Langmuir 1991, 7, 156-161
156
Contact Angle Stability: Reorganization of Monolayer Surfaces? Stephen D. Evans, Ravi Sharma, and Abraham Ulman' Corporate Research Laboratories and Manufacturing Research a n d Engineering Organization, Eastman Kodak Company, Rochester, New York 14659-2109 Received M a y 29, 1990 Contact angle variation was studied on self-assembled alkanethiol monolayers containing mixtures of OH and CH3 groups at their air/monolayer interface. It was found that these high free energy organic surfaces yielded contact angles which were not stable over long periods of time. The extent of the variation was found to be related to the surface free energy (percent OH). The effect of different storage environments and temperature on the changing contact angles is discussed. We propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize, over a time dependent on the chain length, by surface reorganization and the adsorption of contaminants.
Introduction
nitrogen. Gold-coated silicon substrates were made hydrophilic by a brief (-5 min) immersion in the same cleaning solution, The possibility of reorganization of monolayer surfaces followed by a thorough rinsing in Millipore water, immediately was first encountered in our laboratory during the prepbefore monolayer adsorption. Note that long exposures to the aration of hydroxylated surfaces for use in the process of hot H202/H2S04 solution may result in an apparent roughening multilayer formation.' Conflicting with Sagiv's r e p ~ r t s , ~ , ~ of the gold surface. The clean gold surfaces were examined by Auger spectroscopy, and only "normal" carbonaceous-type conwe found that freshlyprepared OH surfaces (by t h e LiAlH4 tamination was found. No chromium was found, indicating that reduction of an ester group at t h e surface of a selfchromium had not diffused from the adhesion layer to the outer assembled monolayer of methyl 23-trichlorosilyltrisurface during the age of the sample (2 weeks). cosanoate) have advancing water contact angles of 30'. Monolayer Preparation. Self-assembledmonolayersof thiAfter these surfaces were exposed to boiling chloroform ols were spontaneously adsorbed by immersing the fresh, hyor C C 4 for 10 min, t h e advancing water contact angles drophilicgoldsubstrate intoa freshlyprepared 2 X 10-3 M solution rose to ~ 6 0 ' . Upon 30 h of immersion of t h e treated of the thiol(s) in THF. The substrate was taken out after 24 h, samples in cold water, t h e water contact angle decreased washed with distilled THF, then with absolute ethanol, followed to - 4 7 O . l Although known for polymer surface^,^ it was by a thorough rinsing in Millipore water and drying (N2). Film difficult to explain this result since the perception was, at thickness was immediately estimated by ellipsometry (five readings), and water contact angles (advancing and receding) the time, that it is impossible to have such a surface were measured (five readings). reorganization in a closely packed, "solid-like" monolayer. Ellipsometry Measurements. Film thickness was estimated In view of this, we decided to investigate the phenomenon by using a Gaertner L116B ellipsometer equipped with a 632.8of contact angle variation in OH surfaces, in different amnm helium-neon laser. Optical constants for the bare substrates bients and time, on monolayers of alkanethiols on gold were predetermined for 5-10 spots on the cleaned, uncoated surfaces. surfaces for each sample immediately prior to immersion in the thiol solution. Measurements employed an estimated film Experimental Section refractive index nf = 1.50. The variation (standard deviation) in measured thickness across the dimensions of an individual NMR spectra were obtained on a GE QE-300 instrument at sample was on the order of f l A. 3 0 MHz for proton and 75 MHz for 1sCspectra in CDCl3solutions (unless otherwise specified), and shifts are referenced to TMS Contact Angle Measurements. Contact angles were deinternal standard. All chemical shifts (6)are in parts per million termined by using a Rame-Hart NRL 100goniometer. The water (ppm), and coupling constants are in hertz (Hz). Boiling and contact angles (advancing, e,, and receding, e,) were measured melting points are not corrected. on droplets which were "captive" between the monolayer surface Substrates. Gold substrates were prepared by resistive and a hypodermic needle (square cut). This was achieved by evaporation of gold (99.99 %),froma tungsten holder, ontosilicon forming a droplet on the end of the needle and carefully lowering wafers (75 mm X 25 mm) at room temperature. A chrome it so that it touched the substrate surface. The advancing contact adhesion layer was evaporated prior to the gold layer. The angle was the maximum angle measured when the volume of the deposition rates and sample thickness were monitored with a droplet was increased without increasingthe solid-liquid interface quartz oscillator. Thicknesses were typically 150/2000A for the area. Likewise, the receding angle was the minimum angle chrome/gold samples. The deposition rates were typically 10 measured for the droplet as its volume was reduced without the A/s. The evaporation chamber was kept at 7.5 X 10"Torrduring solid-liquid interface area decreasing. the evaporation (using a Turbo molecular pump), and the S u r f a c e S p e c t r o s c o p y M e a s u r e m e n t s . 1. X P S substrate temperature was at or slightly above room temperature. Measurements. Measurements were carried out on a Surface After the evaporation, nitrogen was used to backfill the chamber. Science SSX-100 instrument, with a take-off angle of 35", using The silicon wafers were cleaned by heating at 90 "C in a mixture A1 K a radiation. Values were normalized to 100 atomic % for of 30% H202 and concentrated H2SOd (30:70 v/v) for 30 min, the elements detected and were rounded to the nearest atomic rinsed with plenty of clean water, and dried with a stream of percent. XPS concentration values should be used only for comparison between the two samples and are not to be taken as (1)Tillman, N.; Ullman, A.; Penner, T. L. Langmuir 1989,5, 101. absolute. (2) Pomerantz,M.;Segmuller,A.;Netzer,L.; Sagiv,J. Thin SolidFilms 2. Auger Measurements. Measurements were carried out 1985, 232,153. on a PHI Model 590 ESCA-Auger spectrometer, with a take-off (3) Maoz, R.; Netzer, L.; Gun, J.;Sagiv,J. J . Chem. Phys. (Paris) 1989, angle of 35", using A1 Ka radiation. 85, 1059. (4) Holmes-Farley, S. R.; Whitesides, G. M. Langmuir 1987, 3, 62. 21-Hydroxyheneicosane-1-thiol.1. 11-(Trimethylsil-
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0 1991 American Chemical Society
Langmuir, Vol. 7, No. 1, 1991 157
Contact Angle Stability oxy)-1-bromoundecane. 11-Bromoundecan-1-01(Aldrich)(50 g, 0.2 mol) was dissolved in 200 mL of absolute ethyl ether containing 16.6 g (0.21 mol) of dry (KOH)pyridine (Kodak) at 0 O C under nitrogen, and with mechanical stirring. Trimethylsilyl chloride (Aldrich) (28.5 g, 0.21 mol) was added during 30 min, after which stirring at 0 O C was continued for an additional 15 min. Filtration, followed by evaporation of the ether under reduced pressure, left a liquid which was distilled in vacuo: yield 55 g (85%);'H NMR ( 6 , CDC13) 0.10 (s, 9 H), 1.27 (s,12 H), 1.41 (m, br, 2 H), 1.49 (m, br, 2 H), 1.84 (quintet, J = 7.4, 2 H), 3.39 (t, J = 6.8, 2 H), 3.55 (t, J = 6.7, 2 H). 2. 21-Bromoheneicosan-1-01.A solution of 60 g (0.2 mol) of 1,lO-dibromodecane(Aldrich), in 200 mL of dry (CaH) distilled THF containing 8 mL of 0.1 M LirCuCll in THF (Alfa), under nitrogen, was cooled to 0 "C. To that was added, over 3 h, a solution of (11-(trimethylsi1oxy)undecyl)magnesiumbromide (prepared from 32.3 g (0.1 mol) of 11-(trimethylsi1oxy)-1bromoundecane and 2.55 g (0.105mol) of magnesium,in 200 mL of dry THF. The solution was stirred at 0 O C for 1 h, allowed to warm to room temperature, and quenched with 200 mL of saturated ammonium chloride solution. It usually takes a few hours for the copper salts to dissolve, and the solution was left overnight. The organic layer was decanted, and the blue aqueous layer was extracted with ether (2 X 100 mL). The organic layers were combined and washed with saturated NaCl solution, and the solvent was evaporated under reduced pressure. The residue was dissolved in a hot 0.1 M HCl solution in 2:l ethanol-water to ensure complete hydrolysis of the silyl ether. The product was crystallized twice from heptane: yield 25.4 g (65%); mp 75-77 "C; 'H NMR (6, CDCl3) 1.25 (s, 30 H), 1.39 (m, 4 H), 1.61 (quintet,J = 7.3,2 H), 1.85 (quintet, J = 7.1,2 H), 2.52 (q, J = 7.3), 3.41 (t, J = 6.8, 2 H); 13C NMR (6,CDCls) 18.83,21.31, 21.91, 22.58, 22.81, 25.96, 27.20, 56.21. 3. 21-Hydroxyheneicosane-1-thiol.21-Bromoheneicosan1-01(11.73g, 30 mmol) was added to a solution of sodium (0.69 g, 30 mg-atom) and thiolacetic acid (2.4 g, 31.5 mmol) in 100 mL of absolute ethanol, under nitrogen. The solution was refluxed for 1 h, 5 mL of concentrated HC1 was added, and refluxing was continued for 3 h. The solution was cooled to room temperature and was poured on degassed cold water. The product was filtered and chromatographed on silica gel (Aldrich, 230-400 mesh, 60 A) with 25% CHzClz in hexane to give 5.5 g (53%)of white solid: mp 73-75 "C; lH NMR (6, CDC13) 1.25 (s on a broad multiplet,34 H), 1.59 (m, 4 H), 2.52 (q, J = 7.3), 3.64 (4, J = 5.6, 2 H);I3C NMR (6, CDC13) 17.81, 18.90, 21.53,22.22, 22.59, 22.71, 25.93, 27.19, 56.13.
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Figure 1. Advancing (0) and receding ( 0 )water contact angles on a HUT monolayer as a function of monolayer age.
Sample Preparation a n d Characterization. 11Hydroxylundecane-1-thiol(HS(CH2)110H, HUT) forms monolayers on gold (HUT/Au, OH ~ u r f a c e ) with ,~ advancing contact angles of < 5 O for water and Oo for hexa0 for HD when decane (HD) and of -20° for water and ' adsorbed from ethanol6 and T H F solutions, respectively. 1-Dodecanethiol ( C I ~ H ~ ~ DDT) S H , also forms ordered monolayers on gold surfaces (DDT/Au, CH3 surface) with advancing contact angles of 110' for water and 43O for HD. Surfaces containing mixtures of OH and CH3 groups were formed by adsorbing molecules from T H F solutions containing DDT and HUT. By varying the ratio of HUT to DDT in the solution, one can directly vary the surface composition of OH and CH3 groups. It is important to note that for these systems the molecules used are of the same length, and hence the surface roughness due to differences in molecular length is minimal. For similar mixed monolayers adsorbed from ethanol, Bain and Whitesides showed that there was preferential adsorption of
one species over the other a t low concentration^.^^^ In particular, they found that for low OH concentration solutions the OH concentration of the monolayer surfaces was lower than expected, leading to the conclusion that there was preferential adsorption of CH3-terminated molecules from low OH concentration ethanol solutions.* XPS measurements were made in order to calibrate the relative concentrations of OH and CH3 groups on the surface, for a series of samples. It was found that the ratio of OH/CHs-terminated thiols adsorbed onto gold substrates was essentially the same as the ratio of relative concentrations of the solutions from which they were adsorbed. The choice of solvent can, therefore, play an important role in the adsorption of molecules from mixed solutions. In choosing a solvent, T H F ( t = 7.31, which has a dielectric constant somewhere between those of heptane ( t = 1.7) and ethanol ( t = 24), we have demonstrated that it is possible to form monolayers for which the surface composition is very close to that of the solution. We note, however, that the difference between advancing water contact angles on HUT/Au adsorbed from ethanol and T H F solutions, may already be indicative of the effect which the dielectric constant of the solvent may have on the stability of these high free energy OH surface. Contact Angle Measurements. We initially studied the stability of HUT monolayers adsorbed on gold (HUT/ Au) by monitoring the advancing and receding water contact angles (Figure 1). From Figure 1 it is clear that the high free energy OH surface is not stable and that both advancing the receding water contact angles increase with time. The contact angles reaching their limiting values after 16 h in air, with the HUT surface exhibiting an advancing angle of -70'. The question of what is the relationship between surface OH concentration and the contact angle variation was the onset of our detailed study of mixed monolayers. It also became apparent that the mixed monolayers, containing large quantities of OH groups a t the surface, were not stable under ambient conditions (as determined by the changes of contact angles with time). In order to understand more about the processes taking place at the monolayer-air interface, a number of experiments were carried out in which the water contact angles were monitored as a function of monolayer age time and of sample storage. In Figure 2, the effects of storage, for samples kept under high and low relative humidities, for a range of monolayer surfaces are shown. One can see that the contact angles increased with time and seem to be independent of the storage conditions. There are two possible explanations for the observed increase in contact angles (decrease in surface free energy).
(5) Ulman, A.; Tillman, N. Langmuir 1989,5, 1418. (6) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.;Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. SOC.1989, I l l , 321.
(7)(a) Bain, C. D.;Whitesides,G. M. Longmuir 1989,5,1370. (b)Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111,7164. (c) Bain, C. D.; Eval, J.; Whitesides, G. M. J. Am. Chem. SOC.1989,111,7155. (8) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1988,110,6660.
Results and Discussion
Evans et al.
158 Langmuir, Vol. 7,No. 1, 1991 i
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Figure 3. Effect of ambient on samples sotred for 14 h (70% surface OH). 90
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Figure 2. Variation of water contact angles (advancing)as a function of time and relative humidity (0and A represent storage at high and low relative humidities,respectively,for 3 days): (a) contact angles measured on freshly prepared samples; (0, A) contact angles measured 3 days later on samples which had been stored under different humidity conditions (storage at high and low relative humidities,respectively); (e)a samplethat was stored for 2 days after treatment with ambient of acetic acid and washed before measurement; (*) fresh 78% OH sample;(m) same sample after storage at ca. -20 "C for 20 h; (0) the same sample taken from the freezer and stored for 24 h at ambient.
Firstly, and most commonly suggested, it may be due to contamination of the surfaces by adsorbed airborne contaminants such as hydrocarbons. The second possibility, however, is that the increase in water contact angles is intrinsic to the system and that the OH surfaces may be reorganizing. Although quite speculative,the latter may be reasonable since the monolayers have high surface free energies and may try to reduce it by "burying" the polar OH groups in the monolayer surface. If, indeed, the variation of contact angles is dependent on the surface free energy, then the extent of reorganization should increase with increasing surface free energy, Le., percent of OH groups at the surface. A close inspection of Figure 2 shows that this is the case, and the changes appear more dramatic for monolayers containing large numbers of OH groups. Comparing Figures 1and 2, one sees that the 100% OH surface, after 60 min, yields the same contact angles as found for freshly made sampleswith surface OH/CH3 ratios of -1:l. This is equivalent to a decrease in the surface free energy of nearly 30 mJ/mZs9 It is not easy to determine which one of the two options, i.e., surface contamination or reorganization, may be occurring, particularly since we are unaware of any method that would yield direct microscopic evidence of surface reorganization. Further studies have been made to try to distinguish the two possibilities, all of them indirect. In the first experiment, a clean, hydroxylated silicon wafer (oxidized SiOZ/Si, as described in the Experimental Section above) was exposed to ambient laboratory atmosphere overnight. The water contact angles on this aged surface were close to zero and indistinguishable from those for a freshly prepared surface. In the second, a 100% OH monolayer surface (water da = 2 2 O ) was placed in a vacuum chamber. It was then evacuated to mbar and backfilled with clean nitrogen to 1 atm. After 72 h, the advancing water contact angle was 37O. Since there is virtually no possibility of surface reorganization for the surface OH groups in the former case and the monolayer sample was exposed to a minimum of contaminants in the (9) The surface free energy values were calculated from the contact angles of diiodomethane and water by using the geometric mean approximation. Further details are to be published.
latter case, these experiments suggest that surface reorganization may be occurring in the organic monolayers. If surface reorganization does occur, its kinetics should be a function of the ambient (e.g., solvent polarity) and temperature. It is reasonable to expect that a polar solvent should stabilize a polar, high free energy surface and thus decrease the driving force for this process. The effects of storage environment on contact angles were then studied more closely on several samples, made from the same solution, cut from the same substrate, and hence with the same concentration of OH groups at the monolayer surface (in this case 70% surface OH). The samples were placed in different ambients, stored overnight, and then rinsed with methanol and Millipore water, after which the advancing and receding water contact angles were measured (Figure 3). It is evident that only the sample stored in Millipore water and glacial acetic acid (both are polar liquids) gave the same values as found for freshly prepared samples; the other samples all gave higher advancing and receding contact angles (with the exception of the samples stored in pentane, which displayed lower receding angles). I t is interesting to note that samples sent in ambient to the XPS facility at Harvard (usually arriving within 24 h) showed an increase in water advancing contact angles compared to that determined immediately after removal from the THF solutions. On the other hand, samples shipped under water showed no apparent change in their wetting properties.lO Submerging samples in a liquid drastically reduces the possibility of airborne contamination. The question of surface contamination is of crucial importance, especially in view of the speculative nature of the proposed surface reorganization mechanism. By exposing the surface to a known, washable, high-affinity contaminant (CH&OOH), we can protect the surface from airborne contamination. Washing the protected surface and immediately measuring the contact angle reduces the possibility of airborne contaminants affecting the measurements. These data are plotted in Figure 4 before and after washing of the acetic acid protection layer (the diamond in Figure 2 represents a reading of water advancing contact angle after 2 days of storage and a thorough washing with Millipore water). Note that the contact angles did not change, although the contaminant was removed as evident by a reduction of - 5 A in film thickness (Figure 5). One, therefore, can immediately conclude that the observed variation in contact angles is independent of surface contamination and that another mechanism may be operative. In an attempt to reverse the contact angle behavior, a 100% OH sample that has been exposed to ambient for (10) Laibinis, P. Private communication.
(11)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109,3559.
Langmuir, Vol. 7, No. 1, 1991 159
Contact Angle Stability 3 Oe-03
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Figure 4. Variation with time of advancing ( 0 , O )and receding (m, 0 ) water contact angles on an acetic acid contaminated and washed 70 5% OH sample, respectively. I
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Figure 5. Variation of film thickness with time on an acetic acid contaminated (0) and washed (0) 70% OH sample. 16 h was soaked for 96 h in Millipore water, at room temperature. T h e resulting monolayer showed no detectable decrease in its water contact angle. On the other hand, as mentioned already in the introduction, experiments in our laboratory on CClr-treated OH surfaces of reduced self-assembled methyl 23-(trichlorosilyl)tricosanoate monolayers showed that upon 30 h of immersion in water, at room temperature, the water contact angle decreased (before 57', after 47').' This difference between the long (C23) and short ((311) chain systems suggests that the observed changes in contact angles may be related also to the packing and order of the monolayer. It is not clear to us what microscopic events result in this difference, although one can consider transgauche isomerization as one possibility. However, it is reasonable to assume that in the more ordered, solidlike C23 monolayers,disorder probably concentrates at the surface and does not penetrate too deeply into the bulk, while in the C11 mixed monolayer systems the disorder penetrates more deeply into the bulk. As a result, some reversibility of the water contact angles could be observed in the C23 system, while there was no evidence of reversibility in the C11 systems. To examine temperature effects on the contact angle changes, a fresh 78% OH sample was stored at ca. -20 OC for 20 h. A very small increase in water contact angle could be detected (before 42', after 43'). The container was then left in ambient temperature for 16 h, after which time an increase of 5' in the water contact angle was detected (before 43', after 48'). All these results are presented in Figure 2. Note that in an original 78% OH sample that was stored in ambient for 3 days the increase in water contact angle was 21' (before 40°, after 61'). These results bring up the question of what possible mechanism is responsible for the changes observed in the
Figure 6. Grazing angle FTIR spectra of an esterified fresh HUT/Au sample (dotted line) and of a 24-h-aged HUT/Au sample (solid line).
water contact angles. We have already discussed in detail experiments made t o distinguish between surface contamination and the possible surface reorganization. Wetting is a macroscopic phenomenon which, however, has been shown to be very sensitive to microscopic changes in the surfaces studied.417~~Therefore, any surface reorganization mechanism would have to be substantiated by additional evidence, preferably on the microscopic changes in the monolayer assembly. In principle, FTIR could be used to detect trans-gauche isomerization. However, for the C11 mixed monolayers under study, the introduction of gauche bonds, even to every second molecule, will result in less than 5 % gauche bonds in the system. This is not a detectable gauche bond concentration, given the signal-to-noise in the FTIR instrument. Therefore, we decided to examine surface reactions in an attempt to support the reorganization mechanism. The reaction of OH surfaces with acid chlorides (e.g., alkyltrichlorosilanes and acid chlorides) is of interest since it is a key step in the construction of multilayers from alkyltrichlorosilane derivatives.' To quantify the extent of the removal of available OH sites with time,12 we examined the reaction of fresh HUT/Au samples and samples that were stored in ambient for 24 h with acetyl chloride. Samples were exposed to an ambient of acetyl chloride for ca. 30 min, washed thoroughly with Millipore water, and and the FTIR spectra were recorded immedidried (Nz), ately. We believe that the acylation reaction is not retarded by organic contaminants, since acetic anhydride is probably soluble in them a n d t h e reaction is a s p o n t a n eous one. Figure 6 presents the results for the esterified surfaces. The ratio of the areas under the C-0-C peaks, a t -1250 cm-l, gives a slightly lower surface OH concentration for the sample stored in ambient than one would expect from the water contact angle measurements and corresponds to a depletion of available OH groups by -60%, as compared to that of a fresh sample. We note that FTIR is also a macroscopic experiment but has been shown to give detailed structural information.ll However, IR peak intensities are orientation dependent and thus cannot be used for a quantitative study of the changes occurring in surface composition. XPS on the other hand, although not sensitive to structural change at the surface, should provide the most quantitative data on surface reactions. Thus, we repeated the reaction with a freshly prepared (water = 29O, Br = 12') sample and a 3-days aged (e, = 70°, dr = 47') sample in an ambient of trifluoroacetic anhydride (30 min). Water contact angles were (12) We do not mean the physical removal of OH groups from the surface, but rather in their ability to react with the acid chlorides, due to the proposed reorganization, or surface contamination.
Evans et al.
160 Langmuir, Vol. 7, No. 1, 1991 Table I. XPS Results. for Trifluoroacylated Fresh and Aged Surfaces element fresh aged
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Figure 7. Variation of advancing water contact angles on HHT/ Au with temperature and time: (A) contact angles measured at 5 "C; (0) contact angles measured at 25 "C; (0)contact angles measured at 50 O C .
0, = 93",Or = 85" and Oa = 81", Or = 58" for the trifluoroacylated fresh and aged surfaces, respectively. The XPS results for the two samples are summarized in Table I, where elements were detected; their atomic concentrations are given. The difference in fluorine concentration indicates a significant difference in surface trifluoroacetyl concentrations. It is evident from both experiments that there is a considerable difference between the availability of surface OH groups in the fresh and aged samples, and, therefore, the proposed reorganization mechanism is supported. It is clear that if one used a longer chain length and observed similar results the contamination mechanism would be supported. However, if the results were different and could be related to the different chain lengths, then surface reorganization could be supported. We therefore prepared 21-hydroxyheneicosane-1-thiol (HO(CH2)21SH,HHT) and carefully studied the advancing water contact angles on HHT/Au as a function of time and temperature. Figure 7 presents the results. Two important points should be emphasized here: (a) While it took 30 min, a t 25 "C, for the advancing water contact angle on HUT/Au to increase from -25" to -60°, a similar increase in HHT/Au could not be observed a t 25 "C; in fact, after -20 h the contact angle was -35". (b) It took -20 h, at 50 "C, for the advancing water contact angle on HHT/Au to increase to -60'. Although one cannot (and should not) rule out completely some surface contamination, it is clear from these experiments that some surface reorganization took place in the samples under study. Firstly, one can conclude from the difference in results at room temperature that there should be reorganization in the HUT/Au monolayers (since contamination is expected to be the same in both samples). Secondly, t h e behavior of H H T / A u a t different temperatures strongly indicates a process similar to orderdisorder transition (see below) and therefore supports reorganization in both the HUT/Au and HHT/Au samples. The question of reorganization mechanism is of interest, and to address it we have to resort to studies made on order-disorder transition in monolayers. Two studies are
relevent to our problem, The first is by Riegler, who used electron diffraction to study the thermal behavior of LB films and clearly observed a premelting disordering of the films, which was dependent on alkyl chain length.13 A sharp decrease in the diffraction peak intensities was detected at -35 "C for cadmium stearate (c18)a t -55 "C for cadmium arachidate (Czo) and a t -75 "C for cadmium behenate (CzlH&OOH, CZZ), with no apparent change in the hexagonal geometry of the two-dimensional assembly. The author suggested that the intensity decrease may be associated with thermally induced random tilt orientational disorder, or bending &e., trans-gauche isomerization), of the chains. The second is by Nuzzo et al., who studied temperature-dependent phase behavior in longchain docosanethiol (CBH~SH) monolayers on gold(lll).14 In this study, they cooled the monolayers to 80 K and observed significant changes in the IR spectrum. They concluded that gauche conformations, concentrated at the chain termina, existed at temperatures in excess of 200 K. Such conclusions that trans-gauche isomerizations occur predominately near the chain termina are, of course, extremely relevant to the present study. In our case, we observed a fast increase in advancing water contact angles for the C11 monolayer at 25 "C, while for the Cp1 analogue the change was much slower but was accelerated upon heating to 50 "C. Such results are in agreement with the expected correlation to alkyl chain length. This, therefore, suggests that surface reorganization is the result of trans-gauche isomerizations at chain termina, which may start from defects such as pinholes and grain boundaries. While the latter assumption is somewhat speculative, it is in agreement with the behavior observed for the two different systems, since one can assume a more crystalline, close-packed, ordered monolayer, with fewer defects and pinholes, in the case of the longer Czl chain. We also suggest that, in the case of the short C11 chain, surface gauche bonds may penetrate deeply into the bulk thus disrupting the entire assembly. If this is the case, surface reorganization may be an "autocatalytic" process. This is because disruption of long-range order in the twodimensional assembly creates more defects, which in turn makes reorganization easier. With all these experiments, we are still left with one unresolved question: is there a relationship between surface contamination and surface reorganization? That is, can contamination trigger reorganization? While the results described in this report strongly indicate that surface reorganization does occur, they do not address this question, and we leave it open.
Conclusions In conclusion, we propose that monolayer surfaces containing high concentrations of OH groups on mobile organic chains are not stable. Such monolayer surfaces may stabilize, over a time dependent on the chain length, by surface reorganization and the adsorption of contaminants. Although no evidence for the microscopic changes in the monolayer bulk could be provided, XPS results clearly indicated major changes in the availability of OH sites a t the monolayer surface. The question of how general can our observation be made is of importance. We presented studies made on both short (Cll) and long (&I) alkyl chain systems and therefore suggest that this should be a general phenomena, (13)Riegler, J. E. J. Phys. Chem. 1989,93, 6475. (14) Nuzzo, R. G.;Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 161.
Contact Angle Stability
with kinetics strongly dependent on chain length. It is also reasonable to assume that other polar groups such as cyano (CEN), when located at chain termina, should be prone to similar reorganization processes. This is, a t present, under study in our laboratory. Finally, it is evident from the studies presented here that contact angles of organic monolayers may be dependent on the dielectric constant of the solvent from which solution it was adsorbed (e.g., ethanol or THF) and that the contact angles of these surfaces change with time (e.g., Ba for water on HUT/Au increased from 20° to 50° in 30 min). Therefore, extreme caution is called for in reporting contact angle values, and it is suggested that measurements are made immediately after monolayer adsorption. It was also found that the rate of contact angle variation was temperature and ambient dependent. Therefore, mono-
Langmuir, Vol. 7, No. 1, 1991 161
layers should be stored in a clean ambient a t a low temperature (e.g., -20 OC) for further studies.
Acknowledgment. We thank Paul Laibinis and Prof. George M. Whitesides, both of the Department of Chemistry, Harvard University, for the ESCA measurements of surface OH concentrations and for their interest and suggestions. We thank Robert Streber of the analytical division, Eastman Kodak Company, for the ESCA measurements of the fluorinated surfaces. We also want to thank Michael Rubinstein and Tom Penner of the Corporate Research Laboratories, Eastmak Kodak Company, and Prof. Steve Garoff of the Department of Physics, Carnegie-Mellon University, for many useful discussions. Registry No. Au, 7440-57-5;Si, 7440-21-3;Cr, 7440-47-3;21hydroxyheneicosane-1-thiol,129787-62-8.
