Langmuir 1993,9, 3107-3110
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Dependence of Langmuir-Blodgett Film Quality on Fatty Acid Monolayer Integrity. 2. Crucial Effect of the Removal Rate of Monolayer during Langmuir-Blodgett Film Deposition A. P. Girard-Egrot, R. M. Morelis, and P. R. Coulet' Laboratoire de Gknie 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 The quality of LB films was investigated by studying a new parameter, referred to as the monolayer deposition rate (RD). This parameter, corresponding to the actual velocity at which the monolayer is removed from the water surface, takes into account both the dipping rate of the Substrate and its area. It controls the speed of compression of the monolayer and, consequently)influences directly the rate of appearance of potential sites of crystal formation in the monolayer at the air-water interface. During the transfer of the monolayer onto the substrate, this number of potential sites of crystal formation seems to be modified according to the type of the molecule-substrate interaction, hydrophobic or hydrophilic. Introduction We showed previously1 that the collapse of a fatty acid monolayer would start by a nucleation crystal growth at a surface pressure lower than the collapse surface pressure. Then, a t the transfer surface pressure, these crystal defects appear versw time in the monolayer and are subsequently transferred onto the substrates. In order to both quantitatively transfer the monolayer and limit the nucleation crystal growth inherent to the monolayer instability, several conditions must be respected to obtain high-quality LB films. First, the monolayer must be gently compressed to avoid a surface local overpressure, i.e. the speed response of the compression barrier must be low especially during a discontinuous compression. Second, the surface pressure poised for the transfer must enable the molecules just to reach the molecular packing of solid phase. Even if these two conditions are met, it must be kept in mind that the monolayer is in a metastable state and, consequently, the monolayer ages and can lose its integrity during a lengthy transfer. The first layer deposited from the monolayer onto the substrate obviously requires strong interactions between the molecules and the substrate. These interactions may involve modifications in molecular packing2 or produce some defects which will be retained by further layers due to the epitaxy multilayer d e p ~ s i t i o n . ~ With the aim of understanding both the effects of transfer conditions and of molecule-substrate interactions on the rate of appearance of defects in the LB films, two parameters have been investigated the kinds of surface (with two types of substrates, an hydrophilic calcium fluoride CaFz and an hydrophobic silanized glass substrate) and, a new parameter) the monolayer deposition rate (RD). Classically, only the dipping rate is considered and the new parameter we introduce, is defined as the lipid area deposited onto the substrate within 1 min and it is expressed in cm2.min-l. I t depends not only on the dipping
* Abstract published in Advance ACS Abstracts, September 1, 1993.
(1) Morelis, R. M.; Girard-Egrot, A. P.; Coulet, P. R. Langmuir, preceding paper in this issue. (2) Steitz, R.; Mitchell, E. E.; Peterson, I. R. Thin Solid Films 1991, 205,124.
( 3 ) Lesieur, P.; Barraud, A.; Vandevyver, M. Thin Solid Films 1987,
152, 155.
rate of the substrate but also on the coated area of the substrate. Consequently, it is the rate of deposition of molecules onto the substrate which controls the speed motion of the compression barrier maintaining the surface pressure constant by compressing the molecules to compensate the decrease of surface pressure. By modifying either the type of substrates or the dipped area and the dipping rate, we brought out the influence of the deposition rate on the appearance of defects during the transfer and the role of interactions between behenic acid molecules and the substrate. 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 (resistivity1 18.2 Mfbcm) was obtained with a Millipore Milli-Q four-cartridge purification system (Millipore Co.) and used as water subphase. Two different types of substrates for deposition were chosen. CaFa Substrates purchased from Sorem (Frbce) were in the form of a 35 mm x 9.5mm x 2 mm single crystal. These substrates were cleaned with TfDd detergent (Franklab,France) for precise cleanup in a heating ultrasonic bath as described elsewhere.' Glass substrates (50 mm X 20 mm X 1 mm or 50 mm X 10mm x 1 mm) were rinsed in CHCb and cleaned with TfDl detergent. They were immeraed in sulfochromicacid and thoroughly rinsed with Milli-Qwater. To prepare a hydrophobic surface,the cleaned g h substrates were treated with a solution of 1% dichlorodimethylsilane in chloroform for 10 min and then rinsed with fresh CHCL. Prior to use, each substrate waa soaked for a few minutes in CHCL. Behenic acid solutions (2X 10-9 M) in CHCg were prepared and stored at 4 "C, at most for a week, and diluted twice, prior to use.
Langmuir-Blodgett (LB) Film Deposition. A LangmuirBlodgett trough, Model LB-105from ATEMETA,Paris [licence with a Wilhelmy balance was CEA, Patent 83 19770 (12-09-8311, used. It was filled with 7 L of Milli-Q water in equilibrium with atmospheric COz. The temperature was kept at 21 1 "C. The monolayer was prepared as previously described.' Briefly, the surface water was cleaned by suction, a known volume of diluted behenic acid solutionwaa spread, and the discontinuous until a surface compression waa achieved by steps of 2 "em-' pressure ( T ) of 32 mN-m-l waa reached. The gain of the feedback
0743-7463/93/2409-3107$04.00/00 1993 American Chemical Society
Girard-Egrot et a1.
3108 Langmuir, Vol. 9, No. 11,1993
Table I. Qualitynof LB Films Deposited onto Four Successive Silanized Glass Substrates as a Function of Time of Storage of Behenic Acid Solution and Aging Time of Monolayer for Two DeDosition Monolayer Rates (RD) crystals/mm2 time of storage end of RD = RD = of behenic acid substrate transferbof 5.7 i 0.4 10.3 i 0.4 solution (days) (S) 8 layers (min) cm2*min-1 cm2-min-1
s1
38 0 0 60 0 3.8 13.9 85 14.5 110 20.0 25.1 2 s1 35 0 0 s2 68 9 0 s3 103 20 30 s4 130 21 55 4 s1 30 0 0 s2 53 100 2.4 s3 75 126 54.0 s4 95 206 75.0 7 s1 33 0 0 s2 53 210 123 s3 75 463 473 s4 95 1036 939 * Quality of LB films is expressed through number of crystals/ mm2. This time corresponds to the time ellapsed between the end of compression and the end of transfer onto the different substrates at 'lr = 32 "em-', G, = 0.25 au, dipping rate = 2 cmmin-l. 0
s2 s3 s4
~~
0
60
120
180
240
300
Time (min.)
Figure 1. Aging of behenic acid monolayer evaluated by the quality of transfer of 13 layers onto each of three successive CaF2 substrates at ?r = 32 mN0m-l and G, = 0.25 au. Zero corresponds to the end of the compression on the time scale. The shaded areas symbolize the time at which and during which the transfer was performed. servoloop corresponding to the speed response of the compression barrier was poised at a low value (G, = 0.25 au). The compressed monolayer was transferred onto different substrates (hydrophilic or hydrophobic) with the vertical dipping technique. T w o values of dipping rate (1 or 2 cm-min-l) and two areas of substrates (6 or 12 cm2) were used to obtain two values of the monolayer depositionrate (RD).While the surface pressure was maintained at ?r = 32 f 0.3 mNem-l, 7 or 13 layers were successively deposited in Y-type on the same substrate. For the first upstroke, the substrate withdrew in a dry state. The quality of the LB films obtained was estimated through two criteria: the transfer ratio (ratio of removed lipid film area to theoretical substrate area) and the quantitative Nomarski microscopic observations previously defined.l
Results Among the different parameters the quality of LB films is dependingon, monolayer integrity is the most significant. The instability of the fatty acid monolayer led us to determine the parameters allowing the better compromise between a quantitative transfer and a limited rate of defects. The compression of molecules is essentially the step which requires special attention for obtaining a monolayer without defect. Even in optimal transfer conditions (surface pressure maintained a t 32 "em-l and compression gain of the feedback servoloop poised a t 0.25 au), the monolayer is in a metastable state and only the rate of appearance of defects is lowered down. It was thus necessary to appreciate how long the monolayer could be compressed at the transfer surface pressure without losing its integrity. This time is referred here as the aging time. To appreciate the monolayer integrity, parameters like transfer ratio, barrier advance rate, and crystal defects/ mm2 appeared suitable as previously sh0wn.l To determine the aging time, three substrates were successively coated in optimal transfer conditions with 13 layers of behenic acid starting a t 10 (substrate l),130 (substrate 2), and 250 min (substrate 3) after the end of the compression taken as the time origin. As shown in Figure 1, excellent results were obtained for substrate 1: the transfer ratio is equal to 0.96 f 0.06 and no crystal defect was detectable suggestingthe absence of defect in the monolayer. For substrate 2, the transfer ratios were also equal to 1.0 f 0.03 and the dotted line correspondingto the barrier move versus time is parallel
to that of substrate 1,but in this case, crystal defects were found in the LB films. Moreover, between the transfer on substrate 1and on substrate 2, no collapse was detectable (the compression barrier did not move), whereas a slow collapse appeared after the end of transfer on substrate 2. This phenomenon is accountable for the nonparallel dotted line of the barrier advance during the transfer on substrate 3. Consequently, the monolayer stability was considered as acceptable within 2 h after the end of compression. The influence of molecule-substrate interactions on the final quality of LB films was studied with two types of substrates, hydrophilic (CaF2) and hydrophobic(silanized glass). In each case, the monolayer deposition rate correspondingto the actual speed at which the molecules were removed from the water surface was investigated. This parameter depends both on the area of dipped substrate and on the substrate dipping rate. Concurrently, the time of storage of behenic acid solution was examined. Studies were performed by transferring seven or eight layers, depending on the nature of the substrate. Four successive substrates were coated with a monolayer obtained from a behenic acid solution stored during different times. These experiments were conducted to estimate the rate of appearance of defects in LB films within the first 2 h of aging of the monolayer. Table I summarizes the rate of appearance of defects in LB films coating the silanized glass substrates, for two monolayer deposition rates (RD)expressed in cm2.min-l. It is noteworthy that the crystal defects become visible as a function of time whatever the values of both the deposition rate of monolayer and the storage time of the solution. When fresh or two-day stored behenic acid solutions were used, crystal defects appeared more rapidly at high values of the monolayer deposition rate (RD).With an older solution, kinetics of crystal appearanceaccelerated after a 30-min aging time of the monolayer. The older the solution, the higher the number of crystals. Considering that the experiments with a high monolayer deposition rate were performed before those with a lower rate, the influence of the monolayer deposition rate is hidden by the aging of solution.
Fatty Acid Monolayer Integrity Table 11. Quality. of LB Films Deposited onto Four Successive CaFa Substrates as a Function of Time of Storage of Behenic Acid Solution and Aging Time of Monolayer for T w o deposition Monolayer Rates (RD) time of storage end of RD= RD = of behenic acid substrate transfeP of 6.1 & 0.3 12.5 0.3 solution (daw) (S) 7 lavers (min) cm2mir1 cm2.min-l 28 0 1.2 0 s1 53 5.0 2.4 s2 75 32.5 30.0 s3 95 54.0 114.5 s4 s1 30 0 2.7 55 0.9 17.5 52 53 75 15.5 48.5 100 38.5 114.0 s4 s1 28 0 5.5 50 4.8 12.0 s2 75 24.0 54.5 53 95 53.0 113.5 54 a Quality of LB f i i is expressed through number of crystals/ mm2. This time corresponds to the time ellapsed between the end of compression and the end of transfer onto the different substrates at r = 32 mN-m-I, G, = 0.25 au, dipping rate = 2 mmin-l.
Table 111. Quality. of LB Films Obtained after Transfer of Behenic Acid Monolayer onto Three Successive CaFt Substrates in Different Conditions endof no. substrate film dipping deposition area removed rate rateR~ crystals/ transfeP of (min) layers (cm2) area (cm2) (cm-min-1)(cmz-min-1) m m 2 Set a 40 13 6.3 78.3 2 6.5 0 75 13 6.3 79.8 2 6.1 9 110 13 6.3 80.9 2 6.5 18 Set b 30 7 6.3 40.3 2 6.1 0 55 7 6.3 41.4 2 6.1 3.6 75 7 6.3 40.5 2 6.1 24 Set c 30 7 12.7 85.1 2 12.2 3 55 7 12.7 88.0 2 12.2 11 75 7 12.7 85.0 2 12.5 45 Quality of LB films is expressed through number of crystals/ mm2. This time corresponds to the time ellapsed between the end of Compression and the end of transfer onto the different substrates at r = 32 mN-m-', G, = 0.25 au.
The same experiments were performed with the CaF2 substrates (Table 11). The crystal defects became visible during the aging time of the monolayer. In this case, however, the time of storage of the solution has no influence on the rate of appearance of defects; only the monolayer deposition rate has an effect. A comparison between the results in Table I and Table I1 shows that the higher the RD value, the faster the rate of defect appearance is, whatever the type of substrate, hydrophobicor hydrophilic, and that more crystals appeared in LB films on CaF2 substrates. To assess the role of the monolayer deposition rate, the substrate area was changed and other parameters such as breaking of the monolayer, total area of removed film, and time of experiment were also taken into account. Considering the influence of storage of the spreading solution when the monolayer was transferred onto an hydrophobic substrate, the following studies were performed with the CaF2 substrates only. In Table 111,the number of crystals/mm2found in LB films obtained in different conditions is given. The corresponding values obtained at different times exemplifies their rate of appearance. For the same monolayer deposition rate (RD)(comparing sets a and b), when the total area of lipid film removed was higher due to a higher
Langmuir, Vol. 9,No. 11, 1993 3109 Table IV. Quality. of LB Films Obtained after Transfer of Behenic Acid Monolayer onto CaFa in Different Conditions endof no. substrate film dipping deposition trade+ of area removed rate rate& crystals/ (min) layers (an2) area(cm2) (cm-min-9 (cm2.min-9 mm2 _____ Set a 5.8 50 7 40.5 1 3.3 0.03 6.2 85 7 44.2 1 3.5 0.23 Set b w 7 11.1 72.5 1 6.6 0.4 86 7 11.4 87.1 1 6.9 3.1 Set c 50 14 5.9 81.7 2 6.3 1.2 90 14 5.9 86.2 2 6.5 49.0 Set d 50 14 11.7 153.5 2 12.0 6.3 90 14 11.5 156.8 2 12.1 100.5 a Quality of LB f i i is expressed through number of crystals/ mm2. This time corresponds to the time ellapsed between the end of compression and the end of transfer onto the different substrates at r = 32 mN.m-', G, = 0.25 au.
number of deposited layers (set a), the rate of defect appearance was not significantly modified although the total experiment time was not identical. Thus, whatever the removed film total area, the rate of appearance of defects is similar when RD is constant. To increase 2-fold the values of RD keeping constant the same total film area, first, a small area of substrate (ca. 6 cm2)was coated with 13 layers (set a) and, second, a larger area of substrate (ca. 12 cm2)was coated with 7 layers of behenic acid in a shorter time (set c). When the deposition rate was higher (set c), the rate of appearance of crystals increases although the total experiment time was shorter and the breaking of monolayer less important. To eliminate the effects of the monolayer instability, two sets of experiments were performed during identical times. The RD variation was obtained with coating two substrates of different areas by the same number of layers (sets b and c). Again, the defects appeared with faster kinetics when the monolayer was deposited with a faster deposition rate (set c). In order to demonstrate the respective influence of the deposition rate and of the dipping rate, four sets of experiments were performed, as shown in Table IV. In sets a and b, with the dipping rate equal to 1 cm-min-', two different substrate areas were coated leading to two different deposition rates. It can be seen in this case that the number of crystals which appeared uersua time is related to the value of RD. The same conclusion can be drawn from sets c and d with the dipping rate equal to 2 cmmin-'. So, whatever the value of the dipping rate, a decrease of RD induces a slowdown of the appearance of crystal defects. Now, comparing sets b and c, it appears that the dipping rate modifies the rate of appearance of defects only when the deposition rate is constant, but its influence is lower than that of the deposition rate. In conclusion, all these results clearly demonstrate that the depositionrate is really an important parameter which predominates over the dipping rate. It must be pointed out that if both the dipping and the depositionrates are reduced (set a), the rate of appearance of crystal defects is strongly slowed down.
Discussion In order to obtain high-quality LB films, the influence of both the monolayer deposition rate and the moleculesubstrate interactions on the rate of appearance of crystal
3110 Langmuir, Vol. 9,No. 11,1993 defects was studied in conditions previously determined, where the instability of the monolayer was limited. The study of aging time of the monolayer showed that beyond 2 h, the monolayer lost its integrity with a visible collapse and that some crystal defects appeared in the LB f i i s . Thus, the time of experiment needed to investigate the influence of the monolayer deposition rate and the moleculesubstrate interactions never exceeded this time. The nature of molecule-substrate interactions involved in the transfer of the first layer was investigated using hydrophilic and hydrophobic substrates. For both types of substrates, studies of the rate of crystal defects appearancein the LB f i i revealed that the crystal defects became visible as a function of aging of the monolayer and that the kinetics were accelerated when the monolayer deposition rate was higher. In fact this rate, defined as the lipid area deposited onto the substrate within 1min, controls the speed motion of the compression barrier. Moreover, the direct influence of the monolayer compression on both the monolayer integrity and on the cryatal defects appearance was previously demonstrated.' Thus, if the monolayer deposition rate is higher, the removed area within 1min will be larger, the barrier advance will increase, and, consequently,the kinetics of crystal defects appearance will increase too. The effect of the monolayer deposition rate is the same whatever the type of substrate. Nevertheless, the kinetics of appearance of crystals differs in two points according to the type of substrate. First, when the monolayer was transferred onto an hydrophilicsubatrate,the kinetics of appearance of crystals is independent of the time of storage of the spreading solution. On the other hand, with a hydrophobicsubstrate, the time of storage of the solution masks the effect of the monolayer deposition rate and leads to a higher number of crystals. After generation of these defects in the first layer due to miss-interaction, the epitaxy involved in the build-up of LB films keeps these defects visible after deposition of several layers. The second point concerns the differencein the number of crystals found in the LB films according to the type of substrate. When a hydrophilic interaction controls the deposition of the f i s t layer, the number of crystals is higher. Recently, Steitz et aL2 tested the idea that the molecular packing in the monolayer before and after deposition was different depending on the type of the substrate and of deposition conditions and that changing conditions at the monolayer surface can affect its internal surface pressure. According to these authors, when the behenic acid monolayer is deposited at 30 "em-l from the L'z phase, in which the molecules are tilted at the water surface, a vertical packing of molecules in the monolayer deposited onto a Formvar substrate is obtained. Considering first that this vertical packing would occur on the water surface at inaccessible pressures and second that the Formvar substrate is less hydrophilic than water, these authors conclude that the change in interfacial tension at the carboxylic headgroups must be in excess of 25 mN.m-', when water is replaced by Formvar. This change in effective surface pressure in the deposition monolayer could support our results. As the CaF2 sub-
Girard-Egrot et al. strate is also less hydrophilic than water, a higher surface pressure occurring in the deposited monolayer might explain the accelerated kinetics of appearance of crystal defects. According to our results, for an hydrophobic substrate, the difference of the internal surface pressure between the monolayer on water and on substrate is probably leas important. Furthermore, the difference in the number of crystals obtained on two substrates with different kinds of surface is coherent with the idea that the monolayer structure is not entirely preserved on deposition. In the literature, the dipping rate is considered as an important parameter for the build-up of LB f i i s . In this work, we suggest that the role of the deposition rate is preeminent. In fact, the monolayer deposition rate depends not only on the substrate dipping rate but also on the coated area. By varying this area, different monolayer areas were removed with more or less breaking. If the monolayer deposition rate is kept constant, there is no effect on the rate of crystal defects appearance. On the other hand, although the substrate dipping rate poised at 2 cm-min-l is below the maximum drainage speed (1.6 c m d for behenic acid6),when this dipping rate was decreased to 1 cmomin-l and the monolayer deposition rate was kept constant, a consecutive slowing down of appearance rate of defects was produced. So,the enhanced effect due to the hydrophilic substrate surface on the rate of crystal defecta appearance seems to be compensated by a decrease of the dipping rate. At a constant monolayer deposition rate, the influence of the dipping rate provides a weak confirmationthat the transfer might slightlymodify the monolayer structure. Conclusion This study points out the influence of a new parameter on the rate of appearance of defects in the LB films, referred here as the monolayer deposition rate (RD), expressed in cm2.min-'. It takes into account not onlythe dipping rate but also the coated area on the substrate. It corresponds to the actual velocity at which molecules are removed from the water surface; it controls the speed of the monolayer compression and, consequently, influences directly the rate of appearance of potantial sites of crystal formation in the monolayer at air-water interface. During the transfer of the monolayer onto the substrate, the number of potential sites of crystal formation can be modified (sometimes enhanced) according to the type of molecule-substrate interaction. Further, the enhancing effect due to a hydrophilic substrate as CaF2 can be compensated by a decrease of dipping rate. The quality and integrity of the monolayer are obviously the main factors required to obtain high-qualityLB f h . From this study, this new parameter,RD,appears essential for the monolayer integrity. Acknowledgment. The research was partly supported by CNRS-ULTIMATECH and R6gion Rh6ne-Alpes. (4) Vandevyver, M.; Barraud, A. J. Mol. Electron. l988,4, 207. (5) Veale, G.; Girling, I. R.; Peteraon, I. R. Thin Solids Film 1986,127, 293.
