Langmuir 1995,11, 2745-2750
2745
A Transmission FTIR Spectroscopic Study on Mixed Langmuir-Blodgett Films of Cadmium Heptadecanoate- Chloro/Bromohexadecane L. M. Ilharco," A. R. Garcia, A. M. Fidalgo, R. Barros, A. F. Vale, and J. Lopes da Silva Centro de Quimica-Fisica Molecular, Complexo I, Instituto Superior Tkcnico, 1096 Lisboa Codex, Portugal
A. M. GonGalves da Silva Centro de Quimica Estrutural, Complexo I, Instituto Superior Tkcnico, 1096 Lisboa Codex, Portugal Received November 30, 1994. In Final Form: March 15, 1995@ Mixed Langmuir monolayers and multilayers of cadmium heptadecanoate-chlorohexadecane (CdHpClHx)and cadmium heptadecanoate-bromohexadecane (CdHp-BrHx), for molar fractions of alkyl halides up to 0.3,were transferred onto hydrophilicsubstrates (CaF2). Structural information was obtained from surface pressure-area (n-A)isotherms and from FTIR transmission spectra. Frequency shifts in the COO- antisymmetric stretching vibration (at 1535 cm-l for pure CdHp) were induced by the presence of the alkyl halides and have been qualitatively interpreted in terms of local interactions involving the polar groups at the surface. However, the alkyl halides do not modify the in-plane organization of the monolayer for any concentration; this was shown by the vibrational frequency of the CH2 antisymmetric stretch and also by a nonsplit band at 1467 cm-l, assigned to the CH2 bending mode of the hydrocarbon chains and very sensitive to lateral interactions. For mixed multilayers, this band is a doublet, indicating that the structure has changed to a two-parameter (orthorhombic)arrangement, with the same parameters as pure LB multilayers. The structural difference between the first layer and the subsequent ones was proved by studying the spectra of four layers deposited on a CaFz substrate previously hydrophobized with a layer of perdeuterated cadmium stearate (CdDSt);in this case, the band assigned to the CDz bending vibration (at 1090 cm-l) is clearly separated from the two split components of the corresponding CH2 mode. A particular case of a pure CdHp monolayer transferred onto the hydrophobized substrate was also studied: it was proved that it rearranges in an orthorhombic structure, while the first layer (CdDSt) keeps the isotropic packing.
Introduction Organized molecular films such as Langmuir-Blodgett (LB) films have been extensively investigated over the past decade because their properties have made them of interest to physicists, electronic engineers, chemists, and biologists.1,2 LB films are formed by the successive deposition of a series of monolayers of amphiphilic molecules spread at the aidwater i n t e r f a ~ e . ~ , ~ The properties of LB films are determined by the physical state of the precursor monolayers; therefore, it is of major importance to do a n extensive characterization of the floating monolayer, in order to optimize the conditions during the t r a n ~ f e r . Furthermore, ~,~ it is wellknown that metal ions introduced in the subphase adsorb at the interface, inducing a condensation of the monolayer.6-s The presence of minute amounts of di- or trivalent metal ions enhances the stability of the floating
* Author to whom correspondence should be addressed. Abstract published in Advance A C S Abstracts, June 1, 1995. (1) (1)Zasadzinski, J. A Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726. (2) Roberts, G . G. Contemp. Phys. 1984, 25, 109. (3) Kuhn, H. Thin Solid Films 1983, 99, 1. (4) Schoeler, U.;Tews, K. H.; Kuhn, H. J . Chem. Phys. 1974, 61, @
5009. (5) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interface; Wiley-Interscience: New York, 1966. (6)Aveyard, R.; Binks, B. P.; Cam, N.; Cross, A. W. Thin Solid Films 1990, 188, 361. (7) Peltonen, J.;Linden, M.; Fagerholm, H.; Gyorvary, E.; Eriksson, F. Thin Solid Films 1994, 242, 88. (8) Gordziel, S. A,;Flanagan, D. R;. Swarbrick, J.J. Colloid Interface Sci. 1982, 86, 178.
0743-7463/95/2411-2745$09.00/0
monolayer and the adhesion between such layers in the LB film. There is experimental indication that most divalent metal ions interact with carboxyl head groups in the proportion 1:2. This interaction is largely dependent on the properties of the metal ions and it has been shown that cadmium ions produce more stable and condensed monolayers than do magnesium, calcium, or barium ion~.~J~ Structural information on LB films of pure amphiphilic molecules has been obtained in recent years by a number of complementary techniques, namely X-ray and electron diffraction,11J2visible and vibrational s p e c t r o s ~ o p i e s , ~ ~ - ~ ~ and atomic force m i c r o s ~ o p y . l ~In - ~particular, ~ due to the high sensitivity of FTIR spectroscopy, it has been possible to compare between the in-plane organization of (9) M. Lindh, D. J.; Peltonen, J. P. K.; Rosenholm, J. B. Langmuir 1994, 10, 1592. (10) Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990, 6, 1093. (11) Tippmann-Krayer, P.; Kenn, R. M.; Mohwald, H. Thin Solid Films 1992,2101211, 577. (12) Riegler, J. E. J. Phys. Chem. 1989, 93, 6475. (13) Bonnerot, A,; Chollet, P. A.; Frisby, H.; Hoclet, M. Chem. Phys. 1985, 97, 365. (14) Umemura, J.; Kamata, T.; Kawai; T.; Takenaka, T. J. Phys. Chem. 1990,94,62. (15) Blaudez, D.; Buffeteau, T.; Desbat, B.; Escafre, N.; Turlet, J. M. Thin Solid Films 1994, 243, 559. (16) Katayana, N.; Fukui, M.; Ozaki, Y.; Kuramoto, N.; Araki, T.; Iriyana, K. Langmuir 1991, 7, 2827. (17) Schwartz. D. K.: Viswanathan. R.: Zasadzinski. J. A. J . Phvs. Chem. 1992, 96,'10444: (18)Radmacher, M.; Tillmann, R. W.; Fritz, M.; Gaub, H. E. Science 1992,257, 1900. (19) Florsheimer, M.; Steinfort, A. J; Gdnter, P. Thin Solid films 1994,244, 1078. I
0 1995 American Chemical Society
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2746 Langmuir, Vol. 11, No. 7, 1995
one-component LB monolayers and multilayers on different s u b ~ t r a t e s . ~ ~ J ~ , ~ ~ , ~ ~ The transfer and characterization of multicomponent films still represents a new challenge, not only because of the difficulties in achieving stable monolayers but specially because of the extremely interesting possibilities that they open on new molecular architectures with special properties.22-26 The challenge is even greater when nonamphiphilic molecules are incorporated in long chain fatty acid monolayer^.^^,^^ In a previous paper,27some of us have investigated the properties of a mixed heptadecanoic acid-bromohexadecane (HpA-BrHx) monolayer, in order to determine the range of compositions in which stable condensed monolayers can form. It has been proved that, a t 15 "C, condensed monolayers of mixtures can be obtained a t the airlwater interface up to 0.3 mol fraction of alkyl halide. For higher compositions, the guest molecules are squeezed out of the monolayer before the liquid-solid transition occurs. This study was later extended to the temperature effect, to the influence ofheavy metal ions in the subphase, and to the influence of the halogen atom in the guest molecule.28 The purpose of the present work is to report a n FTIR study of LB monolayers and multilayers of two binary systems: cadmium heptadecanoate-bromohexadecane (CdHp-BrHx) and cadmium heptadecanoate-chlorohexadecane (CdHp-ClHx), prepared with alkyl halide molar fractions in the range 0.0-0.3, and transferred onto CaFa substrates. The two components were selected with the same hexadecyl hydrocarbon normal chain linked to different polar groups, in order to study the effect of the interactions between polar groups a t the interface on the structure of LB films. Complementary, and for better clarification of the structural organization of those films, they were also transferred to a hydrophobized CaF2 substrate, obtained by deposition of a monolayer of perdeuterated cadmium stearate (CdDSt).
Experimental Section All the materials used in the present study were purchased in the purest quality commercially available and were used without further purification. Heptadecanoic acid (HpA, better than 99%) and bromohexadecane (BrHx, better than 99%) were supplied by Sigma; chlorohexadecane (ClHx, better than 97%) was from Fluka; cadmium sulfate (p.a. grade) and chloroform (better than 99.4%) were obtained from Rieder-de Haen; perdeuterated stearic acid (DStA, better than 99%)was synthesised and kindly supplied by Dr. A. Barraud's group a t CEA (Saclay). The water used in the subphase was purified with the Millipore Milli-Q system, to obtain resistances as high as 18 MQ cm. The M CdS04 solution a t pH 5.7. subphase consisted of a 6.7 x Calcium fluoride (CaF2) substrates were purchased from Sorem (France) in the form of 15 mm x 40 mm x 1mm single crystals. Surface pressure-area isotherm measurements and deposition experiments were carried out on a full automatic KSV 5000 Langmuir-Blodgett system (KSV Instruments, Helsinki). Sepa(20)Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama, K. Langmuir 1994, 10, 1898. (21) Yarwood, J. Colloids Surf: 1991, 52, 35. (22) Cordroch,W.; Mobius, D. Thin SolidFiZms 1992,2101211,135. (23) Peters, A,; Rogers, K. Thin Solid Films 1994,242, 258. (24)Terashita, S.; Ozaki, Y.; Yageta, H.; Kudo, K.; Iriyama, K. Langmuir 1994, 10, 1807. (25)Puggelli,M.; Gabrielli,G.; Caminati, G. Thin Solid Films 1994, 244, 1050. (26) Gonpalves da Silva, A. M.; Armitage, D. A,; Linford, R. G. J . Colloid Interface Sci. 1993, 156, 433. (27) Botelho do Rego, A. M.; Lopes da Silva,J.;Rei Vilar, M.; Gonpalves da Silva, A. M.; Schott, M. Thin Solid Films 1994, 243, 521. (28) Gonpalves da Silva, A. M.; Guerreiro, J. P.; Rodrigues, T. 0.; Rodrigues, N. G. Manuscript in preparation.
Table 1. Two-Dimensional Compressibilities (k)at n = 0 CdHp-C1Hx CdHp CdHp-BrHx
x2
- f ~ / l O - mN-' ~ m
0.1 0.2 0.3 0.0 0.1 0.2 0.3
1.97 2.31 2.40 2.58 3.61 4.02 4.65
rate stock solutions of the film materials in chloroform were M. Solutions of mixtures prepared with concentration 3 x (with 0.1,0.2, and 0.3 mol fractions of alkyl halides) were prepared by mixing known volumes of stock solutions. Precisely measured volumes of these solutions were spread on the water surface (using a Hamilton microliter seringe). After complete evaporation of the solvent, the floating layers on the subphase were compressed by two mobile barriers a t a speed of 8 mm min-I and transferred to CaFz substrates at a constant surface pressure, using the vertical dipping method. CaFz substrates were always cleaned by immersion in concentrated chromic-sulfuric acid mixture for a few minutes and subsequently rinsed thoroughly with Milli-Q water. Afterward, they were immersed in the ultrapure water for several hours and finally dried in a nitrogen flow just before using. The experimental parameters for the transfer, namely temperature and surface pressure, depend on the film stability requirements: pure monolayers were transferred at room temperature (x20 "C), while for mixed monolayers the transfer occurred a t 15 "C, because the range of miscibility decreases with increasing t e m p e r a t ~ r e .Pure ~ ~ monolayers of cadmium heptadecanoate (CdHp) and perdeuterated cadmium stearate (CdDSt) were transferred at 30 mN m-'; mixed monolayers were transferred a t 16-17 mN m,-l as a t higher surface pressures they are not stable or imiscibility occurs. CaF2 substrates were vertically dipped through the monolayer a t the rate of 4 mm min-l in the upstroke and of 8 mm min-' in the downstroke. Transmission IR spectra were obtained with a Mattson RS1 FTIR spectrometer, purged with dry and CO2 free air and equipped with a wide band MCT detector (wavenumber ranging from 4000 to 400 cm-l). In order to obtain a good signal to noise ratio, the spectra were the result of 1000 scans for monolayers and 500 scans for multilayers, a t resolution 4 cm.-l As backgrounds, recently cleaned CaFz substrates were used.
Results and Discussion Monolayers at the Air/WaterInterface. The surface pressure versus area per CdHp molecule ( n A )isotherms a t 15 "C, for several compositions of CdHp mixed with BrHx and ClHx, are presented in Figure 1. The addition of BrHx (Figure l a ) shifts the isotherm of CdHp to progressively larger areas, a t low surface pressures. The isotherm for the alkyl halide mole fraction ( x ~equal ) to 0.1 is very similar, in shape, to the one of pure CdHp. The isotherm for x2 = 0.3 shows a lower slope for surface pressures above 17 mN m.-l This slope decrease is probably due to BrHx molecules being squeezed out of the monolayer. Upon incorporation of ClHx (Figure lb), the shift to progressively larger areas for low surface pressure does not occur a t low composition. Actually, the isotherm for x2 = 0.1 exhibits an area per molecule of CdHp in the mixture lower than for the pure CdHp. Furthermore, the isotherms for CdHp-C1Hx mixtures are steeper (more resistant to compression) than those of pure CdHp and CdHp-BrHx mixtures, suggesting a closer packing arrangement of the molecules in a CdHp-C1Hx bidimensional monolayer. This is corroborated by the calculated isothermal two-dimensional compressibility, k = --(l/A) aA/an,of the several monolayers at = 0, presented in Table 1. Figure 2 shows the mean molecular area, a t the starting pressure (n= 0), for the two systems studied as a function of x2 up to 0.3. In the absence of Cd2+(open symbols),
Films of CdHp-C1Hx and CdHp-BrHx
Langmuir, Vol. 11, No. 7, 1995 2747
al
4
. 8 a
A
0.20
A
0
$ 0.18
0 0
J
0.0
I
1
I
I
0.1
0.2
0.3
x2 Figure 2. Mean molecular area vs alkyl halide mole fraction at the starting pressure, n = 0: HpA mixed with ClHx in the , in the presence of Cd2+(0);HpA mixed absence of Cd2+(01and with BrHx in the absence of Cd2-(A), and in the presence of Cd2+ (A).
Area per molecule of CdHp / nm2
bl
0.24 0.28 0.32 Area per molecule of CdHp / nrn' Figure 1. Surface pressure-area per CdHp molecule at 15 "C, of CdHp mixed with BrHx (a)and ClHx (b),in the presence of a 6.7 x M CdS04 solution, for several mole fraction values of alkyl halide: pure CdHp (A), 0.1 (B), 0.2 (C), and 0.3 0.20
(D). both systems exhibit approximate mean molecular areas very close to the one of pure HpA. The effect of the smaller size of chlorine is probably opposed by a higher dipole moment of the group CH2C1, with higher repulsion between parallel dipoles a t the aidwater interface. In the presence of Cd2+(solid symbols), the mean molecular areas are considerably lower than in the absence of ions and the two systems behave differently. When BrHx is the guest molecule, the mean molecular area decreases slightly with x2, whereas for the system with ClHx it presents a more pronounced decrease withxz in the range up to 0.1, which could be attributed either to loss of material in the monolayer o r to the formation of cooperative bidimensional solutions. However, the formation of stable and reproducible mixed monolayers seems to reinforce the second possibility. Assuming that, a t low surface pressure, there is no loss of material to the subphase, or to the multilayer, these results indicate that the Cd2+present in the subphase has a condensing effect on the mixed monolayers of HpA
with alkyl halides. The interaction of Cd2+with the groups COO- promotes a tighter packing within the monolayer, with a new and probably more favorable orientation of the polar groups a t the aidwater interface. The reason why the condensing effect is specially pronounced in the system CdHp-C1Hx is not completely understood, but we believe that the smaller size of chloride, associated with a higher local dipole moment of the group CHZC1, plays an important role.22 This argument seems plausible, as intermolecular forces are strongly dependent on the distance. Summarizing, we can say that the presence of chlorine atoms in the guest molecules must contribute to an enhancement of the interactions within the floating monolayer, and between the monolayer and the subphase. On the other hand, these results prove that the alkyl halides in the mixed monolayer orient, as expected, with the halogen atom to the water surface. A similar orientation was observed by Cordroch and Mobiusz2with mixed monolayers of octadecylmalonic acid as the host molecule and different alkyl halides as guests. Infrared Spectra of Monolayers on Hydrophilic CaFz Substrates. Figures 3 and 4 contain two significant regions of the transmission IR spectra obtained for mixed LB monolayers with different concentrations of ClHx and BrHx transferred to hydrophilic CaFz substrates. For comparison, the spectrum referring to the pure CdHp LB monolayer is also included (D). These two regions of the spectra provide complementary information on the structure of the LB films. The C-H stretching region (Figure 3) is dominated by two intense and sharp bands, assigned to chain methylene modes, namely the antisymmetric stretch (2918 cm-l) and the symmetric stretch (2850 cm-l); a third, smaller band is attributed to the antisymmetric stretching mode of the terminal methyl (2960 cm-l). Other authors have proved the correlation between the frequency of the v,,CH2 and the conformational order of aliphatic chains in monolaye r ~On. the ~ other ~ ~hand, ~ the ~ full width a t half-height (fwhh) of this band is a measure of the orientational ~ r d e r . In ~ all ~ ,spectra ~ ~ shown in Figure 2, the frequency of this mode is 2918 cm-l and the fwhh is 16 cm-I; such low values indicate that the chains possess an all-trans (29) Maoz, R.; Sagiv, J. J. Colloid Sci. 1984, 100, 65. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. SOC.1987,109, 3559. (31) Wood, K. A,; Snyder, R. G.; Strauss, H. L. J. Chem. Phys., 1989, 91, 5255.
2748 Langmuir, Vol. 11, No. 7, 1995
Ilharco et al. Table 2. Frequencies of the v,,COO- Mode in Pure CdHp and Mixed Monolayers CdHp x2
vlcm-l
i
'
3050
,
3000
,
'
'
"
2950
I
2900
'
I
2850
'
I
'
2800
I
2750
Wavenumber i cm-' Figure 3. Infrared transmission spectra (C-H stretching region) of LB monolayers on hydrophilic CaFp: (A) CdHpClHx ( ~ =2 0.31, (B) CdHp-C1Hx ( ~ =2 0.21, (C) CdHp-C1Hx ( ~ = O . l ) , (D) pure CdHp, (E) CdHp-BrHx (xp = 0.11, (F)CdHpBrHx (x2 = 0.2), (G) CdHp-BrHx (xp = 0.3).
,
io
1600
'
,
1550
'
,
1500
'
l
1450
'
p
I
1400
1350
Wavenumber i cm" Figure 4. Infrared transmission spectra (1650-1350 cm-') of LB monolayers on CaF2: (A) CdHp-C1Hx (xp = 0.31,(B)CdHpClHx (xp = 0.21, (C) CdHp-C1Hx (x2 = O.l), (D) pure CdHp, (E) CdHp-BrHx (xp = 0.11,(F)CdHp-BrHx ( x 2 = 0.21, (G) CdHpBrHx (xp = 0.3).
conformation and a close-packed arrangement.32 The fact that these bands are not affected by the presence of guest molecules in the film suggests that any changes in lateral interactions between neighbor aliphatic chains induced by the second component are negligible. This result is consistent with the above observation a t the airlwater interface: the halogen atom is oriented in the direction of the polar head groups of the host molecules and this orientation is maintained upon transference to the substrate. Taking this into account, the incorporation of the guest molecules up to mol fractions of 0.3 is expected to affect only the bands due to the carboxylate groups, adjacent to the surface. (32)Byrd, H.; Pike, J.K.; Talham, D. R. Thin Solid Films 1994,242, 100.
CdHp-C1Hx
CdHp-BrHx
0.1 0.2 0.3 0.1 0.2 0.3 0 1535 1534 1532 1530 1537 1539 1541
In Figure 4, the region between 1650 and 1350 cm-l of the same spectra is shown. The main features observed in spectrum D (pure CdHp LB film) are the bands a t 1535 and 1468 cm-l and the small, very broad band at 1408 cm,-l assigned respectively to the COO- antisymmetric stretch (v,,COO-), the scissors mode of the chain methylenes (dCHz), and the symmetric stretch of the COOgroup (v,COO-). Blaudez et obtained similar spectra for cadmium arachidate monolayers on several substrates. A common characteristic to all these spectra is the absence of a band due to the C=O stretching mode, expected a t about 1700 cm,-l which shows that the heptadecanoic acid is mostly deprotonated. The very different intensities of the two carboxylate stretches suggest that this group has a tilted orientation with respect to the substrate, thus accommodating nearly perpendicular alkyl chains. This is also the case for other long-chaincarboxylatemonolayers on different substrate^.^^,^^*^^ Three main observations arise when comparing the series of spectra in Figure 4: (1)The band due to the CH2 scissors mode (1468cm-l) is not shifted and remains sharp, regardless of the composition of the film. These two facts complement the information provided by the antisymmetric stretch of the same groups, indicating that the isotropic structure of the hydrocarbon chains in the mixed monolayer is maintained relative to that of pure CdHp. It has been proved, by a number of different techniques, that this structure is hexagonal or p s e u d ~ - h e x a g o n a l . ~ ~ J ~ J ~ (2) The v,,COO- band (1535 cm-l in spectrum D) is broad (about 39 cm-l), suggesting that there is a certain amount of disorder among the tilted COO- groups at the surface. As expected, this effect is more pronounced as the concentration of the guest alkyl halides increases: e.g., for x2 = 0.3 (spectra A and G respectively), fwhh = 44 cm.-' (3) The same v,,COO- band exhibits a small shift to lower wavenumbers for mixtures with ClHx and to higher wavenumbers for those with BrHx; in both cases, the shifts increase with the concentration of the respective alkyl halide. Table 2 summarizes the values of the frequencies for this mode. These results may be correlated with those at the air/ water interface and confirm that the structure of the monolayer is kept upon transference to the substrate: the incorporation of the bromo-substituted alkane causes an increase in the mean distances between the carboxylate groups a t the surface, thus reducing intermolecular interactions; as a consequence, the force constant associated with the COO- stretching vibration increases with the BrHx concentration. In the case of chloro-substituted alkanes, although the polarizability is lower, there is an additional and prevailing effect of the higher local dipole moment,22acting at shorter distances, in a more condensed monolayer; this effect leads to higher interactions between polar groups and therefore to negative shifts of the band, which depend on x2. A local structure model for the incorporation of ClHx in a close-packed monolayer of CdHp, as viewed from the substrate, is shown in Figure 5. Infrared Spectra of Multilayers. The IR spectra of mixed multilayers depicted in Figure 6 refer to three layers (33) Dote, J.L.; Mowery, R. L. J. Phys. Chem. 1988,92,1571,and references cited therin. (34)Ahn, D. J.;Franses, E. I. Thin Solid Films 1994,242, 971.
Films of CdHp-ClHx and CdHp-BrHx
Langmuir, Vol. 11, No. 7, 1995 2749
d
I Figure 5. Model for the hexagonal structure of the mixed
monolayer of CdHp-ClHx, viewed from the substrate: green, chlorine atom; red, oxygen atom in COO-.
Figure 7. Model for the orthorhombic structure of a bilayer of pure CdHp on hydrophilic CaF2. The two orthogonal planes
are perpendicular to the substrate. The green cadmium ion is not to scale.
1650
1600
1550
1500
1450
1400
1350
Wavenumber / cm-I Figure 6. Infrared transmission spectra of a monolayer of pure CdHp (A) and three layers: (B) pure CdHp, (C) CdHpBrHx (x2 = O.Z), (D) CdHp-ClHx (x2= 0.2).
of CdHp-BrHx (Figure 6C) and CdHp-ClHx(Figure 6D), for x2 = 0.2. Comparison is made with pure CdHp monoand multilayers (Figure 6A,B). The antisymmetric stretch of the carboxylate group (vas COO-) shifts to 1543 cm-l and the symmetric stretch (v,COO-) to 1421 cm-l for all samples, indicating that interactions between polar groups have decreased due to a new and stronger cause, which prevails over the halogen substitution: probably a reorganization which occurs upon the deposition of successive layers. More elucidation is the effect observed on the band assigned to the CH2 deformation mode, which is split in two, respectively at 1472 and 1464 cm-l. As the same effect is observed for pure CdHp multilayers (as shown in Figure 6B),the second component is not responsible for it and does not alter the structure parameters. This observation is compatiblewith a rearrangement of the chains from isotropic to a still close-packed but not so condensed two-parameter structure, probably orthorhombic, as proposed by Umemura et
al.I4 for cadmium stearate and by Blaudez et al.15 for cadmium arachidate multilayers. In both works, the authors estimate a perpendicular orientation of the hydrocarbon chains relative to the substrate. Submolecular details of such structure have recently been detected for the first time by atomic force spectroscopy, for arachidate LB films.lg A schematic model of a bilayer consistent with these observations is shown in Figure 7. With only this set of results, a question remained unanswered: how to distinguish between the organization of the first layer and that of the subsequent ones? Avoiding subtraction of spectra35and following Umemura’s suggestion,14we tried to find an answer to this question by transferring successive LB layers to a CaFz substrate which was previously hydrophobized by deposition of a first layer of perdeuterated cadmium stearate (CdDSt). Infrared Spectra of Mono- and Multilayers on Hydrophobized CaF2. A monolayer of pure CdHp on a CdDSt pre-covered substrate was obtained by removing the floating film at the aidwater interface before the substrate emersion. The correspondingspectrum is shown in Figure 8 and the band assignments are made in Table 3. It is interesting to note that the weak band due to the CD2 scissors mode, now at 1090 cm-l, is sharp, while the corresponding 6CHz of the second layer shows the split characteristic of a two-dimensional structure. This is conclusive evidence that the first layer keeps its isotropic organization, while the second one has a two-parameter (probably orthorhombic) structure. This can be under(35) Kimura, F.; Umemura, J.; Takenaka, T.Langmuir 1986,2,96.
Ilharco et al.
2750 Langmuir, Vol. 11, No. 7, 1995
1480
0.2% T 1
2800
1
~
~
2400
1120
1440
[
'
'
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1
2000
1
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Wavenumber / cm-'
1600
1550
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1500
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,
'
,
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Wavenumber / cm-'
Figure 8. Infrared transmission spectra for a LB monolayer of pure CdHp on a CaFz substrate hydrophobized by a layer of CdDSt. In set, the CH2 scissors doublet (a)and the CD2 scissors mode, at 1090 cm-l (b).
Figure 9. Transmission infrared spectra of four layers on CaF2 hydrophobized with CdDSt: (A) pure CdHp, (B) CdHp-BrHx, and (C) CdHp-ClHx, for 0.2 mol fraction of alkyl halides.
Table 3. Band Assignment for the Spectrum of a CdHp Layer Deposited on a CaFz Substrate Hydrophobized with a Layer of CdDSt
of the composition)suggests that the structure parameters are not altered by the presence of the non-amphiphilic molecules.
vlcm-1
assignment
2957 2920 2851 2216 2195 2156 2089 1541 1472 1466 1422 1090
-CH3 antisymmetric stretch (v,,CH3) -CH2- antisymmetric stretch (v,,CHx) -CH2- symmetric stretch (vsCH2) -CD3 antisymmetric stretch (v,,CD3) -CD2- antisymmetric stretch (v,,CD2) -CD3 symmetric stretch (v,CD3) -CD2- symmetric stretch (v,CDz) -COO- antisymmetric stretch (v,,COO-) -CHz- scissors deformation (dCH2) -COO- symmetric stretch (v,COO-) -CD2- scissors deformation (dCD2)
stood as follows: the interactions between the two layers being tail to tail, the organization of the second one is not determined by the first but rather by the need of two neighboring carboxylate groups to share the same Cd2' ion (for the sake of the electrical neutrality of the film). In other words, ifwe see the CdDSt-covered CaFz substrate as a n hydrophobic surface, the organization of a pure CdHp LB monolayer transferred to it is different from the one obtained on a hydrophilic substrate. This conclusion agrees with Umemura's results,35but not with those reported by Blaudez e t al.,I5probably due to the different hydrophobization process used by these authors (boiling the CaFz substrate in CHC13for 5 min). The shift observed on the frequency of the vas COO- (1541 cm-l) reinforces the idea of a less packed structure of the monolayer on a hydrophobic substrate. In Figure 9, the spectra of four mixed layers ( 3 ~ 2= 0.2) transferred to a CaFz substratehydrophobized in the same manner may be observed. They are compatible with the first isotropic layer of CdDSt (singlet at 1090 cm-l) and two bilayers of mixed CdHp-C1Hx and CdHp-BrHx in a n orthorhombic structure. The wavenumber of the doublet due to the CH2 (1471 and 1466 cm-l, regardless
Summary This study has proved that by only involving surface pressure-area isotherms and transmission FTIR spectra it is possible to obtain detailed information on the structure of mixed LB films. For the two systems CdHp-C1Hx and CdHp-BrHx (x2 up to 0.3), it has been shown that the structure of the mixed monolayers on the hydrophilic CaFz substrate is the same as for pure CdHp: isotropic close-packed (hexagonal, from comparison with the literature). Lateral interactions between the hexadecyl hydrocarbon chains are independent from the presence of the second component, whereas the interactions between polar groups a t the surface are slightly affected, in opposite ways, by bromo- or chloro-substituted alkanes (increased by influence of chlorine and decreased by influence of bromine). This effect was only qualitatively interpreted; a more accurate analysis would need surface potential measurements. It was also demonstrated that the structure of a multilayer is not altered by the guest molecules: the first layer is isotropic and the subsequent ones are arranged a s bilayers in a two-parameter structure (probably orthorhombic), with the same parameters as the pure CdHp multilayers. However, it was possible to conclude that in the case of only a two-layer deposit on a hydrophilic substrate the second one already arranges in a n orthorhombic structure, as revealed by the spectrum of a pure CdHp monolayer deposited on a CaFzsubstrate precovered with a CdDSt layer (hydrophobic substrate). Acknowledgment. This work was supported by Junta Nacional de InvestigaGgo Cientifica e Tecnoldgica (JN1CT)under Project CIENCIA PBIC/C/CEN/1068/92. LA940952G
