Chemistry Reference
In-Depth Information
Imaging surfactant concentration distributions at the
air/water interface
Part 2: Insoluble monolayer concentrations on standing
capillary waves
Gerald M. Korenowski
1
, John R. Saylor
2
, Elizabeth A. van Wagenen
1
,
Joseph S. Kelley
1
,
Mark E. Anderson
1
, and Elizabeth J. Edwards
1
1
Department of Chemistry, Rensselaer Polytechnic Institute,
Troy, NY, USA
2
Naval Research Laboratory, Washington, D.C., USA
Abstract.
Surfactant concentrations at the peaks and troughs of a standing
capillary wave field were measured using a reflected second harmonic
generation (SHG) imaging technique. The results revealed behaviour that
is consistent with a compaction of the insoluble monolayer at the crests of
the capillary waves and a dilation of the monolayer in the troughs. Each
measurement was obtained using a single 3 nanosecond laser pulse, provi-
ding essentially instantaneous measurements of surfactant concentrations.
1 Introduction
In Part 1, we reviewed the utility of reflected second harmonic generation
(SHG) and reflected sum frequency generation (SFG) as ocean surface
probes. We have adapted these laboratory probes to give us the first spec-
troscopic techniques capable of studying the top one or two molecular lay-
ers of the ocean surface
in situ
. The method has been demonstrated in field
studies where we measured surfactant concentration gradients on the ocean
surface (Frysinger et al. 1992, Korenowski et al. 1993, Korenowski 1997).
This ability to measure and determine the chemical composition of the first
two molecular layers is particularly important because surface tension, in-
terfacial elasticity, and interfacial viscosity are a function of the composi-
tion in these layers. While these SHG and SFG probes have proven to be
highly useful, nondestructive, and nonintrusive ocean surface probes, there
are restrictions. In particular, there is a limitation on the spatial resolution
when performing single point measurements on the ocean. The natural di-
vergence in the laser beams and the limited repetition rates (100 Hz or
