Consequences of Water between Two Hydrophobic Surfaces on Adhesion and Wetting Adrian P. Defante, Tarak N. Burai, Matthew L. Becker, and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States * S Supporting Information ABSTRACT: 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 water- free 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 self- assembled 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-Dupre ́ equation, 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. ■ INTRODUCTION Hydrophobic 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 hydro- phobic 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 nano- bubbles, 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. Octadecyltri- chlorosilane monolayer (OTS) and spin-coated semicrystalline ( T m ∼ 50 ° ) poly(vinyl- n -octadecyl carbamate) fi lms (PVNODC) were chosen because these surfaces are considered to be equally hydrophobic. 28 Both of these surfaces are composed of well-ordered all-trans long hydrophobic chains with surface terminal methyl groups, which results in high static water contact angles and low contact angle hysteresis. 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 hydro- phobic 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-Dupré equation. We observed 50% lower work of adhesion for PVNODC compared to OTS surfaces from underwater JKR experiments. Surpris- ingly, the thermodynamic works of adhesion underwater Received: November 24, 2014 Revised: February 8, 2015 Published: February 10, 2015 Article pubs.acs.org/Langmuir © 2015 American Chemical Society 2398 DOI: 10.1021/la504564w Langmuir 2015, 31, 2398-2406