1796
Langmuir 1999, 15, 1796-1801
Surface Characterization of Octadecylamine Films Prepared by Langmuir-Blodgett and Vacuum Deposition Methods by Dynamic Contact Angle Measurements Yuh-Lang Lee* Department of Applied Chemistry, Chia-Nan College of Pharmacy and Science, Tainan 717, Taiwan, Republic of China Received August 24, 1998. In Final Form: December 17, 1998 The behavior of octadecylamine (ODA) monolayers on various subphases is investigated. The monolayer is then transferred to a hydrophilic glass surface to prepare a LB film of one monolayer. The uniformity, stability, and molecular orientation of the LB films were studied by the measurement of dynamic contact angle and compared with characteristics of vacuum-deposited films. The results show that the ODA monolayer is especially stable on alkaline solution. The LB film transferred from the alkaline solution is stable and uniform in wettability. The advancing and receding contact angles of water on one layer of ODA film are about 113° and 62°, respectively. These values are the same as those on a vacuum-deposited ODA film of equivalent thickness. The growth of ODA on a glass surface in a vacuum deposition is thus found to follow a mode of layer growth. The orientation and arrangement of molecules on the LB films are affected by the surface pressure and transferring rate during the transferring procedure. When the transfer is proceeded at low surface pressure and high transferring rate, the ODA molecules are irregularly packed without uniform orientation. However, during the analysis of dynamic contact angle (DCA), the movement of the three-phase contact line has an effect to rearrange the irregularly packed molecules of ODA on the LB films and the surface becomes more and more uniform with the analytic cycles of the DCA. By comparing between the LB films of ODA and stearic acid, the advancing contact angles are identical for both, but the very small value of receding contact angle on stearic acid reflects the exposure of the hydrophilic glass on the stearic acid film. The existence of holes in the stearic acid monolayer is thus proved by the measurement of wettability.
1. Introduction The use of thin organic films has brought more and more interest both as physical objects and as elements for microelectronic devices.1,2 Such films are being used as photoresists, electron beam resists, photoconductors, liquid crystal displays, gas sensors, and protective coatings and for other purposes of surface modifications. Till now, much of the work on these types of films has been with polymeric and amorphous organic thin films in which pinholes and defects have always existed. However, as the thickness of films becomes smaller and smaller in fabrication of very large-scale integration circuits, the need for more ordered and well-defined films arises. The Langmuir-Blodgett (LB) method of organic film deposition provides wide possibilities for preparing highly ordered, densely packed, and defect-free molecular films.3,4 Contrary to the “wet” procedure of the LB method, the vacuum deposition (VD) method, which has been extensively used to prepare inorganic thin films of perfect structure, is also used as an alternative method to obtain epitaxial growth of organic compounds. In particular, the structure and ordering of VD films of amphiphilic organic compounds or fatty acids were investigated previously.5,6
The properties of LB films have been studied for over 50 years, but intensive investigations have only been performed in recent years by the application of newly developed analytical techniques7 such as attenuated total reflection (ATR),8,9 Fourier transform infrared spectroscopy (FTIR),10,11 X-ray photoelectron spectroscopy (XPS),12,13 and atomic force microscopy (AFM).14 With these surface analytical techniques, it is now possible to characterize the molecular orientation, structure, morphology, composition, and defects of LB films. However, due to the limitation of sampling depth of the analytical equipment, it is not easy to obtain information for the top layers which are the immediate interface with the other phases. Since the organic films prepared by the LB method and the VD method are always with amphiphilic molecules, the orientation of the molecules on the interface can be characterized by the analysis of surface wettability. It is well-known that the wetting properties of the surface will be altered when only a single layer of molecules adsorbs on the surface. Therefore, it provides a direct method for examining the orientation and film quality of the molecular layer. The wetting behavior of a liquid on a solid surface has been studied in the literature and is known to have an intimate relation with the
* Fax: 886-6-2666411. E-mail:
[email protected]. (1) Ulman, A. Ultrathin Organic Films; Academic Press: New York, 1991. (2) Roberts, G., Ed.; Langmuir-Blodgett Films; Plenum Press: New York, 1990. (3) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726. (4) Gaines, G. L., Jr. Langmuir 1991, 7, 834. (5) Debe, M. K.; Poirier, R. J.; Kam, K. K. Thin Solid Films 1991, 197, 335. (6) Baran, J.; Marchewka, M. K.; Ratajczak, H.; Borovikov, A. Y.; Byckov, V. N.; Naumovets, A. G.; Podzelinsky, A. V.; Puchkovskaya, G. A.; Styopkin, V. I. Thin Solid Films 1995, 254, 229.
(7) Vickerman, J. C., Ed.; Surface AnalysissThe Principal Techniques; John Wiley & Sons: New York, 1997. (8) Davies, G. H.; Yarwood, J. Spectrochim. Acta 1987, 43A, 1. (9) Song, Y. P.; Petty, M. C.; Yarwood, J.; Feast, W. J.; Tsibouklis, J.; Mukherjee, S. Langmuir 1992, 8, 257. (10) Song, Y. P.; Petty, M. C.; Yarwood, J. Langmuir 1993, 9, 543. (11) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (12) Hazell, L. B.; Rizvi, A. A.; Brown, I. S.; Ainsworth, S. Spectrochim. Acta 1985, 40B, 739. (13) Cave, N. G.; Caylss, R. A.; Hazell, L. B.; Kinloch, A. J. Langmuir 1990, 6, 529. (14) Peachey, N. M.; Eckhardt, C. J. Micron 1994, 25, 271.
10.1021/la981090c CCC: $18.00 © 1999 American Chemical Society Published on Web 02/09/1999
Octadecylamine Films
condition of the solid surface.15-18 This technique has recently been used to characterize a molecularly engineered surface19 and will be used in this work to examine the molecular orientation on LB and VD thin films. In the literature, the fatty acids are the most extensively studied constituents of LB films for both research and application. Heavy-metal ions such as barium or cadmium are always added to the monolayers to produce more stable LB films of fatty acids. However, for certain purposes the presence of heavy metal may be undesirable or an acidsensitive molecule is required, and the fatty acid is thus unusable. The LB films of a long-chain amine, due to its contrary behavior to that of the acid, offer a good substitution to overcome the problem and have a complementary advantage compared with fatty acid films. Although there are several studies dealing with monolayers and LB films of long-chain alkylamines in the literature,20-28 these studies are less extensive compared with those of fatty acids. The quality and stability of LB films are mainly affected by the properties of monolayers on the subphase and by the transfer condition from liquid to the solid support. The surface pressure of the monolayer under which the transfer is carried out and the transfer rate are very important in controlling the arrangement and orientation of the molecules on a solid support. In this work, the stability of the octadecylamine (ODA) monolayer on various subphases and at various surface pressures is studied first to find a suitable system for transferring LB films. The monolayers are then transferred to a hydrophilic glass plate. The effects of surface pressure and transfer rate on the surface wettability, uniformity, and stability of LB films are discussed. These results are compared with those obtained by the method of vacuum deposition and with those for thin films of stearic acid. 2. Experimental Section 2.1. Preparation of Thin Films. Octadecylamine (ODA, CH3(CH2)17NH2 ) was obtained from Aldrich Chemical Co. (purity > 99%) and used without further purification. Hexane was used as the spreading solvent for the monolayer formation. The subphases were (1) pure water, (2) 0.001 M NaOH solution, (3) buffer solution containing 0.01 M NaOH and 0.01 M NaHCO3 (pH ) 10.8), and (4) 0.001 M CdCl2 solution. The solutions of subphase were prepared with bidistilled water, purified with a Milli-Q apparatus supplied by Millipore (resistivity g 18.2 MΩ cm). All experiments were carried out at 25 °C. The spreading isotherms and the LB films were obtained with a Lauda Filmwaage FW2 apparatus. After the hexane solution was spread on the subphase, about 15 min was allowed for complete evaporation of the spreading solvent before compression (15) Chappuis, J. In Multiphase Science and Technology; Hewitt, G. F., Delgaye, J. M., Zuber, N., Eds.; Hemisphere: New York, 1982; p 387. (16) Schwartz, L. W.; Garoff, S. Langmuir 1985, 1, 219. (17) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (18) Schwartz, L. W.; Garoff, S. J. Colloid Interface Sci. 1985, 106, 422. (19) Ulman, A. Thin Solid Films 1996, 273, 48. (20) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons: New York, 1966; pp 226-233. (21) Gaines, G. L., Jr. Nature 1982, 298, 544. (22) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. J. Colloid Interface Sci. 1998, 203, 311. (23) Takahashi, M.; Kobayashi, K.; Takaoka, K.; Tajima, K. Bull. Chem. Soc. Jpn. 1998, 71, 1467. (24) Angelova, A.; Ionov, R. Langmuir 1996, 12, 5643. (25) Sukhorukov, G. B.; Feigin, L. A.; Montrel, M. M.; Sukhorukov, B. I. Thin Solid Films 1995, 259, 79. (26) Li, J. X.; Gardella, J. A. Anal. Chem. 1994, 66, 1032. (27) Bardosova, M.; Tredgold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (28) Puggelli, M.; Gabrielli, G.; Caminati, G. Thin Solid Films 1994, 244, 1050.
Langmuir, Vol. 15, No. 5, 1999 1797 was started. The monolayer was compressed continuously at a constant rate of 2 Å2 molelule-1 min-1. The monolayers were transfer onto optical glass plates (24 × 32 × 0.2 mm3) which were carefully cleaned and have advancing contact angles of 0° with water. One layer of LB film was obtained by initially immersing the glass plate in the subphase (i.e. before spreading of the monolayer), and then, after compression the monolayer to the selected surface pressure, the plate was withdrawn and one layer of monolyaer was transferred. After the deposition process, the LB film was dried in a desiccator with silica gel and the wettability was analyzed immediately after drying. For studying the effects of surface pressure and transferring rate, the selected surface pressures are 15, 20, 30, and 40 mN/m and the transfer rates are 1, 2, 5, and 15 mm/min. The vacuum deposition proceeded in a small coater of model ULVAC VPC-260 made by Sinku-Kiko Co. The base pressure of the vacuum chamber was controlled at 1 × 10-5 Torr, a measurement made with an ionization gauge. An electrically heated tungsten boat was used as evaporator. The deposition rate was controlled at 0.3 nm/s, and the film thickness was monitored by the frequency shifts of a quartz oscillator. 2.2. Dynamic Contact Angle Measurement. The advancing and receding contact angles of water on the thin films were measured by the Wilhelmy plate method with a dynamic contact angle analyzer (Cahn Instruments, Inc., DCA-312). In this method, the force, F, acting on an advancing (or receding) plate which is partially immersed in a liquid is analyzed and can be expressed as
F ) W + Pγ cos θ - B
(1)
where W and P are the weight and perimeter, respectively, of the testing plate, γ is the surface tension of the liquid, and B is the buoyancy force. During the detecting process, the force F varies with the variation of B, which is proportional to the immersion depth of the testing plate. Thus, a linear relationship will be obtained if the plate surface has high uniformity. At zero depth of immersion, that is the position where the solid plate just made contact with the liquid, the buoyancy force is zero and the contact angle can be calculated through eq 1. If the plate is advanced into the liquid, the advancing contact angle θa is obtained; on the contrary, if the plate is receded from the liquid, the receding contact angle θr is measured. Because the contact angles are measured from a moving plate, which is in a dynamic state, they are called dynamic contact angles (DCA’s). During the analysis process, as the plate moves, the three-phase contact line also moves. In that case, the heterogeneity of the plate surface, if any, will result in various contact angles and irregular variation of the acting force. Therefore, from the variation curve of force versus immersion depth, the wettability of the surface is scanned. With this scanning, one can justify the uniformity of the solid surface.
3. Results and Discussion 3.1. Monolayer of ODA on Subphases. The pressure (π)-area (A) isotherms of monolayers of ODA on various subphases are shown in Figure 1. Highly condensed monolayers are formed on the subphases of pure water and alkaline solution, without an obvious transition point of phase change. However, the monolayers on the alkaline solution are more expanded compared with those on pure water. The limiting area per molecule obtained from the isotherm is 22 Å2 for the alkaline solution, and 18 Å2 for pure water. They resemble closely the curves obtained by Gaines21 and Puggelli et al.28 On the alkaline solutions used in this work, the ODA molecules are completely uncharged (NH2) due to the repression of the reaction NH2 + H+ f NH3+. But on pure water, the ODA could be partially ionized. It was reported that when the filmforming molecules are extensively ionized, the monolayer will expand.29 In this work, the small contraction in the (29) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons: New York, 1966; p 193.
1798 Langmuir, Vol. 15, No. 5, 1999
Figure 1. Pressure (π)-area (A) isotherms for monolayers of octadecylamine on various subphases at 25 °C.
Lee
Figure 3. Effect of surface pressure on the relaxation of the monolayers of octadecylamine on a subphase of alkaline solution. Table 1. Transferring Ratio for the Deposition of One Layer of ODA onto a Hydrophilic Glass Surface transferring rate (mm/min) 1 2 5 15
Figure 2. Relaxation curves for the monolayers of octadecylamine at π ) 15 mN/m.
area of the monolayer on pure water, compared with that on the alkaline solution, can be attributed to the interactions between the ionized (NH3+) and unionized (NH2) molecules. With the presence of Cd2+ in the aqueous subphase, which is always used to prepared stable monolyers of fatty acid, a more expanded monolayer is obtained. This is a result of the repulsive force between the ions (NH3+) and Cd2+. The stability of a monolayer can be examined by the relaxation curve. This curve shows the ratio of area per molecule at time t, A(t), to that at time zero, A(0), under constant surface pressure. Figure 2 shows the relaxation curves of monolayers on various subphases at π ) 15 mN/ m. For monolayers on pure water and CdCl2 solution, the ratios of area decrease constantly with the aging time. Within 60 min, over 20% of the area is lost for both curves, which means that the monolayers are not stable. Aggregation, crystallization, or collapse of the molecules may occur which leads to loss of the area under constant pressure. On the contrary, the relaxation curve for the alkaline solution shows little change within 60 min, which represents the high stability of the monolayers. The alkaline buffer solution (0.01 M NaOH and 0.01 M NaHCO3, pH ) 10.8) was thus chosen for the majority of the transfer studies. Figure 3 shows the relaxation curves of monolayers on the alkaline buffer solution under various surface pressures. In general, the monolayers are very stable at surface
transferring ratio at surface pressure 15 mN/m 20 mN/m 30 mN/m 40 mN/m 1.26 1.19 1.10 1.12
1.19 1.10 1.14 1.09
1.08 1.07 1.05 1.06
1.08 1.05 1.03 1.01
pressures between 15 and 40 mN/m. With 60 min, the area loss is less than 1%. This value is much smaller than that of monolayers of stearic acid on a subphase containing CdCl2;30 that is, the monolayer of ODA is more stable than that of stearic acid. Figure 3 also shows that the higher the surface pressure, the more stable the monolayer will be, except for the curve at π ) 20 mN/m. This phenomenon is quite interesting and is different from that for the monolayers of fatty acid,30 in which the increasing of the surface pressure always leads to a more unstable film. At π ) 20 mN/m, the comparative in stability of the ODA monolayers suggests the possibility of a phase transition at this pressure. The transfer ratios for the deposition of LB films onto the glass surfaces are shown in Table 1. These values are larger than 1.0 or, even more, higher than 1.1 for a few cases. A similar result was also found in the literature.28 The high values of transfer ratios mean that the amount of molecule transferred to the solid surface is more than what is presents on a subphase of equivalent area. In general, this phenomenon is always caused by the instability of the monolayer on the subphase, but this effect is excluded due to the observed high stability of ODA monolayers. In this case, the high value of the transfer ratio may be a result of the high attraction force between the glass and the hydrophilic pole (NH2) of ODA (this will be discussed later). When the attraction force between the solid support and the transferred molecules is higher than that between molecules, more molecules will be transferred due to the adsorption effect. Table 1 shows that the transfer ratio will decrease and approach unity when the surface pressure or transferring rate is increased, although there are a few exceptions. A higher transferring rate is supposed to decrease the adsorption time of ODA molecules onto the glass surface and thus (30) Chen, H. Y. A Study of the Wettability Modifications of Solid Surfaces by Using the L-B Film Deposition Technique. M.S. Thesis, National Cheng-Kung University, Taiwan, 1997.
Octadecylamine Films
Langmuir, Vol. 15, No. 5, 1999 1799
Table 2. Advancing and Receding Contact Angles of Water on LB Films of ODA and Stearic Acid surface pressure (mN/m)
transfer rate (mm/min)
15
1 2 5 15
20
1 2 5 15
30
1 2 5 15
θa and θr for respected DCA cycle (deg)
θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr θa θr
1
2
3
4
5
111 55 111 62 109 62 31 33 111 60 104 49 68 37 62 33 108 63 110 55 112 63 105 51 114 61 110 58 113 61 114 62 113 8
109 60 108 62 106 62 109 48 109 65 106 52 102 48 103 46 110 66 108 58 111 66 107 55 110 63 108 60 110 64 110 65 85 0
107 62 107 62 106 60 104 49 107 64 105 55 103 53 106 48 107 66 107 62 110 62 107 59 108 60 107 62 108 59 109 64 47 0
106 62 105 62 104 60 103 51 105 63 103 56 103 53 104 49 104 65 105 63 109 61 105 59 106 59 106 61 105 58 108 62 33 0
103 60 102 60 103 60 102 51 103 61 101 55 101 52 102 47 102 63 104 62 108 61 103 57 104 58 103 59 103 57 106 63 33 0
Figure 4. Variation of force versus immersion depth of the DCA measurement for the LB film of ODA prepared at π ) 40 mN/m and transferring rate ) 15 mm/min. The number of each loop represents the analytic cycle, and the first cycle is described by the bold curve. The letter “a” represents the advancing stage, and “r” represents the receding one.
decrease the transfer ratio. Besides, at higher surface pressure, the densely packed structure of the monolayer leaves less space for the adsorption to happen. 3.2. Wettability on LB films of ODA. The advancing and receding contact angles of water on the LB films of ODA are shown in Table 2. The wettabilities of the films were analyzed by repeating five cycles of the DCA’s loop to examine the stability of the films. Because one layer of the LB films was obtained by withdrawing the hydrophilic glass from the subphase, the ODA molecules are supposed to be arranged with hydrophobic poles facing outward and thus a hydrophobic surface should be obtained. The measured advancing and receding contact angles are about 113° and 62°, respectively, except for a few conditions. The advancing contact angle agrees with a methyl-terminated monolayer obtained by the method of self-assembly (θa ) 114°).31 The receding contact angle was not reported in the literature, and the well-packed and oriented monolayer cannot be confirmed only by the advancing contact angle. But this will be proved in the later discussion. The wettabilities of the ODA films are very uniform, as examined by the variation of the acting force with the dipping depth in the DCA analysis. A typical figure is shown in Figure 4 for the LB film obtained at π ) 40 mN/m and transferring rate ) 15 mm/min. The smooth curves for both advancing and receding reflect the high uniformity of the LB film in wettability. In Figure 4, the curves of the analytical loops shift slightly cycle by cycle
as an effect of the molecular reorganization due to the steady motion of the three-phase contact line during the analysis of the DCA. The advancing and receding contact angles obtained for each loop of the five cycles are also list in Table 2. With increasing of the analytical cycle, the advancing contact angle decreases slightly, but the receding contact angle always increases at the second (or the third) cycle and then decreases slightly. The variation of the contact angles within five cycles is small as compared with that of LB films of stearic acid;30 that is, the films are stable in general. The increasing of receding contact angle with the analytical cycle for the ODA LB films is contrary to the results for other materials in the previous studies.30,32,33 The variation of advancing and receding contact angles can be attributed to different heterogeneous surfaces with various surface compositions. For a heterogeneous surface composed of two materials, one hydrophilic and the other hydrophobic, the advancing and receding contact angles have been studied theoretically in the literature.15-18 If a predominantly high-energy surface (hydrophilic) is considered to be covered with a small fraction of the lowenergy region (hydrophobic), the advancing contact angle will increase when the fractional coverage of the lowenergy region increases, whereas the receding contact angle remains almost constant or slightly increases. On the other hand, if a low-energy surface (hydrophobic) is covered with a small fraction of the high-energy region (hydrophilic), the receding contact angle increases with decreasing the hydrophilic region, but the advancing contact angle remains constant. This model has been verified by experimental data in our previous work.32 The ODA films shown in Table 2 belong to the second case, with a predominantly hydrophobic surface. The monomolecular layer on the LB film may not be perfect but contains few defects. Besides, since the transfer ratios are larger than unity, a bilayer of ODA is likely to be formed locally, as shown in Figure 5a, on which the polar head is exposed outward and the hydrophilic region is increased. During the process of Wilhelmy plate analysis, the ODA molecules may be dragged by the moving contact lines and thus they may cause the rearrangement of the
(31) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665.
(32) Lee, Y. L.; Chen, C. H.; Yang, Y. M. Langmuir 1998, 14, 6980. (33) Lee, Y. L. Colloids Surf. A, in press.
40
1 2 5 15
15a a
2a
Data for LB film of stearic acid.
1800 Langmuir, Vol. 15, No. 5, 1999
Lee Table 3. Advancing and Receding Contact Angles of Water on Vacuum-Deposited Films of ODA and Stearic Acid ODA thickness (nm) 2.5 5 10 20 50 100 a
Figure 5. Schematic illustrations of monolayer structures: (a) for a proper transfer condition, most of the ODA molecules are orientedly packed but with a few defects in the monolayer; (b).when the transfer proceeds at low surface pressure and high transferring rate, the bilayer molecular film or aggregative micelle may be formed.
Figure 6. Variation of force versus immersion depth of the DCA measurement for the LB film of ODA prepared at π ) 20 mN/m and transferring rate ) 5 mm/min. The nonsmooth curve in the first cycle shows that the molecules are irregularly packed.
molecules. As a consequence, the fraction of hydrophilic region is decreased and the receding contact angle increases slightly. The slight decrease of the advancing contact angle, or of the receding contact angle at the later cycle, is a result of the unavoidable adsorption of water molecules on the surface. Several unusual cases shown in Table 2 are at π ) 15 mN/m, transferring rate ) 15 mm/min and π ) 20 mN/m, transferring rate ) 5 and 15 mm/min. In these cases, the contact angles are much smaller than the others in the first DCA cycle, but at the later cycles, they increase to values close to those for the other cases. A typical figure of the DCA curves is shown in Figure 6 (π ) 20 mN/m, transferring rate ) 5 mm/min). The first cycle (θa ) 68°, θr ) 37°) is not smooth and very distinct from the other cycles, which means that the transferred film is nonuniform and hydrophilic. This phenomenon is likely to occur
θa 115 114 115 114 113 114
stearic acid θr
θa
θr
62 47 52 54 53 60
48a
24a 27 52 91 89 94
58 102 110 110 108
Thickness ) 2 nm.
only at the condition of low surface pressure and high transferring rate, and it is especially obvious at the pressure of the phase transition (20 mN/m). Under such circumstances, a hydrophilic bilayer or aggregative micelle of ODA molecules may be formed (as shown in Figure 5b). But at a higher pressure when the monolayer is highly packed, it leaves less freedom for the molecules to tilt and rotate and, thus, this effect will not occur. In Figure 5b, the hydrophilic region (exposed amine head and the uncovered glass) occupies a significant fraction on the surface and thus the contact angles decrease greatly. For these cases, the fraction of the hydrophilic region is reflected by the value of the advancing contact angle. For the film obtained at π ) 15 mN/m and transferring rate ) 15 mm/min, the value of θa is even smaller than that of θr. This unusual result is caused by the deviation during the extrapolation of the nonsmooth curve of DCA to the zero depth point in the calculation of contact angles. However, the close values for θa and θr did mean that the exposure of the hydrophobic tail is negligible. The receding contact angles for the three nonordered films are similar (about 35°) and are higher than that for the glass surface (0°). This phenomenon not only mean that these surfaces are predominantly hydrophilic but also shows that the receding contact angle of water on the amine-exposed surface is about 35°. For the film shown in Figure 6, the advancing and receding contact angles increase abruptly at the second cycle. Obviously, this is also a consequence of molecular rearrangement during the DCA analysis, as described in the previous paragraph. During the DCA analysis, the plate is put into and out of the water, which is quite similar to the procedure of LB deposition. Due to the high affinity of the polar group (NH2) of ODA for water and glass, the molecules on the top layer of the bilayer film will be dragged along the surface by the movement of the threephase contact line and put in the right position to form a regularly packed monomolecular layer. As a result, a uniform and hydrophobic surface is obtained and the surface becomes more and more uniform with the analytic cycle. 3.3. Wettability on Vacuum-Deposited ODA Films. The contact angles of water on vacuum deposited ODA films are shown in Table 3 for films of various thickness. The surface of the vacuum-deposited ODA film is also uniform in wettability, as examined by the smooth curve of DCA analysis. At the film thickness 2.5 nm, the advancing and receding contact angles are 115° and 62°, respectively. These values are the same as the results for the LB film. Because the chain length of a ODA molecule is about 2.5 nm, the amount of deposited ODA on the substrate at 2.5 nm is equivalent to that on a LB film of one monolayer. The identical contact angles for the two methods mean that the initial growth of ODA on glass has a growth mode of layer by layer, which also reflects the higher attractive force between the ODA molecule
Octadecylamine Films
and the glass surface. A contrary example of the island growth mode obtained in the previous work is the vacuum deposition of stearic acid on glass.33 Such results are also shown in Table 3 for comparison. For stearic acid, the slow increasing of contact angles with the increase of film thickness, which is different from that for ODA films, is a typical result to identify the mode of island growth. In Table 3, the advancing contact angles for the two materials are similar due to their equivalent hydrocarbon chain lengths. The slightly higher value of θa for ODA is probably due to the ODA molecules being highly oriented and packed. However, the receding contact angle on a ODA film is smaller than that on a continuous film of stearic acid. This is a result of the stronger attractive force of the (NH2) pole with water. 3.4. Wettability on a Stearic Acid LB Film. It is interesting to compare the wettabilities on LB films of ODA and stearic acid. The advancing and receding contact angles on a LB film of stearic acid were reported to be 115° and 10°, respectively, at π ) 15 mN/m and transfer rate ) 2 mm/min.30 This experiment is repeated with 0.002 M CdCl2 aqueous solution as subphase, and the contact angles are list on the bottom of Table 2. The advancing contact angle on the LB film of stearic acid is equal to that on the ODA at the first cycle, but the receding contact angle is very small and has a value close to that on a hydrophilic glass. Since the attractive force of (COOH) with water is smaller than that of (NH2), the receding contact angle on the stearic acid film should be larger than that on the ODA if the monolayers are densely and regularly packed on both (the results on the vapordeposited films of ODA and stearic acid support this inference). The small value of receding contact angle on the stearic acid film represents the exposure of the glass on the film surface; that is, the LB film of stearic acid is not homogeneous but contains holes. This phenomenon is contrary to the currently accepted concept that the monolayers and the transferred LB films are homogeneous. The presence of holes in the stearic acid monolayer was reported in the literature by observation with electron microscopy.34-36 In this work, the wettability analysis provides an easier method to examine the characteristics of monomolecular films. For the LB film of stearic acid, the advancing contact angle is controlled by the hydrophobic region of the outward hydrocarbon chain, while the receding contact angle is determined by the hydrophilic glass. On the basis of the result in our previous study on the wettability of heterogeneous surfaces,32 the fraction of holes in the stearic acid film has a value between 0.4 and 0.6. For the LB film of ODA, the receding contact angle is much larger than that on glass and, besides, equal to that on a continuous vacuum-deposited film. So, the monomolecular film of ODA should be a homogeneous and closely packed structure without the presence of holes. By further comparison with the receding contact angle on a surface with exposure of the (NH2) head (about 35°), the ODA molecules are highly (34) Uyeda, N.; Takenaka, T.; Aoyama, K.; Matsumoto, M.; Fujiyoshi, Y. Nature 1987, 327, 319. (35) Kajiyama, T.; Oishi, Y.; Kuri, T. Thin Solid Films 1996, 273, 84. (36) Luk, S. Y.; Wright, A. C.; Williams, J. O. Thin Solid Film 1990, 186, 147.
Langmuir, Vol. 15, No. 5, 1999 1801
oriented on the LB films when their advancing and receding contact angles are about 114° and 62°, respectively. The stability of the stearic acid film prepared by the LB method is much less than that of ODA films. The advancing contact angle decreases greatly cycle by cycle during the DCA analysis, which means that the fraction of hydrophobic region becomes smaller and smaller by the dragging effect of the movement of the three-phase contact line. The stearic acid molecules may be peeled off from the glass or aggregate to form multilayer islands during the measurement of DCA. The attractive force between (COOH) and glass is thus weaker than that between (NH2) and glass. By comparing with previous studies on the wettability modification by the LB method,30,37,38 the ODA LB films can provide a surface with a better hydrophobic property, better stability, and a more ordered structure. 4. Conclusion The monolayer of ODA is very stable on a subphase of alkaline solution. The stability of the monolayer increases with increasing surface pressure except for the pressure of the phase transition. Due to the high attraction force between the amine group and the glass, the transfer ratios for the transferring of the monolayer onto hydrophilic glass are higher than unity as an effect of autoadsorption. The LB film of ODA is stable and uniform in wettability, as determined with the dynamic contact angle analyzer. The advancing and receding contact angles of water on a LB film of ODA are about 113° and 62°, respectively. However, when the transfer takes place at small surface pressure (15, 20 mN/m) and high transferring rate, the arrangement of the ODA molecules on the glass may be nonordered and irregular. The LB film is thus nonuniform and hydrophilic. During the analysis of DCA, the movement of the threephase contact line has the effect of rearranging the irregularly packed molecules on the LB film. For the method of vacuum deposition, the growth of ODA on the glass is found to be a mode of layer growth. The advancing and receding contact angles of water on the vapor-deposited films are the same as those on the LB film at a thickness equivalent to one monolayer of ODA (2.5 nm). By comparing the LB films of ODA and stearic acid, the very small value of the receding contact angle on stearic acid reflects the exposure of the hydrophilic glass on the stearic acid film. The existence of holes in the stearic acid monolayer is thus proved by the measurement of wettability. Acknowledgment. The support of this research by the National Science Council of the Republic of China through grant No. NSC 86-2214-E-041-005 is gratefully acknowledged. LA981090C (37) Penacorada, F.; Angelova, A.; Kamusewitz, H.; Reiche, J.; Brehmer, L. Langmuir 1995, 11, 612. (38) Gotoh, K.; Yoshimitsu, S.; Tagawa, M. J. Adhesion Sci. Technol. 1996, 10, 1129.
