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  • 1.
    Liljeblad, Jonathan F. D.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    A Spectroscopic Study of Interfacial Films: Internal Structuring, Stability, and Hydration2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Monolayers of molecules are capable of completely changing the nature of surfaces and their interactions with the surroundings despite their almost negligible thickness. In the research presented in this doctoral thesis the surface specific technique Vibrational Sum Frequency Spectroscopy (VSFS) was combined with the Langmuir trough to investigate various manifestations of monolayers, such as biomimetic membranes, mono methyl branched long chain fatty acids in contact with air, and hydrophobic silane monolayers in contact with water.

    VSFS was applied to in situ studies of the oxidative degradation of Langmuir monolayers of 1,2-diacylphosphocholines with identical C-18 chains featuring various degrees of unsaturation. The monolayer area and vinyl CH signal intensity were probed at constant surface pressure to monitor the degradation. The rapid degradation of the unsaturated lipids in contact with the ambient air is attributed to oxidation of the C=C bonds mediated by reactive species in the air and can be inhibited by purging the compartment surrounding the monolayer with nitrogen.

    The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The monolayers are conformationally well ordered at all surface pressures and the signal intensity increases due to larger molecular number density at increasing surface pressures. Also, water signals with different vibrational frequencies are observed in different polarization combinations.

    Additionally, a selection of common phospholipids (18:0 PC, 18:0 PC-D83 and 18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF2 substrates and the CH- and OH-stretching regions as well as lower wavenumber regions were probed using VSFS. The orientation of the conformationally well ordered aliphatic chains was determined to be approximately perpendicular to the sample surface.

    Monolayers of eicosanoic acid, its iso (19-MEA), and anteiso (18-MEA) analogues were investigated with VSFS, AFM imaging, and the Langmuir trough. The EA forms smooth, featureless monolayers when deposited on silica, while 19-MEA and 18-MEA form 10 -50 nm large domains with homogeneous size distribution. It was not possible to discriminate between the monolayers of racemic and chiral 18-MEA using any of the techniques employed

    The influence of the experimental geometry on the SF spectral shape and en-hancement at and near total internal reflection conditions (TIR) were systematically investigated by comparing simulations with recorded data from a hydrophobized silica / water interface. The data agree qualitatively, but not quantitatively, with the simulations, and the reasons were critically discussed.

    The water structure next to ordered and disordered hydrophobic silane monolayers on silica was investigated using VSFS. The results indicate that the structure of water next to a well ordered hydrophobic monolayer, with the exception of the first layer of water molecules, is not much different from that in the isotropic bulk. This is in contrast to the previous notion where such monolayers were assumed to induce a more ordered interfacial water structure.

  • 2.
    Liljeblad, Jonathan F. D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Bulone, Vincent
    KTH, School of Biotechnology (BIO), Glycoscience.
    Tyrode, Eric C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Johnson, Magnus C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Molecular Structure and Stability of Phospholipid Monolayers Probed by Vibrational Sum Frequency Spectroscopy (VSFS)2012In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 102, no 3, p. 591A-591AArticle in journal (Other academic)
  • 3.
    Liljeblad, Jonathan F. D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    Furo, Istvan
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    Tyrode, Eric C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry.
    The premolten layer of ice next to a hydrophilic solid surface: correlating adhesion with molecular properties2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 1, p. 305-317Article in journal (Refereed)
    Abstract [en]

    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 degrees C, where a plateau marked the adhesive strength having reached the cohesive strength of ice. Interestingly, at temperatures higher than -20 degrees C the ice samples slid on smooth (R-a < 0.4 nm) silica surfaces. This sliding behavior was not observed on rougher silica surfaces (R-a similar to 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 H-2 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 degrees 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.

  • 4.
    Liljeblad, Jonathan F. D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Tyrode, Eric
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Vibrational sum frequency spectroscopy studies at solid/liquid interfaces: Influence of the experimental geometry in the spectral shape and enhancement2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 43, p. 22893-22903Article in journal (Refereed)
    Abstract [en]

    The influence of the experimental geometry, specifically the angles of incidence (AOI) of the exciting beams, on the enhancement of the vibrational sum frequency spectroscopy (VSFS) spectra has been systematically investigated, particularly when approaching total internal reflection (TIR) conditions. Theoretical simulations of the spectral intensity as a function of the AOI and infrared wavelength at three different polarization combinations were critically compared to experimental data obtained from a hydrophobized silica/water interface. The spectra were recorded using a newly established femtosecond VSFS spectrometer designed to allow rapid and accurate changes of the angles of incidence. At the most favorable AOI, when the IR, visible, and/or emitted sum frequency beams were close to TIR conditions, the spectral enhancement could be up to several orders of magnitude. The effect from the IR beam was however less pronounced due to absorption from the aqueous phase. Furthermore, the relative enhancement of the spectral features across the CH and OH stretching regions varied significantly depending on the angles of incidence chosen. The uneven enhancement is a consequence of the large dispersion in the IR refractive index of water. Although the theoretical simulations qualitatively predict this trend, the quantitative agreement with the experimental data is poor. The simulations underestimate the amplification in the free OH range by 2−4 times as the IRbeam approaches TIR, and overestimates it as the visible beam approaches the critical angle. These observations may suggest a need to review the fundamental equations describing the angular dependence of the SF intensity. The practical advantages of selecting specific angles of incidence, as well as the expected variations for two other frequently encountered solid/liquid interfaces (CaF2/water and sapphire/water), are also discussed.

  • 5.
    Liljeblad, Jonathan F. D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Tyrode, Eric
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Thormann, Esben
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. Department of Chemistry, Technical University of Denmark, Kemitorvet 207, DK-2800 Kgs. Lyngby, Denmark.
    Dublanchet, Ann-Claude
    L’Oréal Research and Innovation, Aulnay-sous-Bois, France.
    Luengo, Gustavo
    L’Oréal Research and Innovation, Aulnay-sous-Bois, France.
    Johnson, C. Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Self-assembly of long chain fatty acids: effect of a methyl branch2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 33, p. 17869-17882Article in journal (Refereed)
    Abstract [en]

    The morphology and molecular conformation of Langmuir Blodgett deposited and floating monolayers of a selection of straight chain (eicosanoic acid, EA), iso (19-methyl eicosanoic acid, 19-MEA), and anteiso (18-methyl eicosanoic acid, 18-MEA) fatty acids have been investigated by Vibrational Sum Frequency Spectroscopy (VSFS), AFM imaging, and the Langmuir trough. While the straight chain fatty acid forms smooth, featureless monolayers, all the branched chain fatty acids display 10-50 nm sized domains (larger for 19-MEA than the 18-MEA) with a homogeneous size distribution. A model is suggested to explain the domain formation and size in terms of the branched fatty acid packing properties and the formation of hemispherical caps at the liquid air interface. No difference between the chiral (S) form and the racemic mixture of the 18-MEA is observed with any of the utilized techniques. The aliphatic chains of the straight chain fatty acids appear to be oriented perpendicular to the sample surface, based on an orientational analysis of VSFS data and the odd/even effect. In addition, the selection of the subphase (neat water or CdCl2 containing water buffered to pH 6.0) used for the LB-deposition has a profound influence on the monolayer morphology, packing density, compressibility, and conformational order. Finally, the orientation of the 19-MEA dimethyl moiety is estimated, and a strategy for performing an orientational analysis to determine the complete molecular orientation of the aliphatic chains of 19-MEA and 18-MEA is outlined and discussed.

  • 6.
    Liljeblad, Jonathan F.D.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Biomimetic Membranes:: Molecular Structure and Stability Studies by Vibrational Sum Frequency Spectroscopy2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    In the research presented in this licentiate thesis the surface specific technique Vibrational Sum Frequency Spectroscopy, VSFS, combined with the Langmuir trough has been utilized to investigate Langmuir monolayers and Langmuir-Blodgett (LB) deposited mono- and bilayers of phospholipids. Their molecular structure, stability, and hydration were probed to gain additional understanding of important properties aiming at facilitating the use of such layers as model systems for biological membranes.

    VSFS was applied to in situ studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with identical C-18 chains having various degrees of unsaturation. The time-dependent change of the monolayer area at constant surface pressure as well as the sum frequency intensity of the vinyl-CH stretch at the C=C double bonds were measured to monitor the degradation. It was shown that a rapid degradation of the monolayers of unsaturated phospholipids occurred when exposed to the laboratory air compared to the fully saturated lipid, and that the degradation could be inhibited by purging the ambient air with nitrogen. The degradation was attributed to oxidation mediated by reactive species in the air.

    The molecular structure and order of Langmuir monolayers of 1,2-distearoyl-phosphocholine (18:0 PC) and their hydrating water were investigated at different surface pressures using VSFS. The spectroscopic data indicated a well ordered monolayer at all surface pressures with a more intense signal at higher pressures attributed to the subsequent increase of the number density and more ordered lipid molecules due to the tighter packing. Water molecules hydrating the headgroups or being in contact with the hydrophobic parts were observed and distinguished by their vibrational frequencies, and found to have different average orientations.

    Additionally, monolayers of 18:0 PC, its fully deuterated analogue, and 1,2-distearoyl-phosphoserine (18:0 PS) were Langmuir-Blodgett (LB) deposited on CaF2 substrates and VSFS was used to investigate the structure and order of the films as well as the hydrating water. The CH-region, water region, and lower wavenumber region containing phosphate, ester, carboxylic acid, and amine signals were probed to obtain a complete picture of the molecule. The data indicates that all deposited monolayers formed a well ordered and stable film and the average orientation of the aliphatic chains was determined using the antisymmetric methyl stretch.

  • 7.
    Liljeblad, Jonathan F.D.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface Chemistry.
    Qarternary Ammonium Surfractants Adsorbed at the Solid-Liquid Interface: A VSFS Study2007Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The aim of the research presented in this Master of Science Thesis, was to study the adsorption of quaternary ammonium surfactants at the hydrophilic solid-liquid interface with Vibrational Sum Frequency Generation Spectroscopy (SF-spectroscopy). Studies of both the H-bonded water region and the hydrocarbon region of the SF spectral window have been performed in an attempt to distinguish between a range of different proposed structures for the adsorbed aggregates.

    The adsorption of tetradecyltrimethylammonium bromide, TTAB, with and without NaCl and didodecyldimethylammonium bromide, DDAB, at the solid-liquid (S/L) interface of alumina and water was studied at concentrations above the cmc. It has earlier been indicated by AFM-imaging that those surfactant systems form spherical and cylindrical micelles and a flat bilayer on quartz at concentrations above the cmc and it is assumed that the same applies to alumina. Our results show a very weak, if any, signal in the CH-region for all three systems and may indicate a slightly more loosely ordered water structure at the S/L interface for the case of cylindrical and spherical micelles compared to the planar bilayer. This indicates that SF-spectroscopy can probably not discriminate between flat bilayers, spherical micelles and cylindrical micelles formed by adsorption of ammonium surfactants at a charged surface.

    The adsorption of TTAB at the S/L interface of alumina and water along the adsorption isotherm at high pH was also studied. The SF-spectra did not display any CH-signals of strong intensity at any concentrations. The OH-signals gradually decreased to an almost flat line, as the surfactant concentration increased. The OH-region remained flat also at surfactant concentrations above the point of zero charge and at the cmc where signal was expected. We postulate that adsorption of TTAB might not form a homogeneous layer at the interface; resulting in regions within the probed area, possessing opposite charges (negatively charged sapphire, and positively charge TTAB) aligning water molecules in opposite orientations. This could result in a total net orientation that is zero and a loss of SF signal.

    For all experiments conducted using sapphire at high pH, a strong peak at ~3700 cm-1 was continually observed, especially for the pure water samples. This peak is associated to a non hydrogen bonded OH-group, a “Free-OH” and is not expected to be present at the S/L interface of alumina and water. We have proved by isotope exchange experiments that this peak originates from the S/L interface, but can not provide any explanation for the reason behind.

  • 8.
    Liljeblad, Jonathan F.D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. KTH, School of Chemical Science and Engineering (CHE), Centres, Swedish Center for Biomimetic Fiber Engineering, BioMime.
    Bulone, Vincent
    KTH, School of Biotechnology (BIO), Glycoscience. KTH, School of Chemical Science and Engineering (CHE), Centres, Swedish Center for Biomimetic Fiber Engineering, BioMime.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. KTH, School of Chemical Science and Engineering (CHE), Centres, Swedish Center for Biomimetic Fiber Engineering, BioMime.
    Johnson, Magnus C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. KTH, School of Chemical Science and Engineering (CHE), Centres, Swedish Center for Biomimetic Fiber Engineering, BioMime.
    Supported Phospholipid Monolayers: The Molecular Structure Investigated by Vibrational Sum Frequency Spectroscopy2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 21, p. 10617-10629Article in journal (Refereed)
    Abstract [en]

    The molecular structure, packing properties, and hydrating water of Langmuir-Blodgett monolayers of the phospholipids 1,2-distearoyl-sn-glyercophosphatidylcholine (DSPC, 18:0 PC), its deuterated analogue (18:0 PC-d83), and 1,2-distearoyl-sn-glyerco-phosphatidylserine (DSPC, 18:0 PS) deposited on planar calcium fluoride (CaF2) substrates have been investigated using the surface-specific nonlinear optical technique vibrational sum frequency spectroscopy (VSFS). Compression isotherms were recorded before the deposition of the monolayers at a surface pressure of 35 mN/m, mimicking the conditions of biological cell membranes. The CH and CD stretch regions, the water region, and the lower wavenumber region, containing phosphate, ester, carboxylate, and amine signals, thus partly covering the fingerprint region, were probed to obtain a complete map of the molecules. The data indicate that all deposited monolayers formed a well-ordered and stable film, and probing the water region revealed significant differences in hydration for the different headgroups. In addition, the tilt angle of the aliphatic chains relative to the surface normal was estimated to be approximately 4 degrees to 10 degrees based on orientational analysis using the antisymmetric methyl stretching vibration. Orientational analysis of the ester C=O groups was also performed, and the result was consistent with the estimated tilt angle of the aliphatic chains.

  • 9.
    Liljeblad, Jonathan F.D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Bulone, Vincent
    KTH, School of Biotechnology (BIO), Glycoscience.
    Tyrode, Eric
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. Institute for Surface Chemistry, Stockholm, Sweden.
    Johnson, Magnus C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Phospholipid Monolayers Probed by Vibrational Sum Frequency Spectroscopy: Instability of Unsaturated Phospholipids2010In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 98, p. L50-L52Article in journal (Refereed)
    Abstract [en]

    The surface specific technique vibrational sum frequency spectroscopy has been applied to in situ studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with various degrees of unsaturation in the aliphatic chains. To monitor the degradation of the phospholipids, the time-dependent change of the monolayer area at constant surface pressure and the sum frequency intensity of the vinyl CH stretch at the carbon-carbon double bonds were measured. The data show a rapid degradation of monolayers of phospholipids carrying unsaturated aliphatic chains compared to the stable lipids carrying fully saturated chains when exposed to the ambient laboratory air. In addition, the degradation of the phospholipids can be inhibited by purging the ambient air with nitrogen. This instability may be attributed to spontaneous degradation by oxidation mediated by various reactive species in the air. To further elucidate the process of lipid oxidation in biological membranes artificial Langmuir monolayers probed by a surface specific spectroscopic technique as in this study can serve as a model system for studying the degradation/oxidation of cell membrane constituents.

  • 10.
    Liljeblad, Jonathan F.D.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Rutland, Mark W.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Bulone, Vincent
    KTH, School of Biotechnology (BIO), Glycoscience.
    Johnson, Magnus C.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    The Structure of Model Membranes Studied by Vibrational Sum Frequency Spectroscopy2010In: Progress in Colloid and Polymer Science, ISSN 0340-255X, E-ISSN 1437-8027, Vol. 137, p. 9-12Article in journal (Refereed)
    Abstract [en]

    The structure and order of insoluble Langmuir monolayers consisting of 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC or 18:0 PC) and the surrounding water molecules have been investigated by vibrational sum frequency spectroscopy (VSFS). At surface pressures of 1, 15, and 57 mN/m corresponding to molecular areas of 53, 50, and 43 Å2, respectively, the DSPC molecules formed a well ordered film. Both the VSF signal from the methyl stretching vibrations of the lipid and the surrounding water increased with enhanced surface pressure, as a result of the higher surface density and increased order of the system. Water molecules hydrating the polar parts of the headgroup and in close contact to the hydrocarbon groups of the lipid were observed in all three polarization combinations of the laser beams, and distinguished by their different vibrational frequencies.

  • 11.
    Rutland, Mark W.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science. SP Tech Res Inst, Sweden.
    Liljeblad, Jonathan D. F.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Tyrode, Eric
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Thormann, Esben
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Dublanchet, Ann-Claude
    Luengo, Gustavo
    Johnson, C. Magnus
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Long chained fatty acid monolayers: Influence of methyl branching2014In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 247, p. 61-COLL-Article in journal (Other academic)
  • 12.
    Tyrode, Eric
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Liljeblad, Jonathan F. D.
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
    Water Structure Next to Ordered and Disordered Hydrophobic Silane Monolayers: A Vibrational Sum Frequency Spectroscopy Study2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 4, p. 1780-1790Article in journal (Refereed)
    Abstract [en]

    Vibrational sum frequency spectroscopy (VSFS) has been used to study the structure of water incontact to fused silica surfaces, hydrophobically modified with series of alkyl trichlorosilanemonolayers of varying degrees of order. The interfacial molecular structural information wascomplemented using total internal reflection (TIR) Raman spectroscopy. The silane moleculesconsisted of octadecyltrichlorosilane (OTS) and its shorter chain analogue with eight carbonatoms. The VSFS spectra show a direct correlation between monolayer order and the intensity ofthe free OH mode, characteristic of straddling water molecules vibrating in direct contact to thehydrophobic layer, with a concurrent reduction of the bands associated with hydrogen bondedwater molecules. The results imply that the structure of water in the most ordered monolayers isnot much affected beyond the first layer of water molecules, with bulk isotropic propertiesbecoming apparent already at sub-nanometer distances from the surface. Contact anglemeasurements with both water and hexadecane were also performed in an effort to relatemacroscopically measureable parameters to the molecular surface structure provided by VSFSand TIR Raman. Only the receding contact angles, and in particular those with hexadecane, werefound to correlate with the monolayer order. Finally, to obtain an additional insight into the waterstructure in direct contact to an ordered hydrophobic surface, isotopic dilution experiments withD2O were also performed. They indicate that the vibrational coupling mechanism of watermolecules at the hydrophobized solid/water interface is different from what has been observed atthe liquid/vapor interface.

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