J. Phys. Chem. 1994, 98, 8424-8430
8424
Reflectance Infrared Spectroscopic Analysis of Monolayer Films at the Air-Water Interface Yuan Ren, Curtis W. Meuse, and Shaw L. Hsu' Polymer Science and Engineering Department and The Materials Research Laboratory, The University of Massachusetts, Amherst, Massachusetts 01 003
Howard D. Stidham Department of Chemistry, The University of Massachusetts, Amherst, Massachusetts 01 003 Received: September 15, 1993; In Final Form: June 12, 1994"
The transition moment direction and absorbance are reported for the CH2 stretching vibrations of H(CH2)leOH (I) a t 2918 and 2850 cm-l and for the CF2 stretching vibrations of F ( C F ~ ) ~ W ( C H ~ ) ~ -(11) O Ha t 1151 and 1207 cm-1 in densely packed monolayers at an air/water interface. Packing was monitored by measuring force-area relations for the two systems. Absorbance and transition moment direction parameters are given as estimated by fitting observed s-polarized reflectance spectra obtained as a function of incidence angle to calculated theoretical reflectance spectra by minimum least-squares difference. The parameters determined from s-polarized reflectance spectra were used to calculate p-polarized reflectance spectra. The results compared well with experimental observations of ppolarized reflectance spectra. The results suggest that, a t the packing densities used, the chain axis orientations are 12.9' and 0' for I and 11, respectively.
Introduction Langmuir-Blodgett films have been the focus of numerous studies.' These can generally be divided into two types, those that concentrate on the film at the air-solution interface2-' or on films which have been transferred to a supporting substrate.8-Il Most studies reported to date have focused on the second of these two areas, and there are relatively few studies of the interface. A number of techniques, including ellipsometry,I2 vibrational spectro~copy,~~ and reflectance diffraction,14J5 are capable of yielding structural information for films cast on metallic substrates. Recently, structrual information obtained from fluorescence microscopy and X-ray reflection measurements showed that information concerning the concentration profile or chain orientation can indeed be obtained for molecules at the air-water interface.16-17 In this study we concentrated on localized structural information obtained by vibrational spectroscopy. Force-area relationships have suggested the presence of varying structures (chain conformation, packing, and orientation) at the air-water interface depending on the concentration of the molecules present.' Detailed microscopic information such as changes in chain conformation is still unavailable. The extremely small number of molecules present at the interface requires sensitive measurement techniques. The use of X-ray reflection techniques for the study of these structures is limited by the low scattering intensity arising from monolayer film at the air-water interface. The use of fluorescence microscopy is restricted due to the requirement for a fluorescent tag on some of the molecules in the system. Recent developments in the use of external reflection infrared experiments, however, allow spectra of molecules at the airwater interface to be obtained at nearly any concentration.24 In contrast to the metallic substrate usually employed in reflection experiments, the complex boundary conditions that govern electromagnetic waves interacting with dielectric surfaces such as water make interpretation of spectra from the air-water interface more difficuk2 For such a dielectric surface, both sand p-polarized radiation within the film are present. Even though more spectroscopic information is available, the information obtained is more difficult to analyze. Infrared reflection studies of molecules at the air-water interface have to date concentrated
* To whom correspondence should be sent. @Abstractpublished in Aduunce ACS Abstracts, August 1, 1994. 0022-3654/94/2098-8424%04.50/0
on analysis of isolated high-frequency (-3000 cm-l) C-H streatching vibrations.2+8 Phase transformations that involve chain packing and/or segmental orientation changes have been analyzed. It is generally accepted that vibrations of fluoroalkanes and their derivatives that involve C-F units show intense absorption, and this characteristic aids reflectance infrared spectroscopic experiments done with solute molecules at the air-solution interface. Details of the observed vibrations are not as well assigned as for n-alkanes, particularly in the 700-1 300-cm-1 region. Recently, spectroscopic features sensitive to changes in chain conformation or packing have become better understood, however. 19-2l Several variations of computational methods for optical reflections from dielectric media have been reported.*J**22-23In this study, we present our derivation of the reflected spectrum expected for the monolayer at the air-water interface. This derivation differs slightly from earlier versions in that we seek to analyze the absorption coefficient and transition moment direction of thevibrations observed. There are still uncertainties in details of the experimental parameters to be used in order to obtain the maximum signal/noise ratio for analysis. We have calculated and measured the expected signal/noise ratios for different incidence angles. Rather than using reflected s- and p-polarized spectra at one incidence angle, we have used a much larger data base for a range of 10 incidence angles to calculate spectroscopic and associated structural parameters. These results are presented here. Experimental Section
H(CH2)18-OH (I) obtained from Aldrich was recrystallized five times from hexane. The fluorinated alcohols were obtained from PCR Inc. in Gainesville, FL, and were used as received. The water used was purified with a Millipore ultrapure water system. Hydrogenated Langmuir-Blodgett films were spread by using cyclohexane (Aldrich) as solvent. Fluorinated F(CFz)lr(CH2)2O H (11) Langmuir-Blodgett films were spread with n-hexane as solvent. Generally, solutions with specific concentrations were prepared to produce films with known area per molecule. The stock solution concentration for hydrogenated alcohol was 470.2 mg/L while that for fluorinated alcohol was 376.4 mg/L. Great care was taken in solution preparation. Microsyringes were 0 1994 American Chemical Society
Monolayer Films at the Air-Water Interface
:;:;;
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8425
a -0.01
P SIu0
9 2 I
I
1
1
-0.015
I
c I
-0.02
1,
I
I
Y I
-0.025 1300
b
-I
I
I 1
'
I
I
1
1200 1100 Wavenumbers
0 1
1000
I
-0.012 I
-0.016 ' 3100
'
I
3000
2900
2800
2700
-0.025 1300
I
I
1200
1100
1000
Wavenumbers Figure 1. Experimental reflection spectra at incident angle 30' of H(CH2)lu-OH monolayer at the air-water interface: (a) s polarization and (b) p polarization.
cleaned 10 times using the film solvent before spreading each film. All experiments were repeated several times in order to confirm reproducibility. Each film was allowed to stand for at least 5 min before measurements began. Infrared spectra were obtained with a Nicolet 60 SXB spectrometer equipped with a narrow band MCT detector (Infrared Associates) using an external reflection cell purchased from Specac Ltd. in Milford, CT. The angle of incidence is measured relative to the surface normal and may vary from 0 to -90'. The cell is equipped with a removable Teflon trough. Water evaporation from the solution presents a problem in intensity mismatch between the sample and background spectra. Because of different heights, thus overall reflectivity, associated with sample or background at different degree of evaporation, the resulting absorbance spectrum can be greatly distorted and the integrity of frequency and intensity data compromised. A wavelike base line is found in the spectrum if a mismatch occurs. The time interval between acquisition of sample and background interferograms was usually 10 min or less. The polarizer was placed directly above the water surface to secure the optimum polarizing effect. All spectra were collected by coadding 512 scans at 4-cm-' resolution. Representative spectra (s and p polarizations) at 30' incidence angle are shown for I in Figure 1 and for I1 in Figure 2. The surface pressure vs area curves were measured with a Langmuir-Blodgett monolayer trough (NIMA Type 61 1, Coventry, England). The temperature was maintained at 24 f 1 "C. Typical force area curves are shown in Figures 3 and 4. Method Section
The use of vibrational spectroscopy as an analytical tool is based on measurement and analysis of band frequency, relative
p'i L -10
'
I
0
10
20
30
40
Area/molecule (A*/molecule) Figure 3. Surface pressure versus area isotherm for H(CH2)ls-OH monolayer at the air-water interface.
intensity, and bandwidth. In order to derive structural information from reflection infrared experiments, care must be exercised in analysis of these spectroscopic features in order to separate structural features from optical anomalies. This section provides a few analytical expressions from which reflectivity may be computed for planar electromagnetic waves incident on optically anisotropic and absorbing thin films adsorbed on a dielectric isotropic substrate. The geometry on which the analysis is based is shown schematically in Figure 5. Several variations have been presented in the literature. The treatment first presented by Flournoy and Schaffers was modified slightly.24 The assumptions used in the derivation are the following: (1) A rectangular Cartesian coordinate system with the z axis perpendicular to the interface, they axis directed along the intersection of the plane
8426
Ren et al.
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 I
I
i J x Figure 6. Definition of dipole moment orientation direction in the
laboratory coordinate system. I
I
-10
I
20 30 40 50 Arealmolecule (A*/molecule) Figure 4. Surface pressure versus area isotherm for F(CFZ)IO-(CHZ)~OH monolayer at the air-water interface. 0
10
/
where
B is the phase shift defined as wA2d COS($,)
p=
(2)
C
d is the monolayer thickness, c is the speed of light, and w is the angular frequency of the incoming infrared beam. Other parameters are defined in Figure 5. The reflection coefficients at individual interfaces are h’ - h, i,’ = h, h,
+
i’, =
h 3
- h2
(3)
h3+ h,
~
and h’s defined for p and s polarizations are
A
n3
p:
Ai cos(
fii=-,
ai)
€.
i = 1,2,3
lY
Figure 5. Schematic configuration for the three-phase system and
associated coordinates.
s:
hi=
Ai
cos($,> pi
of incidence and the interface, and the x axis in the interface and perpendicular to the plane of incidence are assumed. (2) The ambient medium (air) and the substrate (water in this case) are assumed to be isotropic. The film at the interface is anisotropic and absorbing. (3) The Cartesian axes are assumed to coincide with principal directions of the dielectric tensor. The expressions derived should specify the reflected, refracted, and transmitted radiation in each medium as a function of incidence angle. Based on the calculated absorbance, the signal/noise ratio is calculated as a function of incidence angle. It is not entirely clear which angle (above, near, or well below the Brewster angle) should yield the highest signal-to-noise ratio. Since both s- and p-polarized waves are present in the anisotropic and absorbing adsorbed film, a quantitative analysis is sought for the absorption coefficient and transition moment direction of vibrational modes relative to the z axis. For many of the samples studied, the distribution of conformations of the alcohol is of importance, as the all-trans conformation may not exist in appreciable surface concentration in the interface. With determination of the absorption coefficient and transition moment direction, structural information in terms of both changing chain conformation and orientation may be derived. As mentioned earlier, several variations for derivation of Fresnel’s law for complex refractive indices exist in the literature. It is not the intent of this section to derive another variation but rather to establish the notations used in these calculations. The reflectivity R is simply the square of the reflection coefficient amplitude which, for the three-phase stratified layer system, was expressed in an earlier study as24 p=
+ i23 exp(2iB) 1 + i , , ~ ,exp(2iB) ,
(1)
, i=l,2,3
(4)
where p is the magnetic susceptibility of the material taken as 1.0. The 6’s are determined by Snell’s law. The anisotropic refractive indices of medium 2 are related to the principal dielectric constants by the well-knonw Fresnel’s law. Different solutions are found for different polarizations: p:
A, =
[iYY(
1-
-
+
s: A’ = t,,l/’ (5) Once refractive indices are determined, the reflection spectra for both s and p polarizations can be determined. The principal refraction indices and extinction coefficients are related by
iZZ = (nr, + ik,,)’
(6) The dipole moment direction (0,cp) is defined in Figure 6 . The principal values of the indices of refraction and absorption coefficients are related to the dipole-orientation angle B and the dipole strength K ( w ) by
k,, = K ( w )( sin’ 0 cos’
cp)
kyy = ~ ( w(sin2 ) 0 sin’
cp)
k,, = K ( w ) (cos’ 0)
(7)
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8427
Monolayer Films at the Air-Water Interface where K(o)= N ( w ) C W . Here, C i s the surface concentration of the chemical units at the interface, M is the dipole moment, and N ( o ) is a parameter which is only a function of frequency. Since there is no specific preferential orientation in the plane of the absorbing medium,2sv26the classical averages (cos2cp) = (sin2 cp) = I / z , and
k,, = kyy = k ,
k,, = K ( w )( COS' 0)
(9)
Consistent with the assumption made above nxx = nyv =
",
n z z = rill (10) The reflection absorbance for either s or p polarization is defined as
A = -log(R/R,) where Ro is the reflectivity of the substrate without the thin layer, which can be obtained by setting thickness d equal to 0. The absorbance at each incident angle is determined by n,, rill, K , and 0. The n , and rill values usually can be estimated by ellipsometry in the visible region. Generally, the bulk refractive index of many organic compounds lies in a range 1.4-1.6. Since very little is known about the optical coefficients in the infrared region, we have used the known values in the visible region as starting points. In principle, the other two parameters, K and 0, can be determined by using both s and p absorbances measured at each angle of incidence. We find only the s curve should be used to calculate the dipole strength K and dipole orientation 0. The p data contain very large error near the Brewster angle and should not be used to calculate K and 0. Instead, using only the absorbance found for one particular angle, we used the s absorbances measured for 10 different angles of incidence. For each K and 0 value used, it is possible to generate a set of absorbance values, A-1, at each experimental angle of incidence. The K and 0 are varied, and the correct values are found by minimizing the sum of the squares of the differences between calculated and measured absorbances. This fitting procedure minimizes the difference between theoretically generated and experimentally observed points on each curve. In all cases, the band maxima in the spectra were used for these calculations. Sample calculations for two model compounds are shown below.
Results and Discussion Determination of K(o)and 8 for Various Transition Moments. The parameters used in the calculation of reflection coefficients were A(air) = 1.0
+ i0.0383 (at 1151 cm-I) #(water) = 1.283 + i0.0361 (at 1207 cm-I) A(water) = 1.415 + i0.0163 (at 2918 cm-I) A(water) = 1.398 + i0.00941 (at 2850 cm-I) ii(water) = 1.269
TABLE 1: Standard Deviations Calculated for Various K and 6 Values (Accepted Values Displayed in Bold) K 9 U K 9 0 3.50 3.20 3.10 3.00 2.90
80 75 80 90 90
2.30 2.50 2.20 2.10
80 70 90 90
1151 cm-1 13.000 3.05 5.849 3.08 5.546 3.03 5.422 3.02 6.743 3.04 1207 cm-L 4.658 2.25 4.909 2.23 4.600 2.22 5.732 2.21
90 85 90 90 90
5.366 5.445 5.332 5.344 5.340
85 90 90 90
4.604 4.579 4.567 4.574
The refractive indices for water in the specific regions of interest were taken from the literat~re.~'The Brewster angle of water is 52.1' in the C-F stretching region and 54.8' in the C-H stretching, as determined by OB = tan-l(n(water)/n(air)). The all-trans chain and that with one gauche conformation were constructed by using a commercial modeling program, POLYGRAF, from Molecular Simulations Inc. A chain with one gauche conformation defect was calculated to have a chain length between 12.70 and 13.52 A, depending upon the position of the gauche conformer. As a first approximation, the thickness of the I1 film at the air-water interface was taken to be 14.57 A, the calculated length of the all-trans chain. The thickness of the I film is set to be 22.57 A, the calculated length of the all-trans conformation. The length of a I chain which has one gauche conformer was calculated to lie between 19.70 and 21.47 A. The principal refractive indices used are 1.3 and 1.6 for nl and nil of the I1 film, respectively, consistent with previous values reported for fluorinated compounds.28 The reflectivity calculated is not especially sensitive to the birefringence associated with our model fluorinated molecule, nor to the upper limit of rill. In fact, the calculation proved to be sensitive only to the lower limit of nl. The values for nl and rill of the I film were taken to be 1.4 and 1.6, as explained in the Method Section. As mentioned in the Method Section, it is possible tocalculate the reflection spectra for both s and p polarizations with these parameters if the absorption coefficient and transition moment direction relative to the chain can be obtained. To obtain these two parameters, the procedure explained in the Method Section was used. Typical calculated results are shown in Table 1. H(CH2),gOiY Analysis. The surface tension isotherm of the molecule displayed in Figure 3 clearly shows a stable monolayer formed at the air-water interface. The molecules began to pack with a cross-sectional area of 23.3 A*/molecule. The film collapses when the area is compressed to 19.9 A2/molecule. The pressure measured at collapse is 47.5 mN/m. The spectroscopic experiment was conducted at a concentration equivalent to 20 A2/ molecule, a well-packed state. Spectra obtained at various incident angles are presented in Figure 7. The infrared spectrum obtained exhibits a narrow intense band due to CHI asymmetric stretching a t 2918 cm-I and a band due to the symmetric stretching vibration at 2850 cm-I. The same types of vibration are found in highly ordered n-alkanes, typically near 2920 and 2850 cm-1, respectively. From the analysis of the angular-dependent reflectance infrared spectra for s polarization, the absorption coefficient and transition moment direction are found. Using the procedure described in the Method Section, K and 8 found for the 2918-cm-I band of I are 1.31 and 80°, respectively. Thus, k, = 0.63 and k, = 0.04. These values are to be compared with ones obtained for a similar molecule, H ( C H Z ) ~ ~ - Opresented H, in an earlier study.I8 For this vibration the reported k, was 0.32 while k, was assumed to be 0.0. The values found for the symmetric stretching vibration at 2850 cm-I are0.81 and 82O, respectively. The fittedcurveand experimental data for the 2918 and 2850-Cm-l bands for both polarizations are shown in Figure 8. The relative intensity of the asymmetric to
Ren et al.
8428 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
200.00 1
I
I
rx"^-"x""-
20'
h
P
45' 50"
- 1 ~
- 53"
{------
-100.00 I\
-200.00 -
'
-300.00 3050
i
55' -
v
65' 70'
4
I
I
I
1
2950
2850
2750
2650
-120
500
__._
-
0; h
-500
*
2
h
s
w
.200.00
-1000
4
4
1
10 20 30 40 50 60 70 80 90 Angle of incidence (')
1000
0.00 *
1
0
b
200.00
i
-
I
^.
1
,. ,. - 0 3 \ \
I
-L--_
,
-1500
-400.00
-2000 -2500
-600.00 3050
2850 2750 2650 Wavenumbers Figure 7. Reflectance infrared spectra measured for H(CH2)ls-OH monolayer at the air-water interface for various incident angles: (a) s polarization and (b) p polarization.
0
2950
symmetric stretching vibration is found to be 1.62, comparable to the value, 1.98, found for highly ordered n - a l k a n e ~ . Since ~~ these vibrations and the chain axis are mutually orthogonal, the orientation of the chain can be obtained using the following expression
cos2 echain-+ cos2 e2920+ cos2 e28s0= 1
i
c
o
'
'
10 20 30 40 50 60 70 80 90 Angle of incidence (")
-
/-
-15
-g
;I '
I
-45
i
(12)
In this case the chain is found to orient at an angle of 12.9' relative to the surface normal.
F(CF2)lcr(CH2)rOHAnalysis.The surface tension isotherm measured for this molecule is shown in Figure 4. The sharp rise indicative of molecular packing is found at -28 A2/molecule. The film collapses at 25.0 A2/molecule at a surface pressure of 40 mN/m. The reflectance experiments were conducted at a concentration equivalent to 27.5 A2/molecule. For this closely packed state, the absorption coefficient and relative transition moment direction associated with the fluorinated segment are also of interest. The infrared spectrum exhibits two intense broad bands in the 1 100-1300-~m-~ region. Another weaker band occurs at 1232 cm-1. Although less obvious, progression bands are found at 1112.7,1087.6,and 1064.5cm-l. Analysisofthev3progression band of perfluoroalkane has been extensively studied.20.21 Unlike CH2 stretching vibrations, a C-F stretch in the 1100-1 300-cm-l region is usually highly coupled to skeletal modes. Despite many studies, the stretching and bending vibrations of fluorinated molecules are not well understood although, in a recent study, the assignment of band features has been significantly ~1arified.l~ There are essentially two sets of bands in this region: the asymmetric and symmetric C F stretches of the CF2 groups. There is general agreement that the 115 1-1 152-cm-I band is assignable to the v3 branch of fluoroalkanes.~9-21~30-33The assignment for other bands is yet to be determined. It has also been well established that these vibrations are extremely sensitive to dipolar-
0
d
10 20 30 40 50 60 70 80 90 Angle of incidence (")
2000 1000
-2000
L
I
I
-3000
10 20 30 40 50 60 70 80 90 Angle of incidence (') Figure 8. Calculated and experimental s and p absorbance angular dependence curve of H(CH2)ls-OH monolayer at the air-water interface: (a) 2918-cm-1 CH2 asymmetric stretching (s polarization); (b) 2918-cm-1 CH2 asymmetric stretching (p polarization); (c) 2850-cm-I CH2 symmetric stretching(s polarization);(d) 2850-cm-1CH2 symmetric stretching (p polarization). dipolar intermolecular interactions.19 Intermolecular interaction is the principal cause of band broadening in this region. Figure 0
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8429 *O'
-100.00
a
1
0
1
1
-20
/
-40 I -200.00
-
w
-2-300.00 v
4 -400.00 -- I
W
-600.00
g
2 2
n
'
I
I
-60
-80 -100 -120
;:;1
1300 1250 1200 1150 1100 1050 1000 950 Wavenumbers
b 600.00
400.00 200.00 *-
0.00
z-200.00 a -400.00 -600.00
V
-1000
-800.00
0
-1000.00 1300 1250 1200 1150 1100 1050 1000 950 Wavenumbers Figure 9. Reflectance spectra obtained for F(CF2)li+CH+rOH monolayer measured at the air-water interface at various incident angles: (a) s polarization and (b) p polarization.
-20
10 20 30 40 50 60 70 80 90 Angle of incidence (')
I
J
/ !
9 presents spectra of various incident angles. For the CF2 units,
K and 0 obtained for the 1151-cm-1 band are 3.03 and 90°, respectively. The same two values for the 1207-cm-l band are 2.22 and 90°. The calculated reflection absorbance as a function of incident angle for both s and p polarizations is shown together with experimental data in Figure 10. As can be seen, calculated values agree well with experimental data. Because the S/N is particularly poor near the Brewster angle, fewer data points are obtained in that region. Comparison of absorption coefficients of CF2 to CH2 stretching vibrations is of interest. The ratio of 5.8 is much smaller than the corresponding values found for fluoroalkanes versus n-alkanes where the relative intensity is in the neighborhood of 200-500.19 The ratios measured by reflection data are consistent with transmission data for this sample. Several aspects should be considered. The oxygen near each end of the chain may significantly perturb the transition dipolesof the nearby units. The very short sequence of CH2 units makes them somewhat different from n-alkanes. The 1151-cm-1 band due to CF2 symmetric stretch is known to be perpendicularly polarized. The polarization of the 1207cm-1 band due to mixed CF2 asymmetric stretch and CF2 bend is less clear. An opposite conclusion was drawn in two earlier studies.33.34 It is physically impossible for these chains to lie on the water surface with a surface concentration of 27.5 A2/ molecule, a value very close to the cross-sectional area of 25 A2/molecule of a fully packed state.7~34In comparison, the crosssectional area of an all-trans hydrocarbon chain is 18.6 A2.35If both these vibrations are assumed orthogonal to the chain axis, the direction cosines must obey the relation COS*
echain+ COS* el,s1 + cos242sl= 1
(13)
-100
3
1 0
d
I
' ' 10 20 30 40 50 60 70 80 90 Angle of incidence (') '
1000
500
1
-
S
2
-500
1 1
-1000 r -1500
I
-2000 0
' ' ' ' I 10 20 30 40 50 60 70 80 90 Angle of incidence (')
Figure 10. Calculated and experimental p and s absorbance angular dependence curves of F(CFz)lr(CH2)AH monolayer at the air-water interface: (a) 1151-cm-I CF2 symmetric stretching, s polarization; (b) 1151-cm-i CF2 symmetric stretching, p polarization; (c) 1207 cm-I, s polarization; (d) 1207 cm-l, p polarization.
and the chain orientation angle is found to be 0' with respect to the surface normal. This value agrees especially well with the
~
Ren et al.
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
8430
15
0
3
0
1
0
" r.
c
3
C 0
Do
; ; 5 -
0
-
n
"
0 0 l
c I
0 0
P polanzation S polanzation
I
3 0 0
I
5 ' I
I 1
I
10 20 30 40 50 60 70 80 90 Angle of incidence (")
Figure 11. Observed signal-to-noise ratio of both p and s polarization reflection spectra (1 151 cm-I) of F(CF~)IO-(CH~)~-OHmonolayer measured a t the air-water interface at various incident angles.
result for perfluorinated carboxylic acids obtained by X-ray ~cattering.1~ Since the absorption of the anisotropic film a t the air-water interface is high, consideration of the change of signal-to-noise ratio as a function of the angle of incidence is extremely important. Dluhy first used an angle of 30° from the surface normal.2 Recently, others have used a value of 52' near the Brewster angle.23 Using a double modulation experiment, the angle of incidence was set at 76°.36 Gericke et al. presented a detailed description for analyzing the structure of an anisotropic film with particular emphasis on a thorough investigation near the Brewster angle region.18 The signal-to-noise ratios obtained as function of incidence angle from 20' (near normal incidence) to 70' (near grazing incidence) are shown in Figure 11. The p-polarized spectra below the Brewster angle do, as expected, exhibit negative absorbance. (Figures 7 and 9). These spectra do not exhibit the high signal/noise ratio found for most transmission data. In addition, a nonideal polarizer, beam divergence, uncertainty in setting incident angle, and low signal make p polarization experiments near the Brewster angle difficult. All originate from the very high sensitivity of the p absorbance to the angle of incidence. Experiments done near the Brewster angle promise significant information. From a practical viewpoint, the signal is simply too low. Instead of utilizing information from one incident angle, the spectra obtained for several angles should prove effective in reducing errors in calculation of the absorption coefficient and transition moment direction. In particular, the s wave angular dependence curve is used to obtain the dipole strength as well as dipole orientation, as explained in the Method Section.
Conclusions Reflectance infrared spectroscopy is an interesting new development for characterization of adsorbed molecules or polymers on dielectric substrates. There are several expressions which describe the reflectance or absorbance of electromagnetic waves from a system involving layered isotropic/anisotropic/ isotropic media. We have derived a different set of expressions which can be used to calculate the absorbance coefficient and transition moment direction for the several vibrations observed. These have been tested on well-defined systems such as I and I1 existing as monolayers at the air/water interface. From analysis of the reflectance infrared spectra for both s and p polarizations for the 2918-cm-I CHI asymmetric stretching and the 2850-cm-I symmetric stretching vibrations, the absorption coefficient and transition moment direction were calculated. Using this procedure, the values of K and t9 for the 2918-cm-' band of I were 1.31 and 80°. The values found for the symmetric stretching vibration at 2850 cm-l were 0.81 and 82O, respectively. These
values agree well with values obtained for the n-alkane systems. Even though fluoroalkanes are not as well understood, the C-F vibrations in the 1100-1300-cm-~ region were analyzed, and the absorbance and transition moment directions were calculated. For the CF2 units, the K and t9 obtained for the 1151-cm-I band are 3.03 and 90°, respectively. The corresponding values for the 1207-cm-' band are 2.22 and 90°. These parameters have shown that the chain orientation is normal to the air-water interface. The results agree well with theoretical modeling studies and previous X-ray diffraction analysis of films of perfluorinated acid^.'^^^^ The angle of incidence most favorable for obtaining data of high signal-to-noise ratio does not lie near the Brewster angle but is instead at higher or lower values with preference for angles near grazing incidence.
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