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AFM of surfactants on surfaces
The spontaneous aggregation of surfactants into micelles of finite size in aqueous solutions was first proposed by
McBain in 1913. Micellization in solution has since been investigated using a variety of experimental techniques
( X-ray scattering, fluorescence quenching, and cryo-transmission electron microscopy, and this process is now
fairly well understood both in terms of the geometry and thermodynamics of aggregation. An analogous aggregation
process at solid-liquid interfaces was first proposed by Fuerstenau in 1955, based on the adsorption characteristics
of anionic surfactants on alumina. However, an understanding of interfacial aggregation has been slower in coming,
partly due to the experimental difficulties in involved in detecting structure in nanometer-scale adsorbate films
in a liquid environment. Surfactant adsorption has been quantified by solution-depletion methods, which measure
the effective surface area per adsorbed molecule, and by surface force apparatus measurements, which measure the
charge density (or surface potential) and thickness of the adsorbate layer; however, these measurements are often
consistent with more than one adsorbate structure. Evidence for lateral structure in adsorbed surfactant films has
come from neutron reflection, fluorescence quenching, surface force measurements and calorimetry, but the size,
shape and lateral organization of aggregates has proven difficult to quantify.
Atomic force microscopy (AFM) has been used to image interfacial aggregates directly, in situ and at
nanometer resolution. The key to this application lies in an unusual contrast mechanism, namely a pre-contact
repulsive force (“colloidal stabilization force”) between the adsorbed surfactant layers on the tip and sample.
In contrast to previous adsorbate models of flat monolayers and bilayers, AFM images have shown a striking variety
of interfacial aggregates – spheres, cylinders, half-cylinders and bilayers – depending on the surface chemistry
and surfactant geometry.
Contact info:
Srin Manne
Physics Dept
PAS 575
520-626-5305
smanne@physics.arizona.edu
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Tapping mode AFM image of surfactant 18-3-1 on mica at 51° C. Image size 400nm x
400nm. Image taken with a Digital Instruments Dimension 3100 with a fluid tapping cell and a silicon
nitride tip (spring constant approx 0.1 N/m). 3mM 18-3-1 in DI H2O. Deflection image, 0.9 nm z-range. Surfactant
synthesized by the lab of Prof. G. D. Stucky, UCSB chemistry.
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Model of 18-3-1 surfactant molecule
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Tapping
mode AFM image of surfactant 18-3-1 on graphite. Image size 200nm x 200nm.
Image taken with a Digital Instruments Dimension 3100 with a
fluid tapping cell and an oxide sharpened silicon nitride tip(spring constant
approx 0.1 N/m). 3mM 18-3-1 in DI H2O. Height image, 4 nm z-range. Surfactant
synthesized by the lab of Prof. G. D. Stucky, UCSB chemistry.
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Model of TTAB (Tetradecyltrimethyammonium bromide C17H38BrN or
C14H29-N-(CH3)3-Br )
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