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Spectroscopic Study of Water Adsorption and Desorption on/from Oligo(ethylene glycol)-Substituted Alkanethiolate Self-Assembled Monolayers Mustafa Sayin, Alexei Nefedov, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02514 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

Spectroscopic Study of Water Adsorption and Desorption on/from Oligo(ethylene glycol)-Substituted Alkanethiolate Self-Assembled Monolayers

Mustafa Sayin1, Alexei Nefedov2* and Michael Zharnikov1* 1

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany 2

Institute of Functional Interfaces, Karlsruhe Institute of Technology (KIT),

Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Adsorption and temperature-driven desorption of water (D2O) on/from the surface of oligo(ethylene glycol) (OEG) substituted alkanethiolate (AT) self-assembled monolayers (SAMs) with a variable length of the OEG segment and variable termination (−OH or −OMe) were studied by synchrotron-based X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The characteristic spectroscopic features of D2O were monitored, with a predominant signature of amorphous ice upon adsorption at 105-125 K and subsequent heating up to desorption temperature. The adsorption occurred exclusively onto the SAM surface, with a strong distortion of the spectral pattern for the first layer (for OH-terminated surfaces) and without any indication for penetration of the D2O molecules into the hydrogel-like OEG part of the SAMs. The thickness of the ice films increased linearly with the D2O dose with the same rate for all OH-terminated SAMs and a somewhat lower rate for the CH3-terminated films. The desorption of D2O in the given quasistatic temperature increase regime occurred at ~150-155 K for all SAMs studied. Specific features of the NEXAFS spectra could be tentatively interpreted as a signature of a hydration phase, formed by temperature-driven diffusion of the adsorbed D2O molecules into the hydrogel-like OEG part of the SAMs.


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1. Introduction A variety of fundamental biological processes as well as different natural and technology relevant phenomena, such as wetting, lubrication, salvation, etc., rely on the interaction of water with organic and biological surfaces. However, whereas there is a significant empirical knowledge about the macroscopic representation of these phenomena, microscopic understanding of the underlying processes is quite limited. On the side of theory, molecular dynamics simulations and quantum mechanics calculations shed some light on these processes1-4 but the results of these calculations – even though quite useful – are frequently questionable because of the limitations regarding the size of the considered systems, uncertainty of potentials and force fields, as well as drawbacks of (mostly commercial) software and numerical codes. On the side of experiment, a variety of different tools are used, including surface force apparatus,5,6 optical methods (infrared and Raman spectroscopy, ellipsometry, etc.),7-13 classical methods of surface science such as temperature-programmed desorption (TPD) etc.,14-17 X-ray scattering,9,14,18 neutron scattering,19 X-ray spectroscopy,20-23 etc. As far as well-defined organic or biology-relevant surface is required, it is frequently mimicked by molecular self-assembly on solid support. The respective molecular films – so called self-assembled monolayers (SAMs) are usually comprised of semi-rigid molecules that are chemically anchored to a substrate and terminated by a specific tail group which defines then the chemical, physical, and biological properties of the surface.24,25 Along these lines, a variety of different SAMs, mostly alkanethiolates (ATs) on Au(111), with the −CH3, −OH, −CH3/−OH, −CF3, −C6H5, (−C=O)OCH3, −COOH, −CH3/−COOH, and pyridine tail groups were studied in context of water adsorption and desorption on the respective organic surfaces differing in their wetting properties.10-12,15-17,20,21,26 According to these studies water adsorbs stronger on a polar than on a non-polar surface, forming two-dimensional ice film in the former case and three-dimensional and drop-like clusters in the latter case. As to the structure, at a low adsorption temperature (T≈100 K) and a moderate coverage water mostly condenses in an amorphous ice phase independent of the character of the organic substrate.10,11,26 The bond pattern in this phase is however distinctly different from the bulk structure of lowdensity amorphous ice and is close to that observed in high-density amorphous ice, exhibiting a certain similarity with high-pressure crystalline phases of ice.20 With increasing temperature, the amorphous ice is believed to transform into a polycrystalline phase. The temperature of the structural transition is determined by the chemical properties of the surface, being ~110 K and 140-150 K for distinctly hydrophobic (−CH3) and hydrophilic (−OH) surfaces, respectively.11,15 The structural transition can be presumably accompanied by 3 ACS Paragon Plus Environment


changes in morphology; in particular, the topmost part of 2D clusters, typical for hydrophilic surfaces, can transform into 3D features.26

Table 1. SAM precursors of the present study and their acronyms. Chemical Formula

Abbreviation

HS-(CH2)11-OH

EG0-OH

HS-(CH2)11-(OCH2CH2)1-OH

EG1-OH


EG3-OH

HS-(CH2)11-(OCH2CH2)3-O-CH3

EG3-OMe


EG5-OH


EG6-OH

HS-(CH2)15-CH3

C16

In the present work we intended to study adsorption and desorption of water on the SAMformed surfaces which not only do possess specific wetting properties, but also exhibit characteristic biorepelling behavior. Along these lines, we addressed a series of oligo(ethylene glycol) (OEG) substituted AT SAMs with a variable length of the OEG segment (EGn-OH and EG3-OMe; see Table 1) as well as the reference systems of hydroxy-substituted and nonsubstituted ATs (EG0-OH and C16, respectively; see Table 1). The EGn-OH SAMs have a persistent packing density and wetting properties over the series, determined by the aliphatic segment and terminal −OH group, respectively, but variable adhesion properties with respect to biomolecules and small organisms, determined by the length of the OEG segment.27-30 Specifically, whereas these films are not biorepulsive at small n (n < 3), they become proteinrepelling and biofouling-resistant if their OEG part consists of more than three or four ethylene glycol units,29,30 showing a typical behavior of poly(ethylene glycol) (PEG) and OEG materials.27,31-35 Interestingly, EG3-OMe exhibits protein-repelling behavior as well, in spite of the terminal methyl group and, consequently, distinctly different wetting properties.27 The adsorption and desorption of water on/from the EGn-OH and EG3-OMe SAMs is not only interesting on its own, as physical phenomena involving specific organic surfaces, but also in context of general understanding of protein-repelling properties of OEG-bearing and 4 ACS Paragon Plus Environment

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PEG systems. Among different explanations of these properties, interaction of OEG/PEG surfaces with water is frequently considered as a key factor, relying on the hydrogel-like character of the PEG and OEG compounds.31,35-37 Accordingly, it is in principle possible that water molecules adsorb not only onto the surface of the EGn-OH and EG3-OMe SAMs but also inside their OEG part or, at least, the adsorption and desorption behavior is affected by the presence and character of this part. Note that water molecules can form both single and multiple hydrogen bonds to the OEG segments, depending on their conformation, with the energies varied from 21 to 39 kJ/mol, according to the model calculations.35 These bonds can however be only formed if water molecules penetrate into the SAM, overcoming an energy barrier associated with the primary interaction with the terminal −OH, −OMe or −CH3 groups. In their turn, whereas an OH-terminated surface allows multiple hydrogen bonds (both donor and acceptor) to the adsorbed water molecules, the ability of CH3-terminated surfaces to form hydrogen bonds is limited.15 The OMe-terminated surfaces represent presumably an intermediate case in terms of hydrogen bond formation, depending on the accessibility of the "buried" oxygen atom. So, the adsorption and desorption of water on/from the EGn-OH and EG3-OMe SAMs, in view of the specific aspects described above, are the questions which we intended to address in the present study. As the experimental tools we selected a combination of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, complementary to our TPD study of the same systems, which will be published elsewhere. Whereas XPS enables to monitor the presence and amount of the adsorbed water, NEXAFS spectroscopy can potentially provide information on the structure of the respective ice films, relying on specific absorption resonances in the spectra.20,22,38-42

2. Experimental The EGn-OH (n = 1, 3, 5, and 6) and EG3-OMe substances were purchased from ProChemia (Poland); EG0-OH and C16 substances were purchased from Sigma-Aldrich. All SAMs were prepared on Au(111) substrates which were purchased from Georg Albert PVD, Silz, Germany. The substrates were fabricated by thermal evaporation of ~30 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 9 nm titanium adhesion layer. The films were polycrystalline, with preferable (111) orientation. The SAMs were prepared according to the literature recipes,29,30,43 by immersion of freshly prepared gold substrates into a 1 mmol solutions of the 5 ACS Paragon Plus Environment


respective precursors in absolute ethanol for 24 h at room temperature. The samples were rinsed with the solvent, blown dry with argon, and either characterized immediately or put into nitrogen-filled plastic containers for transportation to the synchrotron radiation facility (see below). The quality of the SAMs was verified by contact angle goniometry, ellipsometry, synchrotron-based XPS, and NEXAFS spectroscopy. All these measurements were performed at room temperature. The adsorption and desorption of water was monitored by synchrotronbased XPS and NEXAFS spectroscopy. The respective measurements were performed at either 105-120 K (adsorption) or in the range between the adsorption and room temperature (desorption). Contact angle goniometry was performed with a custom-made, computer-controlled goniometer. Static contact angles of millipore water were measured on freshly prepared SAM samples. The measurements were carried out under ambient conditions with the needle tip in contact with the drop. At least three measurements at different locations on each sample were made. The averaged values are reported. Deviations from the average values were less than ±2°. The ellipsometry measurements were carried out with an J.A. Woollam Co. Inc., Typ M44 ellipsometer. The instrument is equipped with a xenon arc lamp as a light source with a spectrum ranging from 400 to 800 nm. The angle of incidence was set to a fixed value of 75°. The intensity of the reflected elliptical polarized light was measured with a 60 Hz rotating polarizer (analyzer). The organic film was modeled as a single Cauchy layer with n0 = 1.45 and A = 0.01 µm2 as Cauchy parameters. A single ellipsometric scan was acquired at three different points for each sample, and the average of those three values was taken to determine the layer thickness of the SAMs. For this purpose we used the modeling software (WVASE™ from J. A. Woollam Co.) assuming a single Cauchy layer. XPS and NEXAFS spectroscopy experiments were carried out at the HE-SGM beamline (bending magnet) at the synchrotron radiation facility BESSY II which is a division of the Helmholtz Zentrum Berlin (HZB).44 A moderate photon flux provided by this beamline is well suitable for such radiation-sensitive samples as OEG-substituted SAMs.45 In addition, we varied the measurement spot and minimized the spectra acquisition time to diminish the radiation load and to avoid noticeable damage of the samples.


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The acquisition of the XPS spectra was carried out in normal emission geometry with an energy resolution of ∼0.3 eV at an excitation energy of 350 eV and somewhat lower resolution at higher excitation energies. The binding energy (BE) scale of these spectra was referenced to the Au 4f7/2 emission at 84.0 eV.46 The spectra were fitted by symmetric Voigt functions and a Shirley-type or linear background. To fit the S 2p3/2,1/2 doublets we used two peaks with the same full width at half-maximum (fwhm), the standard46 spin-orbit splitting of ~1.2 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions. The effective thicknesses of the SAMs and their packing densities were calculated using standard procedures,47,48 based on the C1s/Au4f and S2p/Au4f intensity ratios, respectively. For the thickness evaluation, a standard expression for the attenuation of the photoemission signal was assumed49 and the literature values for attenuation lengths were used.50 The spectrometerspecific coefficients were determined taking the C16 SAM as a reference; the same sample served also as a reference for the packing density (4.63×1014 molecules/cm2)51. NEXAFS spectra were recorded at the carbon and oxygen K-edges in partial electron yield (PEY) mode with retarding voltages of −150 V and −350 V, respectively. As the primary Xray source, linearly polarized synchrotron light with a polarization factor of ~88% was used. The incidence angle of the X-rays was either kept at 55° ("magic angle") or, for the C K-edge, varied between the normal (90°) and grazing (20°) incidence geometry in 10-20° steps. Whereas the former configuration allows to exclude effects of molecular orientation, the angle variation enables a monitoring of these effects, relying on so-called linear dichroism in X-ray absorption.52 The energy resolution was ~0.3 eV at the C K-edge and ~0.6 eV at the O Kedge. The photon energy (PE) scale at the C K-edge was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.53 The PE scale at the O K-edge was referenced to the most prominent absorption resonance of OEG-substituted SAMs at 538.0 eV following the methodology of ref 54 relying on the electron loss spectroscopy measurements of diethyl ether55. The spectra were corrected for the PE dependence of the incident photon flux by division through the spectrum of clean gold and either reduced to the standard form (zero intensity in the pre-edge region and the unity jump in the far post-edge region) or normalized to the pre-edge intensity. The first kind of spectra is better representative of the electronic structure of the samples while the second kind contains information about the amount of the material as well.



The adsorption and desorption experiments were performed under UHV conditions with a base pressure of ~5×10-10 mbar. In order to be able to compare the XPS and NEXAFS spectroscopy data with the results obtained by complementary experimental techniques, such as temperature desorption spectroscopy, etc. (the results will be published elsewhere), D2O instead of H2O was used. Before the experiments, LN2 trap was used to decrease a H2O contamination in the chamber. The temperature of the sample was monitored by a K-type thermocouple directly attached onto the sample surface and kept at 105-125 K during the dosing. The dose was controlled manually by backfilling the analysis chamber over a leak valve and keeping the D2O pressure at a constant value of ~1.3×10-8 mbar. The dose was varied from 1 to 12 L (Langmuir) and calculated by multiplication of the D2O pressure with the dosing time. After the dosing, we waited for ~5 min for stabilization of the base pressure before starting the measurements.

3. Results and discussion 3.1. Characterization of the pristine SAMs The reliability of the results regarding the adsorption/desorption behavior of D2O on the surface of the EGn-OH and EG3-OMe SAMs relies on the quality of these monolayers. The basic parameters of these films, as determined by contact angle goniometry, ellipsometry, and XPS experiments, are compiled in Table 2. These parameters are reasonable and correlate well with the literature values for the analogous systems,27,29 which suggests that the SAMs of this study has sufficiently high quality. Additional protein adhesion experiments, performed following the methodology of ref 45 in combination with ellipsometry measurements, showed, in accordance with the expectations and literature data,27,29 protein (fibrinogen) adsorption onto the EG1-OH SAM and lack of this adsorption for the EGn-OH (n ≥3) and EG3-OMe monolayers. In addition to these numerical data, a qualitative analysis of the XPS and NEXAFS spectroscopy data was performed; the respected spectra served also as a reference data set for the D2O adsorption and desorption experiments. The Au 4f7/2, S 2p, C 1s, and O 1s XPS spectra of the SAMs studied are compiled in Figure 1. The Au 4f7/2 spectra in Figure 1a exhibit expected decrease in the signal intensity with increasing length of the OEG segment and approximately equal signal intensities for the EG3-OH and EG3-OMe monolayers. The S 2p spectra in Figure 1b are dominated by a strong S 2p3/2,1/2 doublet at 162.0 eV (S 2p3/2) characteristic of the thiolate species bound to gold.56 This observation verifies the expected 8 ACS Paragon Plus Environment

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molecular organization in the films, with the docking group bound to the substrate and the terminal −OH/−OMe groups building the SAM-ambient interface. The intensity of the S 2p signal correlates with the film thickness (see Table 2) which defines the attenuation of the respective signal. The normalized C 1s spectra in Figure 1c exhibit two component peaks at 284.7-284.8 eV and 286.6 eV, associated with the alkyl and OEG segments of the SAMs, respectively (terminal −OH group in the case of the EG0-OH film).28,45 The relative spectral weights of these peaks correspond to the film compositions as far as one takes into account the attenuation effects which are quite strong at the given kinetic energy of photoelectrons.49 The O 1s spectra in Figure 1d show only one peak at 532.85-532.9 eV corresponding to the oxygen atoms in the OEG segment and the terminal −OH group. The intensity of the O 1s signal correlates with the molecular composition. Table 2. Basic parameters of the EGn-OH, EG3-OMe and C16 SAMs. The accuracy of the packing density values is ±5%. The packing density and thickness of the C16 SAM are literature values,51 serving as references; the contact angle was measured, it is close to the literature value (110°).10,15 Monolayer

Packing density

Thickness from

Thickness from

Static water

(molecules/cm2)

XPS ( Å)

ellipsometry (Å)

contact angle

EG0-OH

4.5 × 1014

12±3

16±2

28±2°

EG1-OH

4.0 × 1014

13±3

16.5±2

33±2°

EG3-OH

4.2 × 1014

16±3

18±2

31±2°

EG3-OMe

4.3 × 1014

16±3

18±2

61±2°

EG5-OH

4.0 × 1014

21±3

23±2

34±2°

EG6-OH

4.2 × 1014

24±3

25±2

33±2°

C16

4.63 × 1014

18.9±0.2


108±2°


a

HRXPS Au 4f7/2

b

S 2p EG6-OH EG5-OH

EG0-OH

EG3-OH

EG1-OH

EG3-OMe

EG3-OMe

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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EG1-OH

EG3-OH EG5-OH EG6-OH

EG0-OH

87 86 85 84 83 82 81 168 166 164 162 160

c

C 1s

d

O 1s

EG6-OH EG6-OH

EG5-OH

EG5-OH EG3-OH

EG3-OMe

EG3-OMe

EG3-OH

EG1-OH

EG1-OH

EG0-OH

290

EG0-OH

285

536

534

532

530

Binding energy (eV)

Figure 1. Au 4f7/2 (a), S 2p (b), C 1s (c), and O 1s (d) XPS spectra of the EGn-OH (n=0,1,3,5,6) and EG3-OMe SAMs. The C 1s spectra are normalized to the maximum intensity. The Au 4f7/2 and S 2p spectra were measured at a photon energy of 350 eV; the C 1s and O 1s spectra were measured at a photon energy of 580 eV.

The C K-edge NEXAFS data for the EGn-OH and EG3-OMe SAMs are presented in Figure 2. The "magic angle" spectra in Figure 2a exhibit a variety of characteristic absorption resonances, viz. merged peaks at 287.5 eV and 288.1 eV (1) frequently associated with predominantly Rydberg states of the alkyl segments57-59 (see ref 60 for a discussion regarding alternative assignments), a resonance at 289.4 eV (2) corresponding to the 1s→σ*C-O transition and associated with the OEG segment,54 a resonance at 293.3 eV (3) containing contributions from both the alkyl and OEG segments,54 and a variety of broad, joint OEGalkyl resonances at higher photon energies. The relative spectral weights of the resonances 1 and 2 correlate with the molecular composition: whereas the intensity of the former feature decreases with increasing n, the intensity of the latter resonance increases and it becomes a dominant feature at n ≥3. As to the EG3-OH and EG3-OMe SAMs, no large difference between the spectra is observed, which is understandable in view of the same lengths of the OEG segment. 10 ACS Paragon Plus Environment

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The difference spectra of the EGn-OH and EG3-OMe SAMs in Figure 2b reflect the molecular orientation in these films,52 with the specific difference peaks associated with either alkyl or OEG segments or their joint contributions. The change in the amplitude of these peaks over the series stems predominantly from the varying contributions of the alkyl and OEG parts (the spectra were normalized to the entire carbon signal), modulated by the attenuation effects. The signs of these peaks suggest the expected upright orientation of the SAM constituents, in view of the directions of the transition dipole moments (TDMs) associated with the respective molecular orbitals. The character of the difference spectra and their variation with n correlates well with the literature data,54,60 reflecting nearly persistent orientational order within the alkyl part of the SAMs over the series as well as a presence of a certain orientational order within the OEG part. A minor deterioration of the orientational order with increasing n cannot, however, be completely excluded.54

a

2

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

NEXAFS: C K-edge 55°

b

90°-20° EG6-OH

EG6-OH

EG5-OH EG5-OH EG3-OMe

EG3-OMe

EG3-OH EG3-OH EG1-OH EG1-OH EG0-OH EG0-OH

280 290 300 310 320

280 290 300 310 320

Photon energy (eV)

Figure 2. C K-edge NEXAFS data for the EGn-OH (n=0,1,3,5,6) and EG3-OMe SAMs, including the spectra acquired at an X-ray incidence angle of 55° (a) and the difference between the spectra measured at X-ray incidence angles of 90° and 20° (b). Characteristic absorption resonances are marked by numbers (see text for details). Horizontal dashed lines in panel (b) correspond to zero. The O K-edge "magic angle" NEXAFS spectra of the EGn-OH and EG3-OMe SAMs are presented in Figure 3. These spectra are dominated by a rather broad resonance at 538.0 eV which is assigned to the 1s→σ*C-O/Rydberg transitions and is associated with the −OH 11 ACS Paragon Plus Environment


terminal group and the OEG segment of the films.54 The relative intensity of this resonance increases with increasing n and the resonance exhibits a complex shape at large n, suggesting possible "fine structure" behind the joint envelope. NEXAFS: O K-edge

55°

EG6-OH


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

EG5-OH EG3-OH EG3-OMe EG1-OH EG0-OH

520

530

540

550

560

Photon energy (eV)

Figure 3. O K-edge NEXAFS spectra of the EGn-OH (n=0,1,3,5,6) and EG3-OMe SAMs acquired at an X-ray incidence angle of 55°. The extent of linear dichroism is very small at this edge;54 consequently, we only present the 55° spectra.

3.2. D2O adsorption The Au 4f7/2 and O 1s XPS spectra taken in the course of the D2O adsorption onto the EG6OH SAM are shown in Figure 4, representative of the entire series. The Au 4f signal in Figure 4a decreases in intensity with increasing dose because of progressive attenuation by the growing ice film. The S 2p and C 1s spectra (not shown) exhibit similar behavior, showing, at the same time, persistent spectral shape, which suggests that the adsorption of D2O does not trigger any chemical processes or bond cleavage in the SAMs. The O 1s spectra in Figure 4b show a progressive increase in the intensity of the O 1s peak in the course of the D2O adsorption. The BE position of this peak changes in a step-like fashion from ~533 eV to ~533.6 eV upon 1 L D2O exposure and then shifts progressively to a higher BE in the course of further D2O adsorption. The step-like change reflects the formation of the ice film, with the O 1s BE distinctly different from that of the EGn-OH SAMs, in accordance with the literature data,21,42,61 whereas the subsequent progressive shift stems most likely from a partial charging of the thick ice films during the XPS measurements. Interestingly, the O 1s peak corresponding to 1 L D2O dose is almost symmetric, with only a small contribution of the EG6-OH signal at the low BE side, as shown by detailed spectra decomposition (not shown). Such a small contribution is a consequence of the extremely strong attenuation of the O 1s 12 ACS Paragon Plus Environment

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signal at the given kinetic energy of the photoelectrons (~40 eV).49 At the same time, it indicates the expected 2D character of the ice film, covering the entire surface of the EG6-OH substrate.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

XPS: Au 4f7/2

EG6-OH

b

O 1s

T = 104 K

12L 0L 1L 2L 4L 6L 8L 12L

86

85

84

83

82

8L 6L 4L 2L 1L 0L

536

534

532

Binding energy (eV)

Figure 4. Evolution of the Au 4f7/2 (a) and O 1s (b) XPS spectra of the EG6-OH SAM in the course of the D2O adsorption. The spectra were measured at a photon energy of 580 eV. The doses are marked at the respective spectra. The dashed straight lines are guides to the eyes.

The intensity of the Au 4f7/2 signal decreases in a similar, exponential-like fashion with increasing D2O dose for all EGn-OH SAMs studied. This is illustrated in Figure 5a where this parameter, normalized to the value for the pristine monolayers, is plotted as a function of the D2O dose. The difference between the curves for different n is only minor, suggesting that the growth of the ice film occurs similarly on all EGn-OH substrates and is, thus, entirely determined by the hydrophilic character of these surfaces without any visible effect of the OEG segment length. Based on the Au 4f7/2 intensity data and taking the intensities for the bare SAM substrates (dose = 0) as the initial values, the effective thickness of the ice films was evaluated. Upon this evaluation, we used a standard expression for the signal attenuation in photoemission (exponential dependence on the thickness of the overlayer),49 the spectrometer-specific coefficients determined within the measurements on the pristine SAMs (see section 2), and the literature value of the attenuation length in dense organic films for the given kinetic energy of the photoelectrons (15.95 Å)50. The respective results are presented in Figure 5b, along with the analogous data for the EG3-OMe and C16 SAMs. According to these data, the thickness of the ice film increased linearly and similarly with the D2O dose for all EGn-OH SAMs studied. This is one more evidence that the growth of the ice films on these organic surfaces is entirely determined by their hydrophilic character (i.e. by the 13 ACS Paragon Plus Environment


terminal −OH group) without any visible effect of the OEG segment length. As to the absolute values, the derived effective thicknesses correlate well with a reasonable assumption that a D2O dose of 1 L results in the formation of 1 ML of ice. Indeed, the sticking coefficient of D2O on OH-terminated surfaces was estimated at 115 and an estimated thickness of 1 monolayer of ice is believed to be 3.66 Å,15 based on half the value of the c0 lattice constant of hexagonal ice.1 The latter value compares well with the slope of the thickness-vs-dose straight line for the EGn-OH SAMs in Figure 5b, viz. 3.33 Å/L. The difference can be related to the ambiguity in the selection of the attenuation length which was not exactly known and

Norm. Au 4f intensity (a. u.)

could be slightly underestimated.62 1.0

EG6-OH EG5-OH EG3-OH EG1-OH EG0-OH

0.8 0.6

a

0.4 0.2 0.0 C16 EG3-OMe EG6-OH EG5-OH EG3-OH EG1-OH EG0-OH

50 40

Thickness (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

30 20

b

10 0 0

2

4

6

8

10

12

Dose (L)

Figure 5. (a) Normalized intensity of the Au 4f7/2 signal in the course of the D2O adsorption onto the EGn-OH SAMs (n=0,1,3,5,6) and (b) the derived thicknesses of the D2O films as functions of the D2O dose, including the values for the C16 and EG3-OMe SAMs. The colorcoded, exponential-like solid lines in (a) and the dashed straight lines in (b) are guides to the eyes. The legends are given.

Also for the EG3-OMe and reference C16 SAMs, the thickness of the ice film increased linearly with the D2O dose, but the slope of the thickness-vs-dose straight line for both these systems is smaller as compared to that for the EGn-OH case. This can be explained by a lower sticking coefficient for the CH3-terminated surfaces15 but can also be a consequence of the proposed 3D cluster morphology for such substrates, as suggested by IR spectroscopy and TPD studies of non-substituted AT SAMs.10,15 The fact that the EG3-OMe film does not behave as the EG3-OH monolayer but rather as the C16 SAM is particular interesting. On the 14 ACS Paragon Plus Environment

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one hand, the EG3-OMe film is more hydrophobic than the EG3-OH SAM (and the other EGn-OH monolayers as well) but, on the other hand, it is less hydrophobic than the C16 SAM (see Table 2). The intermediate wetting properties stem from a combined effect of the terminal methyl group and the "buried" oxygen atom and the OEG segment. It seems, however, that the former group played a major role regarding the adsorption of D2O and the growth mode of the ice film under the conditions of our experiments. A complementary information is provided by the NEXAFS spectroscopy. The evolution of the O K-edge NEXAFS spectra of the EG6-OH and reference C16 SAMs in the course of D2O deposition is shown in Figure 6. There is a noticeable increase in the NEXAFS signal intensity upon the growth of the ice film (a non-linear character of this increase does not reflect a quasi-linear growth of the ice film)63. For both systems, the spectra of the thick D2O films look similar and exhibit characteristic resonances of ice at 534.3 eV (1; "pre-edge"), 536.6 eV (2; "main edge"), and 540.5 eV (3; "post-edge");22,38-42 see ref 39 for the detailed assignments. Generally, the relative intensities of these resonances are considered as representative of coordination pattern in liquid water and ice. In particular, high intensity of the pre-edge and edge resonances is rated as a fingerprint of broken and strongly distorted hydrogen bonds (HBs) while high intensity of the post-edge resonance is regarded as a signature of intact HB coordination and the degree of crystallinity,40 even though there are noticeable differences for different types of the ice structure.22 In the present case, the overall spectral shape and the relative intensities of the characteristic resonances for the thick ice films are typical of amorphous ice, in view of a moderate intensity of the post-edge resonance, dip in the intensity at ~548 eV, and lack of characteristic fine structure in the 545-565 eV range (4).22,38,39 The spectral shape of the thin (1 L D2O) ice films on the EG6-OH substrate (and on all other OH-terminated SAMs of the present study) is however distinctly different, exhibiting a comparably high intensity of the edge resonance (as compared to the post-edge one), which, as shown by the weighted reproduction of this spectrum (see Figure S1a in the Supplementary Information), cannot be entirely explained by the contribution of the OEG part. Consequently, the first ice layer on the OH-terminated substrates is heavily distorted ("liquid-like") in terms of intact HB coordination, which is understandable in view of its 2D character and strong involvement of the D2O molecules to the bonding to the substrate. This structure transforms into amorphous ice upon the growth of the ice film, as show the spectra in Figure 6a. Interestingly, in contrast to EG6-OH, the spectrum of the thin (1 L D2O) ice films on the C16 substrate looks as that of the amorphous ice (see Figure S1b in the



Supplementary Information), which can be explained by the 3D character of the respective ice film, so that the bulk-like structure can form from the very beginning. 3

a

2

0L 1L 4L 8L

EG6-OH T = 115 K

4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

b

C16 T = 125 K

520

530

1L 2L 4L 8L

540

550

560

Photon energy (eV)

Figure 6. Evolution of the O K-edge NEXAFS spectra of the EG6-OH (a) and C16 (b) SAMs in the course of D2O deposition. The spectra were measured at an X-ray incidence angle of 55°; they are normalized to the pre-edge intensity to reflect an increase of the signal from the growing ice film. The curves are color-coded according to the dose; the legends are given. The adsorption temperature is given in the panels.

It is interesting to compare the NEXAFS spectra of the ice films for the different SAMs. Such a comparison is shown in Figure 7 for the 4 L and 8 L D2O doses. For the 8 L ice films in Figure 7b no difference between the spectra associated with the individual SAM substrates is observed, which suggests that the effect of the substrate (in terms of crystallographic structure), including the EG3-OMe SAM, is mostly neglected as far as the film is thick enough. In contrast, a certain systematic difference, small but beyond the error bar of the spectra, is observed for the 4 L films in Figure 7a, which is additionally highlighted by a magnified part of the spectra. This difference is indicative of a slightly more crystalline character of the ice films on the EGn-OH substrates with small n in contrast to those with larger n.


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a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EG6-OH EG5-OH EG3-OH EG3-OMe EG1-OH EG0-OH

55° 4 L D2 O T = 115-125 K

b

NEXAFS: O K-edge

8 L D2 O T = 115-125 K 520

530

540

550

560

570

Photon energy (eV)

Figure 7. O K-edge NEXAFS spectra of the ice (D2O) films deposited onto the EGn-OH (n=0,1,3,5,6) and EG3-OMe SAMs with a dose of 4 L (a) and 8 L (b). The spectra were measured at an X-ray incidence angle of 55°. Individual spectra are color-coded; the legend is given. The adsorption temperature is given as well. Panel (a) also contains a magnified part of these spectra (marked by the dashed line box) emphasizing difference between the different SAMs.

A further interesting question is whether the adsorption of D2O affects the orientational order in the EGn-OH and EG3-OMe SAMs. In view of the limited linear dichroism for the EG3OH, EG3-OMe, and EG6-OH SAMs (see Figure 2b) and a strong attenuation of the NEXAFS signal by the adsorbed D2O film, we took the EG1-OH monolayer as a test system to answer the question of possible D2O-driven disordering. The respective difference spectra, reflecting the molecular orientation and calculated in the same fashion as those in Figure 2b, are presented in Figure 8 for the pristine and D2O-covered EG1-OH SAMs. Even though the signal-to-noise ratio of these spectra deteriorates with the D2O dose, because of the signal attenuation, the spectra in Figure 8 exhibit no noticeable variation in the sign and intensity of the difference peaks associated with the characteristic resonances of the EG1-OH monolayer (see Figure 2a and the respective discussion). This means that the adsorption of D2O and the respective formation of the ice film on the surface of the EG1-OH SAM do not induce any reorientation or orientational disordering in this monolayer. This conclusion agrees well with the literature IR spectroscopy data for OH-terminated and non-substituted (i.e. CH317 ACS Paragon Plus Environment


terminated) AT SAMs.10,11,15 In these publications, it was concluded that the D2O molecules do not penetrate into the SAMs upon their adsorption at a low temperature and under UHV conditions. This is obviously the case for the EG1-OH SAM and, presumably, for all other EGn-OH monolayers of the present study as well.

NEXAFS: C K-edge

EG1-OH

90°-20°


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4L

1L

0L

280

290

300

310

320

Photon energy (eV)

Figure 8. C K-edge NEXAFS difference (90°-20°) spectra of the EG1-OH SAM in the pristine state and after D2O exposure with doses of 1 and 4 L. The spectrum for 4 L is slightly distorted because of the attenuation of the SAM signal by the ice film and changes in the baseline. The doses are given at the respective spectra. Horizontal dashed lines correspond to zero.

3.3. D2O desorption In contrast to the adsorption case, the desorption experiments were only systematically performed for the EG3-OH, EG6-OH, EG3-OMe, and C16 SAMs. The temperature was varied from the adsorption to room temperature in variable steps and the measurements were carried out after the temperature stabilization following a particular step. Consequently, the spectra were acquired under quasi-stationary conditions but certain kinetic effects, in the temperature range where the desorption was most extensive, cannot be completely excluded. The temperature-driven desorption of D2O adsorbed on the EG6-OH substrate (representative of the other SAMs as well), as monitored by XPS, is illustrated in Figure 9. The Au 4f spectra in Figure 9a exhibit no noticeable intensity change in a temperature range of 115-150 K, a significant increase in intensity at T=155 K, and an even higher and constant intensity at T = 160-300 K. The O 1s spectra in Figure 9b show an expected, inverse behavior, with a high and constant intensity at T = 115-150 K, intensity drop at 155 K, and a low and constant 18 ACS Paragon Plus Environment

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intensity at 160-300 K. The above behavior is additionally clarified in Figure 10, where, for the EG3-OH, EG6-OH, EG3-OMe, and C16 SAMs, the evolution of the Au 4f7/2 and O 1s intensities in the course of the temperature-driven desorption of D2O is shown. The curves for these signals in Figures 10a and 10b, respectively, exhibit inverse behavior, corresponding to the desorption of D2O from the sample surface. The desorption temperature, measured here with a limited precision, in view of the comparably large temperature step, is ~155 K for all SAMs studied. This value correlates coarsely with the peak temperatures in the TPD experiments on the OH-substituted and non-substituted AT SAMs on Au(111),15 underlining its reliability, even though a precise comparison of the desorption temperature for the quasistationary temperature variation in our experiments and a continuous temperature ramp in the TPD case is not exactly strict.

a

XPS: Au 4f7/2 Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

O 1s

EG6-OH

T = 120 K 8 L D2O 115K 140K 150K

300K 200K 170K 160K

155K 160K

155K

170K

150K 140K 115K

86

200K 300K

85

84

83

82

532

534

536

Binding energy (eV)

Figure 9. Evolution of the Au 4f7/2 (a) and O 1s (b) XPS spectra of the EG6-OH SAM exposed to D2O (8 L) at 120 K in the course of the temperature-driven desorption of D2O. The temperature is marked at the respective spectra. Note that the spectra in a and b are presented in different order.

Complementary information is provided by NEXAFS spectroscopy. The temperature-driven desorption of D2O adsorbed on the EG6-OH substrate (representative of the other SAMs as well), as monitored by NEXAFS spectroscopy, is illustrated in Figure 11. Similar to the XPS case (Figure 9), the spectra do not change much in a temperature range 115-150 K, exhibiting a persistent pattern characteristic of the amorphous ice, and then change abruptly at 155 K, showing the characteristic pattern of the EG6-OH SAM, which persists at higher temperatures.


a

EG6-OH EG3-OH EG3-OMe

O 1s intensity (arb. units)

C16

b EG6-OH

EG3-OH EG3-OMe

C16 150

200

250

300

Temperature (K)

Figure 10. Evolution of the Au 4f7/2 (a) and O 1s (b) intensity for the EG3-OH, EG6-OH, EG3-OMe, and C16 SAMs exposed to D2O (8 L) at 115-125 K in the course of the temperature-driven D2O desorption. The type of SAM is marked at the respective curve.

8 L D2O

EG6-OH

300 K


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Au 4f intensity (arb. units)


200 K 170 K 160 K 155 K 150 K 140 K 115 K 520 530 540 550 560

Photon energy (eV)

Figure 11. Evolution of the "magic angle" O K-edge NEXAFS spectra of the EG6-OH SAMs exposed to D2O (8 L) at 115 K in the course of the temperature-driven D2O desorption. The spectra were normalized in the standard fashion. The vertical dashed line marks the resonance at ~531.3 eV (see text for details).

This development is additionally illustrated in Figure 12a, where the data for the EG6-OH SAM are presented in a different fashion, with the spectra normalized to the pre-edge 20 ACS Paragon Plus Environment

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intensity. In this representation, temperature-driven desorption of ice at T = 150-155 K becomes even more obvious. Similar to the XPS case (Figure 10), we plotted the intensity of the most prominent "post-edge" resonance of ice as a function of temperature for both EG6OH SAM and EG3-OH, EG3-OMe, and C16 monolayers in Figure 12b. The resulting curves mimic those for the O 1s XPS signal (Figure 10b), exhibiting an abrupt drop in the intensity of the NEXAFS signal associated with the ice film at 150-155 K for all four SAMs studied.


a

115K 140K 150K 155K 160K 170K 200K 300K

EG6-OH

8 L D2O

520

NEXAFS intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


530

540

550

560

Photon energy (eV)

b EG3-OH EG6-OH EG3-OMe

C16 100

150

200

250

300

Temperature (K)

Figure 12. (a) Evolution of the "magic angle" O K-edge NEXAFS spectra of the EG6-OH SAMs exposed to D2O (8 L) at 115-125 K in the course of the temperature-driven D2O desorption. The spectra were normalized to the pre-edge intensity. (b) Evolution of the normalized intensity of the "magic angle" O K-edge NEXAFS spectra of the EG3-OH, EG6OH, EG3-OMe, EG6-OMe, and C16 SAMs exposed to D2O (8 L) at 110-125 K in the course of the temperature-driven desorption of D2O. The intensity was measured at the position of the post-edge resonance (3; see Figure 6a). The type of SAM is marked at the respective curve.

Apart from this general behavior, two important points should be discussed going back to the spectra presented in Figure 11. First, no transformation to a crystalline-like ice, as claimed on the basis of the IR spectroscopy data,11,15 could be recorded for the EG6-OH SAM at going 21 ACS Paragon Plus Environment


from 115 K to 150 K - the spectra remain nearly identical in this temperature range and do not develop any features typical of crystalline-like ice. We cannot however exclude completely that such a transformation occurs simultaneously with the ice film desorption, slightly above 150 K. This will not disagree with the IR spectroscopy data which suggest that the transition temperature for OH-terminated surfaces lies in the 140-155 K range, i.e. can be well above 150 K.11 The fine structure in the 545-565 eV range, associated with the appearance of crystalline-like ice (see discussion of Figure 6), could, in our case, only be observed for the C16 SAM (see Figure S2 in the Supporting Information) and, to a lesser extent, for the EG3OMe monolayer, in the spectra recorded at 150-160 K. This observation, in contrast to the EG6-OH film (Figure 12a), can be explained by a lower transition temperature for the CH3terminated surfaces as compared to OH-terminated ones.11,15 Summarizing, our NEXAFS data do not contradict the statement made by Engquist et al11,15 regarding the structural transition (amorphous-to-crystalline) upon the heating of the adsorbed ice film, but give higher values for the temperature of this transition which, in the case OH-terminated surfaces, can occur close to or even simultaneous with the outbreak of desorption. The second point to discuss is the appearance of a small but distinct absorption resonance at ~531.3 eV in the temperature range of 155-200 K (Figure 11). Similar features, at slightly different energies (e.g. at ~532.5 eV) have previously been observed in the spectra of ice films and interpreted as contamination or a peak related to an additional adsorbate.38,42 Important circumstances in our case are, however, the facts that (i) this resonance was not observed in the spectra of the pristine SAMs (Figure 3; except for EG0-OH), (ii) this resonance is only observed in a limited temperature range, following the desorption of the ice film, and (iii) this resonance is well-pronounced for the EG6-OH SAM, less intense but perceptible for the EG3-OH and EG3-OMe films, and not present for the C16 monolayer. These facts allow us to speculate that this resonance can be probably associated with the D2O molecules bound within the OEG part of the EGn-OH and EG3-OMe SAMs (hydration phase). Depending on the exact bonding configuration within the OEG matrix, these molecules can have a noticeably higher bonding energy as compared to these in the deposited ice film (wetting phase).35,37 Since all EGn-OH SAMs exhibit similar properties with respect to the D2O adsorption, independent of the length of the OEG segment (see Section 3.2), a direct adsorption into the hydration phase is hardly possible, presumably due to an energetic barrier provided by the SAM-vacuum interface. However, a diffusion from the wetting to the hydration phase should be in principle possible, especially at an elevated temperature, in the course of temperature-driven desorption experiments. 22 ACS Paragon Plus Environment

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4. Conclusions Using a combination of synchrotron-based XPS and NEXAFS spectroscopy we studied adsorption and desorption of water (D2O) onto a series of the OEG-substituted AT SAMs with either −OH or −OMe termination. These monolayers are of particular interests because of the persistent wetting properties but variable biorepulsion ability, determined by the length of the OEG segment. The latter parameter was varied in the present work for the OHterminated SAMs and kept constant for the OMe-terminated monolayers, selecting a particular system for comparison to the −OH case. The quality of the SAMs was verified, providing a reliable basis for the adsorption and desorption experiments. The adsorption of D2O occurred under UHV conditions from the gas phase at a substrate temperature of 105-125 K. The adsorption was found to occur exclusively onto the SAM surface, without any indication for the penetration of the D2O molecules into the hydrogel-like OEG part of the monolayers. The thickness of the ice films increased linearly with the D2O dose with the same rate for all OH-terminated SAMs and a somewhat lower rate for the CH3-terminated monolayers, associated with a lower sticking ability of the latter surfaces and formation of 3D clusters, in contrast to 2D ones in the −OH case. The structure of the D2O ice films was found to be amorphous-like for all SAM-substrates studied, with only small differences (degree of crystallinity) for the different lengths of the OEG segment. At the same time, the first D2O layer on the OH-terminated SAMs exhibited characteristic signatures of strongly distorted and broken hydrogen bonds, associated with the formation of 2D clusters and involvement of most molecules into the bonding to the substrate. The temperature-driven desorption experiments, performed for some selected SAMs only, showed an abrupt change in the intensity and character of the XPS and NEXAFS spectra upon the desorption. No indication of the structural transition to crystalline-like ice was observed for the OH-terminated SAMs in the vicinity of the desorption temperature (150-155 K), but the possibility of such of a transition cannot be completely excluded. For the CH3-terminated SAMs, characteristic signatures of the structural transition were recorded. Other specific features observed in the NEXAFS spectra could be tentatively interpreted as stemming from temperature-driven diffusion of the adsorbed D2O molecules into the hydrogel-like OEG part of the OEG-substituted SAMs and building of a hydration phase. The reliability of this interpretation should be verified in further experiments, performed on a broader set of SAMs and involving a finer variation of temperature.



■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Additional spectroscopic data (PDF).

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (A.N.). *E-mail: [email protected] (M.Z.). ORCID A. Nefedov: 0000-0003-2771-6386 M. Zharnikov: 0000-0002-3708-7571 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We thank Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II and the BESSY II staff for support during the experiments. This work was supported by the German Research Society (DFG; grants ZH 63/21-1 and NE 984/2-1). A.N. additionally acknowledges funding from the “Science and Technology of Nanosystems” program.


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