title: Orientational Dynamics of Water at an Extended Hydrophobic Interface.
abstract: We report on the orientational dynamics of water at an extended hydrophobic interface with an octadecylsilane self-assembled monolayer on fused silica. The interfacial dangling OH stretch mode is excited with a resonant pump, and its evolution followed in time by a surface-specific, vibrationally resonant, infrared-visible sum-frequency probe. High sensitivity pump-probe anisotropy measurements and isotopic dilution clearly reveal that the decay of the dangling OH stretch excitation is almost entirely due to a jump to a hydrogen-bonded configuration that occurs in 1.61 ± 0.10 ps. This is more than twice as fast as the jump time from one hydrogen-bonded configuration to another in bulk H2O but about 50% slower than the reported out-of-plane reorientation at the air/water interface. In contrast, the intrinsic population lifetime of the dangling OH stretch in the absence of such jumps is found to be >10 ps. Molecular dynamics simulations of air/water and hexane/water interfaces reproduce the fast jump dynamics of interfacial dangling OH with calculated jump times of 1.4 and 1.7 ps for the air and hydrophobic interfaces, respectively. The simulations highlight that while the air/water and hydrophobic/water surfaces exhibit great structural similarities, a small stabilization of the OH groups by the hydrophobic interface produces the pronounced difference in the dynamics of dangling bonds.
authors.affiliation: Department of Physics and Astronomy, Michigan State University , East Lansing, Michigan 48824, United States.
authors.forename: Shunhao
authors.initials: S
authors.lastname: Xiao
authors.affiliation: Department of Physics and Astronomy, Michigan State University , East Lansing, Michigan 48824, United States.
authors.forename: Florian
authors.initials: F
authors.lastname: Figge
authors.affiliation: CNRS Laboratoire de Biochimie Théorique, Institut de Biologie Physico-Chimique, Univ. Paris Diderot, Sorbonne Paris Cité, PSL Research University, 13 rue Pierre et Marie Curie, 75005 Paris, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Stirnemann
authors.affiliation: École Normale Supérieure-PSL Research University, Département de Chimie, Sorbonne Universités - UPMC Univ Paris 06, CNRS UMR 8640 PASTEUR, 24, rue Lhomond, 75005 Paris, France.
authors.forename: Damien
authors.initials: D
authors.lastname: Laage
authors.affiliation: Department of Physics and Astronomy, Michigan State University , East Lansing, Michigan 48824, United States.
authors.forename: John A
authors.initials: JA
authors.lastname: McGuire