Interferometric enhancement of x-ray reflectivity from unperturbed Langmuir monolayers
of amphiphiles at the liquid-gas interface
Venkata Krishnan,
1
Joseph Strzalka,
1,2
Jing Liu,
1
Chian Liu,
2
Ivan Kuzmenko,
2
Thomas Gog,
2
and J. Kent Blasie
1,
*
1
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
2
Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
Received 3 August 2009; revised manuscript received 24 November 2009; published 12 February 2010
Langmuir monolayers provide an important system for the investigation of the intramolecular structure and
intermolecular ordering of organic and bio-organic macromolecular amphiphiles at an interface between polar
and nonpolar media, e.g., the liquid-gas interface. Specular x-ray and neutron reflectivity have contributed
substantially to these investigations. However, these reflectivity techniques are generally limited by the absence
of crucial phase information, the relatively small contribution of the amphiphile to the scattering-length density
contrast across the interface, and the relatively limited range of momentum transfer available perpendicular to
the interface. Although several procedures have been developed to provide model-independent solutions to the
phase problem, there remains a limited ability to distinguish features of slightly differing contrast i.e., the
“sensitivity” as well as their minimum allowable separation i.e., the “spatial resolution” along the length of
the scattering-length density profile derived from the reflectivity data via solution to the phase problem. Here,
we demonstrate how the well-known interferometric approach can be extended to the structural investigation of
otherwise unperturbed Langmuir monolayers of these amphiphiles to provide a direct solution to the phase
problem and importantly, substantially enhance both the sensitivity and the spatial resolution in the derived
profiles.
DOI: 10.1103/PhysRevE.81.021604 PACS numbers: 68.18.-g, 61.05.cm
I. INTRODUCTION
Langmuir monolayers of organic and bio-organic am-
phiphilic macromolecules formed at liquid-gas interface pro-
vide an important system for the study of the structure and
properties of these amphiphiles at the interface between polar
and nonpolar environments 1. These two-dimensional 2D
systems can serve not only as models for the study of com-
plex processes exhibited by biological membranes 2, but
also as precursors in the formation of new ultrathin film ma-
terials with superior properties 3. Importantly, these Lang-
muir monolayers offer the possibility of controlling both the
intramolecular structure and intermolecular ordering of the
amphiphilic macromolecules via several available adjustable
parameters including surface pressure, temperature, relative
concentrations of the macromolecular species in mixtures,
and the compositions of both the aqueous subphase and the
gaseous superphase with respect to the interface 4. These
ordered Langmuir monolayers of amphiphiles can also serve
as templates for the directed 2D assembly of components
soluble in the subphase at the amphilphile-subphase inter-
face, ranging from atoms to proteins and nanoparticles 5.
These Langmuir monolayers can subsequently be conve-
niently transferred onto the bare or alkylated surface of solid
inorganic substrates using Langmuir-Blodgett LB6 or
Langmuir-Schaefer LS7 techniques to prepare ultrathin
organic or bio-organic films at the solid-liquid or solid-gas
interface. These so-transferred ultrathin films on solid inor-
ganic substrates are more generally used for investigating
both their structure and particularly their properties 4.
However, LB and LS depositions are nonequilibrium tech-
niques, and therefore the intramolecular structure and inter-
molecular ordering of the macromolecular species prepared
in the precursor Langmuir monolayer may not necessarily be
preserved upon transfer onto the solid substrate surface
8–11. Hence, it is important to study these structural pa-
rameters both prior to and following their transfer onto solid
substrates.
In the structural characterization of Langmuir monolayers
of organic and bio-organic amphiphiles at the liquid-gas in-
terface, both specular and off-specular x-ray and neutron re-
flectivity and grazing-incidence x-ray diffraction are proven
key techniques 4,6. However, the specular reflectivity tech-
niques are generally limited by the absence of crucial phase
information, the relatively small contribution of the am-
phiphile in the absence of heavy-atom or isotopic labeling in
the x-ray and neutron cases, respectively to the scattering-
length density contrast across the interface, and the relatively
limited range of momentum transfer available perpendicular
to the interface. Thus, although several procedures have been
developed to provide model-independent solutions to the
phase problem e.g., 12 and references therein, there re-
mains a limited ability to distinguish features of slightly dif-
fering contrast along the length of the scattering-length den-
sity profile derived from the reflectivity data via solution to
the phase problem, thereby defining the “sensitivity” of the
technique. In addition, the minimum allowable separation of
these features along the length of the scattering-length den-
sity profile, defining the “spatial resolution,” remains low,
e.g., at 10–15 Å level.
In the past, the interferometric approach has been success-
fully employed in specular x-ray and neutron reflectivity
measurements 13,14 of organic and bio-organic monolay-
ers, either transferred onto or self-assembled on the surface
of solid inorganic substrates, including monolayers of vecto-
*
Corresponding author. jkblasie@sas.upenn.edu
PHYSICAL REVIEW E 81, 021604 2010
1539-3755/2010/812/02160410 ©2010 The American Physical Society 021604-1