J. Phys. Chem. 1986, 90, 5830-5835
5830
Time-Resotved Total Internal Reflection Fluorescence Spectroscopy for Surface Photophysics Studies Hiroshi Masuhara,* Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan
Shigeo Tazuke, Research Laboratory for Resources Utilization, Tokyo Institute of Technology, Midori- ku. Yokohama 227, Japan
Naoto Tamai, and Iwao Yamazaki Institute for Molecular Science, Okazaki 444, Japan (Received: February 20, 1986)
Timeresolved total internal reflection fluorescence spectroscopywas proposed and its possibility was demonstrated. A sapphire plate was used as an internal reflection element, and a model system composed of thin and thick polystyrene films were prepared on it. Both films contained different dopants whose fluorescence spectra and rise as well as decay curves were closely investigated as a function of an incident angle. A difference between theoretical prediction and experimental results was ascribed to beam divergence, flatness of optics, and optical condition of the films. On the basis of these results, an analysis method was proposed and applied in order to examine fundamental problems characteristic of the present fluorescence spectroscopy. Effects of polarization of the excitation beam and of a refractive index change due to absorption were examined in detail. It was clearly shown that a good selection of experimental conditions makes it possible to study photophysics of the organic surface with thickness of 0.01 Mm. A potential and the advantages of the present spectroscopy were discussed.
Structure and dynamic behavior of solid surfaces is generally considered to be different from bulk characteristics, which is one of the most attractive topics in chemistry, physics, and technology. Powerful tools for surface investigations are various kinds of electron spectroscopy’ and attenuated total reflection infrared (ATR IR) absorption s p e c t r o ~ c o p y . These ~ ~ ~ have been applied not only to metals and semiconductors but also to organic solids and polymers to give detailed data on their structures. In addition, Iwamoto et al. have demonstrated that Raman spectral measurement under the total internal reflection (TIR) condition is another potential method for surface analysis! However, all these methods cannot give direct information on dynamic processes in the microsecond to picosecond time regions. Recently, the importance of electronic properties was emphasized in the studies on organic solids. Many efforts are being paid to molecular design and evaluation of conductive, magnetic, photoresponsive, and catalytic materials. Their electronic properties are dynamic in nature, and electronic processes in the surface region should be measured directly. However, this cannot be made possible with the spectroscopic methods mentioned above, and a new methodology has been strongly required. In the study on surface photochemistry of silica and semiconductor particles, a time-resolved diffuse-reflectance spectroscopy has proved to be f r ~ i t f u l . ~Electronic absorption spectra of adsorbed molecules in the excited and ionic states were measured, their decay kinetics were analyzed, and reaction as well as relaxation mechanisms were discussed. Another interesting method is time-resolved TIR fluorescence spectroscopy which we demonstrated for the first time.6 Under TIR conditions, a beam penetrates into a material with a smaller refractive index from a material with a larger one. This is called an evanescent wave and is used as an excitation beam for TIR fluorescence spectroscopy. In the case of s polarization an intensity of this wave is given by eq2g6S71 and 2 where z is the depth from E = Eo exp(-yz) y = (2anl/X)(sin2 O - (n2/n1)2)0”
(1)
(2)
the interface between both materials, Eo and E are intensities of *Guest professor of the Research Laboratory of Resources Utilization, Tokyo Institute of Technology (April 1985-March 1986).
0022-3654/86/2090-5830$01 SO10
the evanescent wave a t the interface and at the depth z, respectively, 9 is an incident angle of the beam, A is its wavelength, and n, and n2 are the refractive indices of the denser and rarer materials, respectively. This spectroscopy makes it possible to measure photophysical and photochemical processes of a surface area whose depth is determined by experimental parameters included in eq 1 and 2. In the present work, the optical set and sample selection were examined in detail in order to establish TIR fluorescence spectroscopy, and a practical method of analysis was developed. Factors determining the dynamic range of this spectroscopy and affecting the depth resolution were demonstrated and discussed. In addition, a potential of this spectroscopy and a comparison with other characterization methods are summarized.
Experimental Section Sapphire was selected as an internal reflection element based on the following reasons. It has a high value of refractive index (nl = 1.81 a t 313 nm) compared with organic solids. It is transparent up to the vacuum ultraviolet region which makes it possible to measure the ultraviolet absorption spectra of samples. No appreciable impurity emission was detected, and scattering particles were hardly contained in the sapphire. It is mechanically hard and stable against chemicals, so its handling was very easy. As the birefringence of this internal reflection element is small, 0 can be precisely defined and is almost independent of polarization of an excitation beam. The sapphire was purchased from Toshiba Ceramics Co. Ltd. and polished optically by Eikoh Seisakusho Co. Ltd. Its dimensions were 30 X 10 X 1 mm, and the longest (1) For example: Brewis, D. M. Surface Analysis and Pretreatment of Plastics and Metals; Applied Science: London, 1982. ( 2 ) Hanick, N. J. Internal Refection Spectroscopy; Wiley: New York, 1967. (3) Iwamoto, R.; Ohta, K. Appl. Spectrosc. 1984, 38, 359. (4) Iwamoto, R.;Ohta, K.; Miya, M.; Mima, S. Appl. Spectrosc. 1981, 35, 584. Iwamoto, R.; Miya, M.; Ohta, K.; Mima, S. J . Chem. Phys. 1981, 74, 4780. (5) Kcssler, R. W.; Wilkinson, F. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 309. Beck, G.;Thomas, J. K. Chem. Phys. Lett. 1983, 94, 553. Turro, N. J.; Zimmt, M. B.; Gould, I. R. J. Am. Chem. SOC.1985, 107, 5826. (6) Masuhara, H.; Mataga, N.; Tazuke, S.; Murao, T.; Yamazaki, I. Chem. Phys. Letr. 1983,100, 415. (7) Born, M.; Wolf, E.; Principles of Optics; Pergamon: Oxford, 1974; Chapter 1.
0 1986 American Chemical Society
Time-Resolved Reflection Fluorescence Spectroscopy
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5831
B-f i Im S-f ilm
si4
40
N
Sapphire
I
v
a X
-. PI I
Y
Fluorescence
Figure 1. Optical set for total internal reflection fluorescence spectroscopy. 8: incident angle of an exciting laser beam. S film: thin surface film. B film: thick bulk film.
dimension was along the c axis. The contact surface with the film was the (lOf0) plane. A model system of layered polymer films was prepared and set on the sapphire plate, whose optical set is schematically shown in Figure 1. Polystyrene dissolves high concentrations of dopants, and no appreciable impurity emission was detected even in a commercial one. A thick bulk film (B film) was prepared from a m-xylene solution of 10 wt% polystyrene by evaporating its solvent slowly in a few tens of hours. Its thickness was -30 pm. A thin film (S film) was formed by pouring a dilute toluene solution of polystyrene on the almost vertically inclined sapphire and evaporating the solvent very rapidly. Its thickness was determined by measuring the absorbance of the phenyl groups of polystyrene around 270 nm. The scatter of thickness was within 10%. The 0.1, 1, and 2.6 wt% solutions gave thin films with 0.01-, 0.09-, and 0.4-pm thickness, respectively. These S films were contacted perfectly with the sapphire, while the B film was pressed firmly to the S film in order to obtain an intimate contact. This sapphire-film combination was fixed on a goniometer (Tokyo Kodenshi Co. Ltd.) where 8 was changed arbitrary with precision of less than 0.1O. The refractive index of the polystyrene film (n2)is 1.61 at 313 nm, so that the critical angle (8,) given by sin Oc = n2/n1was calculated to be 68.15'. Possible caqdidates for fluorophores doped in the S and B films were chosen from the following conditions. They can be excited by a common excitation beam but should have a different fluorescence spectrum and different lifetime. Since an absolute number of fluorophores contained in the films is small, their fluorescence quantum yield should be high, especially for the dopant of the S film. Energy transfer between the two films is one of the factors making quantitative analysis difficult. In order to check a contribution of this process, we selected fluorophores so that a molecule with a higher fluorescing energy level hqs a longer fluorescence lifetime compared to a molecule with a lower energy level. When energy transfer occurs,an acceleration of the decay of the donor can be. measured. Examining all these factors, we doped p-bis(2-( 5-phenyleneoxazolyl)benzene) (POPOP) and N-ethylcarbazole in the S and B films, respectively. Their concentrations were calculated by assuming that thexoncentration ratios of fluorophore and polystyrene in m-xylene and toluene were maintained after evaporating the solvent and that the dopan@were homogeneously distributed. A synchronously pumped, cavity-dumped dye laser (Spectra Physics 375 and 344 S) which was operated with a mode-fqked Ar+ laser (Spectra Physics 171-18) was used. The repetition rate of the laser was 800 kHz. The excitation wavelength v a s 315 nm which was obtained by using a KDP crystal. Its beam divergence was 1.7' f 0.4'. A Nikon P250 monochromator was used, and the fluorescence was detected by a HTV R1294U microchannel plate photomultiplier. Time-resolved fluorescence spectra and rise as well as decay curves were measured with a time-correlated singlephoton counting method. This system gave a response function with 60-ps width (fwhm). Details were reported elsewhere.* Absorbance of both films a t the excitation wavelength was smaller than 0.1. Polarizers and filters were set in front of the monochromater. A quartz lens was used to collect the fluorescence efficiently. Excitation polarization was usually ~~
(8) Yamazaki, I.; Kume, H.; Tamai, N.;Tsuchiya, H.; Oba, K. Reu. Sci. Imtrum. 1985, 56, 1187.
h
uL N I Y
P X PI
20
2
68'
69
71'
7(r
e
72'
Figure 2. Calculated relation between B and the contribution ratio of B film over S film. The thickness of the S film is given in the figure. See eq 3-5.
adjusted to be perpendicular to the plane of incidence which includes the c axis of the sapphire and to be normal to the contact surface, using Babinet-Soleit plate. In this case, polarization of the evanescent wave is independent of
Theoretical Consideration Under TIR conditions, an evanescent laser beam penetrating from the sapphire into the polystyrene films excites both POPOP and N-ethylcarbazole. Since absorbance of both fluorophores is small, the fluorescence intensity is proportional to the excitation intensity of each layer, which can be estimated by using eq 1 and 2. Since an effective intensity of the evanescent wave for s polarization is proportional to cos2 8, d
Is = cosz 8 1 E2dz = (cos2
O)Eo21
d
exp(-2yz) dz =
(cos2 O)Eo2(1- exp(-2yd)) (3) where d is the thickness of the S film. The corresponding quantity in the B film with an infinite depth is given in eq 4. Therefore, the excitation intensity ratio of the B film to the S film is calculated by using eq 5, which is shown in Figure 2. ZB = cosz OJmE2 dz = (cos2 8 ) E o 2 X mexp(-2yz) dz = (cos2 O)Eo2exp(-2yd) (4)
= exp(-2yd)/(l - exp(-2yd)) (5) Just over 0, the value of rB/rs is very high, indicating that the zB/ZS
excitation beam penetrates deeply and the thickness of the B film is much larger than that of the S film. When 8 is increased, this value decreases sharply. In the case of the S film with 0.4-pm thickness, IB/zs becomes less than unity at 0 = 68.2O. ZB and Is of the 0.1-pm S film gave almost the same value at 0 = 69". On the other hand, the value of ZB/Zsfor the 0.01-pm S film is larger than unity even at 8, + 4 O . As the thickness of the S film becomes thinner, the dependence of le/& upon 0 is less.
Results and Discussion Fluorescence under the TIR Conditions. In order to show fluorescence decay curves of chromophores used and to demonstrate TIR phenomena, some examples are given in Figure 3. The decay of POPOP fluorescence in the thin S film obeyed a single-exponential decay. which gave 1.2 ns as its lifetime. A deviation from a linear relation was observed in the count region
Masuhara et al.
5832 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
-
-
-
.
J 400 500 nm Figure 4. Timeresolved fluorescence spectra of a bilayer system of the 0.01-pm S film doped with 1 X lo-* mol dm” POPOP and the thick B film doped with 1.3 X 1W2 mol dm” N-ethylcarbazole. 0 is 69.6’. Gated times are (A) 0.4-1.4 ns, (B) 10.4-12.4 ns, and (C) 24.4-44.4 ns.
400 500 nm Figure 5. Time-resolved fluorescence spectra of the same system as that of Figure 4 with 0 = 7 1 . 9 O . Gated times are the same as those in Figure 4.
the S and B films, respectively. The time constant of the slow component ( T ~ was ) identical with that of the N-ethylcarbazole lifetime, which was described above. This indicates that characteristic photoprocesses such as energy transfer from the latter to POPOP can be neglected. Preexponential factors Fs and FB correspond to contributions of both fluorophores at the monitoring wavelength and vary depending on absorbance, fluorescence, spectrum, yield and lifetime. Just above O,, 0 = Oc + 0.27, the contribution of the short component POPOP is slightly detected. The fluorescence intensity of N-ethylcarbazole decreased sharply upon a slight increse of 0. The relative intensity changed by 1 order of magnitude, corresponding to an increase of 0 by about 1’. All these curves were analyzed with eq 6, and the values of FBIFs were plotted against 8 in Figure I. A similar curve was observed at different wavelengths, indicating that a relation between FB/Fs and 8 can be used as a probe for surface analysis. Compared to the theoretical curves given in Figure 2, the following points were noticed. (1) A large increase of the FB/Fs value was observed in the angle region around 8, + 0.7’. This
Time-Resolved Reflection Fluorescence Spectroscopy
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5833 proportional to the thickness ratio of both films is considered to be a practically useful standard. From this viewpoint, we reconsider eq 4 and 5 . Since the thickness of the B film used was about 30 pm, an integration to infinity should be replaced by eq 7. Here 1is the thickness of the B film. Since the fluorescence
&' E' dz = (cos2 O)Eo2& exp(-2yz) dz I
ZB = cos2 0
= (cos' 8)Eo2(exp(-2yd) - exp(-2yl)) (7) intensity is proportional to the excitation one, fluorescence intensities of the S and B films are given by eq 8 and 9. Here a, fs = aZs = A ( l - exp(-2yd)) fB
fB/fS
9.76 nsldiv
Figure 6. Fluorescence decay curves of a bilayer system of the 0.09-gm S film doped with 0.1 mol dm-) POPOP and the thick B film doped with 1.4 X 1F2mol dm" N-ethylcarbazole. Observation wavelength was 385 nm. Incident angles are (a) 8, + 0.27', (b) 8, + 0.68', (c) 8, + 0.90',
+ 1.49',
and (e) OC + 1.89'.
4
0.5
1
e Figure 7. Relation between 8 and the ratio of fluorescence intensity of the B film over that of the S film. The system is the same as that in Figure 6. The monitoring wavelengths are 385 nm (0)and 420 nm (0). 6 8'
69'
70'
(9)
b, A , and B are proportionality constants. Therefore, the fluorescence ratio of both films is given as 10, where the constant C involves all the experimental parameters such as concentration
. -
(d) 8,
= ~ Z B= B(exp(-Zyd) - exp(-2yl))
(8)
71'
is a serious problem, since a delicate dependence of FB/Fs upon 6 should be obtained in the angle region just above 0,. When various experimental conditions are examined, it is considered that a beam divergence of the excitation pulse, optical quality of the polymer films, flatness of the contact face and 45' edge of the sapphire, contact condition of both films, and an error of angle setting of goniomer are the factors leading to this shift. ( 2 ) The FB/Fs values above 69O showed a small change upon 8, while calculated values still indicated a decrease with an increase of 6. This difference is ascribed to the fact that the FB/Fs value approaches the SIN value. If we examine the 6 dependence of the FBIFs value at a shorter wavelength region, the SIN value above 69' will be improved, while the FB/Fs values below 69" will be too large to be determined. This problem is closely related to the dynamic range of the present method. On the basis of the above results, we propose here how to analyze the 6 dependence of fluorescence decay curves. The FB/Fs values were plotted against 0 by normalizing them to the value obtained by excitation under 0 < 6,. The latter value which is
= C(exp(-Zyd) - exp(-W))/(l - exp(-2yd))
(10)
of both fluorophores, molar extinction coefficient at the excitation wavelength, fluorescence spectrum, lifetime, yield, and observation wavelength. In the case of 6 < O,, this equation is approximated to be proportional to l / d . The experimentally determined value FB/Fs can be correlated to be fB/fs. Now we consider the fluorescence dynamic range of the present layered model system. As mentioned before, the lower limit was determined by impurity fluorescence, while the maximum counts were set to lo4 or less, which was decided by considering the machine time. In order to separate the decay curve into two components, the maximum contribution of the dopant with slower decay should be suppressed to about one-half of that of the faster one. This condition is another factor decreasing the effective dynamic range. In consequence, a practical dynamic range of FB/Fs was found to be less than 2 orders of magnitude. In the following sections, we examine the effects of concentration of the fluorophores and polarization of the excitation beam upon the relation between FB/Fsand 0, using the analysis method proposed here. Effect of Excitation Polarization. All the experiments in the present work were performed by using the excitation beam with s polarization. This is because the refractive index for the ordinary wave is available only for this polarization. Furthermore, in the case of p polarization, the polarization of the evanescent wave depends on the incident angle which makes it complicated to analyze the data.5 However, it is experimentally important to examine the FB/Fs-O relation obtained with p polarization. Rotating the Babinet-Soleit plate, we investigated an effect of excitation polarization upon relations between 0 and FB/Fs. The model systems studied were the 0.4-pm S film with 2 X mol dm-3 POPOP and B film with 1.2 X mol dm-3 N-ethylmol dm-3 and B film carbazole, the 0.4-pm S film with 3 X mol dm-3, and the 0 . 0 9 - ~ mS film with 0.1 mol with 1.3 X dm-3 and B film with 1.4 X mol dm-3. Although the FB/Fs values obtained with p polarization were larger than those with s polarization, the relations obtained by the present analysis method were identical with each other within experimental error (0.13') Effect of Fluorophore Concentrkon. As the complex refractive index is a function of absorbance, the 0 dependence of the F,/Fs value will be affected by fluorophore concentration. We examined the following concentration combinations of POPOP of the 0.4-pm S film and N-ethylcarbazole of the B film: 10-2-10-2, 10-3-10-3, 10-1-10-2, and 10-3-10-2mol dm-3. Similar concentration effects were studied for the model film of the 0.09-pmS film and the thick B film. The 0 dependence of the FB/Fs value of the 10-2-10-2 mol dm-3 pair was the same as that of the 10-3-10-3 mol dm-3 pair. This indicates that an effect of the refractive index change due to absorption can be practically neglected under the present experimental condition.
5834
Masuhara et al.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986
9.76 nsldiv
Figure 9. Fluorescence decay curves of bilayer systems of (a) the 0.01pm S film doped with 0.1 mol dm-’ POPOP and the thick B film doped __. 0
6 6’
69‘
70‘
71’
8
mol dm-’ N-ethylcarbazoleand (b) the 0.09-pm S film with 1.4 X doped with 0.1 mol dm-’ POPOP and the thick B film doped with 1.4 X lob2mol dm-’ N-ethylcarbazole. The incident angle was 70’. Observation wavelength is 420 nm.
Figure 8. Concentration effect upon a relation between B and the ratio of fluorescence intensity of the B film over that of the S film. The model systems are (a) the 0.4-pm S film doped with 2 X lo-’ mol dm-’ POPOP mol dm” N-ethylcarbazole and the thick B film doped with 1.3 X and (b) the 0.4-pm S film doped with 3 X lo-’ mol dm-’ POPOP and the thick B film doped with 1.3 X lo-’ mol dm-’ N-ethylcarbazole. Observation wavelength is 420 nm.
On the other hand, the I9 dependence of FB/Fs is dependent upon the concentration ratio of both fluorophores. Since the absorbance of fluorophores is small at the excitation wavelength, the fluorescence intensity is proportional to their concentrations. The FBI& value is increased when the concentration of the dopant in the S film is decreased. However, according to the present analysis, this behavior is replaced by a shift of the FB/Fs-8curve to the large 13 region. One of the examples is shown in Figure 8. When the POPOP concentration of the S film is decreased by seven, the curve is shifted to the right by 0 . 2 5 O , which is larger than the experimental scatter. It is concluded that a concentration difference of 1 order of magnitude between the S and B films can be detected by analyzing the FB/Fs-e curves. Depth Resolution. A similar shift of the FB/Fs-o curve is expected by fixing fluorophore concentrations and changing the thickness of the S film. When this behavior is analyzed, the depth resolution of the present spectroscopy may be examined. We demonstrate how the fluorescence decay curve of the model systems is sensitive to the thickness of the S film. In Figure 9, the result obtained by fixing 6 to 70” is shown as an example. In the system with the 0.01-pm S film, the contribution of the slow component of N-ethylcarbazole was comparable to that of the fast component of POPOP. On the other hand, the former contribution was suppressed to a greater extent in the system with the 0.09-pm S film. A more systematic study was given by investigating relations between I9 and FB/Fs for three model systems with the 0.4-pm, 0.09-pm, and 0.01-pm S films. The results are shown in Figure 10 where the concentration ratios of both fluorophores were fixed. An angle region where the value of sharply decreases with an increase of 0 is shifted to the larger I9 region as the S film becomes thinner. In the case of the 0.4-pm S film a sudden change occurred below 69O, while it was observed around 69.5” for the 0.01-pm S film. This difference in I9 was reproducible and larger than an experimental error of 0.13”. It is safely said that the order of the thickness of the S film can be estimated under the fixed (9) Taniguchi, Y.; Mitsuya, M.; Tamai, N.; Yamazaki, I.; Masuhara, H. J . Colloid Interface Sci. 1985, 104, 596.
68’
69‘
70’
71’
e
Figure 10. Relations between B and the ratio of fluorescence intensity of the B film over that of the S film. Observation wavelength is 420 nm. The model systems are (a) the 0.4-pm S film doped with 2 X lo-’ mol dm” POPOP and the thick B film doped with 1.3 X lo-’ mol dm-’ N-ethylcarbazole, (b) the 0.09-pm S film doped with 2.3 X lo-’ mol dm” POPOP and the thick k film doped with 1.2 X mol dm-’ N-ethylcarbazole, and (c) the 0.01-pm S film doped with mol dm-’ POPOP and the thick B film doped with 1.3 X lo-’ mol dm-’ N-ethyl-
-
carbazole. concentration ratio of both fluorophores. since all these curves are normalized at the FBI& values arpund O,, the SIN value approaches an experimental scatter in the large 0 region. Namely, all the fluorescence decay curves observed in the region of I9 > Of + 3’ are mainly due to the S films. This is consistent with a theoretical prediction for the model systems with the 0.4-pm and 0.1-pm S films. As shown in Figure 2, the excitation intensity ratio of the B film over the S film for these systems is smaller than unity. On the other hand, the contribution of the B film is theoretically larger than that of the S film for the system with the 0.01-pm S film, while experimentally smaller.
J . Phys. Chem. 1986,90, 5835-5841 Namely, the contribution from the S film with thickness of 0.01 pm is larger in the present data compared to that by theoretical estimations. This is ascribed to the following two reasons. First is that the present time-resolved measurement makes it possible to obtain FB and Fs which are more sensitive to B compared to usual fluorescence spectra. Second is that the observation wavelength (420 nm) is just the wavelength of POPOP fluorescence from the S fiim. On the basis of these results, it is concluded that information of the surface with a thickness of 0.01 pm is available by selecting experimental conditions. Conclusion Using layered films, we have demonstrated a possibility of TIR fluorescence spectroscopy. Selection of samples and experimental conditions was described in detail, and the effects of polarization of an excitation beam and concentration of chromophores upon quantitative evaluation were studied by the analysis method proposed here. In the angle region larger than Be 1.5’, the surface with thickness of 0.1 pm is mainly excited, and its structure and dynamics can be investigated. If experimental conditions are carefully set, information on the surface with the 0.01-pm depth is available. We believe that the present TIR spectroscopy is very fruitful in investigating photophysical and photochemical events of the surface of organic solids. Another target system is various organic films prepared by vacuum deposition, the Langmuir-Blodgett
+
5835
method, ion-cluster beam, laser CVD, etc. Particularly layered films are an interesting e ~ a m p l e .Electronic ~ nature and dynamics characteristics of these systems will be revealed. Finally we should compare this spectroscopy with other methods used in surface characterization. The present TIR spectroscopy has the following advantages. (1) No vacuum condition is required, and no damage on the surface is induced. (2) Time resolution of this spectroscopy is just the same as that of the normal single-photon counting measurement. Now a picosecond laser pulse and fast-response detector such as the microchannel plate phototube are available, which gives a resolution of 60 ps. This makes it possible to investigate directly electronic processes and molecular motion on the surface. Selecting an appropriate gated time, we can emphasize the contribution of the surface compared to the bulk. (3) In organic materials, the excitation energy often migrates efficiently and is trapped in some sites from which fluorescence is emitted. Therefore, the present measurement may give a piece of information on the minor sites on the surface. Acknowledgment. The present work was partly defrayed by a Grant-in-Aid from the Japanese Ministry of Education, Science, and Culture to H.M., S.T., and I.Y. (59850146) and to H.M. (6021 1019) and by the Joint Studies Program (1983-1984) of the Institute for Molecular Science. Registry No. POPOP, 1806-34-4; sapphire, 13 17-82-4; polystyrene, 9003-53-6; N-ethylcarbazole, 86-28-2.
Compositions and Mtcroscoplc Structures of Mlcroemulslons in the Single-phase Domain J. Biais, J. F. Bodet, B. Clin,* P. Lalanne, and D. Roux Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence, Cedex, France (Received: February 20, 1986; In Final Form: May 29, 1986)
A first approach to the structural features of hexanol-dodecane-SDSater and pentanol4odecane-SDS-water microemulsions lying within the singlephase domain is made. The experimentalprocedure requires a preliminary determination of the continuous phase via vapor-phase gas chromatography. Then light scattering experiments were performed, and radii as well as second virial coefficients are extracted. Structural features and phase separation behavior are analyzed in light of these new experimental results.
1. Introduction The fact that microemulsion systems are structured in, respectively, organic, aqueous, and interfacial microdomains (see Figure l a ) allows one to understand their extraordinary ability to solubilize substantial quantities of water and oil.’ However, to understand their behavior as a function of composition parameters, temperature, and pressure, i.e, to develop the expression for the free energy allowing one to predict the phase diagram, it is necessary to get precise information about their microscopic behavior. In particular, a microemulsion is well-defined by the composition parameters, but questions remaining are (i) what are the chemical compositions of the microdomains participating in the structure and (ii) what is the structure and what will be its evolution when the composition parameters are varied? 2. Experimental Techniques
2.1. Experimental Determination of Microemulsion Microscopic Compositions via Vapor-Pressure Measurements. 2.1 . I . Pseudophase Hypothesis and Microemulsion Vapor Pressures. The idea that leads to the knowledge of microscopic composition lies in the pseudophase hypothesis.* It consists (i) of considering the relevant microdomains as if they had such dimensions that their composition equilibria obey classical thermodynamics, and *To whom correspondence should be addreased.
0022-3654/86/2090-5835$01 .50/0
(ii) of supposing that the interfacial tension (between microdomains) is low enough for the influence of interfacial curvature on chemical potentials to be neglected. To be consistent with this hypotheses, two disconnected domains of the same kind (oil, water, or interface) have exactly the same composition. If one is only concerned with microdomain composition, a microemulsion, whatever its structure, can be considered as a three-pseudophase system (oil pseudophase Or,membrane pseudophase M’, and water pseudophase W’), in equilibrium with a vapor phase (see Figure lb). Where as in the case under study the surfactant can be considered as entirely contained in the membrane pseudophase (nearly zero solubility in 0’ and W‘ pseudophases2), one can associate to any microemulsion system a corresponding ternary (alcohol, oil, and water) system exhibiting the same vapor pressures. This point clearly constitutes an approximation but has been3 and will (1) Micellisation, Solubilization and Microemulsion: Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2. Bellocq, A. M.; Biais, J.; Bothorel, P.; Clin, B.; Fourche, G., Lalanne, P.; Lemaire, B.; Lemanceau, B.; Roux, D. Adu. Colloid Int. Sci. 1984, 20(3), 167. (2) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J . Dispersion Sci. Technol. 1981, 2, 67. (3) Biais, J.; Odberg, L.; Stenius, P. J . Colloid Int. Sci. 1982,86(2), 350. Biais, J.; Bodet, J. F.;Clin, B.; Lalanne, P. Vapour Pressure Measurements on Microemulsions; Mittal, K. L., Bothorel, P., Eds.; Wiley: New York, in press.
0 1986 American Chemical Society
