Langmuir 1994,10, 1225-1234
1225
Three-Component Langmuir-Blodgett Films with a Controllable Degree of Polarity Johan M. Berg,* L. G. Tomas Eriksson, Per M. Claesson, and Kari Grete Nordli Barvet The Surface Force Group, Department of Physical Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden, and Institute for Surface Chemistry, Box 5607, S-11486 Stockholm, Sweden Received April 26,1993. I n Final Form: October 19,199P Two different series of mixed Langmuir-Blodgett (LB) films with a controllable degree of polarity, deposited on mica, have been studied by wetting and surface force techniques. Both series contain 50% eicosylamine (EA). Films of one series consist of EA, arachidic acid, and docosandioic acid, while those of the other consist of EA, 1-eicosanol,and 1,22-docosandiol. Carboxylic acid groups give lower contact angles than hydroxy groups. Concerning the stability of the LB films in aqueous solutions, repeated exposure to a three-phase line and high salt solutions were found to cause breakdown. Surface force measurements on carboxylic acid-containing films show that films with a 0% (contact angle = 1 1 3 O ) and 25% (contact angle = 90°) content of diacid interact with a long-range (hydrophobic)attraction across water. No similar long-range attraction is observed for the 50% case (contact angle 65"). Surfaceforce measurements also detected instabilities and imperfections of the films.
Introduction During the last few years, several methods to obtain smooth organic surfaces composed of hydrophobic and hydrophilic groups in a controllable ratio have been reported. These methods include diffusion-controlled silanization of glass, resulting in a wetting gradient,' selfassembly of sulfur-containing surfactants onto gold2 and some other metals? and Langmuir-Blodgett (LB) deposition of mixed monopolar-bipolar insoluble monolayers onto mica.'v6 The scope of possible research applications for this type of model surfaces is very wide, e.g., wetting: adhesion: adsorption? biocompatibility? and intermolecular forces.' In this study, we present two types of LangmuirBlodgett films with a controllable degree of polarity, deposited on muscovite mica. Carboxylic acid and hydroxy groups on the outer surface of the LB films are responsible for the polarity. Both systems studied consist of three components: an amine, another monofunctional compound (carboxylic acid or alcohol), and a bifunctional compound (diacid or dialcohol). The amine provides anchoringof the monolayer to the negatively charged mica substrate. The use of a monofunctional compound together with the bipolar substance allows us to control the density of polar groups at the exposed surface of a Author to whom correspondence should be addressed. Department of Chemistry,University of Bergen, AllBgt. 41, N-6007 Bergen, Norway. Abstract published in Advance ACS Abstracts, March 15,1994. (1) Elwing, H.;W e b , S.;Askendal, A.; Nilason, U.; LundstrBm, I. J. Colloid Znterjace Sci. 1987,119, 203. ( 2 )Troughton, E. B.;Bain, C. D.; Whitesides, G.M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4, 365. (3) Ldbiis, P.E.; Whitesides, G.M. J. Am. Chem. SOC.1992,114, t Presentad&
1990. (4) Berg, J. M.; Claeeeon, P. M. Thin Solid F i l m 1989,178, 261. (6) (a) Hato,M.; Miamikawa, H.;Okamoto,K. Chem. Lett. 1991, 1049. (b)Hato,M.; Okamoto, K.; Minamikawa, H. J. Colloid Interface Sci. 1993, 161, 163.
(6) Adameon, A. W.Physical Chemistry of Surfaces., 5th ed.; John Wiley & Sons: New York, 1990. (7) %man, W . A. In Handbook of Adhesives; Skeist, I., Ed.; Van Noetrand New York, 1977; Chapter 3. (8) Elwing, H.;Mlander, C.-G. Ado. Colloid Interface Sci. 1990,32, 317. (9) Ratner, B.D.Adv. Chem. Ser. 1982, 199,9.
deposited monolayer (the monolayer/& interface) without changing the composition of the surface of the monolayer that is in contact with the water subphase and with mica, during and after deposition, respectively. We present wetting and stability studies of the films, as well as some surface force measurements. Work somewhat similar to the present one has recently been published by Hato and co-worker~.~ They used a double-chained quaternary ammonium compound with one of the chains carrying a terminal hydroxy group, mixed with arachidic acid in different ratios, giving ratios of polar groups in the surface of up to 33% . In another study by us,10 mixtures of eicosylamine with docosandioic acid or 1,22-docosandiol were investigated, with emphasis on the properties of the mixed monolayers themselves, not on the LB films with a controllable degree of polarity. That investigation demonstrates a clear difference in behavior between the amine-acid and aminealcohol systems, which can be rationalized in terms of headgroup interaction strength and an almost quantitative proton exchange between carboxylic acid and amine headgroups. For mixtures of amine and carboxylic acid, when the total number of carboxylic acid groups in the monolayer is less than the total number of amine groups, it was found that a considerable loss of monolayer area took place. This is clearly undesirable if one tries to prepare an LB film with a controllable composition and polarity. To avoid these side effects, it appears that the number of carboxylic acid groups must at least equal the number of amine groups. If one wishes to introduce a moderate amount of carboxylic acid groups on the outer surface, this necessitates the use of both monoacid and diacid together with the amine. This is what has been done in this study. The two-component study mentioned abovelo also contains some general considerationsabout the deposition of mixed, amine-containing monolayers on mica and about the interactions between molecules in mixed monolayers, which are relevant to this study as well. They will not be repeated here. ~
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(10) Berg, J. M.; Eriksson, L. G.T. Langmuir, preceding paper in this
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0743-7463/94/2410-1225$04.50/0 0 1994 American Chemical Society
1226 Langmuir, Vol. 10,No.4,1994
Berg et al.
Experimental Section Materials. Arachidic acid (AA) was received from Merck, Germany. Docosandioic acid (DDA), of 98% purity (gas chromatography),was received from Larodan, Sweden. Eicosylamine (EA) was synthesized (as the hydrochloric salt) by reducing eicosanamidewith lithium aluminum hydride. The eicosanamide was prepared by reacting AA with thionyl chloride, after which the resulting eicosanoyl chloride was reacted with aqueous ammonia. 1,22-Docosandiol(DDO)was synthesized by reducing DDA with diborane, using anhydrous tetrahydrofuran as solvent. All compounds synthesized in-house were purified by recrystallization, and their identity was verified by NMR. 1-Eicosanol (EO) was synthesized at Syntestjhst, Sweden. The chloroform was of analytical reagent grade and was distilled before use. KBr and CdClz were of analytical reagent grade. The water used in the experiments was first purified by decalcination, prefiltration, and reverse osmosis. The final purification was carried out by a modified Milli-Q unit, which included two mixed-bed ion exchangers, an activated charcoal cartridge, a 0.2-pm in-line filter cartridge, and a final 0.2-pm filter. The filters were Zetapore products, whereas all other cartridges were from Millipore. Green muscovite mica was obtained from Mica Supplies,UK, and Brown Mica Co., Australia. Surface Balance Experiments, Langmuir-Blodgett Depositions, and Contact Angles. Surface balance experiments and Langmuir-Blodgett depositions were performed as described elsewhere,1° as were some contact angle measurements (using the Wilhelmy plate method, at a speed of 5 mm/min). Some other contact angles in salt solutions were measured using a goniometer. The method used is specified in each figure legend. Under the conditions used, these two methods are expected to give comparableresults.lOJ1 LB fiis were deposited at a constant surface pressure of 30 mN/m, and the deposition speed was 5 mm/min. Surface Forces Measurements. The force versus distance curves, and the pull-off forces, were measured with a Mark I1 surface forces apparatus developed by Israelachvili and Adams.12 This apparatus allows the force between two mica substrate surfaces, in a crossed-cylinder configuration, to be determined as a function of surface separation. The surface separation is determined to within 2 A by using an interferometric technique, employing fringes of equal chromatic order. The force is determined from the deflection of a spring supporting one of the surfaces. The force (FJ between crossed cylinders, normalized by the local geometric mean radius (R), which is 10-20 mm, is related to the free energy of interaction per unit area between flat surfaces (Gf)at distance D via the Derjaguin approximati~nl~
This relation is valid provided that (i) the range of the intermolecular forces (400A) is much less than the surface radius (- 1cm) and (ii)the shapes of the surfaces are not changed due to the action of the surface forces. The theoretical treatment by Parker and Attard" shows that for the rather small and slowly decaying force reported here, surface deformation effects are insignificant and eq 1 is valid. Whenever the gradient of the force with respect to the surface separation, dFJdD, exceeds the spring constant, the mechanical system is unstable. This instability causes the surface separation to change spontaneously until the next stable region has been reached.lb For instance, under the action of a van der Waals force,the surfaces may jump from a small separation to molecular contact. All experiments were performed a t room temperature (approximately 22 OC). (11)Yaminsky, V. V.; Claesson, P. M.; Eriksson, J. C. J. Colloid Interface Sci. 1993, 161,91. (12)Israelachvili,J. N.;Adams,G. E. J. Chem. SOC.,Faraday Trans. 1 1978, 74,975. (13)Derjaguin, B. V. Kolloid-2. 1934, 69,155. (14)Parker, J. L.; Attard, P . J. Phys. Chem. 1992,96, 10398. (15)Horn, R. G.;Israelachvili, J. N. J. Chem. Phys. 1981, 75,1400.
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Area (Az per molecule) Figure 1. P A isotherms for monolayersof the pure components docosandioicacid (DDA),1,22-docosandiol(DDO),arachidic acid (AA), 1-eicosanol (EO), and eicosylamine (EA) on water.
Results We report results obtained for two different threecomponent systems consisting of 50 mol % eicosylamine, a monofunctional C20 compound, and a bifunctional C22 compound. The carboxylic system, or (DDA + AA)/EA, consists of 0 4 0 % docosandioic acid (DDA), 5 0 % arachidic acid (AA), and 50% eicosylamine (EA). The hydroxy system, or (DDO + EO)/EA, consists of 0-60% 1,22-docosandiol (DDO), 50-0 7% 1-eicosanol (EO), and 50% eicosylamine (EA). As can be seen, the difference between the two systems studied is that the carboxylic acid groups (-COOH) of the (DDA + AA)/EA system are replaced with methylene alcohol groups (-CHzOH) in the (DDO+ EO)/EAsystem. The compmition of both systems is referred to as the mole fraction of bipolar compound (DDA or DDO). In general, this mole fraction refers to the initial composition of the monolayer, which may be altered to some extent by preferential loss of one component (usually the bipolar compound) from the monolayer. T-A Isotherms. Surface pressure (*)-area (A) isotherms for the pure components are shown in Figure 1. The bipolar compounds DDA and DDO do not form wellbehaved monolayers by themselves, but lose large amounts of monolayer material on compression, as can be seen from the low apparent molecular area for these compounds when they reach higher surface pressures. AA, EA, and EO, on the other hand, form well-defined monolayers in which a solid condensed phase is visible at the molecular area typical for close-packed hydrocarbon chains (-20 A2), on a pure water subphase of room temperature. All three monofunctional compounds show a liquid condensed phase in pure monolayers, over an increasing mean molecular area range in the order EA < EO < AA, and up to a maximum surface pressure which also increases in the same order. Figure 2 shows typical surface pressure-area isotherms of the carboxylic system for different DDA ratios. The isotherm for 0% DDA (1:lAA/EA) exhibits an expanded phase at large areas and a solid condensed (SC)phase at smaller areas. The extrapolated area of the SC phase, approximately 20 Az/molecule, indicates that there is no appreciable loss of monolayer material during the compression of the monolayer. The same phases are also seen in the isotherms for monolayers with DDA ratios up to 50%. For increasing DDA ratios, there is a tendency toward lower mean molecular areas for the solid condensed phase, indicating increasing loss of monolayer material.
Controllable Polarity LB Film I
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Figure 2. P A isotherms for carboxylic system monolayers, (DDA + AA)/EA, at different DDA ratios. Also shown is a 0% DDA (1:l AA/EA)isotherm where the two components were spread separately. The isotherm for 25% DDA represents the average mean molecular area for two recorded isotherms.
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Figure 4. Area per molecule at u = 30 mN/m for carboxylic and hydroxy system monolayers. The line corresponds to a loss of one hydrocarbon chain for each molecule of bipolar substance added.
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Figure 3. P A isotherms for a few hydroxy system monolayers,
Figure 5. Relaxation data, as A/Ao versus time, for carboxylic system monolayers of different DDA ratios at a constant surface pressure of 30 mN/m, and for the pure components AA and EA.
Isotherms for 0% and 50% DDA monolayers are shown up to collapse. Also included in Figure 2 is an isotherm at 0% DDA (1:l AA/EA) where the two components (Le., arachidic acid and eicosylamine) were spread separately, waiting 8 min after spreading the AA solution before spreading the EA solution, and the same time before starting to compress the monolayer. In this case, there is a prominent liquid condensed phase not seen when using a mixed spreading solution, highly reminiscent of that seen for pure AA (Figure 1). The isotherms of hydroxy system monolayers at some different DDO ratios are shown in Figure 3. At all DDO ratios there is a solid condensed phase present at high surface pressure. However, the isotherms at a very low DDO ratio (including some isotherms not shown in Figure 3) also display a liquid condensed phase at low surface pressure. Also, in the case of the hydroxy system it was observed that the loss of monolayer material increased somewhat with increasing DDO content. Figure 4 shows the mean molecular area at a surface pressure of 30mN/m for various monolayer compositions. Relaxation at Constant Surface Pressure. Figure 5 shows the monolayer relaxation isotherms (loss of monolayer area) at a constant surface pressure, ?r = 30 mN/m, for some carboxylic systems in the 0-50% DDA range together with the corresponding curves for pure AA
and pure EA. Data are expressed as A/Ao versus time t, where A0 is the area of the monolayer after compression to the constant surface pressure and A is the area of the monolayer at time t. The monolayers, except for the pure AA, are remarkably stable, none of them decreasing their area more than about 1.5% during 2 h of relaxation. Note that the 1:l mixture of AA and EA is more stable than any of its pure components. From the shape of the relaxation isotherms it appears that the relaxation mechanisms for all mixed (DDA + AA)/EA monolayers are an initial reorganization, followed by dissolution into the subphase. For pure AA (see also an earlier study by our group9 and EA, however, the mechanism is nucleation and growth of nuclei. However, for the lowest DDA ratios it is hard to draw any conclusionsregarding the relaxation mechanism, due to their very slow relaxation. As a general trend, the relaxation rate increases with increasing DDA ratio. This can also be seen in Figure 7, where the relaxation rate d(A/Ao)/dt at t = 30 min, i.e. after the reorganization process is complete, as a function of DDA ratio is plotted. The area relaxation of hydroxy system monolayers with DDO ratios of 0%,43.75 5%,and 50%,at a constant surface pressure of ?r = 30 mN/m, is shown in Figure 6 together with pure EA. The loss from the monolayers seems to be
(DDO + EO)/EA.
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(16) Pezron, E.;Claesson, P.M.; Berg, J. M.; Vollhardt, D.J. Colloid Interface SCL1990, 138, 245.
Berg et al.
1228 Langmuir, Vol. 10, No. 4,1994 1 .o
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Table 1. Transfer Ratios ( 7 ) for Multilayer Depositions of Carboxylic Systems (DDA + AA)/EA, Films at Various DDA Contents
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Figure 6. Relaxation data, A/Ao, as a function of time, for hydroxy system monolayers of different DDO ratios at a constant surface pressure of 30 mN/m, and for the pure EA. 1000
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layer transfer ratios at the following DDA content of monolayer no." 0% 25% 37.5% 43.75% 50% 1. t 0.92 0.98 0.97 1.00 0.98 2. 1 1.02 0.93 0.90 0.72 0.18 3. t 1.02 1.06 1.15 1.09 1.09 4.1
