Langmuir 1994,10, 2307-2310
2307
Preparation of a Robust Hydrophobic Monolayer on Mica Jonathan Wood? and Ravi Sharma* Materials Science and Engineering Division, Eastman Kodak Company, Rochester, New York 14650-2158 Received October 8, 1993. In Final Form: April 5, 1994@ Mica has been made hydrophobic by a novel approach using Langmuir-Blodgett deposition of prepolymerized octadecyltriethoxysilane (OTE). The mica was activated by treatment in a waterlargon plasma before deposition of the OTE. On baking the OTE-covered mica sheets at 100 "C in a vacuum oven for 2 h, hydrophobic surfaces with an advancing water contact angle = 112"are obtained. Contact angle studies, contact thickness measurements, and evaluation of the quality of contact between two OTEcovered mica surfaces when examined using the surface force apparatus indicate that the OTE deposit is a robust monolayer which is resistant to chemical displacement by potassium ions. Surface energies of these hydrophobic surfaces are in good agreement with published data for hydrocarbon surfaces. The preparation of a robust and dependable hydrophobic layer on mica is key to the direct measurement of the hydrophobic force between macroscopic surfaces.lS2 This is because direct measurement of interaction forces are usually conducted using the surface force a p p a r a t ~ sa, ~ technique in which molecules of interest are adsorbed onto step-free sheets of muscovite mica. Mica is usually used as the supporting surface because it can be cleaved into relatively large step-free sheets and also because it is transparent, both of which are required when using the SFA. Other materials such as ~ i l i c a sapphire5, ,~ and polyethyleneterephthalate6 have been used, but to data none of these have been used as a support surface for depositingorganic molecules. Hydrophobic layers on mica have been extensively studied using the SFA.1,2,7-21 However, these studies have resulted in a puzzling array t Visiting scientist. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, June 15,1994. (1)Christenson, H. K. The Long-Range Attraction between Macroscopic Hydrophobic Surfaces, in Modern Approaches to Wettability: Theory and Applications; Schrader, M. E., Loeb, G., Eds.; Plenum Press: New York, 1992. (2) Tsao,Y.-H.;Yang,S.X.; Evans,D. F.;Wennerstrbm,H.Langmuir 1991, 7, 3154. (3) Israelachvili, J. N.;Adams, G. E. J . Chem. SOC.Faraday Trans. 1 1978, 74, 975. (4) Grabbe, A.; Horn, R. G. J . Colloid Interface Sci. 1993,157, 375. ( 5 ) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J.Mater. Res. 1988, 3, 413. Horn, R. G.; Smith, D. T. J.Non-Cryst. Solids 1990,120, 72. (6)Merrill, W. W.; Pocius, A. V.; Thakker, B. V.; Tirrell, M. V. Langmuir 1991, 7 , 1975. (7) Christenson, H. K.; Claesson, P. M. Science 1988,239, 390. ( 8 )Claesson, P.M.; Christenson, H. K. J. Phys. Chem. 1988, 92, 1650. (9) (a)Rabinovich,Ya, I.; Derjaguin, B. V.; Churaev,N. V. Advances in Colloid and Interface Sci. 1982, 16, 63. (b) Rabinovich, Ya. I.; Derjaguin, B. V. Colloids Surf, 1988, 30, 243. (10) (a) Parker, J. L.; Cho, D. L.; Claesson, P. M. J . Phys. Chem. 1989,93,621. (b) Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.: Tidblad. J.: Blombere. E. J . Colloid Interface Sci. 1990. 134. 449. @
(12) Israelachvili, J. N.; (13) Israelachvili, J. N.;Pashl 98, 500. (14) Pashley, R. N.;McGuiggan,P. M.;Ninham, B. W.;Evans, D. F. Science 1986,229, 1088. (15) K6kicheff,. P.:. Christenson, H. K.: Ninham, B. W. Colloids Surf. 1989, 40, 31. (16) Claesson, P. M.; Blom, P. C.; Herder, P. C.; Ninham, B. W. J. Colloid Interface Sei. 1986, 114, 234. (17) Kurihara. K.:Kunitake. T. J . A m . Chem. SOC.1992.114.10927. (18)Christenson,'H. K.; Parker, J. L.; Yaminsky, V. V. Langmuir 1992, 8, 2080. (19) Christenson, H. K.;Claesson, P. M.; Parker, J. L. J.Phys. Chem. 1992, 96,6725.
of data, raising questions about the stability of the hydrophobic layers.lJ*J9 The surface of mica is devoid of reactive groups; therefore covalent attachment by reactive organic molecules like chloro-, ethoxy-, or methoxysilanes is not possible. However, Parker et al.1° demonstrated that, by treating a mica surface with a plasma produced from water vapor, it was possible to introduce surface silanol groups which could then be utilized to chemically attach silanes. In these experiments the silanes were deposited from the vapor phase, and so they were restricted to the more-volatile (low molecular weight) silanes. The resulting hydrocarbon surfaces exhibited water-advancingcontact angles of -go", but they were not stable in electrolyte (KBr). Furthermore, surface force measurements of these surfaces show that these surfaces were charged, suggesting incomplete coverage of the mica surface by the low molecular weight silanes. It is generally believed that charged hydrophobic surfaces are not optimal for the measurement of the hydrophobic force.lOJ1 Higher molecular weight silanes have been deposited on mica by adsorption from solution ( ~ e l f - a s s e m b l y ) . ~ ~ ~ ~ ~ Carson and GranickZ2 prepared monolayers of OTS (octadecyltrichlorosilane)on mica which has been chemically pretreated by ion-exchanging the surface K+ ions with H30+ions. However, this procedure is cumbersome and requires the right amount of water and hydrochloric acid on the mica surface prior to treatment with OTS. Furthermore, OTS is highly susceptible to hydrolysis, and therefore the presence of water can result in the formation of gelslugs which can then settle on the substrate. In order to circumvent this problem, Kessel and G r a n i ~ k ~ ~ devised a scheme by which octadecyltriethoxysilane, a silane which is less susceptible to premature hydrolysis, could be deposited on mica as a monolayer by submerging mica sheets into a reaction mixture containing prehydrolyzed monomers of an alkyltriethoxysilane [ODSi(OH)3];the triol monomers then self-assembled on the mica surface to produce a monolayer. On baking the monolayer, a robust monolayer with a high wateradvancingcontact angle (111")and stable to cyclohexanone and water is formed. The Kessel and Granick method was attempted, but in our hands a smooth layer was never obtained. When the surfaces were mounted in the surface (20) Christenson, H. K.; Fang,J.; Ninham, B. W.; Parker, J. L.J. Phys. Chem. 1990,94, 8004. (21) Tsao, Y.-H.; Evans, D. F.; Wennerstrom, H. Langmuir 1993,9, 779. (22) Carson, G. A.; Granick, S. J.Mater. Res. 1990, 5, 1745. (23) Kessel, C. R.; Granick, S. Langmuir 1991, 7 , 533.
Q743-746319412410-2307$04.50/00 1994 American Chemical Society
Wood and Sharma
2308 Langmuir, Vol. 10, No. 7, 1994
force apparatus, good contact was not obtained in air, and the surfaces never jumped into contact. Additionally, using Si/SiO2 surfaces (polished silicon wafer with a 20 Anative Si02 coating) as test substrates, a layer in excess of 30A of ODSi(OH)3was invariably found by ellipsometry. This is an indication that the surface was composed of a combination of monolayer and gel slugs (Drobablv mepolymerized oligomers formed b i t h e self-c&densatidn of ODSi(OH)324z25). In this report a novel method of preparing a robust hydrophobic monolayer on mica is presented. Prepolymerized octadecyltriethoxysilane (OTE) is transferred by the Langmuir-Blodgett technique onto a plasma-activated mica surface. After baking the OTE-covered mica surfaces for 2 h a t 100 "C in a vacuum oven, the surfaces had contact angles consistent with a paraffin-like surface and the contact angles were stable over long periods when tested with potassium nitrate solutions. The surfaces also had a smooth jump-in and a flat contact when tested in the SFA.
Experimental Section Materials. Octadecyltriethoxysilane (OTE) was purchased from Petrarch and used without further purification. Hexadecane (Aldrich), chloroform (Baker), potassium nitrate (Baker), 0.1 N nitric acid (Baker), methylene iodide (Kodak), and methanol (Baker) were all used as received. Water was double distilled before being further purified by a Milli-Q water polishing unit to give a final resistivity of 18 MQ cm. In addition the water used in the surface force experiments was deaerated by heating under vacuum forabout 30 min. Surface Pressure Measurements. Surface pressure-area (JC-A)isotherms were obtained for OTE monolayers, over a pH 2 subphase, usinga KSV 5000 Langmuir trough (maximum area 67500 mm2). Solutions of OTE (2 mg/mL) in chlorofodmethanol (9%) were spread on the freshly aspirated subphase surface and allowed to stand for 30 (OTE) min (see text) before compressing the film at a rate of 15 m d s . All isotherms were run at least twice in the direction of increasing pressure using a freshly spread film. Under optimum conditions two isotherms run successively were completely superimposable; however, more typical error values were It 0.2 mN m-' in surface pressure and f0.005 nm2 molecule-' in area. All isotherms were recorded at 24 f 0.5 "C. Mica SurfaceModification. Freshly cleaved mica surfaceseither large sheets for the evaluation of Langmuir-Blodgett (LB) deposition transfer ratios or thin (1-3 pm), partially silvered sheets glued silver side down on quartz discs for use in the surface force apparatus-were employed as substrates for modification. Monolayers of OTE were transferred to mica substrates that had been treated with radio frequency-generated A r / H 2 0 plasma (2 min at %3W [low power setting on a Harrick Plasma Cleaner, PDC-236],500mTorr). In this arrangement, argon was a carrier gas used to introduce water vapor into the plasma chamber. The flow rate of argon was not known, but it was maintained at a level that resulted in a 500 mTorr pressure in the plasma chamber which was continually being evacuated using a rotary vacuum pump. Transfer of OTE monolayers by LB deposition was performed at 20 m N m-l. After drying in a clean airstream for 15min, these samples were baked in a vacuum oven (100 "C, 100 mTorr) for 2 h before use. As we shall see later, we have evidence that this procedure produces smooth and relatively homogeneous (low defect density) OTE surfaces on mica. (24) We note that Kessel and Granick rely on the value of infrared peak area (CHz and CH3 symmetric and asymmetric peaks) to confirm the presence of a monolayer. The infrared data, while supportive of a monolayer, does not eliminate the presence of isolated gel slugs on the surface since it is obtained as an average based on a large area. Furthermore, the uncertainty in integrating the peak areas could hide the presence of a few isolated gel slugs. Films thicker than a monolayer for mono-, di-, and trichloroalkylsilanes were also obtained by Trau et aLZ5using a solution silylation technique. (25) Trau, M.;Murray, B. S.;Grant, K.; Grieser, F. J.Colloid Interface Sci. 1992,148, 182.
.-7 \
1
" I
0.0
0.1
0.2
0.3
r
0.4
0.5
Area per Molecule. A / nmz
Figure 1. Surface pressure-area (PA) isotherm for OTE spread on a loW2 M nitric acid subphase (pH 2), T = 23 "C. Contact Angle Measurements. Advancing and receding contact angles were measured with a Ram6-Hart goniometer using pure water, aqueous potassium nitrate solutions (lo-', 10-2, and 10-3 M), hexadecane, and methylene iodide as test liquids. Advancing contact angles were obtained by increasing the volume of a drop-the maximum angle attained before advancement of the contact line was recorded as the advancing angle. Receding contact angle measurements were made by decreasing the volume of a drop and recording the minimum angle obtained before the contact angle receded. All contact angle measurements were accurate t o Itlo. Contact Thickness Measurements and Contact Evaluation. Contact thickness measurements of the OTE monolayer was carried out using the surface force apparatus designed by Israelachvilli and co-workers3. This apparatus employs fringes of equal chromatic order (FECO) to measure the separation of mica surfaces which are arranged in a crossed cylindrical configuration with a resolution of 0.2 nm. The FECO fringes were examined in order to determine if there was a flat contact between two OTE-covered mica surfaces. Adhesion forces, Fa, were measured by the pull-off force methodZ6sz7 in which the distance to which the two surfacesjump out on separation is recorded and multiplied by the spring constant. Surface energies may then be calculated by application of either the Johnson, Kendall, and Roberts (JKFt)280rDerjaguin, Muller, and Toporov (DMT)29theories of adhesion mechanics.
Results and Discussion OTE A-A Isotherm and OTE Deposition on Activated Mica. The JC-A isotherm obtained for OTE spread on an acidic (pH 2) subphase presented in Figure 1 is similar to that previously recorded for octadecyltrichlorosilane (OTS)spread on a similarly acidic surface.30 In that study, surface X-ray diffraction was performed on the monolayer a t the airlwater interface from which the degree of polymerization was estimated to be 155. The absence of a surface pressure response beyond molecular areas of ca. 0.25 nm2 suggests that the monolayer is composed of isolated, polymerized islands surrounded by pure water. Similarisotherms and their implications have also been reported for phospholipids and fatty acids spread at the airlwater i n t e r f a ~ e . Since ~ ~ , ~the ~ formation ofboth (26) Horn, R. G.; Israelachvili, J. N.; F'ribac, F. J.Colloid Interface Sci. 1987,115,480. (27) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N . J. Phys. Chem. 1991,95,10736. (28)Johnson, K. L.;Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971,324,301. (29) Derjaguin, B. V.;Muller, V. M.; Toporov, Yu. J.Colloid Interface Sci. 1975,53,314. (30) Barton, S. W.; Goudot,A.; Rondelez, F. Langmuir 1991,7,1029. (31) Gaines, G. L. Insoluble Monolayers at LiquidlGas Interfaces; Interscience: New York, 1966. (32) Luckham, P.;Wood, J.; Frogatt, S.; Swart, R. J.Colloid Interface Sci. 1992,153,368.
Robust Hydrophobic Monolayer on Mica
Hydrolysis EtO-Si-OEt OEt
Langmuir, Vol. 10, No. 7, 1994 2309
-1 .I I 1 Condensation
OH-$-OH
OH
OH-$ -0-si -0-$-OH OH OH OH
OH-Si-0-Si-0- Si-OH I
0
P
OH
d
P
Figure 2. Schematic representation of the silylation of an activated mica surface.
monolayers (OTE and OTS) requires hydrolysis of the alkylsilane before linear polymerization can occur in the plane of the aidwater interface (see schematic of silane polymerization, Figure 21, the similarity between the isotherms of these two alkylsilanes should not be surprising.33 The procedure used for depositing OTE on mica was optimized to produce stable and neutral hydrophobic layers in order to eliminate ambiguity in interpreting the interaction forces between hydrophobic surfaces in water. In order to achieve the desired stability, it was decided that the mica would have to be activated in order to provide sites for chemisorption with the monolayer coating. Activation of mica is necessary as there are no reactive sites on the cleavage plane of a mica crystal. The outermost oxygen atoms are part of the aluminosilicate structure.1° Parker et al.1° found that they could activate mica using a water plasma; the procedure introduces silanol groups to the mica surface which can then be reacted with silanating agents to produce a wide variety of surface groups.34 The plasma-treated mica surfaces remain s m o ~ t h . l A ~ ,layer ~ ~ of trialkylchlorosilanes was then deposited on the activated mica surface from the vapor phase. The resulting surfaces were charged and the contact angles were not stable when exposed to electrolyte suggesting incomplete methy1ation.lobJ5 Our rationale was that this deficiency might be overcome by prepolymerizing an OTE36 monolayer at the aidwater interface30a7before covalently attaching this layer to an activated mica surface (Figure 2). Furthermore, because of the cross-linking between the OTE molecules, very few anchoring points with the activated mica surface would be necessary.38 (33) Indeed, we surmise that the reason the initial surface pressure of our OTE monolayer required 30 min to decay back to zero mN m-l after spreading,while OTS monolayerson a water (or extrapolate from Barton's data30this at pH 2) subphase only required less than 5 min, may be explained in terms of the initial stage of hydrolyzingthe three ethoxy groups (rate of hydrolysis, chloro- > methoxy- > ethoxyalkylsilanes). (34) Plueddemann,E. P. Silane CouplingAgents;Plenum: New York, 1982. (35) Senden, T. J.; Ducker, W. A. Langmuir 1992, 8, 733.
.
Trial experiments in which the prepolymerized OTE layer was transferred to either a regular mica surface or activated mica surface had initial advancing water contact angles of 105", but this decayed rapidly over the course of a few minutes as the drop was observed to spread. On curing the surfaces in a vacuum oven at 100 "C for 2 h, stable contact angles were obtained for both types of OTE/ mica preparation. Differences between the two types of preparation were however obvious when tested in the surface force apparatus. Both cured surfaces gave good contact in air, but only the OTE deposited on activated mica gave a good and reproducible contact when tested under water indicating that both thermal treatment and activation of the mica are necessary to make a robust OTE layer. Thus, as a final step in creating a "perfect" and stable OTE monolayer, the OTE-coated mica was cured at 100 "C for 2 h. The baking procedure is expected to iron out possible strained configuration between grain boundaries and also stitch individual islands together across grain boundaries by completing the in-plane polymerization. Additionally, the heating is also expected to remove vicinal water sandwiched between the OTE silanol groups and the mica surface thereby creating a stable monolayer. It is well known that the stability of self-assembled octadecyltrichloroisilane (OTS) monolayers depends on the amount of water present.39 An optimal amount of water is required since an incomplete monolayer is formed when too little water is present; when a thick water layer is present, OTS polymerizes with the surface and this could easily be floated ~ f f . According ~ ~ , ~ ~to spectroscopic e v i d e n ~ e ~following l-~ the reaction between chlorosilanes and silica, temperatures in range 330-500 "C are required for bonding to a s u r f a ~ e , 4however, ~,~ some covalent bonding at temperatures in the range 100-200 "C cannot be ruled out. Furthermore, the spectroscopic work referred to above refer to chlorosilane reacting with silica as opposed to plasma-treated mica. This could be an important difference with respect to surface bonding at lower temperatures, as in plasma-treated mica, surface hydroxyl groups are attachedto silicon atoms (and perhaps aluminum atoms) which makes the surface behave more like a silica surface in which Lewis acid groups have been incorporated. The effect of this is to vastly increase the acidity/activity of the surface silanol gr0ups,4~and this could lead to covalent bond attachment of the ethoxysilane with the plasma-treated mica surface when baked at 100 "C. Our OTE layers were not stable when tested in the surface forces apparatus unless the OTE layer was deposited on an activated mica surface. Thus we conclude that perhaps a small amount of covalent attachment does occur at 100 "C. In this study it is apparent that activation of the mica surface and thermal curing of the transferred (36) OTE was used in preference to OTS (octadecyltrichlorosilane) as it was thought that OTE hydrolyzes at a slow enough rate that it would spread to a monoalyer before hydrolysidpolymerizationbegins. OTS is known to hydrolyze very easily.s7 Extreme care would be necessary to avoid polymerizationbefore spreading, although we note that Barton et aLsl were able to avoid premature polymerization. (37)Okahata, Y.; Yokobori, M.; Ebara, Y.; Ebato, H.; Ariga, K. Langmuir 1990,6,1148. (38)The amount of active silanol sites on the mica will be severely reduced because the activated mica is submerged in water before monolayerdeposition;1oa however,it is believedthat only a few anchoring points are required to stabilize a polymerized network on a surface. (39)Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (40) Silberzan,P.; Leger, L.; Ausserre, D.;Benattar, J. J. Langmuir 1991. 7,. 1647. --(41) Trip~,C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (42) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (43) Tripp, C. P.; Hair,M. L. J.Phys. Chem.ls93,97, 5693. (44) Tripp, C. P.; Hair, M. L. Langmuir, 1991, 7, 923. (45) Hair, M. L. J. Colloid. Interface Science 1977, 60, 154.
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Wood and Sharma
2310 Langmuir,Vol. 10,No. 7,1994
Table 2. Contact Thickness Measurements and Adhesion Force (pull-off force) of L-B Monolayers of OTE Deposited on Mica in Air and When Immersed in Water Using the SFk Surface Energy Values Have Been Calculated Using the Adhesion Force Values in the JKR and DMT Theories of Adhesion Mechanics layer thickness (nm) adhesion force (mN m-l) surface energy in air in water in air in water (mJ m-2) (in air) 4.2 4.2 270 480 29 (22,DMT)
Table 1. Summary of Advancing (0.) and Receding (0.) Contact Angle Data for OTE-Coated Mica Surfaces contact angle, den test liquid water
10-l M KNO3
lo-' M KNo3 10-3 M KNo3 hexadecane methylene iodide
@a
or
112 110 110 110 45 72
93 94 94 94 39 68
we write
OTE monolayer are critical to the formation of a robust hydrophobic OTE monolayer. Contact Angle Characterization. Advancing and receding contact angles for various test liquids on a OTE monolayer deposited on flat mica sheets are presented in Table 1. The advancing contact angles values of 112" for water, 45" for hexadecane, and 72" for methylene iodide are in good agreement with literature data for a closedFurthermore, the packed layer of hydrocarbon monolayers were robust as advancing and receding contact angles did not change over several hours when tested with KN03 solutions. Potassium ions apparently compete for sites on the mica surface by an ion-exchange mechanism." The fact that the contact angles were stable when tested with KN03 solutions implies that the OTE monolayer underneath the drop was not displaced by K+ ions. The contact angle hysteresis of 19"is larger than 10"observed for self-assembled monolayers on siliconwafers& and also larger than 3" for OTE on mica prepared by Kessel and G r a n i ~ k We . ~ ~believe this difference is attributed partly to the method used to measure the advancing and receding contact angles. Wasserman et a1.& and Kessel et ~ 1 . ~ have measured the angle obtained aRer a drop has advanced and &r a drop has receded, neither of which give the maximum (e,) and minimum (6,) contact angle. According to Wasserman et u1.* the advancing contact angle was 2-3" smaller than the maximum angle. If this is used as a guide, the minimum angle would also be 2-3" smaller and the hysteresis increases to 14-16". This is only 3-5" smaller than obtained for the OTE surfaces prepared in this study. Contact angle hysteresis on our OTE surfaces were also measured by advancing and receding a drop. As expected, the "advancing" contact angle reduced to 108" and the hysteresis decreased to 8". Contact Evaluations Using the S F k The contact distance, i.e., the thickness of the two hydrophobic OTE layers was 4.2 nm (or 2.1 nm for each monolayer). This is slightly smaller than the transextended length of 2.6 nm for RSiO (R = C18) and that typically obtained by ellipsometry for RSiO on silica.47 This difference is suggestive of compression of the layer on contact and is in excellent agreement with a similar trend observed by Levins et al.48for alkyl thiols on silver; they obtained a thickness of 2.15 nm for a C18 thiol from contact distance measurements using the surface force apparatus. Adhesion forces in air and water are comparable to previous results (Table 2). Application of JKR and DMT theories give surface energies which are in the appropriate range for hydrocarbon surface^.^' Lastly, from JKR theory ~
~~
(46)(a) Pomerantz,M.;Segmuller,A.; Netzer, L.; Sagiv,J. Thin Solid Films 1986,132, 153. (b) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J . Am. Chem. Soc. lSSf3,110,6136.(c) Wasserman, S.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989,5, 1074. (47)DePalma, V.; Tillman, N. Langmuir 1989,5, 868. (48)Levms, J.M.;Vanderlick, T.K. J.Phys. Chem. 1992,96,10405.
F
$air) = 3nysv
F
where -(air) is the maximum forcelper unit length R required to separate two solid (S)surfaces in their vapor (V). Similarly,
F
+water) = 3nysL R is the maximum forcdunit length required to separate two surfaces when immersed in a liquid (L). UsingYoung's Equation, it follows that
F F $air) - $water) = 3ny,
3
cos 8
Inserting values from Table 2 and using YLV = 72.2 mN m-l for water, a contact angle of 108" is obtained. This is in reasonable agreement with the measured contact angle of 112", as shown in Table 1. This procedure which verifies Young's Equation, was first used by Johnson et a1.,2aand has since been used successfully by ~ t h e r s . ~ ~ - ~ l
Conclusion Mica has been made hydrophobic by L-B deposition of a prepolymerized monolayer of OTE. Two OTE-covered mica surfaces jump into contact in the surface force apparatus without hesitation forming a flat contact indicating the formation of a smooth layer. Contact angle values for various test liquids are consistent with published data for hydrocarbon surfaces. Contact angles measured using potassium nitrate solutions were stable for several hours indicating that the layers were stable. The contact thickness value of a monolayer of OTE on mica was 21A, which is in good agreement with published values for a C18 chain. This value did not change when the surfaces were submerged in water, which indicates that the layers were stable even when fully submerged in water. Using JKR theory and Young's Equation, a water contact angle of 108" was calculated using the adhesion force measurements between two OTE surfaces in air and in water. This compares very well with the measured ~ be shown value of 112". In a future p u b l i c a t i ~ nit, ~will that the OTE surfaces prepared by the novel method outlined in this report were very hydrophobic-spontaneous cavitation (or cavitation on contact) was observed when two OTE surfaces submerged in water were brought into contact. (49)Pashely, R. M.;Israelachvili, J. N. Colloids Surf. 1981,2, 169. (50) Pashely, R. M.; McGuiggan, P. M.; Horn,R. G.; Ninham, B. W. J . Colloid Interface Sei. 1988, 126, 569. (51) Israelachvili, J. N. A d u . Colloid Interface Sci. 1982,16, 31. (52)Wood, J.; Sharma, R. Polym. Prepr. 1993, 34 (2),294.
