Hydration–Dehydration of Acetonitrile and Methanol in Amorphous

Article pubs.acs.org/JPCC

Hydration−Dehydration of Acetonitrile and Methanol in Amorphous Solid Water Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: The interactions of two model dipolar molecules (acetonitrile (ACN) and methanol (MeOH)) with amorphous solid water (ASW) were investigated using temperature-programmed desorption and temperaturedependent secondary ion mass spectrometry of a series of mixed adlayer thin film systems comprising different configurations of monolayers and thin films of ACN, MeOH, and ASW as well as a nonpolar probe molecule, CO2, deposited on a Ni(111) substrate. The temperature-dependent desorption rates and surface compositions of these systems were found to be consistent with the hydrophobic hydration of ACN, wherein the ACN is caged by water molecules, likely in clathrate-like structures. Consequently, the ACN−water hydrogen bonds are limited to the interior surfaces of the cages. The ACN is not incorporated into the ASW hydrogen bonding network, and it does not modify the crystallization kinetics of the ASW. In contrast, MeOH incorporates into the hydrogen bonding network and modifies the water crystallization kinetics. Furthermore, results for mixed ACN−MeOH monolayers indicate that MeOH can mediate inclusion of ACN into the ASW network. These results provide insight into how polar and nonpolar moieties of ACN and MeOH additives play a role in hydration in ASW and also elucidate the respective occurrences of complete and incomplete mixing of MeOH−water and ACN−water binary films.

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surface of amorphous solid water (ASW).8−23 In contrast to solution studies, it is a great advantage that interactions of both polar and nonpolar species with water can be explored in ASW at the molecular level in terms of diffusion, mixing, segregation, hydration, and dehydration by using surface sensitive techniques. The literature includes many studies of ACN−water mixtures in solutions.24−36 The interaction of ACN with water is based mainly on hydrogen bond formation. However, liquid−liquid phase separation occurs at 272 K at an ACN mole fraction, XACN, of 0.38.33 Molecular dynamics simulations35 and X-ray diffraction and infrared spectroscopy studies33 have suggested that ACN−water mixtures are microheterogeneous over an extensive range of compositions and temperatures. In pure ACN, the molecules do not interact strongly with themselves, but 4−6 molecules form a zigzag-shaped cluster.33 After addition of water, hydrogen bonds are formed between ACN and water molecules. However, the ACN clusters remain over the range of 0.2 < XACN < 0.8 because each of the component molecules tends to be self-associated.33 This microheterogeneity does not imply the presence of isolated clusters, but interpenetrating regions of water-rich and ACN-rich clusters do exist. The texture of the interpenetrating regions has a length scale of approximately 0.5−1 nm. The ACN cluster is disrupted

INTRODUCTION Because of their miscibility with water and a range of organic solvents, methanol (MeOH) and acetonitrile (ACN) are respectively the simplest alcohol and organic nitrile used in many fields such as organic synthesis, liquid chromatography, and electrochemistry. The latter, ACN, is a typical aprotic molecule with a large dipole moment of 3.92 D in the gas phase. Therefore, dipole−dipole interaction plays a dominant role in clustering of the ACN molecules and in stabilization of polar species in the condensed phase. Moreover, ACN is expected to form hydrogen bonds with hydrogen-bond donors. In contrast, the small dipole moment of MeOH (1.69 D) is comparable to that of water (1.87 D). Its intermolecular interaction is governed by hydrogen bonding. The ACN−water and MeOH−water mixtures are macroscopically homogeneous, but the interactions between their components are expected to differ largely. Their thermodynamic processes and local structures are determined by competition between hydrogen bonding at the polar groups and reorganization of water molecules near the methyl moiety. According to the view of hydrophobic hydration, the nonpolar solute or headgroup creates an ice-like structure.1 However, hydrophobic hydration is experimentally accessible only with great difficulty because of the low solubility of nonpolar species in water. The local structure of water around hydrophobic entities has been discussed using water-soluble solute species like alcohols2−5 and nonpolar gas−water mixtures under high pressure.6,7 Additional insights into weak intermolecular interactions have been gained from adsorption experiments conducted on the © XXXX American Chemical Society

Received: June 16, 2015 Revised: December 25, 2015

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DOI: 10.1021/acs.jpcc.5b05737 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C at XACN < 0.2, and the water−water hydrogen bond network is enhanced. The increase in the entropy of water−alcohol mixture is much less than that expected from an ideal solution. Therefore, microheterogeneity or an ice-like water structure is likely to occur in the solution.2−5 In fact, a 1:9 molar ratio MeOH− water mixture exhibits the formation of a loose hydrogenbonded cage of water around the MeOH molecule. 2 Furthermore, recent neutron diffraction data show that mixing of a concentrated MeOH−water solution (XMeOH = 0.7) is incomplete at the molecular level.4 Moreover, molecules in pure liquid MeOH form hydrogen bonded chains. The water molecules can bridge MeOH chains to form rings5 because MeOH is also a structured liquid, forming winding polymeric chains.37 To date, ASW has been investigated mainly from a surface science perspective8−23 and because of its relation to astrophysical ice.38−40 The ASW water molecules become mobile at the glass-transition temperature (Tg = 136 K)17 although some controversy persists about whether a liquid-like phase is a supercooled extension of normal water or not.41,42 Porous ASW films can be prepared by water molecule deposition at temperatures well below Tg. Adspecies are trapped in the film interior during heating.43 Examinations of crystallization kinetics of ASW have been undertaken based on temperature-programmed desorption (TPD): nonpolar molecules embedded underneath ASW films are released explosively during water crystallization at Tc = 160 K (the molecular volcano).44,45 Nonpolar additives are believed to have no effect on water crystallization kinetics, but caging in ASW (i.e., hydration) can occur.46−48 In contrast, polar molecules such as MeOH strongly influence the ASW bulk properties.17,49,50 As described in this paper, interactions of ACN with porous and nonporous ASW films are assessed and compared with MeOH−water interactions using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and TPD. Thermal diffusion and segregation of additives in ASW films are analyzed using TOF-SIMS as a function of temperature. The hydration− dehydration processes of the ACN and MeOH additives are discussed based on TPD spectra. Their mutual interactions in ASW are also examined. Thermal desorption of CO2 additives is used to probe the crystallization kinetics of ASW, MeOH, and ACN films as well as intermixing of their binary films. Results demonstrate that ACN is caged by water molecules in the interior of porous ASW films. Actually, MeOH additives enter the hydrogen-bond network of ASW and modify the desorption kinetics of coadsorbed ACN additives. The great difference between ACN and MeOH in interactions with water manifests itself in the complete mixing of MeOH−ASW binary films and incomplete mixing of ACN−ASW binary films.

detected using a microchannel plate after passage through a field-free TOF tube. To extract low-energy secondary ions efficiently, a bias voltage (±500 V) was applied to the sample. The fluence of He+ in TOF-SIMS measurements was maintained as less than 1 × 1012 ions cm−2 to mitigate surface decomposition. A Ni(111) surface, used as a substrate, was heated several times in UHV to approximately 1300 K by electron bombardment from behind. Surface cleanliness was confirmed by the absence of impurities in TOF-SIMS. The substrate was mounted on a Cu coldfinger extended from a closed-cycle helium refrigerator and was cooled to 20 K. The temperature of the coldfinger, monitored near the sample position using Au(Fe)−chromel thermocouples, was controlled using a digital temperature controller and a cartridge heater attached to the finger. The temperature was ramped at a rate of 5 K min−1 for both TPD and TOF-SIMS measurements. Samples of water, acetonitrile (99.8%), and methanol (99.8%) were degassed by several freeze−pump−thaw cycles. Thin films were deposited onto the clean Ni(111) surface by backfilling the UHV chamber with gaseous samples admitted through high-precision leak valves.

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RESULTS AND DISCUSSION Interaction of Acetonitrile and Methanol Additives with Water. The coverage of adspecies on the Ni(111) substrate has been ascertained from TOF-SIMS measurements.16,17 The intensity of fragment ions sputtered from water and methanol adspecies saturated at exposure of ca. 2.5 langmuirs (1 langmuir = 1 × 10−6 Torr s). For acetonitrile adspecies, typical secondary ions were H+, CH3+, HCN+, and H+(CH3CN), which also saturated in intensity at around 2.5 langmuirs (not shown). Therefore, we infer that 1 monolayer (ML) of all molecules studied here is formed during this exposure. First, ASW film porosity effects on interactions with acetonitrile adspecies were assessed. Figure 1 shows experimental results obtained using nonporous ASW film: (a) TOFSIMS intensities from the H2O and CH3CN molecules are displayed as a function of temperature, together with (b) TPD spectra of H2O (18 amu) and CH3CN (40 amu). The water molecules (20 langmuirs) were deposited onto the Ni(111) substrate at 100 K and were then heated to 120 K. The thusprepared ASW film was nonporous.43 The CH3CN molecules (5 langmuirs) were adsorbed onto the film surface at 20 K. Only secondary ions from the acetonitrile adspecies were detected initially. The ion intensities changed steeply at 110− 120 K: H3O+ ions appeared and ions from CH3CN decreased in intensity. These phenomena reflect dewetting and not intermixing because no explosive TPD peak of acetonitrile incorporated in the ASW film was identified at water’s Tc of 160 K. The ion intensities from acetonitrile (water) decrease (increase) further at T > 140 K because the acetonitrile adspecies desorbs from the surface, as evidenced by the concomitant rise in the acetonitrile TPD signal. As a result of ASW film dewetting, the Ni+ ion increased and the H3O+ intensity dropped at T > 160 K. The ion intensities remaining at T > 180 K derived from chemisorbed residues. Water desorption commenced at around 140 K. A characteristic “bump” or shoulder occurred at 160 K because of water crystallization.44 Water film dewetted the Ni(111) substrate during crystallization, thereby increasing Ni+ at Tc = 160 K. At this temperature, pure ASW films are known to dewet the

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EXPERIMENT Experiments were conducted in an ultrahigh-vacuum (UHV) chamber with base pressure of 155 K). The surface segregation is expected as a typical property of amphiphiles or hydrogen bonding solutes in liquid water.53 As described already, the methanol adspecies on the Ni(111) substrate tends to segregate to the ASW film surface.17 The segregation of acetonitrile in ASW resembles that of methanol, suggesting that the −CN group is active for hydrogen bond formation. The pores of ASW collapse at ca. 120 K because of water surface diffusivity,51 but caging of the −CH3 moiety is incomplete at this temperature, thereby ensuring the surface segregation of acetonitrile. However, the following uptake of acetonitrile adspecies at ∼150 K in the ASW film interior implies that the amphiphilic nature is lost or weakened considerably. The incorporation of the acetonitrile adspecies in the porous ASW film (see Figure 2a) is consistent with this behavior. Probably, the −CH3 group of the segregated acetonitrile species is caged by water molecules at T > 150 K. Figure 4 shows that a highly mobile phase of water that induces diffusion of additives through the thin film interior does not engender thermal desorption of additives from the surface because the segregated acetonitrile species interacts with water. Dehydration during the first-order phase transition (i.e., crystallization) is necessary to liberate the hydrated acetonitrile into the gas phase. The interaction of methanol with water in the ASW film interior was investigated using TPD spectra. Figure 5 displays

shoulder of water TPD spectra as a result of water crystallization. The desorption onset of acetonitrile shifts to considerably higher temperature by adsorption of water even in the case of submonolayer coverage (2 langmuirs). This behavior implies that postdeposition of water forms some complex. The water molecules are assembled around the acetonitrile adspecies, which might be a precursor to the hydrates or caged species formed at higher water coverage. The experimentally obtained result shown in Figure 3 implies that at least 2 ML (5 langmuirs) of postdeposited water is necessary to complete the acetonitrile hydration, as inferred from the disappearance of the broad component and occurrence of the sharp peak. This result is consistent with the fact that crystalline hydrates are readily created by codeposition of water and guest molecules, even at cryogenic temperatures. 52 A broad component with a 120 K onset (or 145 K peak) remains when acetonitrile is adsorbed onto the porous ASW film, as presented in Figure 2b. They are assignable as unhydrated (or physisorbed) species. No precursors to the caged acetonitrile species are formed by postdeposition of acetonitrile because the structures of the hydrogen-bonded water molecules are fixed. In this case, the narrow TPD peak occurs at 160 K provided that adspecies incorporated in the film interior are caged during pore collapses that occur during heating.48 The acetonitrile adspecies on Ni(111) stay at the interface immediately after water deposition at 20 K, but they can diffuse into the water film during heating. To investigate this possibility, the surface compositional change of the ASW film (5 langmuirs) deposited onto the acetonitrile (1 langmuir) adsorbed Ni(111) substrate was investigated. Intensities of typical secondary ions in temperature-programmed TOF-SIMS measurements are displayed in Figure 4. The acetonitrile adspecies embedded underneath the ASW film tend to segregate to the surface region at T > 130 K. The onset of surface segregation corresponds well to Tg of water. Therefore, surface segregation of acetonitrile is associated with the water molecule mobility. The segregated acetonitrile species was

Figure 5. TPD spectra of water (18 amu) and methanol (31 amu) for methanol (1 langmuir) adsorbed Ni(111), which was capped with water layers at various thickness. The methanol and water were deposited at 20 K.

results obtained using methanol (1 langmuir) adsorbed on Ni(111) covered by water films of increasing thickness at the substrate temperature of 20 K. The methanol desorption commences at 120 K from the Ni(111) substrate. The desorption rate reaches its maximum at around 140 K. The desorption onset and the peak of methanol shift to higher

Figure 4. Temperature-programmed TOF-SIMS intensities for acetonitrile (1 langmuir) adsorbed Ni(111), which was capped with water (5 langmuirs) at 20 K. D

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phase of water with respect to normal liquid water. The hydrogen bond structure of ASW resembles that of crystalline water.57 Therefore, an ice-like hydration cage with fewer free −OH groups tends to form around the acetonitrile additives, thereby reinforcing the hydrophobicity of acetonitrile in ASW. The hydrogen bonds between the polar −CN group and free −OH groups are limited to the interior surfaces of the cages, so that the cage structure itself is not destroyed by acetonitrile. In ASW, the hydration of acetonitrile can be influenced by methanol additives because the local hydrogen-bond structure of water is modified. To investigate this possibility, ternary systems were investigated based on TPD as depicted in Figure 6. Submonolayers of acetonitrile (1 langmuir) and methanol (1

temperatures with increasing thickness of the capping ASW film. The methanol TPD spectra are broadened by water because the high-temperature cutoffs match those of the water TPD spectra. No sharp peak comparable to that seen at 160 K for acetonitrile is identifiable in the methanol TPD spectra, but a shoulder is apparent in the water TPD spectra at 150−155 K depending on the water film thickness. The water crystallizes at this temperature. The methanol desorption rate during ASW crystallization (20 langmuirs) is about an order of magnitude less than that of the main TPD peak. The same behavior is observed when thicker ASW films (approximately 40 ML) are used.17,49 The extremely low TPD intensity of methanol in the 150−155 K range reflects that the methanol additives remain in the condensed phase during water crystallization because hydrogen bonds are formed with water. The TPD spectra of water from hydrophilic substrates exhibit zeroth-order desorption kinetics before and after film dewetting, as revealed from a common low-temperature leading edge, as well as a shift of the peak desorption rate, with increasing water coverage. The desorption rate of water from the multilayer ASW film is independent of the initial film thickness because water desorbs from the topmost surface.54,55 It is noteworthy that the water TPD spectra in Figures 3 and 5 apparently deviate from the zeroth-order kinetics before the water crystallizes (T < 160 K): The desorption rate (i.e., TPD intensity) of water from the thinner ASW film is considerably low compared to that from the thicker film. The water desorption rate is suppressed by interactions with acetonitrile and methanol adspecies for thinner ASW films. In contrast, the water desorption rate after dewetting (T > 160 K) is apparently independent of the initial film thickness. Figure 5 shows a typical case. For that reason, we infer that the methanol grain boundary phase is segregated after water crystallization and that the water molecules desorb from crystallites without any influence from the methanol additives. The water−acetonitrile interaction resembles that with nonpolar additives in terms of the TPD spectra and uptake of adspecies in porous ASW,46,48 suggesting that hydrogen bonding with the polar −CN group has no marked effect on the water cage formation. The additives are dehydrated during the first-order phase transition (i.e., crystallization) because the cage collapses. For methanol, however, the absence of the peak associated with ASW crystallization at 150−155 K reflects that caging is not the primary interaction with water. The broadened TPD spectra of methanol with the high-temperature cutoffs corresponding to those of the water multilayer TPD spectra are indicative of hydrophilic hydration or of hydrogen bond formation with water. The methanol is both a donor and acceptor of hydrogen bonds, as is water. The methanol hydroxyl group can enter and modify the hydrogen bond network of water. However, such is not the case for acetonitrile because it is merely a hydrogen bond acceptor. According to free energy calculations,53 hydrogen bonding solutes such as methanol and acetonitrile are surface active. They remain at the liquid−vapor interface of water. However, methanol interacts with the water surface more strongly than acetonitrile does. Nevertheless, results show that the acetonitrile adspecies is not surface active against ASW at T > 150 K, as revealed by the uptake of the adspecies in the ASW film interior (Figures 2a and 4). The surface activity of acetonitrile adspecies is expected to decrease if the roles of hydrogen bonds are depressed in interaction with ASW relative to normal water. In the framework of polyamorphism,56 ASW is a distinct amorphous

Figure 6. TPD spectra of water (18 amu), methanol (31 amu), and acetonitrile (40 amu) for methanol (1 langmuir) and acetonitrile (1 langmuir) coadsorbed Ni(111), which was capped with (a) 5 and (b) 20 langmuirs of water at 20 K.

langmuir) were adsorbed onto the Ni(111) substrate. Then (a) 5 langmuirs and (b) 20 langmuirs of water molecules were deposited on them at 20 K. The acetonitrile and methanol TPD spectra retain features of the binary system when the capping ASW film is thin (Figure 6a), although the peaks are shifted slightly to lower (acetonitrile) and higher (methanol) temperatures. The high-temperature component of acetonitrile (approximately 170 K), having the same cutoff as that of water and methanol, is evident in the binary system (Figure 3). This acetonitrile species is trapped in the water crystallites without dehydration at Tc = 160 K. Addition of methanol makes the cutoff peak more prominent than that of the binary system. The peak of acetonitrile and the onset of methanol desorption at ca. 150 K agree with the shoulder of water TPD resulting from crystallization. This behavior is expected to occur when acetonitrile and methanol interact with water almost independently. For the thicker ASW film, however, the TPD spectrum of acetonitrile differs considerably from that for the thinner film, as depicted in Figure 6b. The sharp peak intensity of acetonitrile during water crystallization is depressed strongly. The main peak occurs at 175 K simultaneously with water and E

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The Journal of Physical Chemistry C methanol TPD peaks. In contrast, the methanol TPD spectrum is almost identical to that of the binary system. These results show that effects of methanol on acetonitrile hydration appear to be more remarkable for the thicker ASW film. The concentrations of additives are small relative to the water matrix in the thicker film, but most of the acetonitrile appears to be influenced by methanol additives. Probably, some association of methanol and acetonitrile occurs in ASW, so that the acetonitrile additive tends to be accommodated efficiently in the cage modified by methanol. The fact that the trapped acetonitrile molecule can survive water crystallization and desorb together with water and methanol suggests that the hydration structure induced by methanol is retained despite water crystallization. The acetonitrile additive can be hydrated almost completely by 2 ML (i.e., 5 langmuirs) of pure water (see Figure 3), as revealed by the occurrence of a sharp peak at 160 K. In Figure 6a, however, the broad peak of acetonitrile induced by methanol (approximately 170 K) is very weak relative to the 152 K peak induced by water at 2 ML thickness. The fact that the thicker ASW film is necessary for the dominance of the 170−175 K peak of acetonitrile (Figure 6b) implies that the hydration cage induced by methanol is greater than that of pure water molecules in size because both acetonitrile and methanol are accommodated in the former. Phase Transition of Multilayer Films. Entrapment of nonpolar additives and their release during the first-order phase transition are expected to occur for amorphous molecular solids other than ASW because the interactions between solute and solvent species are fundamentally based on the weak van der Waals force. Figure 7 presents TPD spectra of carbon dioxide

Therefore, the desorption of CO2 is probably related to methanol crystallization as well. For the acetonitrile film, the CO2 peak occurs at around 120 K. We infer that the amorphous acetonitrile crystallizes at this temperature. This inference is supported by the fact that the phase transition of pure acetonitrile is initiated by moderate annealing at 115 K60 and that the phase segregation starts at 115 K for the water− acetonitrile mixtures.23 The phase transition behavior of acetonitrile was investigated using temperature-programmed TOF-SIMS. Figure 8a displays

Figure 8. Temperature-programmed TOF-SIMS intensities of typical ions from an acetonitrile multilayer film (20 langmuirs) formed on Ni(111) with (b) and without (a) water adspecies (2 langmuirs) on the film surface.

intensities of typical ions sputtered from the acetonitrile multilayer (20 langmuirs) deposited onto the Ni(111) substrate at 20 K. The secondary ions from the acetonitrile film (Ni+ ion from the substrate) decreased (increased) in intensity at 120 K, indicating that crystallization changes the film morphology at this temperature. This behavior closely resembles water crystallization at 160 K.17 The thinner acetonitrile film on the nonporous ASW film also exhibited dewetting at 110−120 K (Figure 1). This result suggests strongly that the interfacial interaction of acetonitrile with water has no apparent effect on crystallization kinetics of a thin acetonitrile film (approximately 2 ML). On the porous ASW film (Figure 2), however, no dewetting of acetonitrile was identified at 120 K, suggesting that the nucleation and growth of acetonitrile clusters is suppressed because their surface concentration is much lower than that on the nonporous ASW film. This inference is also supported by the fact that the TPD spectrum of acetonitrile on porous ASW is dominated by the 160 K peak of the dehydrated species rather than the 145 K peak (or 120 K onset) of the unhydrated species. In contrast to the interfacial interaction of acetonitrile with water, a submonolayer of water adspecies (2 langmuirs) on the acetonitrile film (20 langmuirs) surface modifies the dewetting behavior to a considerable degree, as presented in Figure 8b.

Figure 7. TPD spectra of CO2 (44 amu) adsorbed onto Ni(111) with and without a capping layer of water, methanol, or acetonitrile. The CO2 adspecies and the capping layers were deposited respectively by exposure of 1 and 20 langmuirs at 20 K.

(1 langmuir) that is embedded underneath thin films of water, acetonitrile, and methanol formed by exposure of 20 langmuirs. From the Ni(111) substrate, the CO2 molecules desorb at ca. 80 K. The CO2 adspecies capped with the ASW film are released at 160 K as a result of water crystallization. The CO2 TPD peak occurs from the methanol film at 110 K. The respective Tg and Tc of methanol are 103 and 110−115 K.58,59 F

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langmuir) between the two multilayer films. The water TPD spectrum is not influenced markedly by the underlying acetonitrile film. The acetonitrile TPD exhibits a peak at 160 K during water crystallization, but it is not ascribable solely to dehydration because water film dewetting evaporates the acetonitrile multilayer embedded underneath the water multilayer (i.e., the molecular volcano).44 Two main peaks are visible in the CO2 TPD spectrum at Tc of water (160 K) and acetonitrile (120 K). In addition, a small peak is identifiable at 100 K. This species is attributable to CO2 trapped at the water−acetonitrile interface without solvation. In any case, the phase transition behaviors of the binary films are fundamentally identical to the pure acetonitrile and water films. The TPD spectrum of CO2 exhibits no indications of the intermixed layer, in accordance with the slow uptake of the water submonolayer in the acetonitrile film (Figure 8b), as well as the imperfect uptake of acetonitrile adspecies in the porous ASW film (Figure 2a). Incomplete mixing based on the hydrophobic behavior of acetonitrile at cryogenic temperatures might be associated with the microheterogeneous structure of the liquid water− acetonitrile mixture.33 In contrast to acetonitrile, intermixing is clearly identifiable for a water−methanol binary film (20 langmuirs each) using CO2 as a probe. The experimental result is displayed in Figure 10. The CO2 TPD peak during methanol crystallization at 110

The dewetting temperature increases to 135 K by the water adspecies. The water adspecies tends to mix gradually with acetonitrile after the acetonitrile molecules become mobile at ca. 80 K. The water uptake occurs continually across the possible phase transition temperature of acetonitrile at 120 K. Consequently, a submonolayer of water suppresses dewetting of the multilayer acetonitrile film, but acetonitrile crystallization at 120 K is not influenced by water adspecies, as probed by CO2 TPD spectra (not shown here). The suppression of multilayer film dewetting by adspecies has been explained as resulting from their surfactant effects, which has been previously observed for methanol adspecies on water.17 We propose that dewetting of the acetonitrile film can also be suppressed by the water adspecies as surfactants. In this respect, the free energies for water at the vacuum interface of liquid methanol and acetonitrile have been calculated.53 Results show no free energy minimum at the liquid−vapor interface. Water is expected to be incorporated in the film interior. This incorporation is also true for water on the methanol:16 the water adspecies are incorporated almost completely in the methanol film before crystallization. This incorporation is likely to occur because water is stabilized by the hydrogen bond formation with the hydroxyl group of methanol in subsurface layers. In contrast, the uptake of water in the glassy acetonitrile film is apparently gradual and incomplete before crystallization occurs (T < 120 K). This result strongly suggests that the hydrogen bonding is not as strong in the acetonitrile case as in the methanol case at cryogenic temperatures. These behaviors are reportedly associated with phase separation between water and acetonitrile at cryogenic temperature23 and with the occurrence of microheterogeneity in the water−acetonitrile mixture at ambient temperature.33 The interfacial mixing of binary films of water (20 langmuirs) on acetonitrile (20 langmuirs) and its effects on the phase transition were explored using CO2 as a probe molecule. Figure 9 portrays TPD spectra obtained by deposition of CO2 (1

Figure 10. TPD spectra of water (18 amu), methanol (31 amu), and CO2 (44 amu) for a binary film formed by deposition of water (20 langmuirs) on methanol (20 langmuirs) onto the Ni(111) substrate at 20 K. The CO2 probe molecule (1 langmuir) was deposited at the water−methanol interface.

K is missing completely, indicating that intermixing occurs throughout the binary film. The CO2 TPD peak at ca. 140 K, as well as the occurrence of shoulders in the methanol TPD spectrum at this temperature, might be ascribed to eutectoid separation into nearly pure methanol (crystalline) and a waterrich grain boundary phase (amorphous). It is also possible that a clathrate hydrate of methanol can be formed at around this temperature38 although methanol is known to disrupt hydrogen-bonding network of water and inhibit clathrate hydrate

Figure 9. TPD spectra of water (18 amu), acetonitrile (40 amu), and CO2 (44 amu) for a binary film formed by deposition of water (20 langmuirs) on acetonitrile (20 langmuirs) onto the Ni(111) substrate at 20 K. The CO2 probe molecule (1 langmuir) was deposited at the water−acetonitrile interface. G

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whereas those embedded underneath the ASW film diffuse to the surface at T > 130 K because liquid-like water is formed. The surface segregation is characteristic of amphiphiles in liquid water, but uptake of the segregated acetonitrile occurs at T > 150 K because the −CH3 moiety is caged. This behavior indicates that the surface segregated species are not released immediately from the surface: TPD of additives is not simply ascribable to their diffusivity in the liquid-like film interior because the desolvation process during the first-order phase transition plays a decisive role. Further evidence supporting this inference is that the CO2 additives desolvated during crystallization of acetonitrile are released into the gas phase through a multilayer film of ASW. Results of this study are expected to shed light on the hydration and dehydration mechanisms of larger molecules containing polar and nonpolar head groups and elucidate their interaction with nearby water molecules.

formation.61,62 The CO2 TPD peak and the shoulder of water TPD at ca. 160 K result from crystallization of water. One might infer that additives embedded underneath thin films are released immediately when they reach the vacuum− surface interface via diffusion through the highly mobile phase.58,59 However, this supposition is questionable, as described briefly along with interpretation of the experimentally obtained result shown in Figure 4. The results obtained using the binary films appear to conflict with this inference as well. As Figure 9 shows, CO2 from the underlying acetonitrile film is released at 120 K without influence of the water film. The binary film does not dewet at 120 K, as inferred from the effects of water on the acetonitrile film (see Figure 8b), although pinholes or cracks might be created in the ASW film during acetonitrile crystallization. This result indicates that the CO2 additives that are desolvated from the underlying acetonitrile film permeate the ASW film without hydration because hydrogen-bond imperfections of the ASW films disappear at 120 K. Consequently, thermal desorption of embedded additives has nothing to do with the glass−liquid transition or diffusion of additives in a liquid-like phase because desorption is controlled by the desolvation process during the first-order phase transition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSION The interactions of acetonitrile and methanol with water were investigated at cryogenic temperatures using mixed adlayer thin films comprising monolayers and thin films of acetonitrile, methanol, and water. The acetonitrile additives penetrate porous ASW films and become trapped in the film interior upon heating. The trapped species are released during water crystallization, forming a TPD peak at 160 K. The results resemble hydrophobic hydration of nonpolar additives in ASW, indicating that acetonitrile is caged by water molecules. The −CN group of acetonitrile can form hydrogen bonds with the free −OH groups in the cage, but the hydrogen-bond network of water is not modified, thereby enabling cage formation around the polar additives. In contrast to acetonitrile, methanol desorbs along with evaporation of the multilayer water film rather than water crystallization because caging by water molecules is not the primary interaction for methanol. Hydrogen bonds play a role in the methanol−water interaction, so that a small amount of methanol additives exert strong effects on the crystallization kinetics of water. Acetonitrile additives can also survive water crystallization when the methanol additives coexist because acetonitrile can be trapped by the hydration cage modified by methanol. Thin acetonitrile films dewet the Ni(111) substrate at 120 K because of crystallization. The submonolayer of water adspecies improves the acetonitrile film wettability, but water adspecies do not influence the crystallization of acetonitrile. Because of the hydrophobic behavior of the acetonitrile at cryogenic temperatures, the miscibility of acetonitrile with water is poor. In contrast, water and methanol mix at the molecular level. As demonstrated by the present study, the use of TPD at cryogenic temperature yields information related to mixing and intermolecular interactions of liquid-like films through measurements of solvation and desolvation processes of additives. Results also demonstrate that TOF-SIMS measurements yield unique information related to mobility of adspecies on the surface and thin film interior. The acetonitrile adspecies tend to be incorporated in the porous ASW film at T > 80 K because surface diffusivity of free molecules occurs at this temperature,

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