Solvation of 3-Methylpentane in Nanoconfined Water and Methanol at

Dec 13, 2013 - ABSTRACT: The solvation of 3-methylpentane (3MP) in thin films of water ... distinguishable; the distribution of molecules in these thr...
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Solvation of 3‑Methylpentane in Nanoconfined Water and Methanol at Cryogenic Temperatures Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: The solvation of 3-methylpentane (3MP) in thin films of water and methanol, as well as the effect of a graphite substrate, has been investigated based on a multiplicity of thermal desorption peaks. The 3MP molecules desorbed from the free surface, film interior, and substrate interface are clearly distinguishable; the distribution of molecules in these three configurations depends on the thickness and thermal history of the films. The 3MP adspecies is incorporated in the methanol monolayer due to surface diffusion of methanol and the attractive interaction of the graphite substrate irrespective of methanol’s thermal history. In contrast, when 3MP is adsorbed on methanol multilayers, a porous structure is required for 3MP to be solvated. The solvation force overcomes the attractive force of 3MP on graphite when the multilayer of methanol is present. The water multilayer has fundamentally no effects on the solvation of 3MP when 3MP is in direct contact with graphite. However, the 3MP molecules detach from the interface and dissolve in the water film interior when methanol coexists. The 3MP molecules incorporated in the porous water film tend to be stabilized until crystallization occurs because of hydrophobic hydration.

1. INTRODUCTION Dynamics of solvation and chemical reactions in solvents has received more theoretical than experimental attention.1−6 Nanoconfined liquids are of interest in terms of solvation not only because of the finite size effects on a solvation shell but also because molecular motions are influenced by the free surface and substrate interface. Therefore, the solvation force of liquids in thin films is expected to be perturbed by the forces of surface segregation and adhesion to the substrate. Despite such interests, very few experimental studies have been performed in this respect. Thin glassy films are formed by deposition of molecules on cooled substrates; they become supercooled liquid by heating above the glass-transition temperature, Tg. It is known that Tg of supported thin films is modified significantly from the bulk value.7−14 The surface mobility is enhanced significantly in the sub-Tg region because of the existence of a liquidlike layer near the free surface.15,16 The mobility can be depressed at the interface because of the attractive interaction with the substrate.15 Thus, solvation phenomena characteristic of nanoconfined liquids can be explored using initially glassy films upon heating. On the other hand, thermal desorption has been widely used in characterizing the adspecies−substrate interactions especially in terms of adsorption capacities, site distributions, and their binding energies.17−26 A multiplicity of thermal desorption states has been identified in experiments of temperatureprogrammed desorption (TPD) because the adspecies− substrate interaction can be strong enough to produce a monolayer TPD peak at temperatures higher than a multilayer © 2013 American Chemical Society

TPD peak from weakly physisorbed species. Although it cannot be decided generally whether the stabilization of molecules results from a relative increase in binding energy or a decrease in the pre-exponential factor in the desorption rate constant, there is a trend that higher TPD peak temperatures correspond to higher binding energies on the atomically smooth surface.19,20 In this paper, we investigate the interaction of submonolayer 3-methylpentane molecules (as a solute) with thin films of methanol, water, and water−methanol mixtures (as solvents). These molecules are chosen because their surface diffusion and glass−liquid transition of thin films have been studied previously.16 We used highly oriented pyrolytic graphite (HOPG) as a substrate for two reasons. The desorption of molecules in direct contact with metal surfaces is often indistinguishable from recombinative desorption of dissociated species in TPD,19,20 but such an ambiguity is avoided using a chemically inactive substrate. Moreover, a clearly separated monolayer TPD peak is known to appear for hydrocarbons from HOPG.23,24 Because of this behavior, locations (i.e., the surface, interface, and thin film interior) of the solute species during the thermal desorption process are expected to be identified based on the TPD peak temperature. The uptake of the solute into the thin film interior is investigated using timeof-flight secondary ion mass spectrometry (TOF-SIMS) by monitoring the surface compositional change upon heating. Received: July 31, 2013 Revised: December 8, 2013 Published: December 13, 2013 26969

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The solvation characteristic of nanoconfined liquids at cryogenic temperature is explored based on these methods.

2. EXPERIMENTAL SECTION Experiments were performed in an ultrahigh vacuum (UHV) chamber that has a base pressure of 99%, Aldrich), CD3OD (99.8%, Aldrich), and H218O (98%, Taiyo Nippon Sanso) were degassed by repeated freeze−pump−thaw cycles prior to use. Before each dosing, the gas line to the UHV chamber was evacuated and old vapor was replaced by fresh one. Thin films were deposited in situ by exposing the HOPG substrate that was kept at 20 K to a stream of gas by backfilling the chamber through high precision leak valves. The results are reported here based on exposures in langmuirs, where 1 langmuir = 10−6 Torr s. TPD spectra were recorded using a quadrupole mass spectrometer (HIDEN, IDP 300S) placed in a differentially pumped housing by approaching an orifice ∼3 mm apart from the sample surface. For TOFSIMS measurements, a primary beam of 2 keV He+ ions was generated in an electron-impact-type ion gun (Specs, IQE 12/ 38) and was chopped into pulses using electrostatic deflectors. Positive secondary ions ejected perpendicularly to the surface were detected using a microchannel plate after traveling a fieldfree TOF tube. To extract low-energy secondary ions efficiently, a bias voltage (+500 V) was applied to the sample. The temperature was ramped at a rate of 5 K min−1 for both TPD and TOF-SIMS measurements. The fluence of He+ in TOF-SIMS measurements was kept below 1 × 1012 ions cm−2 to minimize surface decomposition. The static SIMS condition was ensured by the experimental fact that no contaminants from the vacuum and fragments of adspecies were detected after finishing the temperature scan.

Figure 1. TPD spectra of (a) methanol-d4 and (b) 3-methylpentane deposited on the HOPG substrate at 20 K. The temperature was ramped at a rate of 5 K min−1.

This behavior might imply that a small intermolecular interaction exists in the monolayer. To date, we have determined the coverage of glassy molecular films using a Ni(111) substrate based on the TOFSIMS measurements.27,28 The intensities of the Ni+ ion and their adducts form a peak at around the same exposure for saturation of secondary ion intensities from adspecies. This occurs at ca. 2.5 and 10 langmuirs, respectively, for methanol27 and 3-methylpentane28 at 20 K. The first monolayers are expected to be formed at these exposures on the HOPG substrate as well because the sticking probability of molecules is thought to be unity at 20 K. These values are apparently greater than the exposures required for completion of the monolayer TPD peaks. This occurs because adsorption structures of molecules are different significantly between the glassy monolayer at low temperature and the high-temperature monolayer phase. In this respect, results of longer-chain nalkane molecules are instructive.29,30 The monolayer TPD peak arises from the molecules whose backbone is aligned parallel to the surface. However, such an ordered structure should not be realized immediately after deposition of molecules. The molecules in glassy films are expected to interact mainly with the neighboring molecules, which is ensured by the fact that the adspecies−substrate bond is weakened when the intermolecular bond is formed.31,32 This behavior not only enables cooperative motion of molecules on attractive substrates (a 2D liquid)16 but also leads to a TPD peak of the glassy monolayer at temperature close to the multilayer peak position. On the other hand, the monolayer peak is thought to occur in TPD only after excess molecules are released with increasing temperature. The glassy monolayer can be regarded as a crowded monolayer, whereas dispersed molecules that interact mainly with the substrate are responsible for the highertemperature monolayer peak in TPD. Consequently, the completion of the glassy monolayer that leads to the 2D liquid

3. RESULTS AND DISCUSSION 3.1. Assignment of the Monolayer TPD Peak. Figure 1 shows TPD spectra of (a) methanol-d 4 and (b) 3methylpentane monitored in the 36 and 56 amu signals after exposing HOPG to the molecules at 20 K. Monolayer and multilayer desorption features are well separated for both adspecies at higher exposure. The monolayer peak is not saturated at exposure of 1 langmuir. The TPD peak area is reproducible within ±10% in the present experiment. The TPD spectra of methanol are fundamentally identical to those reported by Bolina et al.26 A large shift in temperature of the monolayer peak relative to the multilayer peak for 3methylpentane is consistent with the TPD spectra of straight chain alkanes deposited on HOPG.23,24 Here we do not discuss details of desorption kinetics, but the monolayer peak is not simply classified as a first-order process especially for 3methylpentane, as revealed from the asymmetry of the monolayer peak and shift in the peak position with coverage. 26970

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cannot be identified based on TPD. The high-temperature monolayer peak of methanol in TPD is indicative of higher adsorption energy.26 In contrast to 3-methylpentane and methanol, no independent monolayer peak is identified in TPD spectra of water.33 Probably, this is because the intermolecular hydrogen bond of water prevails over the attractive interaction with the substrate. This is true even for small water clusters.34 From these facts, we define 1 monolayer (ML) of glassy and liquid films based on TOF-SIMS rather than TPD. In any case, we can identify whether the submonolayer solute species (3-methylpentane) interacts with the molecular films or the HOPG substrate based on temperatures of the well-separated TPD peaks. No indications of the solvation are observed for binary layers of 3methylpentane and methanol formed at exposures of 1 langmuir (not shown) because the adspecies−substrate interaction prevails over the intermolecular interactions at submonolayer coverage. 3.2. Solvation in Nanoconfined Methanol. Figure 2 shows experimental results of (a) TOF-SIMS and (b) TPD

interacting with HOPG remain after disappearance of CD3OD. The peak occurs at lower temperature than the monolayer peak in Figure 1b because the amount of the 3MP molecules remaining on the substrate is small. Probably, they stayed at the interface via penetration through the CD3OD monolayer without solvation. The TPD peak of CD3OD is almost identical to that shown in Figure 1a because most of 3MP at submonolayer coverage disappears prior to the CD3OD desorption. The molecular films deposited at temperatures well below Tg are characterized by a microporous structure, as evidenced by the ability to incorporate a large amount of simple molecules in the thin film interior.35,36 Although such a phenomenon is not expected in the monolayer regime, it is likely that the molecular structure of the glassy film is dependent on the thermal history. The crystal-like structure can be formed by annealing the glassy methanol film above 120 K.27 Therefore, a measurement similar to that in Figure 2 is made using a CD3OD film (2.5 langmuirs) that is annealed to 120 K before exposure to 1 langmuir of 3MP at 20 K. The experimental results are shown in Figure 3. The

Figure 2. (a) TOF-SIMS intensities of typical ions sputtered from the 3MP and CD3OD molecules on HOPG. The HOPG substrate was exposed to the former (1 langmuir) after deposition of the latter (2.5 langmuirs) at 20 K. (b) TPD spectra of 3MP (m/q = 56) and CD3OD (m/q = 36) from the film prepared in the same manner as in the TOFSIMS measurement.

Figure 3. Same as in Figure 2, but the CD3OD film (2.5 langmuirs) was heated to 120 K and then 3MP (1 langmuir) was adsorbed on it at 20 K.

TOF-SIMS result is similar to that in Figure 2a, although the uptake onset of 3MP increases to 70−80 K. Two TPD peaks of 3MP corresponding to desorption from the surface (120 K) and thin film interior (134 K) are more clearly separated. The 3MP molecules in direct contact with the HOPG substrate are also identifiable. Consequently, mixing of 3MP and CD3OD is not influenced strongly by the thermal history of the CD3OD monolayer. It is likely that crystallization of the monolayer is quenched because of the interaction with HOPG. Moreover, the uptake temperature of 3MP is lower than methanol’s Tg (103 K). It is known that the surface mobility of 3MP and CH3OH occurs at ca. 50 and 70 K, respectively.16 They correspond well to the experimental uptake temperatures of 3MP on as-deposited (Figure 2a) and annealed (Figure 3a)

obtained using a CD3OD film (2.5 langmuirs) on which 3methylpentane (1 langmuirs) was adsorbed at 20 K (referred to as 3MP (1L)/CD3OD (2.5L)/HOPG). At temperature higher than 50−60 K, TOF-SIMS intensities from 3MP (CD3OD) decrease (increase) gradually. The 3MP adspecies are incorporated considerably in the film up to 90 K; then, they tend to reappear at the surface before desorption occurs at 120−130 K. The TPD peak of 3MP at this temperature is bimodal. We tentatively assign them to desorption from the surface (128 K) and interior (132 K) of the CD3OD film. A small amount of the 3MP molecules assignable to those 26971

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CD3OD films. These results indicate that the surface mobility of CD3OD (i.e., formation of the 2D liquid) is responsible for mixing, but mobile 3MP molecules can also penetrate the glassy CD3OD monolayer at lower temperatures. Figure 4 shows TPD spectra from 3MP (1L)/CD3OD (10L)/HOPG. The results are compared using the CD3OD

adspecies than the annealed CD3OD multilayer, as revealed from the 3MP peak intensities at around 133 K. This result is ascribable to the influence of the HOPG substrate: the 3MP molecules can penetrate the monolayer and stay transiently at the interface. The adspecies can be entrapped in the film and interface because the CD3OD monolayer becomes diffusive at 70−80 K prior to the occurrence of 3MP desorption at 110− 120 K as seen in Figures 3a,b. Such species are released during desorption of CD3OD and form the TPD peak at 133 K. In contrast, the 3MP molecules neither enter the film nor reach the interface through the annealed CD3OD multilayer because diffusivity in the film interior is quenched upon crystallization. Even in the case of the glassy films without pores, diffusivity onset in the film interior (Tg = 103 K) is so close to the TPD peak of 3MP (115 K) that the adspecies may not be accommodated efficiently. Thus, the liquidlike properties of the CD3OD monolayer are kept after annealing above the bulk crystallization temperature, so that the 3MP adspecies can be solvated at the interface of the HOPG substrate. The mobility of the topmost layer is expected to occur for the CD3OD multilayer at the same temperature (70−80 K),16 but the 3MP adspecies may not be stabilized at the subsurface site because the attractive force of the underlying CD3OD crystal is weaker than the HOPG substrate. Probably, a thicker layer of CD3OD is required for solvation, so that only the 3MP adspecies that penetrate into deeper layers through pores can be trapped in the multilayer film. How the solvation force of CD3OD influences the adhesive interaction between 3MP and HOPG is investigated using CD3OD (5L)/3MP (1L)/HOPG samples. In Figure 5 are compared the results using 3MP deposited at 20 K (a) and preannealed to 120 K (b) to explore the effects of the initial

Figure 4. TPD spectra of CD3OD (m/q = 36) and 3MP (m/q = 56) from differently tailored binary films. The CD3OD film (10 langmuirs) was deposited on HOPG at 20 K (a) or it was preannealed to 120 K (b); then, 3MP (1 langmuirs) was adsorbed on it at 20 K.

films deposited at 20 K (a) and preannealed to 120 K (b). The 3MP molecules stay on the surface of the annealed CD3OD film until desorption occurs at 115 K, whereas most of them are incorporated in the as-deposited CD3OD film and then released along with the desorption of CD3OD, thereby forming a peak at 133 K. No 3MP molecules remain on HOPG after disappearance of CD3OD. Thus, the HOPG substrate has no effects on the 3MP adspecies beyond the 4 ML CD3OD film (corresponding to exposure of 10 langmuirs); the porous structure of the CD3OD film plays a role in uptake of 3MP. The uptake of 3MP is strongly influenced by the thermal history of the multilayer CD3OD films, providing a striking contrast to the results using monolayer films shown in Figures 2 and 3. The 3MP peak temperature from the annealed CD3OD multilayer (116 K, Figure 4b) is lower than the corresponding peak temperatures from the CD3OD monolayers in Figure 3b (120 K) and Figure 2b (128 K). Moreover, the separation of the 3MP peaks from the surface and film interior becomes smaller for the as-deposited CD3OD monolayer in Figure 2b, suggesting that the 3MP molecules from the surface also undergo solvation to some extent. This result is reasonable because most of the 3MP molecules are once incorporated in the CD3OD monolayer prior to desorption as revealed by the TOF-SIMS measurement (see Figure 2a). From comparison of the results between Figures 3b and 4b, the annealed CD3OD monolayer accommodates a larger number of the 3MP

Figure 5. TPD spectra of CD3OD (m/q = 36) and 3MP (m/q = 56) from differently tailored binary films. The 3MP molecules (1 langmuir) were adsorbed on HOPG at 20 K (a) or it was preannealed to 120 K (b); then, CD3OD film (10 langmuirs) was formed on it at 20 K. 26972

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Most of the incorporated 3MP molecules are released explosively at ca. 154 K where the water desorption rate also changes. These behaviors have been explained as the film morphology change associated with crystallization of water.33 When the 3MP molecules are embedded underneath the ASW film, the 3MP molecules do not desorb during water crystallization (Figure 6b), indicating that the 3MP molecules stay fundamentally at the interface without diffusion into pores of the ASW film. These results suggest that the mobility of 3MP is depressed at the interface relative to the free surface, in agreement with the interfacial stiffening reported by Cowin and co-workers.15 The ASW film has no significant effects on interactions between 3MP and HOPG as far as the main 3MP TPD peak temperature (∼160 K) is concerned. A small hump occurs at ca. 168 K, which is indicative of mixing of a small amount of 3MP with water. Thus, the transport of interfacial 3MP to the thin films’ interior or free surface is ineffective for water relative to CD3OD (see Figure 5). This behavior corresponds to the difference in solvation abilities between water and methanol against 3MP. In contrast, the 3MP molecules incorporated from the free surface into the porous ASW film appear to be stabilized until crystallization occurs (Figure 6a), which contrasts to the result using the porous CD3OD film (Figure 4a). If the simultaneous release of the solute (3MP) and solvent (CD3OD) species is characteristic of solvation, the 3MP molecules are thought to be bound more tightly in water. This result might be attributed to the formation of a cage of hydrogen-bonded water molecules around the solute 3MP species. The liquidlike water formed at Tg = 136 K is distinct from normal liquid, termed low-density liquid (LDL);37 its local structure resembles crystalline ice rather than the normal liquid water.38 Therefore, the 3MP molecules can be incorporated in crystal-like cages of LDL (hydrophobic hydration). The abrupt release of 3MP during crystallization can be explained as the formation of normal liquid water immediately before crystallization (the liquid−liquid phase transition).39 A tertiary film, H218O (20L)/CD3OD (5L)/3MP (1L)/ HOPG, is prepared at 70 K; the TPD spectra from this surface are shown in Figure 6c. The TPD peak of methanol is broadened considerably because mixing occurs with water. The desorption onset of the H/D exchanged species (CD3OH; m/q = 35) corresponds well to water’s Tg (136 K).40 The morphology change of the ASW film during crystallization is quenched, as evidenced by the absence of the hump in water TPD at 154 K. This is because the CD3OD molecules that segregate to the surface play a role in quenching the droplet formation as a surfactant.40 It should be noticed that the desorption kinetics of 3MP from water is influenced significantly by the coexisting CD3OD. The TPD peak at ca. 160 K, resulting from 3MP in direct contact with HOPG, disappears. Instead, the main desorption peak occurs rather steeply at 168 K around the TPD peak maximum of water. The result can be interpreted as that solvation of the 3MP molecules in water is promoted by methanol because of its surfactant effect. In fact, the resemblance of the TPD peak structures between 3MP and CD3OH at around this temperature strongly suggests the formation of their complex. The complex is thought to be created in the liquidlike phase of water (T > 136 K) and confined in crystallites or their boundaries until higher temperatures. The crystalline water is known to be heterogeneous;41 it is characterized as a mixture of solid and liquid phases because of premelting of metastable ice Ic. Therefore, it

adsorption structure of 3MP on HOPG. A marked difference is observed in the TPD peak distributions of 3MP. The bimodal peak in Figure 5a resembles that in Figure 2b. As described earlier, the low-temperature peak at 127 K may originate from the partially solvated species near the surface but is not attributable to simple adsorption on the solid CD3OD surface (the peak at 115 K). The 3MP molecules deposited at 20 K interacts weakly with HOPG because of the coexisting CD3OD; they can move rather freely through the CD3OD layer at low temperature (T > 50 K), thereby resulting in the surface (127 K) and bulk (133 K) solvation peaks. In contrast, when the high-temperature monolayer phase of 3MP is formed initially on HOPG, the surface solvation peak is suppressed considerably relative to the bulk one, suggesting that the molecules are solvated in the thin-film interior without diffusion to the surface. The tightly bound 3MP species are removed from the interface almost completely as revealed from disappearance of the 3MP peak at around 160 K. This result can be explained as that the adspecies−substrate bond is weakened when the intermolecular bond is formed between adspecies.16,31,32 Probably, this is the origin of the solvation force of the CD3OD multilayer. 3.3. Solvation in Nanoconfined Water. The interaction of water with 3MP is also investigated using TPD. The experimental results of (a) 3MP (1L)/H218O (20L)/HOPG and (b) H218O (20L)/3MP (1L)/HOPG are compared in Figure 6. The molecules are deposited at 70 K, so that amorphous solid water (ASW) characterized by the porous structure is created in the film. The 3MP molecules deposited on the surface can diffuse into the film interior, as evidenced by absence of the TPD peak of 3MP from the surface in Figure 6a.

Figure 6. TPD spectra of H218O (m/q = 20), 3MP (m/q = 56), CD3OD (m/q = 36), and CD3OH (m/q = 35) from the films of (a) 3MP (1L)/water (20L), (b) water (20L)/3MP (1L), and (c) water (20L)/CD3OD (5L)/3MP (1L) deposited on HOPG at 70 K. 26973

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released simultaneously with CD3OD desorption. Thus, intermolecular interactions in nanoconfined films are explainable in terms of solvation. This is typically demonstrated by the surfactant effects of CD3OD in interactions between 3MP and water. Because of the presence of CD3OD, the 3MP molecules detach from the interface and dissolve in the water film interior. The 3MP−CD3OD complex formed in water is stabilized until final stage of water desorption where the most stable crystal cores are thought to sublime.

is likely that the liquidlike layer desorbs prior to crystal grains. The fact that shapes of the TPD peaks between water and 3MP are different might suggest that the CD3OD−3MP complex is confined preferentially in crystallites rather than liquidlike phase (i.e., grain boundaries). It is also possible that the 3MP− MeOH complex is so stable in liquidlike water that dissolution is quenched until most of the water molecules disappear from its solvation shell.

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4. CONCLUSION The solvation characteristic of nanoconfined liquids is explored based on the TPD and TOF-SIMS analyses of interactions between the submonolayer 3MP molecules and the thin films of CD3OD and water at cryogenic temperatures. The wellseparated peaks occur for 3MP and CD3OD desorbed from HOPG because of the difference in binding energies between the adspecies−adspecies and adspecies−substrate interactions. The dispersed adspecies that interacts mainly with the attractive HOPG substrate is thought to be responsible for the hightemperature monolayer peak in TPD. The adsorption structure of molecules in the glassy monolayer is thought to be distinct from that in the high-temperature monolayer phase. This occurs because the adspecies−substrate bond is weakened when the intermolecular bond is formed. On the basis of these facts, the solvation environment, as well as the interfacial structure, was discussed from the thermal desorption temperature of molecules. For 3MP interacting with CD3OD, we can distinguish at least three locations. The molecules desorb from the surface and interior of the CD3OD film and from the interface with HOPG. The distribution of the 3MP molecules in these three locations, as well as their desorption temperatures, depends on the thickness of the CD3OD film and its thermal history. The 3MP adspecies tend to be incorporated in the CD3OD monolayer irrespective of its thermal history. Probably, this is because the CD3OD monolayer behaves like a 2D liquid and does not crystallize even at temperatures higher than the bulk crystallization temperature of methanol. The attractive force of the HOPG substrate plays a role in stabilizing 3MP at the interface. In contrast, the interaction of 3MP with the CD3OD multilayer is strongly dependent on the thermal history. The porous structure of the film is required for the solvation of 3MP in the film interior, whereas the molecules stay on the surface of the crystalline CD3OD layer without incorporation. The TPD peak of 3MP from the surface is higher in temperature using the glassy CD3OD film deposited at 20 K than that using the crystalline film, suggesting that “partial solvation” occurs on the surface during the hydrogen-bond formation between the asdeposited CD3OD molecules upon heating. The 3MP molecules at the interface are removed by the 2 ML CD3OD overlayer almost completely because the attractive force of 3MP on the HOPG substrate is weakened by the interaction with the CD3OD molecules. The desorption kinetics of the 3MP molecules on HOPG is not influenced by the presence of the water multilayer; only a small amount of molecules can be incorporated in the water overlayer. On the other hand, the 3MP molecules are trapped in the bulk when 3MP is deposited on the surface of the porous ASW film. The 3MP molecules are stabilized in the film interior and released explosively during crystallization. This behavior can be explained as the occurrence of hydrophobic hydration in LDL. No such a stabilization process is recognizable using the porous CD3OD films; the incorporated 3MP molecules are

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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The Journal of Physical Chemistry C

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