J. Phys. Chem. 1995,99, 12257-12267
12257
Temperature-Programmed Desorption and Infrared Studies of D20 Ice on Self-Assembled Alkanethiolate Monolayers: Influence of Substrate Wettability I. Engquist,” I. Lundstrom, and B. Liedberg Laboratory of Applied Physics, Linkoping University, S-581 83 Linkoping, Sweden Received: January 23, 1995; In Final Form: June 2, 1999
This paper examines the relationship between the thermal desorption of thin overlayers of condensed D20 ice and the wettability properties of the supporting substrate surface. Mixed self-assembled monolayers ( S A M s ) on gold with controlled chemical composition and wettability (-0.4 < cos 8 < 1.0, where 8 represents the static contact angle with water) derived from HS(CH2)160H and HS(CH*)&H3 were used as model surfaces. The D20 ice overlayers were prepared on these substrates by dosing of 0.1-30 langmuirs of D20 in ultrahigh vacuum at 80- 120 K and characterized with temperature-programmed desorption (TPD). Infrared reflectionabsorption spectroscopy (IRAS) was also used to characterize the structural progressions within the overlayers during the course of the TPD experiments, as well as at selected temperatures before and after annealing of the overlayer structure. The IRAS data show that amorphous-like ice is formed at sufficiently low temperatures (5100 K) on all mixed S A M s , regardless of their wettability. A structural transition of the D20 ice from amorphous-like to polycrystalline-like is observed above 100 K. The exact onset of the transition is strongly dependent on the wettability and varies from about 110 K on the extreme hydrophobic (CH3) substrate to 145-150 K on the hydrophilic (OH) substrate. On the most hydrophilic substrates, the strong hydrogen bond interaction with surface hydroxyls prevents completion of the structural transition before desorption of the D20 overlayer. This type of pinning of the D20 molecules to the substrate surface is most likely responsible for the sharp increase in desorption energy of -0.2 kcallmol which is seen at cos 8 FZ 0.6, a value defining the hydrophilicity limit above which, for our set of experimental parameters, the transition is no longer completed. The TPD data also support a model of the D20 overlayer as forming clusters of very different shape depending on substrate wettability-flat, two-dimensional clusters on hydrophilic S A M s and dropletlike, three-dimensional clusters on hydrophobic SAMs.
Introduction
and c o - w ~ r k e r s , ~who ~ - ~have ~ been investigating water adsorption on SAMs terminated by CH3, OH, COOH, CONH2, CH2, The structural properties of liquid water and ice have been and COOCH, groups, using TPD and IRAS. It is found that the subject of several investigation^.'-^ Especially, their surface water binds more strongly to polar than to nonpolar surfaces, and interface properties have attracted considerable interest in that hydrogen bonding may contribute to this increased bonding recent A better understanding of these properties is strength, and that a structural transformation of the condensed important for a number of phenomena, such as the mechanism water from amorphous to polycrystalline ice occurs on these of wetting?.l0 surface induced nucleation,”-I3 surface meltsurfaces between 120 and 130 K. Molecular dynamics simulaing,I4*l5ice microp~rosity,’~-’~ and the structure of liquid water.’ tions by Hautman and Klein of water on methyland hydroxylIndirectly, an understanding of water and ice structure could terminated SAMs9 have also contributed to the understanding help interpreting certain phenomena that involve organic of water behavior on SAMs. The simulations suggest that water molecules and water, e.g., protein adsorption at surfaces,’* the molecules form clusters on both hydrophilic and hydrophobic formation and structure of amphiphilic monolayer^,'^-^' and SAMs regardless of the initial configurationbut that the clusters protein hydration.22 strongly differ in shape, being three-dimensional and dropletlike In recent years, alkanethiolate SAMs on gold surface^^^*^^ on hydrophobic S A M s but two-dimensional and nearly flat on have become a widespread model system for examining hydrophilic SAMs, thus mimicking the shape of the macroscopic interfacial phenomena, many of which are dependent on drops used in contact angle measurements. interaction with water molecules. Examples of such phenomena In this paper we will further investigate the influence of are the absorption and binding of proteins and other organic substrate wettability on the structural properties of adsorbed molecules of biological significance to solid surface^,^^^^^ the water overlayers, since hydrogen bonding and wettability have formation of self-assembled n-alkylsiloxane monolayer^,^^ been shown to be of such large importance to water adsorption. condensation of water vapor,28and pattemed cry~tallization.~~ The investigations by Nuzzo et al. also include substrates with S A M s are excellent substrates for conducting such studies different wettability, achieved by the use of different chainbecause of their ease of preparation, the high quality of the terminating groups. This approach will, however, only result monolayers, and the wide range of surface properties achievable in a few different wettabilities with no possibility to cover the by using different chain-terminating groups. It is therefore whole wettability range. Furthermore, the different chaindesirable to acquire knowledge of the behavior of water on SAM terminating groups have different hydrogen bond acceptorldonor surfaces, both as a step toward understanding the phenomena properties, which is liable to influence the structure of a listed above and as a means of further characterizing the SAMs condensed water overlayer. As an altemative, mixed SAMs, themselves. This issue has been addressed before by Nuzzo prepared from mixed ethanol solutions of HS-(CH2)15CH3 Abstract published in Advance ACS Abstracrs, July 15, 1995. (“TCH3”) and HS-(CH2)160H (“TO,’) and thus terminated @
0022-365419512099-12257$09.0010
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12258 J. Phys. Chem., Vol. 99, No. 32, 1995
by CH3 and OH groups in varying proportion, were preferred as substrates for the measurements in this study. In addition to the advantages shared with all S A M s of alkanethiolates on gold, they make it possible to fabricate and investigate surfaces covering the whole wettability range, from hydrophilic to hydrophobic, without having to use several different chainterminating groups with different chemistries and geometries. With the mixed SAMs used in this study, we keep the number of interaction mechanisms between surface and adsorbate down to an absolute minimum. Of course, the results obtained will only be valid for the TCH3/TOH system. Other systems, e.g., mixed monolayers of CH3- and COOH- or COOCH3-terminated alkanethiols, may exhibit the same contact angles but result in a different structural behavior of the condensed water overlayer. Heavy water, D20, is used as the experimentalprobe throughout this study rather than H20, in order to reduce interference with the residual H20 always present in ultrahigh-vacuum (UHV) systems. A further advantage gained from using D20 is higher quality infrared spectra, because the signal-to-noise ratio of the IRAS equipment is higher in the OD than in the OH stretching region. Using infrared spectroscopy, we first show that water condensed on the S A M surfaces undergoes a structural change with temperature. The exact transition temperature is found to be highly dependent on the wettability of the monolayer. TPD is then used to draw further conclusions about the binding energies of water molecules on these surfaces, and we discuss the results in terms of cluster size and shape according to the model presented by Hautman and Klein.g
Experimental Section Preparation of Thiol Surfaces. The mixed thiol monolayers were prepared as follows. Silicon (100) wafers were cut into pieces, 20 x 20 and 40 x 20 mm2,and cleaned in a mixture of 5/7 H20, 1/7 H202 (30%), and 1/7 NH3 (25%) at 80 "C for 10 min (TL1). The pieces were rinsed in Type I reagent grade deionized ultrafiltrated water (> 18 MQ/cm; 0.2 pm), blown dry with nitrogen, and inserted into an electron beam evaporation system (Balzers U M S 500 P) where they were coated with a 10 A adhesive layer of Cr or Ti (Balzers, 99.9%), followed by 2000 8, of Au (Nordiska Affineriet, Helsingborg, 99.99%). Evaporation rates were 1 and 5 &s, respectively. The base pressure in the system was always < 2 x Torr. Gold films produced in this manner have previously been characterized by X-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy33and found to have grains of the size 200-500 8,with large flat regions having a preferred (1 11) orientation. The films were stored in laboratory atmosphere and cleaned in TL1 before use in adsorption experiments. Atomic force microscopy measurements show that the grain size increases somewhat during the TL1 cleaning, after which grains up to 1000 A in size are frequently seen. Thiol solutions were prepared from HS(CH2)15CH3 (Fluka, 90-95%) and H S ( C H ~ ) I ~ O(Pharmacia H Biosensor, '99.5%) in 99.5% ethanol (Kemetyl, Stockholm). The chemicals were used without further purification. Different ratios of CH3- and OH-terminated thiols were used to obtain solutions with a mole fraction of OH groups,fOH, ranging from 0.0 to 1.0, but always with a total thiol concentration of 2 mh4. Two identical samples, one for UHV measurements and one for characterization with contact angle measurement, ellipsometry and IRAS, were made by immersing one 20 x 20 mm2 and one 40 x 20 nunzgold film in the same solution. Adsorption times of '15 h were employed to ensure thiol films of high d e n ~ i t y . ~Before ~ , ~ ~use, the samples were rinsed with large amounts of ethanol and then ultrasonically cleaned in ethanol for 10 min.
Engquist et al.
Contact Angle Measurements. Quasi-static contact angles were measured by placing a drop of liquid on a horizontal surface and imaging it with a video camera. The angles were measured directly on the video monitor, taking the average of six measurements as the contact angle value for that surface. This procedure gives reproducible contact angles to within f2'. All measurements were performed in laboratory atmosphere; Le., no control of the relative humidity was possible. Normal humidity variations may have caused some spread in the measured contact angles.36 In the case of hexadecane, the contact angle value was assumed to be zero for surfaces where the drop spread to an irregular shape (fingering). Ellipsometry. Ellipsometriccharacterizationwas performed with an automatic ellipsometer (Rudolph Research AutoEl 111), equipped with a He-Ne laser (A = 632.8 nm) as light source at an incidence angle of 70" from the surface normal. The complex refractive index of a freshly TL1-cleaned gold surface was measured prior to modification. This value (mean values are n = 0.14 and k = 3-56), together with the refractive index of the modified sample and for the thiol monolayer (n = 1.50 and k = 0, regardless of composition), was inserted into a calculation program (DAFIBM) to calculate the thickness of the monolayer, following an algorithm by McCrackin3' and using a AdthioVair parallel slab model. The incorporation of another layer in the model to account for the Au-S bond was considered unnecessary, while this only produces slight thickness variations within the error limit of these ellipsometric measurements. IRAS Measurements. The monolayers were characterized with infrared spectroscopy to ensure that they exhibited a welldefined (extended all-trans) structure. These mzasurements were performed on a Bruker IFS 113v Fourier transform infrared (FTIR) spectrometer using an IRAS geometry described previo u ~ l ywithy4 , ~ ~ optics and an angle of incidence of 83" from the surface normal. The spectrometer operated at mild vacuum conditions ( a 1 0 Torr). Spectral resolution was 2 cm-I, and 1000 scans were collected for each sample and reference, switching between sample and reference every 200 scans to compensate for instrument drift. As reference, a TL1-cleaned gold film was used. A noise level of 0.5. Clearly, the surface concentration of OH groups is important also during the D20 adsorption process. What structure, then, do the water clusters assume on our SAM surfaces? There are several alternatives, including no clustering i t all, very small (< IO molecules) clusters, flat, essentially two-dimensional clusters, and multilayered, threedimensional clusters. No conclusive evidence for any of these models has been put forward in this paper. Our assumption is that on hydrophilic (f& 1) surfaces two-dimensional, flat clusters will form, whereas on hydrophobic ( f o ~ 0) surfaces droplet-shaped, three-dimensional clusters will dominate. This model is illustrated by the schematic drawing in Figure 12. The figure is only intended to exemplify the differences that we believe exist for D20 condensation on these surfaces and does again not account for the equilibrium situation that ultimately should occur, but only for the quasi-equilibrium that is reached during the course of our experiments (approximately hours). It is in accordance with the theoretical calculations performed by Hautman and Kleing and supported by some of our experimental results, as listed below. First, the broad TPD peak for CH3 surfaces and the sharper peak for OH surfaces agree with the model insofar as there should be a larger number of sites with different energy on a 3-D cluster (surface, rim, domain boundaries) than on a 2-D cluster. The shoulder seen for larger coverages on OH surfaces could be due to desorption from less strongly binding sites that start to appear when the overlayer thickness increases. Second, if we preanneal the D20 overlayer before TPD, the broad peak seen on CH3 surfaces still remains. This agrees
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with a model where not the crystalline structure, but rather the cluster shape dictates’the distribution of sites. Third, the measurements upon which Figure 10 is based indicate the same sticking coefficient on a TCH3 monolayer for doses from 0 to -7 langmuirs, i.e., up to coverages of around two monolayers given a sticking coefficient of roughly 0.4. If the water formed a uniform monolayer, the sticking coefficient would increase already before one complete layer was formed, because new D20 molecules would adsorb onto already deposited ones. Since this is not the case, we conclude that the water molecules must gather in clusters and leave most of the CH3 surface free. It would have been interesting to continue the measurements with even higher coverages, to find out when the sticking starts increasing. Unfortunately, the operating pressure range of the mass spectrometer prevents us from doing this. IRAS measurements to clarify this point are currently under way. Finally, the static contact angle for HD (Figure 2b) drops to zero above fOH = 0.6, which is consistent with a model where a continuous water layer condenses on the SAM.36 This macroscopic transition in wettability behavior coincides with what we interpret as the shift from three- to two-dimensional clustering upon condensation in UHV, suggesting similar mechanisms at work in both cases.
Conclusions A structural transition from amorphous to polycrystalline ice occurs for D20 deposited onto mixed SAMs of TCH3 and TOH. The transition onset temperature is highly dependent on the composition of the S A M surface and varies from about 110 K for fOH = 0 to -150 K for foH = 1. On some surfaces, the transition to polycrystalline ice is not completed before total desorption of the water overlayer. We believe the limit to be around cos 6 = 0.6, where there is a sudden increase in the D20 desorption peak temperature, Tp. Clusters of water molecules do most likely form on all the investigated surfaces, regardless of wettability. Our results however fit with a model where the clusters are very different in shape-two-dimensional, flat clusters on hydrophilic surfaces and three-dimensional, dropletlike clusters on hydrophobic surfaces. Once again, we believe that the shift between these two structures takes place around cos 8 = 0.6. In performing these measurements, we have also shown that the mixed TCH3ROH monolayers on gold are good model surfaces for examining hydrogen-bonding interactions between surfaces and an adsorbate. We caution that extrapolation of the current results to similar systems, e.g., mixtures of CH3and COOH- or COOCH3-terminated alkanethiols, may not be straightforward,since the hydrogen bonding properties may vary with the chemical nature of the tail group.
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