posted on 2021-08-13, 07:14authored byJiahui Zhang, Junjun Tan, Ruoqi Pei, Shuji Ye, Yi Luo
Hydrophobic-like
water monolayers have been predicted at the metal
and some polar surfaces by theoretical simulations. However, direct
experimental evidence for the presence of this water layer at surfaces,
particularly at biomolecule and polymer surfaces, is yet to be validated
at room temperature. Here we observe experimentally that an ordered
molecular water layer is present at the hydrophobic fluorinated polymer
such as polytetrafluoroethylene (PTFE) surface by using sum frequency
generation vibrational spectroscopy. The macroscopic hydrophobicity
of PTFE surface is actually hydrophilic at the molecular level. The
macroscopically hydrophobic character of PTFE is indeed resulting
from the hydrophobicity of the ordered two-dimension (2D) water layer,
in which cyclic water tetramer structure is found. The water layer
at humidity of ≤40% has a vibrational relaxation time of 550
± 60 fs. The vibrational relaxation time in the frequency range
of 3200–3400 cm–1 shows remarkable difference
from the interfacial water at the air/H2O interface and
the lipid/H2O interface. No discernible frequency dependence
of the vibrational relaxation time is observed, indicating the homogeneous
dynamics of OH groups in the water layer. These insights into the
water layer at the macroscopically hydrophobic surface may contribute
to a better understanding of the hydrophobic interaction and interfacial
water dynamics.