Jonathan F. D.
Liljeblad
,
István
Furó
and
Eric C.
Tyrode
*
Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden. E-mail: tyrode@kth.se
First published on 23rd November 2016
In order to establish the potential correlation between the macroscopic ice adhesion and the molecular properties of the premolten layer (PML), the adhesion strength between ice and hydrophilic silica has been measured as a function of temperature. In addition, temperature-dependent molecular properties have been determined using techniques that are sensitive to different aspects of the PML, specifically total internal reflection (TIR) Raman, vibrational sum frequency (VSFS) and NMR spectroscopies. The ice shear adhesion strength was observed to increase linearly with decreasing temperature until −25 °C, where a plateau marked the adhesive strength having reached the cohesive strength of ice. Interestingly, at temperatures higher than −20 °C the ice samples slid on smooth (Ra < 0.4 nm) silica surfaces. This sliding behavior was not observed on rougher silica surfaces (Ra ∼ 6 nm). By varying the penetration depth of the evanescent field, TIR Raman was used to establish an upper limit to the thickness of the PML in contact with silica (<3 nm even at −0.3 K below the bulk melting temperature). Additional quantitative determination of the temperature-dependent thickness of the PML was obtained from 2H NMR measurements in mesoporous silica particles. Finally, the inherently surface specific technique, VSFS, which probed changes in the hydrogen bond environment, indicated at approximately −25 °C the onset of PML, followed by a marked structural change occurring just a fraction of a degree below the melting temperature. Jointly, the experimental approaches link, strongly and consistently, ice adhesion to the PML properties. Specifically, it is inferred that the premolten layer facilitates sliding and contributes to the observed friction behavior, provided its thickness is comparable to the surface roughness of the underlying silica substrate.
Despite having gained acceptance as a phenomenon, there is no detailed, molecular, experimentally, and consistently tested understanding of premelting.5,6 The core problem is illustrated by the fact that experimental techniques differ by orders of magnitude when it comes to estimating either the extension of the PML or its temperature dependence!3,5,7 This apparent cacophony persists when looking at other derived molecular properties of the PML. It is also reflected by the lack of established terminology – PML is called disordered interface by equally many and quasi-liquid layer or liquid-like layer by some. The lack of agreement in the estimated PML properties has often been attributed to impurities.8 While impurities may certainly be a factor, the apparently divergent results of experiments performed under very similar conditions but with different experimental methods suggest other underlying reasons: firstly, the use of methodologically inherent but, to each other, incompatible definitions of molecular order and dynamics when interpreting the results from different type of experiments and, secondly, experimental artifacts.
The aim of the research presented here is to link the temperature dependent macroscopic properties of adhesion and friction to the corresponding molecular properties of the PML. Specifically, we explore the PML that arises at the silica–ice interface using four different experimental approaches to extract information. The first consist of macroscopic shear adhesion measurements performed on a home-built apparatus. The remaining three are spectroscopic methods: total internal reflection (TIR) Raman spectroscopy,9 NMR spectroscopy, and vibrational sum frequency spectroscopy (VSFS).10,11 Although TIR Raman and NMR are very different from each other in the way they detect water molecules being in a state that is not that of bulk ice, they shall furnish reassuringly consistent information about the extension of the PML. For simplicity and for being able to compare our results to those previously reported, we shall define this extension as “thickness”. Yet, one should be acutely aware that the PML is (despite being often depicted in such manner) probably not a well-defined layer with sharp boundaries. Rather, the PML is an assembly of molecules whose behavior gradually resembles bulk ice as the distance to the defining interface (here, silica) increases. The thickness of the PML is then dependent on the threshold, either given or set up, by a particular experimental method for detecting water molecules behaving differently from molecules in ice. In the NMR studies below, this threshold is set by the reorientational dynamics of molecules. In TIR-Raman the spectrum responds to changes in vibration frequencies influenced by intermolecular interactions, in particular hydrogen bonds whose strength and arrangement defines the state of the material: ice, liquid water, or PML. In contrast to NMR and TIR-Raman, VSFS shall be used to probe in a very sensitive manner the state of the water molecules closest to the silica surface.
Due to its abundance on Earth, silica is a natural choice for a model hydrophilic solid surface that provides strong dipolar interactions and a rich hydrogen bond platform. The silica–ice interface also offers distinct advantages for studying the PML. First, it is available in pure form both as a flat surface that can be assessed by VSFS, TIR Raman, and shear adhesion measurements, but also as a surface in controlled-size pores with the large surface-to-volume ratios required for probing the PML using a relatively surface insensitive method like NMR. Second, it can be made very smooth which, as will be shown below, has important implications for the shear adhesion measurements. That being said, there exist a body of work that addresses the state of PML at the silica–ice interface, which include ellipsometry,12,13 VSFS,14 X-ray,15 NMR,16 and even ice adhesion measurements.17 However, the discord in terms of the description of the PML properties is as vivid as for the ice/vapour interface, with estimations for the PML layer extension ranging from non-existent,13,14 to tens16 and hundreds of nanometers.12,18 We will consider some of the potential causes of these discrepancies, and bring a more consistent understanding of the PML properties at this relevant mineral/ice interface.
Roughness parameters with higher lateral resolution were obtained from AFM topography images using a Dimension FastScan (Bruker) atomic force microscope operated in tapping mode. The images were recorded using two Hz scan rate with FastScan A cantilevers featuring a resonance frequency of 1.4 MHz, a nominal spring constant of 18 N m−1, and a tip radius of 5 nm. For each sample a small and large scale image (10 × 10 μm and 2 × 2 μm, respectively) was recorded. Roughness parameters were calculated using the NanoScope Analysis version 1.5 software (Bruker).
The ice/silica samples were prepared by placing a Teflon cylinder (internal diameter and wall thickness of 10 and 1 mm, respectively) filled with degassed water (0.8 mL) on top of the silica windows. The temperature of the chamber was then reduced to −20 °C to quickly freeze the sample. The intended measuring temperature was subsequently set and the sample was allowed to equilibrate for at least 20 minutes before performing the ice adhesion measurement.
A model function featuring a sum of complex Lorentzians (eqn (1)) has been fitted to the data using the Levenberg–Marquardt algorithm in a commercial software (Origin 9.0) to extract the non-resonant contribution to the signal ANR, as well as the amplitude An, the resonance frequency ωn, and the homogeneous line width Γn of the nth vibrational mode (ωIR represents the tunable IR frequency). This approach is necessary since both constructive and destructive interference between peaks occurs in VSF spectroscopy.
(1) |
Before fitting, the data were normalized by the Fresnel factors which depend on the IR frequency and the specific experimental geometry employed21 (see ESI6† and ESI7† for details).
The ice samples prepared were polycrystalline. Nevertheless, as the crystal sizes were at least ten times larger than the diameter of the laser beams used to probe the interfacial region, all the vibrational spectroscopy data shown here were obtained from single crystals of unknown orientation. The borders between crystals could be easily identified by the grain boundaries at the melting point, as well as by direct inspection of light microscope images using polarized light. SF spectra were collected in several different ice samples, showing an overall similar trend in the variation of the spectral features with temperature. However, for consistency and ease of comparison, the SF data presented here result from measurements carried out in a single ice sample.
The NMR experiments were performed on a Bruker Avance III 500 NMR spectrometer with 76.8 MHz resonance frequency for 2H. A conventional broadband-observe probe was used to detect the time-domain 2H NMR signal as recorded after a single-pulse excitation. The length of the excitation pulse was set to 1 μs that corresponds to a pulse angle of ca. 11° (calculated form the 90° pulse length calibrated using the liquid 2H NMR signal above melting). The actual temperature of the sample versus the set temperature was calibrated in situ with Pt100 temperature sensors. The accuracy of the calibration (within the ±0.1 K precision of the temperature regulator system used) was tested by recording the characteristic signature (the appearance of a narrow peak) of melting in a bulk D2O sample.
One disadvantage with single-pulse detection is that probe ringing introduces transients, which can be significantly suppressed by introducing a few microseconds waiting time after the excitation pulse. However, as a consequence of this waiting time, the broad spectrum of the ice component becomes strongly distorted, a problem that is amplified by the effect of the remaining transients. Hence, extracting the intensity of the peak assigned to the premolten layer requires extensive baseline correction that is nonetheless, rather straightforward at low temperatures, where a simple linear baseline correction suffices. However, closer to the melting point (see the discussion below concerning the melting point in pores) the spectrum of the ice component becomes increasingly motionally averaged23 and thereby narrower. Hence, distinguishing between the still much broader ice peak and the relatively narrower premolten peak becomes less evident. In addition, close to the melting point a “central” peak even in the spectrum of bulk ice can be observed. Yet, at the highest temperature below the melting point explored here, the relative intensity of that peak is more than one order of magnitude lower than that of the premolten peak. These effects jointly form one factor that limits our uppermost temperature to ca. 1.5 °C below the pore melting point of ice.
The other limiting factor is the distribution of pore sizes in the explored porous glass. As described by the Gibbs–Thomson equation,28 materials melt in pores at temperatures inversely proportional to the pore size. Hence, a size distribution leads to a distribution of the pore melting points (indeed, this is the way one often detects pore size distribution in NMR cryoporometry29,30). For this reason, ice in pores smaller than the average size melts at temperatures lower than the average pore melting temperature. This is actually detectable by the appearance of a small and narrower peak on the top of the premelting peak. Coincidentally, the onset of this effect is also around ca. 1–1.5 °C below the average pore melting point of ice.
(2) |
(3a) |
(3b) |
The sliding behavior is particularly worth remarking, because on rougher silica substrates (vide infra) ice does not slide at any temperature, detaching exclusively by adhesive or cohesive failure. The general trend depicted in Fig. 1, with an almost linear increase of the adhesion strength followed by a cohesive regime, as well as the sliding behavior for smoother samples is in agreement with what has been previously reported by Jellinek.17,18 Although not experimentally proven, it was suggested then that the transition from sliding to adhesive/cohesive failure, was influenced by the thickness of the PML relative to the surface roughness. The underlying argument is that the premolten layer promotes sliding, provided it is sufficiently thick to prevent mechanical interlocking by ice engaging groves in the substrate surface: the molecular properties of the premolten layer should then govern the macroscopic adhesion. To establish such correlation we make use of vibrational spectroscopy and NMR.
A closer examination and fitting of the ice spectra in Fig. 2, together with those collected at other polarization combinations (see ESI3†), reveal the presence of at least four resolvable bands, centered at approximately 3050, 3140, 3270, and 3375 cm−1. Although these bands associated with the OH stretching modes have been previously identified,32–35 the detailed assignments remain a source of debate, mainly because of the complicated interplay between OH-bonds of different strengths and in particular, their intermolecular coupling dynamics.36–38 Agreement is found, however, in the assignment of the dominant ∼3140 cm−1 peak in Fig. 2 to the collective in-phase OH stretch of tetrahedrally coordinated water molecules.32,33,35–37 Without going further in the assignment of the remaining bands, for the purpose of the general discussion of the TIR Raman spectra below, it suffices to stress the spectral differences between the solid and liquid forms of H2O, namely sharp bands at lower wavenumbers are linked to strongly hydrogen bonded molecules in ice, while broader features at higher wavenumbers are accordingly associated to more loosely coordinated molecules in the liquid.
Fig. 3 left shows pairs of closely overlapping TIR Raman spectra from the “surface” and bulk collected at three different temperatures, ranging from −35 °C to −0.3 °C. The difference spectra (Fig. 3 right) are significant, and show two clear peaks at ∼3410 cm−1 and ∼3620 cm−1, with essentially temperature-independent centre positions and absolute intensities. The observed lack of temperature dependence is inconsistent with the expected behavior of a premolten layer, particularly when approaching the melting point. In order to determine the origin of the peaks in the difference spectra, experiments were also performed with D2O ice. Replacing the hydrogen atoms of water by the heavier deuterium red-shifts all the ice spectral features by ∼900 cm−1, which should leave in principle, no bands in the OH stretching region. However, the same features observed for H2O ice remained for D2O ice (dashed line in Fig. 3 right), which implies that the bands do not arise from either ice or the PML. They are instead assigned to SiOH groups trapped within the bulk fused silica substrate.42,43
To make certain that potential spectral features from the PML were not obscured by substrate bands, additional TIR Raman measurements were performed with D2O ice (Fig. 4). As previously recognized,32,37,38 the patterns of the spectral features in D2O ice are not identical to those of H2O ice. Notwithstanding, they show obvious resemblance, with a more intense peak at low frequencies (i.e. 2330 cm−1), followed by weaker and broader bands at higher wavenumbers (note that D2O ice melts at +3.8 °C). The difference spectra in the OD stretching region shown in the inset of Fig. 4 may reveal ever slightly higher intensities for the bands associated with weaker hydrogen bonds close to the melting point; however, within error, no significant differences are observed at the two extreme temperatures recorded.
Fig. 4 TIR Raman Sy spectra of D2O ice (melting temperature +3.8 °C) next to a fused silica substrate at various temperatures, collected at two penetration depths: “bulk” and 100 nm. The spectra have been normalized to the peak centred at ∼2330 cm−1. The inset (magnification shown in ESI4†) shows the surface contribution (subtraction of bulk spectrum from 100 nm spectrum for each temperature). |
There are two possible explanations for these results: (i) the PML thickness is very small compared to the shortest penetration depth of the evanescent wave, and/or (ii) its spectral features are not significantly different from those of bulk ice. Option (ii) can be discarded, considering previous IR measurements on thin ice films,40 as well as confocal Raman measurements on porous silica samples (data to be presented elsewhere), that indicate that the PML vibrational spectra clearly differs from that of ice, resembling more that of liquid or supercooled water. This is further substantiated with the sum frequency results described below. Hence, we infer that the thickness of the PML is lower than the detection limit of TIR Raman at all temperatures measured. Specifically, from the subtracted spectra shown in Fig. 3 and 4 and using as model for the PML the spectra of supercooled water, the PML thickness as determined by TIR Raman, is conservatively estimated to be thinner than 3 nm, even at temperatures as close as 0.3 °C from the bulk melting temperature.
Following the procedure described in detail in the Experimental section, the estimated thickness of the premolten layer lPML (see eqn (3b)) as a function of temperature is presented in Fig. 5. The temperature scale in Fig. 5 is relative to the approximate mean pore melting point of 3.3 °C as obtained by conventional cryoporometric experiments (the bulk melting point inside the pores is suppressed as predicted by the Gibbs–Thomson equation).30
Fig. 5 The thickness of the PML layer derived (see lPML in eqn (3b)) from 2H NMR measurements as a function of temperature, measured on porous silica particles imbibed with D2O. The ΔT is relative to the approximate mean pore melting point that is ∼3.3 °C for the silica particles used. The shaded area indicates the customary range given for the thickness of a monolayer of water in the liquid state. |
While premelting has been frequently detected by NMR both in pores but also on free-standing small ice particles,44 the preferred detection method has been 1H NMR.16 Yet, in the 1H NMR spectrum the signals from ice and premolten water cannot be as easily distinguished45 as in the corresponding 2H NMR spectrum, particularly when approaching the melting point where the molecular mobility in ice increases.23 This is illustrated in the ESI10† where it is shown that the 2H NMR signal of the PML layer can be clearly distinguished upon comparison of the spectra recorded in bulk ice and ice imbibed in the porous glass.
Further support for the identification of the PML–NMR signal can be obtained by comparing the signal intensity in porous glasses with varying pore sizes. As indicated by eqn (3a) in the Experimental details, the relative intensity of the signal αPML is inversely proportional to the mean pore diameter, a fact that has been verified both for water45 and other liquids exhibiting premelting.46 For the same reason, premelting in porous materials was typically observed by NMR in systems with small pore sizes. Notwithstanding, as it has been demonstrated the PML thickness is sensitive to the curvature of the pore wall.46 Consequently, porous silica particles with rather large pores (i.e. 187.8 nm) were selected in this study to facilitate the comparison of the NMR data with the TIR Raman and VSFS observations on flat surfaces.
The only comparable temperature-dependent data set we are aware of has been collected in Vycor glasses with 4–50 nm pore diameter;16 Vycor glasses typically exhibit far less regular pore structures and broader pore size distributions30 than those manufactured by Controlled Pore Glasses. In contrast to the data presented in Fig. 5, the data from Ishizaki et al.16 seem to indicate that the premolten layer grows continuously up to complete melting. We believe the data within a few degrees to the pore melting temperature, were wrongly attributed to the PML16 since the contribution of completely molten pore volumes was neglected. Hence, the data from Ishizaki et al.16 reflects not the extension of the premolten layer but instead the width of their pore size distribution (see NMR section in Experimental details). As an additional contributing methodological reason, the 1H NMR spectrum of the premolten phase is narrower (ca. 3 kHz broad45) than the corresponding 2H NMR spectrum (ca. 8–10 kHz, vide supra), while the 1H spectrum of the liquid within molten pores is ca. 6 times broader (>0.5 kHz, caused by magnetic field inhomogeneity to which 1H is more sensitive, in direct proportionality to its high gyromagnetic factor) than the corresponding 2H NMR spectrum. Hence, the NMR signal contributions from the premolten layers and molten pores cannot be easily distinguished by 1H NMR employed by Ishizaki et al.16
The NMR data show a progressively faster increase in the PML thickness as the temperature approaches the melting point but, due to the limitations discussed above, the thickness at temperatures ΔT < 1.5 K, could not be assessed. Still, this latter temperature range was accessible by TIR-Raman, yielding a thickness estimate of <3 nm. On the other hand, the NMR data presented in Fig. 5 shows that around ΔT ∼ 25 K the PML becomes less than a monolayer thick. Notwithstanding, even in that region we find that a very small fraction of water molecules exhibits some rapid re-orientational dynamics, which is the underlying reason for motionally averaging out the static quadrupole coupling and thereby obtaining a broad, yet isotropic peak. The statement “less than a monolayer thickness” implies that, in this temperature range, the premolten fraction is not any longer a continuous layer (and, thereby, may not be called PML) but a collection of individual molecules that behave more like defects in a crystalline structure.
In Fig. 6 the VSF spectra recorded in the SSP, PPP, and SPS polarization combinations for −38 °C ice and liquid water, both in contact with silica, are shown for reference. In contrast to TIR-Raman, the VSF spectra present additional bands at higher wavenumbers, demonstrating that at the interface there is a significant larger fraction of water molecules that form weaker hydrogen bonds. We stress that this, in ice, is not a signature of PML but the expected behaviour at an interface to which water cannot or has more limited options to hydrogen bond. We recall that the experimental geometry used, preferentially enhances the SF response at high wavenumbers making it easier to detect changes in the bands at high frequencies21 (Fresnel factor corrected spectra can be found in ESI7†). When compared with the VSF spectra of liquid H2O, the ice spectra at −38 °C displays lower absolute intensities, but at the same time sharper and more prominent bands at lower wavenumbers. As described in more detail below, this difference in absolute intensities is attributed to the conflicting molecular ordering imposed by the underlying ice crystal and the negatively charged silica surface.
Fig. 6 VSF spectra recorded in the SSP, PPP, and SPS polarization combinations of H2O ice at −38 °C (left) and H2O liquid at +1 °C (right). Note that the intensity in the PPP spectra has been divided by 2 for ease of comparison. The spectra fitted and corrected by the Fresnel factors can be found in ESI7.† |
The features observed at higher wavenumbers, mainly at ∼3350 cm−1, ∼3450 cm−1, ∼3550 cm−1, and ∼3630 cm−1 are in turn linked to under-coordinated water molecules present in the top half bilayer, close or in direct contact to the silica substrate. The first two resonances are near in frequency to features observed at the single crystal ice/vapour interface below 200 K,51,52 which were tentatively assigned to three-coordinated water molecules with dangling lone pairs.51 The remaining bands at the blue end of the spectra (Fig. 6), not observed at the ice/vapour interface, are in turn assigned to weakly hydrogen bonded water molecules in direct contact to the fused silica surface. The high frequency of these modes would indicate that the molecules or moieties participating in the H-bond pair do not accept themselves any additional H-bonds.54,55 Although the peak position of the highest frequency mode closely corresponds with the band assigned in TIR Raman to bulk SiOH groups (see Fig. 3), the OH feature in the SF spectra rapidly exchanges when in contact with D2O,19 which indicates that it originates from accessible surface groups. Nonetheless, SF experiments using silica surfaces with varying Si–OH group densities (to be presented elsewhere), suggest that this latter band may indeed be linked to weakly hydrogen bonded surface silanol groups.
Fig. 7 VSF spectra recorded in the SSP (left), and PPP (right) polarization combinations of H2O ice in contact with fused silica at various temperatures ranging from −38 °C and −0.5 °C. SPS spectra, as well as the Fresnel factor corrected spectra can be found in ESI5 and ESI7.† |
At temperatures exceeding −0.5 °C and in a limited temperature interval before reaching the bulk melting temperature, the VSF spectral features display a contrasting behaviour as shown in Fig. 8, where both SSP and PPP spectra are presented. First and most remarkably, the overall intensity of the SF response rapidly increases, remaining however, substantially lower than when in contact with liquid water (note the liquid spectra in Fig. 8 have been rescaled for ease of comparison). Second, a new band at ∼3400 cm−1, reminiscent of one of the dominant bands in the liquid spectra (see normalized spectra in ESI7† to better apprehend this change), appears gradually. Another detail worth noting is the appearance of a well resolved shoulder in the −0.3 °C spectra at ∼3050 cm−1, that coincides in frequency with a similar feature observed in the bulk Raman spectra of H2O ice (see Fig. 2).
Fig. 8 VSF spectra recorded in the SSP (left), and PPP (right) polarization combinations of H2O ice in contact with fused silica at various temperatures ranging from −38 °C and −0.5 °C. SPS spectra, as well as the Fresnel factor corrected spectra can be found in ESI5 and ESI7.† VSF spectra recorded in the SSP (left), and PPP (right) polarization combinations of H2O ice in contact with fused silica at various temperatures ranging from −0.5 °C to −0.1 °C. The corresponding spectra for liquid water are also added for reference. Note that the liquid water spectra, shown as dashed lines, have been divided by 4 or 2 for ease of comparison. SPS spectra, as well as the Fresnel factor corrected spectra can be found in ESI7.† |
The most important variations of the spectral features as a function of temperature are summarized in Fig. 9, where the fitted amplitudes of selected peaks at different polarization combinations are shown. All fitted parameters, including additional details of the fitting procedure can be found in the ESI11.†Fig. 9 highlights the three distinct temperature ranges with qualitatively different behaviors: stable spectra below −28 °C, and significant changes but with very different character between −25 °C and −10 °C on one hand, and less than a degree below the bulk melting temperature on the other hand.
Fig. 9 Fitted amplitudes of selected modes in the VSF spectra collected in the SSP, PPP and SPS polarization combinations. Note that the amplitudes have been normalized by the corresponding bandwidths. For ease of comparison the amplitude values for the ∼3630 cm−1 band in PPP have been multiplied by −1 as they display an opposite phase. Additional fitted parameters and details of the procedure can be found in the ESI11.† |
The first detectable changes take place in the temperature range between −25 °C and −10 °C with the marked decrease of the bands linked to 4-coordinated water molecules that are not in direct contact to silica, and which in principle connect the surface bilayers (i.e. “stitching” modes). The appearance of changes in the spectral features can be interpreted as the onset of the PML at the ice/silica interface, which we note is curiously ∼60 °C above the estimated onset temperature for the ice/vapour interface based on measurements using the same technique.14,56 A lower SF intensity implies a broadening of the orientational distribution and/or a decrease in the number of molecules having that particular configuration.10,11 Interestingly, during this first transition, the features linked to water molecules in direct contact to the silica substrate, show an opposite behavior, increasing in the PPP, and to a much lower extent, also in the SSP spectra (Fig. 7 and 9), suggesting a narrowing of their average orientation. This behavior can be considered as the first evidence of the conflicting ordering imposed by the underlying ice crystal and the silica substrate, which becomes more apparent at higher temperatures (see below). In this region, the NMR-derived PML thickness is 1–3 monolayers thick, that – in a typical liquid-like material – is sufficient for losing some spatial/orientational correlation. Hence, this is consistent with the orientational distribution inferred above.
From ∼−10 °C and up to approximately −0.5 °C the spectral features, besides some small changes in the high frequency modes, remain largely constant suggesting limited structural changes in this temperature range. On the NMR side, one can note that in the −10 °C to −1.5 °C range there are no dramatic changes in either the line width of the PML signal or in the extension of the PML.
It is only when approaching the bulk melting temperature that dramatic onset of a new behavior is observed (Fig. 8 and 9); unfortunately, we lack NMR data in this interval, but TIR Raman sets a limit of <3 nm for PML thickness at −0.3 °C. The overall increase in spectral intensity is linked to the thickening of the PML, as the negative charge of the silica substrate start imposing an ordering of the interfacial water molecules with their dipole moment oriented towards the silica surface, which is in clear contrast to the ordering enforced by the underlying ice crystal. This conflict is exemplified for example with the increased amplitude and probable change in phase of the ∼3450 cm−1 band, which would essentially indicate a flip of the average relative polar orientation14,20 of the contributing water molecules just a fraction of a degree below the bulk melting temperature (Fig. 8 and 9). Indeed, evidence of this change in orientation upon melting can be found in previous studies of the single crystal ice (basal plane)/silica interface, where a flip of 180° was concluded from examining the SF interference patterns between −20 °C ice and liquid water at a constant frequency of 3150 cm−1.14 We note however, that our fitted data from a single ice crystal but of unknown orientation, do not support such a change in phase for the ∼3125 cm−1 peak upon melting, but instead for that of the neighbouring band at ∼3250 cm−1 when going through the first transition associated to the PML onset (see Fig. 7 and ESI11†).
The significant lower intensity of the SF ice spectra at −0.1 °C compared to the water spectrum at +1 °C (Fig. 8), is an indication that the PML thickness is always smaller that the SF probing depth at the silica/water interface (i.e. just a few molecular layers). The significant increase in SF intensity upon melting is not unique to silica, as a similar behaviour has been recently reported to occur with another negatively charged surface, specifically muscovite mica.57 Finally, we note that as the PML becomes thicker it is also likely that SF signal can be generated from the retreating ice/PML interface. We speculate that this could be origin of the 3050 cm−1 feature resolved in the −0.3 °C spectra (Fig. 8). However, a more conclusive picture and interpretation of the experimental data must await new theoretical calculations and models.
Our data jointly set the onset temperature for PML to approximately −25 °C. Somewhere around this point the VSF spectral parameters start to show a temperature dependent variation and to indicate a growing disorder at the interface while the NMR-derived thickness exceeds that of a monolayer. At the highest explored temperature of 1.5 K below the melting point, the NMR-derived PML thickness is ∼1.5 nm; this is consistent with the upper limit of 3 nm at −0.3 °C determined from TIR-Raman. The information obtained by NMR, TIR Raman, and VSFS, specifically the onset temperature and thickness are also in good agreement with a previous X-ray reflectivity study on the PML properties of single crystal ice in contact with amorphous silica15 and the results of recent theoretical work.58
VSFS yielded richer information still, in the form of the onset of another trend at approximately −0.5 °C, a temperature region that could not be explored by NMR, and which was interpreted in terms of additional growth of the PML. Yet, VSFS also provided evidence that the thickness remains limited to just a few molecular layers even at −0.1 °C. Interestingly, upon formation of the PML, the SF results imply that the net average orientation of the tetrahedrally coordinated water molecules found in the second or third molecular layers from the surface (i.e. bilayer “stitching” modes), broadens, while those molecules in direct contact to the silica substrate show the opposite trend. This behaviour is explained by the conflicting order imposed by the negatively charged fused silica surface on the one side, and the underlying ice crystal on the other. As the melting temperature is approached and the interfacial ice structure relaxes, the influence of the silica surface charge arising from deprotonation of the surface silanol groups increases, aligning more water molecules in the PML, resulting in an increase of the SF spectral intensity, yet without ever reaching the values observed in the presence of liquid water.
Of course, there are also some discrepancies, like for example the apparent doubling in the NMR-derived PML thickness between −10 and −1 °C, a temperature range where limited changes are observed in VSFS. This can be attributed to the fact that the two techniques probe different aspects of the PML, but perhaps also to experimental specificities, as NMR measurements were carried out with polycrystalline D2O ice, while in VSFS they were made with single crystal H2O, though of unknown orientation.
The measured properties of the PML are clearly correlated to the macroscopic ice adhesion measurements. In particular, the transition between the sliding and the non-sliding behaviour observed at approximately −20 °C (Fig. 1). With some caution, commanded by the assumptions involved in the evaluation of the NMR data, the onset of sliding seems to coincide with having a PML that, in addition, has a thickness that exceeds the average surface roughness. In connection to this inference, we note that in rougher silica surfaces with a Ra of ∼6 nm (measured in a 2 × 2 μm area, see ESI9†), ice did not slide at any temperature, which also highlights the potential importance of the surface roughness relative the PML thickness in the sliding mechanism. Having said this, the value of the arithmetic surface roughness, Ra, will depend on the lateral resolution and extension of the probed area (see ESI8 and ESI9†). In this regard, we observed ice to slide at −10 °C on silica surfaces of intermediate roughness, with a Ra of ∼0.7 nm when measured in 2 × 2 μm area, but with a Ra of ∼22 nm when measured in a profilometer for an area of 0.33 × 0.44 mm (see ESI8 and ESI9†). This stresses the importance of the micron or submicron roughness for the PML lubricating sliding to take place.
Our adhesion measurements do not address directly ice friction, particularly not kinetic friction where, except at low sliding speeds, the effects of frictional heating seem to dominate.59 Yet, the lack of consistent information about the premolten layer seems to hinder a more complete analysis just as well as the evaluation of the performance of theoretical models. We hope that our work will provide some progress toward alleviating that shortcoming. In addition, we point out that sliding shear adhesion is, indeed, directly related to static friction.
Footnote |
† Electronic supplementary information (ESI) available: Details of the ice-adhesion measuring device, movie showing the sliding behavior of ice on silica, details of the vibrational spectroscopy sample cell, TIR Raman spectra collected for ice at different polarization combinations, Raman D2O ice spectra at different temperatures, silica/ice SF spectra collected in the SPS polarization combination, Fresnel factor corrected VSF spectra, detailed fitting parameters of the SF spectra, AFM and profilometry images and roughness measurements, and raw 2H NMR spectra of the PML. See DOI: 10.1039/c6cp05303c |
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