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Adsorption of Aerosol-OT to Sapphire: Lamellar Structures Studied with Neutrons
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics.
2011 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 27, no 8, 4669-4678 p.Article in journal (Refereed) Published
Abstract [en]

The adsorption of sodium bis 2-ethylhexyl sulfosuccinate, NaAOT, to a sapphire surface from aqueous solution has been studied by neutron reflection at concentrations above the critical micelle concentration (cmc). Complementary measurements of the bulk structure were made with small-angle neutron scattering and grazing incidence small-angle neutron scattering. At a concentration of about 1% wt (10 X cmc), lamellar phase NaAOT was observed both at the surface and in the bulk. The structure seen at the interface for a solution of 2% wt NaAOT is a 35 +/- 2 angstrom thick bilayer adsorbed to the sapphire surface at maximum packing density, followed by an aligned stack of fluctuating bilayers of thickness 51 +/- 2 angstrom and with an area per molecule of 40 +/- 2 angstrom(2). Each bilayer is separated by a water: at 25 degrees C, this layer is 148 +/- 2 angstrom. A simple model for the reflectivity from fluctuating layers is presented, and for 2.0% wt NaAOT the fluctuations were found to have an amplitude of 25 +/- 5 angstrom. The temperature sensitivity of the structure at the surface was investigated in the range 15-30 degrees C. The effect of temperature was pronounced, with the solvent layer becoming thinner and the volume occupied by the NaAOT molecules in a bilayer increasing with temperature. The amplitude of the fluctuations, however, is approximately temperature independent in this range. The adsorption of NaAOT at the sapphire surface resembles that previously found at hydrophilic and hydrophobic silica surfaces. The coexisting bulk lamellar phase has a spacing of layers similar to that observed at the surface. These observations are an indication that the major driving force for adsorption is self-assembly, independent of the chemical nature of the interface.

Place, publisher, year, edition, pages
2011. Vol. 27, no 8, 4669-4678 p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-152837DOI: 10.1021/la1048985ISI: 000289321000053OAI: oai:DiVA.org:uu-152837DiVA: diva2:414174
Available from: 2011-05-02 Created: 2011-05-02 Last updated: 2012-08-01Bibliographically approved
In thesis
1. Adsorption and Ordering of Surface Active Molecules and Particles at Solid Interfaces and in the Bulk
Open this publication in new window or tab >>Adsorption and Ordering of Surface Active Molecules and Particles at Solid Interfaces and in the Bulk
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Factors that influence the process of adsorption and order of dilute and concentrated systems of molecules and particles are explored in this thesis.  The results are based largely on neutron scattering techniques.  Study of the adsorption from dilute solutions of a common surfactant, AOT (sodium bis (2-ethylhexyl) sulfosuccinate), at a solid/liquid interface showed that AOT molecules adsorbed to the interface in a bilayer structure, with the hydrocarbon tails at maximum packing density even at very low concentrations.  At higher AOT concentrations, a stack of fluctuating layers each separated by large amounts of water next to the dense bilayer was seen.  The driving force for adsorption is dominated by self-assembly of AOT.  It was found that an oriented lamellar phase wets the interface below the bulk concentration for formation of this phase.

Proteins can be viewed as polymeric surfactants.  The adsorption of proteins from seeds of the Moringa oleifera tree to a silicon oxide surface was studied to elucidate the mechanism of the protein as a flocculent in water treatment processes.  The protein was found to adsorb at the interface as dense layers with a thickness suggestive of co-adsorption rather than single isolated molecules.  The strong adsorption and tendency to associate in solution suggest mechanisms for flocculating particulate impurities in water.

As with surfactants, dispersions of colloidal particles can assemble in regular structures by self-assembly.  Polystyrene latex particles were studied and could form large three-dimensional crystals of about 1×1 cm2 in a 2 mm path cell.  The diffraction pattern indicated a close packed structure with the 110 axis perpendicular to the container wall.  The crystal was well-aligned and oriented by the direction of flow.  At the solid interface large two-dimensional domains of about 20 cm2 of highly oriented particles were formed.  The particle-particle separation at the surface and in the bulk was determined by the charge repulsion of the particles.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 56 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 915
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
urn:nbn:se:uu:diva-171739 (URN)978-91-554-8327-2 (ISBN)
Public defence
2012-05-16, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Available from: 2012-04-24 Created: 2012-03-27 Last updated: 2012-08-01Bibliographically approved

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