Article pubs.acs.org/Langmuir
Water Adsorption Isotherms on CH3‑, OH‑, and COOH-Terminated Organic Surfaces at Ambient Conditions Measured with PM-RAIRS Aimee Tu,† Hye Rin Kwag,‡ Anna L. Barnette,§ and Seong H. Kim* Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: The water adsorption isotherms on methyl (CH3)-, hydroxyl (OH)-, and carboxylic acid (COOH)terminated alkylthiol self-assembled monolayers (SAMs) on Au were studied at room temperature and ambient pressure with polarization modulation reflection−absorption infrared spectroscopy (PM-RAIRS). PM-RAIRS analysis showed that water does not adsorb at all on the CH3−SAM/Au at subsaturation humidity conditions. In a dry Ar environment, the OH-SAM/Au holds at least 2 layer thick strongly bound water molecules which exhibit a broad O−H stretch vibration peak centered at ∼3360 cm−1. The peak position implies that the strongly bound water layer on the OH SAM is more like a liquid than an ice. The additional uptake of water in humid environments is relatively weak, and the peak position changes very little. Unlike the OH-SAM/Au, the COOH-SAM/Au does not have strongly bound water layer. This seems to be due to the strong hydrogen bonding between terminal COOH groups in dry conditions. The weak interactions between water and carboxyl groups at low relative humidity (RH) and the solvation of dissociated carboxylic groups in high RH lead to a type III isotherm behavior, based on the BET categories, for water adsorption on the COOH-SAM/Au. The water spectra on the COOH-SAM at RH > 45% are centered at ∼3430 cm−1 and very broad, indicating that the hydrogen-bonding network of water on the COOH-SAM is much different from that on the OH-SAM.
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ature.1,2,15 The O−H stretch vibration spectra of the water layer reported in these studies often revealed an “ice-like” character (peak centered at around 3250 cm−1).15,16 Molecular dynamics (MD) simulation studies predicted that if water is adsorbed on the CH3-terminated hydrophobic surface, the adsorbed water molecules will form small water clusters.15,16 On hydrophobic surfaces with a small fraction of the hydrophilic components, the hydrophilic spots may serve as nucleation sites for small water droplets.15 In contrast, atomic force microscopy studies showed that the well-packed CH3-terminated SAM on mica could block water adsorption, but water could penetrate the hydrophobic SAM through defects and at island edges.17 MD simulation studies also showed that water can easily penetrate in holes with radii greater than about 1 nm in the CH3-terminated SAM and detach silane chains from the surface.18 The “ice-like” feature of the water observed for the CH3-termianted silane SAM on silicon oxide exposed to humid ambience was interpreted as the ingression of water to the SAM/SiO2 interface where water molecules can interact with unreacted silanol groups.19 The penetration of water through the long-chain alkyl silane SAMs
INTRODUCTION Water can be adsorbed on virtually all surfaces exposed to humid atmosphere. The interfacial structure of water in equilibrium conditions can have profound effects on how the solid surface behaves in environmental,1−3 chemical,4,5 electrochemical,6 physical,7,8 and biological processes.9 The type and distribution of organic functional groups exposed at solid surfaces play an important role in the mechanism of water adsorption according to molecular simulations.10,11 Selfassembled monolayers (SAMs) have been considered as a good model system to study the effect of organic functional groups on the water adsorption isotherm since the terminal organic functional group can easily be controlled.12 There are a number of experimental and theoretical studies addressing water adsorption on organic surfaces prepared by SAMs on gold or silicon oxide surfaces. When water is adsorbed on the CH3-terminated SAM at a cryogenic temperature below 100 K in ultrahigh-vacuum (UHV) conditions, amorphous ice is formed.13,14 Upon annealing, the amorphous ice can be converted to polycrystalline ice before it desorbs to vacuum. There are some contradicting reports on whether water adsorbs on the CH3-terminated hydrophobic surface at room temperature. Some experimental works for the CH3-SAMs on silicon oxide or glass surfaces prepared through silane chemistry found a small amount of water to be adsorbed at room temper© 2012 American Chemical Society
Received: July 15, 2012 Revised: September 10, 2012 Published: October 17, 2012 15263
dx.doi.org/10.1021/la302848k | Langmuir 2012, 28, 15263−15269
Langmuir
Article
SiO2 is weak on the OH-SAM/Au. The thick water layer formed on the COOH-surface at RH higher than 40% shows an asymmetric shape with a peak at 3420 cm−1.
on silicon oxide surfaces seems to be driven by the strong affinity between the water molecule and the residual silanol group. Alkylthiol SAMs on gold would be less susceptible to the water penetration since the gold surface has much lower water affinity than the silicon oxide surface. A surface acoustic wave (SAW) sensor study reported that about one monolayer of water could be adsorbed on the CH3-terminated thiol SAM/ Au,20 which was similar to the value reported from quartz crystal microbalance (QCM) measurements of CH3-terminated silane SAM.2 However, on these quartz sensors, water can be adsorbed on the uncoated quartz surface outside the gold electrode region, which could interfere with accurate measurements of water adsorption on the electrode (which is the main sensing area), especially when the amount of water adsorbed on the electrode is small. In the forced dewetting study, a thin liquid water layer was readily ruptured into microdroplets,21 which was consistent with the MD simulation predictions.15,16 But, this study cannot clearly answer if water can adsorb on the CH3-terminated organic surface or not. Water can readily adsorb on hydrophilic surfaces at ambient conditions. The SAW studies reported water adsorption of up to 6 layers at a 90% relative humidity (RH) on OH- and COOH-terminated SAM/Au, which is significantly larger than the value reported for the CH3-SAM measurements.20 Raman spectroscopy studies of the forced dewetting layer found that the water layer on the OH-SAM/Ag shows more ice-like character than that on the COOH-SAM/Ag.21,22 However, the ice-like character for the water layer on the OH-SAM on silver was not as prominent as the one shown for the water layer on SiO2.21,22 On a highly hydroxylated SiO2 surface, the water layer formed at low RH exclusively exhibits the strong ice-like character.19,23,24 The O−H stretch vibration of water on SiO2 at low RH is centered around 3250 cm−1.19,23−25 This ice-like water layer must be due to hydrogen-bonding interactions with silanol groups at the silicon oxide surface. As RH increases, the adsorbed water layer transitions to a liquid-like state (O−H peak centered at around 3400 cm−1).19,23−26 On the basis of these observations, one could assume or question if the water layer adsorbed on all hydrophilic surfaces with hydrogenbonding capability can form the ice-like structure. This paper presents direct experimental assessment for the equilibrium water adsorption behaviors on CH3 (hydrophobic)-, OH (hydrophilic)-, and COOH (hydrophilic and ionizable)-terminated thiol SAMs on gold at room temperature and atmospheric pressure. The thiol SAMs on gold29−31 were chosen for this study because the driving force for water ingression to the substrate surface is weak on gold compared to SiO2. The equilibrium water adsorption for these surfaces exposed to humid argon at room temperature and ambient pressure was measured with polarization modulation reflection−adsorption infrared spectroscopy (PM-RAIRS). The selection rule of the PM-RAIRS allows detection of the adsorbed water layer on SAM/Au in equilibrium with the gasphase water without interference from the gas-phase water. The PM-RAIRS measurements clearly revealed no adsorption of water on the CH3-terminated organic surface under subsaturation humidity at room temperature. Although both are hydrophilic, the COOH-surface can easily be dried with Ar purging, while the OH-surface holds a tightly bound residual water layer. The water layer on the OH-terminated surface exhibited a broad peak centered at 3360−3390 cm−1 with a symmetric shape, implying that the ice-like character seen on
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EXPERIMENTAL METHODS
Gold films on Si wafers (0.8 × 2 cm2) were cleaned with atmospheric rf plasma of oxygen−argon and were immediately immersed in 20 mM solutions of 1-hexadecanethiol (Fluka Chemika), 11-mercapto-1undecanol (Aldrich, 97%), and 12-mercaptododecanoic acid (Aldrich, 96%) in ethanol (Pharmco-Aaper, 200 proof) to prepare the CH3-, OH-, and COOH-terminated SAMs, respectively. After retrieval from the thiol solution, the sample was rinsed with copious amounts of ethanol and dried with argon. It was found that a 30 min immersion time in the thiol solution was sufficient to form a well-packed SAM; a longer immersion time created defects in the SAM prepared in this method (see Supporting Information). The thickness of the 1hexadecanethiol SAM/Au was determined with ellipsometry (Ellipsotech SWE; wavelength = 632.8 nm, incidence angle = 70°) and found to be 2.6 ± 0.2 nm. The water contact angle was >100° for the CH3SAM and
