What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

Ahmet Uysal, Miaoqi Chu, Benjamin Stripe, Amod Timalsina, Sudeshna Chattopadhyay, Christian M. Schlepütz, Tobin J. Marks, and Pulak Dutta
Phys. Rev. B 88, 035431 – Published 17 July 2013

Abstract

The free surface of water and the interface between water and a hydrophobic surface both have positive interface energies. The water density near a free surface drops below the bulk density, and thus it is expected that water near a hydrophobic surface will also show a density depletion. However, efforts by multiple groups to detect and characterize the predicted gap at water-hydrophobic interfaces have produced contradictory results. We have studied the interface between water and fluoroalkylsilane self-assembled monolayers using specular x-ray reflectivity and analyzed the parameter-space landscapes of the merit functions being minimized by data fitting. This analysis yields a better understanding of confidence intervals than the customary process of reporting a unique best fit. We conclude that there are unambiguous gaps at water-hydrophobic interfaces when the hydrophobic monolayer is more densely packed.

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  • Received 13 March 2013

DOI:https://doi.org/10.1103/PhysRevB.88.035431

©2013 American Physical Society

Authors & Affiliations

Ahmet Uysal1,*, Miaoqi Chu1, Benjamin Stripe1,†, Amod Timalsina2, Sudeshna Chattopadhyay3, Christian M. Schlepütz4, Tobin J. Marks2, and Pulak Dutta1

  • 1Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA
  • 2Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
  • 3School of Basic Sciences, Indian Institute of Technology Indore, Indore 452017, India
  • 4X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

  • *Present address: Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
  • Present address: Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.

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Issue

Vol. 88, Iss. 3 — 15 July 2013

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