title: Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles.
abstract: Elucidating the intrinsic water wettability of the graphitic surface has increasingly attracted research interests, triggered by the recent finding that the well-established hydrophobicity of graphitic surfaces actually results from airborne hydrocarbon contamination. Currently, static water contact angle (WCA) is often used to characterize the intrinsic water wettability of graphitic surfaces. In the current paper, we show that because of the existence of defects, static WCA does not necessarily characterize the intrinsic water wettability. Freshly exfoliated graphite of varying qualities, characterized using atomic force microscopy and Raman spectroscopy, was studied using static, advancing, and receding WCA measurements. The results showed that graphite of different qualities (i.e., defect density) always has a similar advancing WCA, but it could have very different static and receding WCAs. This finding indicates that defects play an important role in contact angle measurements, and the static contact angle does not always represent the intrinsic water wettability of pristine graphite. On the basis of the experimental results, a qualitative model is proposed to explain the effect of defects on static, advancing, and receding contact angles. The model suggests that the advancing WCA reflects the intrinsic water wettability of pristine (defect-free) graphite. Our results showed that the advancing WCA for pristine graphite is 68.6°, which indicates that graphitic carbon is intrinsically mildly hydrophilic.
authors.affiliation: Department of Chemical & Petroleum Engineering, Swanson School of Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Andrew
authors.identifier: 0000-0001-9301-2315
authors.initials: A
authors.lastname: Kozbial
authors.affiliation: Department of Chemical & Petroleum Engineering, Swanson School of Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Charlie
authors.initials: C
authors.lastname: Trouba
authors.affiliation: Department of Chemical & Petroleum Engineering, Swanson School of Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Haitao
authors.identifier: 0000-0003-3628-5688
authors.initials: H
authors.lastname: Liu
authors.affiliation: Department of Chemical & Petroleum Engineering, Swanson School of Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Lei
authors.identifier: 0000-0002-8679-9575
authors.initials: L
authors.lastname: Li