Langmuir 1993,9, 3101-3106
3101
Dependence of Langmuir-Blodgett Film Quality on Fatty Acid Monolayer Integrity. 1. Nucleation Crystal Growth Avoidance in the Monolayer through the Optimized Compression Procedure R. M. Morelis, A. P. Girard-Egrot, and P. R. Coulet' Laboratoire de G h i e Enzymatique, EP 19 CNRS-UCBL-ESCIL, 43, Blvd du 11 Novembre 1918,69622 Villeurbanne Cedex, France Received February 3, 1993. In Final Form: July 12,199P
A rational approach for avoiding defects in fatty acid monolayers is exposed. It is demonstrated that the early stages of compression affect the integrity of a behenic acid monolayer prior to transfer. It was shown that defects such as crystals appeared in the monolayer and evolved in heaps when the monolayer is aging. This influences the quality of LB films as shown when transferring the monolayer onto three successive substrates. It is pointed out that the kinetics of appearance of crystals depends not only on the surface pressure but also directly on the way of compression. During this latter step, this evolution can be limited if a local surface overpressure is avoided. A study at different surface pressures in solid phase suggests that the nucleation crystal growth may be a triggering mechanism of slow collapse.
Introduction The Langmuir-Blodgett (LB) technique is known to be an attractive method for preparing high-qualitythin films. However, these films may contain several sorts of defects reviewed by Lesieur et al.l as pinholes2J or microcollapses.415 Without eliminating these defects, several applications as molecular electronic devices having specific functions a t a molecular level cannot be achieved. In fact, many types of defects have been reported to be inherited from the structure of the monolayer itself at the air-water interface. The integrity of a monolayer of amphiphilic materials was traditionally investigated by the studies of ( P A ) isotherm and the quality of transfer of this monolayer by the LB procedure has been often estimated by the calculation of the transfer ratio correspondingto the area of film removed a t the water interface to the area of the covered substrate. In order to form LB films, the properties of the monolayer must be carefully investigated because they condition the possibility of quantitative transfer and an ordered superposition without any defect. The value of surface pressure imposed to the monolayer during the transfer giving the best result depends on the nature of the monolayer and, as reported by Hann,B is established empirically. Generally, to transfer quantitatively the film onto a solid substrate, the monolayer must be in a closepacked state. For fatty acids, this state is reached for a surface pressure fairly higher than the equilibrium spreading pressure (res,,).So, the transfer of a fatty acid monolayer is achieved with an overcompressed monolayer which does not represent an absolutely stable equilibrium with respect to the bulk crystal phase. The monolayer 0 Abstract published in Advance ACS Abstracts, September 1, 1993. (1) Lesieur, P.; Barraud, A.; Vandevyver, M. Thin Solid Films 1987, 152, 155. (2) Arisawa, S.; Arise, T.; Yamamoto, R. Thin Solid Film 1992,209, 259. (3) Yamada, S.; Ishino, F.; Mataushita, K.; Nakadaira, T.; Kitao, M. Thin Solid film 1992,208, 145. (4) Iriyama, K.; Araki, T. Chem. Lett. 1990, 1189. (5) Barraud, A.; Leloup, J.; Maire, P.; Ruaudel-Teixier,A. Thin Solid Film 1985,133,133. (6) Hann, R. A. InLangmuir-BlodgettFlm;Roberta,G., Ed.;Plenum Press: New York, 1990; Chapter 2.
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can remain in such a metastable state for an extended period of time, enabling meaningful experiments to be performed. Nevertheless, it is interesting to determine the suitable conditions to transfer correctly and without any defect the monolayer onto several substrates sequentially even though the monolayer is aging. Moreover, it is noteworthy that the close-packed state can be obtained in a range of surface pressures and the integrity of the film is directly influenced by the surface pressure at which the monolayer is compressed during the transfer. For these reasons,our goal has been to study the integrity of the behenic acid monolayer as a function of time, by transferring 13layers onto three successivesubstrates for close-packed monolayers compressed a t three different surface pressures. The quality of the LB films thus obtained was representative of the integrity of the monolayer present at the water surface. This quality was estimated not only by the transfer ratio of the monolayer onto the substrate but also by scanning the surface structure of the LB films with Nomarski microscopy.
Experimental Section Materials and Sample Preparation. Behenic acid (docosanoic acid) was purchased from Sigma (St Louis, MO) and used without further purification. Chloroform (analytical-reagent grade, Chimie Plus, France, purity = 99%) was used as the spreading solvent for monolayer preparation. Ultrapure water (resistivity 1 18.2 MDcm) was obtained with a Millipore Milli-Q four-cartridge purification system (Millipore Co.) and used as water subphase. Spreading solution was 2 X 1Oa M behenic acid in CHCla. It was stored at 4 "C and diluted twice before use. Calciumfluoride(CaF2)substrateswere purchasedfrom Sorem (France) in the form of 35 mm X 9.5 mm X 2 mm single crystal. They were cleaned through several steps in a heating ultrasonic bath after a 7-min rinsing in CHCL, they were soaked for 40 min in TfD, detergent (Franklab, France) for precise cleanup
and subsequentlyrinsed thoroughly with Milli-Qwater. Finally,
these substrateswere immersed 7 min in CHCb in an ultrasonic bath before use. The CaFz substrateswere foundsuitable for the optical control made with a Nomarski differential interference contrastmicroscope (Carl Zeiss). Isotherm. Langmuir-Blodgett trough, Model LB-105 from ATEMETA, Paris [Licence CEA, Patent 83 19770 (1249-83)l with a Wilhelmy balance was used. The trough and the 0 1993 Americah Chemical Society
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3102 Langmuir, Vol. 9, No.11,1993
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Figure 1. Pressure uemm area isotherm for behenic acid, 20.5' C. compressionbarrier areinPTFE (Teflon). The Wilhelmybalance was close to the dipping site for the substrate transfer and parallel to the compression barrier. The trough (94X 38 cm) was filled with 7 L of Milli-Q water in equilibrium with atmospheric C02. The temperature was kept at 21 f 1 O C and a continuous airdried flowwas maintained on the trough permanently. The water surface was cleaned by suction with capillaries connected to a water vacuum jet pump and cleanliness was verified by reducing the area to a minimum and checking for any increase in surface pressure. The monolayer was prepared by applying a known volume of spreading solution on the water surface and by letting the solvent evaporate (10min); the isotherm was obtained by a discontinuous compression. In this method, the amount of amphiphdic molecules spread on the surface was fiied and the surface area per molecule was varied by discrete and subsequent displacements of the barrier submitted to a feedback servoloop. When the surface pressure reached the poised value,the molecular area was noted. The monolayer was compressed by steps of 2 mN-m-'. The maximal rate of the mobile barrier (3cmamin-l) progressively decreased when the poised surface pressure was reached. The gain of the feedback servoloop corresponding to the speed response of the compression barrier could be modified. The uncertainties were estimated at 0.3 mN-m-l for ?r and 50.01 nm2.molecule-1 for the molecular area. Langmuir-Blodgett (LB) Film Deposition. The monolayer was spread and discontinuously compressed in the same way as for the isotherm until the selected surface pressure for the transfer was reached. Three successive CaF2 substrates were vertically dipped through the monolayer at the rate of 2 cm-min-l. During the transfer, the surface pressure was maintained constant by the feedback servoloop that controlled the motion of the compression barrier. Y-type f i i s consisting of 13 monolayers were formed. The transfer ratio was calculated after each dipping and each withdrawal of the substrate; it corresponds to the ratio of the f i b removed area to the theoretical substrate area. The homogeneity of the LB f i i s was controlled through Nomarski microscopic observation. The typical accuracy is ca. f0.3 nm and the lateral resolution is 1 Mm at a magnification of 500.'
Results Figure 1shows the r A isotherm of behenic acid at 20.5 "C. The isotherm illustrates the condensed-liquid (Lc)/ pseudosolid (S)phase transition observed with un-ionized fatty acids.8 This isotherm reflects the different phases, as reported by Bib0 and Peterson using the HarkinsStenhagen-Lundquist nomen~lature.~J~ For this fatty acid, the condensed-liquid phase consists in fact of two phases with the same molecular packing and two different orientations: the Lz-phase up to 19 mN.m-l (0.21 nm2.molecule-') and the L'z-phase between the kink at 19 mN-m-l and the beginning of the steep rise at =29 (7) Vandevyver, M.; Barraud, A. J. Mol. Electron. 1988,4, 207. (8) Gainea, G. L.,Jr. Imoluble Monolayers at Liquid-Gas Interfaces; Intemience: New York, 1968,Chapter 4. (9) Kenn, R. M.; ah, C.; Bibo, A. M.; Petereon, I. R.; Mbhwald, H.; Ala-Nielsen, J.; Kjaer, K. J. Phys. Chem. 1991,95, 2092. (IO) Bibo, A. M.; Peterson,I. R. Adv. Mater. 1990,2, 309.
"am-l(e0.19 nm2.molecule-'). The more solidlikephase is obtained above -29 "am-'. A,, represents the "zero pressure" area obtained by extrapolating the steepest part of the isotherm to zero pressure. The value of the limiting area is 0.195 f 0.008 nm2.mo1ecule-'. A value of zero pressure area close to 0.2 nm2*mo1ecule-'is traditionally accepted in condensed films. For behenic acid, Bettarini et aZ.ll reported an A, value equal to 0.19 nm2-molecule-l for a monolayer spread on a subphase containing NaC1. This value which corresponds to the cross section of the polar heads indicates an almost perpendicular arrangement of the hydrophobic chains with respect to the interface. The close-packed state was obtained for behenic acid between 29 and 58 mN-m-'. Three values of T (32,36, and 42 "em-') were selected for the transfer of the behenic acid monolayer onto three successive CaFz substrates. The 32 mN0m-l surface pressure was chosen close to the S-phase transition. The resulta are presented in Figure 2. In each case, the behenic acid monolayer was spread on the water surface and the compression is stepwise as described in the Experimental Section. The end of the compression was considered as the initial time of aging. A 10-min lag time was then necessary to enable an homogeneouscompression of the f i i to be reached before the transfer of 13 layers onto the first CaFz substrate. This operation was repeated twice with a 5-min lag time between each substrate. Each transfer took 31.6 min and the total time was about 2 h for the three depositions. This time will be further considered as the aging time of the monolayer on the subphase. The quality of LB films thus obtained was estimated with twocriteria. The f i t one which is the overall transfer ratio calculated after each dipping and withdrawal of the substrate quantifies the quality of the monolayer transfer on the substrate: this parameter takes into account the transfer ratio obtained after the downstroke and the upstroke of the substrate. Even if the transfer ratio at the upstroke is higher than the transfer ratio at the downstroke (data not shown), the overall transfer ratio is a good simplified representation of the quality of the transfer during the backward and the forward motion of the substrate. The second criterion depicts the homogeneity of the LB films. The stress relief patterns can be visualized with a Nomarski differential interference contrast microscope which also is a valuable tool for visualizing scattering centers.12 Although the resolution is close to 1pm, this direct observation has the main advantage of not damaging the LB film structure by either the vacuum stage or the electron beam. The defects observed were light reflecting centers characteristic of behenic acid crystals. To makethe Nomarski microscopicobservations quantitative, 0.22 mm X 0.15 mm areas were delimited in which crystals were counted. The results expressed in crystals/mm2correspond to the average number of crystals found in 18 areas randomly chosen all over the substrate surface. The quality of LB films obtained after transfer at three surface pressures has been studied as a function of a new parameter which, to our knowledge, has never been taken into account until now: the gain of the feedback servoloop during the compression. This compression gain (Gc, expressed in au), controls not only the speed response of the compression barrier but also the slowdown of this barrier when the measured surface pressure is close to the imposed pressure. It appears important to known the (11) Bettarini,S.;Bonoai,F.; Gabrielli,G.; Martini, G.Langmuir 1991, 7,1082. (12) Tippmenn-Krayer,P.;Meiael,W.; Mbhwald,H. Adu.Mater. 1990, 2,589.
Fatty Acid Monolayer Integrity
Langmuir, Vol. 9, No. 11, 2993 3103
Figure 2. Number of crystals obtained per mm2in LB films (a) and overall transfer ratios calculated after each dipping and withdrawal of substrate (b) as a function of aging time of monolayer at the water surface for three surface pressure values ( T ) and two different G,: 0. G, = 0.25 au: 0. G, = 3.0 au. For each T value. 0 refers to the end of the compression on time scale. The shaded areas symbolize the time-at which Ad dwing which the transfer w'as performed.
influence of the G, parameter that is directly related to the monolayer compression. Figure 2.2b shows that at ?r = 36 mN.m-l, the overalltransfer ratio is close to 1for the two chosen compression gain values and the three substrates, indicatinga good quality of transfer. Concerning the number of crystals/mm2,for G, = 0.25 au, the first substrate had no defect,as shownin Figure 3a. (The stripes are due to the ultrasonic vibrations which attack the calcium fluoride during the ultrasonic cleanup.) But the defects appear on the following substrates with a higher intensity for substrate 3 than for substrate 2. Figure 3b shows the defects which appeared on the third substrate for G, = 0.25 au. For a compression with a G, = 3.0 au, the same progressive appearance of crystals is noted with more numerous stress reliefs, even for the first substrate, as shown in photographs of Figure 3. Comparing these four photographs,it can be seen that for the third substrate, the crystals appear not isolated but gathered in heaps present in the whitish region. This latter sometimesexists without crystal. As the number of crystals obtained is not the same for the three substrates, the defects cannot be generated by the transfer of 13layers onto CaF2 substrates. Moreover, when the G, parameter is shiftedfrom 0.25 to 3.0 au, defects become visible early with the first substrate. Obviously, the crystals are generated in the monolayer. Their appearance is kinetically related to the aging time of the monolayer on the aqueoussubphase. A thorough study of transfer ratio shows that this parameter evolves as a function of time and becomes greater than 1. Moreover, when G, is 3.0 au, these values are the highest. The evolution of transfer ratios is in the relationship with the evolutionof crystals. It is generallyaccepted that a transfer ratio higher than 1reflects either a collapse or dissolution phenomenon. This latter possibility is not considered for fatty acids with a long chain.8 In this instance, all the results obtained at ?r = 36 "em-l seem to indicate that the collapse could begin by a nucleation crystal growth. To check this hypothesis, the same experiments were performed with a more compressed monolayer. Since the stability of the monolayer decreases with the increase of surface pressure, this phenomenon will be emphasized at
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Figure 3. Photographs of LB films obtained after transfer of behenic acid monolayer onto CaF2 substrate at T = 36 "em-l: (a) first substrate and (b) third substrate for G, = 0.25 au; (c) first substrate and (d) third substrate for G, = 3.0 au. These substratesare those of Figure 2.2. The stripes are the consequence of ultrasonic cleanup treatment on the substrate. Magnification 625.
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3104 Langmuir, Vol. 9, No. 11, 1993
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Figure4. Advance of the compression barrier and overalltransfer ratios obtained during the transfer of 13 layers of behenic acid onto each of three successive CaFz substrates at u = 42 mN0m-I and G, = 0.25 au. 0 corresponds to the end of the compression on the time scale. Shaded areas symbolize the time a t which and during which the transfer was performed.
= 42 "om-l. Because of the decrease of the monolayer area (characteristicof the collapse by multilayer forming) occurring during the first minutesof the experiment when G, = 3.0 au, only the results for G, = 0.25 au are presented (Figure 2.3). It can be clearly seen that a correlation exists between the increase of overall transfer ratio and the number of crystals which becomes innumerable for the third substrate (Figure 6b). Figure 4 shows the advance of the compression barrier and the overall transfer ratio for G, = obtained during the transfer at ?r = 42 "em-l 0.25 au. If we could consider that the three transfers were rigorously performed in the same conditions, then the values of all the transfer ratios would be identical (close to 1)and the linear graph representing the advance of the barrier as a function of time of transfer would have the same slope for the three substrates. In fact,Figure 4 clearly shows that the advance of the compression barrier is accelerated for the two last substrates. This acceleration is characteristic of the beginning of collapse and is correlated with the increase of the overall transfer ratio. Figure 5 depicts the correlation between the overall transfer ratios and the advance of the compression barrier obtained during the transfer, for two values of the compression gain after a 10- and 20-min waiting-phase followingthe end of compression. After a 20-min waiting, whatever the G, value, the barrier advance (Figure 5a) is not linear; up to 30 min, an acceleration of the barrier occurs,followedby a slowingdown: this behavior expresses the rapid decrease of the monolayer area due, in this case, to a collapsethat is finally limited by the edge of the trough. The high value of the overall transfer ratio (close to 5) confirms the transfer of a collapsed monolayer due to a layer stacking in this case. Although the phenomenon is similarfor both G, values, it is noteworthy that the collapse progresses quicker for G, = 3.0 au than for G, = 0.25 au. This difference is obvious when the transfer is performed 10 min after the end of the compression (Figure 5b). For G, = 0.25 au, the transfer is correctly achieved with an overall transfer ratio close to 1; the advance of the barrier is linear. For G, = 3.0 au, the visible collapse just begins and this is the acceleration phase. That is why the values of overall transfer ratios increase up to 6.4 and decrease after 20 min. When parts a and b of Figure 5 are compared, it is noteworthy that the slowingdown phase starts 20 min after the end of the compression. Figure 6 shows photographs of LB films transferred a t ?r = 42 mN0m-l. For G, = 0.25 au and for a 10-minwaiting time before the first transfer, parts a and b of Figure 6 show the evolution of the crystals appearance between
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Figure5. Advance of the compressionbarrier and overalltransfer ratios obtained during the transfer of behenic acid multilayer onto CaFz substrate at u = 42 mN0m-l and at different times of aging of the monolayer for two G, values: 0 A,for G, = 0.25 au; A,for G, = 3.0 au. 0 correspondsto the end of the compression on the time scale. Arrows indicate the beginning of the transfer. The dotted line between 0 and 10 or 20 min correspondsto the advance of the barrier before transfer.
the second substrate (a) and the third substrate (b),where the crystals are innumerable. For these substrates, the crystals are the highly reflectingcenters in a homogeneous background. Figure 6c shows the substrate relief covered by LB films transferred after a 20-min wait: when the collapse of the monolayer occurs in the slowing phase, the background is inhomogeneous and the crystals are much smaller. The difference between parts b and c of Figure 6 points out a modification in the state of the monolayer on the aqueous subphase. All the results obtained at ?r = 42 mN-m-l clearlysuggest that the well-characterized collapse by the decrease area due to multilayer formation is preceded by a collapse due to a nucleation crystal growth. When the surface pressure is equal to 36 "em-', it is not possible to transfer a multilayer onto three successive substrates without defect. When the transfer is performed a t u = 32 mN-m-l, we do not observe any evolution either of the number of crystals or of the overall transfer ratio value (Figure 2.1): the monolayer is stable enough to limit the formation of crystals. Nevertheless,even if the number of crystalsis low and almost constant,this number is higher when G, = 3.0 au than when G, = 0.25 au. For G, = 0.25 au, the third substrate presents a good homogeneity as shown in Figure 7. The difference of grays is due to the deposition limit. The light gray region correspondsto the behenic acid coating. Taking into account this homogeneity, we can be sure that the first substrate presenting 0 crystal/mm2 would certainly be excellent even if the resolution of the Nomarski microscopy technique is only 1pm. The importance of compression gain with regard to the monolayer integrity has been pointed out (Figures 2 and 5). It appeared interesting to know if the gain of the feedback servoloop could influence the formation of crystals in the monolayer during its transfer onto the CaF2 substrate when the monolayer area variation was small.
Langmuir, Vol. 9, No. 11, 1993 3105
Fatty Acid Monolayer Integrity
Table I. Influence of G on the Quality of LB Films. time after transferb crystals/mm2 on each substrate (min) Gc (au) Gt = 0.25 (au) Gt = 3.0 (au) 4lC 0.25 0 0 78 114
3.00 0.25 3.00 0.25 3.00
61 61 152 182 606
76 61 152 364 606
a Quality of LB f i b s is expressed through number of crystale/ mm2. bThe transfer surface pressure was equal to 36 “em-l. c Transferonto the first substrate began 10 min afterthe compression end.
the compression gain G, and of the transfer gain Gt are given in Table I. Whatever the G, value, there is no increase in the number of crystals when Gt shifted from 0.25 to 3.0 au. Only the compression gain can modify the homogeneityand can influencethe aging of the monolayer.
w 10 pm
Figure 6. Photographs of LB films obtained after transfer of 13 behenic acid layers onto CaF2 substrate at ?r = 42 mN0m-l and G, = 0.25 au: (a) corresponds to substrate 2 in Figure 2.3; (b) corresponds to substrate 3 in the Figure 2.3; (c) LB films when the monolayer was transferred after a 20-min wait (Figure 5b). The stripes are the consequence of ultrasonic cleanup treatment of the substrate. Magnification 625.
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Figure7. Photograph of the third CaF2 substrateobtainedafter transfer of 13 behenic acid layers at ?r = 32 mN0m-l and G, = 0.25 au. The stripes are the consequence of ultrasonic cleanup treatment on the substrate. Magnification 625.
For this study, we defined a parameter, Gt, which correspondsto the gain chosen for the transfer. The results obtained for the transfer of 13layers onto three successive CaF2 substrates at ?r = 36 mN0m-l and for two values of
Discussion Both stability and homogeneity of a spread monolayer are crucial for the preparation of LB films. In fact, it is known that the defects of the monolayer are essentially determined before deposition. In the present work, the integrity of a behenic acid monolayer as a function of the parameters of compression was investigated. The evolution of defects when the monolayer is aging and its influence on the quality of the LB films transferred successivelyonto several substrates was studied. Even if a recent study,13 suggested a modification of the local molecular packing, at least for the deposited monolayers from CS, L2, and L’2, we demonstratedherein by successive transfers that defects such as crystals appeared in the monolayer. They developed in the form of heaps and this evolutionwas related not only to the surface pressure but directly to the way of compression. These crystals were not generated during the deposition although a modification of the molecular packing might occur. We could demonstrate that to correctly transfer the monolayer onto three substrates successively, it was necessary to compress the molecules until the surface pressure where they reached the molecular packing of the Sphase. Even if this surfacepressure is respected, caution must be taken with regard to the control of the speed response of the compression barrier during the discontinuous compression of the monolayer. The advance versus time of the compression barrier was investigated at ?r = 42 “em-l. It is related to the behavior of the monolayer during the successivetransfers and it takes into account not only the decrease of the area film generated by the removal of molecules during the deposition but also the collapse phenomenon. The correlation of the barrier advance with the apparition of the crystallinedefects and the value of transfer ratios suggests that the formation of a three-dimensional multilayer is preceded by nucleation crystal growth. Vollhardt et al.14 have studied directlythe relaxation kineticsof an insoluble monolayer at constant surfacepressure. They showed that for a surface pressure corresponding to the metastable range, the constant surface pressure relaxation can be described by progressive nucleation with hemispherical edge growth. The collapse mechanism is generally considered to be associated with the formation of a three-dimensional multilayer due to an overcompression. This mechanism (13) Steitz, R.; Mitchell, E. E.; Peterson, I. R. Thin Solid Films 1991, 205, 124. (14) Vollhardt, D.; Retter, U.; Siegel, S. Thin Solid Films 1991,199,
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3106 Langmuir, Vol. 9, No. 11, 1993
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Figure 8. (a) Folding mechanism according to Ries and Kimball.14 (b)Postulated form of the multifold ridge according to Barraud et aL6
was investigated by Ries et al.16J6Gainesa described a "slow collapse" and reported further that this type of collapse was due to "greatest hazard, sincemany monolayer depositions are carried out under conditions where the monolayer is unstable".'7 Barraud et aL6have also studied this slow collapse. Introducing heavy ions (silver) in the polar plane of the layers to increse the electron beam absorption, these authors visualized by TEM the defects on LB films of behenic acid transferred at T = 35 mN-m-l (below the collapse pressure). They demonstrated that the defects form the fine structure of randomly oriented ridges made of several very thin parallel dark lines separated by about 45-50 A.6 To explain the multifold nature of the observed ridges, they proposed the formation of close-packed folded ridges as shown in Figure 8. Accordingto this scheme,the close-packedmultifold ridges could appear as crystals with lower magnification of the Nomarski microscopy technique. Moreover, this could explain the gathering in heaps. We showed that crystals must appear before the collapse occurs by formation of double layer platelets in the folding mechanism described by Ries16 at the collapse surface pressure. As we have visualized a certain kind of platelet for the third substrate at T = 36 mN-m-l where crystals are numerous and gathered in heaps, we can postulate that the slow collapse presented in Figure 8b is a prior step for the multifolding which leads to three-dimensional regions (Figure 8a). For a surface pressure fairly below the collapse surface pressure, these two states, crystals and platelets, can coexist in the same time. These results are in agreement with the study of Kat0 et al.ls who investigated the behavior of arachidic acid monolayer, using the "time of observation" (Le. the reciprocal of strain rate of compression) to characterize the T-A isotherm. When the time of observation is longer, the drop of surface pressure occurred in theL, phase and the solid phase cannot exist. Their results are in agreement with the presence of platelets characteristic of the collapse, that we observed with a behenic acid monolayer for a surface pressure lower than the collapse pressure. As according to Gaines? resp for palmitic and stearic acids are close to 7 and 2 mN-m-l, respectively, and reap for arachidic acid is estimated as almost 0 mN.m-l at 20 "C; the res,,for behenic acid at 20 OC can be considered as almost 0 mN-m-l. I t is possibleto postulate that crystals of behenic acid could appear from the early steps of compression before the platelets are present. Our results on the progressive appearance of different structures in the monolayer at a surface pressure below that of collapse (15)Ries, H. E., Jr.; Kimball, W. A. Proceedings of the Second International Congressof Surface Actiuity;Butterworth. London, 1957; Vol 1, p 75. (16) Ries, H. E.,Jr. Nature 1979,281,287. (17) Gaines, G. L., Jr. Thin Solid F i l m 1980,68, 1. (18) Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1991,7, 2208.
complement the recent study of Kato et al.19 This latter study shows also a progressive appearance of two kinds of three-dimensional structures generated after the collapse of the monolayer. In our study, when the surface pressure was poised at 42 "em-' and when the collapse was well characterized by a transfer ratio higher than 1, LB films were very inhomogeneous and the crystals partly disappeared. Those remaining were small reflecting centers (Figure 6c). The same inhomogeneity was reported by Kajiyama et aLmafter the transfer of a partial collapsed baryum stearate monolayer. The different sizes of crystals could be explained as follows: according to Gaines? the freshlyformed collapsed bilayers may contain several molecules of water between and among polar groups. After evaporation of this intercalated water, there is a possible recrystalliiation that generates small crystals in transferred collapsed material. Using different complementary techniques, Kajiyama et aL20 and Takoshima et aLZ1observed that the aggregation of fatty acids in the monolayer starts with the appearance of condensed domains in the gaseous state which grow just after the spreading of molecules. During the compression, these isolated domains gather and fuse together to form larger domains. Since small condensed domains already exist in the monolayer in gaseous state, it is easy to understand that when the monolayer is compressed, crystals could appear in these condensed domains if the compression barrier reacts too rapidly, because a local surface overpressureis then created. Even for arather low surfacepressure limiting the rate of crystals appearance, it is necessary to gently compress the monolayer to avoid a local surface overpressure. The discontinuous compression process with a thorough control of the speed response of the compression barrier seems more adapted to diminish such local surface overpressures, because the barrier stops at each step and the surface pressure gradient is lowered by a relaxation of molecules in a more homogeneous manner.
Conclusions
It was recently suggested6that the characteristics of a nucleation and growth process of collapse observed for arachidic acid monolayer in the L, state near the S-phase transition could be explained if we can expect crystalline material to be generated. In this study, we postulate that the collapse starts slowly by a nucleation crystal growth and is accelerated in the folding mechanism. This phenomenon, which is a direct consequence of the monolayer instability, begins after a time of compression of the monolayer depending on both the surface pressure value and the way of compression. Finally, for applications requiring high-quality LB films,it appears compulsory to take into account the time necessary to transfer numerous layers onto the same or subsequentsubstrates,time within which the monolayer ages and loses its integrity. Acknowledgment. The research was partly supported by CNRS-ULTIMATECH and RBgion Rh6ne-Alpes. (19) Kato, T.; Iriyama, K.; Araki, T. Thin Solid Films 1992,210/211,
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(20) Kajiyama,T.;Umemura, K.;Uchida, M.; Oiehi,Y.; Takei,R.Bull. Chem. Soc. Jpn. 1989,62.3004. (21) Takoshima, T.; Masuda, A.; Mukasa, K. Thin Solid Film 1992, 2101211,51.
