Langmuir 1993,9, 2363-2369
2363
Influence of UV Irradiation on Unsaturated Fatty Acid Monolayers and Multilayer Films: X-ray Diffraction and Atomic Force Microscopy Study J. P. K. Peltonen: P. He,+and J. B. Rosenholm Department of Physical Chemistry, Abo Akademi University, Porthamgatan 3-5, SF-20500 Turku, Finland Received September 10,1992. I n Final Form: April 27,1993 The influence of UV radiation on the state of petroselaidic acid and elaidic acid Langmuir-Blodgett (LB) films and on petroselaidic acid Langmuir monolayers has been investigated using X-ray diffraction in the low angle region and atomic force microscopy. The interlayer spacing including the inclination angle of the molecules in the LB film,the topography of the f i i surface, and the effect of W radiation has been studied. Also the odd-even intensity oscillation of X-ray diffraction peaks of Y-type LB films is discussed. The change in the state of a petroselaidic acid monolayer generated by UV irradiation was monitored by a change in barrier speed under a constant surface pressure. The recorded changes are suggested to be due to a partial polymerization of the double bonds of the fatty acid chains. An apparent activation energy of reaction of petroselaidic acid monolayers was obtained from the maximum change in state at three temperatures and was found to be 192 kJ/mol.
Introduction The lack of stability of Langmuir-Blodgett (LB) films (poor long-term stability, low mechanical and thermal stability,and poor resistance toward solvent)has prevented a large-scale use of LB films in a number of technical applications. The rearrangement of the molecules in the f i circumvents on the other hand the use of LB films as useful scientific model systems. Polymerization offers an interesting method to improve the stability of these films toward mechanical, thermal, and environmental attack.'I2 Moreover, the Langmuir-Blodgett technique itself offers an important alternative method to study the polymerization process and to produce ultrathin polymer films. Due to the very regular arrangement of monomer molecules in an LB film or Langmuir monolayer, the polymerization mechanism is expected to differ from that observed in bulk material.3 Consequently, some polymerization reactions which cannot occur in the gas, solution, or fluid state are activated in an LB film or m~nolayer.~ Unsaturated fatty acids are of special interest since they possess a reactive double bond(@in the chains which may be usedto induce polymerization, e.g., by irradiation. Most investigations on the polymerization in thin films have however been focused on the reactivity of a double bond( 8 ) at the end position of the hydrocarbon chain.Only a few acids with the double bond in the middle of the chain have been tested for their suitability for polymer-
* To whom correspondence should be addressed.
+ On leave from the University of Scienceand Technology of China,
Hefei 230026, Anhui, China. (1)NishikaL,Y.; Kakimoto, M.; Morikawa, A.; Imai, Y. Thin Solid Films 1988, 160, 15. (2) Tippmann-Krayer, P.; Riegler, H.; Paudler, M.; Mhhwald, H.; Siegmund, H.-U.; Eickmana, J.; Scheunemann, U.; Licht, U.; Schrepp,
W. Adu. Mater. 1991,3,46. (3) Kuhn, H.; Mhbius, D.; Biicher, H. In Physical Methods of Chemistry; Webberger, A., Roesiter, B. W., EMS.;John Wiley and Sons: New Work, 1972; Vol. 1, Part 3B, p 577. (4) Tieke,B.In AduancesinPolymer Science:.SDrinnler-Verlan: - - Berlin, 1986; Vol. 71, p 81. (5) Cemel, A.; Fort, T., Jr.; Lando, J. B. J. Polym. Sci. 1972,10,2061. (6) Barraud. A.: Rosilio.. C.:. Ruaudel-Teixier. A. J. Colloid Interface Sci; 1977, 62, 509.. (7) Laschewsky,A.;Ringsdorf, H.; Schmidt,G. Thin Solid Films 1985, 134,153. (8)Fukuda, K.;Shibadci, Y.; Nakahara, H.; Endo, H. Thin Solid Films 1989, 179, 2474.
ization, namely, tram-9-octadecenoic acids and tram-13docosenoic acid.1° Since linoleic acid monolayers showed considerable reactivity under UV irradiation," we decided to continue our study on the reactivity of petroselaidic and elaidic acid LB films under UV irradiation. In this investigation the X-ray diffraction and atomic force microscope (AFM) have been used to monitor radiation-induced periodic structural as well as topographical changes in the LB films. The effect of irradiation on the spacing of molecular layers in the LB film and the inclination angle of the molecules relative to the substrate was recorded. AFM images show clear changes in surface topography for samples treated with UV light. For petroselaidic acid, the W irradiation was also carried out on a floating monolayer, and the apparent activation energy of reaction was calculated from the temperature dependence of the maximum reaction rate. The results should find a wider use when describing the sensitivity of unsaturated LB films toward light. However, as the end product of the reaction could not be exactly defined, it is somewhat early to talk about polymerization as a type of reaction.
Experimental Section Materials. Film substances, trans-6-octadecenoic (petroselaidic) acid and trans-9-octadecenoic (elaidic) acid of reagent grade, were obtained from Fluka and used without further purification. The substances were dissolved in n-hexane to form a solution with a concentration of 1 mg/mL. Until spreading, the solution was protected against light and stored in a refrigerator. Terbium chloride (TbCb)and cadmium chloride (CdCld were also supplied by Fluka and used as subphase salta. Monolayer Formation and LB Film Deposition. A commercially available computer-controlledKSV LB-5OOO Langmuir trough (KSV Instruments, Helsinki, Finland) with a Wilhelmy balance was used. Monolayers were obtained by spreading the acid solutions on a subphase of 0.1 mM TbCb or 0.1 mM CdC12. The Milli-Q fiitration system (MilliporeCorp.) was used to purify the water for the subphase having a resistance of 18 MO cm.The (9) Shih,K.; Rickert, S. E.; Lando, J. B. Paper presented at the 2nd InternationalConference on LB Films, Schenectady,NY,July 1-4, IS@. (10) Tan, J.; Rickert, S. E.; Lando, J. B. Paper presented at the 2nd Intemational Conferenceon LB Films, Schenectady,NY,July 1-4,1986. (11)Peltonen, J. P. K.; He, P.; Rosenholm, J. B. Thin Solid Films
1992,210/211,312.
0743-746319312409-2363$04.00/0 Q 1993 American Chemical Society
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2364 Langmuir, Vol. 9, No. 9, 1993
2
::
PA,Cd
Petroselaidic acid (PA):
PA, Tb
EA,Tb
v1
2 20.
1.03
I
$CHJ.COOH CH=CH H,C-(C/HJt,
EA,Cd
Elaidic acid (EA)
:
(CHJ,COOH
a
I
CH=CH
10
‘30
‘40
‘50
Mean Molecular Area (Az)
28 1.04.
LB films were deposited on 20 X 12 X 1mm3quartz slides. The solid substratewas dipped and liftedacross the floatingmonolayer at a constant velocity of 10 mm/min, except the first layer for which the velocity of 6 mm/min was used. This has been found to increase the quality (stability) of the subsequent layers with enhanced transfer ratios. The surfacepressuresat which the LB f i i were deposited were 25 mN/m for petroselaidicacid and 15 mN/m for elaidicacid. All the experiments were performed under a constant subphase temperature of 20 “C if not otherwise specified. Irradiation. The irradiation was carried out using a 100-W high-pressure mercury lamp placed 0.2 m above the sample for both the LB films and monolayers. The reaction kinetics of the monolayer was monitored as a change in the barrier speed while simultaneouslykeeping the surfacepressure constantat 15mN/m on the subphase of 0.1 mM CdClZ. When using the TbC4 subphase, the reaction kinetics could not be detected by this method because of the very small changes in the molecular area during irradiation. The Langmuir trough was protected against naturalW light with an absorbing yellow semitransparentplastic sheet. Low Angle X-ray Diffraction of the LB Films. The X-ray diffraction was recorded with a Philips APD1700 automated power X-ray diffractometersystem using a proportional detector, Ni fiiter, and automaticdivergenceslit (irradiatedsample length 12.5 nm) with a tube voltage of 50 kV and a tube current of 40 mA. The Cu Ka line (A = 0.1542 nm) was used. Atomic Force Microscope (AFM). A Nanoscope I1 (Digital Instruments, Inc., Santa Barbara, CA) AFM was used to image the substrate surface. A 100-pm cantilever tip with a spring constant of k = 0.38 N/m together with the scanner head ‘A” (1.O-pmscan range) was applied to scan the samples in the height mode (constantforce). A more comprehensive discussionon the AFM experimental details has been given elsewhere.12 Results and Discussion Pressure-Area Isotherms. The pressure-area isotherms of petroselaidic and elaidic acid monolayers on subphases of 0.1 mM TbC13 and 0.1 mM CdC12 are shown in Figure 1. There are large differences between the isotherms recorded on the CdCh and TbCl3 subphases, respectively. When using the CdClz subphase, the isotherm does not reach a region with infinite slope (elaidic acid) or reaches it only within a narrow surface pressure range. Instead, the compression isotherms of both acids on 0.1 mM TbCl3 were comparable to those of stearic acid and arachidic acid. They show a clear a range with low compressibility which may indicate the existence of crystalline or liquid crystalline phases. The corresponding molecular areas refer to a close-packed monolayer with vertically aligned molecules. The deposition from the TbC4 subphase was quite successful with intermediate transfer ratios of 0.95 f 0.04, while the deposition from (12) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. J.Am. Chem. SOC. 1992,114, 7637.
II
I
I
Figure 1. Pressurearea isotherms of petroselaidic acid (PA) and elaidic acid (EA) on a suphase of 0.1 mM TbCla and 0.1 mM CdClz.
I
II
l Ii
(b)
14 h
20
Figure 2. Diffraction profiles of (a) 15-layerpetroselaidic acid LB f i i and (b) 27-layer elaidic acid LB films on quartz after various W irradiation times. the CdClz subphase was muchmore complicated, especially in the case of elaidic acid. This is why we characterized the LB films deposited only from the TbC13 subphase. None of the floating monolayers were ideally stable. If the surface pressure was kept constant a t a predetermined value, an approximately constant barrier speed waa needed to hold the pressure. The influence of this phenomenom on the kinetic measurements is discussed later. X-ray Diffraction of the LB Films. Figure 2 shows the diffraction profiles of 15-layer petroselaidic acid LB fiis (Figure 2a) and 27-layer elaidic acid LB f i b s (Figure 2b) on a quartz slide before W irradiation. The appearance of 10 equally distant Bragg’s peaks in the range 28 = 1.5-20° (for petroselaidic acid LB films, the diffraction angle range is limited to 28 = 2-10°) indicates a strong periodicity of the LB films. As may be expected, the intensities of diffraction peaks decrease with increasing diffraction angle. However, it is interesting to note that the intensities of the odd-numbered diffraction peaks are higher than the neighboring evennumbered peaks for both the petroselaidic acid and the elaidic acid LB films. This corresponds to the so-called “odd-even intensity oscillation” in X-ray diffraction from LB films13J4and will be explained later. The diffraction peaks in Figure 2, exactly speaking the corresponding d ~ values, l can be used to calculate the periodicity d of the LB films as d = Id,, (1) where 1 = 1,2,3, n is the order number corresponding to the number of each diffraction peak. As measuring a plane diffraction, the order numbers k and h in dkhl are zero. The average spacing of the petroselaidic acid double layer was found to be d = 4.72 nm, and for the elaidic acid double layer it was d = 4.82 nm. Usually the period obtained from the d value of the first diffraction peak diverges from the others and will be omitted in the calculation of the average spacing. This is plausible since the first peak appears a t very low diffraction angles (28 = 2O), and thence considerable errors will result from too
...
(13)He, P.;Tim, Q.; Chen, X. New Polym. Mater. 1991,3, 19. (14)Matsuda,M.;Sugi,M.;Fukui,T.;Iixima,S.;Miyahma,M.;Otsubo, Y.J. Appl. Phys. 1977,48, 771.
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Urwaturated Fatty Acid Monolayers and Multilayer F i l m
strong a reflection and possible small displacements from the correct sample position. The layer spacing recorded for the LB films by X-ray diffraction corresponds to two molecular layers. The length of the fully extended petroselaidic and elaidic acid molecules may be calculated from both the bond lengths and the bond angles if their configuration is known. Petroselaidic acid and elaidic acid (Figure 1)are isomers with a trans double bond at different positions in the chain. The molecular lengths should then be equal, Le., 1 = 2.46 nm. Obviously twice this molecule length is larger than the value of spacings obtained here. Therefore, the molecules of petroselaidic acid and elaidic acid are supposed to be tilted from the vertical orientation on the quartz substrate. Comparing the experimentally obtained double layer spacings of 4.72 nm for the petroselaidic acid LB double layer and 4.82 nm for the elaidic acid LB double layer with the theoretical double layer spacing, 4.92 nm, simple calculation gives 16.8' and 12.1O as inclination angles of the molecules from the normal of the quartz substrate, respectively. The only factor that seems to induce the tilt differenceis the position of the double bond in the chain. It is assumed that the conformation along both chains is the same. The moleculesdeposited as LB filmsare frequentlytilted from the perpendicular orientation. Sometimes the angle of inclination is quite large. For instance the angle for a pentacosa-8,lO-diynoic acid LB film was found to be 36O. In this compressed state the film lended itself for polymerization of the diacetylene bonds.13 Odd-Even Intensity Oscillation of the X-ray Diffraction Profiles. Support for the fact that the LB films prepared of petroselaidic and elaidic acid monolayers indeed are of the Y type, i.e., they represent the tail-to-tail and head-to-head vertical packing structure without reorientation (turning up) of the molecules during the deposition, can be found by explaining the odd-even intensity oscillation of the X-ray diffraction profiles. Neglecting the differences in the contribution from the single bond and the double bond as well as the contribution of H atoms to total diffraction intensities, and postulating that the distances of interaction layers are all the same, the diffraction intensities I for the LB films can be expressed by the following equations:
I = lr;12 1+ COS 28 sin2B cos B (3) f(Cd,o,c)
= A exp(-aX2) + B exp(-bX2) + C
X = (sin @/A
(4) (5)
where Iis the diffractionintensity, 8 the Bragg's diffraction angle,001the diffractionindex, and .z a factor havingvalues of l/n, 2/n, ...,nln. Here, n = 1...Nand N is the number of atoms in a unit c e l l 4 The constants A , a, B, b, and C tabulated in the literature16 are used to calculate the scattering factor f for each atom. As the LB films are deposited from a T b subphase, the Tb metal atom layers constitute, of course, the primary scattering layers in the X-ray diffraction measurements. However, the electron density along the long hydrocarbon chains is not homogeneous; the electron density is higher (16)Lee, J. D.Acta Crystallogr. 1969, A23, 712.
E 5
E
\
-___EXPERIMENTAL -CALCULATED
d
DIFFRACTION ANGLE ORDER
Figure 3. Odd-even oscillation intensities of X-ray diffraction for the studied LB fibs.
at the end of the chain than in the middle of the chain.lS The juxtaposition of two hydrophobic ends of the chains in the Y-type films produces an electron-deficient layer which w i l l influence the intensity of the diffraction peaks. Adjusting the thickness of the electron-deficient layers, the intensities of the diffraction peaks can be calculated from the formula above. The best agreement between the calculateddiffraction intensitiesand experimental data (Figure 3) could be obtained when the electron-deficient layer was assumed to be three carbon layers thick. In this way the odd-even intensity oscillation of X-ray diffraction from LB films could be accounted for. There is also an electron inductive effect active from both ends of the molecule to take into consideration. Moreover a double bond influences the properties of the chain as dividing it into two shorter chains." Both effects contribute to the X-ray diffractogram, but have not been taken into account in the calculations. As seen in the next section (alsoFigue 21, the odd-even intensity oscillation phenomenon remains after the W irradiation. This is explained by the lateral reaction taking place only between the neighboring molecules affecting the electron density distribution in the direction perpendicular to the LB film plane to a minor extent. UV Irradiation of the LB Films. The X-ray diffraction profiles of LB films UV-irradiated for various timesarealsoshowninFigure2. The Bragg'speaksremain which suggests that the regular layer structure in the LB multilayer film is maintained after UV irradiation. Indication for the polymerization as a result of UV irradiation comes from the shift of the peaks toward larger angles. The lateral coupling of neighboring molecules in the LB film forces the molecules to reorient themselves, giving rise to shorter spacings of the LB films. This indicates an increased tilt of the monolayer molecules. From this point of view, it is possible to monitor the polymerizationprocess in LB films from the change of diffraction angles or spacings. However, it must be simultaneously assumed that no change in chain conformation takes place. Figure 4 shows the changes in the spacings of petroselaidic and elaidic acid LB films as a function of UV irradiation time. A constant spacing implies that the reaction has ended. Thus, it may be seen in Figure 4 that the reaction of both petroselaidic acid and elaidic acid LB films is almost finished after 12-14 h of UV irradiation. Whether or not (16)Pomerantz, M.;Segmuller, A. Thin Solid Alma 1980,68,33. (17)Lippert, J. L.;Peticolas, W. L. Biochim. Biophys. Acta 1972,282, 8.
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2366 Langmuir, Vol. 9, No. 9, 1993
1
. a
4.6 1
0
,
,
,
,
1
5
1
1
1
1
I
I
I
I
,
10 Time (hour)
Figure 4. Dependence of double layer spacing in the LB film on the UV irradiation time: (a) petroselaidic acid; (b) elaidic
acid. this means that the reaction has been completed, Le., the conversion is 100% ,is difficult to say. At least in the case of petroselaidic acid, the change in spacing is moreover marginal and some other parameter should be used to estimate the degree of conversion. However, the different changes in spacing for the two acids are in agreement with the isotherms where the compressibility of the petroselaidic acid monolayer is much smaller (the T-A curve with almost infinite slope) than that of the elaidic acid at the surface pressures under study. The inclination angles of petroselaidic acid and elaidic acid molecules on the quartz substrate changed during the irradiation. The change of layer spacings was more rapid for the elaidic acid LB film than petroselaidic acid. This shows that the double bond position also affects the reactivity characteristics in addition to the obtained differences in the purely monomeric state. Both petroselaidic acid and elaidic acid LB films have almost the same final double layer spacing of d = 4.71 nm and the same inclination angles of 17.2' after UV irradiation of 12 h. This may indicate that, despite the differences in forming a floating monolayer as well as a deposited multilayer, the reacted multilayers for the acids seem to result in a rather similar final structure as far as interlayer orientation is concerned. Since the experiments were carried out in air, the long UV irradiation time needed should induce some oxidation and decomposition of the LB film of the unsaturated fatty acids. Fortunately, these reactions of oxidation and decomposition affect only the intensities of the diffraction peaks but not their positions. Atomic Force Microscopy Studies of the Surface Structure. Figure 5 shows AFM images of a sample consisting of 15 layers of petroselaidic acid deposited at a surface pressure of 25 mN/m from the TbClB subphase. The surface plot in (a)represents the unirradiated sample, and (b)-(d) the same sample after a UV irradiation of 10 h. The characteristic two-dimensional fast Fourier transform (FFT)diffractogram is inserted in each image. The FFT spectrum of (a) with several bright spots confirms the ordered structure seen in the image and enables the calculation of lattice parameters. It is obvious that the main rows with interrow separation of 1.36 f 0.04 nm and a fine structure of 0.67 f 0.04 nm spacing dominate the surface. The spacing along the rows is 0.45 f 0.06 nm. The structure is not as sharp as what we have found in earlier studies.12 However, those measuremenki were done for films deposited at 20 mN/m instead of the constant surface pressure of 25 mN/m used here. This may be an indication of some reorganization process taking place under higher compression. No single explanation for the effect of UV irradiation on the topography and molecular organization can be given,
and therefore, three resulting packing features locally different in kind are shown (Figure 5b-d). Compared with Figure 5a, parts b and c show the trend of changed packing geometry after UV irradiation. The images have similar features, but the reaction seems to have proceeded with varying speed and strength. In Figure 5b the main rows have become even more dominating with a somewhat enlarged height profile. Some structure can be found across the rows, but this order appears very weak in the FFT spectrum inserted in the image. In Figure 5c the reaction seems to have gone further and also shows a more direction-independent result in the sample plane. Also the FFT diffractogram consists of several bright spots however, being clearly more diffuse than those in Figure 5a. This means that the intermolecular distances have become more heterogeneous, and it is difficult to determine any exact lattice configuration. However, the found FFT maxima still show that the order obtained before UV irradiation has not totally vanished. Compared with Figure 5a, the lattice angle between the two main axes has, according to the FFT diffractogram, changed from perpendicular to 70° f 5 O , indicating a change in packing geometry. Figure 5d is shown in order to demonstrate a "bad" result of the UV-irradiated sample. It is difficult to visualize any ordered structure, and the FFT spectrum looks even more diffuse than in Figure 5c. Figure 6 shows the surface characteristics of 13 layers of elaidic acid before (a) and after (b, c) UV irradiation. The constant deposition pressure from the TbC13subphase was 15 mN/m. The only difference between the acids used in Figures 5 and 6 is the position of the double bond in the hydrocarbon chain. Still the differences in the packing features are obvious. Typically, the spacings between the obtained tops for elaidic acid refer to the doubled value of the expected interchain distance. Also, instead of only one main direction as in Figure 5a, two main axes with a lattice angle of 126O f 3O can be determined. After a UV irradiation of 10 h, the changes in structure are quite dramatic (Figure 6b). In the inserted FFT spectrum, actually only one direction can be exactly determined, while the rest of the spots are rather diffuse. The image shows the existence of longitudinal cracks and even holes and corresponding areas where the density of the hydrocarbon chain ends seems to have locally increased. The same effect can be seen more clearly in Figure 6c on a somewhat larger scale. Firstly, the main rows have a strong curvy nature and simultaneously contain several discontinuities which further result in dislocations. Secondly, the inhomogeneity can be observed in the z direction in the form of bright queue-like structures existing through the surface and being above the average z scale. These kinds of defects are expected in the cases where the reaction between the neighboring hydrocarbon chains leads to area shrinkage and thus results in a failure to retain the perfect planar molecular network.18 This is in agreement with and even expected from the X-ray data where the change in interlayer spacing for elaidic acid LB films was much more pronounced than for petroselaidic acid. Respectively, hardly any holes could be found in the topographs taken for petroselaidic acid multilayers. It should be noted that irradiation tests were also applied for saturated fatty acid multilayer structures of stearic acid. However, no major changes or losses in order could be obtained in the AFM studies. The only differences obtained were very slight, probably thermally induced changes in crystal parameters. (18)For comparison,see: a) Sarkar,M.; Lando, J. B.Thin Solid Films 1983,99,119. b) Meller, P.; Peters, R.; Ringsdorf, H. Colloid Polym. Sci. 1989, 267, 97.
Unsaturated Fatty Acid Monolayers and Multilayer F i l m
Langmuir, Vol. 9, No. 9, 1993 2367
Figure 5. AFM images of 15 layers of petroselaidic acid deposited from the TbC13 subphase a t a surface pressure of 25 mN/m. The characteristic FFT diffractogram is inset in each image. Image a was taken before and images b-d were taken after UV irradiation. All the scales are in nanometers.
Apparent Activation Energy of Reaction of a Petroselaidic Acid Monolayer. In order to compare the reaction in the LB film and in the monolayer state, identical UV irradiation experiments were carried out on a petroselaidic acid monolayer. The reaction kinetics was monitored as changes in the barrier speed.9J1J9~20 Fi 7 shows the progress of polymerization of petroselaidic acid on a 0.1 mM CdCl2 subphase a t three temperatures of 15,20, and 25 "C. The monolayer was compressed until the surface pressure reached a value of 15 mN/m and was then kept constant through the entire experiment. The barrier speed decreased initially when switching on UV light due to the heat generated from the 100-W UV lamp. After several minutes the barrier speed increased clearly and reached a maximum after about 90 min UV irradiation (for a 20 "C experiment) and decreased thereafter slowly. A constant surface pressure on a monolayer forces the molecules close to each other and is assumed to allow the maintenance of a constant intermolecular distance, thus providing grounds for a continuous reaction. It is also a (19)Zhou, H.;Batich, C.; Stern,R.; Duran, R. S.Makromol. Chem., Rapid Commun. 1990,ll,409. (20) Duran, R. S.; Zhou, H. C. Polymer 1992,33,4019.
fact that the monolayer molecules on the subphase surface have more degrees of freedom to reorient than in the case of LB films when fixed in one position on the substrate surface. The monolayer stability was tested under normal conditions, i.e., without any UV irradiation. During a test period of 3 h, a constant barrier speed of about 1mm/min was needed to hold the predetermined constant surface pressure. Thereafter it was possible to determine the purely UV-irradiation-induced area contraction per molecule. At 20 OC, this was found to be 6 A2 from the initial mean molecular area of 22 A2 to the final area of 16 A2 after 180 min of irradiation. The final mean molecular area of 16 A2 seems a little small for a true monolayer. A possible explanation for the large area contraction may be an increased dissolving of the molecules during the irradiation. This may mean that the possible cross-linking taking place is incomplete. The stability of the monolayer can be enhanced by optimizing the subphase content and pH. We have done a lot of studies in this field, and the preliminary results show that the optimum stability can be reached only within a narrow pH range. It seems also evident that the pH of maximum stability correspondsto
Peltonen et al.
2368 Langmuir, Vol. 9, No. 9, 1993
I
'0
'50
'100
Time (min)
'150
I
Figure 7. Barrier speed vs time during the UV irradiation of a petroselaidicacid monolayer on a subphase of 0.1 mM CdC12 at temperatures of 15,220,and 25 "C. The Arrhenius plot of ln(r) vs 1/T is inset in the figure.
Figure 6. AFM images of 13 layers of elaidic acid deposited from a TbCls subphase at a surface pressure of 15 mN/m, before (a)and after (b,c) UV irradiation. In (a)and (b)the characteristic FFT spectrum has been inset in the figure.
a monolayer with a 50% ionization. By pH adjustment and selection of the subphase ions the intermolecular interactions and distances can be tuned, which is crucial
for a successful polymerization of the film. The stability data will be published in full elsewhere. The rate of the reaction increased with increasing temperature, and the time needed to reach the maximum barrier speed appeared to get shorter. The apparent activation energy of reaction in the monolayer state can be roughly estimated from the dependence of the barrier speed on temperature at the maximum speed (Figure 7). The reaction rate constant, r, was determined from the kinetic curve by determining the slope of a line drawn between the barrier speed data points corresponding to times to (UV light on) and t1 (maximum barrier speed), regardless of the fact that the pressure-area isotherms at different temperatures are different. The rate constant actually corresponds to the rate of change in the mean molecular area.19 The apparent activation energy of the monolayer reaction was then calculated from the Arrhenius plot of ln(r) vs 1/T (inset in Figure 7), and was found to be 192kJ/mol. This is considerablyhigher than the values found for the linoleic acid monolayer on a 0.1 mM TbCls subphase (62 kJ/mol) and a 0.1 mM CdC12 subphase (47 kJ/mol).ll Considering that linoleic acid contains two double bonds in the chain with cis configurations the activation energy found for petroselaidic acid with only one trans-type double bond seems reasonable. The validity of the calculated rate constant is based on the observation of a clear starting point and a maximum speed of the reaction. It is obvious from Figure 7 that especially at high temperature the kinetic behavior of the reaction is strongly asymmetric with a quick start followed by a markedly slower period. The observed shape ofthe curve is different from those observed by Zhou et al.19 and Duran et aL20 where the same method gave beautiful polymerization reactions with an almost symmetric shape of the kinetic curves. The asymmetric behavior is assumed to result from a reaction taking place in two steps. We suggest that dimerization occurs at the first stage, which is followed by a clearly slower cross-linking. The slowness of the second step together with the fact that the initial barrier speed was difficult to reach within the measuring time implies that the suggested cross-linking occurs only partially. Support and analog for the presented model can be found from the AFM results where the UV irradiation of the LB f i i samples was found to result in a rather inhomogeneous end product with varying local packing characteristics. We tried gel permeation chromatography (GPC) for the determination of the exact molecular weight of the end product but were not successful. The irradiated sample collected from the subphase surface was not soluble in THF (tetrahydrofuran) most commonly used in GPC.
Unsaturated Fatty Acid Monolayers and Multilayer F i l m
Neither was it soluble in hexane or chloroform. We thus were not able to determine the molecular weight of the product. However, the insolubility indicates that the end product is different from the monomer, which was easily dissolved both in hexane and in chloroform.
Conclusione In this study the structure and topography as well as the effect of UV irradiation have been successfullystudied for two unsaturated fatty acid LB films. The reactivity has been compared with the reactivity of a floating monolayer. The X-ray diffractiondata give the monolayer thickness in the LB multilayer structure. Assuming an equal configuration for the acids studied, the results indicate that the position of the double bond in the hydrocarbon chain is enough to cause some difference in the inclination angle of the monolayer molecules. Also obtained is that the LB films were of the Y type and retained this orientation during UV irradiation; i.e., no reorientation (turning around) of the molecules or threedimensional crystallization takes place. Some thinning of the monolayer occurs, but the extent is so limited that it suggests cross-linking,perhaps partial, to be the process occurring rather than chain The irradiation seems at least partly to destroy the well-ordered packing of the layers. The AFM images clearly show the varying degrees of damage as well as the differences in the behavior (21) Read, R. T.;Young, R. J. J. Mater. Sci. 1984,19, 327.
Langmuir, Vol. 9, No. 9, 1993 2369
between the two acids studied. It seems that petroselaidic acid is more resistant against UV irradiation than elaidic acid, where clear holes and cracks were generated by irradiation. The reactivity seems to be clearly higher in the monolayer state than in the LB films. This is reasonable when considering the flexible and more loose structure of the floating monolayer. The results suggest firstly that the reaction, most probably dimerization followed by a partial cross-linking, is energetically more easily accomplished on a monolayer than in the LB film. Further, to avoid cracks and holes, one might assume it to be ideal to deposit a UV-irradiated monolayer. With this process we have not succeeded so far. Secondly, a detectable degree of order still remains in the irradiated samples. This may be regarded as an advantage of dimerization compared with the formation of long-chain polymers which often results in complicated amorphous, i.e., orderless, structures. The results presented confirm the interlayer and topographical characteristics expected for the studied structures, but additional IR or XPS measurements are needed in order to find the exact irradiation-induced changes in the chemical structure. Also, the reaction kinetics can be more easily and exactly monitored if the monolayer stability can be enhanced.
Acknowledgment. This work was financed by the Technology Development Centre, Tekes, Finland.
