In Situ Characterization of Langmuir-Blodgett Films during a Transfer

Langmuir-Blodgett films of cadmium octadecanoate and other amphiphiles were transferred on ... Abstract published inAdvance ACSAbstracts, August 15,19...
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Langmuir 1994,10, 3255-3259

3255

In Situ Characterization of Langmuir-Blodgett Films during a Transfer Process. Evaluation of Transfer Ratio and Water Incorporation by Using a Quartz Crystal Microbalance1,2 Katsuhiko Ariga3 and Yoshio

Okahata*

Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama 227, Japan Received March 7,1994. I n Final Form: June 6,1994@ Langmuir-Blodgett films of cadmium octadecanoate and other amphiphiles were transferred on a quartz crystal microbalance (QCM, 9 MHz, AT-cut) as a substrate with a vertical dipping method. Frequencies of the QCM substrate were followed with time in air, after the QCM was raised from the interface. The frequency was gradually increased (mass decreased) with time and reached the equilibrium in air, due to the evaporation of water incorporatedbetween layers during a transfer process. From the time courses of these frequency changes at each dipping cycle, the transfer amount of dry LB films (Wl), the incorporated amount ofwater (Wz), and its evaporation speed ( u ) could be obtained at the nanogram level. The LB films transferred on the rougher surface at the lower surface pressure with the higher lifting speed generally showed the greater amount of incorporatedwater (Wz) and the higher evaporation speed ( u ) , which means the disorderedLB films were deposited on the substrate. When the well-oriented LB films were transferred on the substrate, a small amount of water was incorporated and the evaporation speed was small. Both Wz and u values were also affected by chemical structures of amphiphiles: the amount of incorporated water (Wz) increased with increasing polarity of hydrophilic head groups, and the evaporation speed ( u ) decreased with increasing alkyl chain length. Thus, a QCM system will become a useful tool to analyze LB films during the transfer process in situ.

Introduction Interest in Langmuir-Blodgett (LB) films is widespread, and formation of ordered thin organic films by transforming lipid monolayers from a water surface is well The characterization of LB multilayer films has been studied usually in the dry state by various methods such as FT-IR spectroscopy, X-ray diffraction, ellipsometry, and X-ray photoelectron spectro~copy.~-' Recently, the direct observation of the monolayer on the water subphase has been given by fluorescent micros c ~ p y , ~ X-ray - l ~ diffraction, and electron microscopy techniques.11J2However, the in situ evaluation of LB films during a transfer process has not been fully explored.When a monolayer moves from a water subphase to a solid substrate, the behavior of water might be the key to control @

Abstract published inAdvanceACSAbstracts, August 15,1994.

(1)For the preliminary paper, see: Okahata, Y.; Ariga, K. J.Chem. SOC.,Chem. Commun. 1987, 1535. (2) Characterization of Langmuir-Blodgett Films 19. For part 18 of this series, see: Ariga, K.; Okahata, Y. J . Colloid Interface Sci., in

press. (3) Current address: Supermolecules Project, JRDC, 2432 Aikawacho, Kurume, Fukuoka, 830 Japan. (4) (a)Blodgett, K. B. J.Am. Chem. SOC.1936,57,1007. (b)Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51, 964. ( 5 ) Swalen, J. D. J . Mol. Electron. 1986,2, 155. (6) Sasanuma,Y.; Kitano, Y.; Ishitani, A.; Nakahara, H.; Fukuda, K. Thin Solid Films 1990, 190, 325. (7) Uchida, M.; Tanizaki, T.; Oda, T.; Kajiyama, T. Macromolecules 1991,24, 3238. ( 8 )(a) Mohwald, H. Angew. Chem., Int. Ed. Engl. 1988,27,728. (b) Losche, M.; Mohwald, H. Eur. Biophys. J. 1984,11, 35. (9) Weis, R.; McConnell, H. J . Phys. Chem. 1986, 89, 4453. (10)(a) Shimomura, M.; Fujii, K.; Karg, P.; Frey, W.; Meller, P.; Ringsdorf, H. Jpn. J.Appl. Phys. 1988,27, L1761. (b)Shimomura, M.; Fujii, K.; Shimomura, T.; Oguchi, M.; Shinohara, E.; Nagata, Y.; Matsubara, M.; Koshiishi, K. Thin Solid Films 1992,210/211, 98. (11)(a)Kajiyama, T.; Umemura, K.; Uchida, M.; Oishi, Y.; Takei, R. Chem. Lett. 1989,1515. (b)Kajiyama, T.; Hanada, I.; Shuto, K.; Oishi, Y. Chem. Lett. 1989, 193. (c) Kajiyama, T.; Oishi, Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y. Bull. Chem. SOC.Jpn. 1992, 65, 864. (12) Kato, T.; Ohshima, K. Jpn. J. Appl. Phys. 1990,29, L2102

the film quality. It has been reported that a subtle change of the thin water layer under the phospholipid monolayer during the transfer process caused the cry~tallization.'~ The polarized micrograph of the 22-tricosanoic acid LB film showed anisotropy due to water flow.I4 Water content must affect the electrical and the optical properties of LB films. For practical purposes, the drying time of LB films in air during transfer processes has been determined through our own experience and feeling. In this paper, we characterize in situ vertical transfer processes of LB films of cadmium octadecanoate and other amphiphiles by using a quartz crystal microbalance (QCM) as a dipping substrate. The experimental setup is shown in Figure 1. QCMs are known to provide very sensitive mass measuring devices because their resonance frequency changes upon the deposition of a given mass on the e1e~trode.l~ A transfer amount of dry LB films (W1I ng), an incorporated amount of water during a lifting process (Wdng), and an evaporation speed of the water under drying in air (vlng s-l) are obtained from time courses of the frequency change ofthe QCM a t each dipping cycle. The effects of transfer conditions such as surface pressure, number of layers, and dipping speed and chemical structures of amphiphiles on these values (W1, WZ,and v ) are studied. QCMs are widely employed as sensor devices such as for gas sensing,16trace ion detection,17detection of odor compounds and other bioactive compounds,laimmunoas(13) (a) Riegler, H.; Spratte, K. Thin Solid Films 1992,210/211, 9. (b) Riegler, H.; LeGrange, J . D. Thin Solid Films 1990, 185, 335. (14) Peterson, I. R. Thin Solid Films 1984, 116, 357. (15) Sauerbrey, G. 2.Phys. 1959,155, 206. (16) (a) King, W. H., Jr.Ana1. Chem. 1964,36,1735. (b) Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977,49, 1890. (17)Nomura, T.; Iijima, M. Anal. Chim. Acta 1981, 131, 97. (18)(a) Okahata, Y.; Ebato, H.; Taguchi, K. J. Chem. Soc., Chem. Commun. 1987, 1363. (b) Okahata, Y.; Shimizu, 0. Langmuir 1987, 3, 1171. (c)Okahata, Y.; En-na, 0.; Ebato, H. Anal. Chem. 1990, 62, 1431. (d) Okahata, Y.; Ebato, H. Trends Anal. Chem. 1992,11, 344. (e) Ebara, Y.; Okahata, Y. Langmuir 1993, 9, 574.

Q743-7463/94l2410-3255$04.5Q/Q 0 1994 American Chemical Society

3256 Langmuir, Vol. 10,No. 9,1994 Quartz-crystal microbalance(QCM)

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Ariga and Okahata

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LB film-forming amphiphiles

CH3(cb)1f l

Figure 1. Experimental setup for vertical dipping processes of monolayers on a quartz crystal microbalance (QCM) substrate.

say,19 DNA hybridization,20enzyme reaction,21 surface analysis,22gelation m ~ n i t o r i n gliquid , ~ ~ chromatographic detection,24and electrochemical a n a l y ~ i s . ~QCMs ~ - ~ ~are also applied for the detection of phase transition28and swelling behavior29 of organic thin films or LB films. However, there are only a few preliminary uses of the QCM plate to analyze the transfer processes of LB films in

Experimental Section Langmuir-Blodgett Films. Hexadecanoic acid, octadecanoic acid, icosanoic acid, docosanoic acid, octadecanol, ethyl octadecanoate, and octadecanamide were purchased as analytical grade chemicals. Cadmium chloride and 1,1,1,3,3,3-hexamethyldisilazane were purchased as guaranteed grade reagents. Water for the subphase was purified by a Milli-QII system (Nippon Millipore, Ltd., Tokyo) and poured directly into a trough. The specificresistance of the water was ca. 18MQ cm. Measurements of presssure-area (n-A) isotherms and transfers of monolayers (19)Thompson, M.; Arthur, C. L.; Dhaliwal, G. KAnal. Chem. 1986, 58 1206. 420)(a)Okahata,Y.; Matsunobu,Y.; Ijiro, R;Mukai, M.; Murakami, A.; Makino, K. J . Am. Chem. SOC.1992,114,8299. (b) Okahata, Y.; Ijiro, K.; Matsuzaki, Y. Langmuir 1993,9, 19. (c) Yamaguchi, S.; Shimomura, T.; Tatsuma, T.; Oyama, N. Anal. Chem. 1993,65,1925. (d)Ebersole, R. C.; Miller, J.A.; Moran, J. R.; Ward, M. D. J .Am. Chem. SOC.1990,112,3239. (21)Okahata, Y.; Ebara, Y. J . Chem. SOC.,Chem. Commun. 1992, 116. (22)Yang, M.; Thompson, M. Langmuir 1993,9,1990. (23)Muramatsu, H.; Suzuki, M.; Tamiya, E.; Karube, 1.Anal. Chim. Acta 1988,215,91. (24)Konash, P.L.;Bastiaans, G. L. Anal. Chem. 1980,52,1929. (25)(a)Bruckenstein, S.;Shay, M. J . Electroanal. Chem.Interfacial Electrochem.1985,188,131.(b) Bruckenstein, S.;Wilde, C. P.; Shay, M.; Hillman, A. R.; Loveday, D. C . J . Electroanal. Chem. Interfacial Electrochem. 1989,258,457. (26)(a)Ebersole, R.; Ward, M. D. J .Am. Chem.Soc. 1988,110,8623. (b) Ward, M. D. Buttry, D. A. Science 1990,249,1000. (27)Denkin, M. R.; Buttry, D. A.Anal. Chem. 1989,61,1147. (28)(a)Okahata, Y.; Kimura, K.; Ariga, K. J . Am. Chem. SOC.1989, 111,9190.(b) Okahata, Y.;Ebato, H. Anal. Chem. 1989,61,2185. (c) Muramatsu, H.; Kimura, K. Anal. Chem. 1992,64,2502. (29)(a)Okahata,Y.; Ariga, K. Langmuir 1989,5,1261.(b)Okahata, Y.;Ariga, K. Thin Solid Films 1989,178,465. (30)McCaRrey,R. R.;Bruckenstein, S.;basad, P. N.Lungmuir 1986, 2,228.

on a substrate were carried out by using a computer-controlled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD20).18e928p29 The maximum surface area on the trough was 475 x 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 f 0.5 "C). The concentration of lipid solutions was 1 mg/mL, and the spreading amount of lipid solutions was 50-150 pL. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-l. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically in the usual manner . 2 8 3 Quartz Crystal Microbalance (QCM). AT-cut, 9 MHz quartz crystal oscillators were purchased from Kyushu Dentsu, Co., Tokyo, in which Agelectrodes (0.238 cm2)had been deposited on each side of a quartz plate (0.640 cm2). A homemade oscillator circuit was designed to drive the quartz at its resonant frequency in both air and water phase^.^^^^^ The quartz crystal plates were usually treated with 1,1,1,3,3,3-hexamethyldisilazane to obtain a hydrophobic surface unless otherwise stated.31 Frequencies of the QCM were followed continuously by a universal frequency counter (Iwatsu, Co., Tokyo, SC 7201 model) attached to a microcomputer system (NEC, PC 8801 model). The following equation has been obtained for the AT-cut shear mode QCM?

where AF is the measured frequency shift (Hz), 3'0 the parent frequency of the QCM (9 x lo6Hz). Am the mass change on the electrode (g),A the electrode area (0.238 cm2),e, the density of quartz (2.65 g cm-3), and p, the shear modulus of quartz (2.95 x loll dyn cm-2). Thus, the frequency decreases linearly with increasing mass on the electrode area of the QCM. Calibration of the QCM used in our experiment by a polymer-casting or LB film-depositing method gave the following equation. 18928929

Am = -(1.27 f 0.01)x

lo-'

hF

(2)

I t is close to the theoretical equation calculated from eq 1(Am = -1.30 x AF). The stability of the QCM frequency was also examined. The standard deviation of frequencies was ca. 0.5 Hz (0.6 ng) and no frequency drift was confirmed by a statistical method with 95% confidence.

Results and Discussion Typical time courses of frequency changes of the QCM substrate in air during four cycles of vertical dipping processes are shown in Figure 2. The QCM was lowered into the subphase at point A and raised in air at point B with a dipping speed of 100 mm min-l through the cadmium octadecanoate monolayer (20 mN m-l, 20 "C). (31)Farriss, G.;Lando, J.; Rickert, S. J . Mater. Sci. 1983,18,2603.

Characterization of Langmuir-Blodgett Films

. 2 1 n F - l 5

u 1.0

c '0

2 4 6 8 10 0 2 4 6 8 10 0 2 L 6 8 10

Number of Transfer Cycle Figure 3. Total transferred weight (E W1) of cadmium octadecanoate LB films calculated from the QCM method ( 0 )

Langmuir, Vol. 10,No.9,1994 3257

7

-

'0246810

Number of Layers

and the barrier movement (A)during repeated depositions at surfacepressures of(a)20, (b)10,and (c) 5 mN m-l. Solid lines represent the theoretical values calculated from the n-A isotherms (dipping speed, 100 mm min-', 20 "C).

The frequency of the QCM in air gradually increased with time and reached a constant value in 15 min a t point C. From the decrease of frequencies of 183 f3 Hz from points A to C, the increased mass with each cycle was calculated to be Wl = 232 f 3 ng, according to eq 2. This value was consistent with the theoretical mass of four dry monolayers (two layers on each side) of cadmium octadecanoate (225 ng) on the Ag electrode of the QCM, which was calculated from the average area per molecule in the monolayer (0.237 nm2from a n-Aisotherm) and the area of the Ag electrode (0.238cm2). Thus, the frequency decrease is affected only by the masp on the electrode area of the quartz plate. The gradual frequency increase from points B to C is explained by the mass decrease due to the evaporation of water deposited between layers from the subphase. The amount of incorporated water (Wdng)and its evaporation speed ( u h g min-l) were calculated from the frequency change and the initial slope of the time course between points B and C, respectively. The cadmium octadecanoate LB films were observed to incorporate Wz = 209 f 5 ng of water with four layers of LB films (W1 = 232 f 3 ng) a t the first dipping cycle: cadmium octadecanoate was transferred on a substrate with the water of almost the same mass of LB films. Figure 2 also indicates that we should wait ca. 15 min to obtain the dry LB films during transfer processes in these conditions. When the next deposition was carried out before the complete evaporation of water, a transfer ratio of LB films was gradually decreased from 0.9to 0.7 with increasing dipping cycles. Transfer Ratio of LB Films. The transfer process was repeated at least 10 times at different surface pressures (20, 10, and 5 mN m-l). The total transferred weight (XWl) of dry cadmium octadecanoate LB films is plotted against the number of transfer cycles as closed circles in Figure 3. The amount of transferred films was also estimated from the conventional method calculated from a moving area of a barrier, in which the surface pressure was kept constant, and plotted as closed triangles in Figure 3. Straight lines indicate the theoretical mass of two Y-type layers on each side of the QCM. At the high surface pressure of 20 mN m-l, the transferred weight obtained from both the frequency changes of the QCM substrate and the barrier movement was almost equal to the theoretical line, and the obtained transfer ratio was 1.01 f 0.02. McCaffrey et aL30 had reported the deposition of cadmium octadecanoate LB films on a QCM plate and obtained the similar linear correlation between the frequency change and the number of transfer cycles. However, the mass associated with each layer was 20% larger than the theoretical mass of

Number of Layers Figure 4. Effect of number of layers on the amount of incorporated water (WZ) and the evaporationspeed ofthe water (LJ)in the transfer of cadmium octadecanoate LB films. The hydrophilic ( 0 )and hydrophobic (0)substrates were used in this experiment (surface pressure, 20 mN m-l, dipping speed, 100 mm min-', 20 "C). the dry LB films, probably because they deposited LB films with the water lified still incorporated in them. In this study, the weight calculated from the QCM method was also in accord with the value from the conventional barrier movement, which means all the disappeared monolayers from the air-water interface were transferred on the substrate under this condition. At the low surface pressures of 5 and 10 mN m-l, the observed values showed the deviation from the theoretical lines and the transfer ratios were ca. 0.9-0.7. The transferred mass obtained from the barrier movement was smaller than that from the QCM method under these conditions. This means that the barrier motion on the surface cannot compensate the disappeared area of the transferred monolayer sufficiently at the low surface pressure. Thus, the QCM method is useful to estimate in situ the real transfer weight on the substrate in comparison with the conventional method even a t low surface pressure. Effect of Surface of the Substrate and Number of Layers. The amount of incorporated water (WZ)and its evaporation speed (v) were obtained as a function of dipping cycles in the transfer of cadmium octadecanoate LB films, and the results are shown in Figure 4. Both relatively hydrophilic and hydrophobic surfaces of the QCM were employed as a substrate, in which the former was a bare Ag electrode (contact angle for water: 50 f 5 9 , and the later was prepared by treatment with 1,1,l73,3,3-hexamethy1disilazane (contact angle for water: 110 f 5").31 The transfer ratio of LB films (Wd was 0.98f0.05for each dipping cycle on both hydrophilic and hydrophobic substrates. The monolayer could be transferred even on the hydrophilic bare electrode for the first downward process, because the Ag surface is not so hydrophilic (contact angle: 50 f 5"). The amount of incorporated water (WZ)and its evaporation speed (v) decreased as the number of layers

3258 Langmuir, Vol. 10,No. 9, 1994

Ariga and Okahata

f""

Table 2. Effect of Dipping Speeds on the Transfer of Octadecanol LB Films"

300 p

ob

Ib

dipping speed/" min-l 100 80 60 40 20 5

bQ-1 20

30

40

Surface Pressure / mN m" Figure 5. Effect of surface pressure on the amount of incorporated water (WZ) and the evaporation speed ( u ) in the transfer of cadmium octadecanoate LB films (dipping speed, 100 mm min-l, 20 "C, at the fifth transfer cycle).

a

Wdng 393 267 184 111 81 62

WBW,ll/ng

24 25 30 36 44 56

Wz-WSwell/ng

369 242 154 75 37 6

Surface pressure: 20 mN m-l, 20 "C, at the fifih transfer cycle.

Table 3. Effect of Hydrophilic Head Groups on the Transfer of Single-ChainAmphiphiles" amphiphiles Wdng v/ng min-1

Table 1. Effect of Dipping Speeds on the Transfer of Cadmium Octadecanoate LE! Films" dipping speedmm min-1 100 80 60 40

Wdng Ws,u/ng 209 186 212 262

40 50 66 95

Wz-WsWeii/ng

169 136 146 167

v/ng min-' 15.8 14.2 10.0 7.7

Surface pressure: 20 mN m-l, 20 "C, at the fifth dipping cycle.

increased in both hydrophilic and hydrophobic surfaces of the QCM. W Zand u values were particularly large a t the first cycle in the case of the hydrophilic surface. It has been shown that several layers on a substrate are disordered by the influence of the substrate surface, and this effect disappears as the number of layers inc r e a s e ~ . The ~~~ first ~ ~few , ~layers ~ seem to incorporate the large amount of water in the defects of LB films, and the water easily evaporates through the disordered monolayer. The hydrophobic alkyl chains contact with the substrate surface a t the first down stroke. Such a contact has a disadvantage in energy in the case of the substrate having a hydrophilic surface; then the first monolayer on the hydrophilic surface was particularly disordered and shows the large W Zand u values. The dependency of the evaporation speed ( u ) on the number of layers seemed to be larger than that of the amount of incorporated water (Wz), which means u values are more sensitive parameters reflecting the film disorder than the W Z values. When the larger number of layers was deposited, the longer drymg time was required due to the slower evaporation speed of water. Effect of Surface Pressure on Water Incorporation. The W Zand u values at the fifth dipping cycle were obtained a t various surface pressures, and the results are shown in Figure 5. Both Wz and u values increased with decreasing surface pressure. At the low surface pressures below 20 mN m-l, the LB films having many defects were transferred with a low transfer ratio below 1.0, and the large amount of water was incorporated in these defects, and its evaporation speed was fast through the disordered film. Effect of Dipping Speed. Wa and u values at the fifth transfer cycle of cadmium octadecanoate at 20 mN m-l as a function of dipping speed of the QCM substrate are summarized in Table 1. The transfer ratio of LB films (Wd was 0.95 f0.05 a t the dipping speeds of40-100 mm min-l. When the dipping speed was decreased, the evaporation speed ( u ) decreased: the well-oriented LB films could be obtained at the low dipping speed. This is consistent with the report by Pitt et that the lower (32) Tachibana, T.; Fukuda, K. Bull. Chem. SOC.Jpn. 1951, 24, 4. (33) Isemura, T.Bull. Chem. SOC.Jpn. 1940,15,467. (34) Pitt, C. W.; Walpta, L. M. Thin Solid Films 1980,68, 101.

transfer speed was favorable to obtain the higher quality LB films in the first 10 layers. The incorporated amount of water (WZ) seems to increase with decreasing dipping speed. Since the W Zvalue may include both the really incorporated water and the swelling with water when the substrate exists in the water subphase, the effect of dipping speed on W Zvalues should be divided into two factors. We have already reported that cadmium octadecanoate LB films swelled largely in the subphase, compared with other LB films.29 The swelling amount (Wswen) was separately obtained from the frequency decreases (mass increase) when the LB filmdeposited QCM was soaked for a time in the subphase calculated from each dipping speed. The (WZ- Wswell) value reflects the true amount of water pulled into the outer layer during the lifting-up process and was almost independent of dipping speeds of 40-100 mm min-l (see Table 1).The Wz value seems to increase with decreasing dipping speed due to the swelling amount. In the case of the transfer of octadecanol monolayers, the different results were obtained as shown in Table 2. The (WZ - Wswen) value decreased largely with decreasing dipping speed for the octadecanol LB films. The (WZ Wswell) value for octadecanol LB films was more than 2 times larger than those for cadmium octadecanoate (369 and 169 ng a t the dipping speed of 100 mm min-l, respectively), which indicates the OH head groups interact with water strongly compared with the COO- head groups (see the latter section). Therefore, the effect of dipping speed on the incorporated amount of water depends on the hydrophilic head groups of LB films. Effect of Lipid Structures. The W Zand u values were obtained for various single-chain amphiphiles with the same chain length (CIS)and the different hydrophilic head groups, and the results are summarized in Table 3. All LB films could be transferred with the transfer ratio of 1.0 f 0.1 under these conditions. The octadecanol LB film showed particularly large Wz and u values, which means that much water was incorporated into the octadecanol LB film and the water was easily evaporated. The OH groups of octadecanol seem to form the hydrogenbond network in the subphase. The LB films transferred with much water might have defects and disorders in LB films, which make the evaporation speed large. The LB films except octadecanol showed tendencies for W Zto depend on their hydrophilicity of head groups and the u value to be independent of the head groups.

Characterization of Langmuir-Blodgett Films

Langmuir, Vol. 10,No.9,1994 3259

The W Zand u values for LB films prepared from various aliphatic acid cadmium salts having different alkyl chain lengths (c16- CZZ) were also obtained, and the results are shown in Figure 6. All LB films could be transferred with the transfer ratio of 1.0 f0.1 in these conditions. The W Z value was constant and independent of the chain length, but the u value decreased with increasing chain length. This indicates that the chain length oflipids mainly affects the evaporation speed of water in the LB interlayer and has no effect on the amount of the incorporated water. Thus, the incorporated water seems to exist near the hydrophilic head groups, so that the WZvalue depends on their hydrophilicity but not on the alkyl chain length. On the contrary, the evaporation speed u depends on the alkyl chain length but not on the hydrophilic head groups, since the rate-limiting process of evaporation of water is to pass through the hydrophobic alkyl-chain part. The evaporation speed is expressed in the following equation according to Fick's law, where the water evaporation is supposed to occur only from the outer layer, but not from the side part of LB

_u -- D A - w 60

- w, d

(n - 2)

.

s200tu . e

(4)

The curve fitting for the observed value was done with the least square method, and the obtained curve is shown a s a broken line in Figure 6. This model seems to reproduce the experimental value qualitatively. From this curve fitting, the apparent diffusion coefficient D = 9.7 x cm2 s-l was obtained. Since the diffusion coefficient of free water is 2.6 x 10-1 cm2s-l at 25 "C, LB films of the fatty acid cadmium salts are calculated to have the 142.7 x 10") free space for water diffusion to the whole area. The experimental value had a sharper slope than that calculated from the model, which means the diffusion coefficientD decreased as chain length increased. The change in quality depending on the chain length might also affect the water evaporation. According to eq 3, the evaporation speed depends on the humidity in air and is (35)Langmuir, I.; Schlaefer, V. J. J.Franklin Inst. 1943,235,119. (36)Arcker, R. J.; Mer, V. K. L. J. Phys. Chem. 1966, 59,200.

0

16 18 20 22

Acyl Chain Length

n

;I,,/ 16 18 20 22

(3)

where uJ60 (g s-l) is the evaporation speed, D the diffusion coefficient of water in LB film, A the cross sectional area of diffusion (the electrode area), w othe vapor pressure of water outside, w the pressure of water vapor equilibrating with balk water in the LB film, and d the thickness of the monolayer. The w oand w values can be calculated with 60% (outside) and 100% relative humidity (inside, saturated) a t 25 "C. The dependency of the film thickness d on acyl chain length n is expressed in the following equation on the assumption that the chain forms a trans zigzag conformation with the 2.54 A C-C spacing.

d (cm) = 1.27 x

crr 4001 S

Acyl Chain Length

n

Figure 6. Effect of alkyl chain length of aliphatic acid cadmium salt monolayers, [CH3(CH&zCOO-]z Cd2+,on the amount of incorporated water (WZ) and the evaporationspeed of the water (u). The broken line is calculated from eq 3 (surface pressure, 30 mN m-l, dipping speed, 100 mm min-I, 20 "C, at the fifth

dipping cycle).

independent of the amount of water remaining the LB films. Thus, the amount of water in the LB film should decrease by zero order with the amount of water. This is why the frequencies' increase due to the water evaporation was linear in Figure 2 (from point B to C).

Conclusions LB films of various lipids were transferred on a QCM as a substrate under various conditions. The mass of the transferred film (W1, the transfer ratio), the amount of incorporated water (Wz), and the evaporation speed (v) were evaluated from the frequency changes of the QCM during transfer processes in situ. We could estimate the deposition state and structures of LB films from these values. When the LB films were deposited a t the lower surface pressure and a t the higher dipping speed on the more hydrophilic surface, a smaller transfer ratio and a larger amount of incorporated water and a larger evaporation speed of the water were observed, which indicates the deposition ofthe disordered LB films. On the contrary, when the well-packed LB films are obtained, a good transfer ratio (W1) and small Wz and u values are observed. The WZ and u values also reflect the hydrophilic head groups and alkyl chain length of amphiphiles, respectively. A QCM system will become a useful sensor system to evaluate LB films during a transfer process in situ.