure 3. Molecular sizes of an ammonium headgroup (top left) and an entire diC16A+ molecule (bottom left).

Figure 4. Dependence of the edge jump heights of the TR-XAFS spectra for Br- on the surface molecular areas of diCnABr monolayers (A) spread on a 10 mM KBr subphase at 298 K. Solid curves show hyperbolas predicting that the edge jump heights are determined solely by the electrostatic attraction of Br- by the surface monolayers.

subphase can be suppressed with an electrolyte subphase. In actuality, a number of the π-A isotherms have been successfully measured for the diC12ABr monolayer with an electrolyte subphase. In the present case, the monolayer of diC12ABr collapses at a pressure of about 50 mN m-1 similar to that for diC16ABr and diC18ABr, suggesting that the former surface monolayer is stable in the entire range of A similar to the latter. Thus, the dissolution of the amphiphiles to the subphase is negligible. It was reported that the lift-off areas of the diCnABr monolayers correspond to the area occupied by the single molecule lying flat on the solution surface.39 This suggests that the lift-off area becomes larger with increasing the length of the alkyl chains in a diCnABr molecule. However, this consideration obviously does not hold for the results shown in Figure 2. Ha¨hner et al. have studied the structures of the diCnABr monolayer formed on mica by near edge XAFS spectroscopy and the water contact angle measurement.40,41 They evaluated the disorder of the alkyl chains of diCnABr molecules adsorbed onto mica in terms of gauche defects, which led to the high anisotropy of the C-H and C-C bonds. The numbers of the gauche defects of C-H and C-C bonds in a diC12A+ molecule on mica were determined to be 13.2 and 15.4, respectively, and, similarly, 7.4 and 11.2 for diC16A+ and 5.4 and 8.3 for diC18A+, respectively.41 The extent of the disorder of the hydrocarbon chains thus decreases with increasing alkyl chain length. This is consistent with the larger interactions between longer alkyl chains. The larger extent of the gauche defects well explains the larger cross-sectional molecular area of diC12A+ as long as the amphiphile is situated upright at the solution surface with two hydrocarbon chains extended toward the air. The hydrocarbon chains of the diC12A+ molecules with the larger gauche defects come in contact with each other at a relatively large A, at which the extensive contacts of the hydrocarbon chains of the diC18A+ molecules do not occur and the surface pressure also remains low. Thus, the variation of the lift-off area can reasonably be explained by the surface adsorption of the diCnABr molecule in an upright way. The anomaly is seen at A ) 75 Å2 only in the π-A isotherm for diC18ABr. This may also be related to the smaller extent of the contact between hydrocarbon chains for the diC18A+ molecules. For the other monolayers, the extensive contact between hydrocarbon chains may conceal the event found for the diC18ABr monolayer. This aspect will be discussed later. Total Reflection X-ray Spectrometric Measurements. The edge jump height of the XAFS spectrum is basically proportional to the population of a targeted element in a detection volume.

As stated above, the targeted element present just below the solution surface is detected by XR-XAFS. When the X-ray was totally reflected at the surface of an aqueous 10 mM KBr in the absence of the monolayer, the edge jump was negligibly small (the X-ray absorption spectra are shown in the Supporting Information), indicating that Br- in the subphase is not detectable. In contrast, when a monolayer of diCnABr is spread on the same subphase, measurable X-ray absorption signals were observed. This indicates that the population of Br- near the solution surface becomes higher due to its attraction by the positively charged monolayer. The detection volume of this system is estimated to be about 1 nL (5 mm in width, 5 cm in length, and 5 nm in depth). In the absence of the monolayer, 1 pmol Br- is present in 1 nL of 10 mM KBr. At A ) 100 Å2, if all of the amphiphiles comprising the monolayer stoichiometrically bind Br-, the population of Br- in this volume increases to about 400 pmol. The detection of X-ray absorption spectra thus becomes feasible by spreading the surface monolayer of diCnA+ on the solution surface. Figure 4 shows the relations between A and the edge jump height of the TR-XAFS spectra for the diCnABr surface monolayers spread on aqueous 10 mM KBr. The compression of the monolayer (i.e., decreasing A) basically results in an increase in the signal intensity for any type of amphiphiles studied in this work. This increase is basically explained by the higher molecular density of the compressed surface monolayer. However, this simple explanation is not acceptable in a rigorous sense. If the diCnABr surface monolayer stoichiometrically bound Br-, the edge jump heights should be proportional to A-1. However, as shown by the hyperbolas in Figure 4, the experimental values lie below any hyperbolic curve, and the deviation becomes larger with decreasing A, indicating that other factors should also be taken into account to explain these experimental results. Also of importance is that the edge jump height at a given A decreases with an increasing hydrocarbon chain length of a diCnABr molecule, i.e., in the order of diC12ABr > diC16ABr > diC18ABr. The hydrocarbon layer just above the water surface affects the intensity of the X-ray absorbed by Br-. If the hydrocarbon chains adopt the same conformation irrespective of the chain length, the thickness of the hydrocarbon layer should be proportional to the chain length; the hydrocarbon layer above the surface should increase in the order of diC18A+ > diC16A+ > diC12A+. The thicker hydrocarbon layer absorbs more X-rays before it reaches the surface. Thus, this strongly suggests that such damping effects should be taken into account to interpret quantitatively the results shown in Figure 4.

TR-XAFS Studies of Double-Tailed Ammonium Ions

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n ) 1 - δ - iβ

(1)

The real part (δ) and the imaginary part (β) are defined by the following equations

δ ) F

∑

()

(2)

β ) F

∑

()

(3)

NλreFi (Z + fi′) 2πMi i

(4)

NλreFi (f ′′) 2πMi i

(5)

δi )

Figure 5. Schematic illustration of possible paths of X-rays on a surface monolayer. (A) X-ray is partially reflected on the top of the surface diCnA+ molecules. (B) X-ray transmitted through the hydrocarbon layer is reflected on the surface of an aqueous subphase.

i

i

βi ) Discussions Correction of X-ray Reflection and Absorption by the Surface Monolayer. As noted in the above argument, the intensity of the X-ray reaching the surface of the subphase should be different from that of the X-ray irradiating the top of the hydrocarbon layer. In order to estimate the attenuation of the X-ray during transmission through the hydrocarbon layer, two major effects were taken into account: (1) the refection of the incident X-ray at the top of the hydrocarbon layer and (2) the absorption of the X-ray by this layer. These two effects, which reduce the intensity of the X-ray accessible to the surface of the subphase, should be represented as functions of the surface density of the diCnA+ molecules. Figure 5 schematically illustrates the absorption and reflection of the X-ray in the present system. The incident X-ray is partly reflected at the top of the monolayer (at the end of the hydrocarbon chains of the diCnABr molecules on the molecular basis). This reflection induces an evanescent wave, which penetrates into the hydrocarbon layer. The rest passes though the hydrocarbon layer and reaches the subphase surface. The evanescent wave and transmitted X-ray are attenuated when traveling through the hydrocarbon layer. The X-ray transmitted through the hydrocarbon layer is totally reflected on the subphase surface and another evanescent wave is induced. Thus, these two evanescent waves generated at different reflection planes are absorbed by Br- present just below the monolayer. The intensities of the evanescent waves reaching Br- depend on the electronic density of the hydrocarbon layer. When A is relatively large, the diCnABr molecules sparsely distribute over the subphase surface, and thus the incident X-ray mostly reaches the subphase surface. As A decreases, the electronic density of the hydrocarbon layer becomes higher, and the reflection of the X-ray at the top of the hydrocarbon layer becomes more significant. The effective cross-sectional area of a surfactant molecule acting as a reflector is defined as S, which should be smaller than A and must be constant unless the compression of the surface monolayer induces large conformational changes of hydrocarbon chains. Thus, I0 (S/A) represents the intensity of the X-ray reflected on the top of the surface monolayer at a given A, whereas I0 (A - S)/A denotes the intensity of the X-ray transmitted through the hydrocarbon layer. I0 is the total intensity of the incident X-ray. The complex refractive index, n, of an X-ray is given by35,43-48

δi W Fi i βi W Fi i

where N is Avogadro’s number, λ is the wavelength of the X-ray, re is the classical electronic radius (2.818 × 10-6 nm), Fi, Zi, and Mi are the density, atomic number, and atomic masses of the elements comprising a material, f′i and f′′i are anomalous scattering factors, and Wi is the relative mass ratio to the total mass of the material (ΣWi ) 1), respectively.44-47 These i anomalous scattering factors were estimated by the method of 49-52 Cromer and Liberman. The X-ray reflectivity, R(θ), is represented by eqs 6-8.35,42-48

R(θ) )

(θ - X)2 + Y2 (θ + X)2 + Y2

(6)

X)



√(θ - 2δ)2 + 4β2 + (θ - 2δ)

Y)



√(θ - 2δ)2 + 4β2 - (θ - 2δ)

2

2

(7)

(8)

where θ is the incidence angle. The intensity of the evanescent wave at the reflection plane, Ie1(θ,0), is given by

Ie1(θ, 0) ) I0

4θ2 S A (θ + X )2 + Y 2 m m

(9)

where the subscript, m, denotes the surface monolayer. Thus, the intensity of the evanescent wave at the distance of z from the reflection surface is expressed by35,43-48

(

Ie1(θ, z) ) Ie(θ, 0) exp -

4πYm2z λ

)

(10)

The X-ray transmitted through the hydrocarbon layer is also attenuated by the absorption therein before reaching the subphase surface. The intensity of the transmitted X-ray at the subphase surface can be written as

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It(θ, dm) ) I0

(

µmFmdm A-S exp A sin θ

Harada and Okada

)

(11)

where dm is the thickness of the hydrocarbon layer, Fm is the density of the monolayer (38.16/A g cm-3), and µm is the mass absorption coefficient. The thickness, dm, was estimated from a molecular model shown in Figure 3; dm ) 16.7, 21.5, and 23.9 Å for the di12CABr, diC16ABr, and diC18ABr monolayer, respectively. These values agree with those determined by X-ray crystallography. Thus, the intensity of the evanescent wave generated at the subphase surface, Ie2(θ,0), is given by

Ie2(θ, dm) ) It(θ, dm)

Figure 6. Changes in the damping rates of the X-ray with the surface molecular layers (A) of diCnABr molecules.

4θ2 (θ + Xsub)2 + Ysub2

(12)

where Xsub and Ysub are the corresponding parameters for the subphase. Although the incidence angle, θ, at the subphase surface may be different from that at the top of the hydrocarbon layer, the refraction of the X-ray across the air-monolayer interface can be ignored because of the very low density of this layer. The total intensity of the evanescent waves at the subphase surface is thus equal to [Ie1(θ,dm) + Ie2(θ,dm)]

{

(

)

4πYm2dm 4θ2 S + exp A (θ + X )2 + Y 2 λ m m

(13)

Figure 7. Corrections on the edge jump heights depicted in Figure 4 with the damping rates shown in Figure 6. Solid curves show hyperbolas predicting that the edge jump heights are determined solely by the electrostatic attraction of Br- by the surface monolayers (the same as shown in Figure 4).

This equation allows the calculation of the damping rates of the evanescent waves of the diCnABr surface monolayers as functions of A as shown in Figure 6. The X-ray is attenuated to a larger extent when passing through the thicker and/or more compressed hydrocarbon layer. Effects of the density of the hydrocarbon layer on the signal intensities can thus be removed by the corrections with these values. Figure 7 illustrates the edge jump heights corrected with the damping rates stated above. Differences in the edge jump heights between different diCnABr monolayers are substantially reduced at A g 70 Å2, where the surface films do not collapse. This implies that all of the diCnABr monolayers studied here basically have the identical ability in the condensation of Br- from the subphase. In other words, the accumulation of Br- on a monolayer mainly depends on the molecular density of the amphiphiles in the monolayer but not on the hydrocarbon chain lengths of the amphiphiles. The molecular density of the amphiphiles should be proportional to A-1 as mentioned above. The bromide ions are partly dissociated from the amphiphiles, and the dissociation degree will vary with the compression of the surface monolayer. However, as far as the dissociated Br- remains in the detection volume, the corrected TR-XAFS signal intensity should still change with A in a hyperbolic fashion. However, as shown in Figure 7, the corrected values do not fall on any hyperbolic curves, albeit the agreement is much better than that for uncorrected ones (Figure 4). These results strongly suggest that the population of the bromide ions in the detection volume becomes lower than that predicted on the basis of the density of the amphiphiles in the monolayer. It should be noted that the deviation from a hyperbola becomes marked as the monolayer is compressed.

Local Structures of Br- Attracted by diCnABr Surface Monolayers. The TR-XAFS spectra at the Br- K-edge obtained for the diC16ABr monolayers spread onto a 10 mM KBr aqueous subphase surface are shown in Figure 8 as a function of A. Unfortunately, a usual XAFS analysis routine does not give appropriate structural parameters because the XAFS spectra involve serious noises particularly at k > 4.5 Å-1. However, comparison of these spectra with authentic ones gives us an important insight into the local structure of Br- attracted by the cationic surface monolayer from the 10 mM KBr subphase. The spectrum of hydrated Br-, which was measured with a TRXAFS method with a 1 M KBr aqueous solution without a surface monolayer, is illustrated as broken curves in Figure 8 for visual comparison. The TR-XAFS spectra obtained with the diC16ABr monolayer agree with that for hydrated Br- at any A, clearly indicating that Br- is completely hydrated even though it is attracted by the positive charges of the monolayer. Our previous studies of the local structures of Br- in ion exchange resins and micelles have revealed that 2-3 water molecules are stripped from the first hydration shell of Br- in most cases.26,27,53-55 Also, it has been revealed that a part of the bromide ions are bound to the ammonium groups of N-dodecyl-N,N-dimethylammonio-butanesulfonate molecules in the Gibbs monolayer of this zwitterionic surfactant and that a few water molecules are removed from the first hydration shell of Br-.25,30,31 The spectra for partly dehydrated Br- have several common characteristics. As a typical one, the amplitudes of the XAFS spectral oscillation are reduced particularly in the range of k ) 2-3 Å-1 due to the replacement of several water molecules in the first coordination shell of Br- by an ammonium group. Such oscillation reduction is seen in the spectra depicted in Figure 8 as well, albeit the extent is very small. This implies

Ietotal(θ, dm) )

(

µmFmdm 4θ2 A-S exp 2 2 A (θ + X ) + Y sin θ sub sub

)}

I0

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Figure 8. XAFS spectra for Br- attracted by diC16ABr surface monolayers spread onto a 10 mM KBr aqueous subphase at various surface areas. Broken curves represent the spectrum for hydrated Brobtained with TR-XAFS on the surface of a 1 M KBr subphase without a surface monolayer.

that the direct ion pair formation between Br- and the ammonium group of a diC16ABr molecule occurs to a very small extent and most of the bromide ions attracted by the surface monolayer maintain the complete hydration shell. When Br- is more effectively shielded from water by the chains of ammonium groups, a water shortage circumstance is realized and significant dehydration occurs. In the present case, Br- is well-exposed to the subphase and keeps its first hydration shell structure because the shielding by the diCnA+ molecules from water is very weak. Thus, the first hydration shell of Bris almost preserved without undergoing serious perturbations. Submergence of Br- by the Compression of a Surface Monolayer. The π-A isotherm reflects the state of the condensation of amphiphilic molecules in a surface monolayer, whereas the corrected adsorption isotherm of Br- measured by TR-XAFS basically represents its population just below the subphase surface. The comprehensive interpretation of these data having different origins must provide more precise molecular pictures for the surface monolayer than the simple analyses of the individual results. The important findings of the present study with TR-XAFS are summarized as follows: (1) The hydrocarbon chain length (n) is not an important factor governing Brcollection, implying that the electrostatic attraction plays a primary role. (2) Corrected edge jump heights of TR-XAFS spectra are lower than those predicted from the population of Br- electrostatically attracted by the monolayer of diCnA+, and the deviation from the prediction becomes larger as the monolayer is compressed. (3) Br- attracted by the monolayer is almost completely hydrated irrespective of the extent of the monolayer compression. All of these findings can reasonably be explained by assuming the gradual evacuation of Br- into the subphase from the surface monolayer upon its compression. The edge jump height of a TR-XAFS spectrum depends on the location of Br- in the subphase. The signal should be maximized when Br- is present on the subphase surface and then becomes weaker as Br- sinks

Figure 9. Schematic illustrations for the submergence of Br- by the compression of the monolayer: (A) A > 110 Å2and (B) A < 75 Å2.

deeper into the subphase. Because the submergence of Brshould be caused by the compression of the surface monolayer, this phenomenon may appear in the π-A isotherms. The following discussions must most preferably hold for diC18ABr because of its compact hydrocarbon chains. Figure 8 indicates that Br- is almost completely hydrated in the vicinity of the surface monolayer. Our previous XAFS measurements and analyses have indicated that the distance between Br- and the oxygen atom in a coordinating water molecule is about 3.2 Å.53,56 In addition, the radius of a water molecule by spherical approximation is about 1.3 Å. The cross-section area of the first coordination sphere of Br- is thus estimated to be about 60 Å2. If Br- is sandwiched by two ammonium groups, the total area of a single ion pair of diC18ABr is estimated to be about 110 Å2, corresponding to the lift-off area of the diC18ABr monolayer (A ) 110 Å2). When the diC18ABr monolayer is compressed, the first contact may occur at the end of the hydrocarbon chains. However, the relatively compact structure of the C18 hydrocarbon chains results in substantial contact between the adjacent ammonium head groups binding hydrated Br-. The pressure on the ammonium groups facilitates the release of Br- from the ammonium head groups to reduce the molecular area. If dissociated Br- were still present in the detection volume of TR-XAFS, the signal intensity could be explained simply by the molecular density of the amphiphiles in the monolayer. The downward deviation of the corrected TR-XAFS signal intensity from the hyperbolic curves suggests the evacuation of Br- into the subphase. This situation is schematically illustrated in Figure 9. The Br- distribution over the deeper parts of the subphase causes a decrease in the TR-XAFS signal because the evanescent wave adsorbed by Br- becomes weaker in an exponential fashion.

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The same event should take place for the monolayer of diC16ABr and diC12ABr as well because the corrected adsorption isotherms of Br- follow almost the same trace as that for the diC18ABr monolayer. Figure 7 suggests that the release of Br- into the subphase starts at A ) 150 Å2, even for these monolayers. Thus, it should be noted that the submergence of Br- into the subphase occurs even though the pressure from the monolayer compression is not imposed directly on the ammonium groups. Conclusion The adsorption isotherms of Br- based on the corrected TRXAFS spectra strongly imply that it is sandwiched between diCnA+ molecules in the surface monolayers at relatively low surface pressures, whereas the ion associate is dissociated, and Br- is submerged into the subphase when the monolayer is compressed. It should be noted that Br- is almost completely hydrated even when it is attracted by the surface monolayer. A π-A isotherm measurement is commonly used for studies of Langmuir surface monolayers but provides relatively poor information on the event occurring in the subphase. In contrast, though TR-XAFS spectra do not reflect directly the interaction between amphiphilic molecules on the subphase surface, we can get the structural information as well as the concentration of a targeted ion in the vicinity of the subphase surface. Thus, these two methods are complementary, and combined use is much more powerful than individual utilizations. We believe that various features involved in Langmuir monolayers can be revealed by the present approach. Acknowledgment. This work has been performed under the approval of the Photon Factory Advisory Committee (Proposals 98G350, 2001G113, 2003G082, and 2005G040) and was supported by a Grant-in-Aid for Scientific Research (12740406 and 19550080). Supporting Information Available: Comparison of totalreflection X-ray absorption spectra obtained on 10 mM Br aqueous subphases. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Islam, Md. N.; Ren, Y.; Kato, T. Langmuir 2002, 18, 9422. (2) Lee, Y.; Lin, J.; Chang, C. J. Colloid Interface Sci. 2006, 296, 647. (3) Aoki, A.; Umehara, R.; Banba, K. Langmuir 2009, 25, 1169. (4) Chen, Q.; Liang, X.; Wang, S.; Xu, S.; Liu, H.; Hu, Y. J. Colloid Interface Sci. 2007, 314, 651. (5) Koga, T.; Kawazumi, H.; Nagamura, T.; Ogawa, T. Anal. Sci. 1992, 8, 259. (6) Yamada, T.; Yokoyama, S.; Kajikawa, K.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Kakimoto, M.; Imai, Y. Langmuir 1994, 10, 1160. (7) Liu, Y.; Hu, W.; Xu, Y.; Liu, S.; Zhu, D. J. Phys. Chem. B 2000, 104, 11859. (8) Chandra, M.; Sharath; Ogata, Y.; Kawamata, J.; Radhakrishnan, T. P. Langmuir 2003, 19, 10124. (9) Itoh, K.; Oguri, H. Langmuir 2006, 22, 9208. (10) Ogi, T.; Kinoshita, R.; Ito, S. J. Colloid Interface Sci. 2005, 286, 280. (11) Wang, Y.; Du, X.; Miao, W.; Liang, Y. J. Phys. Chem. B 2006, 110, 4914.

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