Molecular Structure of an Alkyl-Side-Chain Polymer−Water Interface:  Origins of Contact Angle Hysteresis

Rangwalla, Hasnain; Schwab, Alexander D.; Yurdumakan, Betül; Yablon, Dalia G.; Yeganeh, M. S.; Dhinojwala, Ali

doi:10.1021/la049073c

Cited by 71 publications

(117 citation statements)

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“…It is more likely that the rearrangement of PVNODC hydrophobic side chains at the grain boundary may be responsible for the interstitial water, resulting in the lowered W a . 31 However, the differences in adhesion energy between the JKR and calculated by the Young−Dupréresults are still not understood.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…The JKR results for PDMS−PVNODC are only similar to PDMS− PDMS and PDMS−OTS under dry conditions. 31,38 However, the W a for PDMS−PVNODC in water is much lower than that for PDMS−PDMS and PDMS−OTS contact. A similar conclusion is reached using the results from zero load measurements, where some PDMS lenses floated atop the PVNODC substrate, while others adhered poorly.…”

Section: ■ Experimental Sectionmentioning

confidence: 99%

“…29−32 When in contact with water, the surface composition of PVNODC changes to expose hydrophobic methylene groups at the crystalline grain boundaries in addition to the ordered terminal methyl groups in the crystalline regions. 31 The Johnson−Kendall−Roberts (JKR) approach was used to measure the adhesion energy between a deformable hydrophobic poly(dimethylsiloxane) (PDMS) lens in contact with OTS and PVNODC in dry and wet conditions. In dry conditions, both surfaces exhibited similar adhesion energies using the JKR approach and the Young−Dupréequation.…”

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confidence: 99%

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Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

et al. 2015

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The contact of two hydrophobic surfaces in water is of importance in biology, catalysis, material science, and geology. A tenet of hydrophobic attraction is the release of an ordered water layer, leading to a dry contact between two hydrophobic surfaces. Although the waterfree contact has been inferred from numerous experimental and theoretical studies, this has not been directly measured. Here, we use surface sensitive sum frequency generation spectroscopy to directly probe the contact interface between hydrophobic poly-(dimethylsiloxane) (PDMS) and two hydrophobic surfaces (a selfassembled monolayer, OTS, and a polymer coating, PVNODC). We show that the interfacial structures for OTS and PVNODC are identical in dry contact but that they differ dramatically in wet contact. In water, the PVNODC surface partially rearranges at grain boundaries, trapping water at the contact interface leading to a 50% reduction in adhesion energy compared to OTS−PDMS contact. The Young−Dupréequation, used extensively to calculate the thermodynamic work of adhesion, predicts no differences between the adhesion energy for these two hydrophobic surfaces, indicating a failure of this well-known equation when there is a heterogeneous contact. This study exemplifies the importance of interstitial water in controlling adhesion and wetting. ■ INTRODUCTIONHydrophobic interactions are used to explain many phenomena prevalent in physical and biological sciences, such as protein folding, 1 self-assembly, 2−6 dewetting, 7 adhesion, 8 friction, 9 adsorption, 10 water transport, 11,12 and chemical reactions. 13 The hydrophobic adhesion is defined as the difference in interfacial energy between two hydrophobic surfaces before and after contact underwater. 14 Experimentally, direct force measurements 15−17 or contact angle measurements 18 have been used to measure adhesion energy where the contact between two hydrophobic surfaces is assumed to be dry. This drying phenomenon has been supported by molecular simulations between hydrophobic surfaces 6,19,20 but never experimentally verified.Recent findings are challenging the concept of dry hydrophobic contact. X-ray crystallography has observed the presence of water within protein cavities of varying hydrophobicity which can affect the strength of protein−ligand binding. 21,22 Simulations have also shown that water can be sequestered between hydrophobic plates with a relatively small centralized hydrophilic patch. 23 Ambiguity also remains as to how dry contact is established underwater. The entropy gained by releasing interstitial water between hydrophobic surfaces prior to contact could be facilitated by a depleted density profile at the hydrophobic water interface, 24,25 the presence of nanobubbles, 26 or the concept of increased fluctuations in interfacial water. 27 To understand the role of water in adhesion and contact angles, we have used surface sensitive sum frequency generation spectroscopy (SFG) to directly study the contact interface between two hydrophobic surfaces underwater....

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Experimental Sectionmentioning

confidence: 99%

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Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

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“…However, the formulas used for interpreting the results of this work are outlined here. 39 The SFG signal intensity in SSP and SPS polarization I(ω 3 = ω 1 +ω 2 ) depends on the electric-field components of the probing visible beam E j (ω 1 ) and IR beam E k (ω 2 ) as follows: 40…”

Section: ■ Experimental Sectionmentioning

confidence: 99%

Molecular Structure of Poly(methyl methacrylate) Surface. I. Combination of Interface-Sensitive Infrared–Visible Sum Frequency Generation, Molecular Dynamics Simulations, and ab Initio Calculations

et al. 2014

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The chemical composition and molecular structure of polymeric surfaces are important in understanding wetting, adhesion, and friction. Here, we combine interfacesensitive sum frequency generation spectroscopy (SFG), allatom molecular dynamics (MD) simulations, and ab initio calculations to understand the composition and the orientation of chemical groups on poly(methyl methacrylate) (PMMA) surface as a function of tacticity and temperature. The SFG spectral features for isotactic and syndiotactic PMMA surfaces are similar, and the dominant peak in the spectra corresponds to the ester-methyl groups. The SFG spectra for solid and melt states are very similar for both syndiotactic and isotactic PMMA. In comparison, the MD simulation results show that both the ester-methyl and the α-methyl groups of syndiotactic-PMMA are ordered and tilted toward the surface normal. For the isotactic-PMMA, the α-methyl groups are less ordered compared to their ester-methyl groups. The backbone methylene groups have a broad angular distribution and are disordered, independent of tacticity and temperature. We have compared the SFG results with theoretical spectra calculated using MD simulations and ab initio calculations. Our analysis shows that the weaker intensity of α-methyl groups in SFG spectra is due to a combination of smaller molecular hyperpolarizability, lower ordering, and lower surface number density. This work highlights the importance of combining SFG spectroscopy with MD simulations and ab initio calculations in understanding polymer surfaces. ■ INTRODUCTIONOne of the simplest and still commonly used technique to characterize surfaces of polymers is the measurement of water contact angle. Although this is a very simple technique to judge the wettability of surfaces, it lacks molecular level sensitivity to identify the chemical composition and molecular structure of surfaces. In addition, the rearrangement of molecules at the surface after exposure to water complicates the interpretation of contact angle results. 1 X-ray photoelectron spectroscopy (XPS), a vacuum-based technique, provides excellent information regarding the elemental composition present at the surfaces. However, to identify different hydrocarbon groups (or bondings) is not trivial and relies on subtle shifts of carbon binding energies in the high-resolution XPS analysis. 2 Attenuated-total-reflectance infrared can be used to pick up molecular fingerprints, but this technique samples a much thicker layer than the surface region that affects wetting, surface energy, and friction. 3 In the past two decades, surface-sensitive infrared−visible sum frequency generation spectroscopy (SFG) has been used to study polymer surfaces, 4−7 polymer/liquid interfaces, 7−11 polymer/metal or metal oxide interfaces, 5,12−15 and polymer/polymer interfaces. 12,16−19 SFG is a second-order nonlinear optical technique, and under electric dipole approximation, the SFG signal is only generated by the ordered molecules at surfaces or interfaces and the contribution from ...

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“…1͒. However, for other organic thin films such as a polymer thin film deposited on a substrate surface, [19][20][21][22][23][24][25][26][27][28] the buried interface may also contribute to the resonant SFG signals. The quantitative treatment of such a thin film is practically important and the model needs to be extended to these more general cases.…”

Section: B Organic Thin Film/gold Substratementioning

confidence: 99%

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems

Tong

Zhao

et al. 2010

The Journal of Chemical Physics

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In this paper, the results of the modeling calculations carried out for predicting the interference effects expected in the sum frequency generation ͑SFG͒ spectra of a specific thin-layer system, described in the accompanying paper, are tested by comparing them with the experimental spectra obtained for a real thin-layer film comprising an organic monolayer/variable thickness dielectric layer/gold substrate. In this system, two contributions to the SFG spectra arise, a resonant contribution from the organic film and a nonresonant contribution from the gold substrate. The modeling calculations are in excellent agreement with the experimental spectra over a wide range of thicknesses and for different polarization combinations. The introduction of another resonant monolayer adjacent to the gold substrate and with the molecules having a reverse orientation has a significant affect on the spectral shapes which is predicted. If a dielectric substrate such as CaF 2 is used instead of a gold substrate, only the spectral intensities vary with the film thickness but not the spectral shapes. The counterpropagating beam geometry will change both the thickness dependent spectral shapes and the intensity of different vibrational modes in comparison with a copropagating geometry. The influences of these experimental factors, i.e., the molecular orientational structure in the thin film, the nature of the substrate, and the selected incident beam geometry, on the experimental SFG spectra are quantitatively predicted by the calculations. The thickness effects on the signals from a SFG active monolayer contained in a thin liquid-layer cell of the type frequently used for in situ electrochemical measurements is also discussed. The modeling calculation is also valid for application to other thin-film systems comprising more than two resonant SFG active interfaces by appropriate choice of optical geometries and relevant optical properties.

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Molecular Structure of an Alkyl-Side-Chain Polymer−Water Interface: Origins of Contact Angle Hysteresis

Cited by 71 publications

References 48 publications

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting

Molecular Structure of Poly(methyl methacrylate) Surface. I. Combination of Interface-Sensitive Infrared–Visible Sum Frequency Generation, Molecular Dynamics Simulations, and ab Initio Calculations

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems

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