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J. Phys. Chem. C 2009, 113, 2128–2133
Effects of Surface Chemistry on Structure and Thermodynamics of Water Layers at Solid-Vapor Interfaces† David B. Asay, Anna L. Barnette, and Seong H. Kim* Department of Chemical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed: July 31, 2008; ReVised Manuscript ReceiVed: October 28, 2008
The effects of surface chemistry on the isotherm thickness and structure of the adsorbed water layer as well as the isosteric heat of adsorption and entropy of adsorption were studied using attenuated total reflection infrared spectroscopy. The degree of hydrophilicity seems to distinctively change the structure and thermodynamic properties of the water layers adsorbed on silicon oxide surfaces. On the highly hydrophilic silicon oxide surface covered with silanol groups, the water layer adsorbed at low humidities exhibits the OH stretching peak at 3230 cm-1 (characteristic of a solid-like water structure), and the isosteric heat of adsorption is much higher than the latent heat of ice sublimation. As the concentration of surface silanol groups decreases, both the initial isosteric heat of adsorption of water and the amount of solid-like water decrease. The water layer adsorbed on the hydrophilic surface at low humidities seems to have much lower entropies than bulk water, while the entropy of the water layer on the partially methylated surface is not much lower than that of bulk water. At high humidities, the liquid water structure becomes dominant in the adsorbed layer. The possible origins of high isosteric heat of adsorption and low entropy are discussed. Introduction Water at interfaces has been a subject of extensive research over many decades.1-8 The physical and structural properties of interfacial water vary from those of the bulk. For example, the viscosity of water under nanoconfinement is observed to vary over many orders of magnitude depending on the surface chemistry.9-13 The structure of water at the interface, as observed with various vibrational spectroscopic techniques, is quite different than that of the bulk.14-20 While these observations are perceived in films that are a few nanometers thick, the effect of this interface is often extended to various macroscale phenomena. For example, the ability of water to wet a flat surface (as measured by the water contact angle) depends greatly on the outermost surface chemistry.8,21-24 Understanding the structure of interfacial water and its impact is therefore of fundamental interest due to its significance in many natural and engineering systems such as geology,5 tribology,25-29 biology,8 and fuel cell research.30 Water has the capacity to hydrogen bond with up to four of its nearest neighbors. This bonding configuration depends on the local physical and chemical environments. When water is completely self-associated, each water molecule forms four hydrogen bonds with its nearest neighbors in a tetrahedral arrangement.31 This structure is observed in solid crystalline ice structures. In IR spectroscopy, the OH stretching vibration of this structured water is observed with a peak centered at ∼3220 cm-1 with a full width at half-maximum (FWHM) of ∼275 cm-1.32 At room temperature, liquid water has two to three hydrogen bonds per water molecule on average.33 This structure of liquid water has a peak position centered at ∼3400 cm-1 with a FWHM of ∼360 cm-1.18,32,34 When the hydroxyl group in water has no hydrogen bonding, this free OH or dangling OH vibration is observed at ∼3640 cm-1 and has a much † Part of the special section “Physical Chemistry of Environmental Interfaces”. * Corresponding author. E-mail:
[email protected].
narrower FWHM. Therefore, the peak position of the OH stretching vibration is very sensitive to the degree of hydrogen bonding and has been used as an indicative tool for investigating the structure of water.14-19,34 At the surface of solid materials, the structure and thermodynamic properties of water deviates from the bulk properties under equilibrium conditions. For example, the structural conformation in the adsorbed water layers varies with the coverage on the NaCl(100) surface at temperatures below 155 K. The salt surface has been extensively investigated because it is an important model surface for sea salts in environmental chemistry studies. At very low coverages, the adsorbed water layers form a two-dimensional well-ordered phase with an isosteric heat of adsorption (qst) of 60-65 kJ/mol.35-37 As the coverage increases above 1 monolayer (ML), a three-dimensional solid structure evolves, and the qst value decreases gradually toward the value of the latent heat of water condensation/sublimation (51 kJ/mol). However, at ambient conditions, the adsorbed water layer structure on NaCl(100) is indistinguishable with the liquid structure with a qst of ∼44 kJ/mol at coverages near 1-3 ML, which is equivalent to the heat of vaporization of bulk liquid water.19 Another important water-solid interface is the adsorption of water on a metal oxide surface. In these systems, water adsorption appears to be very sensitive to the surface chemistry and microporosity of the sample as well as the water vapor exposure conditions. For instance, on a MgO(100) surface in ultrahigh vacuum (UHV) conditions, the adsorbed water layer forms an ordered structure with a low isosteric heat of adsorption (∼40 kJ/mol at ∼0.1 ML) at low coverages and transitions to a different structure with a much higher isosteric heat of adsorption (∼85 kJ/mol at ∼0.5 ML).38,39 Another example showing similar behavior at ambient conditions is bismuth iron molybdate. The qst value for water adsorption is reported to be as low as ∼20 kJ/mol at low relative humidities and gradually increases to the latent heat of bulk water (44 kJ/mol) as the relative humidity increases.40 These behaviors are attributed to
10.1021/jp806815p CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
Structure and Thermodynamics of Water Layers the hydrophobic nature of these metal oxide surfaces.41,42 In contrast, water adsorption on hydrophilic oxide surfaces like silicate shows initially a very high qst (60-80 kJ/mol) at very low relative humidities and a gradual decrease to the latent heat of bulk water as the humidity increases.43,44 A similar trend is also observed for porous carbon materials. In the case of water adsorption in hydrophobic carbon nanopores and nanotubes, the isosteric heat of adsorption is initially very small at low humidities and increases to the value for the bulk condensation at high humidities.45-47 When the carbon surfaces are activated and functionalized with hydrophilic groups, the initial isosteric heat of adsorption shows an opposite trend.48,49 These general trends found in the literature suggest the structure and thermodynamic properties of the adsorbed water layer closest to the surface differ from those of subsequent adlayers. Most UHV studies are limited to sub-ML to 1 or 2 MLs at cryogenic conditions. It is not clear if the trends observed at cryogenic temperatures are applicable to ambient conditions. In the case of ambient experiments, most experiments were carried out with high-surface area samples. It is not clear at what thickness the transition from a high-qst structure to a lowqst structure occurs, and what structures are responsible for these transitions. In this paper, we report a close correlation between the structure and thermodynamic properties of adsorbed water layers on chemically modified silicon oxide surfaces as a function of the adsorbed layer thickness at equilibrium. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to measure these properties. Silicon oxide surfaces were chosen for this study because of their relevance in environmental and nanotechnology systems as well as the large amount of research prevalent in the literature.15,17,21-24,34,50-55 Amorphous SiO2 surfaces with different outermost functional groups were prepared such as (1) a hydroxylated surface via chemical cleaning treatments, (2) a partially methylated surface via hexamethyldisilazane (HMDS) treatments, and (3) a surface covered with octadecyltrichlosilane (OTS) self-assembled monolayer (SAM). Because of changes in the outermost surface chemistry of the silicon oxide surface, the bulk water contact angle differs between these surfaces. It is found that changes in the bulk water contact angle of the silicon oxide surface are reflected in the structure and thermodynamics of the adsorbed water layer in equilibrium with the vapor phase. Experimental Details Water adsorption experiments were performed with a ThermoNicolet Nexus 640 infrared spectrometer with an ATR-IR setup and a MCTA detector. A silicon ATR crystal was used in all experiments. The crystal had a 45° incidence angle, providing a total of 11 internal reflections at the probing surface. At these conditions, the effective penetration depth of the evanescent IR field is ∼240 nm at 3300 cm-1. ATR-IR spectra were taken with 150 scans and a 2 cm-1 resolution. The silicon ATR crystal was cleaned and oxidized in a UV/ ozone chamber for ∼20 min and then placed in a 5:1:1 mixture of water/30% ammonium hydroxide/30% hydrogen peroxide at a temperature of 75 ( 5 °C (commonly known as RCA-1) for 15 min. This cleaning procedure removed organic contaminants and produced a clean silicon oxide layer on the crystal.56 Immediately following this cleaning step, the crystal was rinsed with copious amounts of milli-Q water (resistance ) 18 MΩ/ cm) and dried with high-purity argon. Si 2p X-ray photoelectron spectroscopy (XPS) analysis of the silicon sample cleaned by this method revealed that the silicon crystal was covered with
J. Phys. Chem. C, Vol. 113, No. 6, 2009 2129 a SiO2 layer a few nanometers in thickness. In the silicon oxide surface ATR-IR studies, the sample was promptly mounted onto the ATR holder following cleaning and purged for ∼1 h with dry argon before the adsorption isotherm measurements were taken. In the case of the partially methylated silicon oxide surface, the same cleaning procedure was first used as described above to clean the surface. Following these cleaning steps, the crystal was placed inside two Petri dishes, forming a loose seal. Prior to sealing, 2 or 3 drops of HMDS were placed ∼2 cm from the crystal. The system was gently heated to ∼65 °C for 45 min. This treatment allowed HMDS to react with silanol groups at the silicon oxide surface, leaving trimethylsilyl groups at the surface.52 In the case of the OTS-SAM-coated silicon oxide surface, the ATR crystal surface was cleaned via a piranha solution (3:1 ratio of concentrated sulfuric acid to 30% hydrogen peroxide) for 45 min and then rinsed with copious amounts of milli-Q water. Following this step, the crystal was placed in a 30% hydrogen peroxide solution for 30 min and then dried with dry argon. After the drying process, the crystal was placed in cleaned glassware, and high-purity toluene was added and kept under argon. This step removed residual water on the crystal surface by dissolving it into toluene. Separately, a 1 µM solution of OTS in toluene was mixed and allowed to react with any residual water in the toluene solution over a 24 h period. Then, the OTS-toluene solution was filtered to remove any polymerized OTS contaminates. Next, the toluene was removed from the ATR crystal under dry argon and replaced with the filtered OTS solution. The sample was then heated at 60 °C for 24 h. After this treatment, the crystal was rinsed with a toluene solution containing 10 µM HMDS. The purpose of using the HMDS-toluene solution was to remove any residual water from the rinsing solvent and to help produce a reaction with any hydroxyl groups that had not reacted with OTS. After 10 min in the HMDS-toluene solution, the crystal was rinsed with highpurity toluene 3 times. The crystal was then retrieved and was sonicated in ethanol for ∼10 min. After this step, the crystal was dried with argon and placed into the ATR setup for the water adsorption testing. On the basis of an ellipsometry measurement, the thickness of the OTS SAM was estimated to be ∼3 nm, indicating that the OTS-SAM film is well packed.53 Controlling the partial pressure of water vapor was performed using the method previously described in the literature.15 By mixing two streams, one saturated with water vapor and the other dry, we comtrolled the partial pressure of water of the system. In the case of the silicon oxide surface, adsorption experiments were performed at 10, 22.4, and 35.5 °C. In the case of HMDS-treated silicon oxide surface, adsorption experiments were performed at 9, 23.4, and 35.9 °C. In the case of the OTS-treated silicon oxide surface, adsorption occurred at 11 °C. The temperature of the crystal was maintained within (1 °C, during each test. Results and Discussion Water contact angles were measured on the clean silicon oxide surface, the partially methylated surface, and the OTS-SAM-coated surface. The observed static contact angle of water on each was
