1024
Langmuir 1993, 9, 1024-1027
Mixed Alkanethiol Monolayers on Gold Surfaces: Substrates for Langmuir-Blodgett Film Deposition P. Sanassy and S. D. Evans' Department of Physics, University of Leeds, Leeds LS2 9JT,
U.K.
Received December 8, 1992
Mixed alkanethiolderivativeshave been used to produce high-qualitysurfacessuitablefor use assubstrates for studying the Langmuir-Blodgett (LB) film transfer process. We present results on the correlation of the transfer ratio (TR) with the dynamic contact angle. The latter varies as a function of the relative surfaceconcentrationof hydrophilic(OH)to hydrophobic (CH3)groups. An abrupt transition was observed in the TR correspondingto asurfaceOH concentrationaround 40 % (dynamiccontact angleof approximately 30O). For surfaces with concentrationsof OH groups greater than 35% ,the contact angle is close to 20' and the transfer ratio is essentially unity. Conversely, for surfaces with less than 35% OH, the contact angles were greater than 30° and the transfer ratios were low. The transition in the deposition ratio is ascribed to the formation of a thin prewetting water layer. Introduction The transfer of Langmuir-Blodgett (LB) films from the ahwater interfaceto a solid support is stronglydependent on the nature of the subphase, and the speed of deposition. While the subphase conditions, namely, the pH and temperature, and the deposition speed can be modified continuously,the ability to continuously vary the substrate contribution has been lacking.14 Further, the ability to deposit LB monolayers onto noble metal surfaces is problematic in itself due to the poor adhesion of fatty acids to surfaces with low oxide coverage~.~J~ In contrast alkanethiol derivativesyield highly ordered, close-packed,molecular assemblieson gold." The ability to functionalizethe terminal group (at the monolayer-air interface) has meant that the surface properties of these films can be fine-tuned at the molecular level,thus making them suitable for use as model systems for investigating a wide range of phenomena from wetting to molecular recognition.l2-15 In 1939 Bikerman suggested that for the successful deposition of LB films, i.e. with transfer ratios close to unity, the contact angle of the three-phase line should be less than 90° on the out-stroke and greater than 90' on (1) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. Thin Solid Films 1992,210/221,36-38. (2) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Prog. Colloid Polym. Sci. 1991, 84, 184-188. (3) Petty, M. C.; Barlow, W. B. Langmuir-Blodgett Films; Roberts, G.. Ed.: Plenum Press: London. 1990: DD 93-123. '(4) Ulman, A. Ultrathin Organic F h s ; Academic Press: London,
1991; pp 107-133. (5) Blake, T. D.; Haynes, J. M. J. Colloid Interface Sci. 1969,30,421. ( 6 )Petrov, J. G.; Radeov, B. P. J. Colloid Polym. Sei. 1981,259,753. (7) de Gennes, P. G. Colloid Polym. Sci. 1986,264,463-465. (8)Buhaenko, M. R.; Richardson, R. M. Thin Solid Films 1988, 259, 231-238. (9) Roberta, R. W.; Gaines, G. L. Trans. Natl. Vac. Symp. 1962, 9, 515-518. (10) Spink, J. A. J. Colloid. Interface Sci. 1967,23, 9-26. (11) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (12) Bain, C. D.; Whitesides, G. M. Angew Chem.,Int. Ed. Engl. 1989, 101, 522-528. Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111,7164. (13) Bain, C. D.; Whitesides, G. M. Langmuir 1989,5,1370. Bain, C . D.; Eval, J.; Whitesides, G. M. J. Am. Chem. SOC.1989, 222, 7155. Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6,87. Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (14) Ulman, A.; Evans, S.D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. SOC.1991, 123, 1499-1506. Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156-161. (15) Schmitt, F. J.; Haussling, L.; Ringsdorf, H.;Knoll, W. Thin Solid Films 1992,210, 815-817.
the in-stroke.16 More recentlyAveyard et al. have explored this hypothesis more carefully by studying the correlation between the transfer ratio (TR) and the dynamic contact angle as a function of the pH of the subphase.lT2 They found that the transfer ratio on the out stroke was close to unity if the contact angle was below 30'. In this paper we utilize the ability to form mixed monolayer assemblies to study the deposition of LB monolayers as a function of the dynamic contact angle. By varying the ratio of OH and CH3 groups in the surface of the self-assembledmonolayer (SAM), we can effectively control the surface energy, and hence the substratesubphase and substrate-Langmuir monolayer interaction energies,and through these the contact angle of the threephase line, while keeping ylv(the liquid-vapor interaction energy) constant. The surfaces were formed by the spontaneous adsorption of a thiol mixture from solution onto gold surfaces. The thiols used were 1-dodecanethiol (CHg(CH2)11SH,DDT) and 11-hydroxyundecane-1-thiol (HO(CH2)11SHIHUT) since they have the same molecular length but carry terminal groups that are hydrophobic and hydrophilic,respectively. By varying the HUT/DDT ratio in solution, one can vary the surface composition of OH and CH3 groups.14 The focus of our study was on the withdrawal of such surfaces through the air-subphase (water) interface of an LB trough, with and without the presence of an arachidic acid (CH3(CHz)l&OOH, AA) monolayer, and to correlate the TR with the dynamic contact angle, Figure 1. Note on the Validity of the Interpretation of Dynamic Contact Angles.17J8 Because of the macroscopic nature of the experiment being performed, i.e., the contact angles being determined, we necessarily call into question the types of conclusions which may be inferred from our results. This is, of course, restricted not only to our work but to that of all those working in this area and who make use of dynamic contact angle measurements. In fluid mechanics it is recognized that there are two regions of importance, the so-calledinner region close to the point of contact between the fluid and the solid, and of a microscopic length scale, and the outer region on which most (macroscopic)measurements are made. The shape of the meniscus in the outer region is largely determined (16) Bikerman, J. J. Proc. R . SOC.London, A 1939, 270, 130. (17) Dussan, E. B.; Rame, E.; Garoff, S. J . Fluid Mech. 1991, 230, 97-116. (18) Dussan, E. B. Annu. Reu. Fluid Merh. 1979, 1 1 , :371-JW
Q743-7463/93/2409-1Q24$Q4.00/Q 0 1993 American Chemical Society
Langmuir, Vol. 9, No. 4, 1993 1025
Mixed Alkanethiol Monolayers on Gold Surfaces 100
90 80 .c
' -0
70
a .O 60
Yrn
5
5
50
5
40
9
5 Figure 1. Schematic illustration showingthe three-phase region in the presence of a Langmuir film. The various interfacial energiesimportant for determining film transfer are represented and take their usual meaning (with s, m, 1, and v representing the substrate, the Langmuir monolayer, the liquid (subphase), and the vapor phase, respectively).
30
c
s
20
10
0 0 10 20 30 40 50 60 70 80 90 100
by the properties of the fluid and fluid-substrate interaction in the inner region; however, it is not possible to extrapolate backward from a measurement in the outer region to gain any direct insight into the phenomena occurring in the inner region. Thus, the contact angles reported here are really apparent contact angles rather than true ones and are of limited use for interpreting the underlying phenomena. Not withstanding this the use of this technique is widespread, and such measurements do provide us with an empirical body of information, in our case regarding the Langmuir-Blodgett transfer process, and it is from this persepective that we proceed. Experimental Section Substrate Preparation. Gold substrates were prepared by .the sputtering of gold (99.99%) onto clean glass slides. The thicknesses were typically 400 8, and displayed a predominantly (111)surface construction. Self-Assembled Monolayer Formation. Self-assembled monolayers of the thiols were spontaneously adsorbed by immersing the gold substrates into freshly prepared 2 X 10-3 M solutions of the mixed thiols, in THF, for 2 h. The substrates were subsequently washed with distilled THF and absolute ethanol followed by a thorough rinsing in Millipore water and dried with nitrogen gas. Contact Angle Measurements. Static contact angles on horizontally oriented substrates were measured on sessile drops, which had been allowed to advance across the freshly prepared monolayer surface, with a homemade goniometer, consisting of a rotatable eyepiece and microscope,supplied by Graticules Ltd., U.K. Additional measurements were made with our recently constructed imaging system, with which droplet images were captured using a high-resolution, square pixel, CCD camera (Hamamatsu C3077-Ol), a HRT frame grabber, and the contact angles determined using suitable software. The apparatus described above was also used for the measurements of the dynamic contact angles of vertically oriented substrates. Images of the three-phase region were displayed in real time, and sampled images were saved on the computer hard disk for later analysis. The measurements of the static and dynamic contact angles are the averages of at least three readings per sample and of at least two samples. We note that, for any given substrate, the dynamic contact angle did not change noticeably during the withdrawal through the air-water interface (neither in the presence nor in the absence of the AA monolayer). Langmuir-Blodgett Monolayer Deposition. A Mayer Feintechnik LB trough was filled with Millipore, Milli-Q grade water (specific resistivity of 18 MQ) with no added ions. The measurements were made at 22 "C and at a deposition speed of 5 mm/min. During transfer the Langmuir monolayers were maintained at a constant surface pressure of 25.5 mN m-l.
%OH
Figure 2. Static and dynamic contact angles as a function of the surface OH concentration: static water contact angle (open diamonds), dynamic contact angles of the water surface (filled squares), and the dynamic contact angles of water plus an arachidic acid monolayer a t 25.5 mN/m (filled triangles). All lines are guides to the eye only, and we note that the dynamic water contact angle of the bare water surface was greater than 90" for the 0% OH surface and thus could not be clearly imaged. Transfer Ratio. The transfer ratio was determined in the standard way, i.e., as the ratio of the area film removed from the trough to the geometrical area of the substrate passed through the film. Materials. Arachidic acid (99%)was obtained from Aldrich Chemical Co. and was filtered twice before use. The monolayers were spread from chloroform (aristar grade) as supplied by BDH chemicals. The alkanethiol derivatives were kindly supplied by Dr. A. Ulman, Eastman Kodak, and details of chemical purity are given in ref 14. The THF (99%) was obtained from Aldrich.
Results and Discussion HUT forms monolayers on gold (HUT/Au, OH surface) with advancing water contact angles of 35%. The analysis in terms of work of adhesion between the substrate and the polar
groups of the Langmuir monolayer has not been pursued because of the lack of a clear indication of the physical meaning of such an approach, i.e., of the splitting of the interfacial energies such as ylv = ylP+ ?pa + yav(where 1, p, a, and v represent the liquid, polar portion of the monolayer, hydrophobic portion of the monolayer, and vapor regions, respectively) when not at T = 0 K.20 If we follow such an approach, we predict a decreasein the work of adhesion between the Langmuir film and the substrate (mixed SAM) as the percent OH is increased; this clearly seems contrary to our expectations and to the TR measurements. From measurements of the transfer ratio as a function of OH concentration, Figure 4, we see even more unusual behavior. It appears that there is an abrupt jump in the TR for a surface OH concentration between 30 and 40%. Such a transition although not expected is explicable in light of our previous work on these systems where we found a similar transition in the static control angles for hexadecane (HD) at a similar surface OH ~0ncentration.l~ Our physicalexplanationfor the observed behavior was that the HD was really sensingthe formationdestruction of a thin adsorbed water layer on the SAM surface. Below the transition point, at low OH concentrations, the OH groups were sufficiently far apart that they could not sustain the formation of a continuouswater layer, while above the transition point they could. The explanation was supported by mean field calculations which illustrated the abrupt nature of the formationdestruction of the condensedwater 1a~er.l~ It seems highly plausible that the same underlying physical phenomenon is taking place here, indicating that good transfer only starts to take place once there is a continuous, thin prewetting water layer present on the SAM surface. Without this prewetting layer the film transfer is poor. We note, however, that if the prewetting layer were too thick we would also expect poor transfer, in analogy with the idea of a maximum dipping speed above which macroscopic,continuousfilmsof water are present between the substrate and the LB monolayer and preclude its successful transfer to a solid ~ u p p o r t . ~ Comparison of Figures 3 and 4 does not appear to yield any simple correlation between the transfer ratio and the
(19) Johnson, R. E.; Dettre, R. H. J . Phys. Chem. 1964,68, 1744.
(20) Riegler, H.; Engel, M. Ber. Bunsen-Ces. Phys. Chem. 1991, 95, 1424-1430.
Mixed Alkanethiol Monolayers on Gold Surfaces 1.o
0.9 0.8
0.7
.2 0.6 a & 0.5 .&C
0.4 0.3
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I
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I 0.6
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.
0.7
I
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I
I 0.9
I
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cos 8
Figure 5. Transfer ratio versus dynamic contact angle. The lines are guides to the eye.
work of adhesion. This is illustrated in Figure 5 where the TR is plotted as a function of the cosineof the dynamic contact angle. It is evident that the TR increases rapidly as cos 8 (W,) increases, for cos 8 > 0.85 (8< 30°),and that below this there is essentially no, or at least very poor, deposition. Such results are in general agreement with the recent papers by Aveyard in which they found that for TRs close to unity the contact angles were less than 30'.
Conclusions We have demonstrated that mixed self-assembled monolayers may be useful for modifying surfaces, in a
Langmuir, Vol. 9, No. 4, 1993 1027
controlledmanner, for studyingthe LB depositionprocess. By controllingthe ratio of OH to CH3groups on the surface, we can control the dynamic contact angle and hence influencethe transfer ratio. In particular we have, at least for this system, found that the presence of a thin adsorbed water layer may be needed for the deposition to occur with a high TR. By using SAMs as a first layer, it is possible to create good LB mono- and multilayer films on gold substrates which, although highly useful, have tended to be avoided due to the problem of poor LB film adhesion. We have at present built LB films with transfer ratios of unity, using 100% CH3 surfaces in cases where the first LB layer is to be deposited on the in stroke and 100%OH surfaces when it is to be deposited on the out stroke. In light of the restrictionson the informationobtainable from the types of dynamic contact angle measurements usually made (i.e., in the outer region) caution must be applied in their general use as a tool for studying the transfer process. The inclusion of such results serves to build our empirical understanding rather than to further our knowledge at a fundamental level. Future work will look at mixed SAMs containing acid groups in the surface and study the deposition speedcontact angle relationship.
Acknowledgment. We are grateful to A. Ulman, Eastman Kodak, Rochester, NY, for the.kind gift of the thiol derivativesand to J. Davies, Eastman Kodak, Harrow, ClinicalDiagnostics, U.K., for the preparation of the gold substrates. We would also like to thank J. R. Henderson, University of Leeds, for valuable discussions. This work was supported by the University of Leeds, University Research Fund.
