Liquid–Vapor
Interface of Formic Acid Solutions
in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic
in Situ X‑ray Photoelectron Spectroscopy Measurements
- Publication date
- Publisher
Abstract
The liquid–vapor interface
is difficult to access experimentally
but is of interest from a theoretical and applied point of view and
has particular importance in atmospheric aerosol chemistry. Here we
examine the liquid–vapor interface for mixtures of water, sodium
chloride, and formic acid, an abundant chemical in the atmosphere.
We compare the results of surface tension and X-ray photoelectron
spectroscopy (XPS) measurements over a wide range of formic acid concentrations.
Surface tension measurements provide a macroscopic characterization
of solutions ranging from 0 to 3 M sodium chloride and from 0 to over
0.5 mole fraction formic acid. Sodium chloride was found to be a weak
salting out agent for formic acid with surface excess depending only
slightly on salt concentration. In situ XPS provides a complementary
molecular level description about the liquid–vapor interface.
XPS measurements over an experimental probe depth of 51 Å gave
the C 1s to O 1s ratio for both total oxygen and oxygen from water.
XPS also provides detailed electronic structure information that is
inaccessible by surface tension. Density functional theory calculations
were performed to understand the observed shift in C 1s binding energies
to lower values with increasing formic acid concentration. Part of
the experimental −0.2 eV shift can be assigned to the solution
composition changing from predominantly monomers of formic acid to
a combination of monomers and dimers; however, the lack of an appropriate
reference to calibrate the absolute BE scale at high formic acid mole
fraction complicates the interpretation. Our data are consistent with
surface tension measurements yielding a significantly more surface
sensitive measurement than XPS due to the relatively weak propensity
of formic acid for the interface. A simple model allowed us to replicate
the XPS results under the assumption that the surface excess was contained
in the top four angstroms of solution